Isotope geochemistry of stratified Lake "A," Ellesmere Island, N.W.T. ...

Isotope geochemistry of stratified Lake "A," Ellesmere Island, N.W.T., Canada M. 0 . JEFFRIES Department of Geography, The University of Calgary, Calgary, Alta., Canada E N IN4

H. R. KROUSE AND M. A. SHAKUR Department of Physics, The University of Calgary, Calgary, Alta., Canada T2N IN4 AND

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S. A. HARRIS Department of Geography, The University of Calgary, Calgary, Alta., Canada T2N IN4 Received September 6, 1983 Revision accepted April 24, 1984 Ionic composition, salinity, temperature, pH, tritium, and stable isotope compositions of water and ions were determined for samples collected in 1969 and 1982 from different depths of stratified Lake " A on Ellesmere Island. Tritium contents and ionic and stable isotope compositions were diagnostic of recent fresh water overlying older, deeper trapped seawater. A temperature maximum occurs at 15 m in the freshwater-seawater transition zone. Salinity, S'", and SD data suggest that the lake evaporated slightly and acquired about 12% fresh water prior to stratification. Individual ion concentrations reveal a slightly modified cation composition and marked depletions in sulphate and enrichments in dissolved carbonate compared with modern ocean water. The S34S,S1'O, and 8°C data for SO:- and HCO; attest to extensive anaerobic S O : reduction duringthe lake's history. La composition ionique, la salinitt, la temperature, le pH, le tritium et la composition en isotopes stables de I'eau et des ions furent dCterminCs pour les Cchantillons prklevks en 1969 et en 1982 a diffkrentes profondeurs du lac "A" stratifie sur I'ile Ellesmere. Les concentrations de tritium et les compositions en ions et en isotopes stables sont caractCristiques d'une eau douce rkcente sus-jacente h de I'eau de mer plus ancienne piCgCe en profondeur. La tempkrature maximale apparait 15 m dans la zone de transition eau douce - eau de mer. La salinitt, les donnCes de S1'O et SD indiquent une 1Cgkre Cvaporation du lac et un enrichissement d'environ 12% d'eau douce avant la stratification. Les concentrations des ions individuels rCvklent une composition cationique Ikgkrement modifiCe, une nette riduction des sulfates et un enrichissement en carbonate dissous par rapport h I'eau de mer actuelle. Les donnCes de S34S, S1'O et S13C pour SO:- et HCO, dCmontrent une forte rkduction anakrobique des SO:- durant I'histoire du lac. [Traduit par le journal] Can. I. Earth

Sci. 21,

1008-1017 (1984)

Introduction Numerous lakes, particularly at high latitudes, are stratified with respect to temperature and salinity. Of those in the Antarctic, Lake Vanda and Lake Bonney have been the subjects of numerous chemical and isotopic studies (e.g., Angino and Armitage 1963; Nakai et al. 1975; Matsubaya et al. 1979). Many stratified lakes have been discovered on Ellesmere Island in the Canadian Arctic, including Lake Tuborg, 80°50'N, 70°00'W (Hattersley-Smith and Serson 1964), and four lakes on its northwest coast designated "A," "B," "C," and " E by Hattersley-Smith et al. (1970) (Fig. 1 ) . Lake Tuborg is 120 m deep and formed when a glacier advanced across Greely Fiord. Its deeper, trapped seawater and calcilutite bottom were I4C dated at about 3000 and 25 000 years old, respectively (Long 1967). The other four lakes range in elevation from 2 to 23 m above sea level and from 40 to 68 m in depth. The above authors related the existence of these lakes to postglacial uplift of the north coast of Ellesmere Island as described by Christie (1967). Lake A, the subject of our study, is 4.9 km2 in area at an elevation of 3.3 m above sea level. The basin associated with this lake has an area of 36 km2. The bedrock is the McClintock Formation (Ordovician) and consists mainly of pyroclastics, volcanic flow rocks, and small amounts of volcanogenic conglomerate along with sandstone, mudrock, and local carbonates (Trettin 1981). The perennial ice cover is about 2 m thick and very clear. A depth profile of 6"0 for water from Lake A sampled in May 1969 was found to be consistent with the transition from

overlying fresh water to deeper, trapped seawater (Krouse 1970). Lake A was investigated again in May 1982 and was found to have remained stratified (Jeffries 1982). In this recent investigation, many chemical and isotopic parameters were measured. The geochemistry of this lake is now discussed using data from both the 1969 and 1982 sample sets.

Field sampling and laboratory procedures Methods for sampling in 1969 were described by HattersleySmith et al. (1970). In 1982, a 20 cm diameter hole was drilled through the ice in the middle of the lake. Water depths refer to the depth below the ice surface. Water samples were obtained with a 1 L Knudsen bottle to which three reversing thermometers were attached. Prior to tripping, the bottle was left at the selected depth for 7 min to allow the thermometers to reach a steady value. Upon recovery, each thermometer was read by two observers and calibration data were used to correct the measurements to an accuracy of ?0.02"C. A 5 mL aliquot was used for measuring salinity (to an accuracy of *O. lYoo)with an Endeco refracting salinometer. The remaining water was placed in two 500 mL polyethylene air-tight bottles, frozen, and shipped to Calgary. Upon receipt in Calgary, one bottle was allowed to thaw to room temperature. Fifty millilitre water samples were used for conductivity-salinity determinations on a Yellowstone Scientific Instruments Model conductivity bridge (accuracy, -+ 1%). A further 50 mL was used for pH measurements with a Fisher Model Accumet 750 selective ion analyzer (accuracy, 20.001 pH units).

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JEFFRlES ET AL.

OCEAN

STRATIFIED LAKES

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0 I

ARCT\C

ICE SHELVES

km

110

YELVERTQN

BWRAELI FIORD

80

75.

FIG. 1 .

Location of stratified lakes on Ellesmere Island, Northwest Territories (after Hattersley-Smith et al. 1970).

Aliquots (25 mL) sealed in polyethylene bottles were sent to the Department of Earth Sciences, University of Waterloo, for tritium analyses by scintillation counting (accuracy, -t-8tritium units (TU)). Ten millilitres of water was equilibrated with C02 for oxygen isotope analyses. In the case of the 1969 samples, the water was frozen with liquid N2 in the equilibration flask, which was evacuated to remove air. The water was thawed, refrozen using dry ice, and evacuated again prior to admitting COz. In the case of the 1982 samples, a preparation line with capillary pumping, which avoided the use of cold baths during evacuation, was used. The latter line was attached directly to a stable isotope ratio mass spectrometer based upon Micromass 903 components, which simultaneously collected masses 44, 45, and 46. For H/D analyses, 3 pL samples of water were injected into a line containing uranium heated to 800"C, and the product Hz was introduced directly to a mass spectrometer built around Micromass 602 components. Oxygen and hydrogen isotope abundances are expressed according to the internationally accepted S1'O and 6D scales defined as

where (180/'60)and (D/H) are abundance ratios, Yoo is parts per thousand (per mil), and subscript x designates the sample. SMOW (standard mean ocean water) is an international standard that approximates the mean isotopic composition of the modem ocean (Craig 1961). Distilled seawater and tap-water samples are used as secondary standards in the laboratory. Their isotopic composition has been directly compared with International Atomic Energy

Agency (IAEA) standards. The second bottle was thawed and 1.0 N BaCl, added to precipitate BaSO,. The SO:- concentration was determined gravimetrically. For SI80analyses of SO:-, C02was generated at 1000°C from intimate 1: 1 mixtures of BaS04 and spectrographically pure graphite in resistance-heated platinum boats in a reactor that is an improved version of that described by Sakai and Krouse (1971). Minor amounts of CO produced in the reaction were converted to C02 with a high-voltage electrical discharge unit in a trap immersed in liquid nitrogen. For sulphur isotope analyses, BaSO, was converted to Ag2S using the techniques of Thode et al. (1961). Cu20 intimately mixed with AgzS in the mass ratio 4: 1 was heated at 900°C to generate SOz, which after purification by vacuum distillation was directly introduced into a mass spectrometer built around Micromass 602 components. For some samples, carbon isotope analyses were determined for dissolved carbonate species. Ba(OH), was used to precipitate both BaCO, and BaSO, in a N2 atmosphere. The supernatant was decanted and the precipitate dried. A portion of the precipitate was reacted with H3P04 to yield C 0 2 for carbon isotope analyses. The remainder was treated with HCI to decompose BaCO, prior to conversion of the BaSO, to Ag2S as described above. The S3,S and 8I3C scales, analogous to those for SI80 and SD, are defined in terms of [34S/32S]and ['3C/12C]abundance ratios. The standard for sulphur isotope abundances is meteoritic troilite. Secondary standards in the laboratory include a natural pyrite close to the meteorite composition and commercial Ag,S powder, which is near the isotopic composition of modem seawater sulphate. The standard for SI3C values is PDB, a belemnite fossil from the Peedee Formation of South Carolina, U.S.A. In our laboratory, a secondary standard prepared from vacuum pump oil has a SI3Cvalue of -27.5%". Ion concentrations were spectrophotometrically determined by Chemex Laboratories of Calgary.

CAN. J. EARTH SCI. VOL. 21, 1984

TABLE1. Salinity and ion concentrations (ppm) of water from Lake A, May 1982

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Depth (m) 3 5 7 9 11 13 15 20 30 40 50 60 68

Field salinity

Laboratory salinity

(700)

(%o)

0.6 0.6

0.2 0.3 0.4 1.7 2.7 6.7 12.4 19.7 28.0 30.4 31.5 30.8 30.5

1.7 6.8 12.10 20.60 28.40 30.30 30.40 30.80 30.90

K+

Na'

MgZ+ C1-

Ca2'

SO:-

HCO,

~0:-

Seawater

FIG.2. Comparison of 1982 salinity data for Lake A, Ellesmere Island, Northwest Territories, with those of Hattersley-Smith et al.

FIG.3. Depth profiles for ions and pH in water from Lake A.

(1970).

Results and discussion Ionic composition Salinity data for the 1982 sampling are given in Table 1 and compared with those of Hattersley-Smith et al. (1970) in Fig. 2. Although there is reasonable agreement between the field and laboratory determinations, some data fall outside the accuracy quoted for the field unit (Table 1). The differences may relate to the use of refractive index for the field measurements and conductivity for the laboratory determinations. Variations in ionic com osition with depth caused by processes such as bacterial SO,4- reduction may also affect the salinity determinations. The 1982 salinities were not quite as high as those determined in 1969 (Table 1; Fig. 2). We believe that this reflects analytical uncertainties rather than actual changes in the lake. All three sets of salinity data attest to trapped seawater underlying fresh water derived from melting snow and ice. The concentrations of individual ions as a function of depth are plotted in Fig. 3. Most depth profiles are similar to those of salinity and are also diagnostic of fresh water overlying trapped seawater. One expects the lighter ions to diffuse preferentially

upwards in the transition zone. However, the mass dependency of the diffusion coefficients is not simply the atomic mass because the ions drag along nearby water molecules and possibly other species (Anderson and Graf 1978). The situation is more complex where several ion species of different masses and charges are involved, as the solution strives to maintain electroneutrality (Vinograd and McBain 1941; Li and Gregory 1974). Toth and Lerman (1975) computed salt diffusion coefficients from salinity profiles and stratification ages for five lakes. For four of the five lakes, including Lake A, they found an order of magnitude agreement with coefficients reported for aqueous solutions and concluded that the salinity profiles were consistent with vertical molecular diffusion. It should be noted that the above authors used a stratification age of 600 years for Lake A, whereas Lyons and Mielke (1973) quoted a radiocarbon date of over 4000 years for the deepest water. Whereas the relative concentrations of Na', K', and C1- are similar to those of average seawater, Mg2+is enriched (Fig. 4). [Ca2+]and [SO:-] are comparatively higher in the upper levels, but depleted at depth. HCO, is greatly enriched at all levels. The odour of H,S that accompanied some deeper samples indicates that bacterial SO:- reduction lowered the [SO:-] and

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!

/ so:

FIG.4. SO:-, Cl-, Mg2+,K', HCO,, and CaZ' ion concentrations plotted against Na' concentrations. The bold, straight lines represent the ion ratios expected of a simple freshwater-seawater mixing model.

TEMPERATURE ( " C )

ION RATIOS

FIG.5. Behaviour of [ ~ ~ : - ] / s a l i n i t ~(circles), HCO;/salinity (squares), [Ca2+]/salinity (triangles), and [SO,-]/[HCO;] (diamonds) ratios with depth in Lake A. All ratios have been standardized to the 0- 15 ion ratio scale. increased the [HCO,]. It will be shown that sulphur and carbon isotope data are also consistent with microbial activity at depth. The excess Ca2+ and SO:- in the transition zone and excess Mg2+throughout the water column suggest either the original trapped seawater was slightly different in composition from

FIG. 6. Behaviour of temperature with depth in Lake A. Data for 1969 from Hattersley-Smith et al. (1970). For comparison, data from Lake Bonney, Antarctica (Shirtcliffe and Bensemen 1964) are also plotted.

"average" seawater or some dissolution of salts has occurred. The relative Mg2+enrichment occurs around 15-20 m where the salinity gradient is greatest. The [Ca2+]/salinity and [SO:-]/salinity ratios go through maximum values in the transition zone (Fig. 5). Simple mixing of the deeper, modified seawater with surface water will not produce these maxima. They are, however, consistent with the predictions of ionic

CAN. J, EARTH SCI. VOL. 21, 1984

TABLE2. Temperature, pH, and radio- and stable isotope compositions of water, sulphate, and bicarbonate in water from Lake A, May 1982 -.

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Depth (m)

Temperature

("c)

'H (TU)

PH

(H20) (%o)

SD (HzO)

(%)

s34S (SO:-)

(7001

S1'0 (SO:-) (%o)

SI3C (HCO,) (%o)

diffusion because the diffusion coefficients at 25°C in free solution for C1-, K + , Na+, HCO,, SO:-, Ca2+,and Mg2+have been quoted as 11.1, 10.7, 7.2, 6.4, 5.2, 4.3, and 3.9 cm2/s X respectively (Li and Gregory 1974). Thus SO:-, Ca2+, and Mg2+ should have diffused upwards at a slower rate than most other ions, which could account for their greater relative enrichments in the depth interval where the concentration gradient is largest. The [HCO,]/salinity ratio goes through a minimum value in the transition region that coincides with the maximum [ ~ ~ : - ] / s a l i n i tratio ~ (Fig. 5). This suggests that sulphate reduction was minimal in the transition region and dominated at depth. Temperature The temperature-depth profiles are shown in Fig. 6. A distinct maximum-in temperature (8.28"C) is noted at 15 m (Table 2). Similar maxima have been observed in other stratified lakes. For example, a Tmaxof 7.4"C was measured at about 13 m depth in Lake Bonney (Shirtcliffe and Benseman 1964). This phenomenon has been attributed qualitatively to solar energy trapping by the density gradient. Many physical parameters such as indices of refraction, electrical conductivity, relative coefficients of heat flow and diffusion, and duration of surface snow are probably required for a more detailed explanation of this phenomenon. The lower temperature beneath the ice surface is to some extent due to heat exchange with the ice itself and heat transfer by horizontal freshwater flow. The decrease in temperature with depth below the maximum is consistent with an exponential decrease in transmitted energy associated with absorption. It is interesting to note that the temperature in Lake A remains around 4°C in the deepest waters (down to 68 m), in contrast to Lake Bonney, where a temperature of -2°C was recorded at 30.5 m (Shirtcliffe and Benseman 1964) (Fig. 6). This is possibly due to greater solar energy attenuation by the higher salt content of the latter ([Cl-] is five times that of ocean water). The higher salt content also permits this lower temperature without freezing. PH The near surface waters are slightly basic (pH 8.6) and a pH maximum of 9.0 is noted at 5 m. With depth, pH decreases to a minimum value of 7.1 at 13 m (Table 2). Thereafter, it increases slightly and achieves a value near 7.7 at 68 m. Hence

-

FIG. 7. Depth profiles for tritium and pH in water from Lake A. The dashed line represents water with minimal tritium concentration.

the shallow and deep waters have pH values higher and lower, respectively, than seawater (-8.0; Mason 1966). The lower pH values in the deeper waters may have resulted from bacterial C02 and H2Sproduction. Similar trends have been noted in Lake Vanda (Nakai et al. 1975) and Ace Lake (Burton and Barker 1979), Antarctica, with even lower pH values for deeper, more saline water. The surface waters may have interacted with alkaline rocks. but this is difficult to substantiate. considering the varied lithology (see Introduction). In Lake A, the pH minimum occurs at the depth where the temperature is near its maximum and the salt concentration stark to increase rapidly (Fig. 3). Although this might be a diffusion phenomenon, another explanation is related to bacterial populations. Although the relevant study was not carried out in Lake A. maximum bacteria counts and sometimes associated suspended colloidal sulphur have been found at about 10 m depth in some lakes (e.g., Ace Lake; Burton and Barker 1979). It should be noted that this reflects the activity of photosynthetic sulphide oxidizing bacteria; sulphate reducers dominate in deeper waters. The pH maximum at 5 m depth is more difficult to explain. It is an interesting question whether it bears any relationship to the 3Hmax recorded in the same sample (Fig. 7). If the shallow water input is alkaline, then interaction with the ice under-

JEFFRIES ET AL.

TABLE3 . Temperature, salinity, 6'*0 (H20), and 634S(SO:-) of water from Lake A, May 1969

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Depth (m)

Temperature ("'2)

Salinity

%

6'" (H,O)

(xO)

634S (SO:-)

(“hO)

FIG. 8. Depth profiles of 6D and 6180 in water samples from Lake A.

surface could coincidently reduce the tritium concentration and the alkalinity below the ice. However, to produce the maxima some 3 m below the ice would require significant long-term net ablation of the ice undersurface. Tritium The tritium data are given in Fig. 7 and Table 2. Below 11 m, the water appears to be uncontaminated by bomb tritium since natural trjtium levels fall in the 4-25 TU range (Gat 1980; 1 TU represents one tritium atom per 1018 hydrogen atoms). The 3H maximum generated during the testing of thermonuclear devices from 1961 to 1963 is readily detected and interpreted as such in Arctic and Antarctic ice cores (e.g., H , in the water Ravoire et at. 1970; Theodorsson 1977). The ' column at 5 m greatly exceeds the values of 12-80 TU (mean, 36 TU) measured in contemporary, local precipitation samples. This implies that tritium was introduced shortly after the bomb tests and has been subsequently displaced downwards by meltwater streams flowing just beneath the ice cover. The maximum of the TU versus depth curve has probably been flattened by vertical diffusion. The step-function transition to the minimum values at about 12 m depth will be discussed in the conclusion. Hydrogen and oxygen isotopic composition of HzO The 6D and 6180 values for water samples are given in

FIG.9. Concentrations of K + , SO:-, and HCO, versus 6D, Lake A. Tables 2 and 3. Their depth profiles are consistent with seawater overlain by freshwater runoff (Fig. 8). The agreement between the 6180 data sets is gratifying considering the laboratory techniques differed considerably for the two sampling occasions. The extreme 6 values in the upper and lower depths are comparable, whereas departures between the two profiles occur in the transition zone. These probably represent variations with location and (or) the influence of horizontal freshwater throughputs below the ice surface during the 13 years between samplings. It is noted that the depth sampled in 1982 (68 m) is 11 m greater than that sampled in 1969. This discrepancy is identified with two different sampling locations rather than a change in water level. The depth profiles for 6D and 6180 (Fig. 8) are very similar to those obtained for salinity (Fig. 2). Further, plots of some ion concentrations against 6180 or SD approach linearity (Fig. 9a). The linear equations for Na+, Kt,Mg2+,Ca2+,and C1- concentrations versus 6D and S1'O are given in Table 4, column 2. The correlation coefficients (r) are 0.96 or better. The ion concentrations corresponding to an original trapped seawater with isotopic composition SMOW are given in columns 3 and 4. Although the agreement is better for 6D = 0 than for 6180 = 0, the extrapolated Kt,Na+, Ca2+,and C1concentrations are lower than those identified with average

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CAN. J. EARTH SCI. VOL. 21, 1984

FIG. 10. 6D versus 6180 for water samples from Lake A.

seawater in both cases. Alternatively, with the exception of Mg2+, average seawater concentrations of these ions correspond to positive SD and S1'O values (Table 4, columns 7 and 8). (The extrapolated SD and S i 8 0values based on [Ca2+]are exceptionally high!) These equations suggest that the trapped seawater may have become isotopically heavier than SMOW prior to stratification. Columns 5 and 6 list ionic concentrations for average seawater and the water at 68 m depth in Lake A. The numbers in parentheses in columns 3-5 are the ratios of ion concentrations at 68 m depth to those in average seawater and to those predicted for SMOW by the regression equations in column 2. The ratios range from 0.79 to 1.05, with a mean of 0.89. This suggests that the deepest water in Lake A is an 8 : 1 mixture of seawater and fresh water. [SO:-] and [HCO,] depart considerably from a linear relationship with SD and FIRO(Fig. 9 b and c ) . It is shown that IS@-] per se is much higher at 20 m (transition zone) than predicted by simple mixing of seawater with fresh water. This is consistent with the behaviour of the [ ~ ~ : - ] / s a l i n i tratio ~ previously noted (Fig. 5). The SO:- depletions and large HCO; enrichments for 6D values >-807~0(Fig. 9 b and c ) are consistent with extensive bacterial SO:- reduction. Much precipitation in the Northern Hemisphere tends to obey the "meteoric line," SD = 8.1 fii80 + 1 I (Craig 1961; Dansgaard 1964). With evaporation, a plot of 6D versus S"0 approaches a slope of 5.5 (Dansgaard 1964). The bulk of the data for Lake A ialls below the meteoric line (Fig. 10). At 6 values near zero, this behaviour is predicted by a simple mixing model because SMOW, representative of the ocean, is below the meteoric line. However, the departure is greatest in the transition region, with the plot displaying a downward curvature. Data for shallow-water samples fall closer to the meteoric line, albeit with considerable scatter. A number of phenomena may have influenced the SD versus S180relationship. The scatter in data for the shallow samples may reflect water-ice interactions. In laboratory experiments, O'Neil(1968) found ice freezing at O°C to be enriched in D and "0 by about 19 and 3"/0,, respectively, compared with the liquid phase. The downward curvature in the transition region could relate to diffusion phenomena since the order of increasing masses is H2I60< HDI60 < H2180.Thus any change in S i 8 0is expected to be larger than that in SD even if the mass dependence of diffusion departs significantly from that of individual water molecules. Alternatively, a downward curvature

-1015

ET AL.

r

R a y l e i g h Fractionation?

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Mlxlng L i n e ?

0

I

-240

-160

0

-80

d D(%ol

RG. 1 1 . Depth profiles for 634Sand 6"0 in SO:-, Lake A.

in the 6D versus 6180 plot could result if, early in the lake's history, the meteoric line had a larger slope. This water would have to remain in the lower part of the transition zone and become overlain by progressively younger water with a steadily decreasing meteoric line slope approaching 8. Whereas the tritium data show that the freshwater component in the lower part of the transition zone is old, the required change in the slope of the meteoric line seems unrealistically large.

634Sand 6180 values of SO:The depth profiles for 634S and 6180 values for SO:- are shown in Fig. 11. Consistent with the isotopic compositions of water, the agreement in the 634Sdata attests to the stability of the stratified character of the lake over the 13 years between the two sampling times (Tables 2, 3). Whereas the 634Sand 6I8O values for SO:- tend to increase with depth, the deepest samples are slightly less positive than the maxima. The average aMS and 6180 values for the deep samples are unexpectedly high in comparison with those of respectively). The modem ocean SO:- (+21 and + extent to which the S3,S data depart from a simple seawaterfreshwater mixing model based on 6D values is shown in Fig. 12. It seems unlikely that isotopically heavy SO:- was introduced into the lake from surrounding mineralizations. Evaporites have not been reported in the catchment area. Further, the comparatively low [Ca2+]in the deeper waters (Table 4, column 5) is not consistent with evaporite dissolution. A sample of gypsum and selenite collected about 25 km southeast of Lake A had a a3,S value of +20.0"/00. The high 634S and 6 1 8 0values are characteristic of SO:remaining after preferential bacterial reduction of the lighter isotopic species. The low SO:- concentrations and the odour of H2S in some of the deeper samples are consistent with this mechanism. Unfortunately, sufficient HS- could not be extracted for sulphur isotope analyses. Even if it possessed very negative E3,S values, the amount is not sufficient to provide an isotopic or chemical mass balance. Following Rees (1973) attempts were made to fit Rayleigh fractionation curves to the 634S and [SO:-] data, assuming that the latter represents the remaining reactant. For the deeper water samples, the ratio of the isotopic rate constants, k3,/k3,, is consistently near 1.055 if the modern oceanic 634S value of +21T0 is chosen for the initial SO:-. In the transition zone, the data do not fit a Rayleigh fractionation curve. This would seem to relate to the observation that in this zone the sulphate concentrations exceed those of a marine-freshwater mixing model (Fig. 9b). A pos-

FIG. 12. 6 3 4values ~ for SO:- versus 6D of water.

Slope 0.6 1

FIG. 13. Plot of S1'O versus 6"s of sulphate in Lake A. SW represents the isotopic composition of modem seawater.

sible explanation for higher [SO:-] is the oxidation of dissolved sulphide from SO:- reduction at lower levels migrating u wards. However, in that case, the g 4 S for the original SO,l)r plus secondary SO:- should be less than +21& rather than the +25"/00measured. There is a suggestion in Fig. 12 that shallower waters (6D < -220T0) fit a mixing line where the freshwater and seawater components possess a3,S values of + 10 and +25%0, respectively. For deeper waters with 6D values >-220%0 and associated 634Svalues >+25"/00the data tend to be more consistent with Rayleigh fractionation. A plot of 6180 and 6"s for SO:- is shown in Fig. 13. A line of slope 0.25 has been drawn through modem seawater values because in some laboratory experiments and field studies this relationship has been identified with bacterial sulphate reduction (Hunt 1974; Rafter and Mizutani 1967). However, Mizutani and Rafter (1973) have shown that the oxygen isotope composition of SO:- is influenced by the 6180 value of the water because of isotopic exchange among intermediates and back reaction during the reduction. With the exception of the two bottom samples, the data below 20 m appear to linearly extrapolate, with a slope of 0.5, through the modern seawater point. There also appears to be a linear relationship in the shallow waters that coincides with the mixing line in Fig. 12.

1016

CAN. 1. EARTH SCI. VOL. 21. 1984

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However, in the transition zone (15-20 m) there is a marked vertical shift. Despite this shift, the SI80of the SO:- is linearly related to 6"0 and SD of the water throughout the column, with the exception of the two bottom samples

The departure of the two bottom samples from these lines perhaps reflects interactions with the sediment. Whereas the linear relationships are consistent with simple mixing, the absence of similar relationships for S34Stends to rule out mixing. Further, the [SO:-] in the influent shallow water is too low to be a significant factor in the plot in Fig. 13 or the above equations. It seems more likely that the depletions in "0 and 34 S in the shallow SO:- are the consequence of reoxidation of the biogenic sulphide. During such oxidations, oxygen is incorporated from the water molecule (van Everdingen et al. 1982). This would establish a sympathetic relationship between the 6180 values of the SO:- and H20. If isotopic equilibrium had been established between S O : and H2Sthrough exchange with intermediates of SO:- reduction and (or) sulphide oxidation, the S180so:-versus S180H,oline would have a slope of unity because the temperature variations are small.

6I3C values of dissolved carbonate Unfortunately complete sequences of dissolved carbonate were not analyzed for both SI3C and concentration (Table 2). However, the available data are shown in Table 5, and a distinct trend is evident. The bicarbonate ion is essentially the sole carbonate species in the overlying fresh water. Its SI3C values of -5.4 and -7.4T0 are typical of fresh water. Upon going deeper into the underlying ocean water, CO: increases in concentration, but HCO, still dominates. Instead of apas expected for ocean water, the SI3Cvalues proaching + 1.O"/oo fall rapidly to an average of -22.0"/,,. These isotopic compositions and ion concentrations four to five times that of modem ocean water identify the dissolved carbonate as the product of oxidized organic matter. This is consistent with extensive bacterial sulphate reduction as deduced from the sulphur and oxygen isotope data for SO:-. Conclusion Lake A on Ellesmere Island was created by. .postglacial uplift and isolation of a seawater basin a few thousand vears a i o In comparison with average seawater the ionic composition of the underlying water in Lake A is slightly modified in cationic com~osition and drasticallv modified in anion composition, dith marked depletions bf SO:- and enrichments of dissolved carbonate species. The S34S,Si80, and SI3C data reveal that the latter are the consequence of bacterial SO:reduction and associated oxidation of organic matter. Questions arise concerning the sulphur isotope balance because of the low quantities of dissolved sulphide and S34Svalues for SO:- exceeding +25.0"/0,. It appears that some sulphide was reoxidized, as witnessed by the lower S34Svalues in the transition zone. It is possible that prior to stratification there were extensive H2S and perhaps COz emissions to the atmosphere. Alternatively, sulphide minerals may have been precipitated. Despite these uncertainties, the marked depletions in SO:coupled with the isotopic data indicate that anaerobic microbiological activity has been extensive during the lake's history. Mixing phenomena in the transition zone are difficult to

.,

TABLE5. Dissolved carbonate concentrations and 6I3C values of the three water zones in Lake A Depth (m) Surface fresh water (1982) Transition zone (1982) Underlying seawater (1982) (1969) (1982)

7.0 11.0

[HCO,]

+ [co:-1

78+1 = 79 98 +0 = 98

6I3C

(yoo) -5.36 -7.43

13.0 15.0 30.0

160+0 = 160 231+0 = 231 289+15.5 = 304.5

-11.83 -11.51 -12.50

40.0 57.0 68.0

555+95 = 650 -

-20.90 -22.00

692+84.5

=

776.5

-

assess. Macroscopic processes such as entrainment mixing during " convection or vertical advection should not alter relative ionic concentrations or cause large departures from simple linear mixing models. It is more reasonable to relate the concentration and isotopic data to upward ionic and molecular diffusion with attending isotopic fractionation. These processes, however, are difficult to measure and model because of large temperature gradients in the transition zone. For species that can be readily examined isotopically the isotopic dependence of diffusion is masked by bacterial isotope fractionation. However, comparison of the tritium profile with those of ionic and stable isotope abundances suggests that diffusion plays more than a minor role during mixing of the shallow fresh water with the deep seawater. The step function behaviour of the decrease of tritium with depth is consistent with its recent iniection: i.e.. the time has been too short to establish a smoother transition at its concentration front. In contrast, the curved concentration profiles for the ionic and stable isotope data are consistent with diffusion phenomena that have been operative for a few thousand years. The ionic and stable isotope compositions of the deeper trapped seawater indicate some modification of the original seawater in the basin. The SD and SI80 data for the water suggest that Lake A evaporated slightly and acquired about 12% fresh water, which was thoroughly mixed prior to stratification. This possibly occurred after uplift and trapping of seawater during a warm period. Alternatively, the water might have had this composition prior to trapping as the result of dilution of nearshore Arctic Ocean water bv meltwater runoff. Bowman and Long (1968) suggested thit the presence of certain copepods in Lake Tuborg indicates more widespread brackish water conditions greater than 3000 years ago. This is consistent with other evidence for open-water conditions and a warmer climate in the Canadian Arctic Archipelago prior to 3500 years ago (Blake 1970; Nichols 1970). Trapping during or after this warm ~eriodand the subseauent formation of a surface ice cover and inflow of fresh water probably produced the present stratified conditions.

Acknowledgments M.O.J. acknowledges the generous financial and logistic support of the following: Dome Petroleum Ltd.; Gulf Canada Ltd.; Supply and Services Canada, contract No. 065B, 97708-1-0993 (through the Arctic Acoustics Section, Defence Research Establishment Pacific); Polar Continental Shelf Project; Arctic Institute of North America; and the University

;

ET AL.

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Renmin University of China on 06/05/13 For personal use only.

of Calgary (the Alberta Research Council scholarship). The stable isotope laboratory is supported by Natural Sciences and Engineering Research Council of Canada (NSERC) grants to H.R.K. W e thank Dr. G. Hattersley-Smith for originally drawing attention to the stratified lakes on Ellesmere Island and J. Mielke for sending samples from the 1969 collection. M.O.J. thanks Harold Serson for his assistance in the field. ANDERSON, D. E., and GRAF,D. L. 1978. Ionic diffusion in naturally occurring aqueous solutions: use of activity co-efficients in transition state models. Geochimica et Cosmochimica Acta, 42, pp. 251-262. K. B. 1963. A geochemical study of ANGINO,E. E., and ARMITAGE, Lakes Bonney and Vanda, Victoria Land, Antarctica. Journal of Geology, 71, pp. 89-95. BLAKE,J. W., JR. 1970. Studies of glacial history in arctic Canada, I: pumice, radiocarbon dates, and differential, postglacial uplift in the eastern Queen Elizabeth Islands. Canadian Journal of Earth Sciences, 7, pp. 634-664. T. E., and LONG,A. 1968. Relict populations of DrepaBOWMAN, nopus bungei and Lirnnocalanus macrurus grimaldii (Copepoda: Calanoida) from Ellesmere Island, N. W.T. Arctic, 21, pp. 172- 180. BURTON,H. R., and BARKER,R. J. 1979. Sulfur chemistry and microbiological fractionation of sulfur isotopes in a saline Antarctic lake. Geomicrobiology Journal, 1, pp. 329-340. CHRISTIE,R. L. 1967. Reconnaissance of the surficial geology of northeastern Ellesmere Island, Arctic Archipelago. Geological Survey of Canada, Bulletin 138, 50 p. CRAIG,H. 1961. Standard for reporting concentrations of deuterium and oxygen-18 in natural waters. Science, 133, pp. 1833- 1834. DANSGAARD, W. 1964. Stable isotopes in precipitation. Tellus, 16, pp. 436-468. GAT, J. P. 1980. The isotopes of hydrogen and oxygen in precipitation. In Handbook of environmental isotope geochemistry. Vol. 1. The terrestrial environment. Edited by A. P. Fritz and J. Ch. Fontes. Elsevier Scientific Publishing Company, Amsterdam, The Netherlands, pp. 21 -47. HAT~ERSLEY-SMITH, G., and SERSON,H. 1964. Stratified water of a glacial lake in northern Ellesmere Island. Arctic, 17, pp. 109- 110. HAT~ERSLEY-SMITH, G., KEYS, J. E., SERSON,H., and MIELKE, J. E. 1970. Density stratified lakes in northern Ellesmere Island. Nature, 225, pp. 55-56. HUNT,R. N. 1974. Oxygen isotope studies on sulfates. Ph.D. thesis, The University of Alberta, Edmonton, Alta., 221 p. JEFFRIES,M. 1982. The Ward Hunt Ice Shelf, spring 1982. Arctic, 35, pp. 542-544. KROUSE,H. R. 1970. Application of isotope techniques to glacier studies. In Glaciers, the proceedings of workshop seminar. Canadian National Committee, the International Hydrological Decade, pp. 49-59. LI, Y. H., and GREGORY, S. 1974. Diffusion of ions in sea water and in deep sea sediments. Geochimica et Cosmochimica Acta, 38, pp. 703-714. LONG,A. 1967. Trapped sea water at the bottom of Lake Tuborg, Ellesmere Island (abstract). Eos, 48, p. 136. LYONS,J. B., and MIELKE,J. E. 1973. Holocene history of a portion of northernmost Ellesmere Island. Arctic, 26, pp. 314-323. MASON,B. 1966. Principles of geochemistry. 3rd ed. John Wiley and

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