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1

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Location of NIPR

3

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5

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6

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7

8

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Session 1

9

Paleoclimate modeling perspective on understanding the Dynamics of Antarctic Ice Sheet Change Implications from experiments with MIROC AOGCM

Ayako Abe-Ouchi1,2,3, T. Obase1, S. Sherrif-Tadano1,W.-L., Chan1, A. Oka1, F. Saito2, K. Kusahara2, M. Yoshimori4,1 and R.

Greve4 1The University of Tokyo

2JAMSTEC, 3 NIPR and 4Hokkaido University Multiple lines of Paleoclimate evidence show that the Antarctica ice sheet retreated as much as 3 to 7 meters equivalent of sea level at the Last interglacials, about 130- 110 thousand years ago. This is important to be used as a test for understanding the tipping point like behavior of Antarctic ice sheet. The cause is related to the climate warming at that time, the ultimate cause of the warming and the mass loss and the understanding of the link to future projection is still missing. During the last termination of ice age cycle (deglaciation), Heinrich event 1 as well as B/A, Antarctic Cold Reversal (ACR) and Younger Dryas occurred as millennial scale climate changes, while the glacial termination looked different for the penultimate termination around 135 to 130 thousand years ago. Here we ran and plan to run several experiments on glacial terminations as well as sensitivity experiments using a coupled atmosphere and ocean GCM (MIROC4m AOGCM) developed in Japan. We analyze the stability of Southern ocean and AMOC (Atlantic Meridional Overturning Circulation) and climate. Each model run was run for more than 10000 years under many different conditions with different Greenhouse Gas levels, Milankovith forcing with and without glacial (global) ice sheets and with and without freshwater flux into North Atlantic region. The MIROC AOGCM results show large self-sustained oscillation of AMOC and high latitude temperature change similar to B/A and D-O cycles, comparable with ice core data and deep-sea data for some cases. We show that the D-O like oscillation and B/A-Y/D type change occur under limited range of CO2 and freshwater forcing. The implication for the response of past Antarctica ice sheet to the paleoclimate change and the perspective on understanding the dynamics, such as tipping points of Antarctic ice sheet are discussed.

10

Roles of warm and cold sea water pumps along the coast of East Antarctica

Shigeru Aoki1,2 1 Institute of Low Temperature Science, Hokkaido University

2 National Institute of Polar Research Along with the anthropogenic climate change, changes in meridional overturning circulations and ice mass of the Southern Ocean and Antarctic Ice Sheet are listed as the problems with huge social implications. For Amundsen Sea, West Antarctica, accelerations of basal melting of ice shelves and loss of ice mass have been observed (eg., Paolo et al., 2015; IMBIE team, 2018) and heat supply of warm sea water are considered to play a pivotal role in their processes. The acceleration of ice discharge could affect the downstream ocean freshening of cold shelf and bottom waters of the Ross Sea (Jacobs et al., 2010). In the Weddell Sea, which is the major source of the Antarctic Bottom Water, possibility of substantial reduction in bottom water formation is proposed due to a change in behaviors of waters on the continental shelf in connection with the global warming (Hellmer et al., 2012). Both of these processes – warm water intrusion and cold/dense water formation – characterizes the ocean sectors off East Antarctica (Fig. 1). Beneath the Totten Glacier Ice Shelf off Wilkes Land, ice discharge is accelerating, and a pathway of substantially warm water access has been discovered (Rintoul et al., 2016; Silvano et al., 2018). The ice mass behind the Totten Glacier is estimated to be 3.5 m of global sea level equivalent, and the bed rock configuration reveals a vulnerability of the maritime ice sheet instability (eg. Greenbaum et al., 2015). Geological evidence has been discovered that points to the substantial disintegration of ice sheets on George V Land during the Pliocene, when the surface temperature was higher by several degrees than that of the present climate. Recently, in the Lützow-holm Bay in the Weddell-Enderby Basin, access of the warm water to the inflowing glacier, Shirase Glacier, and subsequent outflow of melt water has been revealed. The region could be accumulating ice mass in the recent decades, and the ice tongue of Shirase Glacier is also known to have decadal/multi-decadal cycle of disintegration (Ushio, 2006). Hence revealing the behaviors of the oceans and continental ice are important in investigating the ice-ocean interactions and freshwater budgets over the whole Antarctica. Fig.1 Circumpolar Ice conditions around Antarctica. Warm color range denotes the amount of annual sea ice production. Blue denotes the landfast sea ice area. Red arrows are the schematic pathways of Antarctic Circumpolar Current. Based on the map of Nihashi and Ohshima (2015).

11

Along the East Antarctic coast, coastal polynyas are ubiquitous. Sea ice formation and subsequent brine rejection in the polynyas, including Cape Darnley and Vinceness Bay Polynyas as well as Mertz Polynya, result in the production of Dense Shelf Water and lead to the export of bottom water (Ohshima et al., 2013; Kitade et al., 2014). The bottom water off Adélie Land Coast affects the water from the Ross Sea, and the Adélie Land Bottom Water consists one of the basis of the bottom water off Cape Darnley. The Cape Darnley Bottom Water transformed into a variety of the Weddell Sea Deep Water. In addition to the near surface pathway, property changes such as excessive freshwater from the continent can propagate through the pathway near the bottom. In the recent decades, warming and freshening of the Antarctic Bottom Water and changes in shelf waters are reported from the repeat hydrographic observations (eg., Purkey and Johnson, 2010; 2013; Schmidtko et al., 2014). Since the East Antarctic coast is located in the bridging regions of the prominent warm water and cold water signals –Amundsen Sea, Ross Sea and Weddell Sea-, it is indispensable to investigate the water mass exchange process and long-term change in this region to fully understand the circumpolar budgets of the heat and freshwater storage. Together with the multi-decadal trends/variations, presence of variabilities on relatively shorter time scale of one to two decades have been proposed on the continental shelf regions, and their connection with continental ice are considered to be important (Jenkins et al., 2016; Aoki, 2017). To understand of the mechanism of the ice-ocean interaction processes and predict the future change of Antarctic ice mass balance, efforts in East Antarctica are needed to realize the circumpolar observational network of Antarctic ocean and ice. References Aoki, S., Breakup of land-fast sea ice in Lützow-Holm Bay, East Antarctica and its teleconnection to tropical Pacific sea-surface temperatures, Geophys. Res. Lett., 44, 3219-3227., 2017. Jenkins, A., P. Dutrieux, S. Jacobs, E.J. Steig, G.H. Gudmundsson, J. Smith, and K.J. Heywood, Decadal ocean forcing and Antarctic ice sheet response: Lessons from the Amundsen Sea. Oceanography 29(4):106–117, 2016. Greenbaum, J.S., et al., Ocean access to a cavity beneath Totten Glacier in East Antarctica. Nature Geoscience, 8, 294-298, 2015. Hellmer, H., F. Kauker, R. Timmermann1, J. Determann and J. Rae, Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current, Nature, 485, 225-228, 2012. Jacobs, S. S., and C. F. Giulivi, Large multidecadal salinity trends near the Pacific-Antarctic continental margin, J. Clim., 23, 4508-4524, 2010. Kitade, Y., et al., Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica, Geophys. Res. Lett., 41, 3528-3534, doi:10.1002/2014GL059971, 2014. Nihashi, S., and K.I. Ohshima, Circumpolar mapping of Antarctic coastal polynyas and landfast sea ice. Relationship and variability. J. Clim., 28, 3650-3670, 2015. Ohshima, K.I., Y. Fukamachi, G.D. Williams, et al., Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley Polynya, Nature Geoscience, doi:10.1038/ngeo1738, 2013. Paolo, F. S., H.A. Fricker, and L. Padman, Volume loss from Antarctic ice shelves is accelerating. Science 348, 899-903, 2015. Purkey, S. G., and G. C. Johnson, Antarctic Bottom Water warming between the 1990s and the 2000s: Contributions to global heat and sea level rise budgets, J. Clim., 23, 6336-6351, 2010. Purkey, S.G. and G.C. Johnson, Antarctic Bottom Water warming and freshening: contributions to sea level rise, ocean freshwater budgets, and global heat gain. J. Clim., 26, 6015-6122, 2013. Rintoul, R.S., A. Silvano, B. Pena-Molino, E. van Wijk, M. Rosenberg, J. S. Greenbaum, D. D. Blankenship, Ocean heat drives rapid basal melt of the Totten Ice Shelf. Sci. Adv., 2, e1601610, 2016. Schmidtko, S., K.J. Heywood, A.F. Thompson, S. Aoki, Multi-decadal warming of Antarctic waters, Science, 346 (6214), 1227-1231, 2014. Silvano, A., et al., Freshening by glacial meltwater enhances melting of ice shelves and reduces formation of Antarctic Bottom Water, Science Adv., 4, eaap9467, 2018. The IMBIE team, Mass balance of the Antarctic Ice Sheet from 1992 to 2017, Nature 558, 219-222, 2018. Ushio, S., Factors affecting fast-ice break-up frequency in Lützow-Holm Bay, Antarctica, Ann. Glaciol., 44, 177–182, 2006.

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Session 2

13

Impact of ice-shelf buttressing on Antarctic ice-sheet dynamics

Frank Pattyn1, Sainan Sun1

1Laboratoire de Glaciologie, Université libre de Bruxelles, Brussels, Belgium The reaction of the Antarctic ice sheet to atmospheric and ocean forcing happens to a large extent through weakening of ice shelves, concomitant reduction in ice-shelf buttressing, leading to grounding-line retreat, inland ice acceleration and loss of grounded ice mass. While the processes governing ice-shelf weakening are quite complex, due to specific interactions with atmosphere (surface melt, meltwater percolation, refreezing) and ocean (CDW circulation changes, ice-shelf- ocean interactions), uncertainties on the response of the grounded ice sheet in response to decreased buttressing is therefore harder to assess. We present the results of two experiments where ice-shelf buttressing was totally or greatly removed either due to complete removal of ice shelves or extreme sub-shelf melt, with the f.ETISh ice sheet model (Pattyn, 2017). Results with different models and model setups reveals that grounding line retreat is dominated by the sensitivity to basal friction characteristics and processes defining basal properties. A model comparison (ABUMIP) on similar experiments also demonstrates the wide range of responses to the applied de-buttressing; However, the range of ice-sheet model response is significantly narrower compared to SeaRISE (Bindschadler et al., 2013), demonstrating major improvements in ice sheet models to cope with grounding line dynamics. References Bindschadler, R. A., Nowicki, S., Abe-Ouchi, A., Aschwanden, A., Choi, H., Fastook, J., Granzow, G., Greve, R., Gutowski, G., Herzfeld, U., Jackson, C., Johnson, J., Khroulev, C., Levermann, A., Lipscomb, W. H., Martin, M. A., Morlighem, M., Parizek, B. R., Pollard, D., Price, S. F., Ren, D., Saito, F., Sato, T., Seddik, H., Seroussi, H., Takahashi, K., Walker, R., and Wang, W. L.: Ice-sheet model sensitivities to environmental forcing and their use in projecting future sea level (the SeaRISE project), J. Glaciol., 59, 195–224, 2013 Pattyn, F.: Sea-level response to melting of Antarctic ice shelves on multi-centennial timescales with the fast Elementary Thermomechanical Ice Sheet model (f.ETISh v1.0), The Cryosphere, 11, 1851-1878, https://doi.org/10.5194/tc-11-1851-2017, 2017.

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Ice sheet evolution in the past and into the future, and the role of interactions with the ocean

Nicholas R. Golledge1,2

1Antarctic Research Centre, Victoria University of Wellington, Wellington 6140, New Zealand2GNS Science, Avalon, Lower Hutt 5011, New Zealand

Even with government pledges to reduce greenhouse gas emissions as part of the Paris Agreement, we are likely committed to

3-4°C surface warming above pre-industrial levels by 2100 CE, leading to enhanced ice-sheet melt. But the mechanisms that

drive oscillations of the Greenland and Antarctic ice sheets differ, so it isn't always clear how sensitive each might be to future

warming amounts or to the predicted rates. In this presentation I will describe some of the controls on ice sheet dynamics, and

will use examples spanning the past, present, and future to illustrate the degree to which these systems are understood, and

where the key uncertainties remain. Focusing primarily on Antarctica, this presentation will explore the way that ice sheets

interact with adjacent oceans, and will illustrate some of the ways in which geological data can be used to constrain model

parameterisation. Looking to the future, I will demonstrate how satellite-based measurements of recent ice mass change can be

employed to constrain Greenland and Antarctic ice-sheet simulations, and how ice-ocean interactions can be simulated in a

way that allows feedbacks to be incorporated that may enhance future melting. We will also look at how ice sheet meltwater

will impact the global climate, for example, by slowing the Atlantic overturning circulation or trapping warm water below the

sea surface around Antarctica, with the latter leading to thermosteric changes that substantially affect regional sea level.

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Modeling activities for ocean–cryosphere interactions over the Southern Ocean with a Japanese OGCM

Kazuya Kusahara1

1Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Ice shelves are marine termini of ice sheets and are afloat in the ocean. Antarctic Ice Sheets have many ice shelves in the coastal margins, being 11% of all Antarctic Ice Sheets in the areal extent and 44% of the coastline. There are two processes, iceberg calving at ice shelf fronts and basal melting/freezing, which induce mass exchange between the ice sheet and ocean. These two processes are the main ablation mechanisms in the total mass balance over the Antarctic Ice Sheets. The mass exchange between the Antarctic Ice Sheets and the Southern Ocean has drawn much attention scientifically and societally because it directly results in a global sea level change (i.e., a large uncertainty in future sea-level projection). Melting of ice shelves by itself contributes little to the sea level change because they are already afloat in the ocean. However, a change of the shape of ice shelf could alter the stress distribution of the ice shelf. Subsequently, the information could be rapidly transmitted to the ice sheet interior through ice sheet dynamics and could be a trigger for an abrupt change of the ice sheets and sea level. Recent satellite analyses show a significant increase of ice discharges from the Antarctic Ice Sheets, suggesting a negative mass balance of the Antarctic Ice Sheet. Thinning of the ice sheet interior near coastal regions has been observed, and its primary cause is considered to be enhanced ice shelf-ocean interactions. Water properties of temperature and salinity in the Southern Ocean have also changed significantly in the recent decades. Mass losses from the Antarctic ice shelves result in additional freshwater flux into the ocean. Basal melting of ice shelves can be considered to have a large impact on dense water formation because coastal polynyas for high sea ice production and the associated salt input into the ocean are often located next to ice shelves. Over the recent decades, freshening of the Antarctic Bottom Water has been reported, and several studies have pointed out that enhanced melting of the continental ices contributes to the freshening of the Antarctic Bottom Water. In order to investigate ocean-cryosphere interaction over the Southern Ocean, an ice shelf component was incorporated into a Japanese OGCM, named COCO. The OGCM has been used for several purposes of the global climate system. The model has been developed by Atmosphere Ocean Research Institute, the University of Tokyo (AORI) and Japan Agency for Marine-Earth Science and Technology (JAMSTEC). It solves the primitive equations with the Boussinesq and hydrostatic approximations. The equations are formulated on the generalized curvilinear coordinate in the horizontal direction (with the B-grid system) and on a hybrid of sigma- and z-coordinates in the vertical direction. A sea ice component can be optionally used. The steady-shape ice shelf component was incorporated in the z-coordinate system. In the last five years, the coupled ocean-sea ice-ice shelf model has been utilized for several applications from regional to circumpolar Southern Ocean scales. In this presentation, I would like to briefly introduce some of our recent modeling activities which links to basal melt at Antarctic ice shelves. A circumpolar Southern Ocean configuration with the horizontal resolution of 10-15 km for the Antarctic coastal regions has been used for estimating basal melt of the ice shelves in different climate conditions (present-day and LGM) and tracing pathways of basal meltwater and newly-formed coastal dense waters. These experiments suggest that the relative strength between cold and water masses (i.e., Dense Shelf Water, Circumpolar Deep Water (CDW), Antarctic Surface Water) over the continental shelf regions is important to understand basal melt of Antarctic ice shelves, regarding the regional difference and the response to climate changes. The OGCM can also be used for regional applications with the horizontal resolution of a few kilometers. Since the equations are formulated on the generalized curvilinear coordinate, the model can increase the horizontal resolution by placing the singular points near the focal region, keeping the circumpolar Southern Ocean domain. Using this method, we have configured two regional applications for Adelie and George V Land (AGVL) and Lützow-Holm Bay (LHB), East Antarctica. In the AGVL application, we estimated the impacts of the Mertz Glacier Tongue calving event on sea ice production, basal melt, and dense shelf water formation. In the LHB application, the model reproduced a pronounced intrusion of warm CDW onto the continental shelf which is strongly guided by the north-south oriented trough in the LHB. The model clearly shows that the CDW intrusion results in strong basal melting at Shirase Glacier Tongue which is located in the south end of the LHB. The model results for the LHB application are consistent with the recent in-situ oceanographic observations.

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Bipolar-seesaw climate changes during the last two deglaciations and implications for Antarctic ice sheet extent during the following interglacials

Takashi Obase1, Ayako Abe-Ouchi12

1Atmosphere and Ocean Research Institute, the University of Tokyo 2Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

Reconstructions indicate that the surface climate was warmer than the present-day in Antarctic regions during the Last Interglacial (LIG). The retreat of West Antarctic Ice Sheet during LIG can be reproduced if warmer Antarctic ocean is take into account (Deconto and Pollard 2016). Previous studies suggested that a weakened Atlantic Meridional Overturning Circulation (AMOC) contributed to the warm Antarctic climate by the “bipolar seesaw” mechanism of a weakened meridional heat transport of the ocean (Holden et al. 2010; Stone et al. 2017). However, the warm Antarctic climate was not reproduced if the atmospheric greenhouse gas concentrations, insolation, and glacial meltwater flux at the LIG were considered (Lunt et al. 2013).

First, we focus on the reconstructed AMOC change during the last two deglaciations, indicating the AMOC was weak during the deglaciation before LIG (Deaney et al. 2017), different from the last deglaciation in the point there is an early recovery in the AMOC (Liu et al. 2009). We show that the speed of Northern Hemisphere ice sheet melting during deglaciation has a significant impact on the Antarctic climate, by transient cliamte simulations from the glacial to the interglacial using MIROC, an atmosphere-ocean coupled general circulation model. The changing insolation, atmospheric greenhouse gas concentrations were applied to the climate model based on reconstructions, and glacial meltwater fluxes were applied. The magnitude of simulated temperature differences in the Antarctic regions was close to reconstructions of the early LIG. Second, we estimated the impact of the deglaciatial climate change on the mass balance change of Antarctic ice shelves using a regional ocean model resolving Antarctic ice shelves (Kusahara and Hasumi 2013; Obase et al. 2017). The outputs of MIROC is used as a atmospheric and oceanic boundary conditions for the regional ocean model. We discuss the factors that affected climate changes during the last two deglaciation, and the impact on the extents of Antarctic ice sheet retreat during the following interglacial. References Deaney, E. L., S. Barker and T. van de Flierdt (2017), Timing and nature of AMOC recovery across Termination 2 and magnitude of deglacial CO2 change, Nature Communications, 8, doi:10.1038/ncomms14595. Deconto, R. M., and D. Pollard (2016), Contribution of Antarctica to past and future sea-level rise. Nature, 531 (7596), 591-597, doi:10.1038/nature17145. Holden, P. B., N. R. Edwards, E. W. Wolff, N. J. Land, J. S. Singarayer, P. J. Valdes and T. F. Stocker (2010), Interhemispheric coupling, theWest Antarctic Ice Sheet and warm Antarctic interglacials, Climate of the Past, 6, 431-443, doi:10.5194/cp-6-431-2010 Kusahara, K., and H. Hasumi (2013), Modeling Antarctic ice shelf responses to future climate changes and impacts on the ocean. Journal of Geophysical Research: Oceans, 118, 2454-2475, doi:10.1002/jgrc.20166. Liu, Z., et al. (2009), Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød warming, Science, 325(5938), 310-314, doi:10.1126/science.1171041. Lunt, D. J. et al. (2013), A multi-model assessment of last interglacial temperatures, Climate of the Past, 9, 699-717, doi:10.5194/cp-9-699-2013. Obase, T, A. Abe-Ouchi, K. Kusahara, H. Hasumi, R. Ohgaito (2017), Responses of basal melting of Antarctic ice shelves to the climatic forcing of the Last Glacial Maximum and CO2 doubling, Journal of Climate, 30(10), 3473-3497, doi:10.1175/JCLI-D-15.0908.1. Stone, E. J., E. Capron, D. J. Lunt, A. Payne, J. S. Singarayer, P. J. Valdes and E. W. Wolff (2016), Impact of meltwater on high-latitude early Last Interglacial climate, Climate of the Past, 12, 1919-1932, doi:10.5194/cp-12-1919-2016.

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On the interaction of millennial climate variability, Southern Ocean and Antarctic ice sheet Sam Sherriff-Tadano1, Takashi Obase1 and Ayako Abe-Ouchi1

1Atmosphere and Ocean Research Institute, the University of Tokyo Paleoclimatic records show evidences of climate variabilities on millennial time scale during the glacial period. These climate variabilities accompanied significant changes in surface air temperature, sea ice and ocean temperature over the Southern Ocean, which could have impacted on the Antarctic ice sheet through basal melting of the ice shelf. Furthermore, the melting of the Antarctic ice sheet in turn affects the ocean salinity and convection, which impacts on the deep ocean circulation and climate. This implies important interactions between the Southern Ocean and the Antarctic ice sheet in understanding the past climate changes. However, the interaction of the millennial climate variability, the Southern Ocean and the Antarctic ice sheet remains elusive. Here, we show results of a 10,000-year simulation conducted with MIROC4m AOGCM under mid-glacial boundary conditions, which exhibits internal variability of climate on millennial time scale. Analysis focuses on the responses of sea ice, surface air temperature and subsurface ocean temperature, which affect the Antarctic ice sheet through basal melting of the ice shelf. Furthermore, we will present results of sensitivity experiments, which aim to evaluate the role of freshwater discharge from the Antarctic ice sheet on the climate, and to evaluate the effect of changes in sea ice over the Southern Ocean on the frequency of the millennial climate variably.

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Simulations of glacial ocean carbon cycle with a parameterization of brine rejection

Hidetaka Kobayashi1 and Akira Oka1 1Atmosphere and Ocean Research Institute, the University of Tokyo

Atmospheric carbon dioxide concentration (pCO2) at the Last Glacial Maximum (LGM) is about 100 ppm lower than that in the pre-industrial period (Petit et al., 1999). The ocean carbon cycle is recognized to play the primary role in this glacial reduction of atmospheric pCO2 (Broecker, 1982). However, previous paleoclimate modeling studies using an ocean general circulation model (OGCM) have failed to reproduce all the variation of atmospheric pCO2. One of the reasons is that these studies cannot sufficiently reproduce strong salinity stratification inferred from paleo-ocean proxy data (Adkins et al., 2002; Skinner et al., 2010; Burke and Robinson, 2012). Enhanced stratification can increase carbon storage in the deep ocean by isolating carbon sinking from the surface ocean. In this study, we apply a simple parameterization of brine rejection during sea ice production to an OGCM and attempt to reproduce bottom-water salinity and ventilation ages reconstructed from paleo-ocean proxy data at the LGM. The brine parameterization transports part of the salt released to the surface ocean during sea ice production to the deep ocean. An LGM simulation including this brine parameterization expresses a vertical transport of salinity associated with more extensive sea ice production at the LGM and reproduces the reconstructed distribution of bottom-water salinity. As a result, more carbon is stored in the deep ocean due to the enhanced salinity stratification, reducing atmospheric pCO2. This study shows that applying the brine parameterization is a simple but useful way to improve the simulated distribution of oceanic tracer. References Adkins, J.F. et al., The Salinity, Temperature, and δ18O of the Glacial Deep Ocean, Science, 298(5599), 1769–1773, 2002. Broecker, W. S., Glacial to interglacial changes in ocean chemistry, Prog. Oceanogr., 11(2), 151–197, 1982. Burke, A. and L. F. Robinson, The Southern Ocean’s Role in Carbon Exchange During the Last Deglaciation, Science, 335(6068), 557–561, 2012. Petit, J. R. et al., Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, Nature, 399(6735), 429–436, 1999. Skinner, L. C. et al., Ventilation of the Deep Southern Ocean and Deglacial CO2 Rise, Science, 328(5982), 1147–1151, 2010.

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Timing of penultimate glacial-interglacial transition from gas measurements of the Dome Fuji ice core, Antarctica

Ikumi Oyabu1, Kenji Kawamura1, 2, 3 and Kyotaro Kitamura1 1National Institute of Polar Research

2SOKENDAI (The Graduate University of Advanced Studies) 3Japan Agency for Marine Science and Technology (JAMSTEC)

Paleoclimatic proxies provide information about the variations of climate forcings and responses, such as atmospheric CO2

concentration, temperature and sea level, as well as their magnitudes and phase relationships. Precise reconstructions are particularly important for numerical simulations, both as input and validation data, to better understand the long-term, transient behaviors of climate and ice sheets. During the last interglacial period (~130 to ~120 thousand years before present (kyr BP)), the peak global mean surface temperature was ~1˚C higher than the preindustrial period, and the global sea level was ~6 – 9 m higher than today with significant contribution from the Antarctic ice sheet, whereas atmospheric CO2 levels were similar to those in the preindustrial period (Dutton et al., 2015). This warm period and the preceding transition from the penultimate glacial period are important targets for the paleoclimatic research to investigate the mechanisms for the warming and ice-sheet reductions. Previous proxy and model studies have revealed that the last deglaciation (~20 – 11 kyr BP) was not a period of monotonic warming and ice-sheet disintegration but involved complex interplay between ice sheets, ocean circulation, and inter-hemispheric climate changes on centennial to millennial time scales. The penultimate deglaciation is much less well understood, partly because the number of proxies and time resolution are limited, and the chronological uncertainties are relatively large.

We here analysed air in the Dome Fuji (DF) ice core around the last interglacial period for establishing an accurate chronology for Antarctic climate and global atmospheric histories. The DF chronology for the last 340 kyr BP was constructed by synchronizing variations in O2/N2 of occluded air with local summer insolation, with 2σ uncertainty of generally ~2 kyr, based on the link between local insolation, snow metamorphism and gas fractionation at bubble close-off (Kawamura et al., 2007). However, relatively large errors were recently identified around the last interglacial (at ~100 kyr BP and ~129 kyr BP, Fujita et al., 2015) possibly due to low quality of the O2/N2 data. We improved the methods for sample treatment, air extraction and mass spectrometry, and re-analyzed O2/N2 ratio between 57 and 165 kyr BP to revise the DF chronology. The new chronology agrees within 1.2 kyr with a radiometric (U-Th) chronology of Chinese speleothems, whose stated error is less than 1 kyr (Cheng et al., 2009), suggesting successful improvement of the ice-core chronology.

Using δ18O of atmospheric O2 (δ18Oatm) and CH4 records measured on the same ice samples as used for the O2/N2 data, we investigate climatic variations during the penultimate glacial period and deglaciation. Variations in the δ18Oatm and CH4 are mainly controled by rainfall in low latitudes in the Northern Hemisphere. δ18Oatm gradually shifts to lower values while CH4 concentration stays high during warm phases (interstadials) in the North Atlantic region, and vice versa during cold phases (stadials), due to latitudinal migrations of intertropical convergence zone (ITCZ) associated with the strength of Atlantic meridional overturning circulation (AMOC) (Severinghaus et al., 2009). During massive iceberg discharge into the North Atlantic (Heinrich event, HE) and resulting AMOC collapse, however, opposite relationship between δ18Oatm and CH4 is observed; δ18Oatm starts to become higher while CH4 abruptly increases, due possibly to extremely southward shift of ITCZ to enhance CH4 emission from the Southern Hemisphere tropics (Rhodes et al., 2015). Although the time resolution is limited in our data, we found the same relationships between δ18Oatm and CH4 variations during the penultimate glacial period as seen for the stadial/interstadial in the last glacial period, and near the beginning of penultimate deglaciation (at 138.4 kyr BP) as seen for the HE 2 (at ~24 kyr BP) in the last glacial maximum.

We compare the timing of Heinrich-like event in our data with other proxies. An abrupt weakening of the Asian monsoon occurred at 138 kyr BP (Wang et al., 2008). The onset of relative sea level rise was suggested at 139±1 kyr BP from seawater δ18O of Red Sea, whose age was tuned to U-Th dating of a Mediterranean speleothem (Grant et al., 2012), and at 137 kyr BP from U-Th dating of Tahiti fossil corals (Thomas et al., 2009). From the similarity of theses ages, we suggest that a massive iceberg discharge event occurred at ~138 kyr BP and it marked the onset of penultimate deglaciation. We point out that the similar iceberg discharge events occurred near the minimum of Northern Hemisphere summer insolation in the last two glacial maxima, but the last glacial maximum continued after HE 2 until ~20 kyr BP whereas the deglaciation seems to have followed the 138-kyr HE. The difference might be related to a much more rapid rise of Northern Hemisphere summer insolation after its minimum for the penultimate deglaciation. The Antarctic temperature (from the DF core) and atmospheric CO2 concentration (from the EPICA Dome C core on our chronology) started to increase at around 138-kyr HE and peaked at ~130 kyr BP when CH4 concentration abruptly increased. Our new data may provide better chronological constraints on the climatic forcings and

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responses for numerical simulations, for better understanding of the sea-level contributions from both polar ice sheets during the last interglacial period. References Cheng, H. et al. (2009), Science, 326, 248-252. Dutton et al. (2015), Science, 349, aaa4019. Fujita, S. et al. (2015), Clim. Past., 11, 1395-1416 . Grant, K. M., et al. (2012), Nature, 491, 744-747. Kawamura, K., et al. (2007), Nature, 448, 912-916. Rhodes, R. H.et all. (2015), Science, 348, 1016-1019. Severinghaus, J. P. et al., (2009), Science, 324, 1431-1434. Thomas, A. L.et al. (2009), Science, 324, 1186-1189. Wang, Y. et al. (2008), Nature, 451, 1090-1093.

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Session 3

23

Ocean forcing of West Antarctic Ice Sheet retreat during the Late Quaternary: Clues from marine sedimentary records from the Amundsen Sea continental margin

Claus-Dieter Hillenbrand1, Mervyn Greaves2, James A. Smith1, Elaine M. Mawbey3,4, Rosalind E.M. Rickaby5, David A.

Hodell2, Henry Elderfield2,*, Thorbjørn Joest Andersen6, Katharine E. Hendry3, Thomas Williams1,7, Gerhard Kuhn8, Johann P. Klages8, Patrycja E. Jernas9, Matthias Forwick9 and Robert D. Larter1

1British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK 2Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK

3Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK 4University College London, Earth Sciences, 5 Gower Place, London WC1E 6BS, UK 5Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK

6Center for Permafrost (CENPERM), University of Copenhagen, DK-1350 Copenhagen K, Denmark 7Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, Florida 32611, USA

8Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, D-27568 Bremerhaven, Germany 9Department of Geosciences, University of Tromsø - The Arctic University of Norway, N-9037 Tromsø, Norway

*deceased The marine-based West Antarctic Ice Sheet (WAIS) is considered as the most vulnerable part of the Antarctic Ice Sheet. Far-field data together with results from numerical models and a study on sub-glacial sediments from the base of the modern ice sheet suggest that the WAIS collapsed at least once - and possibly even multiple times - during warm interglacial periods of the Late Quaternary. Moreover, modern observations combined with modelling experiments predict that current thinning, flow acceleration and retreat of Thwaites Glacier, which together with Pine Island Glacier forms the so-called “weak underbelly” of the WAIS and drains into the Amundsen Sea Embayment (ASE), may lead to a complete draw-down of its drainage basin in the future, potentially starting a total WAIS collapse. Sub-ice shelf melting caused by the upwelling of relatively warm Circumpolar Deep Water (CDW) onto the ASE shelf has been identified as the main driver of the present mass loss of Thwaites Glacier and other ice streams feeding into ice shelves fringing the ASE coast. This sub-ice shelf melting, which reduces the buttressing effect of the ice shelves, is held responsible for most of Antarctica’s contribution to the ongoing global sea-level rise. The crucial role of ocean forcing for ice-sheet dynamics has been furthermore confirmed by results from ice-sheet/ice-shelf models indicating that this mechanism was the most important driver of WAIS retreats that took place at the end of Late Quaternary glacial periods. Over recent years, several studies were undertaken on marine and sub-ice shelf sediment cores from the ASE continental shelf and slope to evaluate the role of ocean forcing in driving WAIS changes during the Late Quaternary climatic cycles, since the last ice age (ca. 25-11.5 ka before present) and during the last ca. 150 years. Although several marine expeditions to the ASE, especially since 2006, had collected a considerable number of sediment cores, the recovered palaeo-environmental and palaeo-oceanographic archives were difficult to decipher. This was mainly a consequence of the fact that calcareous foraminifer fossils rarely occur in the predominantly terrigenous glacimarine sediments from the Antarctic continental margin. The issue does not only hamper the establishment of reliable age models for the sedimentary sequences by applying AMS 14C dating and/or oxygen isotope (δ18O) stratigraphy but also the analysis of proxies required for reconstructing CDW advection and/or temperature changes in the water column. However, a few of the retrieved sedimentary sequences proved suitable for such investigations. Here we summarise recent efforts to reconstruct the relative changes in CDW advection and ocean temperatures along the ASE margin during the past. The different studies used a variety of proxies, including measurements of the trace metal composition (i.e., magnesium/calcium and boron/calcium ratios) and the stable carbon isotope composition (i.e., δ13C ratios) of benthic and planktic foraminifer shells as well as the analysis of benthic foraminifer assemblages. The results of these investigations demonstrate that phases of WAIS retreat on glacial-interglacial timescales, at the end of the last ice age and since the 20th century were associated with phases of increased CDW upwelling, supporting the conclusion that ocean forcing has been the main driver of WAIS dynamics. Furthermore, we give an overview over the potential and the limitations of the used proxies in providing reliable reconstructions of past CDW advection and palaeo-seawater temperatures.

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Ice sheet retreat in the Amundsen Sea sector: precursor for a WAIS collapse?

Karsten Gohl Alfred Wegener Institute Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany

The West Antarctic Ice Sheet (WAIS) rests on a bed that typically deepens toward the interior of the Antarctic continent. This fore-deepened basis is largely below sea level. Therefore, the marine-based WAIS is sensitive to global sea level rise and regional oceanographic and atmospheric changes, and its history has been highly dynamic. A complete WAIS collapse would raise the global sea level by 3.3 to 4.3 m, whereas the collapse of its Amundsen Sea drainage sector would raise the sea level by about 1.5 m. Over the most recent decades, glaciers draining into the ASE thinned at an alarming rate, their flow speed dramatically increased, and their grounding lines retreated significantly, thereby contributing to present sea level rise at a faster rate than any other glacier on Earth. The present ice loss in the Amundsen Sea Embayment (ASE) of West Antarctica is mainly attributed to sub–ice shelf melting induced by relatively warm CDW upwelling onto the shelf and spreading through deep bathymetric troughs toward the grounding zones. It is unclear, however, whether the current ice loss results from ongoing deglaciation since the Last Glacial Maximum (LGM), recent climatic/oceanographic warming, or recent internal ice sheet dynamics. If the WAIS has undergone similar thinning and retreat in the past, the factors driving that retreat can be compared to modern conditions. The reconstruction and quantification of WAIS collapses during the Neogene and Quaternary will provide constraints for ice sheet models predicting future WAIS behavior and resulting sea level rise. Numerous modeling studies have tried to link the waxing and waning of the WAIS to various forcing mechanisms. However, large uncertainties exist regarding the spatial and temporal variability of past ice sheet advance and retreat. These uncertainties are mainly caused by the lack of data from cores drilled proximal to the WAIS. The only existing drill cores along the Pacific Antarctic margin outside the Ross Sea are from Deep Sea Drilling Project (DSDP) Leg 35 in the Bellingshausen Sea and Ocean Drilling Program (ODP) Leg 178 on the Antarctic Peninsula. Results of Leg 178 Site 1097, drilled on the shelf, revealed a major late Miocene change in sequence geometry on the outer shelf, which may indicate a change in the typical extent of glacial advances, the dynamic behavior of ice streams, or glacial sediment transport. This information was used to correlate seismic horizons to Leg 178 Sites 1095 and 1096 on the continental rise and to interpreted transitions from pre-glacial to intermediate and full glacial conditions from the eastern Bellingshausen Sea to the Amundsen Sea. In a similar fashion, seismic correlation could be done across the western Amundsen Sea rise to connect the seismic horizons of the eastern Ross Sea to those of the Amundsen Sea. However, an exact stratigraphic sequencing is hampered by the lack of deep drill sites. The dense seismic network of the Amundsen Sea enabled us to propose drilling by the International Ocean Discovery Program (IODP). This expedition (IODP Exp. 379) will be conducted in early 2019 and is expected to reveal paleo-ice sheet dynamics records from early glaciation to the Plio/Pleistocene warm-cold phases. This presentation will focus on the existing geophysical, glacial morphological and geological data from the Amundsen Sea Embayment and will demonstrate that the modelling hypothesis of the past and current ASE ice sheet retreats being precursors for a partial or total WAIS collapse is a viable one.

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Sedimentary 10Be signature of ice retreat history in Pine Island Bay, West Antarctica

Yusuke Suganuma1,2, Gerhard Kuhn3, Claus-Dieter Hillenbrand4, and Albert Zondervan5 1National Institute of Polar Research, 10-3 Midoricho, Tachikawa, Tokyo 190-8518, Japan

2Department of Polar Science, SOKENDAI, 10-3 Midoricho, Tachikawa, Tokyo 190-8518, Japan 3Alfred Wegener Institute (AWI), Helmholtz Center for Polar and Marine Research, Bremerhaven, Germany

4British Antarctic Survey (BAS), Cambridge, United Kingdom 5GNS Science, National Isotope Centre, Lower Hutt, New Zealand

Reconstruction of past glacial dynamics of the Antarctic ice sheets, by studying records from their margins, is essential to evaluate their stability and to anticipate their contribution to future sea level rise. Recent reconstructions of the West Antarctic Ice Sheet (WAIS) in Pine Island Bay (PIB) revealed that grounded ice had retreated to within ~100 km of the modern grounding-line by 11.7 kyr, and that an extensive ice shelf had disintegrated around 7.5 kyr due to intensified warm Circumpolar Deep Water inflow (Hillenbrand et al., 2013; 2017). In addition, the first direct evidence for a paleo-subglacial lake on the Antarctic continental shelf was reported from a small bedrock basin in PIB, based on a distinct sediment facies and geochemical pore water signatures detected in marine sediment core PS69/288 recovered from this basin (Kuhn et al., 2017). Here, we report further evidence for Holocene ice-shelf retreat and the existence of the reported -and possibly another- paleo-subglacial lake in PIB, based on changes in bulk 10Be concentrations and authigenic 10Be/9Be ratios recorded in sediment cores PS69/288 and PS75/214. Our data show significant up-core increases in bulk 10Be concentrations and authigenic 10Be/9Be ratios in the uppermost sections of the cores (at 128–158 cm depth in PS69/288 and at 50–100 cm depth in PS75/214). The average 10Be concentration (5.57–4.77 × 108 atom/g) in these core intervals is roughly comparable to those of Late Holocene open marine sediments from the Ross Sea sector of Antarctica (Sjunneskog et al., 2007). In contrast, these same 10Be parameters in sediments below these core depths are an order of magnitude lower, a clear indication of a much reduced input of meteoric 10Be to the water column when the corresponding sediments formed. These down-core changes in bulk 10Be concentrations and authigenic 10Be/9Be ratios are thought to mirror an environmental and depositional change from a sub-ice shelf to an open marine setting (cf. Yokoyama et al., 2016). Based on the 14C ages of the cores (Hillenbrand et al., 2013; Kuhn et al., 2017), this ice-shelf retreat occurred at some time after 8.0 kyr, probably in the mid-Holocene. The low authigenic 10Be/9Be ratios in the lowermost section of core PS69/288 (518–908 cm) support the conclusion that the corresponding sediments were deposited in a subglacial lake and a sub-ice shelf cavern (Kuhn et al., 2017). Furthermore, 10Be concentrations in the lower section of core PS75/214 (470–750 cm) are as low as in the subglacial lake/ice-shelf cavern sediments of core PS69/288, suggesting that another subglacial lake may have existed in PIB. This is consistent with the porewater signature recorded at the base of core PS75/214 (Kuhn et al., 2017). References Hillenbrand, C.-D., Kuhn, G., Smith, J.A., Gohl, K., Graham, A.G., Larter, R.D., Klages, J.P., Downey, R., Moreton, S.G., Forwick, M., Vaughan, D.G., 2013. Grounding-line retreat of the West Antarctic Ice Sheet from inner Pine Island Bay. Geology 41, 35–38. doi:10.1130/G33469.1. Kuhn, G., Hillenbrand, C.-D, Kasten, S., Smith, J.A., Nitsche, F.O., Frederichs, T., Wiers, S., Ehrmann, W., Klages, J.P., Mogollón, J.M. (2017). Evidence for a palaeo-subglacial lake on the Antarctic continental shelf. Nature Communications, 8, doi: 10.1038/ncomms15591. Hillenbrand, C.-D., Smith, J.A., Hodell, D.A., Greaves, M., Poole, C.R., Kender, S., Williams, M., Andersen, T.J., Jernas, P.E., Elderfield, H., Klages, J.P., Roberts, S.J., Gohl, K., Larter, R.D., Kuhn, G., 2017. West Antarctic Ice Sheet retreat driven by Holocene warm water incursions. Nature 547, 43-48. Sjunneskog, C., Scherer, R., Aldahan, A., Possnert, G. (2007). 10Be in glacial marine sediment of the Ross Sea, Antarctica, a potential tracer of depositional environment and sediment chronology. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 259(1), 576–583. Yokoyama, Y., Anderson, J.B., Yamane, M., Simkins, L.M., Miyairi, Y., Yamazaki, T., Koizumi, M., Suga, H., Kusahara, K., Prothro, L., Hasumi, H., Southon, F.R., Ohkouchi, N. (2016). Widespread collapse of the Ross Ice Shelf during the late Holocene. Proceedings of the National Academy of Sciences of the Unites States of America, 113, 2354–2359.

26

Reconstruction of sea surface temperature through the last 150 kyrs in the Indian sector of the Southern Ocean

Hiroki Matsui1 and Minoru Ikehara1

1Center for Advanced Marine Core Research, Kochi University Understanding the Antarctic Circumpolar Current (i.e., Antarctic polar front and Subantarctic front) is a key to climatic changes in the Southern Ocean and Antarctic ice sheets. Modern polar front ranges from 45°S to 60°S (Freeman et al., 2016), and the boundary between carbonate and siliceous sediments (Dutkiewicz et al., 2015) closely matches the front position. Thus, the position of the polar front can be estimated from the plankton assemblages of deep-sea sediments, and the changes in the front position should be recorded as sea surface temperature (SST) anomalies. Here we conducted planktic foraminifera census counts to reconstruct SST and the position of the polar front since the last interglacial (last 150 kyrs). Sediment samples are collected from the DCR-1PC (46°S, 44°E, 2632 m water depth) recovered on the Del Caño Rise in the Indian sector of the Southern Ocean. Planktic foraminifera were abundant during the Marine Isotope Stage (MIS) 1 compared to the MIS 2. High latitude species such as Globigerina bulloides (28 ~ 51%) and Neogloboquadrina pachyderma (19 ~ 43%) comprised assemblages during the MIS 1, while N. pachyderma (>75%) dominated during the MIS 2. Based on the foraminifera census data, we reconstructed the SST by the modern analogue technique in the R environment. The Southern Ocean summer SST through the last 30 kyrs ranged from 10.2°C to 2.5°C, and ~4.9°C anomaly was observed at the transition from the MIS 2 to the MIS 1. We extracted characteristic assemblages of the MIS 1 and MIS 2 from the surface sediments database (Siccha and Kucera, 2017) to compare with the modern polar front position. The typical assemblages of the MIS 1 and MIS 2 were restricted to the north and south of the Antarctic polar front, respectively. Because the DCR-1PC lies north of the modern polar front, we suggested that the front moved southward at the last deglaciation. We also reconstructed the MIS 5 and the MIS 6, focusing on the last interglacial (MIS 5e). During the MIS 5e, samples yielded G. bulloides (28 ~ 51%) and N. pachyderma (14 ~ 39%), whereas N. pachyderma (>88%) dominated during the MIS 6. The summer SST varied from 9.8°C to 2.7°C and increased by ~6.3°C at the deglaciation. As in the case of the last deglaciation, the Antarctic polar front seemed to move southward at the transition from the MIS 6 to the MIS 5. We note that a subtropical species Orbulina universa presented throughout ~7 thousand years during the first half of the MIS 5e. The species live in the SST range from 10°C to 30°C (e.g., Kucera et al., 2005), which is consistent with the reconstructed summer SST. Since the O. universa was not present throughout the MIS 1, we consider that the MIS 5e was likely warmer than the MIS 1 in the studied site.

Figure 1. Last 30 kyrs SST reconstruction. References Dutkiewicz, A., R. D. Müller, S. O’Callaghan, and H. Jónasson, Census of seafloor sediments in the world’s ocean, Geology, 43(9), 795–798, 2015. Freeman, N. M., N. S. Lovenduski, and P. R. Gent, Temporal variability in the Antarctic Polar Front (2002-2014), Journal of Geophysical Research: Oceans, 121(10), 7263–7276, 2016. Kucera, M., M. Weinelt, T. Kiefer, et al., Reconstruction of sea-surface temperatures from assemblages of planktonic foraminifera: multi-technique approach based on geographically constrained calibration data sets and its application to glacial Atlantic and Pacific Oceans, Quaternary Science Reviews, 24(7–9), 951–998, 2005. Siccha, M. and M. Kucera, ForCenS, a curated database of planktonic foraminifera census counts in marine surface sediment samples, Scientific Data, 4, 170109, 2017.

27

Antarctic Ice Sheet change before Marine Isotope Stage 3 using a glacial isostatic adjustment model

Takeshige Ishiwa1, Jun'ichi Okuno1,2, Pippa L. Whitehouse3, Yusuke Suganuma1,2, Hideki Miura1,2 1 National Institute of Polar Research

2 Department of Polar Science, SOKENDAI 3 Durham University

The continental ice sheet volume fluctuated before Marine Isotope Stage 3 (MIS 3), heading to its maximum in the Last Glacial Maximum (LGM: ~21,000 years ago). The rate of ice-sheet growth toward the LGM is essential to understand the mechanism of glaciation (Pico et al., 2017). Past ice-sheet and sea-level history can be reconstructed using a glacial isostatic adjustment (GIA) model, but ice-sheet expansion during the LGM and sea-level regression during deglaciation make it challenging to obtain the field-based evidence of ice-sheet and sea-level change before MIS 3. Sedimentary records from the Soya Coast region in East Antarctica provides valuable sea-level records demonstrating relative sea level at MIS 3 close to the present level (Miura et al., 1998). However, previous GIA modeling studies cannot explain these MIS 3 sea-level highstands enough (ref., Nakada et al., 2000; Argus et al., 2014). Here we demonstrate that the revised Antarctic Ice Sheet (AIS) history based on W12 model (Whitehouse et al., 2012) can explain MIS 3 sea-level highstands in East Antarctica. We ran over 100 GIA experiments with changes in the timing of AIS growth before MIS 3. The preliminary results suggest that the timing and rate of ice volume change in AIS play an important role in MIS 3 relative sea level along with Soya Coast. We will also discuss the relationship between the previously reported variability of sea ice area and sea surface temperature in the Southern Ocean and the glaciation of AIS before MIS 3. References Argus, D.F., Peltier, W.R., Drummond, R., Moore, A.W. The Antarctica component of postglacial rebound model ICE-6G_C (VM5a) based on GPS positioning, exposure age dating of ice thicknesses, and relative sea level histories. Geophysical Journal International 198, 537-563 (2014). Miura, H., Maemoku, H., Seto, K., Moriwaki, K. Late Quaternary East Antarctic melting event in the Soya Coast region based on stratigraphy and oxygen isotopic ratio of fossil molluscs. Polar geoscience 11, 260-274 (1998). Nakada, M. et al. Late Pleistocene and Holocene melting history of the Antarctic ice sheet derived from sea-level variations. Marine Geology 167, 85–103 (2000). Pico, T., Creveling, J.R. Mitrovica, J.X. Sea-level records from the U.S. mid-Atlantic constrain Laurentide Ice Sheet extent during Marine Isotope Stage 3. Nature Communications 8, 15612 (2017). Whitehouse, P.L., Bentley, M.J. Le Brocq, A.M. A deglacial model for Antarctica: geological constraints and glaciological modelling as a basis for a new model of Antarctic glacial isostatic adjustment. Quaternary Science Reviews 32, 1–24 (2012).

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Session 4

29

Exploring the ocean under ice shelves

Martin Truffer1

1Geophysical Institute, University of Alaska Fairbanks

The biggest potential sea level rise over the next several decades originates from ice grounded below sea level and delivered to

the ocean via outlet glaciers. At many locations these outlet glaciers are bounded by ice shelves. These ice shelves provide

resistance to outlet glacier flow, an effect known as buttressing. When ice shelves disintegrate the buttressing effect is lost and

outlet glaciers can accelerate substantially, leading to the loss of grounded ice and hence raising sea level. The vulnerability of

ice shelves depends on both surface and basal conditions. On the surface, melt water can lead to ponding and eventually

drainage through the shelf, thus weakening it. At the bottom, increasing water temperatures or enhanced circulation can very

effectively thin an ice shelf and also lead to a retreat of the grounding zone, where grounded ice meets the ocean. A sufficiently

thinned ice shelf will lose structural integrity and can disintegrate in its entirety. This was very effectively demonstrated when

the Larsen B Ice Shelf on the Antarctic Peninsula collapsed, leading to the acceleration of several outlet glaciers (Scambos et

al., 2004). Other well known examples include Jakobshavn Isbrae in Greenland, where the loss of the floating tongue led to a

more than three-fold acceleration of the glacier (Joughin et al., 2004).

Monitoring sub-shelf ocean conditions is thus paramount to the assessment of ice shelf health and thus the likelihood of outlet

glacier acceleration and the onset of possible positive feedbacks, which can occur on retrograde glacier beds. But such

observations are difficult to obtain due to the extremely difficult access to sub-shelf environments. Some success has been

achieved with autonomous submarine vehicles, such as Autosub, which can undertake prolonged excursions underneath the

ice. A successful campaign led to new insights about the bathymetry and recent ungrounding of the Pine Island Glacier and the

first documentation of warm water under the shelf (Jenkins et al., 2010). Such measurements provide temporal snapshots with

great spatial coverage. An alternative approach to access this environment is to use hot water drills to install sub-shelf

oceanographic moorings. This yields time series, but is constrained to one or a few point measurements only. Boreholes enable

additional measurements, such as ice temperatures and sediment cores from the ocean floor.

Hot water drilling is a relatively mature technology, but it comes at great logistical expense. This is exacerbated by the

remoteness and generally difficult surface conditions on ice shelves. Due to fast flow and large strain rates, ice shelf surfaces

are often heavily crevassed. Uneven sub-shelf melting leads to channelization, which is then expressed at the surface in rough

topography. Access is often restricted to helicopters or small airplanes. It is therefore important to keep hot water drills

sufficiently modular to be compatible with these modes of transport.

The UAF hot water drill was derived from earlier drills used in Greenland and adapted for drilling through 700 m or more of

temperate mountain glaciers in Alaska. It has since been enhanced and used to penetrate several ice shelves in Antarctica to

depths exceeding 400 m and with hole diameters of 20 cm. Ocean conditions have ranged from relatively cold waters

underneath the Nansen Iceshelf to relatively warm at Pine Island Glacier. The drill has been deployed by helicopters as well as

Twin Otter aircraft. It will also be deployed as part of the USA/UK International Thwaites Glacier Collaboration to track warm

water reaching the front of Thwaites Glacier, the location of the largest body of marine ice on the planet (Scambos et al.,

2017). In past campaigns we have collected a variety of measurements. An acoustic ranger was used to directly monitor melt

rates underneath Pine Island Iceshelf of sub-daily time scales. This was combined with a profiler that collected temperature and

salinity measurements in the underlying ocean and turbidity measurements in the boundary layer (Stanton et al., 2013).

Thermistor strings in the ice were allowed to freeze in and provide ice temperatures as well as a documentation of melt, as they

gradually became exposed to sea water. The boreholes were also used to access the ocean floor and extract several sediment

cores, which were interpreted to show the retreat history of Pine Island Glacier through the twentieth century. (Smith et al.,

2016). At the Nansen Iceshelf we used our hot water drill to add an oceanographic component to the AMIGOS stations, which

are weather stations with added cameras and GPS (Scambos et al., 2013). The added components included temperature and

salinity sensors, current meters, and a fibre optic temperature that allows a quasi continuous measurement of temperatures

within the ice shelf and in the underlying ocean. A similar setup will be used for the Thwaites Glacier program.

I will discuss results from these previous borehole campaigns and present an outlook for our planned observations in the

Amundson Sea.

30

References

Jenkins, A., Dutrieux, P., Jacobs, S. S., McPhail, S. D., Perrett, J. R., Webb, A. T., & White, D. (2010). Observations beneath

Pine Island Glacier in West Antarctica and implications for its retreat. Nat. Geosci., 3(7), 468–472.

https://doi.org/10.1038/ngeo890

Joughin, I., Abdalati, W., & Fahnestock, M. (2004). Large fluctuations in speed on Greenland’s Jakobshavn Isbrae glacier.

Nature, 432(7017), 608–610. https://doi.org/10.1038/nature03130

Scambos, T. A. (2004). Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica.

Geophysical Research Letters, 31(18), L18402. https://doi.org/10.1029/2004GL020670

Scambos, T. a., Ross, R., Haran, T., Bauer, R., Ainley, D. G., Seo, K. W., De Kayser, M., Behar, A., and MacAyeal, D. R.

(2013). A camera and multisensor automated station design for polar physical and biological systems monitoring: AMIGOS.

Journal of Glaciology, 59(214), 303–314. https://doi.org/10.3189/2013JoG12J170

Scambos, T. A. and 22 others (2017). How much, how fast?: A science review and outlook for research on the instability of

Antarctica’s Thwaites Glacier in the 21st century. Global and Planetary Change, 153, 16–34.

https://doi.org/10.1016/j.gloplacha.2017.04.008

Smith, J. A. and 14 others. (2016). Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier.

Nature, 541(7635), submitted. https://doi.org/10.1038/nature20136

Stanton, T. P., Shaw, W. J., Truffer, M., Corr, H. F. J., Peters, L. E., Riverman, K. L., Bindschadler, R., Holland, D. M. and

Anandakrishnan, S. (2013). Channelized ice melting in the ocean boundary layer beneath Pine Island Glacier, Antarctica.

Science (New York, N.Y.), 341(6151), 1236–1239. https://doi.org/10.1126/science.1239373

31

Hot water drilling at Langhovde Glacier, East Antarctica

Shin Sugiyama1, Masahiro Minowa2, Masato Ito1, Shiori Yamane1, Takeshi Tamura3 and Shigeru Aoki1 1Institute of Low Temperature Science, Hokkaido University

2 Institute of Physics and Mathematics, Austral University of Chile 3 National Institute of Polar Reearch

Basal melting of ice shelves is an important ablation process of the Antarcic ice sheet. Recent studies have shown that increase in the basal melting is a driver of recent ice mass loss in Antarctica. It is thus important to study water properties and circulations underneath Antarctic ice shelves and floating tongues of outlet glaciers. Moreover, subshelf ocean and sea floor are unique environments for studying a marine ecosystem and sedimentation. Despite the importance, such data and samples are available only at a limited number of ice shelves because in-situ measurements are difficult. To better understand subshelf environments of Antarctic outlet glaciers, we performed hot water drilling and subshelf measurements at Langhovde Glacier in East Antarctica. Langhovde Glacier is located on the Soya Coast in Lutzow-Holm Bay, approximately 20 km south from a Japanese Antarctic base Syowa Station (Fig. 1). The glacier discharges ice at a rate of 130 m a−1 from a ~3 km wide calving front (Fukuda et al., 2014). The lower part of the glacier forms a floating tongue over several kilometers from the front (Sugiyama et al., 2014). Field activity was carried out from December 2017 to February 2018 as a part of the 59th Japanese Antarctic Research Expedition. We drilled boreholes using a hot water drilling system at four locations within 0.5–2.5 km from the ice front. The drilling revealed that the sea floor under the drilled region deepens downglacier from 380 to 510 m below sea level, whereas ice thickness decreases from 410 to 235 m (Fig. 2). The boreholes were utilized to lower down a CTD (conductivity, temperature and depth) profiler, current meter, video camera, and water and sediment samplers into the subshelf ocean. Two boreholes were permanently instrumented with mooring systems for long-term measurements of temperature, salinity, current and pressure. In this contribution, we report the overview of the drilling activity and the results of the borehole measurements and sampling. This study was carried out under the ROBOTICA (Research of Ocean-ice BOundary InTeraction and Change around Antarctica) project. Figure 1. Figure font size is also 9pt.

References Fukuda, T., S. Sugiyama, T. Sawagaki and K. Nakamura. 2014. Recent variations in the terminus position, ice velocity and surface elevation of Langhovde Glacier, East Antarctica. Antarctic Science, 326(6), 636–645. Sugiyama, S., T. Sawagaki, T. Fukuda and S. Aoki. 2014. Active water exchange and life near the grounding line of an Antarctic outlet glacier. Earth and Planetary Science Letters, 399C, 52–60.

Fig. 1. Location of the study site. Fig. 2. Vertical cross section of Langhovde Glacier along the drilling sites.

32

Direct measurements of water properties underneath a floating tongue of Langhovde Glacier, East Antarctica

Masahiro Minowa1,2, Shin Sugiyama1, Masato Ito1, Shiori Yamane1 and Shigeru Aoki1

1Institute of Low Tempeature Science, Hokkaido University 2Instituto Ciencias Físicas y Matemáticas, Universidad Austral de Chile

Acceleration in the basal melting of ice shelves and glacier floating tongues is considered as the driver of recent ice mass loss in Antarctica. Despite the importance and interests, in-situ subshelf ocean data are limited because measurements are difficult under several hundred-meter-thick ice. To better understand subshelf water properties and basal melting of an Antarctic outlet glacier, we directly measured subshelf water properties through boreholes, by lowering a CTD (conductivity, temperature, and depth) profiler, current meter, and water sampler at Langhovde Glacier in East Antarctica. Two boreholes were instrumented with mooring systems for long-term measurements of temperature, salinity, current and water pressure. These boreholes were located in 0.5–2.5 km from the ice front. The uppermost borehole is within several hundred meters from the grounding line. Potential temperature of the seawater near the ice sole was between –1.4 and –1.1ºC, which was 0.65–0.95ºC warmer than freezing temperature. Temperature and salinity were slightly warmer and saltier towards the ocean floor at all of the drilling sites. Mooring measurements over four weeks showed that water generally flowed towards the ocean at a mean rate of 1.8 cm s–1 in the layer directly below the ice shelf. These results imply that the freshening of seawater due to basal melting drives the subshelf water circulation. The mean basal melt rate was calculated as 2.5 m a–1 by solving the heat and salt flux balance equations at the ice-ocean boundary. The melt rate increases as it approaches to the grounding line. Our direct measurements of subshelf ocean properties demonstrate that basal melting occurs everywhere of the floating ice tongue of Langhovde Glacier due to a warm ocean water intrusion.

Figure 1. Cross-sectional potential temperature and salinity profiles underneath the floating tongue of Langhovde Glacier.

34.2934.29

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33

Estimation of ice flow velocities of Shirase Glacier and its surrounding fast ice in Antarctica using ALOS-2/PALSAR-2 data

Kazuki Nakamura1, Shigeru Aoki2, Tsutomu Yamanokuchi3, Takeshi Tamura4, Shuki Ushio4 and Koichiro Doi4

1College of Engineering, Nihon University 2Institute of Low Temperature Science, Hokkaido University

3Remote Sensing Technology Center of Japan 4National Institute of Polar Research

Shirase Glacier is the one of East Antarctica's icestream which is known to be a fast moving glacier. The glacier flows into the southernmost of Lützow-Holm Bay which is surrounded by the fast ice in the bay which suppresses direct calving. When the southernmost fast ice in Lützow-Holm Bay starts to disintegrate, the terminus of the ice stream of the glacier such as the form of a floating ice tongue breaks away from the tongue as releasing icebergs. While the dynamics of Shirase glacier are associated with the condition of fast ice stability or instability in Lützow-Holm bay, the details of its outlet mechanism remain unknown. In recent years, it is remarkable that the floating ice tongue broke away and then that suddenly drifted away from the glacier as icebergs. To observe not only the ice flow velocity of the glacier but also displacement of the fast ice in the bay as well is important for understanding that phenomenon. The floating ice tongue of Shirase Glacier broke away from the glacier which is subsequent to break up the fast ice of the southward advance in Lützow-Holm Bay in the autumn of 1998 (Nakamura et al., 2007). After this release, no large scale disintegration or drift away of the growing conglomerate had been confirmed up to 2014. However, between March and June 2015, the fast ice in Lützow-Holm bay broke up, 45 km far from the terminus of Shirase Glacier and flowed away (Nakamura et al., 2017). After the discharge, the disintegration of unstable fast ice progressed in March 2016, furthermore, that reached the calving front of the glacier in April 2016. Part of the calving front flowed away in the end of March 2017 as once more the disintegration and discharge were occurred. This study presents that temporal variations in ice flow velocities for Shirase Glacier and its surrounding fast ice in Lützow-Holm Bay, East Antarctica were estimated using synthetic aperture radar (SAR) images acquired by the Phased Array L-band SAR-2 (PALSAR-2) onboard Advanced Land Observing Satellite-2 (ALOS-2) in 2015-2018. The images were analyzed using image correlation (e.g. Nakamura et al., 2017). The profile of ice flow velocity was obtained on the central streamline which was analyzed using image correlation (e.g. Nakamura et al., 2007). The flow velocity at the 20 km downstream from the grounding line (GL) of Shirase Glacier is 2.32±0.03 km a–1 (mean and standard deviation of 2015–2018), which show that flow velocity is considered to be less related to the fast ice stability. However, the flow velocity around 50 km downstream from the GL show changes related to fast ice condition. The flow velocity difference between the 20 km downstream from the GL and the vicinity of the calving front is about 0.2 km a–1 under the stable fast ice condition and about 0.5 km a–1 under the unstable fast ice condition. Therefore, the flow velocity increases about 0.3 km a–1 under the unstable fast ice condition. The flow velocities of fast ice tend to be decreased from the vicinity of calving front of Shirase Glacier to the offshore ,which are 1.69–0.27 km a–1 in 2015. The flow velocity of fast ice after the discharge on April 2016 is similar to the trend of that before, but the velocity change after the discharge of 2016 is smaller than before that in 2015. Those velocities are 0.56–0.27 km a–1 and 0.34–0 km a–1 in spring of 2016 and 2017, respectively. Acknowledgements ALOS-2's SAR data were provided by Japan Aerospace Exploration Agency with the ALOS research announcement (PI No.1191, 3049). Our research was supported by MEXT Grant-in-Aid for Scientific Research on Innovative Areas (17H06321) and JSPS Grant-in-AidScientific Research (C) (18K11627). This study was also supported by the Grant for Joint Research Program of the Institute of Low Temperature Science, Hokkaido University (18G033) and the National Institute of Polar Research (NIPR) through General Collaboration Project (28-21, 29-31). References Nakamura, K., K. Doi, K. Shibuya, Estimation of seasonal changes in the flow of Shirase Glacier using JERS-1/SAR image correlation, Polar Sci, 1, 73-83, 2007. Nakamura, K., T. Yamanokuchi, S. Aoki, K. Doi and K. Shibuya, Temporal variations in the flow velocity for Shirase Glacier in Antarctica over a 20-year period, J. Japanese Soc. Snow and Ice (SEPPYO), 79(1), 3-15, 2017.

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Session 5

35

Exploration of the Ocean Cavities Beneath Antarctica’s Floating Ice Shelves Using Autonomous Underwater Vehicles

Adrian Jenkins1, Pierre Dutrieux2, Stan Jacobs2, Hartmut Hellmer3, Markus Janout3, Mike Schröder3, Steve McPhail4 and Rob

Templeton4 1British Antarctic Survey, Natural Environment Research Council, Cambridge, UK

2Lamont-Doherty Earth Observatory of Columbia University, New York, USA 3Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany

4National Oceanography Centre, Natural Environment Research Council, Southampton, UK Direct interaction between the Antarctic Ice Sheet and the Southern Ocean occurs at the base of the floating ice shelves that form at the margins of the ice sheet. The phase changes that occur at the ice-ocean interface play a key role in determining both the mass balance of the ice sheet, with implications for global sea levels, and the water mass transformations that occur on the Antarctic Continental Shelf, with implications for the global meridional overturning circulation. However, with an ice cover that ranges in thickness from hundreds to thousands of metres, the seawater cavities beneath the Antarctic ice shelves remain among the most inaccessible and unexplored regions of the ocean. Until a decade ago, the only possibilities for direct observation beneath ice shelves were provided by drilling access holes through the ice shelf. Such time consuming operations yielded immensely valuable time-series observations, but could never provide the necessary spatial coverage for a fuller understanding of the sub-ice-shelf system. Recently Autonomous Underwater Vehicle (AUV) technology has advanced to the point where AUVs can provide that spatial coverage. This presentation reviews the achievements of the UK Natural Environment Research Council’s Autosub AUVs on campaigns in the Antarctic over the past decade and discusses how the resulting datasets can expand our understanding of the sub-ice-shelf environment and the physical processes by which the Southern Ocean forces change in the Antarctic Ice Sheet.

36

Autosub Long Range AUV Deployments Beneath the Ronne and Filchner Ice Shelves - The Engineering Challenges

Steve McPhail, Rob Templeton, Miles Pebody, Daniel Roper, Richard Morrison

National Oceanography Centre, Natural Environment Research Council, Southampton, UK The Autosub Long Range (ALR) AUV is a 3.6 m long, 800 kg AUV with a depth rating of 6000 m. Known by some as“Boaty McBoatface”, it is capable of endurances of more than a month, and runs at speeds of 2.5 km hr 1. It isdesigned to be a relatively low cost AUV; for example using a magnetic rather than gyro compass for headingestimation. In February 2018 it measured turbulence, CTD and currents during multi day and dangerousexplorations under the Ronne and Filchner Ice Shelves in the Weddell Sea, Antarctica.

There were many technical challenges to be overcome to successfully and safely execute these missions. TheAUV Navigation must be accurate (figure 1), critically so where, because of ice cover, the AUV cannot simplysurface at the end its mission (figure 2). Achieving this accuracy was encumbered by high currents in theoperating area (figure 3), the scientific requirement to make measurements a long distance from the sea bed (outof Doppler sea bed lock) and the use of a low cost compass. Another challenge is to control the AUV depthtrajectory, safely avoiding, in an unknown environment, the ice shelf overhead and the seabed below.

Finally recovery was made more complex by the sea ice and freezing temperatures (figure 4)

In my talk I will discuss how, from an engineering perspective, we have tackled these and other technical issues. Iwill also introduce the recently commissioned 1500 m rated ALR AUV, which trades depth rating for greaterenergy storage. With planned implementation of more sophisticated navigation and collision avoidance systems,we will be able to execute long distance polar ocean missions in complex environments. A very exciting prospect!

Figure 1 The real time, and post processed Navigation for ALR Mission, which went 25 km under the Ronne ice shelf, taking 24 hours. USBL positioning at the start of the dive facilitated reconstruction of the trajectory.

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Figure 2 Conditions at the end of Mission 47, a 2 day dive under the Filchner Ice shelf. It was necessary for the ship to execute ‘figures of 8’ to break up the ice while the AUV circled 800 m under the ship.

Figure 3 Output of the tidal model display (Laurie Padman), displaying the magnitude of tidal ellipses. Tidal flow conditions were difficult at exactly the positions we wanted to operate in (particularly at the Ronne Ice shelf.

Figure 4 Recovery onto the RV Polarstern was somewhat hampered by the Sea-ice

38

Preparing for the new wave of Autonomous Underwater Vehicle exploration in Antarctica

Guy Williams 1�Institute for Antarctic and Marine Studies, University of Tasmania

2�Antarctic Climate and Ecosystem Cooperative Research Centre, University of Tasmania 3�The Australian Research Council Antarctic Gateway Partnership

For over a decade the development and deployment of long-range Autonomous Underwater Vehicles (AUV) for ocean research beneath Antarctic ice shelves has been led by the United Kingdom’s Autosub program at the National Oceanographic Facility. Recently Australia, Sweden and Japan have acquired AUVs, ushering in a new era of multinational, interdisciplinary research programs in key regions of ocean/ice shelf interaction around Antarctica. This talk will focus on the University of Tasmania’s AUV ‘nupiri muka’ and the preparations for its first Antarctic campaign this summer (2018/19). There will also be a discussion of future proposals for Antarctic research with this AUV and the opportunity it presents for the international Antarctic science community as it urgently seeks to address critical knowledge gaps in our cryosphere.�

39

Design of the Variable and Compact AUV “MONACA” for Antarctic Survey

Hirokazu Yamagata1 , Toshihiro Maki1, Hiroshi Yoshida2, Yutaka Ohta2 and Yoshihumi Nogi3

1Institute of Industrial Science, The University of Tokyo 2Japan Marine Science and Technology Center (JAMSTEC)

3National Institute of Polar Research (NIPR) Autonomous Underwater Vehicles (AUVs) have been used for under ice explorations of the Antarctic Ocean [1,2]. Most of the current under ice AUVs are large and are used for long distance survey. Survey results of these AUVs are extremely precise, but there is a trade-off between precision and running cost and/or risk of losing AUVs. In this presentation, the authors propose a variable and compact AUV "MONACA: Mobility Oriented Nadir AntarctiC Adventurer " which can take the necessary and sufficient compositions by its modular design. Although the survey range is limited compared with large AUVs, it can conduct a highly precise survey with lower costs. ! The three missions are considered as shown in Fig. 1. ( A: Seafloor tracking, B: Ice tracking, C: Water survey in constant depth). In the missions A and B, the shape of the seafloor or the ice is mapped by a multi-beam sonar while tracking it by a probabilistic approach using a scanning sonar [3]. In the mission C, MONACA navigates at a constant depth, measuring water quality by CTD. In order to realize these three missions at low-cost, MONACA (Fig. 2) has the sensor unit that can be turned upside down. The sensor unit includes the multi-beam sonar, an INS, and a DVL. Another feature of the vehicle is a modular structure. As the front and tail parts can be apart from the center part, it is easy to add new sensors or functions as necessary. In addition, the modular structure reduces a transportation cost. MONACA is currently under construction, to be launched in 2019.!

Figure 1.! Mission of MONACA Figure 2.! Overview of MONACA

References [1] Andrew S. Brierley, et al., Antarctic Krill Under Sea Ice:Elevated Abundance in a Narrow Band Just South of Ice Edge, Science, 295(5561), 1890-1892 , 2002. [2] Peter W. Kimball, et al., The ARTEMIS under-ice AUV docking system, Journal of Field Robotics, 35(2), 299-308, 2017. [3] T. Maki, et al., AUV Hattori: a Lightweight Platform for High-speed Low-altitude Survey of Rough Terrain, OCEANS17 MTS/IEEE Anchorage, Anchorage, 2017.

40

An Elecro agnetic nder ce ositioning ste and An nderwater rone for nder ce

Hiroshi Yoshida1, Shojiro Ishibashi1, Kiyotaka Tanaka1, Ryo Sato1 1Japan Agency for Marine-Earth Science and Technology(JAMSTEC)

Research under the ice in polar region are an important issue to understand mechanisms of ice melting. Autonomousunderwater vehicles (A Vs) or underwater drones are a promising platform which carries sensors. Observation data must include the observed position. But it is not easy to deploy an underwater localization system from the ice surface. In open seaan acoustical super short baseline (SSBL) system installed on a vessel or a boat is widely utilized. In ice-covered sea a localization system have to deploy from the ice surface because the ice prevents the entry of vessels. We need much effort and cost to dig a hole on the ice if we deploy SSBL systems since acoustic waves does not propagate in the ice. We propose an underwater localization system for A Vs or underwater drones. It consists of two major units a short range positioning (SRP) unit using electromagnetic (EM) waves and a DVL (Doppler velocity log) - INS (inertial navigation system) hybrid unit. The SRP unit uses low frequency EM waves which can propagate in both of the ice and sea water. It is set on the ice, receiving GPS signal to fix self-location. It will transmit EM waves into the sea water through the ice up to several tens meters deep. A DVL-INS hybrid unit can measure relative position from a reference point within error of a few meters per km if the DVL receives valid reflection wave from the ice bottom. If the SRP unit covers the range of over 30 meters from the ice bottom, it will be placed in every 10 km on the ice. We have been studying which localization method is suitable for the proposed system. An underwater dorone (about 200 kg in weight) has also developed since 2017. We describe the drone system as well as the localization system in the conference.

Figure 1. Concept image of the under Ice Positioning System.

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Session 6

43

The Filchner-Ronne Ice Shelf System

H. H. Hellmer

Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany

The Filchner-Ronne Ice Shelf (FRIS), the largest (by volume) floating extension of the Antarctic Ice Sheet (AIS), fringes the southern Weddell Sea known to be the dominant source of the globally relevant Antarctic Bottom Water. As a link between ocean and ice sheet, this ice shelf plays an important role for the stability of the AIS and the preconditioning of water masses participating in the global thermohaline circulation. The dominant process serving this pivotal role of FRIS is the exchange of heat, salt and tracers at the base of the ice shelf. While the southern Weddell Sea has been considered as largely invulnerable to climate warming, recent projections point to a potential tipping of the ocean state from cold to warm by the end of this century. The lack of detailed knowledge about the ocean underneath FRIS and the possibility of dramatic changes in the near future brought together scientists from the UK, Norway, and Germany. In the framework of the Filchner Ice Shelf Project, they intensively investigate the southern Weddell Sea continental shelf, including the FRIS cavity, by means of ship-based observations, moorings in front of and beneath the ice shelf, sub-ice shelf water sampling, and numerical modeling. This presentation reviews the achievements of the Alfred Wegener Institute over the past 6 years focused on observation, modeling, and comprehensive understanding of on-shore flow, dense water formation, sub-ice shelf circulation, meltwater production, and Ice Shelf Water spreading on the southern Weddell Sea continental shelf. All together has an impact on the ice shelf mass balance and thus on the discharge of inland ice with consequences for global sea level rise.

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Antarctica’s Great Sub-Ice Basins: A Natural Wonder or a Global Threat?

Donald D. Blankenship1

1University of Texas Institute for Geophysics, University of Texas at Austin, Austin, TX 78758

Internationally collaborative aerogeophysical exploration over the last two decades has revealed Antarctica as a geologically diverse continent underlying an ice sheet with significant sea level potential, parts of which are currently undergoing rapid change. The sub-continental scale Byrd, Wilkes and Aurora Subglacial Basins, the three largest reservoirs of sea level potential on Earth, are broader, deeper, and more susceptible to marine ice sheet instability than previously known and have been found to collectively represent a potential sea-level contribution of up to twenty meters. In particular, we have discovered that the Wilkes and Aurora Subglacial Basins of East Antarctica share a similar geometry to the Byrd Subglacial Basin underlying the West Antarctic ice sheet, with a large proportion of the ice sheet bed lying one to two kilometers below sea level and sloping towards the interior. The morphology and coastal connections of the ASB indicate a dynamic ice sheet with a significant erosional history and multiple semi-stable ice sheet configurations. Recent results imply routine disintegration of the West Antarctic ice sheet overlying the Byrd Subglacial Basin and significant retreat of the East Antarctic ice sheet into the Wilkes Subglacial Basin during Pliocene warming. Our findings indicate that only small coastal ridges halt irreversible discharge from both the Byrd and Wilkes Subglacial Basins.

Our aerogeophysical studies have also unveiled complex contemporary subglacial landscapes beneath all three basins suggesting a varied forcing regime on the ice providing new challenges and opportunities to ice sheet modelers attempting to predict both the timing and rates of future sea-level rise. For instance, geothermal heat flow beneath all three basins varies spatially on multiple scales in a continental crust that was long assumed to be homogeneous. For example, a large, active, subglacial hydrological system modulated by rift controlled heterogeneous geothermal flux flows through the Byrd Subglacial Basin of West Antarctica whereas in East Antarctica’s Aurora Subglacial Basin, subglacial hydrological pathways likely predate large-scale glaciation.

Geological proxies indicate four to eight meters of global sea level rise during the last interglacial period with ice core results constraining the amount of sea level rise from Greenland to two meters implying a potential two to six meter sea-level contribution form ice overlying the great sub-ice basins of Antarctica. New space- and airborne altimetry data along the Antarctic coast reveal extensive contemporary lowering of the glaciers over both the Byrd and Aurora Subglacial Basins while satellite gravity indicates a variable but persistent record of negative regional mass loss. These discoveries provide a new baseline as the international community increases its focus on glacier change over these basins resulting from both ocean and atmospheric forcing.

Questions driving our current work include:

• What is the character and distribution of subglacial boundary conditions and water systems upstream of the grounding line where these sub-ice basins meet the ocean?

• How much subglacial water discharges into sub-ice shelf cavities downstream of these basins and how does this water modulate ocean forcing, ice surface elevation change and grounded ice mass budget?

• How does ice shelf cavity geometry and atmospheric forcing affect, respectively, sub-ice shelf circulation and ice shelf calving in these areas of significant potential sea level impact?

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200m

North South

1200m(1) warm mCDW flows southward at the deep layer of trough leading into the region beneath SGT(2) mCDW meets to melt the base of SGT, then a mixture of mCDW and glacial MW is transported upward as a buoyant melt plume (ice pump)(3) the mixture exports northward at surface and sub-surface layers

Strong ice-ocean interaction at Shirase Glacier Tongue, East Antarctica

Daisuke Hirano1, Takeshi Tamura2, Kazuya Kusahara3, Kay I. Ohshima1, Shuki Ushio2, Daisuke Simizu2, Kazuya Ono1, and Shigeru Aoki1

1Institute of Low Temperature Science, Hokkaido University 2National Institute of Polar Research, 3Japan Agency for Marine-Earth Science and Technology

Shirase Glacier Tongue (SGT) is a thick floating slab of ice that forms where the glacier flows down onto the ocean surface at the southern closed-section of Lützow-Holm Bay (LH Bay) off Enderby Land, East Antarctica. Compared with other major ice shelves/tongues around Antarctica, SGT is smaller in area but its basal melt rate was estimated to be relatively high at a rate of ~7 ± 2 m per year (Rignot et al., 2013) based on presence of warm deep water. Although comprehensive hydrographic observations in LH Bay is indispensable for understanding the SGT-ocean interaction, they are extremely limited since the bay is usually covered with heavy fast ice even in summer. To explore in detail the SGT-ocean interaction, summer comprehensive hydrographic observations in LH Bay were conducted during JARE58th in 2017 under the project called ROBOTICA. LH Bay has a deep glacial trough in its center, connecting the regions from shelf break to SGT. Cold, fresh, and oxygen-rich Winter Water (WW: remnant of winter mixed layer) is overlying warm, saline, and oxygen-poor modified Circumpolar Deep Water (mCDW) along the deep trough from the shelf break to the ice front. This indicates mCDW inflow beneath the SGT, and the inflowing mCDW temperature exceeds the in-situ freezing point by more than 2.7oC. At surface/sub-surface layers, water properties become warmer and lower oxygen content toward the ice front. On !18O-salinity space, this anomalous warm and oxygen poor layer at the ice front is distributed along the line connecting mCDW with glacier end-members, with glacial melt water fraction estimated to be 0.5-1.7%. In addition, the anomalous layer contains relatively high mCDW fraction even at surface/sub-surface layers, indicating the glacial melt water outflow beneath the SGT as a mixture with mCDW. The above observational results suggest a 3-dimentional circulation, associated with SGT-ocean interaction (i.e., basal melting of SGT by mCDW; Figure 1), that comprises: (1) warm mCDW flows southward at the deep layer of glacial trough leading into the region beneath SGT, (2) mCDW meets to melt the base of SGT, then a mixture of glacial melt water and mCDW is transported upward as a buoyant melt plume, and (3) the mixture exports northward at surface/sub-surface layers. As is the case with Totten Ice Shelf, the SGT is also characterized as a warm ice cavity atypical in East Antarctica, which resulted from an absence of coastal polynya (i.e., cold Dense Shelf Water) as well as a presence of deep trough serving as a pathway of mCDW toward the SGT in LH Bay.

Figure 1. Schematic illustrating a 3-dimentional circulation associated with SGT-ocean interaction. References Rignot, E., S. Jacobs, J. Mouginot, & B. Scheuchl (2013), Ice-shelf melting around Antarctica, Science, 341(6143), 266-270, doi:10.1126/science.1235798.

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Pathway of Circumpolar Deep Water into Pine Island and Thwaites ice shelf cavities and to their grounding lines�

Yoshihiro Nakayama1,2, Georgy Manucharayan2, Patrice Kelin2,4, Hector G. Torres2, Michael Schodlok5, Eric Rignot 2,6, Pierre Dutrieux 7, Dimitris Menemenlis2

1 Hokkaido University, Institute of Low Temperature Science, Hokkaido, Japan

2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, 3 California Institute of Technology, Pasadena, CA, USA

4 Laboratoire de Physique des Océans, IFREMER-CNRS-IRD-UBO, Plouzané, France, 5 JIFRESSE, University of California Los Angeles, CA, USA

6 Earth System Science, University of California Irvine, CA, USA 7 Lamont-Doherty Earth Observatory Ocean and Climate Physics, NY, USA

Melting of Pine Island and Thwaites ice shelves is caused by Circumpolar Deep Water (CDW) intruding onto the Amundsen Sea continental shelves via submarine glacial troughs located at the continental shelf break. Despite existing works on on-shelf CDW transports with horizontal grid resolutions of ~1-2 km or coarser, it has been difficult to fully resolve sub-ice shelf environments with steep changes in both bathymetry and ice shelf shapes. In this study, we use a regional Amundsen Sea configuration of the Massachusetts Institute of Technology general circulation model (MITgcm) with horizontal and vertical grid spacings of 200 and 10 m, respectively. We calculate time-mean and time-evolving fields of velocity and investigate the mechanisms of how CDW is transported into the ice shelf cavities and to their grounding lines. We find a prominent submesoscale variability in the ice cavity, with scales of motion O(1-5km) and Rossby numbers O(1). This study is the first step towards understanding the importance of sub-mesoscale processes for sub-ice shelf circulation and thus transport of warm CDW to the grounding lines and ice shelf bases.

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Two recirculations in the Australian-Antarctic Basin revealed by improved ice-free monthly absolute dynamic ocean topography using CryoSat-2 radar altimeter

Kohei Mizobata1, Keishi Shimada2

1 Department of Ocean Sciences, Tokyo University of Marine Science and Technology 2 Center for Marine Research and Operations, Tokyo University of Marine Science and Technology

The bottom melting of ice sheet and the supression of Antarctic bottom water (AABW) formation have been reported [e.g., Williams et al., 2016; Kusahara and Hasumi, 2013]. The key driver of both phenomena is thought to be the warm and salty Circumpolar Deep Water (CDW). The transport of CDW and its access to continental shelf break area should depend on the ocean circulation. The ocean circulation in the seasonal sea ice zone, however, is still unclear due to the difficulty of hydrographic observation covering regions of interest. To elucidate the ocean circulation, absolute dynamic ocean topography (ADT) is one of the important parameters. Recently, Mizobata et al. [2016] developed ice-free monthly-mean ADT in the Arctic Ocean using the measurements of the CryoSat-2 (CS-2) radar altimeter, and now several works show similar products in the Southern Ocean [e.g, Armitage et al. 2018]. Unlike the Arctic Ocean, ADT products in the Southern Ocean faces the problem on the spatial interpolation. The distance between the orbits of the polar orbiting satellites like CS-2 becomes larger as going to the lower latitudes. The continental shelf and shelfbreak areas in the Southern Ocean locate around 64S or higher so that the distance between the orbits, i.e., the spatial resolution of original dataset is sometimes not enough due to smaller Rossby radius comparing to it in lower latitude ocean. Also CS-2 only cannot covers the mid-latitude ocean where the Antarctic Circumpolar Current (ACC) flows. To reveal the ocean circulation in both shelf area and offshore basin area (40S-70S), we developed “Ice-free” Monthly ADT using the measurements of satellite altimeters, CS-2/SIRAL (Geophysical Data Record, BASELINE-C) and Jason-2 (AVISO/CorSSH products), based on the methods described in Mizobata et al. [2016]. Also we employed the interpolation method with a topographic constraint scheme (TCS) proposed by Shimada et al. [2017] to overcome “interpolation” problem. Developed dataset still has uncertainty which will be due to the accuracy of radar altimeter retrieval. Then we applied the empirical orthogonal functions (EOF) to reduce noises and to understand the variability of the ocean circulation field. Although the dataset we developed covers entire Southern Ocean but this study mainly focuses on the Australian-Antarctic Basin (A-A Basin) to avoid the discussion diverging. Our products reconstructed by using dominant EOF modes, show that the clockwise circulations locate along 63.5S latitude line from 0E to 150E. In the A-A Basin, Bindoff et al. [2000] showed relatively widely distributed “ACC recirculation gyre” from 90E to 110E. But our product show that two recirculation gyres stationary locates at the eastern side of the ridge around 107E and 115E. Those recirculations indicate that there are many possible routes of poleward CDW transport more than expected. Unsolved question is what controls the variability of recirculation field. Through the comparative study, the correlation between First EOF mode of ADT explaining 16% variance and Southern Annular Mode (SAM) index turns out -0.4, i.e., weak sensitivity to atmospheric forcing related to SAM. References Kusahara, K., and H. Hasumi (2013), Modeling Antarctic ice shelf responses to future climate changes and impacts on the ocean, Journal of Geophysical Research Oceans, 118, 2454–2475, doi:10.1002/jgrc.20166. Mizobata K., E. Watanabe and N. Kimura (2016), Wintertime variability of the Beaufort Gyre in the Arctic Ocean derived from CryoSat-2/SIRAL observations, Journal of Geophysical Research-Oceans, doi:10.1002/2015JC011218. Shimada K., S. Aoki and K.-I. Ohshima (2017), Creation of a Gridded Dataset for the Southern Ocean with a Topographic Constraint Scheme, 34, 511-532, doi: 10.1175/JTECH-D-16-0075.1. Williams G. D., L. Herraiz-Borreguero, F. Roquet, T. Tamura, K.I. Ohshima, Y. Fukamachi, A. D. Fraser, L. Gao, H. Chen, C .R. McMahon, R. Harcourt and M. Hindell (2016), The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay, Nature Communications, 7:12577, doi:10.1038.

Fig. 1: First EOF of ADT in the A-A basin

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Subsurface cross-slope exchange in the Australian-Antarctic Basin

Kaihe Yamazaki 1, 2, Shigeru Aoki 1, Keishi Shimada 3, Yujiro Kitade 3 1. Institute of Low Temperature Science, Hokkaido University,

2. Graduate School of Environmental Science, Hokkaido University, 3. Tokyo University of Marine Science and Technology.

Autonomous profiling float (Argo) data during the last decade, including under-ice data, were analyzed to describe the mean

structure of the subpolar gyre in the Australian-Antarctic Basin. According to their trajectories, the gyre is meridionally regulated by the westward Antarctic Slope Current and the eastward flow maximum along the 4000 m isobath, which corresponds to the southernmost jet of the Antarctic Circumpolar Current (ACC). The Southern Boundary of ACC, commonly defined by the southern limit of the 1.5 °C isotherm, travels inside the gyre in contrast to the Weddell and Ross Gyres, so that Circumpolar Deep Water (CDW) offshore can deliver relatively much heat to the shelf. The Antarctic Slope Current flows generally along the continental slope and is deflected by the topographic features, leading to meridional flows near the slope. The meridional flows were accordingly observed with the potential temperature; northward and southward flows are correlated with cold and warm waters, respectively, indicating the heat transport by mean flows.

Potential temperature for a subsurface density range 27.7-27.8 kg/m3 (in σθ) is presented in Fig.1. Since the southward intruding warm waters are accompanied by the shallow temperature maximum, they shall correspond to CDW intrusions onto the shelf. The distinctive intrusions are recognized at 113 and 120 °E. Meanwhile, the seaward excursions of cold water can be the signal of either Shelf Water export or deepened Winter Water. To discern the nature of watermass, the depth of permanent pycnocline was examined. Subsequently, as winter mixing is not likely to reach the isopycnal of 27.7 kg/m3 as reported in Wong and Riser (2011), the cold signals can be attributed to water originated from the shelf. Furthermore, the density of 27.7 kg/m3 well agrees with the upper limb of CDW signals, whereas 27.8 kg/m3 is the typical density for bottom on the shelf (Nistche et al. 2017). We conclude that this subsurface layer is responsible for the cross-slope exchange in the region.

Figure 1. Potential temperature averaged for the isopycnal range 27.7-27.8 kg/m3 in study region. Bathymetry is drawn for every 1000 m, while the isobath of 3000 m is highlighted by white contour. Red and Blue lines denote the Southern Boundary and the Antarctic Slope Front, respectively, calculated from the gridded climatological dataset of Shimada et al. (2017). References Wong, A. P. S., & Riser, S. C. (2011). Profiling Float Observations of the Upper Ocean under Sea Ice off the Wilkes Land Coast of Antarctica. Journal of Physical Oceanography, 41(6), 1102–1115. Nitsche, F. O., Porter, D., Williams, G., Cougnon, E. A., Fraser, A. D., Correia, R., & Guerrero, R. (2017). Bathymetric control of warm ocean water access along the East Antarctic Margin. Geophysical Research Letters, 44(17), 8936–8944. Shimada, K., Aoki, S., & Ohshima, K. I. (2017). Creation of a Gridded Dataset for the Southern Ocean with a Topographic Constraint Scheme. Journal of Atmospheric and Oceanic Technology, 34(3), 511–532.

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Wintering habitat of Weddell seals along the continental shelf off Enderby Land, East Antarctica

Nobuo Kokubun1, 2, Yukiko Tanabe1, 2 , Takeshi Tamura1, 2,Vigan Mensah3, Daisuke Hirano3, Shigeru Aoki3 and Akinori Takahashi1, 2

1National Institute of Polar Research 2 SOKENDAI (The University of Advanced Studies)

3Hokkaido University Weddell seals (Leptonychotes weddellii) inhabit the southernmost part of the Southern Ocean where land-fast ice covers the sea surface. Due to their high diving ability Weddell seals deployed with tracking devices can provide an unique opportunity to collect environmental data in the coastal waters of Antarctica during autumn and winter, when conventional oceanographic observations are difficult. Although winter tracking data have been collected and reported elsewere across the Antarctic circumpolar regions (Weddell Sea, Antarctic Peninsula Region, Ross Sea, Adelie Land and Prytz Bay) until now, no studies have investigated the Lützow-Holm Bay region, where large inter-annual variations in sea ice extent was observed. This study aims at characterizing the wintering habitat of Weddell seals using oceanographic data collected via bio-logging. To achieve this, we deployed conductivity-temperature-depth satellite relay data loggers (CTD-SRDL) on 7 Weddell seals near Syowa Station (69.0oS 39.6oE) from late March to ealy April 2017. Among them, 3 were adult females, 1 was a sub-adult male and 2 were adult males. An additional deployment on a sub-adult male was conducted in late September 2017. The deployment duration was 166.0 ± 97.2 days (mean ± SD, n = 7 seals deployed during autumn). Between March and May, breakup events of land-fast ice were observed in Lützow-Holm Bay. The averaged distance travelled by the seals from the deployment location was 321.9 ± 201.5 km (range, 99.9 - 633.1 km). Most of the seals showed eastward movement, and remained in areas where the bottom depth was less than 1000 m, suggesting the seals prefered the continental shelf off Enderby Land. The total number of profiles covered by the seals was 1,302 (a total of 17,245 CTD data). During autumn and winter, high temperature (≧-1.0oC) and high salinity (≧34.4‰) water were observed at the deepest points of some dives in the Lützow-Holm Bay and on the edge of the continental shelf east of Syowa Station, which possibly originated from Modified Circmpolar Deep Water (MCDW). In addition, a subsurface temperature maximum (at maximum ≧-1.0oC) was observed in the 50- 400 m layer during autumn and winter across a wide range of the continental shelf off Enderby Land. The seals dived to depths of 162.8 ± 96.2 m (maximum: 632 m), with relatively high proportion of benthic dives (in that 64.6 ± 21.5% of all dives). We will explore further the diving behaviour and habitat use of the seals in relation to the sea ice characteristics and water masses. Overall, the Weddell seals from Syowa Station showed long distance migration through the winter and provided valuable oceanographic data off Enderby Land where winter sea ice extends over the continental shelf.

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Poster Session

53

Reconstruction of the last glacial-interglacial cycle in the Kerguelen Plateau’s PFZ and SAZ, Southern Ocean.

Civel Matthieu1, Crosta Xavier2, Cortese Giuseppe3, Michel Elisabeth4, Mazaud Alain4, Thöle Lena5, Jaccard Samuel5, Ikehara

Minoru1, Itaki Takuya6.

1Center for Advanced Core Research, Kochi University, Kochi 2UMR CNRS EPOC, Université de Bordeaux, France

3GNS Science, Lower Hutt, New Zealand 4LSCE-IPSL, CEA-CNRS-UVSQ, Gif-sur-Yvette, France

5Institute of Geological Sciences & Oeschger Centre for Climate Change Research, University of Bern, Switzerland 6National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba

The Southern Ocean (SO) connects the Atlantic, the Indian and the Pacific oceans and therefore influences the thermohaline circulation (THC) (Rintoul et al., 2001). The latitudinal position of the SO hydrological fronts also impacts the transfer of heat and salt to the THC by the Agulhas Current (Caley et al., 2012). Finally, the SO strongly influences global climate due to its control over atmospheric CO2 content via surface and deep-water stratification and SO productivity (Kohfeld et al., 2017). Deglaciations are fast transition periods between glacial and interglacial mean states. Deglaciations appear different over the last 400.000 years in term of shape and amplitude (EPICA, 2004), which may have been modulated by the interplay of complex mechanisms (sea ice, fronts migration, bipolar seesaw, heat transfer from lower latitudes) initiated in the SO (Deaney et al., 2017). However, little is still known about the exact role of the SO in deglaciations because of the lack of adequate records in the Indian sector, especially after the last 70 kyrs. The main objectives of the project are to (1) document variations in the Antarctic Circumpolar Current and its associated fronts, (2) reconstruct variations in surface and subsurface ocean temperatures and sea ice extent, in order to better understand their impact over global circulation during the last 400 kyrs and (3) reconstruct variations in biosiliceous productivity. These parameters will allow to document the timing and unfolding of the past 4-5 deglaciations. For this project I will focus on two sediment cores, one taken from the Kerguelen Plateau in 2011, the MD11-3353 and one that will be recovered on the Crozet Plateau in 2019. I will reconstruct sea-surface temperature and sub-surface temperatures by using advanced statistics applied to diatom and radiolarian census counts (Crosta et al., 2004; Cortese et al., 2007). These two microfossil groups, being the two most important silica producers groups in the SO, will also provide information on global siliceous productivity in these areas. I will focus on the last 4-5 deglaciations (transition MIS12-MIS11 at ~400 kyrs) to encompass very different transitions. This presentation mainly focus on the first radiolarian counts obtained on MD11-3353 sediment core and their comparison to similar results obtained on core MD12-3396CQ that I studied for my Master thesis. Core MD12-3396CQ was taken east of the Kerguelen Plateau while core MD11-3353 was retrieved west of the Plateau. Hence my results will help describe and compare the unfolding of the last deglaciation at a sub-millennial time scale around the Kerguelen Plateau along with driving mechanisms. References Bazin, L., A. Landais, B. Lemieux-Dudon, H. Toyé Mahamadou Kélé, D. Veres, F. Parrenin, P. Martinerie, C. Ritz, E. Capron,

V. Lipenkov, M-F. Loutre, D. Raynaud, Bo M. Vinther, A. Svensson, S. Rasmussen, M. Severi, T. Blunier, M. Leuenberger, H. Fischer, V. Masson-Delmotte, A. Chappellaz, E. Wolff, delta Deuterium measured on ice core EDC on AICC2012 chronology, doi:10,1594/PANGAEA,824891, 2013.

Caley, T., J. Giraudeau, B. Malaizé, L. Rossignol, and C. Pierre, Agulhas leakage as a key process in the modes of Quaternary climate changes, Proc. Natl. Acad. Sci. U. S. A., 109(18), 6835–6839, doi:10.1073/pnas.1115545109, 2012.

Cortese, G., and R. Gersonde, Morphometric variability in the diatom Fragilariopsis kerguelensis: Implications for Southern Ocean paleoceanography, Earth Planet. Sci. Lett., 257, 526–544, doi:10.1016/j.epsl.2007.03.021, 2007.

Crosta, X., A. Sturm, L. Armand, and J.J. Pichon,, Late Quaternary sea ice history in the Indian sector of the Southern Ocean as recorded by diatom assemblages. Mar. Micropal., 50, 209-223, 2004

Deaney, E. L., S. Barker, & T. van de Flierdt, Timing and nature of AMOC recovery across Termination 2 and magnitude of deglacial CO2 change. Nature Communications, 8, 14595. https://doi.org/10.1038/ncomms14595, 2017.

Kohfeld, K.E. & Z. Chase, Temporal evolution of mechanisms controlling ocean carbon uptake during the last glacial cycle. Earth and Planetary Science Letters 472, 206–215. https://doi.org/10.1016/j.epsl.2017.05.015, 2017.

54

Rintoul, S. R., C. Hughes, & D. Olbers, in Ocean Circulation and Climate (eds Siedler, G. et al.) 271–302 Academic Press, Cambridge, 2001.

55

Millennial-scale sea ice expansion in the glacial Southern Ocean driven by Antarctic warming

Minoru Ikehara1 and Kota Katsuki2 1Kochi University (ikehara@kochi-u.ac.jp)

2Shimane University The Southern Ocean has played an important role in the evolution of the global climate system. Area of sea ice shows a large seasonal variation in the Southern Ocean. Sea ice coverage on sea surface strongly affects the climate of the Southern Hemisphere through its impacts on the energy and gas budget, on the atmospheric circulation, on the hydrological cycle, and on the biological productivity. However, millennial-scale sea ice coverage and its impacts are not well understood. Here we show high-resolution records of sea ice-rafted debris (SIRD) and diatom assemblage to reveal a rapid change of sea ice distribution in the Indian sector of the Southern Ocean for last 43,500 years. The depositions of rock-fragment SIRD excluding volcanic glass and pumice were associated with increasing of sea-ice diatoms, suggesting that the millennial-scale events of sea-ice expansion and cooling were occurred in the glacial South Indian Ocean. The extent of sea ice in the Southern Ocean is occurred during the warming events, Antarctic isotope maximum (AIM). Sea ice expansion might be caused by enhanced production of sea ice in the Weddell Sea due to Antarctic ice sheet melting during the small Antarctic warming in the glacial climate.

56

Tidally controlled vertical ice motion generate seismicity near a terminus of a floating tongue, East Antarctica

Masahiro Minowa1,2, Evgeny A. Podolskiy3 and Shin Sugiyama1

1Institute of Low Tempeature Science, Hokkaido University 2Instituto Ciencias Físicas y Matemáticas, Universidad Austral de Chile

3Arctic Research Center, Hokkaido University Glacier microseismicity is providing new insights into the glacial dynamics. However, physical mechanisms generating icequakes are not well understood, especially near the terminus of ice shelves and floating ice tongues. We observed ice speed and icequakes at the floating tongue of Langhovde Glacier in East Antarctica. We installed four GPS stations on the ~3-km long floating tongue at 0.3–2.5 km from the ice front to the grounding line. A seismic array formed of three seismometers was installed at 0.5 km from the terminus. Diurnal and semidiurnal variations in the ice speed were observed at all the GPS and seismic stations. A linear correlation was found between the vertical ice motion and occurrence of icequakes. A larger number of icequakes was observed during rising and high tide. The correlation coefficient increased as tidal amplitude increased. The tidally controlled vertical ice motion was 7.6 times larger than the horizontal motion at the lowermost GPS. These results suggest that events of basal origin, such as basal crevassing, are the likely source of the icequakes. Further detection and monitoring of the seismicity near the terminus may help to understand how the tidally controlled ice motion fractures the ice shelf near the ice front.

Figure 1. (a) and (b) the study area in East Antarctica. (c) Enlarged image of the glacier with instrument sites. (d) Cross-sectional profile of the floating tongue.

57

Seasonality in sulfur isotopic compositions of atmospheric sulfate in East Antarctica

Sakiko Ishino1, Shohei Hattori1, Joel Savarino2, Michel Legrand2, Emmanuelle Albalat3, Francis Albarede3, Susanne Preunkert2, Bruno Jourdain2, and Naohiro Yoshida1

1 Tokyo Institute of Technology, Japan 2 Univ. Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, F-38000 Grenoble, France

3 Ecole Normale Supérieure de Lyon, Université de Lyon, CNRS, France Sulfur stable isotopic compositions of sulfate in the Antarctic snow and ice cores have been used to investigate the contribution of its sources such as marine biogenic activity and volcanic emissions, as well as its formation pathways. However, temporal variability of those signatures in the present Antarctic atmosphere has never been examined. Here we report a year-round observation of sulfur isotopic compositions of sulfate in aerosol samples collected in the year 2011 at Dome C (75°10’S, 123°30’E; 3233 m a.s.l.) and Dumont d’Urville Station (66°40’S, 140°01’E; 40 m a.s.l.), inland and coastal sites in East Antarctica. The d34Snss values showed clear seasonal variations with summer maxima and winter minima, and were in good agreement between inland and coastal sites. In summer periods, d34Snss value were similar to the values observed in dimethyl sulfide (DMS) produced by marine biota, in contrast to 34S depletion during winter, which suggest the contribution of other sources or unknown processes. Combined with the uniform d34Snss values observed in surface snow in a different sector of East Antarctica, the spatial variation suggests that the net isotopic fractionation through SO2 oxidation during the transportation is insignificant for changes in d34Snss values. In the presentation, we will discuss the factor controlling d34Snss values and the relative importance of various sulfur sources including marine biogenic, volcanic, stratospheric, and anthropogenic sulfate.

58

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The effect of basal melting of the Shirase Glacier on the CO2 system

in Lützow-Holm Bay, East Antarctica

Masaaki Kiuchi1, Daiki Nomura1, Daisuke Hirano2, Takeshi Tamura3, Tomohide Noguchi4, Gen Hashida3, Shigeru Aoki2

1Faculty of Fisheries Sciences, Hokkaido University 2Institute of Low Temperature Science ,3National Institute of Polar Research, 4Marine Works Japan

In order to clarify the effect of the basal melting of Antarctic ice sheet on the CO2 system in the coast of the Southern Ocean, dissolved inorganic carbon (DIC), total alkalinity (TA), nutrients, oxygen isotopic ratio were measured off Shirase Glacier Tongue (SGT) in the ice-covered Lützow-Holm Bay, east Antarctica during summer 2017 and 2018. Vertical profiles of temperature, salinity, DIC, and TA showed that the modified Circumpolar Deep Water (mCDW) characterized by high-temperature, -salinity, -DIC and -TA flowed southward along to the deep layer of submarine canyon and encountered at the bottom of SGT. Due to the freshwater supply by the basal melting, salinity, DIC, and TA decreased at the subsurface layer near SGT, and the melt water fraction estimated from oxygen isotopic ratio and salinity was 1.0 %, which corresponded to the decreases of 0.3 for salinity and 30 µmol kg-1 for DIC and TA. Higher fraction of melt water (about 2.0 %) was observed in the ice covered surface water with high chlorophyll a concentration and low-salinity, -DIC and -TA near SGT. Our results suggest that the basal melting of Antarctic ice sheet supply the fresh water into mCDW at the subsurface layer, and subsequent transport to the surface layer provides conditions favorable for under-ice bloom presumably due to the supplement of iron contained fresh water into the surface water.

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59

Treatments of ice-ocean interaction in ice sheet models and implications for Antarctic ice sheet retreat in the past and future

Takashi Obase1, Ayako Abe-Ouchi12 and Ralf Greve3

1Atmosphere and Ocean Research Institute, the University of Tokyo 2Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

3Institute of Low Temperature Science, Hokkaido University

Basal melting beneath ice shelves is an important factor in the retreat of grounding lines of Antarctic ice sheet, and it may siginificanly affect the volume of Antarctic ice sheet. Physically, basal melting of ice shelf is determined by upward heat transfer across ice shelf-ocean boundary, that is determined by ocean circulations beneath ice shelves and Antarctic continental shelf seas. Recent ocean modeling (includes ice shelf cavity component) studies suggest a climate change affect basal melting rate by not only increasing ocean temperature itself but also by changing water mass formations and ocean circulations in the Antarctic continental shelf seas, which may lead to drastic increase in basal melting (Hellmer et al. 2012; Obase et el. 2017). In turn, basal melting of ice shelves is an essential mass balance term for ice sheet models. However, as it is difficult to conduct long-term simulation using such ocean models, basal melting rate of ice shelves are often parameterized with ocean temperature, which is derived from outputs of GCMs (without ice shelf component) or reconstructions near Antarctic continent (e. g. Deconto and Pollard 2016). In the present study, at first, we review formulations of basal melting in the previous ice sheet modeling studies and discuss current limitations of such parameterizations, using perspectives from oceanographical and glaciological observations, and ocean modeling studies. Second, using a 3-dimensional Antarctic ice sheet model (SICOPOLIS), we investigate the responses of grounding lines of Antarctic ice sheet to simplified atmospheric surface mass balance and oceanic basal mass balance changes, similary to Golledge et al. (2017). Based on Southern Ocean temperature reconstructions in the past and climate model simulations, we discuss uncertainties in the treatment of basal melting and its implications for the retreats of Antarctic ice sheet in the past and future. References Deconto, R. M., and D. Pollard (2016), Contribution of Antarctica to past and future sea-level rise. Nature, 531 (7596), 591-597, doi:10.1038/nature17145. Golledge, N. R., R. H. Levy, R. M. McKay and T. R. Naish (2017), East Antarctic ice sheet most vulnerable to Weddell Sea warming, Geophysical Research Letters, 44, doi:10.1002/2016GL072422. Hellmer, H. H., F. Kauker, R. Timmermann, J. Determann, and J. Rae (2012), Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current. Nature, 485, 225-228, doi:10.1038/nature11064. Obase, T, A. Abe-Ouchi, K. Kusahara, H. Hasumi, R. Ohgaito (2017), Responses of basal melting of Antarctic ice shelves to the climatic forcing of the Last Glacial Maximum and CO2 doubling, Journal of Climate, 30(10), 3473-3497, doi:10.1175/JCLI-D-15.0908.1.

60

Provenances of ice-rafted detritus in sedimentary core from the Conrad Rise

Hiroshi Sato1, Minoru Ikehara2 and Takuya Matsuzaki2 1Senshu University

2 Center for Advanced Marine Core Research, Kochi University

Ice-rafted debris (IRD) is a terrigenous material transported within a matrix of ice and deposited in marine sediments when the ice matrix melts (Kuijpers et al., 2014). IRD is one of proxy of glacial variability because both sea ice and iceberg are ice rafting agents. Previous studies suggest that iceberg transports continental materials mainly from Antarctica, while sea ice transports volcanic rocks mainly from island and/or volcano around Antarctica. Therefore, if we determine provenances of IRD, we can estimate Antarctic glacial dynamics.In this study, IRD in sedimentary core (COR-1bPC) from the Conrad Rise were analyzed with Electron Probe Micro Analyzer (EPMA) and their provenances were preliminary estimated.

Based on compositions and mineral stoichiometry, IRD grains were classified into two groups: volcanic glasses including groundmass of volcanic rocks and mineral grains. Volcanic glasses including groundmass of volcanic rocks: Based on SiO2-total alkalies classification, these grains have compositions from basalt to rhyolite. Most of grains have similar compositional range to volcanic rocks from the South Sandwich Islands, except for grains with rhyolite compositions. On SiO2 vs K2O plot, compositional trend shows low-K series, tholeiitic series to calc-alkaline series, which are recognized for volcanic rocks from the South Sandwich Islands. Compositions of volcanic glasses including groundmass of volcanic rocks reveals that they derived from volcanic islands in the South Sandwich Islands. Mineral Grains: Volcanic glasses including groundmass of volcanic rocks contains plagioclase, clinopyroxene, and orthopyroxene, corresponding the idea that volcanic glasses derived from volcanic islands in the South Sandwich Islands. Quartz and orthoclase grains are contained in several section. According to the description of volcanic rocks from the South Sandwich Island by Pearce et al. (1995), volcanic rocks including orthoclase were not reported. They also described that “quartz is present as occasional angular xenocrysts in some of the Leskov island andesite and also forms a late interstitial mineral in the dioritic blocks”. If quartz grains derived from the South Sandwich Island, they were only from the Leskov island. Alternate hypothesis is that they derived from continental crust rather than volcanic islands because both quartz and orthoclase are dominant minerals in felsic plutonic rocks and metamorphic rocks. Previous studies reported garnet grains as a line of evidence for continent origin of IRD (e.g., Teitler et al., 2010). Garnet grains were not recognized in this study, suggesting that garnet-free rocks might be distributed at provenance. Table 1. Mineral and compositions of IRD form Conrad rise.

Sample ID Depth in core (m) Quartz Orthoclase Plagioclase clinopyroxene orthopyronexe 437 RF 4.953 dominant Or>85 An60-80 Mg#=68 437 VC 4.953 An80 507 RF 5.751 dominant Or>85 An60-70 Mg#=74 731 RF 8.278 An>85, 60-70, 40 Mg#=46.5, 60-70, 83.9 Mg#=67.8 731 vol 8.278 present An70-82 Mg#=75 739 RF 8.369 An60-70, >80, Up to 96 Mg#=68.5 Mg#=70 739 vol 8.369 An 50-66 Mg#=58 887 RF 10.051 present An80-90, 53 Mg#=75 Mg#=75 887 vol 10.051 An80-89 Mg#=75 Mg#=70

References Kuijpers A., Knutz P., Moros M. (2014) Ice-Rafted Debris (IRD). In: Harff J., Meschede M., Petersen S., Thiede J. (eds) Encyclopedia of Marine Geosciences. Springer, Dordrecht Pearce, J.A., Baker, P.E., Harvey, P.K. & Luff, I.W. (1995) Geochemical evidence for subduction fluxes, mantle melting and fractional crystallization beneath the South Sandwich island arc. Journal of Petrology, 36, 1073–1109. Teitler, L., Warnke, D. A., Venz, K. A., Hodell, D. A., Becquey, S., Gersonde, R., and Teitler, W. (2010) Determination of Antarctic Ice Sheet stability over the last ∼500 ka through a study of iceberg‐rafted debris. Paleoceanography 25, doi:10.1029/2008PA001691

61

Response of the Antarctic ice sheet to increased sub-ice-shelf melt rates

Ralf Greve1, Fuyuki Saito2, Shun Tsutaki3, Takashi Obase3, Ayako Abe-Ouchi3 1Institute of Low Temperature Science, Hokkaido University

2Japan Agency for Marine-Earth Science and Technology (JAMSTEC) 3Atmosphere and Ocean Research Institute, University of Tokyo

The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6, Nowicki et al. 2016) brings together a consortium of international ice sheet and climate models to explore the contribution from the Greenland and Antarctic ice sheets to future sea level rise (SLR). LARMIP (Linear Antarctic Response Model Intercomparison Project) and ABUMIP (Antarctic Buttressing Model Intercomparison Project) are initiatives within ISMIP6 to explore the sensitivity of the Antarctic ice sheet to increased basal melting rates under the ice shelves, which was identified as the process to which the ice sheet is likely most vulnerable by the SeaRISE project (Bindschadler et al. 2013, Nowicki et al. 2013). We contribute to LARMIP and ABUMIP with the ice sheet model SICOPOLIS (e.g., Greve and Blatter 2016), thus investigating the effect of the full range from moderately increased basal melting rates to extreme scenarios that melt away all floating ice rapidly. As shown in Fig. 1, over the next 200 years, the mass loss of the ice sheet (contribution to SLR) depends approximately linearly on the melting-rate increase up to ~4 m/a, while for larger melting rates, some saturation shows up (sub-linear response). An extreme scenario with 400 m/a basal melting removes almost all floating ice within a few years, which leads to contributions to SLR of ~0.7 metres after 100 years and ~1.2 metres after 200 years. The sensitivity to regionally increased basal melting rates (region definitions by LARMIP) decreases in the order Weddell > East Antarctica > Ross > Amundsen > Antarctic Peninsula.

Figure 1. Simulated response of the Antarctic ice sheet to increased sub-ice-shelf melt rates ranging from 1 to 32 m/a, applied simultaneously for the floating ice all around the grounded ice sheet. The mass loss is shown in sea-level equivalents (contribution to sea level rise). References

Bindschadler, R. A. and 27 others, Ice-sheet model sensitivities to environmental forcing and their use in projecting future sea level (the SeaRISE project), J. Glaciol., 59(214), 195-224, 2013, doi: 10.3189/2013JoG12J125.

Greve, R. and H. Blatter, Comparison of thermodynamics solvers in the polythermal ice sheet model SICOPOLIS, Polar Science, 10(1), 11-23, 2016, doi: 10.1016/j.polar.2015.12.004.

Nowicki, S. M. J. and 30 others, Insights into spatial sensitivities of ice mass response to environmental change from the SeaRISE ice sheet modeling project I: Antarctica, J. Geophys. Res. Earth Surf., 118(2), 1002-1024, 2013, doi: 10.1002/jgrf.20081.

Nowicki, S. M. J. and 8 others, Ice Sheet Model Intercomparison Project (ISMIP6) contribution to CMIP6, Geosci. Model Dev., 9(12), 4521-4545, 2016, doi: 10.5194/gmd-9-4521-2016.

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Role of clouds on the Southern Ocean sea surface temperature bias and its impact on climate simulations

Sam Sherriff-Tadano1, Ayako Abe-Ouchi1, Haruka Hotta1, Maki Kikuchi2, Takanori Kodama1 and Kentaroh Suzuki1 1Atmosphere and Ocean Research Institute, the University of Tokyo

2Earth Observation Research Center, JAXA Antarctic and Southern Ocean are key regions for the climate system. A precise simulation of these regions in climate models are necessary to improve our understandings of the climate system, confidence of the future climate predictions and performances of paleoclimate simulations. However, most climate models suffer from warm sea surface temperature biases over the Southern Ocean in the simulations of modern climate (warm SST bias). Previous studies suggest that the warm SST bias is associated with an overestimation of a downward short wave radiation at the sea surface, which is partly related to an underestimation of supercooled cloud water in the model. In this study, by improving the representation of supercooled cloud water in MIROC4m AOGCM based on satellite data, we aim to improve the warm SST bias. We also assess the impact of improvements in the warm SST bias on the modern climate and a simulation of the Last Glacial Maximum. Model simulations show a reduction in the downward short wave radiation at the Southern Ocean in response to the modification in the supercooled cloud water. As a result, the warm SST bias at the Southern Ocean is also improved. The simulation of the Last Glacial Maximum with the improved MIROC4m AOGCM also shows improvements in sea ice cover over the Southern Ocean and the deep ocean circulation compared to reconstruction data. Hence, this study clarifies the importance role of clouds over the Southern Ocean in simulating both the climate of modern and past.

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Present-day crustal motion and gravity change around the Lützow-Holm Bay region based on GIA modeling

Jun’ichi Okuno1, 2, Koichiro Doi1, 2, Yuichi Aoyama1, 2, Takeshige Ishiwa1, Akihisa Hattori2 and Yoichi Fukuda3

1Natinal Institute of Polar Research 2SOKENDAI (The Graduate University for Advanced Studies)

3Kyoto University Geodetic and geomorphological surveys in the Lützow-Holm Bay region, East Antarctica have been conducted by Japanese Antarctic Research Expedition (JARE) to evaluate the crustal deformation induced by Glacial Isostatic Adjustment (GIA) in various time scales. In particular, several geodetic observations (e.g., Global Navigation Satellite Systems: GNSS and absolute gravity observations) have been carried out on outcrop rocks in this area since the 1990s to monitor crustal movements related to the Antarctic Ice Sheet (AIS) change (e.g., Ohzono, 2006, Kazama et al., 2013). These observations include the components of the GIA induced by the melting of AIS from Last Glacial Maximum (~21,000 years before present) and elastic deformation due to present surface mass balance. Therefore, to estimate the change of AIS based on the geodetic signals, it is crucial for the separation of the components of viscous and elastic signals in comparison of these observations with numerical predictions based on the GIA modeling. In this presentation, we will show the crustal deformation rates and gravity changes calculated by the GIA modeling using the previously published deglaciation histories and the comparisons with these observations obtained by JARE for about 20 years. Preliminary results show that the numerical predictions of the crustal deformation rates due to only last deglaciation cannot explain the recent observations of GNSS sufficiently. We intend to discuss the separation of the components between recent ice mass change and the last deglaciation, and estimate influences of recent AIS mass changes on the geodetic measurements in the Lützow-Holm Bay region. References Ohzono M., T. Tabei, K. Doi, K. Shibuya and T. Sagiya, Crustal movement of Antarctica and Syowa Station based on GPS measurements. Earth, Planets and Space, 58(7), 795-804, 2006. Kazama T., H. Hayakawa, T. Higashi, S. Ohsono, S. Iwanami, T. Hanyu, H. Ohta, K. Doi, Y. Aoyama, Y. Fukuda, J. Nishijima and K. Shibuya, Gravity measurements with a portable absolute gravimeter A10 in Syowa Station and Langhovde, East Antarctica. Polar Science, 7(3-4), 260-277, 2013.

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Subshelf environment of Langhovde Glacier, Antarctica

Shiori Yamane 1, 2, Shin Sugiyama 1 Masahiro Minowa 3 Masato Ito 1 and Daisuke Hirano 1 1 Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan

2 Graduate School of Environmental Science, Hokkaido University, Sapporo, Japan 3 Institute of Physics and Mathematics, Austral University of Chile

Around Antarctic continental margin, floating ice shelves and

outlet glaciers drain out ice from an ice sheet over the ocean.

Recently, hydrographic observations have been carried out

under the ice shelves in several coastal regions, and some

living things were observed there 1). However, it is difficult to

observe the regions under the ice shelf directly, understanding

of subshelf environment and its biota are limited. Here, we

document the results from hydrographic observations under the

ice shelf of Langhovde glacier in Lutzow-Holm Bay, East

Antarctica (figure. 1). To better understand subshelf ocean

characteristics and its biota, we carried out the field activity

from December 2017 to February 2018 as a part of the 59th

Japanese Antarctic Research Expedition. We obtained subshelf

ocean properties such as temperature and the video of borehole

camera through the four boreholes located 0.5–2.5 km from the

ice front. As the result, there are several kinds of smaller zooplanktons within ~10 m below the base of ice shelf where

temperature is about －1.3℃. Also, there are several kinds of larger zooplanktons such as krills and bentos within ~1 m above

the seabed where temperature is －1.2℃ (Figure. 2). In the document, we report the result of the borehole measurements and

discuss implications for subshelf biodiversity and environment of Langhovde glaciers.

References

1) Sugiyama, S. et al., Active water exchange and life near the grounding line of an Antarctic outlet glacier. Earth and

Planetary Science Letters, 399, 10.1016, 2014.

2) L, Neal. et al., Comparative marine biodiversity and depth zonation in the Southern Ocean: evidence from a new large

polychaete dataset from Scotia and Amundsen seas, Marine Biodiversity, 48, 581–601, 2018.

69°12’ S 39°48’ E

Figure 1. Satellite image of Langhovde glacier (Landsat 8, 16, Feb,

2018), ice front of every year and four borehole sites.

Figure 2. (a) Subshelf in-situ temperature, (b)-(d) observed living things from the video of borehole camera. Dashed lines represent the

location of four boreholes.

points.

Ice shelf

Ocean

Seabed

~5 mm ~5 mm

(b) Zooplanktons

~50 mm

(c) Krill (d) Polychaeta

In-s

itu

tem

per

atu

re [℃

]

Image: Neal et. al., 2018

Upper

65

南大洋インド洋区における食物網ベースラインの特徴と食段階構造

山本あゆ 1、真壁竜介 2,3、佐野雅美 2、立花愛子 1、高尾信太郎 2,3、小達恒夫 2,3、茂木正人 1,2 １東京海洋大学（海洋大）、２国立極地研究所（極地研）、３総合研究大学院大学（総研大）

Characteristics of baseline isotope ratios in food web and trophic structure in Indian sector of the Southern Ocean

Ayu Yamamoto1, Ryosuke Makabe2,3, Masayoshi Sano2, Aiko Tachibana1, Shintaro Takao2,3, Tsuneo Odate2,3, Masato Moteki1,2

1 Tokyo University of Marine Science and Technology (TUMSAT), 2 National Institute of Polar Research (NIPR) 3 The Graduate University for Advanced Studies (SOKENDAI)

It is suggested that krill independent food web is dominant especially in the Indian Sector of the Southern Ocean (SO) because

of much smaller krill biomass than that in the west antarctic regions, although its ecosystem structure still have been unclear . In

this study, we used carbon and nitrogen stable isotopes to understand the characteristics of the food web structure in the Indian

sector of the SO. Samples of suspended particulate organic matters (POM) in the water column were collected at two Umitaka-

maru cruises in 2007/08 and 2016 and the Hakuho-Maru cruise in 2007. Zooplankton, fish larvae samples, and Sea ice floes

were collected at the Umitaka-maru cruise in 2016. Carbon and nitrogen stable isotope ratios in sea ice floes were analyzed to

determine the baseline other than that of suspended POM in the water column. δ13C of suspended POM was positively correlated

with latitude, although no clear relationship with longitude and sampling period (season, year) was found. This would be

explained by temperature dependent isotopic fractionation of δ13C. However, in southmost stations, δ13C was clearly high

compared to the liner relationship determined from all δ13C data. This might be due to contribution of POM originated from sea

ice. At the station where the sea ice were recently melted, δ13C in several carnivorous species were high, suggesting that they

have been depended on POM originated from sea ice. On the other hand, δ13C of most suspension feeders indicated that they

have mainly fed on suspended POM. These facts would be due to rapid response of suspension feeders to changing baseline. In

these two trophic categories, myctophid Electrona sp larvae was identified as a suspension feeder rather than carnivorous.

南大洋インド洋区ではナンキョクオキアミを介さない生態系が多くの場所で存在すると考えられている。しかし、その生態系構造に関する知見は乏しい。また、季節海氷域に生息する一次消費者は海水由来, 海氷由来 2 つの基礎生産物を捕食している可能性が考えられる。そこで本研究では安定同位体比を用いて南大洋インド洋区における食物網のベースラインの特徴を把握し、同海域の動物プランクトン、魚類の食物網構造の推定を行った。海水中の粒状有機物（POM）試料は 2007/08年と 2016年の 12月-1月に行われた海鷹丸航海、2007年の 12月-1月に行われた白鳳丸航海で採取した。また、2016年の海鷹丸航海では動物プランクトン及び、仔稚魚試料、海氷を採取し、海氷は海水中の POMとは異なる同位体比のベースライン取得を目的として分析を行った。ベースラインの δ13C値は緯度及び水温との間に正の相関が認められたが、経度や季節による違いは明瞭でなかった。また、海氷を除く懸濁粒子のδ15Nでは海域、季節間の違いはほとんど見られなかった。これを考慮し、ベースラインの値は動物を採取した緯度帯に留意し決定した。採集直前まで海氷が残っていた測点では、海氷に食起源をもつ可能性が見られる種も多かった。海氷の影響が明確に表れた種は肉食性が強い生物が多く、確認されなかった生物では懸濁物食性が多かった。これは、懸濁物食者はベースラインの変化の影響を受けやすく、安定同位体比の更新時間が短いため、

66

採集した段階では影響が消えていた可能性が考えられる。一方、食段階の高いナンキョクダルマハダカの仔魚で海氷依存の可能性がほとんど見られなかった。これはナンキョクダルマハダカの仔魚が肉食より懸濁物食性に近いことを示唆している。

67

AI based automatic microfossil counting system and its appication for the Pleistocene Cycladophora davisiana curve in the Southern Ocean

Takuya Itaki1 and Minoru Ikehara2 1Geological Survey of Japan, AIST

2Kochi University Cycladophora davisiana is a radiolarian species widely distributed in the world ocean. Because it is known that this species significantly increased its abundance during the Pleistocene glacial intervals in the sub-arctic and sub-antarctic areas, the abundance curve has been used as an important tool for the Pleistocene stratigraphy or paleoceanography in these areas. However, it is not always collect such dataset due to recent lack of micropaleontologists who has a high degree of skill for identification of radiolarian species. In addition, traditional mictroscopic work needs a large effort and time for observing microfossils, even if the micropaleontologist agree with your collaboration. In order to avoid these problems, we therefore have developped a new instrument for automatic microscopic image collector equipped with an AI (deep-learning) software. The instrument used in this study is composed of a digital microscopic system with motorized X-Y stage, and the deep-learning software is instolled in the operating computer. A large amount of microscopic image can be collected by the microscopic system and identify them by the deep-learning. In this study, automatic counting of radiolarian species Cycladophora davisiana for estimation of its relative abundance (%) in total assemblage was conducted using this system. Microscopic images as tranning dataset were collected from Holocene and the last glacial sediments of core DCR-1PC in the Southern Ocean. The deep-learning classification model was constructed based on ~10,000 images divided into 5 categories of C. davisiana, C. bicornis, other radiolarians, shell fragments and diatoms. In results of classification based on this model for several test slides, microscopic images more than 90 % classified as C. davisiana were correct (>95 % confident level). Moreover, the model based C. davisiana % [=C. davisiana / (C. davisiana+ C. bicornis + other radiolarians)] ranging between 2 and 20 % shows high correlation (r=0.98) with real data counted by researcher. Therefore, high-resolution record of the C. davisiana % curve in the Southern Ocean can be obtained efficiently using our system.

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How mineral dust aerosol during LGM affects temperature surrounding of the Antarctica

Rumi Ohgaito1, Ayako Abe-Ouchi2,1, Toshihiko Takemura3, Akinori Ito1, Ryouta O’ishi2, Tomohiro Hajima1, Shingo Watanabe1 and Michio Kawamiya1

1JAMSTEC, 2AORI, U. Tokyo, 3RIAM, Kyushu Univ.

Deposition flux of mineral dust aerosol (dust) synchrnously fluctuates with temperature through glacial-interglacial cycle of the Quaternary, which is known from various proxy data (Dome Fuji Ice Core Project members 2017, Sci. Adv., Winckler et al. 2008, Science). Here we evaluate the effect of high dust flux due to enhanced glacial activity during Last Glacial Maximum (21,000 years before present, LGM) using numerical experiments.

We use an Earth System Model, MIROC-ESM (Watanabe et al. 2011, GMD), which consisits of atmosphere, land, river (AGCM) and ocean modules. Each module calcurates physical variables, and carbon cycle every discretized time steps and exchange these values through a flux coupler. Additional experiments are conducuted using the AGCM part of MIROC-ESM.

We conducted a LGM experiment (LGMglac.e) (Sueyoshi et al. 2013, GMD) following the protocol of Paleoclimate Modelling Intercomparison Project phase 3 (PMIP3) (Abe-Ouchi et al. 2015, GMD). However, LGM.e failed to represent the enhancement of dust deposition recoreded in the proxy data especially in the southern hemisphere. Hence, we performed an additional experiment, which is the identical to the LGM.e but with additional “glaciogenic dust” emissions (Mahowald et al. 2006, JGR) to achieve better fitting for the proxy data archive (Kohfeld et al. 2013, QSR, Albani et al. 2014, JAMES) (hereafter, called LGMglac.e). See Table 1 for the explanation. LGMglac.e obtained better representation of dust distribution at LGM. Especially, a strong glaciogenic dust from Pampas region in LGMglac.e spreads over the Southern Ocean ranging up to the higher troposphere. Additional sensitivity experiments using the AGCM are listed in Table 2. LGM.a corresponds to LGM.e and LGMglac.a to LGMglac.e, only sea surface is prescribed. There were no significant differences between these two paired (.e) and (.a) experiments in the southern hemisphere.

Dust loading is enhanced in LGMglac.a compared to LGM.a (Fig. 1), centered in the South America, distributed over the Southern Ocean and the Antarctica. And, warming of LGMglac.a is observed around the Antarctica compared to LGM.a, i.e., less cooling compared to the present day (Fig. 2). The resulting temperature anomaly at the LGM is pronounced at the surrounding of the Antarctica, but not very significant over the high plateau of the Antarctica. The model results of the sensitivity experiments suggest that the aerosol-cloud interaction in the upper troposphere plays an important role in warming the lower atmosphere. On the other hand, ageing effect of snow by dust deposition is not the reason of this temperature change around the Antarctica. These experiments and analyses are detailed in Ohgaito et al. (2018, CPD). It should be noted that we need further improvement on the amount and method of glaciogenic dust emission for quantitative conclusions. Table 1. Experiments using MIROC-ESM

Table 2. Experiments using the AGCM part of MIROC-ESM

Experiment names Explanation LGM.e The lgm experiment

submitted to PMIP3. LGMglac.e LGM.e + adding

glaciogenic dust flux following Mahowald et al. (2006)

Experiment names

Explanation

LGM.a The same with LGM.e but the sea surface temperature and sea ice are taken from LGM.e

LGMglac.a LGM.a + adding glaciogenic dust flux following Mahowald et al. (2006)

LGM.naging.a LGM.a + no ageing of snow albedo

LGMglac.naging.a

LGMglac.a + no ageing of snow albedo

Figure 1. Anomaly of column loading of dust in the atmosphere for LGMglac.a-LGM.a (kg m-2). Circled alphabets over the Antarctica show the locations of 4 ice core sites, E for EDML, F for Dome Fuji, V for Vostok, and C for Dome C.

Figure 2. Air temperature anomaly at 2 m height for LGMglac.a-LGM.a (kg m-2). Circled alphabets are the same with Fig. 1

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Observing seasonality and driver of carbon sequestration in seasonal ice zone

Ryosuke Makabe1,2, Shintaro Takao1,2, Kohei Mizobata3, Itsuki Suto4, Norio Kurosawa5, Masato Moteki1,3, Tsuneo Odate1,2

1National Institute of Polar Research 2 The Graduate University for Advanced Studies (SOKENDAI)

3Tokyo University of Marine Science and Technology 4Nagoya University

5Soka University Carbon sequestration is primarily influenced by the primary production and efficiency of the biological carbon pump. In the Southern Ocean, it is thought that ice edge phytoplankton bloom is one of the most important events to regulate primary production, which is strongly related with the seasonal prevalence of the sea ice. Thus, relationship between sea ice dynamics and biological activity is a critical factor for understanding not only ecosystem structure and its dynamics but also carbon sequestration. A full year observation using mooring arrays with time-sequential sediment traps is a possible solution to reveal the relationship in Polar regions. We designed a mooring observation, which will be conducted for one year from January 2019 along the 110°E transect off Wilkes Land, East Antarctica, during the two cruises by the training vessel Umitaka-maru, Tokyo University of Marine Science and Technology.

We organized to deploy three mooring arrays at 61°S (ice edge area during winter-spring), 63.5°S (upwelling area around Southern Boundary of Antarctic Circumpolar Current) and 65°S (ice edge area in January) along the 110°E. Each array is equipped with two sediment traps at 500 m and bottom-500 m (500 m above the sea floor) depths. The shallow and deep traps aim at determining the export flux from winter mixed layer and sinking particles just before reaching the sea floor, respectively. Buffered formalin is used to preserve all sediment trap samples. Furthermore, we apply neutral Lugol solution for half of sample series from the twin traps at 500 m depth for DNA and microzooplankton analyses. At 63.5°S, a long-ranger ADCP is deployed just above the shallow trap to quantify the biomass and vertical distribution of macrozooplankton and fish, which is likely main contributors for vertical fluxes.

Additionally, we try to establish the sea ice proxy for determining the past sea ice distribution. A previous paper reported that the morphological characteristic of a diatom species was different between those in sea ice and water column. Our colleagues also found similar phenomena in two diatom species from our target area. In order to discover the sea ice-form of the diatom, samples in the deeper traps during ice melting season is applied for microscopic analyses. Observing the sea ice-form in deeper traps could contribute to accurate reconstruction of paleoenvironment.

70

Reconstructing ice sheet fluctuation in Skarvesnes, southern part of Soya Coast, East Antarctica

Moto Kawamata1, Yusuke Suganuma2, 1 , Koichiro Doi2, 1, Takanobu Sawagaki3 1SOKENDAI, The Graduate University for Advanced Studies

2National Institute of Polar Research 3Hosei University

Geomorphological studies in Antarctica are important to understand past fluctuation of the Antarctic Ice Sheet, which is essential to evaluate the stability of ice sheet and to anticipate their contribution to future sea level rise. Recent studies have reported a retreat ages of the ice sheet based on surface exposure dating (SED) in Skarvsnes, southern part of Soya Coast, East Antarctica (Yamane et al, 2011). However, the past thickness and retreat processes of the ice sheet remain unclear, because reported ages are only from four locations and the glacial retreat history was not well discussed. In this study, we reconstruct detail retreat history of the ice sheet at the Skarvsnes based on geomorphological field survey and systematically obtained SED samples for each difference altitude and distance from the current ice sheet margin. As a result of the field survey, the basement rocks at the top of 400 m altitude in Skarvsnes were weathered extensively, whereas weathering of the basement rocks below 250 m altitude were relatively weak. These differences indicate upper limit ice sheet height at LGM in Skarvsnes. Also, at the several points in Skarvsnes, two directions of glacial striations were found. From these field observations, the ice sheet elevation was declined and the flow direction was changed under the influence of the basement topographical roughness since the LGM in Skarvsnes. Surface exposure ages newly obtained from the three locations are consistent with these geomorphological considerations and it suggests that near current ice sheet edge has been exposed from ice about 9 ka. Consequently, the ice sheet retreat probably has already completed at the early Holocene in Skarvsnes. These data almost consistent with retreat ages at other regions in East Antarctica (Mackintosh et al. 2014). References Yamane, M., Yokoyama, Y., Miura, H., Maemoku, H., Iwasaki, S. and Matsuzaki, H, The last deglacial history of Lützow-Holm Bay, East Antarctica. Journal of Quaternary Science, 26, 3–6, 2011. Mackintosh, A., Verleyen, E., O'Brien, P.E., White, D.A., Jones, R.S., McKay, R., Dunbar, R., Gore, D.B., Fink, D., Post, A.L., Miura, H., Leventer, A., Goodwin, I., Hodgson, D.A., Lilly, K., Crosta, X., Golledge, N.R., Wagner, B., Berg, S., van Ommen, T., Zwartz, D., Roberts, S.J., Vyverman, W., Masse, G, Retreat history of the east Antarctic ice sheet since the last glacial maximum. Quaternary Science Revews, 100, 10-30, 2014.

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海海氷氷融融解解がが植植物物ププラランンククトトンンのの現現存存量量おおよよびび生生産産力力にに及及ぼぼすす影影響響

高尾信太郎 1,2, 影沢歩友子 2, 真壁竜介 1,2, 茂木正人 1,3, 小達恒夫 1,2 1国立極地研究所

2総合研究大学院大学 3東京海洋大学

Effects of melting sea ice on phytoplankton abundance and productivity

Shintaro Takao1,2, Ayuko Kagesawa2, Ryosuke Makabe1,2, Masato Moteki1,3, Tsuneo Odate1,2

1National Institute of Polar Research, Tachikawa, Japan 2The Graduate University for Advanced Studies (SOKENDAI), Tachikawa, Japan 3Tokyo University of Marine Science and Technology (TUMSAT), Tokyo, Japan

Sea ice in the Southern Ocean is an important factor to control phytoplankton abundance and productivity. Although several studies have assessed functions of sea-ice on phytoplankton dynamics in the Ross Sea and the west Antarctic Peninsula regions, the effects of melting sea-ice still remain elusive in the Indian sector of the Southern Ocean. For the better understanding the effects on phytoplankton abundance and productivity, we conducted an incubation experiment by the 13C method in the austral summer. Sea ice and seawater were collected at an ice edge (64º41.9’S, 109º56.0’E) on 11 January, 2018 by the training vessel Umitaka-maru of TUMSAT, and then sea ice cubes (~20 kg) were melted in filtered seawater (180 L) in a dark container on deck. Incubations were carried out in the following four test sections: 1) seawater, 2) sea-ice melting water, 3) the blended, sea-ice melting water plus seawater, and 4) the blended, filtered sea-ice melting water plus seawater, for approximately 24 hours under a natural light condition in a water bath on the upper deck. In addition to the net primary production, we also obtained samples for bulk and size-fractionated chlorophyll a (chl a) concentrations, nutrients, and phytoplankton and micro-zooplankton community compositions before and after the incubation. We found significant differences in chl a ratios of final concentration to initial concentration (chl a Fin/chl a Init) of the bulk and <10 μm size class among four test sections (Fig. 1). These results suggest that dissolved and particulate materials in sea ice could affect phytoplankton abundance in <10 μm size class, which is potentially preferred food size by micro-zooplankton.

Fig. 1. Bulk and size-fractionated chl a ratios of final concentration to initial concentration in (A) seawater, (B) sea-ice melting water,

(C) sea-ice melting water plus seawater, and (D) filtered sea-ice melting water plus seawater. The same letters above each bar in bulk

and size-fractionated chl a ratios denote no significant difference according to Tukey multiple range test at the 0.05 alpha level.

0

1

2

3

Bluk > 10 µm < 10 μm

Chla

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72

Ocean-driven thinning of Totten and Denman Glaciers, the two primary outlets of the Aurora Subglacial Basin in East Antarctica

Jamin S. Greenbaum1, J.L. Roberts2,3, Noel Gourmelen4, C. Grima1, D.M. Schroeder5, C.F. Dow6, Patrick Heimbach1, Sun Bo7, Guo Jingxue7, C. Zappa8, T.D. van Ommen2,3, and D.D. Blankenship1

1University of Texas Institute for Geophysics, University of Texas at Austin, Austin, TX 78758 2Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart, Tasmania

7001, Australia 3Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia

4School of Geosciences, University of Edinburgh, Edinburgh, EH8, Scotland 5Department of Geophysics, Stanford University, California, USA

6Department of Geography and Environmental Management, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada.

7Polar Research Institute of China, 451 Jinqiao Road, Pudong, Shanghai, 200136, China 8Lamont Doherty Earth Observatory, Columbia University, Palisades, New York, USA

The Aurora Subglacial Basin (ASB) in East Antarctica contains at least 3.5 meters of eustatic sea level potential in ice grounded below sea level primarily draining through the Totten and Denman Glaciers located along the Sabrina and Knox Coasts, respectively. The ice surface elevation in the Totten and Denman grounding zones has been lowering steadily since the beginning of the satellite altimetry record (Zwally et al., 2015) and their grounding lines are retreating (Konrad et al., 2018). The ice within the ASB is believed to have collapsed and advanced multiple times since the onset of largescale glaciation (Aitken et al., 2016; Young et al., 2011) so it is imperative to understand what is driving the contemporary changes.

Basal melting of the Totten Glacier Ice Shelf (TGIS) was recently shown to be driven by warm, modified Circumpolar Deep Water (mCDW) that enters the ice shelf cavity through a system of seafloor troughs (Greenbaum et al., 2015; Rintoul et al., 2016) connecting to a reservoir of mCDW that has been observed on the nearby continental shelf since 1996 (Bindoff et al., 2000). Totten Glacier is also susceptible to seasonal surface melting owing to its relatively northern latitude. Using airborne ice penetrating radar data we show that the ocean actively melts large channels into the ice shelf base that grow to over two kilometers wide and 350 meters deep with steep walls and flat terraces characteristic of rapid melt. We also apply airborne surface radar analyses to show that the near surface of the ice shelf (within the firn layers) undergoes widespread melting in warm years as indicated by the closest available automatic weather stations. The natural vulnerability of the TGIS to surface and basal melting is concerning given recent numerical modeling results indicating that ocean melting and surface melt-induced hydrofracture and ice cliff failure could cause substantial retreat into the ASB (DeConto and Pollard, 2016).

Denman Glacier drains through the Shackleton Ice Shelf (SIS), the seventh largest and the most northern ice shelf in Antarctica outside the Antarctica Peninsula. Denman’s rapid coastal thinning and the high rate of basal melt rate observed on the SIS (Rignot et al., 2013), combined with our knowledge of mCDW access to the TGIS, suggest that ocean-driven thinning could be responsible for changes in the ASB’s western outlet, as well. However, the sub-ice shelf bathymetry and nearby ocean state of the SIS have not been known well enough to confirm that hypothesis. Here we present a new sub-ice shelf bathymetry compilation for the eastern SIS derived from an airborne gravity inversion using a geological model constrained by seafloor depth estimates. Depth constraints were estimated from airborne-deployed bathythermograph sensors and depth to basement solutions from airborne magnetics data. The new bathymetry reveals at least one seafloor trough deep enough to allow mCDW observed by Autonomous Pinniped Bathythermographs (tagged seals) near this location to reach the grounding line. Finally, we use ice core data acquired nearby to confirm that atmospheric temperatures have not been high enough since at least 1931 to explain the thinning observed in the Denman grounding zone. These results confirm that both outlets of the ASB, via Totten and Denman Glaciers, are vulnerable to ocean-driven retreat also known to be responsible for rapid thinning of several glaciers in West Antarctica.

References Aitken, A., Roberts, J., Van Ommen, T., Young, D., Golledge, N., Greenbaum, J., Blankenship, D., Siegert, M., 2016. Repeated large-scale retreat and advance of Totten Glacier indicated by inland bed erosion.

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Bindoff, N.L., Rosenberg, M.A., Warner, M.J., 2000. On the circulation and water masses over the Antarctic continental slope and rise between 80 and 150°E. Deep Sea Research Part II: Topical Studies in Oceanography 47.

DeConto, R.M., Pollard, D., 2016. Contribution of Antarctica to past and future sea-level rise. Nature 531, 591.

Greenbaum, J., Blankenship, D., Young, D., Richter, T., Roberts, J., Aitken, A., Legresy, B., Schroeder, D., Warner, R., Van Ommen, T., 2015. Ocean access to a cavity beneath Totten Glacier in East Antarctica. Nature Geoscience 8, 294.

Konrad, H., Shepherd, A., Gilbert, L., Hogg, A.E., McMillan, M., Muir, A., Slater, T., 2018. Net retreat of Antarctic glacier grounding lines. Nature Geoscience 11, 258.

Rignot, E., Jacobs, S., Mouginot, J., Scheuchl, B., 2013. Ice-Shelf Melting Around Antarctica. Science 341, 266-270.

Rintoul, S.R., Silvano, A., Pena-Molino, B., van Wijk, E., Rosenberg, M., Greenbaum, J.S., Blankenship, D.D., 2016. Ocean heat drives rapid basal melt of the Totten Ice Shelf. Science Advances 2, e1601610.

Young, D.A., Wright, A.P., Roberts, J.L., Warner, R.C., Young, N.W., Greenbaum, J.S., Schroeder, D.M., Holt, J.W., Sugden, D.E., Blankenship, D.D., van Ommen, T.D., Siegert, M.J., 2011. A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes. Nature 474, 72-75.

Zwally, H.J., Li, J., Robbins, J.W., Saba, J.L., Yi, D., Brenner, A.C., 2015. Mass gains of the Antarctic ice sheet exceed losses. Journal of Glaciology 61, 1019-1036.

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