Multiple sulfur isotopes (δ34S, Δ33S) of organic sulfur and pyrite from Late Cretaceous to Early Eocene oil shales in Jordan

We present the first multiple sulfur isotope study (32S, 33S, 34S, 36S) of bulk kerogen sulfur (KS) and disulfides (‘chromium-reducible sulfur’, CRS) from the oil shale of the Umm Rijam Chert-Limestone and Muwaqqar Chalk Marl Formation, Jordan (Late Cretaceous to Early Eocene, appr. 50–70 Ma). Analysis of the sulfur isotopic composition of KS (δ34SKS) shows values ranging from 0.3 to 17.9‰, which are 34S-enriched compared to the δ34SCRS ranging from −23.5 to −3.7‰. Values for CRS and KS are significantly 34S-depleted compared to seawater sulfate sulfur which suggests a major input of early-diagenetic, microbially-generated sulfide. A minor contribution of assimilated seawater sulfate to KS is assumed. The 34S-enrichment of KS compared to CRS can be partly explained by (1) sulfide oxidation to intermediate sulfur species prior to its incorporation into organic material, by (2) fractionations during organic sulfur generation, as well as by (3) a post-depositional timing of formation. Additionally, we hypothesize, based on parallel depth trends of maturity parameters (e.g., vitrinite reflectance) and δ34SCRS and δ34SKS values, that the sulfur isotopic compositions were influenced by thermal maturation (catagenesis). We suggest that the CRS pool comprises a contribution of sulfide released during the thermal decomposition of KS. Overall, our study highlights the importance of organic sulfur in sulfur isotopic studies and the potential of multiple sulfur isotope analyses in maturated sedimentary successions.