Fire and brimstone: the microbially mediated formation of elemental sulfur nodules from an isotope and major element study in the paleo-Dead Sea

Abstract

We present coupled sulfur and oxygen isotope data from sulfur nodules and surrounding gypsum, as well as iron and manganese concentration data, from the Lisan Formation near the Dead Sea (Israel). The sulfur isotope composition in the nodules ranges between -9 and -11‰, 27 to 29‰ lighter than the surrounding gypsum, while the oxygen isotope composition of the gypsum is constant around 24‰. The constant sulfur isotope composition of the nodule is consistent with formation in an 'open system'. Iron concentrations in the gypsum increase toward the nodule, while manganese concentrations decrease, suggesting a redox boundary at the nodule-gypsum interface during aqueous phase diagenesis. We propose that sulfur nodules in the Lisan Formation are generated through bacterial sulfate reduction, which terminates at elemental sulfur. We speculate that the sulfate-saturated pore fluids, coupled with the low availability of an electron donor, terminates the trithionate pathway before the final two-electron reduction, producing thionites, which then disproportionate to form abundant elemental sulfur.

Figure 1. The enzymatic steps involved in bacterial sulfate reduction.

Figure 2. Isotope and major ion results from sulfur nodules 1 (left) and 2 (right).

In the top panels we show sulfur isotopes in gypsum (black circles) and elemental sulfur (open circles) and oxygen isotopes in gypsum (black squares). Error bars on the oxygen isotopes in gypsum are based on replicate measurements. In the middle panels we show total iron concentration (stars) and total manganese concentration (diamonds). Photos of the nodules are included at the bottom.

Figure 3. A schematic of the sulfur isotope composition during bacterial sulfate reduction in closed versus open systems.

In closed systems the sulfur isotopes in the reduced product (pyrite or elemental sulfur, for example) may have significant isotope variability as they are growing from a pool that is evolving isotopically with time. In contrast in an open system with constant replenishment of the source of sulfate the isotope composition of the reduced product would not be expected to vary. We use our data to conclude that the sulfur nodules in the Lake Lisan formed in an open system. The symbol ε_s is the sulfur isotope fractionation during sulfate reduction.

Figure 4. Our hypothesized model for the geochemical environment leading to the formation of the sulfur nodules.

Our precursor gypsum is +19‰ and the elemental sulfur is -8‰; these are the two ‘knowns’ as we are able to measure them today. In theory the pore fluid sulfate would be significantly heavier than the gypsum since this would be the mobile pool from which sulfate is reduced to form the sulfur nodule. Thus the sulfur isotope fractionation could be much larger than the 28‰ difference between the gypsum and the nodule.

Figure 5. A diagram of the crystal proteins and the proposed pathway to make elemental sulfur.

Sulfite is formed during the first two-electron reduction of sulfate within a microbial cell (see Figure 1). Sulfite can be further reduced to sulfide through three two-electron reductions (pathway 1, sulfur species in red boxes). During this, sulfite binds to the DrsA-B and DrsC proteins, where it is sequentially reduced. It has been hypothesized that the DrsC protein plays a particularly important role in the terminal two-electron reduction of elemental sulfur to sulfide. Excess sulfite (blue box) in the cell can further attack S²⁺ and S⁰ while they are bound in the DsrA-B-C complex, forming thiosulfates (green boxes). We propose that these thiosulfates are disproportionated (or oxidized) forming elemental sulfur (yellow circles) and sulfite again, allowing for large amounts of elemental sulfur to accumulate (pathway 3). The specific conditions that permit this to happen are the supersaturated sulfate in the pore fluids coupled with low electron donor.