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Cl-37 in the Dead Sea system---preliminary results
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Applied Geochemistry, Vol. 13, No. 8, pp. 953±960, 1998 # 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain S0883-2927(98)00017-1 0883-2927/98 $ - see front matter
Cl-37 in the Dead Sea systemÐpreliminary results Mariana Stiller and Arie Nissenbaum* Weizmann Institute of Science, 76100 Rehovot, Israel
Ronald S. Kaufmann Department of Geology, Northern Illinois University, DeKalb, IL 60511, U.S.A.
Austin Long Department of Geosciences, University of Arizona, Tucson, AZ 85721, U.S.A. (Received 7 January 1997; accepted in revised form 10 November 1997) AbstractÐThis study presents the ®rst set of d37 Cl measurements in the Dead Sea environment. d37 Cl values for the meromictic (long term strati®ed) Dead Sea water column prior to its complete overturn in 1979 were ÿ0.47- SMOC for the UWM (Upper Water Mass) and +0.55- SMOC for the LWM (Lower Water Mass). The d37 Cl values for the pre-overturn Dead Sea cannot be explained by the prevailing model on the evolution of the Dead Sea during the last few centuries and require corroboration by more measurements. The 1979 overturn wiped out almost completely the isotopic dierences between the UWM and LWM. Even so, Cl isotope data could be used to decipher physical processes related to the overturn such as incomplete homogenization of the deep water mass. Inputs into the lake, comprising freshwaters (springs and the Jordan River) and saline springs gave a range of ÿ0.37to +1.0- with the freshwater sources being more enriched in d37 Cl. Based on the d37 Cl measurements of the End-Brine (the euent from Dead Sea evaporation ponds) and of recent Dead Sea halite, the Cl isotopic composition of the originating brines have been estimated. They gave a narrow isotopic spread, +0.01- and +0.07- and fall within the same range with Dead Sea pore water (+0.13-) and with the post-overturn Dead Sea (ÿ0.03- and +0.16-). Rock salt from Mount Sdom gave a value of ÿ0.59- indicating its formation at the last stages of halite deposition from evaporating sea water. The hypersaline En Ashlag spring gave a depleted d37 Cl value of ÿ0.32-, corresponding to a residual brine formed in the very latest stages (including bisho®te deposition) of seawater evaporation. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
The study of Cl stable isotope distribution has become a valuable tool for the study of formation waters (Kaufmann et al., 1988, 1993) and the quantitative evaluation of mixing between dierent sources of chloride in brines and aquifers (Long et al., 1993). The unique environment of the Dead Sea, a chloridic hypersaline lake, presents a case study for the applicability of this new tool. The question of how did the present-day hypersaline Dead Sea acquire its chemical composition has been debated for many years. A range of hypotheses has been proposed. Lowengart (1962) suggested that the salts are of atmospheric origin; Bentor (1961) proposed that the quantity of most salts can be accounted for by inputs from the Jordan River and surface water sources; while Starinsky (1974) has described the various stages by which Pliocene *Corresponding author. 953
seawater had supposedly evolved into Dead Sea brines. The current paradigm on the source of salts in the lake is that the brine has multiple sources, and it is quite probable that for determining the relative contribution of each one a multi-parameter approach will be needed. This report, based on a very limited number of samples, is intended to investigate the capabilities of one such parameter, the stable Cl isotopes, which has not been previously applied to this system. The Dead Sea Basin is located in the deepest part of the East African Rift Valley. The Dead Sea itself is the lowest exposed surface on the face of the earth, with the lake level being today, in the mid1990s, around 410 m below mean sea level (MSL). The Dead Sea is a terminal lake fed by the Jordan River, by surface runo during the wet season and by a few perennial, fresh and saline springs and streams (Fig. 1); it is a 320 m deep water body, characterized by its extreme hypersalinity. The average total dissolved salt content of the lake in 1990
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Fig. 1. Map of the Dead Sea with location of in¯ows and sampling stations of Dead Sea brines and cores. The depth contours for the Dead Sea are in meters below mean sea level (MSL). The locations of Mount Sdom and En Ashlag are also shown.
was 340 g lÿ1. The chemical composition is unusual by having Mg as the dominant cation (46.4 g lÿ1) followed by Na (36.7 g lÿ1), Ca (17.1 g lÿ1) and K (7.9 g lÿ1). The dominant anion is, by far, Cl (225.6 g lÿ1), distantly followed by Br (5.46 g lÿ1). Sulfate and CO3 are very minor (Beyth et al., 1990; Gavrieli, 1997) at less than 1.0 g lÿ1. The Dead Sea location in the center of an active tectonic rift results in a very complex hydrological situation, and the springs along its coast demonstrate an extreme range of salinities, from freshwater (e.g. Nahal Arugot) with a chloride content of 112 mg lÿ1 to hypersaline brines such as En
Ashlag with 260 g Cl lÿ1, thus spanning a range of 3 orders of magnitude. The Dead Sea basin has been the locus of a series of paleo-lakes which existed there since the Pliocene. These lakes were of varying salinities, but some of them were hypersaline enough to precipitate gypsum and halite. It has been proposed that some of the salts in the Dead Sea and associated saline springs originate from dissolution of these evaporates. It is also possible that residual brine from the Lisan Lake which preceded the Dead Sea and disappeared not earlier than 18 000 a ago (Begin et al., 1985; Kaufman et al., 1992) may have contributed salts to the present lake.
Cl-37 in the Dead Sea system METHODOLOGY Description of samples Samples for this study were selected as to represent the brines of the Dead Sea and the wide spectrum of salinities of the in¯owing waters (Fig. 1). Samples from the Dead Sea water column are from two dierent periods in its recent history. The samples from 1976 represent a period when the lake was permanently strati®ed (meromictic) with a slightly less saline layer (273 g dissolved salts kgÿ1) ¯oating over a denser, anoxic layer (276 g dissolved salts kgÿ1). The salinity was uniform in the upper 80 m and from 100 m to the bottom with a 20 m thick transition layer (Steinhorn, 1985). In February 1979 the lake level dropped to ÿ403.5 m below MSL, the water column overturned and became completely mixed (Steinhorn et al., 1979). The Dead Sea samples from 1981 and 1982 represent the post-overturn situation (Anati et al., 1987). The End-Brine sample represents Dead Sea brine from which part of the chloride has been removed by precipitation, ®rst as halite and then as carnallite. The End-Brine sample was taken from a carnallite evaporation pond that is used by the ``Dead Sea Works'' Company in the process of potash production, and is situated near the southern end of the lake. Pore water brines are represented by a sample from core B1 (Stiller et al., 1983) which was taken from the northern part of the deep water basin (Fig. 1). The Nahal Arugot, En Bokek and the Jordan River water are taken as representative of freshwater inputs into the lake. En Feshka is a complex of saline springs with varying salinities (Mazor and Molcho, 1972) to which a mixed salt source has been attributed. Hamme Mazor is a warm (428C) hypersaline brine, with about 1/2 the salinity of the Dead Sea which is typical of several springs of varying, but high, salinities, which issue along the Dead Sea coast (Mazor et al., 1969). The En Ashlag brine is from a small seepage near the southern basin of the Dead Sea and is associated with the Mt. Sdom diapir. This seepage has a salinity exceeding that of the Dead Sea but with a dierent chemistry (Kawamura and Nissenbaum, 1992). Two halite samples were also analyzed in the present study. One, halite crystals found at 20 cm depth in a sediment core taken in 1981 (Levy, 1988) and one from a
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quarry of rock salt from the exposed Mt. Sdom diapir, near the southern part of the Dead Sea (Fig. 1). This latter salt sample is assumed to have originated from the Sdom Formation of Pliocene (Zak, 1967) or Miocene age (Steinitz and Bartov, 1991; Tannenbaum and Charrach, 1993).
Analytical procedures Isotopic measurements of stable Cl isotopes used methyl chloride (CH3Cl) produced as follows: with the sample heated to near boiling, the chloride was precipitated as AgCl by addition of 0.1 M silver nitrate solution, approximately 8% more than is required for complete precipitation. The precipitate was then quantitatively reacted with CH3I for two days at 1208C. The CH3I was puri®ed by gas chromatography and the isotopic composition + measured as CH337 Cl+/CH35 on a VG 602 mass spec3 Cl trometer with double Faraday cups and a dual inlet port. The precision of the method based on averaging dierences of replicate samples is 0.1-. The results of the measurements are reported with respect to standard mean ocean chloride (SMOC).
RESULTS AND DISCUSSION
The Dead Sea system Inputs into the Dead Sea. The number of samples analyzed is far too small for a high resolution evaluation of the source of salts in the Dead Sea in¯ows. However, the 37 Cl data does provide partial information on some of these sources. The two samples of the Jordan River water have d37 Cl values of +0.63- and +0.39- and the other freshwater in¯ows into the Dead Sea, Nahal Arugot and En Boqeq, +1.1- and +0.27-, respectively (Table 1). The inverse relationship which is apparent between d37 Cl and salinity (Fig. 2)
Table 1. Chlorine stable isotopes in the Dead Sea system Sample type Freshwater Freshwater Brine Brine Brine Brine Brine Halite Brine Freshwater Freshwater Brackish Brine Brine Halite 1
Sample description
Date of sampling
The Jordan in¯ow and the Dead Sea Jordan River, Abdallah Bridge Feb. 18, 1982 Jordan River, Abdallah Bridge July 27, 1995 Dead Sea, 25 m depth (pre-overturn1) June 25, 1976 Dead Sea, 310 m depth (pre-overturn) June 25, 1976 Dead Sea, 10 m depth (post-overturn1) Nov. 23, 1981 Dead Sea, 270 m depth (post-overturn) Nov. 23, 1982 Dead Sea ± End-Brine July 1982 1981 Crystals at 20 cm depth in Core EG2502 3 Pore water from 155±160 cm depth in Core B1 Dec. 1980 Other in¯ows to the Dead Sea Nahal Arugot Jan. 24, 1983 En Boqeq spring 1984 En Feshka spring 1984 Hamme Mazor spring 1984 Mount Sdom environment En Ashlag June 21, 1984 Rock Salt from Mount Sdom
Cl mg lÿ1
d37 Cl- SMOC
440 2400 220 300 223 300 219 200 224 600 344 800 224 800
+0.63 (20.04) +0.39 +0.55 (20.04) ÿ0.47 (20.01) ÿ0.03 (20.04) +0.16 (20.04) ÿ0.08 +0.33 +0.13
112 525 1300 93 700
+1.0120.20 +0.27 ÿ0.37 0.00
260 000
ÿ0.3220.02 ÿ0.59
Pre-overturn and post-overturn refer to the overturn of February 1979 (Steinhorn et al., 1979). Sediment core taken from the Dead Sea by Y. Levy at a water depth of 250 m (Levy, 1988). Core B1 was taken at a water depth of 320 m (Stiller et al., 1983) in December 1980.
2 3
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Fig. 2. The relationship between the Cl content (mg lÿ1), expressed as 1/Cl, and d37 Cl in in¯ows to the Dead Sea. J stands for the Jordan River, A for Nahal Arugot, EBq for En Boqeq, EF for En Feshka. Separate lines connect between the Jordan River samples and between the in¯ows from the western shore of the Dead Sea.
suggests that chloride-rich sources in the area are relatively depleted in 37 Cl. Despite the small number of samples, the data also suggest that the chloride sources feeding the Jordan River are dierent Ð relatively less depleted in 37 Cl Ð than those that supply chlorides to freshwater in¯ows and to saline springs on the western Dead Sea coast such as En Feshka and Hamme Mazor. The Dead Sea brines and halite from sur®cial sediments. The Dead Sea brines from the deep central part of the lake (Fig. 1) display a widespread range of d37 Cl values: +0.55- and ÿ0.47- in 1976, +0.16- in 1981 and ÿ0.03- in 1982 (Table 1). The brines sampled in 1976 were taken at about 10 km distance from those sampled in the 1980s (Fig. 1). Lateral inhomogeneity in chemical parameters has not been observed in this part of the lake and therefore we assume also that horizontal variability in d37 Cl is very unlikely. But, the samples taken in June 1976 represent a long-term stable strati®cation (a meromictic state) while those taken in 1981 and 1982 represent a vertically mixed water column, after the 1979 overturn has taken place. Mass balances which were carried out (see below) show that the apparently wide range of d37 Cl values can be explained by the drastic change in the strati®cation pattern. For mass balance calculations it has been assumed that the Dead Sea brines sampled in 1976 at 25 m and 310 m do represent the chloride concentrations and d37 Cl values of the upper and lower water masses, respectively, and that the deep water
sample (270 m) from November 1982 represents the unstrati®ed water column. In June 1976 the upper water masses, from 0 m to 80 m, and the lower water mass, from 100 m to 320 m had homogeneous salinities (Steinhorn, 1985). The respective water volumes were 51.54 km3 and 84.94 km3, the lake level was at 401 m below MSL (Steinhorn, 1981) and the entire lake volume was 147.1 km3. For the transition layer between 80 m and 100 m (10.63 km3) intermediate chloride concentrations and d37 Cl values have been assumed. Using these data, the average chloride concentration of the water column in 1976 was calculated to have been 222.2 g. In 1982 the lake level dropped to about 403.5 m below MSL (Anati et al., 1987) and the lake volume decreased to about 145.4 km3. If one assumes a conservative chloride inventory and the decrease in lake volume from 1976 to 1982 is attributed solely to excess evaporation over in¯ow (negative hydrological balance), then the calculated average Cl concentration becomes 224.9 g. This latter value is in good agreement with that actually measured at 270 m depth (224.6 g) in the post-overturn, mixed water column of 1982. The somewhat lower chloride concentration at 10 m depth in 1981 is due to seasonal dilution of surface layers by freshwater (which would not signi®cantly aect its d37 Cl value). The mass balances for d37 Cl were performed in a similar way as for chloride. The calculated average d37 Cl of the 1976 water column being ÿ0.08-, is
Cl-37 in the Dead Sea system
very close to the post-overturn value of ÿ0.03-, measured at 10 m in 1981. If compared to the other sample representing post-overturn brines, taken at 270 m in 1982, and in which d37 Cl is +0.16-, the agreement with the calculated average d37 Cl is less good. One possible explanation for the discrepancy could be that vertical mixing in February 1979, although causing destruction of the meromictic structure, was rather weak and short-lived. Thermal strati®cation developed soon after the overturn and prevented a complete homogeneous mixing of the water column. This process could not be observed by using tracers such as density and chloride concentration, for which the pre-overturn gradients were rather small, but was indicated by tracers for which large pre-overturn gradients existed such as 14 C and 210 Pb±210 Po (Talma et al., 1997; Stiller and Kaufman, 1984). If this explanation is correct, then d37 Cl joins the evidence suggested by these tracers and thus emphasizes the potential of d37 Cl as a tracer for the degree of mixing in cases where the dierence in chloride concentration between two water masses is very small. According to the 14 C data, it was only after the winter of 1982/1983 when a turnover and mixing of longer duration (three months) took place that the water column became thoroughly mixed (Talma et al., 1997). Although the above calculations oer an explanation for the wide range of d37 Cl values and a reasonable agreement emerges between the d37 Cl data of 1976 and of 1981/1982, for reasons described below, it is dicult to understand the isotopic values themselves measured in the two water masses of 1976, namely +0.55- and ÿ0.47-. The present concept about the more recent history of the Dead Sea is the following: at the beginning of the 18th century the lake was relatively low, about 404.5 m below MSL (Klein, 1981). At that time the lake was probably monomictic (the entire water column mixed every year), and the lake waters were at saturation with respect to halite. Precipitation of halite was widespread and it deposited all over the lake bottom. Then the lake level began to rise again and a meromictic (strati®ed) structure of the water column became stabilized about 300 a ago (Stiller and Chung, 1984). During this meromictic phase the halite layer underlying the more diluted upper water mass probably redissolved, as can be shown (see below) from the chemical composition of the Dead Sea measured during 1963±1965 by Nissenbaum (1969), while the halite underlying the deep basin became covered by several tens of centimeters of sediments (Levy, 1988). The halite crystals (+0.33-; Table 1) found at a depth of 20 cm in core EG 250 (taken at 250 m water depth) are believed to be about 500 a old (Levy, 1988). The Cl isotopic composition of the halite crystals allows an estimate of the composition
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of the saturated brine from which they originate. With a 37 Cl/35 Cl isotope fractionation of +0.26at 228C, between halite and a saturated NaCl solution (Eggenkamp et al., 1995), it can be calculated that the Cl isotopic composition of the brine from which the halite crystals have been formed was d37 Cl= + 0.07-. If the calculated value of +0.07- is indeed that of a mixed Dead Sea about 300 a ago, just before the initiation of its meromictic stage, then this value should have been preserved in the LWM. However, we have measured for the 1976 LWM (Table 1) a much more depleted value of ÿ0.47-. It is evident that more halite crystals from the bottom sediments as well as more preoverturn, deep water brines must be analyzed for their Cl isotopic composition. In the meantime the depleted pre-overturn d37 Cl value of the deep water remains poorly understood. The pre-1979 overturn value of the upper water mass, +0.55-, is puzzling. By examining the chemical composition of 24 samples taken in 1963± 1965 (Nissenbaum, 1969) from the Dead Sea, it turns out that the Br/Mg and K/Mg ratios throughout the water column are constant. Moreover, the concentration ratios between the upper and lower water masses of these conservative ions, Mg, K and Br, is 0.87, exactly the same for all 3 elements. The constant ion ratios throughout the water column indicate the common origin of both water masses. The concentration ratio of 0.87 for each conservative element indicates that the UWM has been formed by surface dilution of the original brine with freshwater. It is worthwhile noting that the 3 conservative elements also indicate that the transition layer in 1963±1965 was a mixture of about 1:2.2 of the UWM and the LWM. Examination of Na and Cl concentrations in the 1963±1965 Dead Sea brines reveals that the Cl/Mg and Na/Mg ratios are not constant with depth, but are higher in the upper water mass compared to the lower one. These ``excesses'' of Na and Cl are probably due to salts introduced by the Jordan River in¯ow into the upper layer during the meromictic phase and to dissolution of halite from the lake ¯oor at shallow depths. The amount of halite which has been dissolved into the UWM (Clhalite) has been calculated as follows: Clhalite Clmeas ÿ Clexpc ClJ where: Clmeas is the Cl content measured in the UWM in 1963±1965; Clexpc is the expected Cl content of the UWM which was calculated by taking the Cl content of the LWM during the meromictic period multiplied by 0.87, i.e. the concentration ratio between the UWM and the LWM (see above), ClJ is the amount of chloride added annually by the Jordan in¯ow (Neev and Emery, 1967) over the last 300 a, the length of the meromictic phase.
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A similar calculation has been performed for the transition layer. The total amount of halite dissolved in both layers was 320106 ton. The same type of calculations have been done with the Na data and yielded 276106 ton. It follows that the UWM brine sampled in 1976 at 25 m (+0.55-) represents a mixture of 95% deep water-derived chloride, 3.4% chloride brought in by the Jordan River and 1.7% derived by dissolution of halite from the littoral areas. By assigning d37 Cl values of ÿ0.47- to chloride derived from the deep water mass, +0.63- to chloride derived from the Jordan in¯ow and +0.33- for dissolved halite, it is not possible to obtain a mixture in the above proportions, having a ®nal value of +0.55-. Even by assigning instead of ÿ0.47-, our estimated value of +0.07-, which is the d37 Cl of DS in equilibrium with halite crystals (see above and Fig. 3), for chloride derived by dilution of the relict deep water mass, the measured value of +0.55- cannot be obtained. It should be mentioned that in these calculations contributions of chloride and of d37 Cl from freshwater in¯ows from the western (like Nahal Arugot) and eastern (Nahal Arnon) coasts of the Dead Sea and from saline springs have not been taken into account. It could be that they in¯uence the isotopic composition of the upper layer to a lar-
ger extent than envisaged by our tentative calculations which did not consider them. End-Brine and pore water. The End-Brine sampled in 1982 has a d37 Cl value of ÿ0.08-. It represents Dead Sea brine which spent about 2 a in solar evaporation ponds to deposit halite and then somewhat less than 1 a in carnallite evaporation ponds. From about 2.4 l of original Dead Sea brine, 1 l of End-Brine is left after the carnallite precipitation stage and pumped back into the Dead Sea. During the evaporation process, about 152 g chloride are deposited as NaCl and about 16 g chloride as carnallite (Epstein, 1976). The remaining fraction (f) of soluble chloride in the End-Brine is 0.688. Rayleigh type behavior was applied to the evaporation/halite deposition process: dEB dDS e ln f where dEB and dDS are the d37 Cl of the End-Brine and of the initial Dead Sea brine from which the End-Brine was produced, respectively; e = (a ÿ 1)1000 where a = 1.00026 (Eggenkamp et al., 1995) is the isotopic fractionation factor between NaCl and a saturated solution. It follows that the isotopic composition of the original Dead Sea brine was dDS= + 0.02-. In this estimate it was assumed that all the chloride was lost from sol-
Fig. 3. Schematic representation of the d37 Cl values measured in this study. The d37 Cl calculated average of the Dead Sea water column in 1976 is marked with an arrow. The estimated d37 Cl of the Dead Sea brines from which the End-Brine and from which the halite crystals found in the sediments originate, are also shown with arrows. J stands for Jordan River, A for Nahal Arugot, EBq for En Boqeq, HM for Hamme Mazor, EF for En Feshka, DS for Dead Sea, EB for End-Brine, EA for En Ashlag.
Cl-37 in the Dead Sea system
ution as halite. Re®ning the calculations, and estimating separately the fractionations for the depositions of halite and of carnallite, and by using the fractionation factor a = 1.00002, given by Eggenkamp et al. (1995) for carnallite, leads to dDS3 + 0.01- for the isotopic composition of the initial Dead Sea brine. The above calculations show that during the entire potash production process the isotopic composition of the Dead Sea becomes more enriched in 37 Cl by only 0.1-. The calculated value for the original Dead Sea brine of about +0.02- to +0.01- is very close to that measured in the surface waters of the Dead Sea sampled after the 1979 overturn (in 1981) namely ÿ0.03- (Table 1). Since the residence time in the evaporation ponds is about 2±3 a, this fact con®rms that the End-Brine sampled in 1982 originated from a post-1979-turnover brine. The pore waters of core B1, taken in 1980 at a water depth of 320 m, had a rather constant salinity pro®le (Stiller et al., 1983). Their salinity was very similar (within analytical error) with that of the isolated pre-1979 LWM. We expected therefore the pore waters of core B1 to re¯ect the Cl isotopic composition of these isolated pre-1979 deep waters. However, the d37 Cl of the pore water at 155± 160 cm depth in sediment, +0.13- (Table 1 and Fig. 3) is very dierent from the 1976 deep waters (ÿ0.47-). But, on the other hand, the d37 Cl of the pore water is very close to the d37 Cl value predicted for the isolated deep waters (+0.07-) by the isotopic composition of the halite crystals from core EG 250 (see above and Fig. 3). Evidently the 37 Cl/35 Cl measurements of the Dead Sea system performed so far, poses questions as to our present concepts about the recent history and evolution of this environment. More information is required in order to decipher this problem.
Mount Sdom The halite from Mount Sdom (ÿ0.59-; Table 1) has the most depleted d37 Cl measured so far in the Dead Sea-Rift Valley area. This value is very close to the calculated minimum value of ÿ0.55- that evaporates can reach (Eggenkamp et al., 1995) after precipitating about 88% of the chloride of the evaporating seawater as halite, kainite and carnallite. During the following stage of bischo®te deposition the trend of continuously decreasing d37 Cl is reversed. Depleted d37 Cl values similar to the Sdom Salt, of ÿ0.56- and ÿ0.58-, were encountered only in two samples, at 1738 m depth and 1783 m depth, respectively, in the Zechstein III formation (Eggenkamp et al., 1995). In these samples halite was mixed with other chloride minerals.
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The hypersaline En Ashlag seepage-spring (ÿ0.32-) is less depleted in d37 Cl than the nearby rock salt. This result precludes the possibility that En Ashlag represents the relict brine from which Mount Sdom rock salt was formed, because in such a case the d37 Cl of the seepage should be more depleted than that of the halite sample (expected value ÿ0.85-). This possibility has also been dismissed by Yechieli et al. (1996) who found that the 36 Cl/Cl ratio is signi®cantly higher in En Ashlag brine (meaning it is much ``younger'') than in nearby salt. The extreme salinity of En Ashlag and the very low Na/Cl molar ratio of about 0.1 (Kawamura and Nissenbaum, 1992) suggest that it is associated with evaporite deposits formed in very late stages of evaporation. The d37 Cl value of En Ashlag may indicate that it represents the residual brine from a more advanced evaporitic stage than the Sdom rock salt itself. This explanation is supported by recent drilling in Mt. Sdom which revealed the existence of K and Mg minerals (J. Charrach, pers. commun.). This hypothesis does, however, not exclude the possibility that En Ashlag also displays some degree of ``dilution'' of a hypersaline residual brine (even more depleted in d37 Cl) with a relatively more enriched d37 Cl and ``younger'' water source. The 37 Cl/35 Cl ratios seem to provide an additional useful tool for studying the geological history of the Mount Sdom diapir.
CONCLUSIONS
All the measured and calculated d37 Cl values are presented schematically in Fig. 3. (1) The chloride isotope composition of the Jordan River and of the other in¯ows present a wide range of d37 Cl values. A detailed d37 Cl survey of all in¯ows, saline springs and seepages and of their respective discharges is therefore needed in order to enable an estimate of their contribution to the Cl isotopic composition of the Dead Sea. (2) The post-1979 overturn Dead Sea, the pore water, the estimated Dead Sea values derived from halite crystals found in recent Dead Sea sediments and from End-Brine, are all within the same narrow range of d37 Cl, from ÿ0.03- to +0.16-. But, there is a large dierence in Cl isotopic composition between the pre-overturn lower and upper water masses: ÿ0.47- and +0.55-, respectively. This isotopic dierence cannot be explained by the current hypotheses for the behavior of the Dead Sea over the last several centuries. If con®rmed by additional data, then the sub recent history of the Dead Sea will have to be reevaluated and if correct, these extreme values may shed some light on the origin of the Dead Sea brines.
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(3) The d37 Cl data of the En Ashlag seepage suggest that it probably represents residual brines from advanced stages of evaporation that occurred after the deposition of Mount Sdom halite. AcknowledgementsÐWe thank Dr Y. Levy from the Geological Survey of Israel, Jerusalem, for making available to us halite crystals from core EG 250. Professor Miriam Kastner (SIO) kindly ran two samples for the present study. We are grateful to Professor P. Fritz for many constructive comments. The study was supported by a research grant from the Directorate of Earth Sciences, Ministry of National Infrastructures, Israel. We are grateful to Mrs S. Newman for typing the manuscript. Editorial handling:ÐP. Fritz
REFERENCES Anati D. A., Stiller M., Shasha S. and Gat J. R. (1987) Changes in the thermohaline structure of the Dead Sea: 1979±1984. Earth Planet Sci. Lett. 84, 109±121. Begin Z. B., Broecker W., Buchbinder B., Druckman Y., Kaufman A., Magaritz M. and Neev D. (1985) Dead Sea and Lake Lisan levels in the last 30 000 years. Geol. Survey Israel Report GSI 29, 85. Bentor Y. (1961) Some geochemical aspects of the Dead Sea and the question of its age. Geochim. Cosmochim. Acta 25, 239±260. Beyth M., Gavrieli I. and Bauman N., (1990) Mixing of End-Brines in the Lynch Strait, Dead Sea (16±19 July, 1990). Geological Survey of Israel, Report GSI/48/90. Eggenkamp H. G. M., Kreulen R. and Koster van Groos A. F. (1995) Chlorine stable isotope fractionation in evaporites. Geochim. Cosmochim. Acta 59, 5169±5175. Epstein J. A. (1976) Utilization of the Dead Sea minerals. Hydrometallurgy 2, 1±10. Gavrieli I. 1997. Halite deposition in the Dead Sea: 1960± 1993. In: The Dead Sea, ed. T. Niemi, Z. Ben-Avraham and J. R. Gat. Cambridge University Press, U.K., pp. 161±170. Kaufman A., Yechieli Y. and Gardosh M. (1992) Reevaluation of the lake-sediment chronology in the Dead Sea basin, based on new 230 Th/U dates. Quatern. Res. 38, 292±302. Kaufmann R. S., Long A. and Campbell D. J. (1988) Chlorine isotope distribution in formation waters Texas and Louisiana. Am. Assoc. Petroleum Geol. Bull. 72, 839±844. Kaufmann R. S., Frape S. K., McNutt R. and Eastoe C. J. (1993) Chlorine stable isotope distribution of Michigan Basin formation waters. Appl. Geochem. 8, 403±407. Kawamura K. and Nissenbaum A. (1992) High abundance of low molecular weight organic acids in hypersaline spring water associated with a salt diapir. Org. Geochem. 18, 469±476. Klein C. (1981) The in¯uence of rainfall over the catchment area on the ¯uctuations of the level of the Dead Sea since the 12th century. Israel Metereol. Pap. 3, 29± 58.
Levy Y. (1988) Recent depositional environments in the Dead Sea. Geological Survey of Israel, Report GSI/42/ 88. Long A., Eastoe C. J., Kaufmann R. S., Martin J. G., Wirt L. and Finley J. B. (1993) High precision measurement of chlorine stable isotope ratios. Geochim. Cosmochim. Acta 57, 2907±2912. Lowengart S. (1962) The geochemical evolution of the Dead Sea basin. Bull. Res. Counc. Israel 11G, 85±96. Mazor E., Rosenthal E. and Eckstein J. (1969) Geochemical tracing of mineral water sources in the South-Western Dead Sea basin. Israel J. Hydrol. 7, 246±275. Mazor E. and Molcho M. (1972) Geochemical studies on the En Feshcha Springs Dead Sea basin. J. Hydrol. 15, 37±47. Neev D. and Emery K. O. (1967) The Dead Sea, depositional processes and environments of the Dead Sea. Israel Geol. Survey Bull. 41, 147. Nissenbaum A. (1969) The geochemistry of the Jordan River ± Dead Sea system. Unpublished Ph.D. thesis, UCLA. Starinsky A. (1974) Relation between Ca±chloride brines and sedimentary rocks in Israel. Ph.D. thesis, The Hebrew University, Jerusalem (in Hebrew, English abstract). Steinhorn I. (1981) A hydrographical and physical study of the Dead Sea during the destruction of its long-term meromictic strati®cation. Ph.D. thesis, Weizmann Institute of Science, Rehovot. Steinhorn I. (1985) The disappearance of the long term meromictic strati®cation of the Dead Sea. Limnol. Oceanogr. 30, 451±472. Steinhorn I., Assaf G., Gat J. R., Nishry A., Nissenbaum A., Stiller M., Beyth M., Neev D., Garber R., Friedman G. M. and Weiss W. (1979) The Dead Sea: Deepening of the mixolimnion signi®es the overture to overturn of the water column. Science 206, 55±57. Steinitz G. and Bartov Y. (1991) The Miocene±Pleistocene history of the Dead Sea segment of the Rift in the light of K±Ar of basalts. Israel J. Earth Sci. 40, 199±208. Stiller M. and Chung Y. C. (1984) Radium in the Dead Sea: A possible tracer for the duration of meromixis. Limnol. Oceanogr. 29, 574±586. Stiller M. and Kaufman A. (1984) Lead-210 and polomium-210 during the destruction of strati®cation in the Dead Sea. Earth Planet. Sci. Lett. 71, 390±404. Stiller M., Kaushansky P. and Carmi I. (1983) Recent climatic changes recorded by the salinity of pore waters in the Dead Sea sediments. Hydrobiologia 103, 75±79. Talma S., Vogel J. C. and Stiller M. (1997) The radiocarbon content of the Dead Sea. In: The Dead Sea, ed. T. Niemi, Z. Ben-Abraham and J. R. Gat, pp. 193±198. Cambridge University Press, U.K., in press. Tannenbaum E. and Charrach J. (1993) Oil shows in Mt. Sdom and their implications to the age of the potential oil source. Proc. Israel Geol. Soc. Ann. Meeting, Arad, Israel. Yechieli Y., Ronen D. and Kaufman A. (1996) The source and age of groundwater brines in the Dead Sea area, as deduced from 36 Cl and 14 C. Geochim. Cosmochim. Acta 60, 1909±1916. Zak I. (1967) The geology of Mt. Sdom. Unpublished Ph.D. thesis, Hebrew University, Jerusalem (in Hebrew, English summary).
Applied Geochemistry, Vol. 13, No. 8, pp. 953±960, 1998 # 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain S0883-2927(98)00017-1 0883-2927/98 $ - see front matter
Cl-37 in the Dead Sea systemÐpreliminary results Mariana Stiller and Arie Nissenbaum* Weizmann Institute of Science, 76100 Rehovot, Israel
Ronald S. Kaufmann Department of Geology, Northern Illinois University, DeKalb, IL 60511, U.S.A.
Austin Long Department of Geosciences, University of Arizona, Tucson, AZ 85721, U.S.A. (Received 7 January 1997; accepted in revised form 10 November 1997) AbstractÐThis study presents the ®rst set of d37 Cl measurements in the Dead Sea environment. d37 Cl values for the meromictic (long term strati®ed) Dead Sea water column prior to its complete overturn in 1979 were ÿ0.47- SMOC for the UWM (Upper Water Mass) and +0.55- SMOC for the LWM (Lower Water Mass). The d37 Cl values for the pre-overturn Dead Sea cannot be explained by the prevailing model on the evolution of the Dead Sea during the last few centuries and require corroboration by more measurements. The 1979 overturn wiped out almost completely the isotopic dierences between the UWM and LWM. Even so, Cl isotope data could be used to decipher physical processes related to the overturn such as incomplete homogenization of the deep water mass. Inputs into the lake, comprising freshwaters (springs and the Jordan River) and saline springs gave a range of ÿ0.37to +1.0- with the freshwater sources being more enriched in d37 Cl. Based on the d37 Cl measurements of the End-Brine (the euent from Dead Sea evaporation ponds) and of recent Dead Sea halite, the Cl isotopic composition of the originating brines have been estimated. They gave a narrow isotopic spread, +0.01- and +0.07- and fall within the same range with Dead Sea pore water (+0.13-) and with the post-overturn Dead Sea (ÿ0.03- and +0.16-). Rock salt from Mount Sdom gave a value of ÿ0.59- indicating its formation at the last stages of halite deposition from evaporating sea water. The hypersaline En Ashlag spring gave a depleted d37 Cl value of ÿ0.32-, corresponding to a residual brine formed in the very latest stages (including bisho®te deposition) of seawater evaporation. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
The study of Cl stable isotope distribution has become a valuable tool for the study of formation waters (Kaufmann et al., 1988, 1993) and the quantitative evaluation of mixing between dierent sources of chloride in brines and aquifers (Long et al., 1993). The unique environment of the Dead Sea, a chloridic hypersaline lake, presents a case study for the applicability of this new tool. The question of how did the present-day hypersaline Dead Sea acquire its chemical composition has been debated for many years. A range of hypotheses has been proposed. Lowengart (1962) suggested that the salts are of atmospheric origin; Bentor (1961) proposed that the quantity of most salts can be accounted for by inputs from the Jordan River and surface water sources; while Starinsky (1974) has described the various stages by which Pliocene *Corresponding author. 953
seawater had supposedly evolved into Dead Sea brines. The current paradigm on the source of salts in the lake is that the brine has multiple sources, and it is quite probable that for determining the relative contribution of each one a multi-parameter approach will be needed. This report, based on a very limited number of samples, is intended to investigate the capabilities of one such parameter, the stable Cl isotopes, which has not been previously applied to this system. The Dead Sea Basin is located in the deepest part of the East African Rift Valley. The Dead Sea itself is the lowest exposed surface on the face of the earth, with the lake level being today, in the mid1990s, around 410 m below mean sea level (MSL). The Dead Sea is a terminal lake fed by the Jordan River, by surface runo during the wet season and by a few perennial, fresh and saline springs and streams (Fig. 1); it is a 320 m deep water body, characterized by its extreme hypersalinity. The average total dissolved salt content of the lake in 1990
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Fig. 1. Map of the Dead Sea with location of in¯ows and sampling stations of Dead Sea brines and cores. The depth contours for the Dead Sea are in meters below mean sea level (MSL). The locations of Mount Sdom and En Ashlag are also shown.
was 340 g lÿ1. The chemical composition is unusual by having Mg as the dominant cation (46.4 g lÿ1) followed by Na (36.7 g lÿ1), Ca (17.1 g lÿ1) and K (7.9 g lÿ1). The dominant anion is, by far, Cl (225.6 g lÿ1), distantly followed by Br (5.46 g lÿ1). Sulfate and CO3 are very minor (Beyth et al., 1990; Gavrieli, 1997) at less than 1.0 g lÿ1. The Dead Sea location in the center of an active tectonic rift results in a very complex hydrological situation, and the springs along its coast demonstrate an extreme range of salinities, from freshwater (e.g. Nahal Arugot) with a chloride content of 112 mg lÿ1 to hypersaline brines such as En
Ashlag with 260 g Cl lÿ1, thus spanning a range of 3 orders of magnitude. The Dead Sea basin has been the locus of a series of paleo-lakes which existed there since the Pliocene. These lakes were of varying salinities, but some of them were hypersaline enough to precipitate gypsum and halite. It has been proposed that some of the salts in the Dead Sea and associated saline springs originate from dissolution of these evaporates. It is also possible that residual brine from the Lisan Lake which preceded the Dead Sea and disappeared not earlier than 18 000 a ago (Begin et al., 1985; Kaufman et al., 1992) may have contributed salts to the present lake.
Cl-37 in the Dead Sea system METHODOLOGY Description of samples Samples for this study were selected as to represent the brines of the Dead Sea and the wide spectrum of salinities of the in¯owing waters (Fig. 1). Samples from the Dead Sea water column are from two dierent periods in its recent history. The samples from 1976 represent a period when the lake was permanently strati®ed (meromictic) with a slightly less saline layer (273 g dissolved salts kgÿ1) ¯oating over a denser, anoxic layer (276 g dissolved salts kgÿ1). The salinity was uniform in the upper 80 m and from 100 m to the bottom with a 20 m thick transition layer (Steinhorn, 1985). In February 1979 the lake level dropped to ÿ403.5 m below MSL, the water column overturned and became completely mixed (Steinhorn et al., 1979). The Dead Sea samples from 1981 and 1982 represent the post-overturn situation (Anati et al., 1987). The End-Brine sample represents Dead Sea brine from which part of the chloride has been removed by precipitation, ®rst as halite and then as carnallite. The End-Brine sample was taken from a carnallite evaporation pond that is used by the ``Dead Sea Works'' Company in the process of potash production, and is situated near the southern end of the lake. Pore water brines are represented by a sample from core B1 (Stiller et al., 1983) which was taken from the northern part of the deep water basin (Fig. 1). The Nahal Arugot, En Bokek and the Jordan River water are taken as representative of freshwater inputs into the lake. En Feshka is a complex of saline springs with varying salinities (Mazor and Molcho, 1972) to which a mixed salt source has been attributed. Hamme Mazor is a warm (428C) hypersaline brine, with about 1/2 the salinity of the Dead Sea which is typical of several springs of varying, but high, salinities, which issue along the Dead Sea coast (Mazor et al., 1969). The En Ashlag brine is from a small seepage near the southern basin of the Dead Sea and is associated with the Mt. Sdom diapir. This seepage has a salinity exceeding that of the Dead Sea but with a dierent chemistry (Kawamura and Nissenbaum, 1992). Two halite samples were also analyzed in the present study. One, halite crystals found at 20 cm depth in a sediment core taken in 1981 (Levy, 1988) and one from a
955
quarry of rock salt from the exposed Mt. Sdom diapir, near the southern part of the Dead Sea (Fig. 1). This latter salt sample is assumed to have originated from the Sdom Formation of Pliocene (Zak, 1967) or Miocene age (Steinitz and Bartov, 1991; Tannenbaum and Charrach, 1993).
Analytical procedures Isotopic measurements of stable Cl isotopes used methyl chloride (CH3Cl) produced as follows: with the sample heated to near boiling, the chloride was precipitated as AgCl by addition of 0.1 M silver nitrate solution, approximately 8% more than is required for complete precipitation. The precipitate was then quantitatively reacted with CH3I for two days at 1208C. The CH3I was puri®ed by gas chromatography and the isotopic composition + measured as CH337 Cl+/CH35 on a VG 602 mass spec3 Cl trometer with double Faraday cups and a dual inlet port. The precision of the method based on averaging dierences of replicate samples is 0.1-. The results of the measurements are reported with respect to standard mean ocean chloride (SMOC).
RESULTS AND DISCUSSION
The Dead Sea system Inputs into the Dead Sea. The number of samples analyzed is far too small for a high resolution evaluation of the source of salts in the Dead Sea in¯ows. However, the 37 Cl data does provide partial information on some of these sources. The two samples of the Jordan River water have d37 Cl values of +0.63- and +0.39- and the other freshwater in¯ows into the Dead Sea, Nahal Arugot and En Boqeq, +1.1- and +0.27-, respectively (Table 1). The inverse relationship which is apparent between d37 Cl and salinity (Fig. 2)
Table 1. Chlorine stable isotopes in the Dead Sea system Sample type Freshwater Freshwater Brine Brine Brine Brine Brine Halite Brine Freshwater Freshwater Brackish Brine Brine Halite 1
Sample description
Date of sampling
The Jordan in¯ow and the Dead Sea Jordan River, Abdallah Bridge Feb. 18, 1982 Jordan River, Abdallah Bridge July 27, 1995 Dead Sea, 25 m depth (pre-overturn1) June 25, 1976 Dead Sea, 310 m depth (pre-overturn) June 25, 1976 Dead Sea, 10 m depth (post-overturn1) Nov. 23, 1981 Dead Sea, 270 m depth (post-overturn) Nov. 23, 1982 Dead Sea ± End-Brine July 1982 1981 Crystals at 20 cm depth in Core EG2502 3 Pore water from 155±160 cm depth in Core B1 Dec. 1980 Other in¯ows to the Dead Sea Nahal Arugot Jan. 24, 1983 En Boqeq spring 1984 En Feshka spring 1984 Hamme Mazor spring 1984 Mount Sdom environment En Ashlag June 21, 1984 Rock Salt from Mount Sdom
Cl mg lÿ1
d37 Cl- SMOC
440 2400 220 300 223 300 219 200 224 600 344 800 224 800
+0.63 (20.04) +0.39 +0.55 (20.04) ÿ0.47 (20.01) ÿ0.03 (20.04) +0.16 (20.04) ÿ0.08 +0.33 +0.13
112 525 1300 93 700
+1.0120.20 +0.27 ÿ0.37 0.00
260 000
ÿ0.3220.02 ÿ0.59
Pre-overturn and post-overturn refer to the overturn of February 1979 (Steinhorn et al., 1979). Sediment core taken from the Dead Sea by Y. Levy at a water depth of 250 m (Levy, 1988). Core B1 was taken at a water depth of 320 m (Stiller et al., 1983) in December 1980.
2 3
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Fig. 2. The relationship between the Cl content (mg lÿ1), expressed as 1/Cl, and d37 Cl in in¯ows to the Dead Sea. J stands for the Jordan River, A for Nahal Arugot, EBq for En Boqeq, EF for En Feshka. Separate lines connect between the Jordan River samples and between the in¯ows from the western shore of the Dead Sea.
suggests that chloride-rich sources in the area are relatively depleted in 37 Cl. Despite the small number of samples, the data also suggest that the chloride sources feeding the Jordan River are dierent Ð relatively less depleted in 37 Cl Ð than those that supply chlorides to freshwater in¯ows and to saline springs on the western Dead Sea coast such as En Feshka and Hamme Mazor. The Dead Sea brines and halite from sur®cial sediments. The Dead Sea brines from the deep central part of the lake (Fig. 1) display a widespread range of d37 Cl values: +0.55- and ÿ0.47- in 1976, +0.16- in 1981 and ÿ0.03- in 1982 (Table 1). The brines sampled in 1976 were taken at about 10 km distance from those sampled in the 1980s (Fig. 1). Lateral inhomogeneity in chemical parameters has not been observed in this part of the lake and therefore we assume also that horizontal variability in d37 Cl is very unlikely. But, the samples taken in June 1976 represent a long-term stable strati®cation (a meromictic state) while those taken in 1981 and 1982 represent a vertically mixed water column, after the 1979 overturn has taken place. Mass balances which were carried out (see below) show that the apparently wide range of d37 Cl values can be explained by the drastic change in the strati®cation pattern. For mass balance calculations it has been assumed that the Dead Sea brines sampled in 1976 at 25 m and 310 m do represent the chloride concentrations and d37 Cl values of the upper and lower water masses, respectively, and that the deep water
sample (270 m) from November 1982 represents the unstrati®ed water column. In June 1976 the upper water masses, from 0 m to 80 m, and the lower water mass, from 100 m to 320 m had homogeneous salinities (Steinhorn, 1985). The respective water volumes were 51.54 km3 and 84.94 km3, the lake level was at 401 m below MSL (Steinhorn, 1981) and the entire lake volume was 147.1 km3. For the transition layer between 80 m and 100 m (10.63 km3) intermediate chloride concentrations and d37 Cl values have been assumed. Using these data, the average chloride concentration of the water column in 1976 was calculated to have been 222.2 g. In 1982 the lake level dropped to about 403.5 m below MSL (Anati et al., 1987) and the lake volume decreased to about 145.4 km3. If one assumes a conservative chloride inventory and the decrease in lake volume from 1976 to 1982 is attributed solely to excess evaporation over in¯ow (negative hydrological balance), then the calculated average Cl concentration becomes 224.9 g. This latter value is in good agreement with that actually measured at 270 m depth (224.6 g) in the post-overturn, mixed water column of 1982. The somewhat lower chloride concentration at 10 m depth in 1981 is due to seasonal dilution of surface layers by freshwater (which would not signi®cantly aect its d37 Cl value). The mass balances for d37 Cl were performed in a similar way as for chloride. The calculated average d37 Cl of the 1976 water column being ÿ0.08-, is
Cl-37 in the Dead Sea system
very close to the post-overturn value of ÿ0.03-, measured at 10 m in 1981. If compared to the other sample representing post-overturn brines, taken at 270 m in 1982, and in which d37 Cl is +0.16-, the agreement with the calculated average d37 Cl is less good. One possible explanation for the discrepancy could be that vertical mixing in February 1979, although causing destruction of the meromictic structure, was rather weak and short-lived. Thermal strati®cation developed soon after the overturn and prevented a complete homogeneous mixing of the water column. This process could not be observed by using tracers such as density and chloride concentration, for which the pre-overturn gradients were rather small, but was indicated by tracers for which large pre-overturn gradients existed such as 14 C and 210 Pb±210 Po (Talma et al., 1997; Stiller and Kaufman, 1984). If this explanation is correct, then d37 Cl joins the evidence suggested by these tracers and thus emphasizes the potential of d37 Cl as a tracer for the degree of mixing in cases where the dierence in chloride concentration between two water masses is very small. According to the 14 C data, it was only after the winter of 1982/1983 when a turnover and mixing of longer duration (three months) took place that the water column became thoroughly mixed (Talma et al., 1997). Although the above calculations oer an explanation for the wide range of d37 Cl values and a reasonable agreement emerges between the d37 Cl data of 1976 and of 1981/1982, for reasons described below, it is dicult to understand the isotopic values themselves measured in the two water masses of 1976, namely +0.55- and ÿ0.47-. The present concept about the more recent history of the Dead Sea is the following: at the beginning of the 18th century the lake was relatively low, about 404.5 m below MSL (Klein, 1981). At that time the lake was probably monomictic (the entire water column mixed every year), and the lake waters were at saturation with respect to halite. Precipitation of halite was widespread and it deposited all over the lake bottom. Then the lake level began to rise again and a meromictic (strati®ed) structure of the water column became stabilized about 300 a ago (Stiller and Chung, 1984). During this meromictic phase the halite layer underlying the more diluted upper water mass probably redissolved, as can be shown (see below) from the chemical composition of the Dead Sea measured during 1963±1965 by Nissenbaum (1969), while the halite underlying the deep basin became covered by several tens of centimeters of sediments (Levy, 1988). The halite crystals (+0.33-; Table 1) found at a depth of 20 cm in core EG 250 (taken at 250 m water depth) are believed to be about 500 a old (Levy, 1988). The Cl isotopic composition of the halite crystals allows an estimate of the composition
957
of the saturated brine from which they originate. With a 37 Cl/35 Cl isotope fractionation of +0.26at 228C, between halite and a saturated NaCl solution (Eggenkamp et al., 1995), it can be calculated that the Cl isotopic composition of the brine from which the halite crystals have been formed was d37 Cl= + 0.07-. If the calculated value of +0.07- is indeed that of a mixed Dead Sea about 300 a ago, just before the initiation of its meromictic stage, then this value should have been preserved in the LWM. However, we have measured for the 1976 LWM (Table 1) a much more depleted value of ÿ0.47-. It is evident that more halite crystals from the bottom sediments as well as more preoverturn, deep water brines must be analyzed for their Cl isotopic composition. In the meantime the depleted pre-overturn d37 Cl value of the deep water remains poorly understood. The pre-1979 overturn value of the upper water mass, +0.55-, is puzzling. By examining the chemical composition of 24 samples taken in 1963± 1965 (Nissenbaum, 1969) from the Dead Sea, it turns out that the Br/Mg and K/Mg ratios throughout the water column are constant. Moreover, the concentration ratios between the upper and lower water masses of these conservative ions, Mg, K and Br, is 0.87, exactly the same for all 3 elements. The constant ion ratios throughout the water column indicate the common origin of both water masses. The concentration ratio of 0.87 for each conservative element indicates that the UWM has been formed by surface dilution of the original brine with freshwater. It is worthwhile noting that the 3 conservative elements also indicate that the transition layer in 1963±1965 was a mixture of about 1:2.2 of the UWM and the LWM. Examination of Na and Cl concentrations in the 1963±1965 Dead Sea brines reveals that the Cl/Mg and Na/Mg ratios are not constant with depth, but are higher in the upper water mass compared to the lower one. These ``excesses'' of Na and Cl are probably due to salts introduced by the Jordan River in¯ow into the upper layer during the meromictic phase and to dissolution of halite from the lake ¯oor at shallow depths. The amount of halite which has been dissolved into the UWM (Clhalite) has been calculated as follows: Clhalite Clmeas ÿ Clexpc ClJ where: Clmeas is the Cl content measured in the UWM in 1963±1965; Clexpc is the expected Cl content of the UWM which was calculated by taking the Cl content of the LWM during the meromictic period multiplied by 0.87, i.e. the concentration ratio between the UWM and the LWM (see above), ClJ is the amount of chloride added annually by the Jordan in¯ow (Neev and Emery, 1967) over the last 300 a, the length of the meromictic phase.
958
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A similar calculation has been performed for the transition layer. The total amount of halite dissolved in both layers was 320106 ton. The same type of calculations have been done with the Na data and yielded 276106 ton. It follows that the UWM brine sampled in 1976 at 25 m (+0.55-) represents a mixture of 95% deep water-derived chloride, 3.4% chloride brought in by the Jordan River and 1.7% derived by dissolution of halite from the littoral areas. By assigning d37 Cl values of ÿ0.47- to chloride derived from the deep water mass, +0.63- to chloride derived from the Jordan in¯ow and +0.33- for dissolved halite, it is not possible to obtain a mixture in the above proportions, having a ®nal value of +0.55-. Even by assigning instead of ÿ0.47-, our estimated value of +0.07-, which is the d37 Cl of DS in equilibrium with halite crystals (see above and Fig. 3), for chloride derived by dilution of the relict deep water mass, the measured value of +0.55- cannot be obtained. It should be mentioned that in these calculations contributions of chloride and of d37 Cl from freshwater in¯ows from the western (like Nahal Arugot) and eastern (Nahal Arnon) coasts of the Dead Sea and from saline springs have not been taken into account. It could be that they in¯uence the isotopic composition of the upper layer to a lar-
ger extent than envisaged by our tentative calculations which did not consider them. End-Brine and pore water. The End-Brine sampled in 1982 has a d37 Cl value of ÿ0.08-. It represents Dead Sea brine which spent about 2 a in solar evaporation ponds to deposit halite and then somewhat less than 1 a in carnallite evaporation ponds. From about 2.4 l of original Dead Sea brine, 1 l of End-Brine is left after the carnallite precipitation stage and pumped back into the Dead Sea. During the evaporation process, about 152 g chloride are deposited as NaCl and about 16 g chloride as carnallite (Epstein, 1976). The remaining fraction (f) of soluble chloride in the End-Brine is 0.688. Rayleigh type behavior was applied to the evaporation/halite deposition process: dEB dDS e ln f where dEB and dDS are the d37 Cl of the End-Brine and of the initial Dead Sea brine from which the End-Brine was produced, respectively; e = (a ÿ 1)1000 where a = 1.00026 (Eggenkamp et al., 1995) is the isotopic fractionation factor between NaCl and a saturated solution. It follows that the isotopic composition of the original Dead Sea brine was dDS= + 0.02-. In this estimate it was assumed that all the chloride was lost from sol-
Fig. 3. Schematic representation of the d37 Cl values measured in this study. The d37 Cl calculated average of the Dead Sea water column in 1976 is marked with an arrow. The estimated d37 Cl of the Dead Sea brines from which the End-Brine and from which the halite crystals found in the sediments originate, are also shown with arrows. J stands for Jordan River, A for Nahal Arugot, EBq for En Boqeq, HM for Hamme Mazor, EF for En Feshka, DS for Dead Sea, EB for End-Brine, EA for En Ashlag.
Cl-37 in the Dead Sea system
ution as halite. Re®ning the calculations, and estimating separately the fractionations for the depositions of halite and of carnallite, and by using the fractionation factor a = 1.00002, given by Eggenkamp et al. (1995) for carnallite, leads to dDS3 + 0.01- for the isotopic composition of the initial Dead Sea brine. The above calculations show that during the entire potash production process the isotopic composition of the Dead Sea becomes more enriched in 37 Cl by only 0.1-. The calculated value for the original Dead Sea brine of about +0.02- to +0.01- is very close to that measured in the surface waters of the Dead Sea sampled after the 1979 overturn (in 1981) namely ÿ0.03- (Table 1). Since the residence time in the evaporation ponds is about 2±3 a, this fact con®rms that the End-Brine sampled in 1982 originated from a post-1979-turnover brine. The pore waters of core B1, taken in 1980 at a water depth of 320 m, had a rather constant salinity pro®le (Stiller et al., 1983). Their salinity was very similar (within analytical error) with that of the isolated pre-1979 LWM. We expected therefore the pore waters of core B1 to re¯ect the Cl isotopic composition of these isolated pre-1979 deep waters. However, the d37 Cl of the pore water at 155± 160 cm depth in sediment, +0.13- (Table 1 and Fig. 3) is very dierent from the 1976 deep waters (ÿ0.47-). But, on the other hand, the d37 Cl of the pore water is very close to the d37 Cl value predicted for the isolated deep waters (+0.07-) by the isotopic composition of the halite crystals from core EG 250 (see above and Fig. 3). Evidently the 37 Cl/35 Cl measurements of the Dead Sea system performed so far, poses questions as to our present concepts about the recent history and evolution of this environment. More information is required in order to decipher this problem.
Mount Sdom The halite from Mount Sdom (ÿ0.59-; Table 1) has the most depleted d37 Cl measured so far in the Dead Sea-Rift Valley area. This value is very close to the calculated minimum value of ÿ0.55- that evaporates can reach (Eggenkamp et al., 1995) after precipitating about 88% of the chloride of the evaporating seawater as halite, kainite and carnallite. During the following stage of bischo®te deposition the trend of continuously decreasing d37 Cl is reversed. Depleted d37 Cl values similar to the Sdom Salt, of ÿ0.56- and ÿ0.58-, were encountered only in two samples, at 1738 m depth and 1783 m depth, respectively, in the Zechstein III formation (Eggenkamp et al., 1995). In these samples halite was mixed with other chloride minerals.
959
The hypersaline En Ashlag seepage-spring (ÿ0.32-) is less depleted in d37 Cl than the nearby rock salt. This result precludes the possibility that En Ashlag represents the relict brine from which Mount Sdom rock salt was formed, because in such a case the d37 Cl of the seepage should be more depleted than that of the halite sample (expected value ÿ0.85-). This possibility has also been dismissed by Yechieli et al. (1996) who found that the 36 Cl/Cl ratio is signi®cantly higher in En Ashlag brine (meaning it is much ``younger'') than in nearby salt. The extreme salinity of En Ashlag and the very low Na/Cl molar ratio of about 0.1 (Kawamura and Nissenbaum, 1992) suggest that it is associated with evaporite deposits formed in very late stages of evaporation. The d37 Cl value of En Ashlag may indicate that it represents the residual brine from a more advanced evaporitic stage than the Sdom rock salt itself. This explanation is supported by recent drilling in Mt. Sdom which revealed the existence of K and Mg minerals (J. Charrach, pers. commun.). This hypothesis does, however, not exclude the possibility that En Ashlag also displays some degree of ``dilution'' of a hypersaline residual brine (even more depleted in d37 Cl) with a relatively more enriched d37 Cl and ``younger'' water source. The 37 Cl/35 Cl ratios seem to provide an additional useful tool for studying the geological history of the Mount Sdom diapir.
CONCLUSIONS
All the measured and calculated d37 Cl values are presented schematically in Fig. 3. (1) The chloride isotope composition of the Jordan River and of the other in¯ows present a wide range of d37 Cl values. A detailed d37 Cl survey of all in¯ows, saline springs and seepages and of their respective discharges is therefore needed in order to enable an estimate of their contribution to the Cl isotopic composition of the Dead Sea. (2) The post-1979 overturn Dead Sea, the pore water, the estimated Dead Sea values derived from halite crystals found in recent Dead Sea sediments and from End-Brine, are all within the same narrow range of d37 Cl, from ÿ0.03- to +0.16-. But, there is a large dierence in Cl isotopic composition between the pre-overturn lower and upper water masses: ÿ0.47- and +0.55-, respectively. This isotopic dierence cannot be explained by the current hypotheses for the behavior of the Dead Sea over the last several centuries. If con®rmed by additional data, then the sub recent history of the Dead Sea will have to be reevaluated and if correct, these extreme values may shed some light on the origin of the Dead Sea brines.
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M. Stiller et al.
(3) The d37 Cl data of the En Ashlag seepage suggest that it probably represents residual brines from advanced stages of evaporation that occurred after the deposition of Mount Sdom halite. AcknowledgementsÐWe thank Dr Y. Levy from the Geological Survey of Israel, Jerusalem, for making available to us halite crystals from core EG 250. Professor Miriam Kastner (SIO) kindly ran two samples for the present study. We are grateful to Professor P. Fritz for many constructive comments. The study was supported by a research grant from the Directorate of Earth Sciences, Ministry of National Infrastructures, Israel. We are grateful to Mrs S. Newman for typing the manuscript. Editorial handling:ÐP. Fritz
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