Significance of δ34S and evaluation of its imprint on sedimentary organic matter: I. The role of reduced sulfur species in the diagenetic stage: A conceptual review

Abstract

Both carbon and sulfur cycles in the geosphere are biogenically and chemically interwoven. Sulfate is the main source for sulfur in marine sediments. The incorporation of sulfur into biogenic organic matter (OM) via the assimilatory process has very little isotopic discrimination. The pioneering work by Kaplan and Rittenberg showed that sulfate-reducing bacteria (SRB) oxidize organic carbon to CO2 while producing H2S depleted in the 34S isotope. The use of sulfate as an electron acceptor during bacterial dissimilatory processes produces H2S that can be up to 72% depleted in 34S relative to the sulfate. Carbon source, SRB species and hence rate of sulfate reduction may influence the overall isotopic fractionation ΔSO42- → S2-. In addition, the supply of sulfate (open versus closed system) is important for determining isotopic fractionation. The H2S formed by the SRB quickly reacts with available iron to form pyrite (FeS2) via the precursor FeS. Sulfide-oxidizing bacteria may form elemental sulfur, that at pH ~7–9, reacts with sulfide to form polysulfides. Polysulfides were found to be chemically the most reactive species of sulfur with OM. Isotopically, polysulfides carry the dissimilatory δ34S value. Hence, if the secondary sulfur enrichment in sedimentary organic matter (SOM) is by chemical reaction with the polysulfides, then the δ34S values for the OM, rich in sulfur, will gradually be imprinted by the dissimilatory process. Most of the information on δ34S values of the reduced sulfur in sediments derives from both acid “volatile” sulfide (FeS) and pyrite (FeS2). In a few cases, both sulfate and other sulfur species were isotopically compared. Due to analytical difficulties, organic sulfur and elemental sulfur isotope (δ34S) ratios were studied only in cases where the secondary enrichment led to OM rich in sulfur. This secondary enrichment forms type II-S kerogens.Three such natural cases of secondary enrichment of OM will be discussed:(a)Solar lake—young cyanobacterial mat (Sinai, Egypt);(b)Dead Sea—immature asphalts and bituminous rocks (Senonian Ghareb Formation, Israel);(c)Monterey Formation selected samples (Miocene Formation, California, USA). Solar lake—young cyanobacterial mat (Sinai, Egypt); Dead Sea—immature asphalts and bituminous rocks (Senonian Ghareb Formation, Israel); Monterey Formation selected samples (Miocene Formation, California, USA). These three case studies are typified by low-medium maturity. The Solar Lake mats are at the early stages of diagenesis, the Monterey and the Ghareb Formations have already formed type II-S kerogens. In most cases, the pyrite records the most 34S-depleted sulfur in the sediments and sedimentary rocks, whereas sulfate is the most 34S enriched. The organically bonded sulfur has a wider range of isotopic compositions probably due to its dependence on timing and multiple step reactions discussed in this review. It is our intention in this review to offer a feasible mechanistic approach to connect δ34S ratios recorded with depositional environment and diagenetic processes.