THE SIGNIFICANCE OF d34S DETERMINATION AND EVALUATION IN ORGANIC RICH SEDIMENTS, INSIGHT TO THE MECHANISTIC APPROACH, I. THE ROLE OF SULFUR IN THE DIAGENETIC STAGE: A CONCEPTUAL REVIEW

The formation of sedimentary rocks rich in organic matter is controlled by the depositional conditions. Both carbon and sulfur cycles in the geosphere are biogenically and chemically interweaved. Sulfate is the main source for sulfur of marine sediments. The incorporation of sulfur into biogenic organic matter via the assimilatory process has very little isotopic signature. The use of sulfate as an oxygen source for the dissimilatory process produced reduced sulfides that can be 40‰ lighter than the d³⁴S value of the sulfate. The pioneering work by Kaplan and Rittenberg (1964) showed that sulfate reducing bacteria (SRB) oxidize organic carbon while producing sulfide depleted in the ³⁴S isotope. Carbon source, SRB types and hence rate of sulfate reduction may influence the DSO₄^2- ® S^2- isotopic discrimination. In addition, the supply of sulfate is important. The sulfides formed by the SRB are kinetically taken by available iron to form pyrite (FeS₂) . Under pH~8-9 and sulfide forms polysulfides. Polysulfides were found to be chemically the most active species of sulfur in reactions with organic matter. Isotopically polysulfides carry the dissimilatory d³⁴S values signature. Hence, if the secondary sulfur enrichment is by chemical reaction between the sedimentary organic matter (SOM) with the polysulfides, then the d³⁴S values for the organic matter, rich in sulfur, will gradually be imprinted by the dissimilatory process.

Sulfate and other sulfur species were isotopically scantly compared. Organic sulfur and elemental sulfur isotope (d³⁴S) ratios were studied only in cases where the secondary enrichment led to SOM rich in sulfur. Under these conditions type II-S kerogens are formed. Three such cases will be discussed: (a) Solar lake - young cyanobacterial mat (Sinai, Egypt). (b) Dead Sea immature asphalts and bituminous rocks (Senone Formation, Israel). (c) Monterey (Miocene Formation, California, USA).

In most cases, the pyrite records in the sediments and sedimentary-rocks the most ³⁴S depleted sulfur, whereas its sulfate that remains will be the heaviest. The organically bonded sulfur has a wider range in between the two. It is our intention in the present review and evaluation to offer a feasible mechanistic approach to connect d³⁴S ratios recorded with depositional environment and diagenetic processes.