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

Abstract

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.