J....zontale V.I" est supposee decroitre proportionnellcment it ) 1. De pl us. it une profondeur y...

Journal o/Glaciologl' , Vol. 8 , ~o. 53, 1969

A FLOW MODEL CORE FROM

AND A CAMP

TIME SCALE FOR THE GREENL A ND CE T U RY ,

By W. DANSGAA RD and S. J. j O H NS EN

ICE

(Fysisk L a boratorium II , H .C . 0rsted Institu te t, K 0benhavns U niversite t, K 0benhav n, D enma rk)

ABSTRACT. A Aow model is described fo r th e Ca m p Century a rea in G reenla nd . T he hori zonta l \'eloc ity pro fil e a long the core is assumed to be uniform from the surface down to y = 400 m a bove the bottom . Below thi s leve l. the hori zonta l veloc ity v" is assumed to decrease p roportiona ll y to y . Furt hermore. a t a g iven y , V.r is ass umed to be proportiona l to the dista nce x ri'om the ice d ivid e. T he result ing vert ica l stra in ra te under stead y-sta te cond it ions g ives the age of the ice as a funct ion ofy . T he Aow model has exp la ined t he measured tem pera ture p ro fi le, a nd the time sca le has been veri fi ed by com pa ri son between observed sta ble isotope va ri a t ions a nd past cl ima ti c changes (a t least 70000 yea rs back in t ime) estima ted by other methoels .

RESUMIC. Un modele d'deolliemenl et II lle dehelle de lem/)s /)ow· la carotic de glace dll Camp Cen/llry . Croellialld . Les a uteurs decri vent un moelele d 'ecou lement d e la glace pour la zone el u Camp Century a u Croenla nel. Le profi l hori zon ta l des vitesses to ut a u monel d e la ca rotte d e g lace est suppose ct re uni forme d e la sur face j usq u ' it une profondeur de y = 400 m a u-dessus d e sod e rocheux. En-dessous d e ce n ivea u, la vitcsse horizonta le V.I" est supposee decroitre p roportionnellcment it ) 1. D e pl us. it un e p rofond eur y donn ee. Vx est supposee et re proport ionn elle it la d ista nce x d e la ligne de separa tion des g laces . L a vitesse de defo rma t ion ver tica le qu i en rcsul te sous des cond itions sta tionna ires do nne I' age d e la g lace en fonct ion el e }l. Le moelele eI'ecoulement a ex plique le p ro fi l mesure el es tempera tu res et I' echcllc des temps a ete veri fi ee pa r compa ra ison des va ria tions o bservees el es concent ra t ions eI ' isotope sta ble et les cha ngements clima t iques pa c,sees (j usq u' it 70 000 a ns en a n 'iere ) est imes pa r eI 'a ut res mcthodes .

ZUSA~lM E:-I FASSUNG . E ,n FliesslIlodell wu/ e ill ~ <eilskala fill' dell B ohrkem van Camp Cm/lIr ), . Gronland. FO r d as Gebiet \'on Ca mp Century in Gronland wi rd ein F liessmode ll besc hrieben . Das H or izonta lgeschwind igke itsprofi l en t lang d es Kerns wird von der OberA ache b is zu y = 400 m Uber dem Boden a ls g ie ichfo rmig a ngenommen . D a ru n ter wird e in e Abna hme del' H ori zonta lgescbwind igkeit v.,. propor tiona l zu ) ' vora usgesetzt . vVe iterhin wi rd untcrstell t. el ass sich flU' ein gegebenes)! V.r proport iona l zur Entfern ung x von cler E isscbeicle verha lt. Die result ierende vert ika le Ver form ungsgeschwindigkeit unter stet igen Bed ingungcn crg ibt das Alter el es E ises a ls eine F unkt ion von ) !. D as F liessmoel ell erkJa rt cl as gemessene T em pera turpro fi l : el ie Zeitska la wird clu rch einen Vergle ich zwischen beobachleten Va ri a tionen sta b iler Iso tope und fr l1 heren I,- li maschwan kungen (zumi nelest in el en letzten 70 000 J a hren) . d ie m it a ndcren !\ [ethoden abgescha t?r werden , besta t igt .

I NTROD UCTIO N

I ce cores con tain information on clim a ti c a nd geochemica l conditions of the past, because the isotopic and chemica l comp 03ition of fa lling snow rem a ins uncha nged in glac ier ice for long periods of time. H owever, the value of isotopic a nd chemica l a na lysis is, of course, closely related to a knowledge of the age of the ice in ques tion . T his emphasizes the necessity for es ta blishing a time versus dep th relation .

. Macroscopic stratigraphic studies have been used by m any in ves tiga tors on the upper strata of ice which a re no more than a few hundred yea rs old . H owever, the techniq ue based on seasona l va riation in li ght t ransmissivity (La ngway, 196 7) migh t be used fo r ice seve ra l thousa nd yea rs old (Johnsen and o thers, unpu b li shed ) .

Another possible way of es ta blishing a time sca le a long a n ice core wo uld seem to be th e measurem en t of seasona l oscilla tions in the isotopi c composition of the ice (Epstein and Sharp, 1959) and counting isotopic maxima from the surface. H owever , various processes tend to diminish the isotopic gradients in snow a nd ice. For example, m olecular diffusion in the solid ice, accelera ted by the thinning of the layers, g radua ll y oblitera tes the stable iso tope oscillations tha t rem ain a fter firnifica tion (John en and Dansgaard , unpubli shed ) .

Four radioacti ve isotopes have been used for dating ice. T ritium (Aeger ter a nd o thers, 1969) and 2l OPb (Goldberg , 1963; Crozaz and others, 1964 ; Crozaz and Langway, 1966) reach only 100 years back in time. The other two, 3

2 Si (Dansgaard and others, 1966) and 14 C (Scholander and others, 1962 ), may be detected in 3000 and 20000- 30000 year-old ice, respectively, but both of these techniques require several tons of ice . T he technique [or sampling 14C from ice in situ , developed by O eschger and others ( 1967), might in the future be perfec ted to be applied to d eep bore holes.

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JO U R NA L OF GLACIOLOGY

But at present a time scale for the Camp Century core must be based on calculations on a chosen flow model.

THE NYE M ODEL

Most of the previously evaluated flow models imply a uniform vertical strain-rate along any vertical line in an ice cap (Nye, 1951 , 1957, 1959) . Furthermore, assuming that the horizontal velocity components are parallel and there is negligible melting at the bottom, Nye (1963) expressed the ratio between the reduced and initial thicknesses, A and AH, of an annual layer (e.g. in cm of ice) by

A ) - - -AH H'

) and H being its present and initial distance from the bed . With this model, we get the change of A per unit of time

dA AH d) AH A dt = H dt = - HT '

T being I year. Integration gives A as a function of time

and from Equation ( I) we get the depth of a layer age t formed at a height H above the bottom

HA (AH t) ) = AH = H exp - HT .

H ence, the age ofa layer (H - y) below surface (cr. Haefeli, 1961 )

HT ) t = - - In

AN H'

Figure I shows A as a function of time (Equation ( 2 )) for H = I 400 m and AN = 0.2

and 0.35 m . Paradoxically, the highest value of AN corresponds to the thinnest layers after 5 000 years. The explanation is that, for a given value of H, the horizontal movement and, therefore, the vertical strain-rate will be the faster the higher the rate of accumulation.

mr----------.----------~------_,

0.4 A

0.1

5000 10000 years

Fig. I. Thickness '\ of an annual layer as a function if age t, according to the Nye flow model. Initial thicknesses 0.35 and 0.20 m. T hickness of the ice sheet H = 1400 m.



FLOW MODEL AND TIME SC AL E FOR ICE CORE FROM CAMP CEN T U RY 217

The Nye model describes the flow conditions at the bedrock by sliding and /Ol- by rapid shear strain-rates being concentrated in a thin bottom layer (Nye, 1963). However, at Camp Century the sliding is probably negligible, because the temperature at the bottom is as low as - 13°C (Hansen and Langway, 1966). Furthermore, the idea of relative motion being essentially concentrated in a thin bottom layer was advanced in view of the fact that the flow law of ice depends critically upon the temperature, at the same time as the temperature gradient at the bottom was assumed to be as high as rooC/ roo m (Nye, 1959). However, at Camp Century the temperature gradient was later m easured to be on ly r.8°C / roo m (Hansen and Langway, 1966). Consequently, Nye's flow model is not necessarily appl icable in the case of the Camp Century area.

NON- UN IFORM VERTICAL STRAIN-RATE MODEL

Another approach would be to assume, like Nye (1963) , that the horizontal velocity profil e a long a vertical line in the distance x from the ice divide may be written as

vx (y ) = k ·f(y ) . x

and also to calculate vx(Y) by integrating Glen 's law (Haefeli , 196 1)

l'

vAy ) = J k ' an dy , (5)

o

a being the shear stress, k' and n being constants that depend on the temperature and therefore ony. T his integration is justified only if the longitudinal strain-rate is small compared with the shear strain-rate at any y , which is only the case in the lower part of the ice sheet. i'\evertheless, we consider Weertman 's (1968) calculation of Equation (5), shown as the full curve in Figure 2 , as an improvem ent compared with calculations based on a uniform verti cal strain-rate, because Weertman's procedure accounts for the important fact , that vx(Y) = 0

for y = o. In this work, we use the simple approximation shown as the dashed curve in Figure 2 , i.e. Vx proportional to y from y = 0 to y = h, and Vx independent of y from y = h to y = H or, in other words

f (y ) = y /h, o :C; y :C; h, } (6) f (y ) = I h :C; y :C; H.

m 1400

1200

1000

h 800

600

400

200

0 0 m year-1

Fig. 2. Full ClIrve: horizontal veloci~y profile calculated by integration of Glen's law, assuming vx (o) = 0 and v.r;( H ) = 3.3 m .year- ' (from Weertman, 1968). Dashed Cl/rve : adopted approximation.



218 JO URNAL OF GLACIOLOGY

The incompressibility of the ice is expressed by

oV y oV x --;:;-+ --;;- = 0 , ~y ux

and the vertical velocity component is calculated as

OVy

oy

Vy =

OVx - --;:;- = - kf(y ),

ox

y

- k I f (Y ) dy Q

and, according to Equation (6)

For y = h :

For y = H :

Vy = {

- kYZ /2h,

-~ ('2Y - h),

o ~y ~ h

h ~ y ~ H.

o ~y ~ h

h ~ )I ~ H,

H ~----------------------------~

/ /

/ /

/ /

/

h f----zJ"--/

/ /

/ /

/

/ /

/

?' >'

/'

o ~--------------------------~ o "HIT.

(8)

Fig. 3. Vertical velocity Vy as afunction of the distance y from the bottom. D ashed line,' the Nye flow model ; full curve ,' the flow model described on p. 2 [7. The dashed curve close to the full curve corresponds to the horizontal veloci£v profile calculated by W eertman ( /968) ; cf. Fig. 2.



FLOW MOD EL AND TIM E SCALE FOR ICE CO R E FROM CA MP CEN T U RY 2 19

which is shown in F igure 3 (cf. discussion on p . 2 1 7 - I 8) . The straigh t line in th e upper range inter ects the y -axis at y = h/2. Consequently, in the same range, h ~y ~ H, we can use

h h Nye' m od el a nd Equations ( I), (2) and (3), if Hand y are replaced by H - - and y - -

2 2

2y - h All, A = 2H - h AH = (2J - h) h '

A == AH exp [ ( 2 ;~:) Tt ] = AJj exp ( -~» , (2H - h) T 2y - h hT 2y - h

( = In-- = - -In--- h ~y ~ H. 2Af/ 2H- h 2Ah 2H - h'

In the range 0 ~ y ~ h, Equation (10) gives

dy A AI/, V y = dt = - -:; = - h 2 T

Y'·

In tegration over the intervals h -'>' Y and th -'>' t (th being the age of ice at y = h g iven by Eq uation ( 12 )) leads to

h y =

1 + A,,(t - th )/hT

or t- t" = h2T (~_~) = (2H - Iz )T (~_ I),

A" y Iz A/-l Y o ~ )I ~ h

and from Equations (13) and (14)

m ~ S ~S'

1300

1200

1100

1000

900

800

700

600

500

"': "':

"': ~ ~

A = )..11[1 + Ah(t- t,, )/hT] - 2.

" , " Eq. (1 2) ,

Nye 's mOdel -->', , , '

'-Age '-',

• 400 L-___ -L _ ___ L-_ __ -L ____ ~ __ _=~ __ ~~

o 1000 2000 3000 4000 5000 years BP

Fig. 4. Age of the ice in the Camj) Cent1lry core as a function of the distance y from the bollom. 'p ;;' h. S is the /966 sw/ace ( y = [ 387.5 Ill) and S' is the 1966 surface correctedfor low densities ill the u/JIJer layers ( f = H = / 367.5 In of ice ) .



220 JOU R NAL OF GLACIOLOGY

Using the equations way back in time implies that H and AN are independent of x and, fu rthermore, steady-state conditions in a broad sense, i.e. H, AN and ice temperature Tare constant in time.

The former assumption seems justified (i) by the direction of the movement at Camp Century being approximately parallel to the iso-accumulation curves in the area (Mock, 1968), and (ii) by the site of formation of even a 15 000 year old section of the core being barely more than 50 km from Camp Century (the surface velocity is 3.3 m /year (Mock, referred to by W eertman (1968)). As to the changes in the non-independent parameters, H, AN and T, many thousand years back in time, it might be reasonable to assume lower AN and T, but higher H during the glaciation. Lower AN in that period would correspond to a closer packing of the annual layers, which, on the other hand, would be counteracted by slower thinning of the layers due to lower temperature (i.e. higher viscosity of the ice) and higher H. However, we have no means of verifying such assumptions, so a ll that we can do at present is to calculate the time scale by using the present constants and try, by other means, to check if and when an error enters (these remark, of course, should also be considered in connection with the thinning of the layers (Fig. 3)) .

The mean value of AN over the last roo years has been 0.35 ± 0.03 m of ice, as shown by Crozaz and Langway (1966) by the 2IOPb method. The surface to bottom distance, H , was measured as I 387.5 m in 1966 (Hansen and Langway, 1966), corresponding to I 367 .5 m

1000 r---~~~~~----------------~-----------------------+--~ " -....:::

" " " "-"-"

500 -- h - - - --'\

'\ \

\ \

200 \ \ \ \ \

100 r---~--------------------- \ -------

50

20

1000

\

in m

Age in years S.P

\ \ \

~

2000 5000 10,000

\ \ \ \

Eq. (12) and (15)

\ Nye 's model \ \ \ \

20,000 50,000 100,000

Fig. 5. Age of the ice in the Camp Century core as afunction of the distance)'from the bottom ( I 160 m ~ y ~ 20 m), both on a logarithmic scale.



FLOW MODEL AND TIM E SC AL E FOR ICE COR E FROM CAMP CENT U RY 221

of ice in view of the low densiti es in the upper layers (personal communication from C. C . Langway, Jr. ) .

Thus, we can use Equations ( 12) and ( 15) with H = I 3 67.5 m, h = 400 m , AH = 0.35 rn , and T = 1 year. For t > 6000 years, we can write

(2.670 X 106)

t = Y - 800 years B.P .

The result is shown by the full curves in Figures 4 and 5. According to the Nye model (dashed curve; Equation (4 )) , only the lowest few metres of ice should be more than 2 0 000 years old , whereas in our model the ice reaches this age 130 m above the bottom.

-45 -40 -35 -30 -25

6(18 0 ) in %. '10

10 - .. 20

20

30 50

50 100

100 200 j Depth Age in in

200 meters years 500

S.P. 300 1000 400

600 2000

800

5 000 1000 1100 10,000

~AlIer<l>d 1200 B<l>lI in g

----- La scaux 20,000 I

1300 - - Paudorf

1330 Br<l>rup J Amersf oor t 50,000

..J

1360 100,000

m years B.P.

- 45 -40 -35 -30 ' / .. Fig. 6. The heavy oxygen isotope concentratioll , 0('80 ), of sections of the Camp Century core versus the age of the ice (Equations

( 12) and (15)) plotted on a logarithmic scale to the right. The outer scale to the left gives the corresponding depth below the 1966 suiface. The isotope data are given as relative deviation of the ,80 / ,60 ratio from that of standard mean ocean water. (From Dansgaard and others, unpublished. )

EXPERIMENTAL EVIDENCE

The validity of the flow model described above has been checked in two independent ways. First, the flow model was used as a basis for calculating the temperature profile down the bore hole (Dansgaard and Johnsen, 1969) ' The result fitted the profile measured by



222 JO U RNAL OF GLACIOLOGY

Hansen and Langway ( 1966) within ± 0 .6°C. The calculated temperature difference between surface and bottom deviated only 0.3°C from the measured difference. Using the Nye model, W eertman (1968) found a deviation of 2. 7°C.

Secondly, when stable oxygen-isotope data for about I 600 samples from the core were plotted against the age of the ice in our time scaJe (Fig. 6), they showed a variat ion wh ich was in complete agreement with practically a ll known climatic changes within the last 70000 or 100000 years (for furth er details, cr. Dansgaard and others, in press) . The geochemical explanation for this is the fact that the isotopic composi tion of precipitation at high latitudes is mainly determined by the temperature offormation (Dansgaard, 1954, 1964 ; Gonfiantini and Picciotto, J 959), which causes seasonal isotopic osci llations in the ice strata , as well as longperiod isotop ic variations in phase wi th climatic changes.

The agreement with other quite independent climatological estima tes, covering nearly [00 000 years, leads us to the conclusion that the time scale and therefore our flow model is basically correct down to 30 or 35 m above the bottom. Below that level, the flow pattern is probably influenced by the irregular topograph y of the bottom , which is a lso indicated by visua l observation of the lowest ilty core sections.

M oreover, unless the influences of the various pa ram eters, which determine the flow pattern , quite accidentally counterba lance each other (e .g. as ou Liined on p . 220), it wo uld seem as if AH and J-J have not devia ted considerab ly from th eir present values in the Camp Cen tury area.

One might ask how the time scale is influenced by the choice of h (cf. Fig. 2). To check this, we consider a horizon y = 209.0 m above the bottom. In our time scale (h = 400 m ) the age is / 209 = [ 2 000 years with an un certain ty of barely more than 1%, * whereas, for h = 300 m (500 m ), / 209 would be 10400 years B.P. ( [3600 years n.p. ) . Thus, ifl() l) is accepted in accorda nce with Equation (6), h must be very close to 400 m.

A CKNOWLEDGEMENT

The authors are indeb ted to the Carlsberg Foundation, Copenhagen , for fin a ncial support.

MS. received J 5 December J 968

REFERENCES

Aegerter , S ., and others. 1969. Studies based on the t r it ium content of ice sa mples, by S. Aegerter, H. Oeschger , A. R ena ud a nd E. Schum acher. Meddelelser om Gronland, Bd. 177, Nr. 2, p . 76- 92.

Crozaz, G. , and La ngway. C. C .. jr. 1966. Da ting G reenland firn-i ce cores with Pb"o. Earth and Planetary Science Lellers, Vol. I , No. 4, p. 194- 96 .

C rozaz, G. , and others. 1964. Antarctic snow chronology with Pb" o, by C. Crozaz , E. lE.] Pi cciotto a nd W. de Breuck. J ournal of Geo/Jh) 'sical Research, Vol. 69, No. 12, p. 2597- 604.

Dansgaard, W. 1954. The Ol 8-abundance in fresh water. Geochimic!l et Cosmo,himica Acta. Vol. 6, Nos. 5- 6, P·24 1- 60 .

Dansgaard, 'vV. 1964. Sta ble isotopes in precipitation. T ellus, Vol. 16, No. 4, p . 436- 68. D :tnsgaard , W. , andJ ohnsen , S. J. 1969. Comment on paper by J . Weertman. "Com parison between measured a nd

theoretical temperature profi les of the Camp Century. G reen la nd , borehole". Journal of GeojJ}rysical R esearch, Vol. 74, No. 4, p . I 109- 10. [See a lso " Erra ta" , J ournal of GeoIJ}I)'sicai R esearch , Vo!. 74.]

D ansgaard , 'vV. , and others. 1966 . The Si]' fa llout in Sca ndinav ia; a new method for ice dating, by "V . Dansgaard, H. B. C la usen a nd A. Aarkrog. T ellus, Vol. 18, Nos. 2- 3, p. 187- 9 1.

Dansgaard , W ., and others. In press. O ne thousand centuries of climatic record from Camp Century, by W . Dansgaard, S. J . J ohnsen, J . M01ler a nd C. C. Langwa y, Jr. Science .

D ansgaa rd , W. , and others. Unpublished. Oxygen isotope a nalysis of a core representing a complete verti ca l profi le of a polar ice sheet , by W. Da nsgaard, S. J. J ohnsen , J. M011 er a nd C. C. Langway, Jr. [Pa per read at International Symposium on Antarctic G laciological Explorat ion, H anover, N.H ., U .S.A., 3- 7 September 1968 .]

Epstein, S., and Sharp, R . P . 1959. Oxygen isotope studies. Transactions . American Geophysical Union , Vo!. 40, No. I , p. 81-84.

* Because, in the same range, the la te interstadia ls (Aller0d and B0lling) are reflected by the stab le-isotope curve in time intervals (in our time scale) on ly 100 years from the "C da tes ( I I 800- 1 I 000 yea rs B.P. and 12 400-12000 yea rs B.P . ) .



F LOW M O D E L AND TIM E SC ALE FO R ICE C OR E F ROM C AM P C ENT U RY 22 3

Goldberg, E. D. 1963. Geochronology with lead-2l o . (In Radioactive dating . Proceedings of the symposium on radioactive dating held ~)! the In ternational Atomic Energy Agency in co-o/lerat;on with the J oint Commission on All/llied R adioactivity (lCS U) in Athens, 19- 23 November 1962. Vienna, Internat ional Atomic Energy Agency, p. 12 1- 3 I. )

Gonfia ntini , R. , and Picciotto, E . E. 1959. Oxygen isotope va ria tions in Anta rcti c snow sa mples. Nature. Vo!. 184, ~o. 4698, p. 1557- 58.

H aefeli , R. 1961. Contribution to the m ovem ent and th e form of ice sheets in th e Arcti c a nd Anta rcti c. J ournal of Glaciology, Vo!. 3. No. 30, p. II 33-5 I.

H a nsen, B. L. , and Langway, C. C .. jr. 1966 . Deep core drilling in ice a nd core a na lys is a t Ca mp Century. Greenl and, 196 1- I 966. Antarctic J ournal of the United States . Vo!. I. No. 5. p . 207- 08.

J ohnsen, S. j. , alld Da nsgaard , W. U npublished . Obli tera tion of sta ble isotope osc illa tions in snow a nd ice. [In prepa ra tion. 1969. )

J ohnsen, S . J. , and others. U npublished . Comparison between visua l and isotopic st ra tigraphy of the Camp Century ice core, by S. J . J ohnsen. VV . Da nsgaa rd a nd C. C . La ngway. Jr. [In prepa ration. 1969 .J

Langway, C. C. , jr . 1967. Stratigraphic a na lys is of a d eep ice core fi'om G reenla nd . US. Cold R egions Research and E ngineering Laborato~JI . Research Re/)art 77 .

lVfock. S. J . 1968. Snow accum ula tion studies on t he Thule peninsul a . Greenla nd. J ournal of Glaciology, " 01. 7. No. 49 , p . 59- 76.

Nye, J . F. 1951. The fl ow of glac iers a nd ice-sheets as a problem in plasti city . Proceedings q( the Royal SocifU, Ser. A, Vo!. 207. No. 109 1. p . 554- 72.

:\iye . J . F . 195 7. The distributi on of stress a nd veloc ity in glac iers a nd ice-sheets. Proceedings of the R~)lal Socie{r. Ser. A. Vo!. 239. :\io . 12 16. p . J 13-33.

Nye, J . F . 1959. T he motion of ice sheets a nd glaciers. J ournal of Glaciology. Vo!. 3. No. 26. p . 493- 507 . Nye, J . F. 1963. Correction factor for accumulation measured by the thickness of the a nnua l layers in a n Ice

shee t. J ournal of Glaciology, Vo!. 4, No . 36, p. 785- 88. Oeschger, H ., alld others. 196 7. Instruments and methods. An ill situ gas-ex traction system to radioca rbon d ate

glacier ice, by H . Oeschger. B. Alder a nd C. C . La ngway. Jr. J ournal of Glaciology. Vol. 6. 1'\0. 48. p . 939- 42 . Schola nder. P. F .. and others . 1962. Radio-ca rbon age a nd oxygen-18 content of Greenla nd icebergs , by P . F .

Schola nder, VV. Dansgaa rd , D. C . N Ull. H . deVries. L. K. Coachma n a nd E . H emm ingsen. Nleddelelser om Gronland, Bd. 165, Nr. I.

' ,Veertma n, J. 1968 . Compa ri son between measured a nd theoretica l temperature profil es of the Ca mp Century. Greenla nd, borehole. J ournal of Geo/)h)!sica! Re<mrch. Vo!. 73 . No. 8. p . 269 1- 700.



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