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The exchange of hydrogen isotopes between ice and water in temperature glaciers
EARTH AND PLANETARY SCIENCE LETTERS 6 (1969) 423-430. NORTH-HOLLANDPUBLISHINGCOMP., AMSTERDAM
THE EXCHANGE OF HYDROGEN ISOTOPES BETWEEN ICE AND WATER IN TEMPERATE GLACIERS Bragl/(RNASON Science Institute, University o f Iceland, Reykl"av~, lceland Received 23 July 1969 Isotopic exchange between ice and water is found to take place in temperate glaciers. This exchange causes homogenization of deuterium in snow during summer thaws, together with a general increase in deuterium concentration. Ice in a temperate glacier is therefore more homogeneous and has a higher deuterium concentration than the precipitation on the glacier. The difference between the average deuterium concentration in ice and precipitation gives information about the runoff ratio.
1. I n t r o d u c t i o n Although much work has already been done in studying oxygen and hydrogen isotopes in glaciers in the polar regions (Dansgaard [I] ; Epstein and Sharp [2] ; Merlivat et al. [3] ), such a study of temperate glaciers is still in a formative stage. The difference between the polar and the temperate glaciers, which makes the study of the latter more complicated, is mainly the modifying influence of rain and meltwater percolating through the temperate glaciers, thus causing homogenization which until now has not been fully understood. Several authors (Epstein and Sharp [4] ; Sharp et al. [5] ; Deutsch et al. [6] ; Macpherson and Krouse [7] ) measured the oxygen isotopes in temperate glaciers. These authors observed that the great variations in the winter layers, caused by varying isotopic content in the winter precipitation, become smaller as the melting period passes. Some enrichment in 180 was found to take place. According to the authors this homogenization of ice is mainly due to freezing of rain and meltwater and capture of snow in crevasses. Sharp et al. [5] also mentioned that a part of the 18Oenrichment may be due to freewater content in the snow layers. This freewater, which is thought to be in the neighbourhood of 10% by weight, is considered to be summer rain and therefore en-
riched in 180 relative to the winter snow. The authors do not consider exchange of oxygen isotopes between ice and water important for the homogenization. The distribution of the deuterium concentration in the precipitation in SW Iceland has already been measured. A part of this work has been reported Q(rnason and Sigurgeirsson [8]). The present paper describes some measurements of hydrogen isotopes in ice samples from boreholes on the Langi6kull glacier and the Vatnaj6kull glacier and one model experiment carried out in the laboratory.
2. Ice core m e a s u r e m e n t s The measurements are carried out with a mass spectrometer of Nier type in a way similar to that described by Friedman [9]. The results are expressed as 5 = per mille deuterium enrichment (depletion negative) relative to Smow (Standard Mean Ocean Water). All samples are prepared and analysed at least in duplicate. The standard deviation for a sample analysed in duplicate is 0.7%0. On 13 August 1967 ice cores were collected from a hole in the northern part of the Langj6kull glacier at 1230 m altitude. The location is shown in fig. 1 as L-1. The hole was 6.86 m deep and probably reached the winter layer of 1962-1963. The hole was dug
424
B.~RNASON
22"
2'0o
14•
18°
HveraveUir Langj6ku[I.~J~ ~
%y
k .~,~'/
---
VatnajiikutI ( °V-1
• Vegatunga upnahaed
•6 4 •
24.
220
zo °
--
10-
le°
14. I
Fig. 1. Map of Iceland showing the two locations on the Lang]bkull glacier, L-1 and L-2 and the location on the VatnajSkull glacier V-l, where ice cores were collected. The three meteorological stations: Rjupnahaed, Vegatunga and Hveravellir are also shown. down to 3.40 m depth and continuous ice cores were collected from the wall. F r o m 3.40 m to 6.86 m a mechanical drill was used to collect ice cores. Fig. 2 shows the deuterium content of samples taken from the hole plotted as a function of depth. Each vertical column in the upper part of the hole represents a composite mean value for respective depth. Each p o i n t in the lower part o f the hole represents a value for cores o f approximately 10 cm length. On 21 April 1967 composite samples o f the uppermost winter layer were collected in the vicinity of L-1. The deuterium content of these samples results in 8 = + 90.0%0 as a mean value for the winter layer o f 1 9 6 6 - 1 9 6 7 at L-I. This value is shown together with the results from L-1 in fig. 2, as a d o t t e d vertical line. On 20 May 1968 ice cores were collected from another hole on the Lang]bkull glacier at 1300 m altitude. The location o f the hole is shown in fig. 1 as L-2. This hole reached a depth of 30 m. A t 26 m the ice became almost clear and below 27 m there was only clear ice with air bubbles 1 to 2 m m in diameter. A pit was dug down to 4.3 m and 10 cm long ice cores collected continuously from the wall. F r o m 4.3 m down to 8.3 m the mechanical drill was used and ice
cores from 5 - 1 5 cm in length were collected continuously. Below 8.3 m down to 30 m a thermal drill was used. The thermal drill oonsists o f an iron tube approximately 1.2 m in length and 7 cm in o.d. Its lower end is electrically heated. The collected ice cores are 4.6 cm in diameter. A similar drill has been described by Shreve and Kamb [ 10]. Unfortunately the meltwater from the thermal drilling seems to change the deuterium concentration o f fine crystalline and perhaps also coarse crystalline snow. Therefore there are no reliable measurements from 8.3 to 26 m depth. But as will be shown below the meltwater has no effect on almost clear ice. Below 26 m the ice core, cut into lengths of 20 cm, has been used for deuterium measurements. Fig. 2 shows the deuterium concentration o f samples from the hole L-2 plotted as a function o f depth. On 13 June 1968 ice cores were collected from a hole on the Vatnajbkull glacier at an altitude of 1280 m. The location is shown in fig. 1 as V-1. A pit was dug down to 2.4 m and 30 cm long ice cores collected continuously from the wall. Below 2.4 m down to 7 m the mechanical drill was used and ice cores up to 40 cm length were collected continuously.
HYDROGEN ISOTOPES
t
'
425
0" L-1
I-
V-1
~
.
•
3"
Summer E su rf ace 1965 .C
E
o
6
,1
/-t
" 5
\
.
l
-70 -50 -90 Deuterium %. SIdOW
"
7 .~
1967
8.
8 \x
26
2?
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s
.r1
lo.
211
~/\
\ /
11
x
-6o - ~ " -t0o Deuterium %. 5MOW
29
-~
" -lib
' -a'O
' -IdO
" -I:~o
" -I?,o
Deuterium "/.. 5MOW
Fig. 2. Variations of deuterium concentration with depth in the three boreholes L-l, L-2 and V-1. Each of the four upper points in L-I represents the average deuterium content of a snow column, whose lengths are indicated by vertical bars. Each point in the lower part of L-1 represents a value for cores of 10 cm length. Each point in L-2 and V-1 is placed in the middle depth of the respective core piece. The ice cores in L-2 and V-1 are collected continuously. Samples below 26 m in L-2 and samples marked X in V-I are collected by thermal drill. Other samples are collected from the wall of a pit or by a mechanical drill. The reason for the great variations in the upper part of I.-2 compared to the variations in V-l, is that each point represents a 10 cm long core in L-2 but a 30 cm long core in V-1. The dotted vertical lines represent weighted mean value of the winter precipitation.
Below 5 m the ice changed to a greyish coarse crystalline tim. Samples were collected simultaneously from 5 m down to 7 m with the thermal drill. The thermal drill was then further used to collect samples down to 11.6 m. Fig. 2 shows the results o f deuterium measurements at V-I as a function o f depth. Comparison o f the samples collected simultaneously from 5 m to 7 m depth with the mechanical and thermal drills
shows that the meltwater from the thermal drill does not change the deuterium concentration in the tim. The samples collected from 7 m to 11.6 m with the thermal drill should therefore give results o f deuterium measurements comparable to the results above 7 m collected with the mechanical drill.
I
426
B.ARNASON the amount of precipitation in mm for each period in question. It is seen from table 1 that the mean annual precipitation at Rjupnahaed is only 1.2%o higher in deuterium than the winter precipitation (OctoberApril). At Vegatunga and Hveravellir the corresponding difference is 2.2%0 and 3.2%o respectively. The period October-April, or even October-May, is the part of the year when the precipitation on the glaciers in question falls as snow and conditions are similar to those on polar glaciers. Samples collected from the winter layer on the glaciers before the beginning of the melting season thus give the mean deuterium concentration during the winter, and may give reasonable information about the deuterium concentration in the mean annual precipitation. The distance between L-1 and Hveravellir is only 15 km. Although there must be some difference in meteorological conditions because 1,1 lies 630 m higher than Hveravellir, the deuterium content of precipitation at the two places is similar. The mean winter precipitation at Hveravellir has 6 ( O c t . - A p r , ) = + 87.8 %0 and at L-1 6(Oct_Apr~ ) = + 90.0%0. As the summer precipitation at Hveravellir has ~i(May_Sept) = + 7 8 . 5 % o , 1,1 might be expected to have 5(May_Sept ) = + 80.7%0. However, the deuterium concentration below the uppermost winter layer at I~1 is 6 = + 72.4%0, which is 6.1%o higher than the f-value for the summer precipitation at Hveravellir. In other words, the deuterium concentration in the ice at 1,1 is higher than in precipitation anywhere in the neighbourhood, even if only the summer precipitation is taken into consideration. It is therefore clear that the deuterium enrichment in the remaining ice at L-1 cannot be explained by refreezing of meltwater or rain in the winter layers. The
3. Discussion of the ice core measurements
Considering the results from the two boreholes 1-2 and V-1 the variations in.the last winter layer, collected before the beginning of the melting season, seem to be of the same magnitude as in the precipitation (/~rnason and Sigurgeirsson [8] ). Below the uppermost winter layer an extensive homogenization has apparently occurred. These are about the same conclusions as drawn by Epstein et al. [4], Sharp et al. [5], Macpherson and Krouse [7]. An additional interesting phenomenon is that the ice is considerably enriched in deuterium relative to the uppermost winter layer. The deuterium enrichment observed in the remaining ice on the LangjSkuU and VatnajSkull glaciers, is too great to be explained in the same way as the 180 enrichment by Epstein and Sharp [4], Sharp et al. [5], Deutsch et al. [6], Macpherson and Krouse [7]. Enrichment of deuterium in the remaining ice through evaporation from wet surface is unlikely to be of importance on glaciers. Macpherson and Krouse [7] have expressed the same opinion. Measurements of deuterium concentration in monthly composite samples of precipitation in SW Iceland show only small differences between the mean annual precipitation and the mean precipitation in the months October-April. This is probably due to oceanic climate. This can be seen by considering the deuterium concentration in the precipitation at three meteorological stations in SW Iceland. These stations, Rjupnahaed, Vegatunga and Hveravellir are shown in fig. 1. Table 1 shows the mean deuterium concentration in the precipitation at these stations for the periods May-September, October-April and the mean annual deuterium concentration. Table 1 also gives
Table 1 Mean annual deuterium concentration and mean deuterium concentration for October-April and May-September at three meteorological stations in SW Iceland. The amount of precipitation in mm is also given for each period in question. Meteorological stations
Altitude
Total sampling period
Annual mean 6~
mm
Oct.-April
May-Sept.
a%o
mm
a~
mm
Hveravellir
620
Sept. '66-Aug. '68
- 84.6
700
- 87.8
455
- 78.5
245
Vegatunga
100
Jan. '63-Dec. '64
+ 57.5
1099
+ 59.7
749
+ 52.9
350
Rjupnahaed
120
Jan. '58-Dec. '65
-57.6
923
-58.8
628
-55.1
295
HYDROGEN ISOTOPES
only possible explanation is that some isotopic exchange occurs as the ice recrystallizes and thus tends to reach equilibrium conditions. Similar considerations will also hold for L-2 and V-1.
427
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~
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=
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4. Isotopic fractionation between ice and water One model experiment has been carried out in the laboratory as an approximation to the exchange process which goes on in temperateglaciers during the summer. A column 60 cm high and 3 cm in inside diameter was Filled with dry fine-grained snow. The snow was collected in such a way that good mixing and homogeneous deuterium concentration were ensured throughout the sample. Two samples taken while the column was being filled were analysed for their deuterium content. These two samples gave = + 87.6%o and 5 = + 88.2%0 respectively in good agreement with 5 = + 88.1%o obtained as weighted mean for the snow in the column when totally melted and each sample analysed for its deuterium content. The column was thermally insulated and melting occurred only at the top. The meltwater percolated through the snow, after which it was collected and the deuterium concentration measured. After 3 hr, approximately -$ 2 of the snow had melted. The remaining snow, which had changed from a fine crystalline to a more coarse crystalline form, was then melted and measured for its deuterium concentration. In fig. 3 the deuterium content of each drain water sample is plotted against the total collected drain water. The first four data points in fig. 3, obtained at the beginning of the experiment, are scattered. This is probably because the meltwater percolation takes a finite time to reach a regular flow pattern. Later, as the column becomes thoroughly wet, the meltwater approaches an even flow rate resulting in the linear part of fig. 3. From the beginning of the experiment until ~ of the ice had melted, the drain water in each case has approximately 8%o lower f-value than the remaining ice. This means an effective fractionation factor K e = 1.008. Towards the end of the experiment the fractionation factor seems to change to a lower value of K e = 1.005. This lower K e may be due to the fact that the snow has recrystallized to a coarser form. It is therefore clear that although equilibrium conditions are not obtained in the experiment, an isotopic
.
~ -85.c_
~ -90-
"
-g5
o
o'.5
,,~
Fig. 3. Results of the model experiment. The deuterium content of each drain water sample versus the fraction of melted snow. 8o is the composite 8-value of the initial snow in the column. Qo is the amount of snow initially in the column. Qv is the amount of snow that had melted at each measurement.
fractionation occurs between the snow and water as the water percolates through the column and the remaining snow changes from f'me to coarse crystalline. Friedman et al. [11] discussed changes in the deuterium concentration in water undergoing any change of state. The fractionation constant
K=
([HDO]/[H,2 O] ) ice ( [HDOI / [H 2 O] ) water
for deuterium between ice and water has been estimated by several authors both theoretically (Weston [12], Kuhn and Thiirkauf [13] ) and experimentally (Posey and Smith [14], Kuhn and Thiirkauf [13], Merlivat and Nief [15] and O'Neil [16]). The data obtained show some disagreement. The value used below K = 1.0208 + 0.0007 is obtained from experiment carried out in this laboratory where ice was frozen out from continuously stirred water (/(mason [17]).
428
B.,~RNASON
5. Results and conclusion The conclusion drawn from the model experiment is that recrystanization followed by an isotopic exchange between ice and water must be an important factor in the homogenization processes in temperate glaciers. Considering the values obtained for the ice core measurements the consistent data from below the uppermost winter layer down the boreholes show that the homogenization occurs mainly during the first melting period. The deuterium concentration in the uppermost winter layer of L-1 collected on 13 August 1967 indicates that the exchange process is already completed in the upper part of the layer, but the lower part may be subject to further exchange, probably until the end of September. The data obtained also suggest that equilibrium conditions may possibly be nearly reached between the remaining ice and the percolating meltwater. Certainly the model experiment shows lack of equilibrium, but the whole experiment took only 3 hr while in the glaciers the exchange process is going on throughout the summer. The previously mentioned experiment in which the equilibrium constant has been measured (,~rnason [ 17]), also shows that ice will be frozen out under equilibrium conditions if the freezing rate is sufficiently low. In August 1966 two samples were collected on the northern part of the Langjt~kuil glacier at an altitude of 1100 m. An ice sample at the surface gave 6ice = + 66.7%0 and a water sample at the surface gave 6warer = - 82.1%~. This would mean an effective fractionation factor K e = 1.017, which is considerably lower than the equilibrium constant. If in accordance with Sharp et al. [5] we assume a 10% freewater content in the snow when it froze to ice and further assume the deuterium content of this freewater to be the same as that of the water sample, this would give K e = 1.019. The conclusion drawn from the available data is, therefore, that although equilibrium conditions are neither verified in the model experiment nor on the glaciers in question, the effective fractionation factor, Ke, for deuterium exchange in the glaciers may have a value not far from that of the equilibrium constant K = 1.0208. This again means that the effective fractionation factor or even the equilibrium constant together with (a) the deuterium content of the annual precipitation, and (b) the deuterium content of the ice from below the uppermost winter layer,
might be used to give reasonable information about the extent to which the yearly precipitation remains as ice in Icelandic glaciers. Direct measurements of the deuterium concentration in the annual precipitation on Icelandic glaciers have not been carried out, but as the difference between the deuterium concentration in the annual and the winter precipitation is small (see table 1) 6(annual) can be calculated from 6(winter) without a great error. Further measurement of the water content of annual layers below the uppermost winter layer should give the amount of annual precipitation. As a first step it is possible to set up the balance equation:
Riq + R w ( 1 - q ) = Rp where q is the fraction of total annual precipitation remaining as ice, R i is the mean relative deuterium concentration in the remaining ice, R w is the mean relative deuterium concentration of the water which escapes during the summer, Rp is the relative deuterium concentration of the annual precipitation. R w may be derived from the equation K e = Ri/Rw, where K e is the effective fractionation factor. Using the available data for L-1 on the Langib.kull glacier, 6 i = + 72.4%o and 6p = + 86.8% , and using the equilibrium constant K = 1.0208 instead of Ke, it follows that 24% of the yearly precipitation remains as ice and 76% escapes as water during the warm period. The amount of ice between the summer surface 1966 and the summer surface 1965 is 75 g per cm 2. This would mean that the yearly precipitation is 3100 mm at L-I. There are no direct measurements available for yearly precipitation on glaciers in Iceland, but according to the Icelandic Meteorological Office (Sigftasd6ttir, private communication) the mean yearly precipitation at L-1 for the period 1931-1960 has been estimated at 3000 mm. A lack of equilibrium cannot be the reason for a too high calculated value for yearly precipitation at L-I. Such a lack would mean an effective fractionation factor lower than the equilibrium constant used here, and this would again give a value of yearly precipitation higher than 3100 ram. It is difficult to make any reliable estimate of the accuracy of the available meteorological data for precipitation. Considering meteorological data from several stations in Iceland from 19311960 it is found that at none of these stations does
HYDROGEN ISOTOPES the precipitation in a single year exceed 1.5 times the mean annual precipitation. I f we accordingly set 4500 m m as an upper limit o f precipitation for the year in question at L-1 and use this in the balance equation together with other available data, it would give an effective fractionation factor K e = 1.0190. Thus there is a reason to believe that the effective fractionation factor for deuterium exchange at L-1 on the Langj6kull glacier will lie between the limits o f 1.0208 and 1.0190. These calculations are o f course only valid for L-1 on the Langjbkull glacier at an altitude o f 1230 m. Locations which differ in altitude and other conditions may yield different values of K e. Similar calculations can be made for L-2 and V-1. The mean 8-value in the uppermost winter layer is ~L-~2(Oct.-May) = + 8 1 . 4 ~ and ~V-l(Oct.-May) = + 81.4%o. The mean 8-values in the ice below the uppermost winter layer are 8L. 2 ice = + 69.0%o and 8v-1 ice = + 69.1%o. L-2 and V-1 are both at an altitude o f approximately 1300 m. Assuming that the deuterium concentration is 3.2%0 higher in the annual precipitation than in the winter precipitation, the mean 8-value in the annual precipitation might be as high as 8 = + 78.2%o in b o t h cases. Using these data with K = 1.0208 instead o f K e the balance equation indicates that 52% o f the yearly precipitation would remain as ice at L-2 and V-1. While the thickness o f the ice for the next year below the summer surface 1967 has not been measured at L-2 it is n o t possible to calculate the amount o f yearly precipitation. A t V-1 the water content o f the ice core between the summer surface 1967 and the summer surface 1966 is 172 g/cm 2. Thus our m e t h o d would give about 3200 m m yearly precipitation at V-1. According to the Icelandic Meteorological Office (private communication) the mean yearly precipitation at V-I for the period 1 9 3 1 - 1 9 6 0 should be about 3000 mm. It should be stressed here that this m e t h o d will probably only work in cases where the seasonal variation o f deuterium concentration in the precipitation is small. A more extensive study o f the isotopic exchange phenomenon is now being carried out on several glaciers in Iceland. The detailed results will be presented later.
429
Acknowledgements The author wishes to express his appreciation to Prof. Th.Sigurgeirsson, head o f the Physics Department o f the Science Institute, for his advice and encouragement, and to Mr. Th.Bfason of the Mathematics Department, for many fruitful discussions. F u r t h e r more the author is indebted to Mrs. A.B.Sigffsd6ttir, o f the Icelandic Meteorological Office, for making available unpublished meteorological data. The field work was made possible by a number o f volunteers who participated in the glacier expeditions.
References [1] W.Dansgaard, The isotopic composition of natural waters, with special reference to the Greenland ice cap, Medd. Groenland 165 (1961) 2. [2] S.Epstein.and R.P.Sharp, Oxygen and hydrogen-isotope variations in a firn core, Eights Station, Western Antarctica, J. Geophys. Res. 72, 22 (1967) 5595-5598. [3] L.Merlivat, C.Lorius, M.Majzoub, G.Nief and E.Roth, Etudes isotopiques en profondeur d'un glacier en Antaxctique, Proc. Symp. on Isotopes in Hydrology (International Atomic Energy Agency, Vienna, 1967) 671681. [4] S.Epstein and R.P.Sharp, Oxygen isotope variations in the Malapsina and Saskatchewan glaciers, J. Geol. 67 (1959) 88-102. [ 5 ] R.P.Sharp, S.Epstein and I.Vidziunas, Oxygen-isotope ratios in the Blue Glacier, Olympic Mountains, Washington, J. Geophys. Res. 65, 12 (1960) 4043-4059. [6] S.Deutsch, W.Ambach and H.Eisner, Oxygen isotope study of snow and firn on an alpine glacier, Earth Planet. Sci. Letters 1 (1966) 197-201. [7] D.Macpherson and H.R.Krous¢, O18/O 16 ratios in snow and ice of the Hubbard and Kaskawulsh glaciers, Proc. Syrup. on Isotope Techniques in the Hydrologic Cycle, ed. G.E.Stout (American Geophysical Union, Washington, 1967) 180-194. [ 8 ] B..g,rnason and Th. Sigurgeirsson, Hydrogen Iso topes in hydrological studies in Iceland, Proc. Syrup. on Isotopes in Hydrology (International Atomic Energy Agency, Vienna, 1967) 35-47. [9] I.Friedman, Deuterium content of natural water and other substances, Geochim. Cosmochim. Acta 4 (1953) 89-103. [10] R.L.Shreve and W.B.Kamb, Portable thermal core drill for temperate glaciers. Instruments and Methods, J. Glaciol. 5, 37 (1964) 113-117.
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B.ARNASON
[11] l.Friedman, A.C.Redfield, B.Schoen and J.Hartis, The variation of the deuterium content of natural waters in the hydrologic cycle, Rev. Geophys. 2, 1 (1964) 177224. [12] R.E.Weston Jr., Hydrogen isotopic ftactionation between ice and water, Geochim. Cosmochim. Acta 8 (1955) 281-284. [13] W.Kuhn and M.Thtirkauf, Isotopentrennung beim Gefrieren yon Wasser und Diffusionskonstanten yon D und O 18 im Eis, Helv. Chim. Acta 41, 110 ( 1958) 938-971. [ 14] J.C.Posey and H.A.Smith, The equilibrium distribution of light and heavy water in a freezing mixture, J. Amer. Chem. Soc. 79 (1957) 555-557.
[ 15] L.Merlivat and G.Nief, Fractionnement isotopique lots des changements d'6tat solide-vapeur et liquide-vapeur de l'eau ~ des temp6ratures inf6rieures ~ 0°C, TeUus 19, 1 (1967) 122-127. [ 16[ LR.O'Neil, Hydrogen and oxygen isotopic fractionation between ice and water, J. Phys. Chem. 72 (1968) 36833684. [ 17] B./~rnason, Equilibrium constant for the ftactionation of deuterium between ice and water, J. Phys. Chem. (1969) in press.
THE EXCHANGE OF HYDROGEN ISOTOPES BETWEEN ICE AND WATER IN TEMPERATE GLACIERS Bragl/(RNASON Science Institute, University o f Iceland, Reykl"av~, lceland Received 23 July 1969 Isotopic exchange between ice and water is found to take place in temperate glaciers. This exchange causes homogenization of deuterium in snow during summer thaws, together with a general increase in deuterium concentration. Ice in a temperate glacier is therefore more homogeneous and has a higher deuterium concentration than the precipitation on the glacier. The difference between the average deuterium concentration in ice and precipitation gives information about the runoff ratio.
1. I n t r o d u c t i o n Although much work has already been done in studying oxygen and hydrogen isotopes in glaciers in the polar regions (Dansgaard [I] ; Epstein and Sharp [2] ; Merlivat et al. [3] ), such a study of temperate glaciers is still in a formative stage. The difference between the polar and the temperate glaciers, which makes the study of the latter more complicated, is mainly the modifying influence of rain and meltwater percolating through the temperate glaciers, thus causing homogenization which until now has not been fully understood. Several authors (Epstein and Sharp [4] ; Sharp et al. [5] ; Deutsch et al. [6] ; Macpherson and Krouse [7] ) measured the oxygen isotopes in temperate glaciers. These authors observed that the great variations in the winter layers, caused by varying isotopic content in the winter precipitation, become smaller as the melting period passes. Some enrichment in 180 was found to take place. According to the authors this homogenization of ice is mainly due to freezing of rain and meltwater and capture of snow in crevasses. Sharp et al. [5] also mentioned that a part of the 18Oenrichment may be due to freewater content in the snow layers. This freewater, which is thought to be in the neighbourhood of 10% by weight, is considered to be summer rain and therefore en-
riched in 180 relative to the winter snow. The authors do not consider exchange of oxygen isotopes between ice and water important for the homogenization. The distribution of the deuterium concentration in the precipitation in SW Iceland has already been measured. A part of this work has been reported Q(rnason and Sigurgeirsson [8]). The present paper describes some measurements of hydrogen isotopes in ice samples from boreholes on the Langi6kull glacier and the Vatnaj6kull glacier and one model experiment carried out in the laboratory.
2. Ice core m e a s u r e m e n t s The measurements are carried out with a mass spectrometer of Nier type in a way similar to that described by Friedman [9]. The results are expressed as 5 = per mille deuterium enrichment (depletion negative) relative to Smow (Standard Mean Ocean Water). All samples are prepared and analysed at least in duplicate. The standard deviation for a sample analysed in duplicate is 0.7%0. On 13 August 1967 ice cores were collected from a hole in the northern part of the Langj6kull glacier at 1230 m altitude. The location is shown in fig. 1 as L-1. The hole was 6.86 m deep and probably reached the winter layer of 1962-1963. The hole was dug
424
B.~RNASON
22"
2'0o
14•
18°
HveraveUir Langj6ku[I.~J~ ~
%y
k .~,~'/
---
VatnajiikutI ( °V-1
• Vegatunga upnahaed
•6 4 •
24.
220
zo °
--
10-
le°
14. I
Fig. 1. Map of Iceland showing the two locations on the Lang]bkull glacier, L-1 and L-2 and the location on the VatnajSkull glacier V-l, where ice cores were collected. The three meteorological stations: Rjupnahaed, Vegatunga and Hveravellir are also shown. down to 3.40 m depth and continuous ice cores were collected from the wall. F r o m 3.40 m to 6.86 m a mechanical drill was used to collect ice cores. Fig. 2 shows the deuterium content of samples taken from the hole plotted as a function of depth. Each vertical column in the upper part of the hole represents a composite mean value for respective depth. Each p o i n t in the lower part o f the hole represents a value for cores o f approximately 10 cm length. On 21 April 1967 composite samples o f the uppermost winter layer were collected in the vicinity of L-1. The deuterium content of these samples results in 8 = + 90.0%0 as a mean value for the winter layer o f 1 9 6 6 - 1 9 6 7 at L-I. This value is shown together with the results from L-1 in fig. 2, as a d o t t e d vertical line. On 20 May 1968 ice cores were collected from another hole on the Lang]bkull glacier at 1300 m altitude. The location o f the hole is shown in fig. 1 as L-2. This hole reached a depth of 30 m. A t 26 m the ice became almost clear and below 27 m there was only clear ice with air bubbles 1 to 2 m m in diameter. A pit was dug down to 4.3 m and 10 cm long ice cores collected continuously from the wall. F r o m 4.3 m down to 8.3 m the mechanical drill was used and ice
cores from 5 - 1 5 cm in length were collected continuously. Below 8.3 m down to 30 m a thermal drill was used. The thermal drill oonsists o f an iron tube approximately 1.2 m in length and 7 cm in o.d. Its lower end is electrically heated. The collected ice cores are 4.6 cm in diameter. A similar drill has been described by Shreve and Kamb [ 10]. Unfortunately the meltwater from the thermal drilling seems to change the deuterium concentration o f fine crystalline and perhaps also coarse crystalline snow. Therefore there are no reliable measurements from 8.3 to 26 m depth. But as will be shown below the meltwater has no effect on almost clear ice. Below 26 m the ice core, cut into lengths of 20 cm, has been used for deuterium measurements. Fig. 2 shows the deuterium concentration o f samples from the hole L-2 plotted as a function o f depth. On 13 June 1968 ice cores were collected from a hole on the Vatnajbkull glacier at an altitude of 1280 m. The location is shown in fig. 1 as V-1. A pit was dug down to 2.4 m and 30 cm long ice cores collected continuously from the wall. Below 2.4 m down to 7 m the mechanical drill was used and ice cores up to 40 cm length were collected continuously.
HYDROGEN ISOTOPES
t
'
425
0" L-1
I-
V-1
~
.
•
3"
Summer E su rf ace 1965 .C
E
o
6
,1
/-t
" 5
\
.
l
-70 -50 -90 Deuterium %. SIdOW
"
7 .~
1967
8.
8 \x
26
2?
'i
s
.r1
lo.
211
~/\
\ /
11
x
-6o - ~ " -t0o Deuterium %. 5MOW
29
-~
" -lib
' -a'O
' -IdO
" -I:~o
" -I?,o
Deuterium "/.. 5MOW
Fig. 2. Variations of deuterium concentration with depth in the three boreholes L-l, L-2 and V-1. Each of the four upper points in L-I represents the average deuterium content of a snow column, whose lengths are indicated by vertical bars. Each point in the lower part of L-1 represents a value for cores of 10 cm length. Each point in L-2 and V-1 is placed in the middle depth of the respective core piece. The ice cores in L-2 and V-1 are collected continuously. Samples below 26 m in L-2 and samples marked X in V-I are collected by thermal drill. Other samples are collected from the wall of a pit or by a mechanical drill. The reason for the great variations in the upper part of I.-2 compared to the variations in V-l, is that each point represents a 10 cm long core in L-2 but a 30 cm long core in V-1. The dotted vertical lines represent weighted mean value of the winter precipitation.
Below 5 m the ice changed to a greyish coarse crystalline tim. Samples were collected simultaneously from 5 m down to 7 m with the thermal drill. The thermal drill was then further used to collect samples down to 11.6 m. Fig. 2 shows the results o f deuterium measurements at V-I as a function o f depth. Comparison o f the samples collected simultaneously from 5 m to 7 m depth with the mechanical and thermal drills
shows that the meltwater from the thermal drill does not change the deuterium concentration in the tim. The samples collected from 7 m to 11.6 m with the thermal drill should therefore give results o f deuterium measurements comparable to the results above 7 m collected with the mechanical drill.
I
426
B.ARNASON the amount of precipitation in mm for each period in question. It is seen from table 1 that the mean annual precipitation at Rjupnahaed is only 1.2%o higher in deuterium than the winter precipitation (OctoberApril). At Vegatunga and Hveravellir the corresponding difference is 2.2%0 and 3.2%o respectively. The period October-April, or even October-May, is the part of the year when the precipitation on the glaciers in question falls as snow and conditions are similar to those on polar glaciers. Samples collected from the winter layer on the glaciers before the beginning of the melting season thus give the mean deuterium concentration during the winter, and may give reasonable information about the deuterium concentration in the mean annual precipitation. The distance between L-1 and Hveravellir is only 15 km. Although there must be some difference in meteorological conditions because 1,1 lies 630 m higher than Hveravellir, the deuterium content of precipitation at the two places is similar. The mean winter precipitation at Hveravellir has 6 ( O c t . - A p r , ) = + 87.8 %0 and at L-1 6(Oct_Apr~ ) = + 90.0%0. As the summer precipitation at Hveravellir has ~i(May_Sept) = + 7 8 . 5 % o , 1,1 might be expected to have 5(May_Sept ) = + 80.7%0. However, the deuterium concentration below the uppermost winter layer at I~1 is 6 = + 72.4%0, which is 6.1%o higher than the f-value for the summer precipitation at Hveravellir. In other words, the deuterium concentration in the ice at 1,1 is higher than in precipitation anywhere in the neighbourhood, even if only the summer precipitation is taken into consideration. It is therefore clear that the deuterium enrichment in the remaining ice at L-1 cannot be explained by refreezing of meltwater or rain in the winter layers. The
3. Discussion of the ice core measurements
Considering the results from the two boreholes 1-2 and V-1 the variations in.the last winter layer, collected before the beginning of the melting season, seem to be of the same magnitude as in the precipitation (/~rnason and Sigurgeirsson [8] ). Below the uppermost winter layer an extensive homogenization has apparently occurred. These are about the same conclusions as drawn by Epstein et al. [4], Sharp et al. [5], Macpherson and Krouse [7]. An additional interesting phenomenon is that the ice is considerably enriched in deuterium relative to the uppermost winter layer. The deuterium enrichment observed in the remaining ice on the LangjSkuU and VatnajSkull glaciers, is too great to be explained in the same way as the 180 enrichment by Epstein and Sharp [4], Sharp et al. [5], Deutsch et al. [6], Macpherson and Krouse [7]. Enrichment of deuterium in the remaining ice through evaporation from wet surface is unlikely to be of importance on glaciers. Macpherson and Krouse [7] have expressed the same opinion. Measurements of deuterium concentration in monthly composite samples of precipitation in SW Iceland show only small differences between the mean annual precipitation and the mean precipitation in the months October-April. This is probably due to oceanic climate. This can be seen by considering the deuterium concentration in the precipitation at three meteorological stations in SW Iceland. These stations, Rjupnahaed, Vegatunga and Hveravellir are shown in fig. 1. Table 1 shows the mean deuterium concentration in the precipitation at these stations for the periods May-September, October-April and the mean annual deuterium concentration. Table 1 also gives
Table 1 Mean annual deuterium concentration and mean deuterium concentration for October-April and May-September at three meteorological stations in SW Iceland. The amount of precipitation in mm is also given for each period in question. Meteorological stations
Altitude
Total sampling period
Annual mean 6~
mm
Oct.-April
May-Sept.
a%o
mm
a~
mm
Hveravellir
620
Sept. '66-Aug. '68
- 84.6
700
- 87.8
455
- 78.5
245
Vegatunga
100
Jan. '63-Dec. '64
+ 57.5
1099
+ 59.7
749
+ 52.9
350
Rjupnahaed
120
Jan. '58-Dec. '65
-57.6
923
-58.8
628
-55.1
295
HYDROGEN ISOTOPES
only possible explanation is that some isotopic exchange occurs as the ice recrystallizes and thus tends to reach equilibrium conditions. Similar considerations will also hold for L-2 and V-1.
427
~u
-7S"
~
-80"
=
C
4. Isotopic fractionation between ice and water One model experiment has been carried out in the laboratory as an approximation to the exchange process which goes on in temperateglaciers during the summer. A column 60 cm high and 3 cm in inside diameter was Filled with dry fine-grained snow. The snow was collected in such a way that good mixing and homogeneous deuterium concentration were ensured throughout the sample. Two samples taken while the column was being filled were analysed for their deuterium content. These two samples gave = + 87.6%o and 5 = + 88.2%0 respectively in good agreement with 5 = + 88.1%o obtained as weighted mean for the snow in the column when totally melted and each sample analysed for its deuterium content. The column was thermally insulated and melting occurred only at the top. The meltwater percolated through the snow, after which it was collected and the deuterium concentration measured. After 3 hr, approximately -$ 2 of the snow had melted. The remaining snow, which had changed from a fine crystalline to a more coarse crystalline form, was then melted and measured for its deuterium concentration. In fig. 3 the deuterium content of each drain water sample is plotted against the total collected drain water. The first four data points in fig. 3, obtained at the beginning of the experiment, are scattered. This is probably because the meltwater percolation takes a finite time to reach a regular flow pattern. Later, as the column becomes thoroughly wet, the meltwater approaches an even flow rate resulting in the linear part of fig. 3. From the beginning of the experiment until ~ of the ice had melted, the drain water in each case has approximately 8%o lower f-value than the remaining ice. This means an effective fractionation factor K e = 1.008. Towards the end of the experiment the fractionation factor seems to change to a lower value of K e = 1.005. This lower K e may be due to the fact that the snow has recrystallized to a coarser form. It is therefore clear that although equilibrium conditions are not obtained in the experiment, an isotopic
.
~ -85.c_
~ -90-
"
-g5
o
o'.5
,,~
Fig. 3. Results of the model experiment. The deuterium content of each drain water sample versus the fraction of melted snow. 8o is the composite 8-value of the initial snow in the column. Qo is the amount of snow initially in the column. Qv is the amount of snow that had melted at each measurement.
fractionation occurs between the snow and water as the water percolates through the column and the remaining snow changes from f'me to coarse crystalline. Friedman et al. [11] discussed changes in the deuterium concentration in water undergoing any change of state. The fractionation constant
K=
([HDO]/[H,2 O] ) ice ( [HDOI / [H 2 O] ) water
for deuterium between ice and water has been estimated by several authors both theoretically (Weston [12], Kuhn and Thiirkauf [13] ) and experimentally (Posey and Smith [14], Kuhn and Thiirkauf [13], Merlivat and Nief [15] and O'Neil [16]). The data obtained show some disagreement. The value used below K = 1.0208 + 0.0007 is obtained from experiment carried out in this laboratory where ice was frozen out from continuously stirred water (/(mason [17]).
428
B.,~RNASON
5. Results and conclusion The conclusion drawn from the model experiment is that recrystanization followed by an isotopic exchange between ice and water must be an important factor in the homogenization processes in temperate glaciers. Considering the values obtained for the ice core measurements the consistent data from below the uppermost winter layer down the boreholes show that the homogenization occurs mainly during the first melting period. The deuterium concentration in the uppermost winter layer of L-1 collected on 13 August 1967 indicates that the exchange process is already completed in the upper part of the layer, but the lower part may be subject to further exchange, probably until the end of September. The data obtained also suggest that equilibrium conditions may possibly be nearly reached between the remaining ice and the percolating meltwater. Certainly the model experiment shows lack of equilibrium, but the whole experiment took only 3 hr while in the glaciers the exchange process is going on throughout the summer. The previously mentioned experiment in which the equilibrium constant has been measured (,~rnason [ 17]), also shows that ice will be frozen out under equilibrium conditions if the freezing rate is sufficiently low. In August 1966 two samples were collected on the northern part of the Langjt~kuil glacier at an altitude of 1100 m. An ice sample at the surface gave 6ice = + 66.7%0 and a water sample at the surface gave 6warer = - 82.1%~. This would mean an effective fractionation factor K e = 1.017, which is considerably lower than the equilibrium constant. If in accordance with Sharp et al. [5] we assume a 10% freewater content in the snow when it froze to ice and further assume the deuterium content of this freewater to be the same as that of the water sample, this would give K e = 1.019. The conclusion drawn from the available data is, therefore, that although equilibrium conditions are neither verified in the model experiment nor on the glaciers in question, the effective fractionation factor, Ke, for deuterium exchange in the glaciers may have a value not far from that of the equilibrium constant K = 1.0208. This again means that the effective fractionation factor or even the equilibrium constant together with (a) the deuterium content of the annual precipitation, and (b) the deuterium content of the ice from below the uppermost winter layer,
might be used to give reasonable information about the extent to which the yearly precipitation remains as ice in Icelandic glaciers. Direct measurements of the deuterium concentration in the annual precipitation on Icelandic glaciers have not been carried out, but as the difference between the deuterium concentration in the annual and the winter precipitation is small (see table 1) 6(annual) can be calculated from 6(winter) without a great error. Further measurement of the water content of annual layers below the uppermost winter layer should give the amount of annual precipitation. As a first step it is possible to set up the balance equation:
Riq + R w ( 1 - q ) = Rp where q is the fraction of total annual precipitation remaining as ice, R i is the mean relative deuterium concentration in the remaining ice, R w is the mean relative deuterium concentration of the water which escapes during the summer, Rp is the relative deuterium concentration of the annual precipitation. R w may be derived from the equation K e = Ri/Rw, where K e is the effective fractionation factor. Using the available data for L-1 on the Langib.kull glacier, 6 i = + 72.4%o and 6p = + 86.8% , and using the equilibrium constant K = 1.0208 instead of Ke, it follows that 24% of the yearly precipitation remains as ice and 76% escapes as water during the warm period. The amount of ice between the summer surface 1966 and the summer surface 1965 is 75 g per cm 2. This would mean that the yearly precipitation is 3100 mm at L-I. There are no direct measurements available for yearly precipitation on glaciers in Iceland, but according to the Icelandic Meteorological Office (Sigftasd6ttir, private communication) the mean yearly precipitation at L-1 for the period 1931-1960 has been estimated at 3000 mm. A lack of equilibrium cannot be the reason for a too high calculated value for yearly precipitation at L-I. Such a lack would mean an effective fractionation factor lower than the equilibrium constant used here, and this would again give a value of yearly precipitation higher than 3100 ram. It is difficult to make any reliable estimate of the accuracy of the available meteorological data for precipitation. Considering meteorological data from several stations in Iceland from 19311960 it is found that at none of these stations does
HYDROGEN ISOTOPES the precipitation in a single year exceed 1.5 times the mean annual precipitation. I f we accordingly set 4500 m m as an upper limit o f precipitation for the year in question at L-1 and use this in the balance equation together with other available data, it would give an effective fractionation factor K e = 1.0190. Thus there is a reason to believe that the effective fractionation factor for deuterium exchange at L-1 on the Langj6kull glacier will lie between the limits o f 1.0208 and 1.0190. These calculations are o f course only valid for L-1 on the Langjbkull glacier at an altitude o f 1230 m. Locations which differ in altitude and other conditions may yield different values of K e. Similar calculations can be made for L-2 and V-1. The mean 8-value in the uppermost winter layer is ~L-~2(Oct.-May) = + 8 1 . 4 ~ and ~V-l(Oct.-May) = + 81.4%o. The mean 8-values in the ice below the uppermost winter layer are 8L. 2 ice = + 69.0%o and 8v-1 ice = + 69.1%o. L-2 and V-1 are both at an altitude o f approximately 1300 m. Assuming that the deuterium concentration is 3.2%0 higher in the annual precipitation than in the winter precipitation, the mean 8-value in the annual precipitation might be as high as 8 = + 78.2%o in b o t h cases. Using these data with K = 1.0208 instead o f K e the balance equation indicates that 52% o f the yearly precipitation would remain as ice at L-2 and V-1. While the thickness o f the ice for the next year below the summer surface 1967 has not been measured at L-2 it is n o t possible to calculate the amount o f yearly precipitation. A t V-1 the water content o f the ice core between the summer surface 1967 and the summer surface 1966 is 172 g/cm 2. Thus our m e t h o d would give about 3200 m m yearly precipitation at V-1. According to the Icelandic Meteorological Office (private communication) the mean yearly precipitation at V-I for the period 1 9 3 1 - 1 9 6 0 should be about 3000 mm. It should be stressed here that this m e t h o d will probably only work in cases where the seasonal variation o f deuterium concentration in the precipitation is small. A more extensive study o f the isotopic exchange phenomenon is now being carried out on several glaciers in Iceland. The detailed results will be presented later.
429
Acknowledgements The author wishes to express his appreciation to Prof. Th.Sigurgeirsson, head o f the Physics Department o f the Science Institute, for his advice and encouragement, and to Mr. Th.Bfason of the Mathematics Department, for many fruitful discussions. F u r t h e r more the author is indebted to Mrs. A.B.Sigffsd6ttir, o f the Icelandic Meteorological Office, for making available unpublished meteorological data. The field work was made possible by a number o f volunteers who participated in the glacier expeditions.
References [1] W.Dansgaard, The isotopic composition of natural waters, with special reference to the Greenland ice cap, Medd. Groenland 165 (1961) 2. [2] S.Epstein.and R.P.Sharp, Oxygen and hydrogen-isotope variations in a firn core, Eights Station, Western Antarctica, J. Geophys. Res. 72, 22 (1967) 5595-5598. [3] L.Merlivat, C.Lorius, M.Majzoub, G.Nief and E.Roth, Etudes isotopiques en profondeur d'un glacier en Antaxctique, Proc. Symp. on Isotopes in Hydrology (International Atomic Energy Agency, Vienna, 1967) 671681. [4] S.Epstein and R.P.Sharp, Oxygen isotope variations in the Malapsina and Saskatchewan glaciers, J. Geol. 67 (1959) 88-102. [ 5 ] R.P.Sharp, S.Epstein and I.Vidziunas, Oxygen-isotope ratios in the Blue Glacier, Olympic Mountains, Washington, J. Geophys. Res. 65, 12 (1960) 4043-4059. [6] S.Deutsch, W.Ambach and H.Eisner, Oxygen isotope study of snow and firn on an alpine glacier, Earth Planet. Sci. Letters 1 (1966) 197-201. [7] D.Macpherson and H.R.Krous¢, O18/O 16 ratios in snow and ice of the Hubbard and Kaskawulsh glaciers, Proc. Syrup. on Isotope Techniques in the Hydrologic Cycle, ed. G.E.Stout (American Geophysical Union, Washington, 1967) 180-194. [ 8 ] B..g,rnason and Th. Sigurgeirsson, Hydrogen Iso topes in hydrological studies in Iceland, Proc. Syrup. on Isotopes in Hydrology (International Atomic Energy Agency, Vienna, 1967) 35-47. [9] I.Friedman, Deuterium content of natural water and other substances, Geochim. Cosmochim. Acta 4 (1953) 89-103. [10] R.L.Shreve and W.B.Kamb, Portable thermal core drill for temperate glaciers. Instruments and Methods, J. Glaciol. 5, 37 (1964) 113-117.
430
B.ARNASON
[11] l.Friedman, A.C.Redfield, B.Schoen and J.Hartis, The variation of the deuterium content of natural waters in the hydrologic cycle, Rev. Geophys. 2, 1 (1964) 177224. [12] R.E.Weston Jr., Hydrogen isotopic ftactionation between ice and water, Geochim. Cosmochim. Acta 8 (1955) 281-284. [13] W.Kuhn and M.Thtirkauf, Isotopentrennung beim Gefrieren yon Wasser und Diffusionskonstanten yon D und O 18 im Eis, Helv. Chim. Acta 41, 110 ( 1958) 938-971. [ 14] J.C.Posey and H.A.Smith, The equilibrium distribution of light and heavy water in a freezing mixture, J. Amer. Chem. Soc. 79 (1957) 555-557.
[ 15] L.Merlivat and G.Nief, Fractionnement isotopique lots des changements d'6tat solide-vapeur et liquide-vapeur de l'eau ~ des temp6ratures inf6rieures ~ 0°C, TeUus 19, 1 (1967) 122-127. [ 16[ LR.O'Neil, Hydrogen and oxygen isotopic fractionation between ice and water, J. Phys. Chem. 72 (1968) 36833684. [ 17] B./~rnason, Equilibrium constant for the ftactionation of deuterium between ice and water, J. Phys. Chem. (1969) in press.