Ittai Gavrieli

The sulfur system in anoxic subsurface brines and its implication in brine evolutionary pathways: the Ca-chloride brines in the Dead Sea area

Abstract Important elements in the evolutionary history of saline groundwater might be overlooked when they involve both sulfate removal through reduction and input of sulfate via dissolution. These two simultaneous and apparently contrasting processes can result in a negligible net effect on the sulfate concentration. Isotopic composition of sulfur in sulfate and sulfide can be applied to identify the bacterial sulfate reduction (BSR) though the extent of the process is difficult to quantify. Saturation with respect to gypsum may suggest that gypsum dissolution also occurs. However, a more definite identification of these processes and their quantification can be achieved through the use of ammonium concentration in the anoxic brines. This approach assumes that the ammonium is derived only from the oxidation of organic matter through BSR and it requires that the C:N ratio in the oxidized organic matter be known. A minimum estimate for the sulfate reduction can be obtained when the Redfield C:N ratio (106:16) is assumed. Several calculation methods are presented to identify the extent of sulfate reduction prior to, concomitant with, or following gypsum dissolution that are based on combining sulfur isotopic compositions, Rayleigh distillation equation, and calculated gypsum saturation indices. The required assumptions are presented and their validation is discussed. The subsurface hypersaline Ca-chloride brines in the vicinity of the Dead Sea are taken as a case study. Here sulfur isotope compositions of sulfate and sulfide, and high ammonium concentrations indicate BSR occurs in the subsurface. The sulfur isotopic composition of the sulfate makes it possible to distinguish between two major groups of brine and their recent evolutionary histories: (1) the Qedem–Shalem thermal brines (δ 34 S SO4 =21–24‰) which emerge as springs along the shores and are slightly undersaturated with respect to gypsum; (2) DSIF–Tappuah brines (δ 34 S SO4 =30–60‰) which are found in shallow boreholes and are saturated to oversaturated with respect to gypsum. Calculations based on their ammonium content suggest that both groups of brine require apparent unreasonably high oversaturations with respect to gypsum prior to the onset of the reduction. This implies that the groundwater systems were open with respect to sulfate, and that the sulfate reservoir was replenished continuously or intermittently during the BSR. The DSIF–Tappuah brines continue to dissolve gypsum during their BSR. The dissolving sulfate is derived from relatively isotopically enriched gypsums (δ 34 S SO4 >20‰), such as found in the Lisan Formation. These brines approach the steady-state isotopic composition (δ 34 S ss ) dictated by the combination of the δ 34 S of the dissolving gypsum and the fractionation factor accompanying BSR. The sulfur isotopic composition of the Qedem–Shalem brines implies that most of their ammonium content is derived from an earlier phase of BSR and that the last phase of BSR takes place during the brines’ rapid ascent to the surface. Prior to this stage they evolved through either: (1) dissolution of gypsum with δ 34 S SO4 ≤20‰ which occurred after the main BSR in the subsurface; (2) a previous phase in which the brines were part of a lake and later percolated to the subsurface. As such, their isotopic composition and ammonium content were determined by the combined effect of freshwater sulfate input to the lake and BSR in the stratified lake.

The Dead Sea as a Dying Lake

Anthropogenic intervention has severely disturbed the Dead Sea as an ecosystem. The water level has been decreasing at a rate of nearly one meter per year during the last decade. In 1979 the water column overturned, thereby ending centuries-long stratification. Since then the lake is mostly holomictic, with annual built up of stratification in spring and its destruction in late autumn. The negative water balance of the lake results in a gradual increase in its salinity. The lake is saturated with respect to NaCl, and halite is precipitating to the bottom. The main components of the Dead Sea biota are the unicellular green alga Dunaliella and several red halophilic Archaea. Massive microbial development is possible only when the upper water layers become diluted with more than 10% fresh water and phosphate is available. Dense microbial blooms occurred in 1980 and in 1992. In both cases the archaeal community imparted a reddish color to the lake. Today the lake is virtually devoid of microbial life.

COPRECIPITATION OF TRACE AND MINOR ELEMENTS IN MODERN AUTHIGENIC HALITES FROM THE HYPERSALINE DEAD SEA BRINE

Abstract Modern halite samples were collected from the Dead Sea (post 1983) and analyzed for their minor (Br, Mn, K, Ca) and trace (Cd, Pb, Zn, Ni) element concentrations. The halites collected include sedimentary halites with different morphologies and halites which crystallized on ropes suspended in the water body (rope-halites). The mechanisms of minor and trace element coprecipitation with these halites are discussed, and their apparent distribution coefficients are calculated (DMn = 0.09, DK = 3.6 × 10−4, DCa = 5 × 10−5, DBr = 0.011, DCd = 9.3, DPb = 3.5, DZn = 0.041, DNi = 0.11) mainly based on the sedimentary halites, which represent the slowest crystallization rates and are therefore crystallized at or closest to equilibrium with the brine. Several types of halites which crystallized at different crystallization rates exhibit large variations in coprecipitated Mn and to a lesser degree of coprecipitated K and Ca. The wide range of calculated DBr and DK values in halites precipitated from the Dead Sea brine and from evaporated seawater are probably due to kinetic factors and not to differences in the parent brine compositions. The present study apparent distribution coefficient values probably best represent, both the Dead Sea brine and evaporated seawater at similar ionic strengths. The continuous precipitation of halite since 1983 played a major role in the removal of Cd and Pb from the Dead Sea brine and only a minor role in the removal of Zn and Ni. Based on estimates of inventories in the Dead Sea, the removal flux of Cd and Pb are discussed.

Anaerobic oxidation of methane by sulfate in hypersaline groundwater of the Dead Sea aquifer

Geochemical and microbial evidence points to anaerobic oxidation of methane (AOM) likely coupled with bacterial sulfate reduction in the hypersaline groundwater of the Dead Sea (DS) alluvial aquifer. Groundwater was sampled from nine boreholes drilled along the Arugot alluvial fan next to the DS. The groundwater samples were highly saline (up to 6300 mm chlorine), anoxic, and contained methane. A mass balance calculation demonstrates that the very low δ13CDIC in this groundwater is due to anaerobic methane oxidation. Sulfate depletion coincident with isotope enrichment of sulfur and oxygen isotopes in the sulfate suggests that sulfate reduction is associated with this AOM. DNA extraction and 16S amplicon sequencing were used to explore the microbial community present and were found to be microbial composition indicative of bacterial sulfate reducers associated with anaerobic methanotrophic archaea (ANME) driving AOM. The net sulfate reduction seems to be primarily controlled by the salinity and the available methane and is substantially lower as salinity increases (2.5 mm sulfate removal at 3000 mm chlorine but only 0.5 mm sulfate removal at 6300 mm chlorine). Low overall sulfur isotope fractionation observed (34ε = 17 ± 3.5‰) hints at high rates of sulfate reduction, as has been previously suggested for sulfate reduction coupled with methane oxidation. The new results demonstrate the presence of sulfate-driven AOM in terrestrial hypersaline systems and expand our understanding of how microbial life is sustained under the challenging conditions of an extremely hypersaline environment.

Sources and distribution of trace and minor elements in the western Dead Sea surface sediments

Abstract Twenty Dead Sea surface sediment samples were analyzed for their major, minor and trace element compositions. The samples represent muddy sediments along the western parts of the lake, from water depths of 8–250 m. These sediments were deposited after 1983, under oxic conditions, following the overturn of the water column in 1979, which ended about 300 years of meromictic stratification with an anoxic lower water mass. The changes in their metal concentrations are discussed in view of the different brine oxidation state. The sediments consist of detrital minerals—carbonates, quartz and clays and authigenic minerals—aragonite, halite and traces of gypsum. Calculations indicate that all mud samples contain more than 3.6% authigenic aragonite, which was found to precipitate preferentially in near shore sediments. An increase in Ca and a decrease in Al concentrations with decreasing water depths and in a transect from N to S were observed. These are attributed to differential settling of detritus and authigenic carbonates close to the shore and fine Al-silicates in the deep waters, and to the southward decrease in the contribution of clay minerals, mostly derived from the Jordan river. Fe, Ce, Be and Eu were found to exhibit conservative behavior with respect to Al during the transition from stream sediments in the drainage basin to lake sediments. When compared to normal marine sediments, the Dead Sea sediments have similar Cu, Ni, Zn, Be, Ce and Eu concentrations, whereas Cd is enriched by nearly 1 order of magnitude. A good correlation exists between Cd and P, suggesting that the Cd enrichment arises from outcrops of Cd-rich phosphate rocks that are found in the Dead Sea basin. The somewhat depleted Pb concentrations in the lake muddy sediments might be explained by the somewhat high Pb concentrations (normalized to salinity) in the Dead Sea water column, as compared to seawater and by its removal, mainly through halite precipitation. The unusual distribution of Mn concentrations in the surface sediments and its association with authigenic aragonite imply, as has already been suggested, that Mn co-precipitates with aragonite.

Gypsum saturation degrees and precipitation potentials from Dead Sea-seawater mixtures

Environmental context. Since the 1960s the Dead Sea water level has dropped by nearly 30 m and over the last decade the rate of decline accelerated to over 1 m per year. Conveying seawater to the Dead Sea to stabilise or even raise its water level is currently being considered but may result in ‘whitening’ of the surface water through the formation of minute gypsum crystals that will remain suspended in the water column for a prolonged period of time. This paper is a first step in attaining the relevant physical and chemical parameters required to assess the potential for such whitening of the Dead Sea. Abstract. Introduction of seawater to the Dead Sea (DS) to stabilise its level raises paramount environmental questions. A major concern is that massive nucleation and growth of minute gypsum crystals will occur as a result of mixing between the SO42–-rich Red Sea (RS) water and Ca2+-rich DS brine. If the gypsum will not settle quickly to the bottom it may influence the general appearance of the DS by ‘whitening’ the surface water. Experimental observations and theoretical calculations of degrees of saturation with respect to gypsum (DSG) and gypsum precipitation potentials (PPT) were found to agree well, over the large range but overall high ionic strength of DS–RS mixtures. The dependency of both DSG and PPT on temperature was examined as well. Based on our thermodynamic insights, slow discharge of seawater to the DS will result in a relatively saline upper water column which will lead to enhanced gypsum precipitation.

Formulating A Regional Policy for the Future of the Dead Sea — The ‘Peace Conduit’ Alternative

The Dead Sea is a severely disturbed ecosystem, greatly damaged by anthropogenic intervention in its water balance. Since the beginning of the 20th century, the Dead Sea level have dropped by more than 20 meters, and presently (2006) it is about 419 meters below mean sea level. The rate of water level drop over the last 10 years is about 1.0 m/yr, representing an annual water deficit of about 650 million cubic meters. The sharp level drop reflects the annual interception by riparian countries of over 1000 million cubic meters of freshwater which in the past drained to the Dead Sea. In addition to the water interception upstream, the Israeli and Jordanian mineral industries contribute to this deficit by artificially maintaining extensive evaporation surfaces in the otherwise now dried southern Dead Sea basin.

Water Sources and Quality along the Lower Jordan River, Regional Study

The Lower Jordan River received in the past a large volume of freshwater from Lake Tiberias, the Yarmouk River, and local runoffs. Currently, a much smaller flow-rate of mostly poor-quality fluids enters the river. The severe reduction of inflow and the poor-quality flows have led directly to the degradation of the water quality along the river. According to the regional peace agreements, both sewage and saline waters will be treated and used. Carrying out these agreements will result in a dramatic reduction of input flow-rates into the river. Under these circumstances, the almost sole available source will be drainage and groundwater. The objective of this study is to evaluate the different components that presently control the quality of water in the Lower Jordan River. In particular, the study is looking for ways to assess the role played by the subsurface contributions. We present here preliminary results of an ongoing research, which involves researchers from Israel, Jordan, and the Palestinian Authority. By means of water sampling, chemical analysis, isotope analysis, flow-rate measurements, and mass balance calculations, the study improves our understanding of the hydrology and hydrochemistry of the river system.

Concurrent Salinization and Development of Anoxic Conditions in a Confined Aquifer, Southern Israel

An ancient, brackish, anoxic, and relatively hot water body exists within the Yarqon-Tanninim Aquifer in southern Israel. A hydrogeological-geochemical conceptual model is presented, whereby the low water quality is the outcome of three conditions that are met simultaneously: (1) Presence of an organic-rich unit with low permeability that overlies and confines the aquifer; the confining unit contains perched horizons with relatively saline water. (2) Local phreatic/roofed conditions within the aquifer that enable seepage of the organic-rich brackish water from above. The oxidation of the dissolved organic matter in the seeping water consumes the dissolved oxygen and continues through bacterial sulfate reduction, with H2 S as a product. These exothermic reactions result in some heating. (3) The seeping water comprises a relatively large portion of the water volume. In the presented case study, the latter condition first developed in the Late Pleistocene following climate change, which led to a dramatic decline in recharge. Consequently, water flow in the local basin has nearly ceased, as evident by old water ages, specific isotopic composition, and nearly equipotential water levels. The continuous seepage from above into the almost stagnant water body has resulted in degraded water quality. Seepages of organic-rich brackish water exist at other sites throughout the aquifer but have limited impact on the salinity and redox conditions due to the dynamic water flow, which flushes the seeping water, that is, the third condition is not met. The coexistence of the above three conditions may explain the development of anoxic and saline groundwater in other aquifers worldwide.