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A study of strontium isotopes in lakes and surficial deposits of the ice-free valleys, southern Victoria Land, Antarctica
Chemical Geology, 22 (1978) 107--120
107
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A STUDY OF STRONTIUM ISOTOPES IN LAKES AND SURFICIAL DEPOSITS OF THE ICE-FREE VALLEYS, SOUTHERN VICTORIA LAND, ANTARCTICA*
LOIS M. JONES and G U N T E R F A U R E
Department o f Geology and Mineralogy and Institute o f Polar Studies, The Ohio State University, Columbus, OH 43210 (U.S.A.) (Received December 15, 1976; accepted for publication May 13, 1977)
ABSTRACT Jones, L.M. and Faure, G., 1978. A study of strontium isotopes in lakes and surficial deposits of the ice-free valleys, southern Victoria Land, Antarctica. Chem. Geol., 22 : 107--120. The ice-free valleys of southern Victoria Land, Antarctica, contain saline lakes including Lake Vanda in Wright Valley and Lake Bonney in Taylor Valley. The source of the salts dissolved in the brines has not yet been identified. Strontium in water-soluble salts of soil samples in Wright Valley has STSr/8'Sr ratios ranging from 0.7119 to 0.7157 with an average of 0.7144 + 0.0008 (1 a). This value is very similar to the 8~Sr/S6Sr ratio of brines in Lake Vanda (STSr/86Sr = 0.7149 +- 0.00017), but differs significantly from the 87Sr/8~Sr ratios of seawater (0.7094 -+ 0.00012) and basaltic rocks of the McMurdo Volcanics (0.7044 _+ 0.00046). The Sr in Lake Vanda therefore could not have originated from seawater or from volcanic rocks of the area, but may have been derived by chemical weathering of the igneous and metamorphic rocks exposed in Wright Valley. The 87Sr/86Sr ratios of Lake Bonney are invariant with depth and average 0.7130 -+ 0.00014 (1 ~), whereas the Sr concentrations increase from 0.7345 ppm near the surface to a maximum of 39.92 ppm at a depth of 25 m. The Sr concentrations at greater depth decrease slightly to 35.88 ppm at 30 m. The Sr in water-soluble soil salts near Lake Bonney and of the Taylor Red Cone is similar isotopically to the Sr in the lake. The 87Sr/86Sr ratios of soil salts and of meltwater in Taylor Valley decrease systematically from Lake Bonney toward the coast. The water in Lake Fryxell, which is closest to the coast, has an 87Sr/8'Sr ratio of 0.7090 and is identical to that of seawater. The evidence from this and similar studies in other parts of the world indicates that the 87Sr/8'Sr ratios of saline brines in closed continental basins are representative of the Sr in the rocks underlying the basin. The Sr in carbonate and sulfate minerals precipitated from such brines preserves the 87Sr/8'Sr ratios that existed at the time of deposition. Because of these relationships we suggest that stratigraphic variations of the 87Sr/8'Sr ratios of non-marine carbonate or sulfate rocks reflect changes in the geology of the drainage basin. The isotopic composition of Sr of such rocks may therefore provide useful information about the geologic histories of continental basins.
*Laboratory for Isotope Geology and Geochemistry Contribution No. 24.
108 REVIEW OF PREVIOUS STUDIES The ice-free valleys of southern Victoria Land (Fig. 1) contain several lakes and ponds including Lake Vanda and Don Juan Pond in Wright Valley and Lake Bonney in Taylor Valley (Tedrow and Ugolini, 1963; Goldman et al., 1967; Wilson, 1967). Both lakes are permanently ice-covered b u t contain h o t brines at depth whose concentrations and densities greatly exceed those of seawater. The elevated temperatures are attributable to the entrapment of solar radiation (Wilson and Wellman, 1962; Hoare et al., 1964, 1965; Ragotzkie and Likens, 1964; Shirtcliffe, 1964; Shirtcliffe and Benseman, 1964; Hoare, 1966, 1968). However, the source of the salts and the origin and evolution of the brines in the lakes are still uncertain (Angino and Armitage, 1963; Angino et al., 1964; Wilson, 1964). Several investigators have used ratios of cations and/or anions to determine the source of the salts in Lake Vanda, but have reached contradictory conclusions. Angino et al. (1965) concluded that the brines in Lake Vanda were discharged b y hot springs, whereas Boswell et al. (1967) cited ratios of several trace metals (Zn, Pb, Bi, Fe, Mn and Mo) to support their suggestion that the brines originated as glacial meltwater. Yamagata et al. (1967) reported concentrations of halogens and alkali metals, b u t drew no firm conclusions, whereas Morikawa et al. (1975) favored a marine source for Mg 2÷ and K ÷ in Lake Vanda. The question regarding the origin of the brines has also been investigated by means of isotopic data. Ragotzkie and Friedman (1965) reported D/H ratios for Lake Vanda and concluded that the water is of non-marine origin. However, Craig (1966) disputed this interpretation and stated that the data do not exclude a marine source for the brine. More recently, Nakai et al. (1975) reported 5'sO values of a b o u t - - 3 1 % 0 which indicates a non-marine source for the water. However, 534S values of sulfate in the brines and 513C and 61so values of calcite in the sediment are not easily reconciled with a simple history of progressive concentration of glacial meltwater. Measurements of 234U/23sU activity ratios by D.L. Thurber of brines taken at different depths in Lake Vanda, yielded values of 2.5 (10 m), 3.3 (50 m), and 4.5 (57 m) (E.E. Angino, pets. commun., 1968. The presence of excess 234U indicates that the U originated as a weathering product of U-bearing minerals in the watershed. Jones and Faure (1967a) measured STSr/S68r ratios and Sr concentrations in a suite of water samples taken at different depths in Lake Vanda. Their results showed that the Sr concentrations increase from 0.141 ppm just below the ice to 67.1 ppm near the b o t t o m of the lake at a depth of 60 m. However, the STSr/S6Sr ratios of the brines are c o n s t a n t throughout and average 0.7149 -+ 0.00017 (1 a). Jones and Faure (1967a) also analyzed Sr in seawater from the Ross Sea (STSr/S6Sr = 0.7094 + 0.00012), in Tertiary basalts of the McMurdo Volcanic Province (STSr/S6Sr = 0.7044 + 0.00046), and concluded that the Sr in Lake Vanda could n o t have originated from seawater or from the volcanic rocks. On the other hand, their analyses showed that the Sr in the Onyx River (Fig. 1) and in soil salts have S~Sr/S6Sr ratios that are identical to that of Lake
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WRIGHT AND TAYLOR VALLEYS, SOUTHERN VICTORIA LAND
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Vanda. The silicate fraction of one soil sample from Wright Valley was found to have the same 87Sr/86Sr ratio (0.7148) as salts leached from that soil with dilute HC1 (pH = 3--4). These results demonstrated that the Sr in Lake Vanda could have been derived predominantly by chemical weathering of bedrock and regolith in Wright Valley. The apparent unimportance of seawater or of marine deposits as sources of Sr in the brines of Lake Vanda requires c o m m e n t in view of the conclusion of Webb (1972) that Wright Valley was a fiord during the Pliocene Epoch. When uplift occurred in latest Pliocene or earliest Pleistocene time (Victoria Orogeny), the basin of Lake Vanda was presumably occupied by trapped seawater. We believe that this water evaporated and may have deposited a thin layer of salts containing marine Sr. This deposit was subsequently covered with detrital sediment and non-marine evaporites which have prevented the marine Sr from entering the brines that now occur in Lake Vanda. We plan to measure 87Sr/S6Sr ratios of evaporite minerals in sediment cores from Lake Vanda in order to identify the marine c o m p o n e n t whose presence is implied by the geologic history of Wright Valley as it is currently understood. We now report additional measurements of STSr/86Sr ratios of leachable soil salts from Wright Valley in order to d o c u m e n t the importance of chemical weathering to the isotopic composition of Sr in water and leachable salts in this closed continental basins. We also present data for Lake Bonney and for soils and meltwater ponds in Taylor Valley, located a b o u t 25 km south of Wright Valley (Fig. 1). The results of this study are applicable to the interpretation of 87Sr/S6Sr ratios of saline lakes in continental basins within which non-marine carbonate rocks m a y be deposited. The concentrations of Sr were measured by isotope dilution using a calibrated spike solution enriched in ~6Sr. Measurements of 87Sr/~6Sr ratios were made on unspiked samples on a solid-source mass spectrometer (Nuclide ® , model 6-60-S). Duplicate determinations of isotope ratios of three brine samples from Lake Bonney indicate a reproducibility of -+ 1.3 • 10 -4 (1 G). The Eimer & Amend ® SrCO3 isotope standard was analyzed repeatedly during the course of this study and has an STSr/86Sr ratio of 0.70825 + 0.00044 (1 o). All measured values of the STSr/S6Sr ratio were corrected for isotope fractionation to a standard value of 0.1194 for the S6Sr/SSSr ratio. The analytical procedures were described in more detail by Jones {1969). A general account of the isotope geology of Sr was published by Faure and Powell (1972). SOILS OF WRIGHT VALLEY Wright Valley is underlain primarily by igneous and metamorphic rocks described by McKelvey and Webb (1961, 1962), Allen and Gibson (1962), Bull et ah (1962), Gunn and Warren (1962), Nichols (1962), and by Haskell et al. (1965). These rocks were metamorphosed in Early Paleozoic time (Faure and Jones, 1974; Jones and Faure, 1967b) and are u n c o n f o r m a b l y overlain by sandstones and shales of Middle to Late Paleozoic age (Beacon Supergroup).
IIi
In addition, there are several large sills of the Ferrar Dolerite of Jurassic age {Gunn, 1962). The youngest rocks in Wright Valley are small basaltic cinder cones of Late Tertiary age. The b o t t o m of Wright Valley is covered by a blanket of unconsolidated rock debris most of which was derived from the bedrock exposed higher up on the valley walls and the surrounding mountain ranges (McCraw, 1967; Calkin and Nichols, 1972; Linkletter et al., 1973). This "soil" contains small lenticular deposits and efflorescences of salt which are c o m m o n throughout the area and consist primarily of mirabilite (Na2SO4 • 10H20) and thenardite (Na2SO4) (Smith, 1965; Wellman and Wilson, 1965). The occurrence and origin of these salts has been discussed more recently by Black and Bowser (1967), Bowser et al. (1970) and Dort and Dort (1970a, b). Most of the salt deposits that have been studied in detail are located close to the coast and are generally believed to be of marine origin. Wright Valley is separated from the coast by the Wilson Piedmont Glacier (Fig. 1) and actually slopes a w a y from the coast. The salt in the soil of this valley, therefore probably does n o t have a direct marine source. Table I contains S~Sr/S6Sr ratios for sixteen samples of soil and meltwater collected along the entire length of Wright Valley (Fig. 1). Most of the soil samples were leached with double-distilled and demineralized water in order to simulate the removal of Sr from the soil by glacial meltwater. Several samples were treated with dilute V y c o r ® -distilled HC1 and two soils were TABLEI STSr/S'Sr ratios of soluble
salts in the soil of Wright Valley
Sample No.
Locality
Water-soluble soil salts
HCl-soluble soil salts
Silicate fraction (salt-free)
201-V9 201-V16 206
18 m N of Lake Vanda 100 m N of Lake Vanda between Lake Vanda and Bull Pass Bull Pass east of Bull Pass near Meserve Glacier near Meserve Glacier near Meserve Glacier near Meserve Glacier near Meserve Glacier near Bartley Glacier eastern Wright Valley eastern Wright Valley Wright Lower Glacier Clark Glacier water, Goodspeed Glacier
0.7144 0.7148
---
---
-0.7148 -0.7144 0.7145 0.7142 0.7142 0.7157 -0.7145 0.7145 0.7149 0.7119 0.7145
0.7144" -0.7148" -----0.7141 ------
--0.7148" --------0.7144 ---
BP-0-12 207 202 9-2-12 23-2-9 23-9-19 24-7-36 208 204 205 0-12 203 015 *Reported
by Jones
and Faure
(1967a).
112
analyzed in bulk, after prior removal of water-soluble salts. The results indicate that the 87Sr/86Sr ratios of water-soluble salts are very similar throughout the valley. The values range from 0.7119 to 0.7157 and have a mean of 0.7144 + 0.0008 (1 o). The Sr removed b y treating three soils with dilute HC1 has exactly the same average 87Sr/86Sr ratio as Sr liberated by leaching with water. The two soils that were analyzed after removal of leachable salts have 87Sr/86Sr ratios of 0.7148 (near Bull Pass) and 0.7144 (near Wright Lower Glacier). These values are indistinguishable from those of the soil salts and therefore confirm the conclusion of Jones and Faure (1967a) that the Sr in Lake Vanda and in the Onyx River could have been derived from the silicate minerals of the soil by chemical weathering. Since the leachable Sr has the same isotopic composition as that in the silicate minerals of the soil, it follows that Sr isotopes are not fractionated appreciably during chemical weathering of the c o m m o n rock-forming silicate minerals in Wright Valley. This conclusion is consistent with studies by Dasch (1969), Brass (1975) and Fullagar and Ragland (1975) who obtained the same result for a wide range of rock types exposed to weather. ing under a variety of climatic conditions in other areas of the world. All of these studies support the statement that the isotopic composition of Sr in solution in surface water in closed basins with interior drainage is a weighted average of the Sr in the bedrock and overburden. The application of this generalization to the study of non-marine carbonate rocks and evaporites will be developed later in this report. DONJUANPOND Don Juan Pond is located in a small basin at the extreme western end of Wright Valley (Fig. 1). It contains a highly concentrated CaCl~ brine only a few centimetres deep. We measured the Sr content of this brine and found it to be 825.1 ppm, with an 87Sr/86Sr ratio of 0.7183. The basin in which Don Juan Pond is located, is underlain primarily by a granite of Early Paleozoic age (Vida Granite, 481 +- 44 m.y.B.P.). Faure and Jones (1974) analyzed the Sr in seven whole-rock samples of this granite and obtained 87Sr/86Sr ratios ranging from 0.7145 to 0.7232. The 87Sr/86Sr ratio of Don Juan Pond is therefore well within the range of isotope ratios of the most abundant rock unit in its drainage basin. LAKE BONNEY AND THE SOIL OF TAYLOR VALLEY Lake Bonney, shown in Fig. 1, is located at the western end of Taylor Valley a b o u t 30 km from the coast. It is a b o u t 6 km long, up to 1 km wide and has a maximum recorded depth of 32 m. It consists of t w o lobes separated by a narrow channel whose width has increased since the discovery of Lake Bonney by Scott's 1901--1904 expedition (Shirtcliffe, 1964). The widening of the channel is due to recent increases in the level of the lake. Taylor Valley also contains Lake Fryxell (Angino et al., 1962a) and "Suess P o n d " which are
113
located east of Lake Bonney and are closer to the coast (Fig. 1). Lake Bonney is perennially covered by ice whose thickness averages about 4 m. The water temperature increases with depth to a maximum of + 8°C at a depth of about 15 m and then declines toward the bottom where it is --2°C (Angino et al., 1964; Hoare et al., 1964; Torii et al., 1967). The temperature distribution within Lake Bonney probably results from the entrapment of solar radiation by the brine (Shirtcliffe, 1964; Shirtcliffe and Benseman, 1964). Lake Bonney contains a MgC12--NaC1 brine (Angino et al., 1964; Jones, 1969) overlain by fresh water. At a depth of 32 m, the brine contains 26,030 ppm Mg2+; 36,000 ppm Na÷; 1,540 ppm Ca2*; 2,870 ppm K÷; 154,500 ppm C1- and 2,950 ppm SO4:- (Yamagata et al., 1967). Table II lists measurements of density, Sr concentrations, and 87Sr/86Sr ratios for Lake Bonney. The density inTABLE
II
Density, Sr concentrations Depth below surface
a n d 8~Sr/S~Sr r a t i o s i n L a k e B o n n e y , T a y l o r V a l l e y
Density at 20°C
Sr (ppm)
0.9992 0.9987 0.9994 0.9994 1.0023 1.0080 1.0075 1.0283 1.00559 1.0775 1.1103 1.1259 1.1410 1.1509 1.1613 1.1643 1.1699 1.1703 1.1744 1.1744 1.1750 1.1787 1.1764 1.1791 1.1820 1.1820 1.1932
0.7345 0.5827 0.5949 0.8124 0.7150 3.026 3.502 7.066 12.28 15.95 21.84 24.98 33.69 -37.19 -38.89 -38.70 39.37 38.93 39.92 39.02 39.41 38.21 37.02 35.88
s~Sr/S6Sr
(m) 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
0.7130 ------0.7131 ---0.7131 ---0.7129 ---0.7130 ----0.7133 --
a v e r a g e STSr/S6Sr = 0 . 7 1 3 0
± 0.00014
(1 a )
114
I0
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Fig. 2. Variation of the Sr concentration and 87Sr/86Sr ratios of Lake Bonney, Taylor Valley, as functions of depth.
creases from 0.9992 below the ice to a m a x i m u m value of 1.1932 at the b o t t o m (30 m). The concentrations of Sr (as well as those of other elements) vary similarly from 0.7345 to 35.88 ppm (Fig. 2). The water directly under the ice (at 4 m) is slightly denser and contains more Sr than the water 1 m below the ice (at 5 m). We attribute this reversal to the effect of freezing of water at the underside of the permanent ice layer which increases its solute c o n t e n t and hence its density. The denser water presumably sinks until it reaches a level at which it is stable. We also note that the highest Sr c o n t e n t (39.92 ppm) occurs at a depth of 25 m and that the abundance of this element below that depth actually decreases toward the bottom. Since the density of the brine rises smoothly all the way to the bottom, we attribute the decrease of the Sr concentration to the precipitation of an unidentified Sr-bearing phase from the brine. The 87Sr/S6Sr ratios of water at different depths in Lake Bonney are constant and average 0.7130 + 0.00014 (1 a). This value is lower than the STSr/S6Sr ratio of Lake Vanda but is significantly greater than the STSr/86Sr ratios of seawater and the McMurdo Volcanics (Jones and Faure, 1967a). Consequently, the Sr in Lake Bonney was n o t derived from either seawater or from volcanic rocks. Since the 87Sr/S~Sr ratios of both Lake Bonney and Lake Vanda are
115
constant and independent of depth we conclude that either isotopic homogenization has taken place in these lakes or that the Sr entering them has originated primarily from sources whose respective isotopic compositions have remained constant. SOILS IN TAYLOR VALLEY
Taylor Valley differs significantly from Wright Valley in the sense that it slopes through a series of bedrock basins toward the coast and opens onto McMurdo Sound. It is, therefore, much more accessible to marine salts either directly by incursion of seawater or by transport of sea spray through the air. The geology of Taylor Valley is very similar to that of Wright Valley, although minor differences probably exist in the abundances of certain rock types or their mineralogical compositions. (McKelvey and Webb, 1959; Angino et al., 1962b; Hamilton et al., 1962; McCraw, 1962; Hamilton, 1965; Armstrong et al., 1967). Lake Bonney receives meltwater from snow fields and glaciers in its immediate vicinity. In addition, a brine is discharged intermittently from the terminus of Taylor Glacier and results in the formation of the so-called "Taylor Red Cone". {Black et al., 1965; Black, 1969). Recent studies by Weand et al. {1975) suggest that fresh meltwater enters Lake Bonney intermittently at depths below 20 m. Water from the Taylor Red Cone has an 87Sr/S6Sr ratio of 0.7136 (Table III). Sr leached from a soil sample in the immediate vicinity of Lake Bonney also has a ratio of 0.7136, as does the salt-free silicate fraction of that soil. These results suggest that the Sr in Lake Bonney could also have originated as a weathering product of the bedrock and soil in its drainage basin. However, the 87Sr/86Sr ratios of soil salts in Taylor Valley decline systematically in the direction of the coast and become indistinguishable from marine Sr in the vicinity of Lake Fryxell which has a ratio of 0.7090. The striking decrease TABLE III 87Sr/86Sr ratios of water, salts and soil samples, Taylor Valley Sample No.
Locality
87Sr/8'Sr
TRM 66-019 68-001 68-201 68-201 68-202 68-203 68-204
water, "Taylor Red Cone" water, Suess Pond water, Lake Fryxell soil, Lake Bonney, water leach silicate fraction, salt-free soil, LaCroix Glacier, water leach soil, Canada Glacier, water leach soil, Lake Fryxell, water leach
0.7136 0.7112 0.7090 0.7136 0.7136 0.7125 0.7101 0.7092
116
87S¢ / 86Sr
~
9
p
O
~--"Toylor
i
j~ -- Loke
S
Red
_/
Cone =
Bonney
LaCrolx
Glacier
"Sues$
Pond"
Canada Glacier
l
LakeFryxell "
McMurdo Sound
Fig. 3. Systematic decrease of the 878r/86Sr ratio of water-soluble soil, salts and meltwater in a longitudinal (W--E) profile of Taylor Valley.
of the STSr/S6Sr ratios in the soil salts and meltwater is illustrated in Fig. 3. We attribute this p h e n o m e n o n to the presence of Sr of marine origin which becomes dominant in the eastern portion of Taylor Valley. NON-MARINE CARBONATES
We have shown that the Sr in Lake Vanda has the same STSr/86Sr ratio as water-soluble soil salts in Wright Valley and that the Sr in the soil salts is a representative sample of the Sr residing in the silicate minerals of that soil. The results o f this study, together with those of other workers, therefore support the hypothesis that the Sr in saline lakes in closed continental basins is a weighted sample of the isotopic composition of Sr in the rocks and overburden existing in a particular basin (Clauer and Tardy, 1972; Faure and Barrett, 1973). When St-bearing minerals are precipitated from such saline waters, their STSr/S6Sr ratios are the same as those of the aqueous phase because Sr isotopes are n o t measurably fractionated in nature. Consequently, we may conclude that the STSr/S6Sr ratios of non-marine evaporites are representative of the Sr that existed in the rocks and overburden of the drainage basin at the time of deposition. Non-marine carbonate rocks are particularly important in this c o n t e x t because of their abundance in the geologic record and because their R b concentrations are very low. Therefore, the STSr/a6Sr ratios of non-
117
marine carbonate rocks depend only on the isotopic compositions and concentrations of Sr in the rocks and minerals that were weathering in the basin at the time of deposition and do not change subsequently. These considerations suggest that systematic stratigraphic changes of the 87Sr/86Sr ratios of carbonate minerals in non-marine evaporites indicate changes in the geology of the drainage basin and may therefore provide useful information about its history. For example, a decrease in the 87Sr/86Sr ratio of a sequence of non-marine carbonate rocks may be caused by the deposition and weathering of young volcanic rocks which have low 87Sr/86Sr ratios (Faure and Powell, 1972) and thus signals the occurrence of volcanic activity within the drainage basin or in adjacent areas. The 87Sr/86Sr ratio may increase due to unroofing of old crystalline basement rocks or due to enlargement of the watershed to include rocks enriched in radiogenic 87Sr. The presence of marine carbonate rocks is particularly important in controlling the isotopic composition of Sr released by weathering because such rocks have high Sr concentrations and weather more rapidly than silicate rocks. Nevertheless, time-dependent variations in the STSr/86Sr ratio in a sequence of non-marine carbonate rocks can only be caused by changes in the geology of the basin in which they were deposited. Neat (1975) measured STSr/86Sr ratios of carbonate phases from a section of the Flagstaff Formation (Eocene) at Fairview Canyon, near Ephraim, Utah, and reported values ranging from 0.7097 to 0.7126. These preliminary results suggest that the isotopic composition of Sr in Lake Flagstaff varied significantly and may be interpretable in terms of changes in the geology of its drainage basin. ACKNOWLEDGEMENTS
We are grateful to D.D. Koob for sharing with us the water samples he collected from Lake Bonney. R. Montigny, K.R. Everett and R.E. Behling collected the soil samples and C. Bull reviewed a preliminary draft of this paper. This study was supported by the Division of Polar Programs of the National Science Foundation through Grants GA-713, GA-898X, and OPP7200459-A02. REFERENCES Allen, A.D. and Gibson, G.W., 1962. Outline of the geology of the Victoria Valley region. N . Z . J . Geol. Geophys., 5: 234--242. Angino, E.E. and Armitage, K.B., 1963. A geochemical study of lakes Bonney and Vanda, Victoria Land, Antarctica. J. Geol., 71: 89--95. Angino, E.E., Armitage, K.B. and Tash, J.C., 1962a. Chemical stratification in Lake Fryxell, Victoria Land, Antarctica. Science, 138: 34--36. Angino, E.E., Turner, M.D. and Zeller, E.J., 1962b. Reconnaissance geology of lower Taylor Valley, Victoria Land, Antarctica. Geol. Soc. Am. Bull., 73: 1553--1561. Angino, E.E., Armitage, K.B. and Tash, J.C., 1964. Physicochemical limnology of Lake Bonney, Antarctica. Limnol. Oceanogr., 9 : 207--217.
118
Angino, E.E., Armitage, K.B. and Tash, J.C., 1965. A chemical and limnological study of Lake Vanda, Victoria Land, Antarctica. Univ. Kansas Sci. Bull., 45: 1097--1118. Armstrong, R.L. Hamilton, W. and Denton, G.H., 1967. Glaciation in Taylor Valley, Antarctica, older than 2.7 million years. Science, 159: 187. Black, R.F., 1969. Saline discharges from Taylor Glacier, Victoria Land, Antarctica. Antarct. J. U.S., 4 (3): 89--90. Black, R.F. and Bowser, C.J., 1967. Salts and associated phenomena of the termini of the Hobbs and Taylor Glaciers, Victoria Land, Antarctica. Comm. of Snow and Ice, General Assembly, Bern, Sept.--Oct. 1967, pp. 277--338. Black, R.F., Jackson, M.L. and Berg, T.E., 1965. Saline discharge from Taylor Glacier, Victoria Land, Antarctica. J. Geol., 73: 175--181. Boswell, C.R., Brooks, R.R. and Wilson, A.T., 1967. Some trace elements in lakes of McMurdo Oasis, Antarctica. Geochim. Cosmochim. Acta, 31: 731--736. Bowser, C.J., Rafter, T.A. and Black, R.F., 1970. Geochemical evidence for the origin of mirabilite deposits near Hobbs Glacier, Victoria Land, Antarctica. Mineral. Soc. Am., Spec. Pap., 3: 261--272. Brass, G.W., 1975. The effect of weathering on the distribution of strontium isotopes in weathering profiles. Geochim. Cosmochim. Acta, 39: 1647--1654. Bull, C., McKelvey, B.C. and Webb, P.N., 1962. Quaternary glaciations in southern Victoria Land, Antarctica. J. Glaciol., 4: 63--78. Calkin, P.E. and Nichols, R.L., 1972. Quaternary studies in Antarctica. In: R.J. Adie (Editor), Antarctic Geology and Geophysics, Universitetsforlaget, Oslo, pp. 625--643. Clauer, N. and Tardy, Y., 1972. Discrimination between continental and marine sedimentary environments from the isotopic composition of strontium. C.R. Acad. Sci., 273: 2191--2194 Craig, H., 1966. Origin of the saline lakes in Victoria Land, Antarctica. Trans. Am. Geophys. Union, 4 7 : 1 1 2 - - 1 1 3 (abstract). Dasch, E.J., 1969. Strontium isotopes in weathering profiles, deep-sea sediments, and sedimentary rocks. Geochim. Cosmochim. Acta, 33: 1521--1552. Denton, G.H., Armstrong, R.L. and Stuiver, M., 1970. Late Cenozoic glaciation in Antarctica: the record in the McMurdo Sound region. Antarct. J. U.S., 5: 15--21. Dort, Jr., W. and Dort, D.S., 1970a. Sodium sulfate deposits in Antarctica. Modern Geol., 1: 97--117. Dort, Jr., W. and Dortl D.S., 1970b. Low-temperature origin of sodium sulfate deposits, particularly Antarctica. In: J.L. Rau and L.F. Dellwig (Editors), Third Symposium on Salt. North Ohio Geol. Soc., Columbus, Ohio, pp. 181--203. Faure, G. and Barrett, P.J., 1973. Strontium isotope compositions of non-marine carbonate rocks from the Beacon Supergroup of the Transantarctic Mountains. J. Sediment. Petrol., 43: 447--457. Faure, G. and Jones, L.M., 1974. The isotopic composition of strontium and the geological history of the basement rocks of Wright Valley, southern Victoria Land, Antarctica. N . Z . J . Geol. Geophys., 1 7 : 6 1 1 - - 6 2 7 (with an Appendix by L.B. Owen). Faure, G. and Powell, J.L., 1972. Strontium Isotope Geology. Springer, Berlin, 188 pp. Fullagar, P.D. and Ragland, P.C., 1975. Chemical weathering and Rb--Sr whole-rock ages. Geochim. Cosmochim. Acta, 39: 1245--1252. Goldman, C.R., Manson, D.T. and Hobble, J.E., 1967. Two Antarctic desert lakes. Limnol. Oceanogr., 12: 295--310. Gunn, B.M., 1962. Differentiation in Ferrar dolerites, Antarctica. N . Z . J . Geophys., 5: 820--863. Gunn, B.M. and Warren, G., 1962. Geology of Victoria Land between the Mawson and Murdock Glaciers, Antarctica. N.Z. Dep. Sci. Ind. Res.', N.Z. Geol. Surv. Bull., 71, 157 pp. Hamilton, W., 1965. Diabase sheets of the Taylor Glacier region, Victoria Land, Antarctica. U.S. Geol. Surv., Prof. Pap., 456B: 1---71.
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Hamilton, W., Frost, I.C. and Hayes, P.T., 1962. Saline features of a small ice platform in Taylor Valley, Antarctica. U.S. Geol. Surv., Prof. Pap,, 450B: 73--76. Haskell, T.R., Kennett, J.P., Prebble, W.M., Smith, G. and Willis, I.A., G. 1965. The geology of the middle and lower Taylor Valley of South Victoria Land, Antarctica. Trans. R. Soc. N.Z., 2: 169--186. Hoare, R.A., 1966. Problems of heat transfer in Lake Vanda, a density-stratified Antarctica lake. Nature (London), 210: 787--789. Hoare, R.A., 1968. Thermohaline convection in Lake Vanda, Antarctica. J. Geophys. Res., 73: 607--612. Hoare, R.A., Popplewell, K.B., House, D.A., Henderson, R.A., Prebble, W.M. and Wilson, A.T., 1964. Lake Bonney, Taylor Valley, Antarctica: a natural solar-energy trap. Nature (London), 202: 886--888. Hoare, R.A., Popplewell, K.B., House, D.A., Henderson, R.A., Prebble, W.M. and Wilson, A.T., 1965. Solar heating of Lake Fryxell, a permanently ice-covered Antarctic lake. J. Geophys. Res., 70: 1555--1558. Jones, L.M., 1969. The application of strontium isotopes as natural tracers: The origin of the salts in the lakes and soils of southern Victoria Land, Antarctica. Ph.D. dissertation, Ohio State University Columbus, Ohio, 336 pp. Jones, L.M. and Faure, G., 1967a. Origin of the salts in Lake Vanda, Wright Valley, southern Victoria Land, Antarctica. Earth Planet. Sci. Lett., 3: 101--106. Jones, L.M. and Faure, G., 1967b. Age of the Vanda porphyry dikes in Wright Valley, southern Victoria Land, Antarctica. Earth Planet. Sci. Lett., 3: 321--324. Linkletter, G., Bockheim, J. and Ugolini, F.C., 1973. Soils and glacial deposits in the Beacon Valley, southern Victoria Land, Antarctica. N . Z . J . Geol. Geophys., 16: 90--108. McCraw, J.D., 1962. Volcanic detritus in Taylor Valley, Victoria Land, Antarctica. N.Z. J. Geol. Geophys., 5: 740--745. McCraw, J.D., 1967. Soils of Taylor Dry Valley, Victoria Land, Antarctica, with notes on soils from other localities in Victoria Land. N . Z . J . Geol. Geophys., 10: 498--539. McKelvey, B.C. and Webb, P.N., 1959. Geological investigations in south Victoria Land, Antarctica, Part II. Geology of upper Taylor Glacier region. N . Z . J . Geol. Geophys., 2: 718--728. McKelvey, B.C. and Webb, P.N., 1961. Geological reconnaissance in Victoria Land, Antarctica. Nature (London), 189: 545--547. McKelvey, B.C. and Webb, P.N., 1962. Geological investigations in southern Victoria Land, Antarctica, Part 3. Geology of Wright Valley. N . Z . J . Geol. Geophys., 5: 143--162. Morikawa, H., Minato, I. and Ossaka, J., 1975. Origin of magnesium and potassium ions in Lake Vanda, Antarctica. Nature (London), 254: 538--584. Nakai, N., Wada, H., Kiyosu, Y. and Takimoto, M., 1975. Stable isotope studies on the origin and geological history of water and salts in the Lake Vanda area, Antarctica. Geochem. J., 9: 7--24. Neat, P.L., 1975. Strontium-isotope geology of a section of the Flagstaff Formation in Fairview Canyon, Utah. M.Sc. Thesis, Ohio State University, Columbus, Ohio (unpublished). Nichols, R.L., 1962. Geology of Lake Vanda, Wright Valley, South Victoria Land, Antarctica. Am. Geophys. Union Monogr., 7: 47--52. Ragotzkie, R.A. and Friedman, I., 1965. Low deuterium content of Lake Vanda, Antarctica. Science, 148: 1126--1227. Ragotzkie, R.A. and Likens, G.E., 1964. The heat balance of two Antarctic lakes. Limnol. Oceanogr., 9 : 412--425. Shirtcliffe, T.G.L., 1964. Lake Bonney, Antarctica: cause of the elevated temperatures. J. Geophys. Res., 69: 5257--5268. Shirtcliffe, T.G.L. and Benseman, R.F., 1964. A sun-heated Antarctic lake. J. Geophys. Res., 69: 3355--3359. Smith~ G.J., 1965. Evaporite salts from the dry valleys of Victoria Land, Antarctica. N.Z. J. Geol. Geophys., 8: 381--382.
120
Tedrow, J.C.F. and Ugolini, F.C., 1963. An Antarctic saline lake. N . Z . J . Sci., 6: 150--156. Torii, T., Yamagata, N. and Cho, T., 1967. Report of the Japanese summer parties in dry valleys, Victoria Land, 1963--1965, II. General description and water temperature data for the lakes. Antarct. Rec. (Jpn.), 28: 2225--2238. Weand, B.L., Fortner, R.D., Hoehn, R.C. and Parker, B.C., 1975. Subterranean flow into Lake Bonney, Antarct. J. U.S., 10: 15--19. Webb, P.N., 1972. Wright Fjord, Pliocene marine invasion of an Antarctic dry valley. Antarct. J. U.S., 7(6): 226--234. Wetlman, H.W. and Wilson, A.T., 1965. Salt weathering, a neglected geological erosive agent in coastal and arid environments. Nature (London), 205: 1097--1098. Wilson, A.T., 1964. Evidence from chemical diffusion of a climatic change in the McMurdo Dry Valleys 1200 years ago. Nature (London), 201: 176. Wilson, A.T., 1967. The lakes of the McMurdo dry valleys. Tuatara, 15: 152--164. Wilson, A.T. and Wellman, H.W., 1962. Lake Vanda as a solar trap. Nature (London), 196: 1171--1173. Yamagata, N., Torii, T. and Murata, S., 1967. Report of the Japanese summer parties in the dry valleys, Victoria Land, 1963--1965, V. Chemical composition of lake waters. Antarct. Rec. (Jpn.), 29: 2339--2361.
107
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A STUDY OF STRONTIUM ISOTOPES IN LAKES AND SURFICIAL DEPOSITS OF THE ICE-FREE VALLEYS, SOUTHERN VICTORIA LAND, ANTARCTICA*
LOIS M. JONES and G U N T E R F A U R E
Department o f Geology and Mineralogy and Institute o f Polar Studies, The Ohio State University, Columbus, OH 43210 (U.S.A.) (Received December 15, 1976; accepted for publication May 13, 1977)
ABSTRACT Jones, L.M. and Faure, G., 1978. A study of strontium isotopes in lakes and surficial deposits of the ice-free valleys, southern Victoria Land, Antarctica. Chem. Geol., 22 : 107--120. The ice-free valleys of southern Victoria Land, Antarctica, contain saline lakes including Lake Vanda in Wright Valley and Lake Bonney in Taylor Valley. The source of the salts dissolved in the brines has not yet been identified. Strontium in water-soluble salts of soil samples in Wright Valley has STSr/8'Sr ratios ranging from 0.7119 to 0.7157 with an average of 0.7144 + 0.0008 (1 a). This value is very similar to the 8~Sr/S6Sr ratio of brines in Lake Vanda (STSr/86Sr = 0.7149 +- 0.00017), but differs significantly from the 87Sr/8~Sr ratios of seawater (0.7094 -+ 0.00012) and basaltic rocks of the McMurdo Volcanics (0.7044 _+ 0.00046). The Sr in Lake Vanda therefore could not have originated from seawater or from volcanic rocks of the area, but may have been derived by chemical weathering of the igneous and metamorphic rocks exposed in Wright Valley. The 87Sr/86Sr ratios of Lake Bonney are invariant with depth and average 0.7130 -+ 0.00014 (1 ~), whereas the Sr concentrations increase from 0.7345 ppm near the surface to a maximum of 39.92 ppm at a depth of 25 m. The Sr concentrations at greater depth decrease slightly to 35.88 ppm at 30 m. The Sr in water-soluble soil salts near Lake Bonney and of the Taylor Red Cone is similar isotopically to the Sr in the lake. The 87Sr/86Sr ratios of soil salts and of meltwater in Taylor Valley decrease systematically from Lake Bonney toward the coast. The water in Lake Fryxell, which is closest to the coast, has an 87Sr/8'Sr ratio of 0.7090 and is identical to that of seawater. The evidence from this and similar studies in other parts of the world indicates that the 87Sr/8'Sr ratios of saline brines in closed continental basins are representative of the Sr in the rocks underlying the basin. The Sr in carbonate and sulfate minerals precipitated from such brines preserves the 87Sr/8'Sr ratios that existed at the time of deposition. Because of these relationships we suggest that stratigraphic variations of the 87Sr/8'Sr ratios of non-marine carbonate or sulfate rocks reflect changes in the geology of the drainage basin. The isotopic composition of Sr of such rocks may therefore provide useful information about the geologic histories of continental basins.
*Laboratory for Isotope Geology and Geochemistry Contribution No. 24.
108 REVIEW OF PREVIOUS STUDIES The ice-free valleys of southern Victoria Land (Fig. 1) contain several lakes and ponds including Lake Vanda and Don Juan Pond in Wright Valley and Lake Bonney in Taylor Valley (Tedrow and Ugolini, 1963; Goldman et al., 1967; Wilson, 1967). Both lakes are permanently ice-covered b u t contain h o t brines at depth whose concentrations and densities greatly exceed those of seawater. The elevated temperatures are attributable to the entrapment of solar radiation (Wilson and Wellman, 1962; Hoare et al., 1964, 1965; Ragotzkie and Likens, 1964; Shirtcliffe, 1964; Shirtcliffe and Benseman, 1964; Hoare, 1966, 1968). However, the source of the salts and the origin and evolution of the brines in the lakes are still uncertain (Angino and Armitage, 1963; Angino et al., 1964; Wilson, 1964). Several investigators have used ratios of cations and/or anions to determine the source of the salts in Lake Vanda, but have reached contradictory conclusions. Angino et al. (1965) concluded that the brines in Lake Vanda were discharged b y hot springs, whereas Boswell et al. (1967) cited ratios of several trace metals (Zn, Pb, Bi, Fe, Mn and Mo) to support their suggestion that the brines originated as glacial meltwater. Yamagata et al. (1967) reported concentrations of halogens and alkali metals, b u t drew no firm conclusions, whereas Morikawa et al. (1975) favored a marine source for Mg 2÷ and K ÷ in Lake Vanda. The question regarding the origin of the brines has also been investigated by means of isotopic data. Ragotzkie and Friedman (1965) reported D/H ratios for Lake Vanda and concluded that the water is of non-marine origin. However, Craig (1966) disputed this interpretation and stated that the data do not exclude a marine source for the brine. More recently, Nakai et al. (1975) reported 5'sO values of a b o u t - - 3 1 % 0 which indicates a non-marine source for the water. However, 534S values of sulfate in the brines and 513C and 61so values of calcite in the sediment are not easily reconciled with a simple history of progressive concentration of glacial meltwater. Measurements of 234U/23sU activity ratios by D.L. Thurber of brines taken at different depths in Lake Vanda, yielded values of 2.5 (10 m), 3.3 (50 m), and 4.5 (57 m) (E.E. Angino, pets. commun., 1968. The presence of excess 234U indicates that the U originated as a weathering product of U-bearing minerals in the watershed. Jones and Faure (1967a) measured STSr/S68r ratios and Sr concentrations in a suite of water samples taken at different depths in Lake Vanda. Their results showed that the Sr concentrations increase from 0.141 ppm just below the ice to 67.1 ppm near the b o t t o m of the lake at a depth of 60 m. However, the STSr/S6Sr ratios of the brines are c o n s t a n t throughout and average 0.7149 -+ 0.00017 (1 a). Jones and Faure (1967a) also analyzed Sr in seawater from the Ross Sea (STSr/S6Sr = 0.7094 + 0.00012), in Tertiary basalts of the McMurdo Volcanic Province (STSr/S6Sr = 0.7044 + 0.00046), and concluded that the Sr in Lake Vanda could n o t have originated from seawater or from the volcanic rocks. On the other hand, their analyses showed that the Sr in the Onyx River (Fig. 1) and in soil salts have S~Sr/S6Sr ratios that are identical to that of Lake
45 162"
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Vanda. The silicate fraction of one soil sample from Wright Valley was found to have the same 87Sr/86Sr ratio (0.7148) as salts leached from that soil with dilute HC1 (pH = 3--4). These results demonstrated that the Sr in Lake Vanda could have been derived predominantly by chemical weathering of bedrock and regolith in Wright Valley. The apparent unimportance of seawater or of marine deposits as sources of Sr in the brines of Lake Vanda requires c o m m e n t in view of the conclusion of Webb (1972) that Wright Valley was a fiord during the Pliocene Epoch. When uplift occurred in latest Pliocene or earliest Pleistocene time (Victoria Orogeny), the basin of Lake Vanda was presumably occupied by trapped seawater. We believe that this water evaporated and may have deposited a thin layer of salts containing marine Sr. This deposit was subsequently covered with detrital sediment and non-marine evaporites which have prevented the marine Sr from entering the brines that now occur in Lake Vanda. We plan to measure 87Sr/S6Sr ratios of evaporite minerals in sediment cores from Lake Vanda in order to identify the marine c o m p o n e n t whose presence is implied by the geologic history of Wright Valley as it is currently understood. We now report additional measurements of STSr/86Sr ratios of leachable soil salts from Wright Valley in order to d o c u m e n t the importance of chemical weathering to the isotopic composition of Sr in water and leachable salts in this closed continental basins. We also present data for Lake Bonney and for soils and meltwater ponds in Taylor Valley, located a b o u t 25 km south of Wright Valley (Fig. 1). The results of this study are applicable to the interpretation of 87Sr/S6Sr ratios of saline lakes in continental basins within which non-marine carbonate rocks m a y be deposited. The concentrations of Sr were measured by isotope dilution using a calibrated spike solution enriched in ~6Sr. Measurements of 87Sr/~6Sr ratios were made on unspiked samples on a solid-source mass spectrometer (Nuclide ® , model 6-60-S). Duplicate determinations of isotope ratios of three brine samples from Lake Bonney indicate a reproducibility of -+ 1.3 • 10 -4 (1 G). The Eimer & Amend ® SrCO3 isotope standard was analyzed repeatedly during the course of this study and has an STSr/86Sr ratio of 0.70825 + 0.00044 (1 o). All measured values of the STSr/S6Sr ratio were corrected for isotope fractionation to a standard value of 0.1194 for the S6Sr/SSSr ratio. The analytical procedures were described in more detail by Jones {1969). A general account of the isotope geology of Sr was published by Faure and Powell (1972). SOILS OF WRIGHT VALLEY Wright Valley is underlain primarily by igneous and metamorphic rocks described by McKelvey and Webb (1961, 1962), Allen and Gibson (1962), Bull et ah (1962), Gunn and Warren (1962), Nichols (1962), and by Haskell et al. (1965). These rocks were metamorphosed in Early Paleozoic time (Faure and Jones, 1974; Jones and Faure, 1967b) and are u n c o n f o r m a b l y overlain by sandstones and shales of Middle to Late Paleozoic age (Beacon Supergroup).
IIi
In addition, there are several large sills of the Ferrar Dolerite of Jurassic age {Gunn, 1962). The youngest rocks in Wright Valley are small basaltic cinder cones of Late Tertiary age. The b o t t o m of Wright Valley is covered by a blanket of unconsolidated rock debris most of which was derived from the bedrock exposed higher up on the valley walls and the surrounding mountain ranges (McCraw, 1967; Calkin and Nichols, 1972; Linkletter et al., 1973). This "soil" contains small lenticular deposits and efflorescences of salt which are c o m m o n throughout the area and consist primarily of mirabilite (Na2SO4 • 10H20) and thenardite (Na2SO4) (Smith, 1965; Wellman and Wilson, 1965). The occurrence and origin of these salts has been discussed more recently by Black and Bowser (1967), Bowser et al. (1970) and Dort and Dort (1970a, b). Most of the salt deposits that have been studied in detail are located close to the coast and are generally believed to be of marine origin. Wright Valley is separated from the coast by the Wilson Piedmont Glacier (Fig. 1) and actually slopes a w a y from the coast. The salt in the soil of this valley, therefore probably does n o t have a direct marine source. Table I contains S~Sr/S6Sr ratios for sixteen samples of soil and meltwater collected along the entire length of Wright Valley (Fig. 1). Most of the soil samples were leached with double-distilled and demineralized water in order to simulate the removal of Sr from the soil by glacial meltwater. Several samples were treated with dilute V y c o r ® -distilled HC1 and two soils were TABLEI STSr/S'Sr ratios of soluble
salts in the soil of Wright Valley
Sample No.
Locality
Water-soluble soil salts
HCl-soluble soil salts
Silicate fraction (salt-free)
201-V9 201-V16 206
18 m N of Lake Vanda 100 m N of Lake Vanda between Lake Vanda and Bull Pass Bull Pass east of Bull Pass near Meserve Glacier near Meserve Glacier near Meserve Glacier near Meserve Glacier near Meserve Glacier near Bartley Glacier eastern Wright Valley eastern Wright Valley Wright Lower Glacier Clark Glacier water, Goodspeed Glacier
0.7144 0.7148
---
---
-0.7148 -0.7144 0.7145 0.7142 0.7142 0.7157 -0.7145 0.7145 0.7149 0.7119 0.7145
0.7144" -0.7148" -----0.7141 ------
--0.7148" --------0.7144 ---
BP-0-12 207 202 9-2-12 23-2-9 23-9-19 24-7-36 208 204 205 0-12 203 015 *Reported
by Jones
and Faure
(1967a).
112
analyzed in bulk, after prior removal of water-soluble salts. The results indicate that the 87Sr/86Sr ratios of water-soluble salts are very similar throughout the valley. The values range from 0.7119 to 0.7157 and have a mean of 0.7144 + 0.0008 (1 o). The Sr removed b y treating three soils with dilute HC1 has exactly the same average 87Sr/86Sr ratio as Sr liberated by leaching with water. The two soils that were analyzed after removal of leachable salts have 87Sr/86Sr ratios of 0.7148 (near Bull Pass) and 0.7144 (near Wright Lower Glacier). These values are indistinguishable from those of the soil salts and therefore confirm the conclusion of Jones and Faure (1967a) that the Sr in Lake Vanda and in the Onyx River could have been derived from the silicate minerals of the soil by chemical weathering. Since the leachable Sr has the same isotopic composition as that in the silicate minerals of the soil, it follows that Sr isotopes are not fractionated appreciably during chemical weathering of the c o m m o n rock-forming silicate minerals in Wright Valley. This conclusion is consistent with studies by Dasch (1969), Brass (1975) and Fullagar and Ragland (1975) who obtained the same result for a wide range of rock types exposed to weather. ing under a variety of climatic conditions in other areas of the world. All of these studies support the statement that the isotopic composition of Sr in solution in surface water in closed basins with interior drainage is a weighted average of the Sr in the bedrock and overburden. The application of this generalization to the study of non-marine carbonate rocks and evaporites will be developed later in this report. DONJUANPOND Don Juan Pond is located in a small basin at the extreme western end of Wright Valley (Fig. 1). It contains a highly concentrated CaCl~ brine only a few centimetres deep. We measured the Sr content of this brine and found it to be 825.1 ppm, with an 87Sr/86Sr ratio of 0.7183. The basin in which Don Juan Pond is located, is underlain primarily by a granite of Early Paleozoic age (Vida Granite, 481 +- 44 m.y.B.P.). Faure and Jones (1974) analyzed the Sr in seven whole-rock samples of this granite and obtained 87Sr/86Sr ratios ranging from 0.7145 to 0.7232. The 87Sr/86Sr ratio of Don Juan Pond is therefore well within the range of isotope ratios of the most abundant rock unit in its drainage basin. LAKE BONNEY AND THE SOIL OF TAYLOR VALLEY Lake Bonney, shown in Fig. 1, is located at the western end of Taylor Valley a b o u t 30 km from the coast. It is a b o u t 6 km long, up to 1 km wide and has a maximum recorded depth of 32 m. It consists of t w o lobes separated by a narrow channel whose width has increased since the discovery of Lake Bonney by Scott's 1901--1904 expedition (Shirtcliffe, 1964). The widening of the channel is due to recent increases in the level of the lake. Taylor Valley also contains Lake Fryxell (Angino et al., 1962a) and "Suess P o n d " which are
113
located east of Lake Bonney and are closer to the coast (Fig. 1). Lake Bonney is perennially covered by ice whose thickness averages about 4 m. The water temperature increases with depth to a maximum of + 8°C at a depth of about 15 m and then declines toward the bottom where it is --2°C (Angino et al., 1964; Hoare et al., 1964; Torii et al., 1967). The temperature distribution within Lake Bonney probably results from the entrapment of solar radiation by the brine (Shirtcliffe, 1964; Shirtcliffe and Benseman, 1964). Lake Bonney contains a MgC12--NaC1 brine (Angino et al., 1964; Jones, 1969) overlain by fresh water. At a depth of 32 m, the brine contains 26,030 ppm Mg2+; 36,000 ppm Na÷; 1,540 ppm Ca2*; 2,870 ppm K÷; 154,500 ppm C1- and 2,950 ppm SO4:- (Yamagata et al., 1967). Table II lists measurements of density, Sr concentrations, and 87Sr/86Sr ratios for Lake Bonney. The density inTABLE
II
Density, Sr concentrations Depth below surface
a n d 8~Sr/S~Sr r a t i o s i n L a k e B o n n e y , T a y l o r V a l l e y
Density at 20°C
Sr (ppm)
0.9992 0.9987 0.9994 0.9994 1.0023 1.0080 1.0075 1.0283 1.00559 1.0775 1.1103 1.1259 1.1410 1.1509 1.1613 1.1643 1.1699 1.1703 1.1744 1.1744 1.1750 1.1787 1.1764 1.1791 1.1820 1.1820 1.1932
0.7345 0.5827 0.5949 0.8124 0.7150 3.026 3.502 7.066 12.28 15.95 21.84 24.98 33.69 -37.19 -38.89 -38.70 39.37 38.93 39.92 39.02 39.41 38.21 37.02 35.88
s~Sr/S6Sr
(m) 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
0.7130 ------0.7131 ---0.7131 ---0.7129 ---0.7130 ----0.7133 --
a v e r a g e STSr/S6Sr = 0 . 7 1 3 0
± 0.00014
(1 a )
114
I0
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/ C~l
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d
d
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i I0
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STRONTIUM,
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87Sr/Sesr
Fig. 2. Variation of the Sr concentration and 87Sr/86Sr ratios of Lake Bonney, Taylor Valley, as functions of depth.
creases from 0.9992 below the ice to a m a x i m u m value of 1.1932 at the b o t t o m (30 m). The concentrations of Sr (as well as those of other elements) vary similarly from 0.7345 to 35.88 ppm (Fig. 2). The water directly under the ice (at 4 m) is slightly denser and contains more Sr than the water 1 m below the ice (at 5 m). We attribute this reversal to the effect of freezing of water at the underside of the permanent ice layer which increases its solute c o n t e n t and hence its density. The denser water presumably sinks until it reaches a level at which it is stable. We also note that the highest Sr c o n t e n t (39.92 ppm) occurs at a depth of 25 m and that the abundance of this element below that depth actually decreases toward the bottom. Since the density of the brine rises smoothly all the way to the bottom, we attribute the decrease of the Sr concentration to the precipitation of an unidentified Sr-bearing phase from the brine. The 87Sr/S6Sr ratios of water at different depths in Lake Bonney are constant and average 0.7130 + 0.00014 (1 a). This value is lower than the STSr/S6Sr ratio of Lake Vanda but is significantly greater than the STSr/86Sr ratios of seawater and the McMurdo Volcanics (Jones and Faure, 1967a). Consequently, the Sr in Lake Bonney was n o t derived from either seawater or from volcanic rocks. Since the 87Sr/S~Sr ratios of both Lake Bonney and Lake Vanda are
115
constant and independent of depth we conclude that either isotopic homogenization has taken place in these lakes or that the Sr entering them has originated primarily from sources whose respective isotopic compositions have remained constant. SOILS IN TAYLOR VALLEY
Taylor Valley differs significantly from Wright Valley in the sense that it slopes through a series of bedrock basins toward the coast and opens onto McMurdo Sound. It is, therefore, much more accessible to marine salts either directly by incursion of seawater or by transport of sea spray through the air. The geology of Taylor Valley is very similar to that of Wright Valley, although minor differences probably exist in the abundances of certain rock types or their mineralogical compositions. (McKelvey and Webb, 1959; Angino et al., 1962b; Hamilton et al., 1962; McCraw, 1962; Hamilton, 1965; Armstrong et al., 1967). Lake Bonney receives meltwater from snow fields and glaciers in its immediate vicinity. In addition, a brine is discharged intermittently from the terminus of Taylor Glacier and results in the formation of the so-called "Taylor Red Cone". {Black et al., 1965; Black, 1969). Recent studies by Weand et al. {1975) suggest that fresh meltwater enters Lake Bonney intermittently at depths below 20 m. Water from the Taylor Red Cone has an 87Sr/S6Sr ratio of 0.7136 (Table III). Sr leached from a soil sample in the immediate vicinity of Lake Bonney also has a ratio of 0.7136, as does the salt-free silicate fraction of that soil. These results suggest that the Sr in Lake Bonney could also have originated as a weathering product of the bedrock and soil in its drainage basin. However, the 87Sr/86Sr ratios of soil salts in Taylor Valley decline systematically in the direction of the coast and become indistinguishable from marine Sr in the vicinity of Lake Fryxell which has a ratio of 0.7090. The striking decrease TABLE III 87Sr/86Sr ratios of water, salts and soil samples, Taylor Valley Sample No.
Locality
87Sr/8'Sr
TRM 66-019 68-001 68-201 68-201 68-202 68-203 68-204
water, "Taylor Red Cone" water, Suess Pond water, Lake Fryxell soil, Lake Bonney, water leach silicate fraction, salt-free soil, LaCroix Glacier, water leach soil, Canada Glacier, water leach soil, Lake Fryxell, water leach
0.7136 0.7112 0.7090 0.7136 0.7136 0.7125 0.7101 0.7092
116
87S¢ / 86Sr
~
9
p
O
~--"Toylor
i
j~ -- Loke
S
Red
_/
Cone =
Bonney
LaCrolx
Glacier
"Sues$
Pond"
Canada Glacier
l
LakeFryxell "
McMurdo Sound
Fig. 3. Systematic decrease of the 878r/86Sr ratio of water-soluble soil, salts and meltwater in a longitudinal (W--E) profile of Taylor Valley.
of the STSr/S6Sr ratios in the soil salts and meltwater is illustrated in Fig. 3. We attribute this p h e n o m e n o n to the presence of Sr of marine origin which becomes dominant in the eastern portion of Taylor Valley. NON-MARINE CARBONATES
We have shown that the Sr in Lake Vanda has the same STSr/86Sr ratio as water-soluble soil salts in Wright Valley and that the Sr in the soil salts is a representative sample of the Sr residing in the silicate minerals of that soil. The results o f this study, together with those of other workers, therefore support the hypothesis that the Sr in saline lakes in closed continental basins is a weighted sample of the isotopic composition of Sr in the rocks and overburden existing in a particular basin (Clauer and Tardy, 1972; Faure and Barrett, 1973). When St-bearing minerals are precipitated from such saline waters, their STSr/S6Sr ratios are the same as those of the aqueous phase because Sr isotopes are n o t measurably fractionated in nature. Consequently, we may conclude that the STSr/S6Sr ratios of non-marine evaporites are representative of the Sr that existed in the rocks and overburden of the drainage basin at the time of deposition. Non-marine carbonate rocks are particularly important in this c o n t e x t because of their abundance in the geologic record and because their R b concentrations are very low. Therefore, the STSr/a6Sr ratios of non-
117
marine carbonate rocks depend only on the isotopic compositions and concentrations of Sr in the rocks and minerals that were weathering in the basin at the time of deposition and do not change subsequently. These considerations suggest that systematic stratigraphic changes of the 87Sr/86Sr ratios of carbonate minerals in non-marine evaporites indicate changes in the geology of the drainage basin and may therefore provide useful information about its history. For example, a decrease in the 87Sr/86Sr ratio of a sequence of non-marine carbonate rocks may be caused by the deposition and weathering of young volcanic rocks which have low 87Sr/86Sr ratios (Faure and Powell, 1972) and thus signals the occurrence of volcanic activity within the drainage basin or in adjacent areas. The 87Sr/86Sr ratio may increase due to unroofing of old crystalline basement rocks or due to enlargement of the watershed to include rocks enriched in radiogenic 87Sr. The presence of marine carbonate rocks is particularly important in controlling the isotopic composition of Sr released by weathering because such rocks have high Sr concentrations and weather more rapidly than silicate rocks. Nevertheless, time-dependent variations in the STSr/86Sr ratio in a sequence of non-marine carbonate rocks can only be caused by changes in the geology of the basin in which they were deposited. Neat (1975) measured STSr/86Sr ratios of carbonate phases from a section of the Flagstaff Formation (Eocene) at Fairview Canyon, near Ephraim, Utah, and reported values ranging from 0.7097 to 0.7126. These preliminary results suggest that the isotopic composition of Sr in Lake Flagstaff varied significantly and may be interpretable in terms of changes in the geology of its drainage basin. ACKNOWLEDGEMENTS
We are grateful to D.D. Koob for sharing with us the water samples he collected from Lake Bonney. R. Montigny, K.R. Everett and R.E. Behling collected the soil samples and C. Bull reviewed a preliminary draft of this paper. This study was supported by the Division of Polar Programs of the National Science Foundation through Grants GA-713, GA-898X, and OPP7200459-A02. REFERENCES Allen, A.D. and Gibson, G.W., 1962. Outline of the geology of the Victoria Valley region. N . Z . J . Geol. Geophys., 5: 234--242. Angino, E.E. and Armitage, K.B., 1963. A geochemical study of lakes Bonney and Vanda, Victoria Land, Antarctica. J. Geol., 71: 89--95. Angino, E.E., Armitage, K.B. and Tash, J.C., 1962a. Chemical stratification in Lake Fryxell, Victoria Land, Antarctica. Science, 138: 34--36. Angino, E.E., Turner, M.D. and Zeller, E.J., 1962b. Reconnaissance geology of lower Taylor Valley, Victoria Land, Antarctica. Geol. Soc. Am. Bull., 73: 1553--1561. Angino, E.E., Armitage, K.B. and Tash, J.C., 1964. Physicochemical limnology of Lake Bonney, Antarctica. Limnol. Oceanogr., 9 : 207--217.
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