STEADY STATE SULFUR ISOTOPIC COMPOSITION DURING CONCURRENT BACTERIAL SULFATE REDUCTION AND GYPSUM DISSOLUTION

Gypsum dissolution by gypsum-saturated water can be induced through the removal of sulfate from the water by bacterial sulfate reduction (BSR). The concurrent BSR and gypsum dissolution maintains a constant sulfate concentration while the isotopic composition changes. A numerical model shows that a steady state isotopic composition is approached in such a system after reduction and replacement of the equivalent of about 4 times the original dissolved sulfate content. The steady state isotopic composition is determined by the fractionation during reduction and sulfate dissolution and can be described by:

d³⁴S_SS=d³⁴S_gypsum + D^SO4_H2S

Where: d³⁴S_SS: Steady state isotopic composition; d³⁴S_gypsum: Isotopic composition of dissolving sulfate; D^SO4_H2S : Isotopic fractionation, d³⁴S_SO4 - d³⁴S_H2S

The concurrent BSR and gypsum dissolution, and approach to steady state were examined by two systems in the Dead Sea basin:

1. The anoxic Ca-Chloride subsurface brines found in shallow boreholes around the Dead Sea have salinities similar to the Dead Sea (TDS=330 g/L). They have relatively low sulfate concentration of about 500 mg/L and are saturated with respect to gypsum. The brines are found in the gypsum-rich Holocene to Late Pleistocene sediments deposited by the Dead Sea and its precursor Lake Lisan, respectively. The isotopically enriched sulfate composition of the brines, up to d³⁴S=60‰, and the presence of H₂S in some, suggest that BSR occurs in the subsurface. Saturation with respect to gypsum indicates that this process is accompanied by gypsum dissolution while the enriched sulfate isotopic composition indicates that steady state is approached.

2. Sulfate in Lake Lisan sediments is present as gypsum layers (14 to 28‰) and as disseminated gypsum (-13 to 1‰) in aragonite layers. The former precipitated as gypsum during overturn events while the latter, based on its isotopic composition, precipitated as sulfide, indicating that BSR occurred in the hypolimnion (lower water mass). The isotopically enriched compositions of some of the gypsum layers suggest that sulfate in the hypolimnion, prior to overturn, reached relatively high concentrations and was isotopically highly enriched. Such conditions are feasible provided that gypsum precipitated from the upper water body and was redissolved in the anoxic, sulfate reducing hypolimnion.