Geography and Environmental Science

Silent witnessesFreshwater bivalves as archives of environmental

variability in the Rhine-Meuse delta

Emma A. A. Versteegh

H. B. Vonhof, S. R. Troelstra, D. Kroon

Rhine and Meuse

Floods

Droughts

Holocene

Freshwater mussels

Species: Unio crassus, U. pictorum & U. tumidus

Common

Archaeological finds

Museum collections

Record water chemistry in growth bands in their shells

Oxygen isotopes

Oxygen isotopes

Meuse

Low δ18O values in winter

through winter

precipitation with little 18O

High δ18O values during

summer through

evaporation and summer

precipitation enriched in 18O

Rhine

Additional factor: Melt water from the Alps during summer → low δ18O values

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δ18O values of the rivers

Meltwater

High discharge

Dry summer

Research questions

• Do these unionids record δ18O values of river water in

their shells?

• Can we use this as a proxy for past river dynamics?

– Floods?

– Droughts?

Project plan

• Monitoring experiment

• 20th century shells (museum collections)

• Late Holocene shells (archaeological finds)

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Monitoring experiment: collection and tagging of shells

Monitoring experiment: cages in fish ladders

Methods

• Micro sampling

– Merchantek Micromill

• Stable isotopes

– Thermo Finnigan MAT 252

– Thermo Finnigan Delta+

• Water samples

Results: average 18O per shell

Seasonal patterns

Slowdown of growth

Water δ18O & temperature shell δ18O

1000 ln = 2.559 (106 T-2) + 0.175

Grossman & Ku (1986); Dettman et al. (1999)

Predicted shell δ18O valuesLek

Shell 18O values

Freshwater mussels record ambient18O values in their shell.

Reconstructed water 18O values

Reconstruction of discharge

Meuse Rhine

Back in time

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North Atlantic Oscillation

Conclusions

Freshwater mussels record ambient water 18O values in their shell.

Shell 18O values can be used to determine from which river a shell originates.

It is not possible to quantitatively reconstruct river discharge.

Extreme droughts and possibly melt water peaks can be detected.

NAO-variability during the Middle Ages??

So what am I doing here?

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Some other silent witnesses…

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…who also produce CaCO3

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Thank you!

Questions?