The Sources and Evolution of Sulfur in the Hypersaline Lake Lisan (Paleo-Dead Sea)

Earth and Planetary Science Letters 236 (2005) 61–77 www.elsevier.com/locate/epsl

The sources and evolution of sulfur in the hypersaline Lake Lisan (paleo-Dead Sea)

Adi Torfsteina,b,*, Ittai Gavrielia,1, Mordechai Steina,2

aGeological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem 95501, Israel bInstitute of Earth Sciences, the Hebrew University of Jerusalem, Giv’at-Ram Campus, Jerusalem 91904, Israel Received 18 October 2004; received in revised form 19 April 2005; accepted 25 April 2005 Available online 17 June 2005 Editor: E. Bard

Abstract

d34S values in gypsum are used to evaluate the fate of sulfur in the hypersaline Lake Lisan, the late Pleistocene precursor of the Dead Sea (70–14 ka BP), and applied as a paleo-limnological tracer. The Ca-chloride Lake Lisan evolved through meromictic periods characterized by precipitation of authigenic aragonite and holomictic episodes characterized by enhanced gypsum precipitation. The lake deposited two major gypsum units: the bLower Gypsum unitQ (deposited at ~56 ka) showing d34S values of 18–20x, and the bUpper Gypsum unitQ (deposited at 17 ka) displaying significantly higher d34S values of 26– 28x. Laminated and disseminated gypsum, residing within the aragonite, exhibit d34S values in the range of 26x to 1x. The isotopic composition of the gypsum was dictated by freshwater sulfate input that replenished the upper layer of the lake (the mixolimnion), bacterial sulfate reduction (BSR) that occurred under the anoxic conditions of the lower brine (the monimolimnion), and mixing between these two layers. During meromictic periods, the sulfate reservoir in the lower brine was replenished by precipitation of gypsum from the upper layer, and its subsequent dissolution due to sulfate deficiency induced by BSR activity. This process describes a bsulfur pumpQ mechanism and its effect on d34S in the water can be modeled by a modified Rayleigh distillation equation. Steady state d34S values (~40x) were reached in the lower brine after long meromictic periods. Following overturn episodes, induced by diminishing freshwater input and lake level decline, large quantities of d34S enriched gypsum precipitated. The negative d34S values in laminated and disseminated gypsum provide evidence for BSR activity in the lower brine that removed isotopically depleted sulfides from the water column, causing significant isotopic enrichment of remaining sulfate. Following the lake desiccation, the sediments were exposed and the latter sulfides oxidized and re-crystallized as gypsum. D 2005 Elsevier B.V. All rights reserved.

Keywords: sulfur isotopes; bacterial sulfate reduction; Lake Lisan; Rayleigh distillation; brines

* Corresponding author. Geological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem 95501, Israel. Tel.: +972 2 5314311; fax: +972 2 5314332. E-mail addresses: [email protected] (A. Torfstein), [email protected] (I. Gavrieli), [email protected] (M. Stein). 1 Tel: +972 2 5314201. 2 Tel: +972 2 5314296.

1. Introduction (Fig. 1). These water bodies evolved from the ancient (probably Pliocene) Sedom lagoon that derived its The isotopic composition of sulfur in open water chemical constituents from evaporated seawater that bodies is dominated by the rate of sulfur input from ingressed into the rift valley and interacted with the external sources, and changes in the rate of geochem- carbonate wall rocks of the ancient Dead Sea basin, ical processes and removal fluxes within the water leading to dolomitization and production of Ca-chlo- body, including the deposition of sedimentary sulfides ride brine [7,11–13]. This brine is relatively low in and sulfates, and bacterial sulfate reduction (BSR) [1– 6]. Of the above, the latter has the most significant

35º Mt. effect on the isotopic composition of water column Hermon sulfate under anoxic conditions. The development of N anoxia in open water bodies is typical of estuarine 1 stratified water columns, where BSR tends to occur n

Jordan R. 2 33º a 33º e . continuously resulting in sulfate isotopic enrichment, 3 R n Sea of k ou and lowered sulfate concentrations. In closed system a m r Galilee ar r a Haifa Y environments the BSR and sulfate enrichment can e e t 4 i S follow the Rayleigh distillation curve [1–6]. d 5 Here, we discuss the fate of sulfur in the paleo-Dead e Sea (Lake Lisan) lacustrine system that follows a mod- M ified track of sulfate isotopic enrichment and Rayleigh distillation scheme. The Dead Sea brine evolved from Extent of an ancient Ca-chloride brine [7] (a general summary of Lake Lisan closed-basin brines is given in [8,9]). This brine is Tel Aviv highly enriched in calcium and relatively depleted 32º 32º with respect to sulfate, and involves an uncommon Jerusalem mechanism of gypsum precipitation. It has been suggested that the precipitation of massive gypsum layers 6 in Lake Lisan was induced by major lake level falls and water column overturns, that occurred during arid per- 9

7 Dead Sea iods in the region [10]. The purpose of this work is to W. Mujib study the sulfur system and the gypsum precipitation 8 mechanism in Lake Lisan, and its significance as a paleo-limnological tracer. We use the sulfur isotopic PZ-1 section 10 composition in saline and fresh waters and in various types of gypsum to evaluate the sulfur sources and 31º W. Hasa 31º initial isotopic composition as well as the processes Sinai that influenced its isotopic and chemical evolution. On the basis of this data we present a conceptual model 010 20 30 40 50 km describing the sulfur cycle under the particular limno-

logical–geochemical configuration of Lake Lisan. 35º Fig. 1. Location map of sampling sites along the Dead Sea basin– Jordan Valley. Maximum extent of Lake Lisan is marked in grey. 2. Geological setting Sediments were sampled in the Perazim Valley (PZ-1 section, marked by D). Water sampling sites are marked in numbers as 2.1. Lakes and brines in the Dead Sea basin follows: 1, Jordan River at Yosef Bridge; 2, Golan Springs; 3, Jordan River at Arik Bridge; 4, Hamme Tiberias; 5, Hamat Gader (Yarmouk River); 6, Enot Zuquim (brackish); 7, En-Kedem/En Several water bodies occupied the tectonic depres- Shalem saline springs; 8, En Gedi spring (freshwater); 9, Wadi sions along the Dead Sea rift during the Quaternary Mujib; 10, Wadi Hasa. A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 63

HCO3 and SO4 and is characterized by Ca/ In Lake Lisan, changes in lake level and mode of (SO4 +HCO3)N1 (equivalent ratio) [7,11,13]. After stratification are reflected in several geochemical the disconnection of the Sedom lagoon from the parameters such as the Sr/Ca and 87Sr/ 86Sr ratios open sea, the limnological and geochemical history [10,14,26,27] and the two limnological configura- of the now closed and terminal water bodies was tions (stratified and mixed) are characterized by spe- controlled by the interaction between the brine and cific sedimentation. The Ca-chloride composition of freshwater input from the drainage area of the Dead the brine, along with the bi-carbonate and Ca content Sea basin [14]. of the runoff water (draining carbonate terrains) led The volume of the freshwater input, and the lakes’ to saturation of the upper (mixed) layer of the lake limnological configuration (e.g., stratified vs. homo- with respect to CaCO3 (similar to modern condi- geneous lakes) reflect the climatic–hydrological his- tions). Saturation with respect to CaCO3 was main- tory of the region, which fluctuated during the tained through annual precipitation of aragonite Quaternary between arid to semiarid conditions whose crystallization, rather than calcite, was con- [10,14–17]. These changes were correlated to the trolled by the high Mg/Ca ratio of the brine [10,28]. global climate records of Greenland ice and deep- The slight excess of bi-carbonate over calcium in the sea cores [10,14,16,18]. runoff water required that aragonite precipitation The depth of the northern basin of the Dead Sea is consumed some calcium from the brine. However, 730 msl. The water level of the late Pleistocene because the brine was highly enriched in calcium, Lake Lisan fluctuated between 160 msl (highest this had little effect on its concentration. Sulfate that stand during the last glacial maximum) to less than reached the lake with the runoff water accumulated 450 msl (at the end of the Pleistocene) [14–17]. For in the upper layer, which was initially under-saturat- comparison, the water level of the Dead Sea is signif- ed with respect to gypsum. Provided sufficient time icantly lower, fluctuating around 400 msl during of lake stratification and sulfate accumulation, gyp- most of the Holocene. sum saturation was eventually attained. Alternatively, During times of enhanced rain precipitation, lake gypsum saturation could have also been achieved as level rose and a stratified water column developed. To lake level declined and the salinity of the upper water maintain stratification, a positive or balanced water layer increased. Nevertheless, relatively a few gyp- balance was required, i.e. evaporation was compen- sum layers are found in Lake Lisan sediments, while sated by at least the equivalent amount of freshwater those that are found were interpreted to have preci- inflow. Stability and thickness of the stratification pitated during overturn events [10]. According to must have been sufficient as not to allow time and Stein et al. [10], these gypsum layers, however, winds to cause significant mixing of the upper layer cannot account for the total calculated supply of (mixolimnion) with the lower brine (monimolim- sulfate to the lake. They speculated that the missing nion). A change to a negative water balance resulted sulfate must be present in the form of disseminated in water level drop and a decrease in the thickness of gypsum within the Lisan formation. the upper layer. The consequent increase in salinity and density of the upper waters weakened the stability 2.2. The Lisan formation at Perazim valley of stratification, leading to overturn and mixing of the entire water column. Such an overturn was recorded The Lisan formation (Fm.), deposited between ~70 in the Dead Sea in 1979 [19] following several to 14 ka BP [14,18,29,30], is exposed in the Dead Sea hundreds of years of stable stratification [20], that basin from south of the Dead Sea, to the Sea of were terminated by a sharp lake level drop that Galilee in the north (Fig. 1). The formation comprises began in the 1960s [21]. Prior to overturn, the thick- primarily of three lithologies: aragonite, gypsum and ness of the upper water column was 40 m [22]. Since silty detritus (made mainly of quartz, calcite, and then the lake is mostly holomictic, experiencing an- some dolomite and clay). Aragonite appears in lami- nual stratification and overturn. Short meromictic nated sequences where ~1 mm laminae alternate with periods (3–4 yr) develop following particularly rainy silty–detritus ones. The aragonite precipitated chemi- winters [22–25]. cally from the lake, whereas the detritus laminae are 64 A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 mainly eolian quartz and calcite, and erosion products that can be correlated over a large distance in the Dead of the rock cover from the Dead Sea rift shoulders Sea basin [37]. The Middle Member contains more [14,31,32]. It is important to note that in none of the abundant clastic beds (sand, silt and clay) alternating previous studies of the Lisan Fm., have pyrite or other with aad sequences and some gypsum. The Upper sulfide phases been identified in the sediments Member comprises mainly the aad facies with mas- [26,27,32,33]. sive gypsum layers at the top (bUpper Gypsum unitQ). In the Perazim Valley, west of Mt. Sedom diapir, U-series dating of Lisan aragonite obtained by several sections of Lisan Fm. have been measured and TIMS provides a high-resolution chronology of the described in details [18,29,32,34–37]. Here we focus PZ-1 section [18,29]. The ages range between 70 ka on the PZ-1 section (Fig. 2), which was divided into BP at the bottom to 14 ka BP at the top of the section. three stratigraphic members. The Lower Member con- The Lower Gypsum unit precipitated at 56 ka BP and sists mainly of balternating aragonite–detritus the upper one at 17 ka BP (Fig. 2). The boundaries laminaeQ (baadQ facies) and is topped by three thick between the Lower, Middle and Upper Members, co- (10–20 cm) gypsum layers (bLower Gypsum unitQ) incide with the transitions from marine isotopic stages

Lisan Formation Perazim Valley Section (PZ-1)

Marine δ18O Stages 40 Upper Gypsum Unit 2

35 Mainly laminated aragonite, a few prominent gypsum layers 30

25 3

20 Gypsum laminae Mix of laminated aragonite, gypsum, and clastic layers 15 Middle Member Upper Member Height above base (m) 10 Lower Gypsum Unit

5 4 Mainly laminated aragonite Lower

0 Member plus a prominent gypsum unit

Legend

Laminated aragonite Thick clastic layers Laminated aragonite Thick gypsum layers abundant thin clastic layers

Fig. 2. Lithology and stratigraphy of the PZ-1 section at the Perazim Valley (after Hasse-Schramm et al. [18]). The Lower Gypsum unit and the Upper Gypsum unit are the main gypsum units in the section, while thin gypsum laminae are abundant throughout the Middle Member. A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 65

(MIS) 4, 3 and 2 in the global record, respectively. The and saline water (representing the brines) inputs. d34 aad sequences that characterize the Lower and Upper Table 1 lists SSO4 values of representative water Members, were deposited during high lake-level con- samples from the drainage basin of the Dead Sea. The ditions, corresponding to the colder MIS 4 and 2, sampling sites are marked in Fig. 1. It can be assumed respectively, while the abundant clastic layers of the that most of these sources were also relevant during Middle Member reflect lower lake-stand conditions the Lisan period (~70–14 ka BP), when the lake was that prevailed during the warmer MIS 3 [14,18]. significantly larger than the present Dead Sea. A major water source to Lake Lisan was probably the upper Jordan River, which originates from the springs 3. Methods discharging at the foot of Mt. Hermon. The Jordan River was sampled at Yosef Bridge before it enters the The different types of gypsum that appear in the artificially dried Hula basin. The d34S value obtained Lisan PZ-1 section (discussed in Section 4.2) were there was 13.7x reflecting contributions of subsur- dissolved in de-ionized water, filtered, and the SO4 face Triassic gypsum [40,41]. Nissenbaum [41] precipitated as BaSO4. The precipitate was then col- reported lower values of 6.5–10.6x for the upper lected on a Whatmann ashless filter paper (#42), Jordan River. Further downstream, the Jordan River thoroughly washed with de-ionized water and burned receives water from the springs of the Golan Heights at 850 8C. The BaSO4 was introduced into a sulfur (basaltic aquifers and streams), which exhibit values vacuum extraction line at the Geological Survey of of d34S=12–13x. None of these water sources are Israel, where SO2 gas was produced and separated suspected of being contaminated and therefore their following the procedure described by Coleman and isotopic composition should represent the late Pleis- Moore [38]. The SO2 gas was collected in gas tocene solutions. The southern leg of the Jordan River ampoules, which were shipped to the British Geolog- between the Sea of Galilee and the Dead Sea was not ical Survey where the isotopic analyses were carried sampled because its current base flow is not of natural out on a VG Sira II mass spectrometer. The overall composition [42]. analytical reproducibility is F0.2x as determined on Other major freshwater sources that drained into repeated replicates of laboratory internal standard. The Lake Lisan include the Yarmouk River, the major international reference material NBS-127 gave an average of d34S=20.4x (n =5). Water samples were treated according to the pro- Table 1 cedures described by Stein et al. [10] and Gavrieli et d34S values in major water sources in the drainage area of the Dead al. [39]. Samples were collected directly into double- Sea and Lake Lisan 34 seal polyethylene bottles and refrigerated. Before Water source Date d S SO4 further treatment they were filtered to remove sus- (x) (mg/l) pended particles. SO4 concentrations in the water and North Jordan River 7/1994 13.7 26 [10] brine samples were measured in the Geological Sur- (Yosef Bridge) vey of Israel using an ICP-AES (Perkin Elmer Op- Golan basaltic springs 9/2000 11.9–12.7 4–8 tima 3300). Jordan River before Sea 3/1994 21 [10] of Galilee (Arik bridge) Sea of Galilee (surface) 7/1996 11.8 52 [10] Yarmouk River 3/2002 10.4 [42] 77 [42] 4. Results Dead Sea 1997 14.5 [39] 494 [39] Wadi Mujib 3/1996 6.9 60 4.1. d34S values in waters from the drainage area of Wadi Hasa 3/1996 5.5 120 Dead Sea saline springs 1996– ~22 [39] ~1000 [39] Lake Lisan 1997 En Gedi spring 5/1996 9.6 [39] 28 [39] The sulfur isotopic compositions of various waters (En Shulamit) from the drainage area of the lake are required to Enot Zuqim 10/1996 4.8 [39] 78 [39] constrain the composition of the freshwater input (freshwater spring) 66 Lake Lisan level (m below msl)

400 360 320 280 240 200 160 Age Massive gypsum (ka B.P.) Laminae 5 Disseminated 10

15 61–77 (2005) 236 Letters Science Planetary and Earth / al. et Torfstein A. pe ebr Lower Member MemberUpper Upper Gypsum Unit

35 Middle Member

55 Lower Gypsum Unit

70 -28 -24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 δ34S (‰ )

Fig. 3. Variationsin d34S values along the PZ-1 section. The main gypsum units display d34S values in the range of 14–28x, whereas the disseminated and gypsum laminae have depleted d34 S values in the range of 26x to +1x. Also shown is the reconstructed level curve of Lake Lisan (modified after Bartov et al. [16]). The Upper Member was deposited during high- stand period; the Middle Member during low stands. The deposition of the main gypsum units occurred following water column overturn during arid periods and water level drop. A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 67 tributary of the Lower Jordan River south of the Sea Table 2 34 of Galilee (d34S=10.4x), Wadi Mujib (d34S=6.9x) d S of sulfates in sediments of the Lisan Formation and Wadi Hasa (d34S=5.5x), the latter two dischar- Sample Height Lithology d34S ging directly to the Dead Sea from the Edom Moun- (cm)* (x) tains to the east. The wadis on the western escarpment Upper Member have no base flow and are characterized by winter IG-1A 3826 Laminated (4–6 mm) gypsum 26.4 IG-4 3776 Massive gypsum 25.7 flashfloods. Freshwater springs, such as En Gedi and IG-3 3770 Laminated gypsum 27.1 the brackish water at Enot Zuqim, yielded sulfur IG-2 3769 Alternating gypsum and detritus 27.1 isotopic composition in the range of 4.7–9.6x [39], IG-10 3690 Massive gypsum 28.5 typical to waters of carbonate terrains in Israel. d34S IG-11 3671 Mainly laminated gypsum 25.7 values in rainwater from the Dead Sea basin area lie in with detritus x the range of 0.6–8.5 [43–45]. Middle Member Another water source in the Dead Sea–Jordan IG-12 2909 Red laminae, mostly halite 26.1 basin are thermal saline springs, such as Hamme with some gypsum Tiberias at the Sea of Galilee, Hamat Gader in the IG-15 2540 Gypsum laminae with 23.5 Yarmouk River, and En-Shalem and En Kedem halite within aragonite IG-17 2380 Gypsum laminae 25.9 springs located on the shores of the Dead Sea. IG-18 2335 Laminated gypsum 18.0 These springs have sulfur isotopic compositions in IG-19 2335 Laminated gypsum 21.8 the range of 22–23x [39,46]. Based on hydrological IG-20 2330 Laminated gypsum 20.7 considerations, their chemical composition, low IG-21 2327 Laminated gypsum 22.2 87Sr/ 86Sr ratios and d34S values, it has been suggested IG-21a 2327 Laminated gypsum 22.2 IG-21b 2327 Laminated gypsum 21.8 that some of the saline springs are diluted remnants of IG-22 2325 Laminated gypsum 21.9 the Sedom Ca-chloride brine that percolated into the IG-23 2119 Gypsum and detritus 15.8 Cretaceous wall rocks of the basin and remained there IG-24 2092 Laminated gypsum 18.0 for a substantial time period [10,39,47]. These, or IG-25a 2090 Columnar gypsum within detritus 16.2 chemically similar brines had limited contribution to IG-25b 2090 Gypsum and detritus 15.0 IG-26 1950 Columnar gypsum within detritus 14.1 the chemical budget of Lake Lisan [10]. IG-27 1412 Gypsum laminae 13.4 L-8-T 1247 Disseminated in aragonite 12.8 4.2. d34S values in gypsum samples from the Lisan PZ-1 section Lower Member IG-31 450 Laminated gypsum 18.3 IG-30 390 Massive gypsum 19.3 Samples for the present study were collected from IG-29 330 Laminated gypsum 17.9 the three distinct types of gypsum that appear in the L-3 290 Disseminated in aragonite 4.4 PZ-1 Lisan section: (1) massive and laminated gyp- L-7 82 Disseminated in aragonite 1.1 sum units that are 10–20 cm in thickness (the major PZ-1-0 0 Disseminated in aragonite 0.9 gypsum units are marked in Fig. 2), (2) thin (less than * Height refers to elevation above base line of PZ-1 section in 5 mm) gypsum laminae, which are abundant in the Fig. 2. Middle Member of the formation, (3) disseminated gypsum, extracted by water leaching of aragonite laminae. layers from the Middle Member yielded d34S values The d34S values of the gypsum samples from the in the range of 14–22x. The gypsum laminae and the PZ-1 section are illustrated in Fig. 3 and are listed in disseminated gypsum are characterized by negative Table 2 along with the lithological description of the d34S values in the range of 26x to 13x and samples. The Lower Gypsum unit (~4 m above the 13x to +1x, respectively. The d34S values of the base of the formation) is characterized by d34S=18– thick gypsum layers appear to increase from bottom to 20x while the Upper Gypsum unit (capping the top of the section, while combined, the disseminated Upper Member) displays significantly more 34S and thin laminae of gypsum seem to form a negative enriched compositions of d34S=26–28x. Gypsum trend. 68 A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77

5. Discussion The saline springs, with high sulfate concentrations, could contribute sulfate with enriched isotopic values 5.1. d34S values in the ancient Dead Sea brine (~22x; Table 1; [39]) to the lake. However, it was argued that the saline springs could have only limited The d34S isotopic value of the ancient brine in the contribution to the chemical budget of the lake [10]. Dead Sea basin was dictated by the composition of Moreover, considering the isotopic composition of the the Sedom lagoon, which was filled by seawater that saline springs, changes in their relative sulfate contri- entered the basin during the Pliocene. The evaporated bution cannot account for the isotopic enrichment of seawater precipitated gypsum, interacted with the Cre- the Upper Gypsum unit nor for the isotopic depletion taceous wall rocks of the basin, dolomitized them, of the disseminated gypsum. precipitated more gypsum and returned to the lagoon For the discussion and the conceptual model that as Ca-chloride brine [7,11,13,39]. Precipitation of gyp- follows we assume an inflowing sulfate average iso- sum involves only limited fractionation of sulfur isotopic composition of 10x. While this composition topes whereby 34S is enriched in the solid phase may be offset by a few per mill either way, it would leading to slight isotopic depletion in the residual not change the results of the proposed model and the solution (D34S=1.65xF0.12x; [4,48]). The Sedom conclusions regarding the unique sulfur cycle that Fm. salts display d34S values of 16.5–20.8x [11], i.e., operated in Lake Lisan. the same to somewhat depleted values relative to Neogene seawater (~21x; [2,49]), most likely indi- 5.3. Sulfate in the modern Dead Sea brine and in Lake cating some contribution of freshwater in the early Lisan stage of the lagoon history. After the disconnection of the Sedom Lagoon from Sulfate concentration in the Ca-chloride brines of the open sea, BSR at the bottom of the lakes and the Dead Sea basin (modern Dead Sea water and mixing between the Sedom brine and freshwater could saline springs on the lake margins) is limited by the have modified the sulfur isotopic composition. In- high Ca concentration and gypsum saturation: the deed, BSR has been identified in the modern Dead more saline and Ca-rich the brine is, the lower is its Sea and its sediments when the lake was still stratified sulfate concentration. [33,45]. Thus, changes in the d34S values that char- In the modern Dead Sea (TDS=340,000 mg/l) acterize the lacustrine bodies in the Dead Sea basin the high calcium content (18,000 mg/l) and gypsum have been governed by the sulfate content and d34S saturation dictate sulfate concentration of only 450– values in the inflowing freshwater, in the brines, the 500 mg/l. Even at these relatively low sulfate con- product of their mixing, and processes within the centrations, the brine is somewhat over-saturated lakes. with respect to gypsum but kinetic factors limit its precipitation [50,51]. Higher sulfate concentrations 5.2. d34S of the inflowing sulfate in the Dead Sea brines were reported in the 1950s. At that time, the Dead Sea was still a stratified lake, The relative contributions of the freshwater sources with a relatively dilute upper layer (TDS=300,000 to the Dead Sea/Lake Lisan were estimated by Stein et mg/l, Ca=16,400 mg/l; [22]). The lower Ca con- al. [10] on the basis of strontium isotopes and chem- centration enabled a higher sulfate concentration of ical composition of the Lisan aragonite and contrib- ~580 mg/l, while still maintaining saturation with uting solutions. They suggested that northern Jordan respect of gypsum. As the Dead Sea level declined, River and runoff waters from the basin shoulders the salinity of the upper layer as well as Ca con- contributed about 1:1 mixture of the freshwater en- centration increased, and gypsum precipitated from tering the lake. Applying this ratio, and considering the brine [22]. Because of the extremely low SO4 / the sulfate concentration of the various freshwater Ca ratio in the brine, the precipitation of gypsum sources (~20–120 mg/l; Table 1), the sulfur isotopic lowered the SO4 concentration, while Ca concentra- composition of freshwater sulfate that flowed into tion continued to increase along with the overall Lake Lisan was in the range of 6–14x (Table 1). salinity. A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 69

The Ca-chloride brine of the Dead Sea and Lake tion of the brine. This process takes place under Lisan, and the freshwater sources derived from the anoxic conditions following the very simplified carbonate terrain in the drainage basin, have different overall reaction: SO4 and Ca contents, but rather similar HCO3 abun- 2 dances [10]. Thus, when the calcite-saturated fresh- 2CH2O+SO4 Z 2HCO3 +H2S. (1) water enters the aragonite-saturated lake, its HCO3 is removed from the upper water layer as aragonite. BSR is accompanied by isotopic fractionation 32 Most of the calcium that precipitates as aragonite is whereby preferential reduction of SO4 results in also derived from the freshwater. Yet, because there is the remaining sulfate becoming typically enriched in 34 some excess of HCO3 over Ca in the inflowing fresh- S. The fractionation associated with BSR varies water (i.e. Ca/HCO3 b1), some contribution of Ca over a wide range of 5x to (46)x [53–59] and is from the brine is also required. Thus, Ca concentration dependent on various factors, including temperature, remains nearly unchanged, or decreases as a function rate of reduction, bacteria species, and availability of of the dilution of the upper water column. On the sulfate. Under closed system conditions and known other hand, as long as gypsum does not precipitate fractionation factor, the evolving isotopic composition from the upper water column, sulfate accumulates and of the remaining sulfate follows a Rayleigh distillation its concentration rises. equation which can be approximated by [3]: The saline springs around the Dead Sea, which are 34 34 saturated with respect to gypsum, have a salinity d S ¼ d Si þ 1000ðÞa 1 lnf ð2Þ approximately half that of the Dead Sea, and contain lower Ca and higher sulfate concentrations (about where: f is the fraction of sulfate remaining in solu- 34 34 34 11,500 and 1000 mg/l, respectively; [39]). These tion; d Si, d S are the initial and measured d SSO4, compositions probably resemble those of Lake respectively; and a is the sulfur fractionation factor. Lisan, whose salinity ranged between a third and a During stratification, anoxic conditions develop in half that of the Dead Sea [27,32,52]. the lower brine as oxygen is consumed through the The differing isotopic (87Sr/ 86Sr ratios) and chem- oxidation of organic matter, which sinks to the deep ical (Sr/Ca ratios) compositions of aragonites and water from the upper layer. Once all or most of the the corresponding disseminated salts in the Lisan oxygen has been consumed, the oxidation of the Fm. provide evidence for the preservation of differ- remaining organic matter continues through bacterial ence in compositions between the upper and lower reduction of nitrate, Mn- and Fe-oxyhydroxides and water bodies during stratified periods [10,14,26,27]. sulfate [60 and references therein]. Prolonged stratifi- This does not imply that some diffusion and mixing cation periods, lasting tens to a few thousands of years between the two waters did not take place at the are possible in deep saline lakes such as Lake Lisan interface and transition layer. In fact, it is possible and the Dead Sea, when freshwater inflow dilutes the that some gypsum precipitated at this interface due to upper waters and the salinity of the upper layer is mixing between the relatively sulfate-rich upper significantly lower than that of the lower brine. As layer, and more concentrated and calcium-rich mentioned above, during meromictic periods limited lower waters. However, we consider this process to mixing probably occurs across the transition layer. be minor as it still enabled the two water bodies to Nevertheless, the composition of the lower brine dur- develop a different chemical and isotopic signature ing meromictic periods changes very little. At the over a prolonged time period. As will be discussed same time, it is likely to develop anoxic conditions hereafter, this is particularly true of the sulfur isoto- which favor BSR activity. Indeed, prior to the 1979 pic composition. overturn, the lower water body of the Dead Sea contained H2S [22,33,45], and coexisting sulfate and 5.4. Bacterial sulfate reduction sulfides showed sulfur isotope fractionation of 25– 30x, suggesting H2S production by BSR [33,45]. Bacterial sulfate reduction (BSR) is a major pro- Similar sulfur isotope fractionations were found in cess that can change the sulfate isotopic composi- various hypersaline springs and groundwater around 70 A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 the Dead Sea [33,39] as well as in sediments of the lake and at the same time the accumulation of entering Dead Sea [45,61], indicating that BSR can thrive in sulfate in the upper layer (Fig. 4a). Periods of positive the hypersaline environment of the Dead Sea brine water balance resulted in lake level rise, development system. Some of these anoxic groundwaters have of relatively dilute upper water column and the main- chemical compositions very similar to the Dead Sea tenance of prolonged water column stratification. The water, yet their sulfate isotopic composition is highly build-up of sulfate concentrations in the upper layer is 34S enriched, attaining values as high as d34S=60x imperative for the precipitation of massive gypsum [39]. layers during the following overturn events. Given Evidence for the occurrence of BSR in Lake enough time and sulfate input, the upper layer even- Lisan is provided by the isotopic composition of tually attained saturation with respect to gypsum. the disseminated gypsum and the thin gypsum lam- From this stage onward, additional freshwater inflow inae. The occurrence of these gypsum types in the to the lake would result not only in precipitation of sediments, in addition to the massive gypsum layers, aragonite, but also with that of gypsum (Fig. 4b). was previously suggested by Stein et al. [10] on the However, the isotopic composition of the laminated basis of mass balance calculations (i.e., the bmissingQ 34 2- δ34 ≈ sulfate). The low d S values of both (13x to a. SO4 ; S 10‰ inflow +1x in the disseminated gypsum and 26x to 13x in the thin gypsum laminae; Table 2; Fig. 2- 3) indicate that the sulfate sulfur is of biogenic Upper Water Layer [SO4 ] ➔ origin. In fact, the most depleted d34S values may δ34S ➔

point to further isotope fractionation during bacterial ➔ 2- [SO4 ] disproportionation of sulfur intermediate products Lower Brine δ34 produced during anoxygenic phototrophic oxidation S ➔ of H2S [59,62,63]. On the other hand, the more 2- ⇒ - 2CH2O + SO4 2HCO3 + H2S enriched d34S values determined for the disseminated gypsum could be an analytical artifact. During the dissolution of the disseminated salts, some aragonite SO 2- ; δ34S ≈ 10‰ also dissolves, thereby contributing sulfate that co- b. 4 precipitated with the aragonite from the upper layer.

As outlined below, this aragonite-derived sulfate is Ca2+ + SO 2- +2H O ⇒ CaSO .2H O 2- 4 2 4 2 [SO4 ] Const probably characterized by isotopic composition clos- δ34S SS er to the freshwater sulfate composition, i.e., 34 c 2- d S 10x, and far removed from the depleted [SO4 ] Const . ⇒ 2+ 2- compositions found in the thin gypsum laminae or δ34S SS CaSO4 2H2O Ca + SO4 measured in the disseminated gypsum. The source of 34 2- ⇒ - the isotopically depleted d S values of the thin 2CH2O + SO4 2HCO3 + H2S gypsum laminae and disseminated gypsum is discussed in the following sections. Fig. 4. Schematic representation of the bsulfur pumpQ model. b Q Arrows denote trends of increasing (upward) or decreasing (down- 5.5. The sulfur pump in Lake Lisan 34 ward) SO4 concentrations or d S values: a) Onset of stratification. Sulfate from inflowing freshwater accumulates in the upper water Due to the high salinity of Lake Lisan, the contri- layer while sulfate is consumed in the lower brine through bacterial bution of most incoming ions to the salt budget of the sulfate reduction (BSR). b) Full operation of the bsulfur pumpQ. The water column was generally relatively small, enabling upper water layer attains saturation with respects to gypsum, which the preservation of the ionic ratios typical to the crystallizes and precipitates to the lower brine where it dissolves. BSR consumes sulfate but its concentration rises due to gypsum original Ca-chloride brine [10]. The unique Ca-chlo- dissolution until saturation with respect to gypsum is reached. d34S ride composition dictated a swift removal of the rel- rises until steady state isotopic composition (SS), defined by Eq. (4), atively large quantities of bi-carbonate that entered the is attained. A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 71 and disseminated gypsum, characterized by d34S bsulfur pumpQ, by its nature, leaves little evidence in values of 26x to +1x, which are much lower the sedimentary record of the Lisan Fm., and its d34 x d34 than SSO4 in the freshwater (~10 ) or in the identification relies on the interpretation of S brine, are not consistent with direct deposition of values in the various types of gypsum. gypsum from the upper water layer to the sediments. Massive gypsum precipitation occurs during over- More likely, these depleted isotopic compositions re- turn, when the gypsum-saturated upper and lower flect sulfide production by BSR as argued above. To water bodies mix and out-salting occurs. The d34S resolve this problem we propose a mechanism termed composition of the gypsum will be that of the mixed the bsulfur pumpQ. water body which in turn depends on the relative The rate of oxygen consumption in the lower brine volumes, concentrations and isotopic compositions during stratified periods depends largely on the supply of the two water layers before the overturn. This of organic matter from above. It is reasonable to boverturn gypsumQ should have higher d34S values assume that by the time gypsum begins precipitating compared to gypsum precipitating pre-overturn from from the upper layer, the lower brine has already the upper layer. The latter might not be present at all developed anoxic conditions and BSR is active. In- in the sediments since it may have dissolved in the tensive BSR in the lower brine would decrease the lower brine, or at the bottom of the lake, as might sulfate concentration in the brine, which may become have been the case for the Dead Sea [22,33]. under-saturated with respect to gypsum. Gypsum that It has been suggested that gypsum precipitation precipitates from the upper layer and sinks through the took place during meromictic periods due to some lower brine is then susceptible to dissolution. Accord- mixing across the transition layer between the water ingly, the inflowing sulfate is transported via contin- bodies [10]. The isotopic composition of this gypsum uous gypsum precipitation from the upper layer to the would represent a mixture between the two water lower brine where it re-dissolves. Concurrent BSR in bodies at proportions that are different than during the lower brine results in 34S-depleted sulfide and 34S- full overturn or when gypsum precipitates solely from enriched sulfate (Fig. 4b). This bsulfur pumpQ can the upper water column, and may therefore smear the work as long as sulfur supply and gypsum precipita- change in the isotopic signal expected during the tion from the upper layer continues. When the rate of transition from a stratified to a homogeneous water gypsum precipitation from the upper layer and its column. dissolution in the lower brine outweighs the rate of sulfate reduction, sulfate concentration recovers. 5.6. Sulfide precipitation and oxidation Eventually the lower brine attains its original sulfate concentration, and becomes saturated with respect to Stein et al. [10] showed, based on mass balance gypsum. Reduction of sulfate at this stage is compen- considerations, that not all the sulfate that reached sated by gypsum dissolution, while the excess gyp- Lake Lisan precipitated in the form of the massive sum reaches the bottom sediments. gypsum layers. They suggested that this bmissingQ Anoxic conditions and the bsulfur pumpQ will pre- sulfate, which according to them precipitated due to vail for some time even after a change to a negative mixing and grazing of the interface between the lower water balance, and will cease only when overturn and upper water layers, is present in the Lisan Fm. in occurs and the entire water column oxidizes. Prior the form of disseminated gypsum. The isotopic data to the overturn, gypsum will precipitate from the presented in this paper indicates that the bmissingQ upper layer as an evaporitic mineral, in response to sulfate was first reduced to sulfide and only then increasing salinity, and less due to sulfate inflow. removed from the water column. Sulfate concentration will decrease as the general The 34S depleted sulfides were most probably re- salinity and Ca content increases, and the upper moved from the lake system through precipitation of layer approaches a chemical composition similar to Fe-sulfide phases and burial in the sediments. The high that of the lower brine. At this stage, the most signif- sedimentation rate of Lake Lisan decoupled these icant difference between the two water layers could be phases from the aqueous system soon after deposition. their sulfate content and isotopic composition. The Thus, during water column mixing and oxidation the 72 A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77

34 reduced sulfides were not susceptible to oxidation and Eq. (3) is plotted in Fig. 5 for different d Si SO4 x therefore were not recycled back to the brine. This compositions and with a =0.97 (DH2S=30 ). The enabled the preservation of an enriched isotopic com- latter value is based on a fractionation factor that position also after overturn and oxidation. characterized the meromictic Dead Sea prior to the Oxidation of the Fe-sulfides (whether pyrite, grei- 1979 overturn and is similar to the values found in the gite or any other sulfide mineral or amorphous phase), hydrothermal springs around the Dead Sea [39,45,61]. occurred following the lake desiccation and sediment It can be assumed that the BSR in Lake Lisan was exposure. Such oxidation involves only minor isoto- accompanied by the same fractionation factor. With pic fractionation [59,64,65], enabling the preservation time, continuous BSR leads to a steady state sulfur of the original depleted values. Given the abundance isotopic composition, as observed in Fig. 5 and de- of calcium in the exposed sediments, the oxidized fined by Gavrieli et al. [39]: sulfur quickly precipitated and formed the disseminat- d34S ¼ d34S þ DSO4 ð4Þ ed and thin laminae of gypsum. Though no Fe–S ss precipitate H2S species were identified in the Lisan Fm., their occur- 34 rence was reported in the Holocene Dead Sea sedi- where: d Sss is the steady state isotopic composition; SO4 ments [61]. Thus, contrary to the Lisan Fm., the DH2S is the isotope fractionation associated with the freshly exposed Holocene sediments have not yet BSR. undergone full oxidation and still contain the original reduced phases. 5.8. The evolution of sulfur in Lake Lisan

5.7. Deviation from Rayleigh distillation process The steady state d34S values of the lower brine are dictated completely by the BSR fractionation The bsulfur pumpQ describes an open system mech- factor and the isotopic composition of the replenish- anism that causes the isotopic composition of the ing sulfate, and are not affected by the initial isotopic sulfate in the lower brine to deviate from a simple composition of the brine (Fig. 5). Based on present- Rayleigh distillation behavior described by Eq. (2). day d34S values in the main freshwater sources in the Rather, we consider a bmodified Rayleigh distillation Dead Sea drainage system (Table 1), the waters processQ for an open system, previously suggested by entering Lake Lisan were in the range of d34S=6– Gavrieli et al. [39] for the groundwater around the 14x. Under prolonged stratification and sulfate in- Dead Sea. In this process, the fractionation between flux, the upper layer would approach the isotopic the reactant sulfate and product sulfide remains con- composition of this inflowing sulfate. Thus, follow- stant, and the sulfate content of the lower brine does ing Eq. (4) and Fig. 5, the steady state d34S values of not change. At each time step of the modified Ray- the lower brine during prolonged stratification periods leigh distillation the reservoir of sulfur in the lower could have attained values as high as ~40x and still brine is replenished by the dissolving sulfate and remain saturated with respect to gypsum. The steady attains the following sulfur isotopic composition: state composition is attained when 3–4 times the original amount of dissolved sulfate have been re- 34 34 d S ¼ d Si þ 1000ðÞa 1 lnðÞ 1 F duced (Fig. 5). In the following discussion, we assume that the 34 þ d SprecipitatedF ð3Þ lower brine occupied the deepest depression in the basin and its volume could not have been significantly 34 where: d Si is the S–SO4 isotopic composition of the different than that of the present day Dead Sea (pres- reservoir (the lower brine) prior to current BSR time ent water level of 420 msl), and therefore we use the step; F is the fraction of sulfate that is reduced; Dead Sea properties as representative of those of Lake 34 d Sprecipitate is the S–SO4 isotopic composition of Lisan’s lower brine. The sulfate reservoir of the Dead the sulfate that replenishes the reservoir (i.e., the Sea amounts to ~6 g/cm2 [10]. Assuming: 1. sulfate gypsum that precipitates from the upper layer and concentrations in incoming freshwater of 20–120 mg/ dissolves in the lower brine). l, and 2. an annual freshwater influx of 2–5 m/cm2 A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 73

34 Sss ) ‰ S ( 34 Initial=10‰; Input=5‰

Initial=10‰; Input=15‰ Most probable scenario for Lake Lisan Initial=15‰; Input=10‰ Rayleigh distillation

Fig. 5. Changes in d34S composition of sulfate in open systems where bacterially reduced sulfate is replenished by sulfate input (e.g. gypsum dissolution), thereby maintaining constant sulfate concentration. The calculations are based on a fractionation factor of 30x with initial sulfate 34 compositions of 10x and 15x, and sulfate inputs of 5x,10x and 15x (Eq. (3)). Note that the d Sss (Eq. (4)) is approached when the 34 equivalent of 3–4 times the initial sulfate content has been reduced. The d Sss depends only on the fractionation factor and the input values. Assuming an initial isotopic composition of a Dead Sea like brine (~15x) and an input composition of 10x (see text), the designated graph 34 34 describes the most probable scenario for Lake Lisan. The d Sss value for Lake Lisan is approximately 40x. Higher d Sss values can be attained through input of more enriched sulfate. The units at the bottom abscissa (N) are the number of times the original sulfate content was replaced. The latter may also represent time units, depending on the residence time of sulfate in the lake (see text). The units at the upper abscissa indicate the fraction of sulfate (between f =1 and f =0) according to the Rayleigh distillation in a closed system.

[10], then the residence time of sulfate in the Dead Gypsum unit took place at ~56 ka BP following ~5 ka Sea is in the range of 100–1500 yr. Thus, the time it of aragonite–silty detritus deposition, which in turn would take the lower brine to reach a steady state came after a major depositional hiatus at ~61 ka BP composition, assuming it is reached when the amount [18]. Between 61 and 56 ka BP water level was much of sulfate reduced is about four times the original lower than during the long meromictic period preced- dissolved sulfate (Fig. 5), may range between 400 ing the deposition of the Upper Gypsum unit [16,18]. and 6000 yr. These time periods could be somewhat Therefore it is not clear whether the ~5 ka time longer in the case of Lake Lisan because its lower interval preceding the deposition of the Lower Gyp- brine could have been slightly dilute relative to pres- sum unit represents a stable period during which ent day Dead Sea conditions. Thus, the lower Ca prolonged stratification was established. 34 concentrations would allow higher SO4 concentra- The difference between the d S values of the tions at gypsum saturation, and the arrival time to Lower Gypsum unit and the isotopically heavier steady state could be longer accordingly. Upper Gypsum unit could therefore reflect the follow- Since the precipitation of the Upper Gypsum unit ing scenarios: occurred after at least ~10 ka of high water level stand (27–17 ka BP; [18,66]), suggesting prolonged mero- 1. The steady state isotopic composition (described mictic conditions, it is reasonable to assume that prior by Eq. (3) and Fig. 5) was not reached in the lower to its deposition the water column approached steady brine prior to the deposition of the Lower Gypsum state values. In contrast, the deposition of the Lower unit because of frequent mixing events during 74 A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77

lower lake stands. This would result in a less flux of sulfate to and from the water body, and con- effective or no BSR operation in the lower brine. current bclosed systemQ characteristics of the lower Alternatively, a ~5 ka meromictic period, that water layer can coexist. Sulfate replenishment is pro- might have preceded the Lower Gypsum unit de- vided in the form of solid gypsum, requiring no water position, might have been too short to attain steady circulation. Such a unique setting enables the attain- state conditions. ment of enriched isotopic composition without deple- 2. During the high stand period of 27–17 ka BP, the tion of the sulfate reservoir. amount of organic matter supplied from the upper water layer to the lower brine and available for BSR was higher. This is due to an increase in 6. Conclusions bioactivity enabled by the lowered salinity of the upper water column and from increased inflow of a) Lake Lisan, the late Pleistocene precursor of the detrital material. This organic matter could support Dead Sea (70–14 ka BP), evolved through cycles enhanced BSR activity in the lower waters prior to of stratifications, overturn episodes and periods of the deposition of the Upper Gypsum unit. In- relatively well-mixed water column. In the present creased bioactivity in the Upper Member period study, d34S in gypsum has been established as a is supported by higher d13C values in correspon- paleo-limnological tracer that can be used to con- ding aragonite laminae compared to the Lower strain modes of lake configuration. Member [67,68]. b) Three distinct types of gypsum appear in the PZ-1 Lisan Fm.: massive and laminar gypsum layers The behavior of the sulfur cycle in Lake Lisan, as (d34S=14–28x), thin gypsum laminae (d34S= illustrated by the bsulfur pumpQ mechanism, is a direct (26)–(13)x), and disseminated gypsum result of the lake’s unique Ca-chloride composition. (d34S=(13)–1x). The accumulation of sulfate from freshwater sources c) The d34S value in the gypsum is dictated by the results in attainment of gypsum saturation in the upper composition of the freshwater sulfate input to the water column. Once this is achieved, any additional upper water layer, bacterial sulfate reduction (BSR) incoming sulfate would lead to gypsum crystallization in the lower brine and the degree of mixing be- and sinking to the lower brine where it is susceptible tween the two water layers. Freshwater input is to dissolution. Accordingly, the replenishment of the characterized by d34S values of 6–14x, while 34 sulfate reservoir in the lower water column does not d SSO4 in the lower brine is significantly higher, require any additional mass transfer (i.e., water circu- due to BSR. lation) to the lower waters. d) Evidence for BSR is provided by negative d34S In many non-closed-system anoxic water bodies, values of the disseminated gypsum and thin gyp- such as the Black Sea, sulfate removal by reduction is sum laminae (26x to +1x). This gypsum is the compensated at least partially through water circula- product of oxidation of the sulfide that was depos- tion (Mediterranean Sea water, in the case of the ited during stratified-anoxic periods in the lake’s Black Sea). Despite long periods of anoxia, sulfate history. Oxidation took place after the exposure of concentration remain steady but at lowered levels, the sediments, during low stands, such as those that while the isotopic values are enriched but do not prevailed during the Holocene. reach the steady state values described by Eq. (4) e) We propose a bsulfur pumpQ mechanism, whereby [69-72 and references therein]. In contrast, sulfate gypsum precipitates from the upper layer and re- reduction in closed systems, as proposed for example dissolves in the anoxic lower brine where BSR in bsnowball earthQ models, follows the Rayleigh operates. This process enables a steady state sulfate distillation curve. Accordingly, the sulfate is progres- concentration and isotopic composition in the sively exhausted to a minimum and reaches extremely lower brine. The steady state d34S value is defined high isotopic values [73–75]. by the composition of the inflowing freshwater It is only in a Ca-chloride environment, such as sulfate as well as the fractionation associated with Lake Lisan, that the open system behavior of constant the BSR (30x in the Dead Sea brine system). A. Torfstein et al. / Earth and Planetary Science Letters 236 (2005) 61–77 75

Thus, the lower brine may attain sulfur isotopic [5] M.B. Goldhaber, I.R. Kaplan, The sulfur cycle, in: E.D. Gold- composition as high as ~40x, following prolonged berg (Ed.), The Sea, Vol. 5, Marine Chemistry, Wiley, New York, 1974, pp. 569–655. stratification, and yet remain saturated with respect [6] H. Strauss, The isotopic composition of sedimentary sulfur to gypsum. through time, Palaeogeogr. Palaeoclimatol. Palaeoecol. 132 f) Two limnological configurations are revealed by (1997) 97–118. the d34S values in Lake Lisan gypsum: (I) Pro- [7] A. Starinsky, Relationship Between Ca-chloride Brines and longed stratification and efficient bsulfur pumpQ Sedimentary Rocks in Israel, Ph.D, Hebrew University, Jeru- salem, 1974 (in Hebrew, Engl. abstract). operation that enables attainment of steady state 34 [8] L.A. Hardie, H.P. Eugster, The evolution of closed-basin values. This scenario is illustrated by the high d S brines, Mineral. Soc. Am. Spec. 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