Secular Change In the Stable Sulfur Isotope Composition of Seawater Sulfate Inferred from Organic Sulfur In Marine-Sourced Crude Oils

Secular changes in the stable sulfur isotope composition of seawater sulfate are based on that of evaporite minerals and, more recently, on sulfate incorporated into carbonate shell. Changes in the stable sulfur isotope composition of marine organic matter, however, have rarely been studied. This is primarily because of the potential complexity of diagenetic pathways for the incorporation of sulfur into organic matter subsequent to biosynthesis. Given the range of stable sulfur isotope compositions for seawater sulfate during the Phanerozoic, (25 to 30 per mil), we suggest that this variability will be reflected in the stable sulfur isotope composition of marine organic matter, irrespective of diagenesis. To test this hypothesis, we determined the stable sulfur isotope compositions of nondegraded crude oils ranging in geologic age from Neoproterozoic to Neogene. The oils were primarily sourced from marine carbonates/marls of low to moderate maturity.

The stable sulfur isotope values for the crude oils and their respective aromatic, NSO and asphaltene fractions tracked the secular changes for seawater sulfate previously inferred from evaporite minerals, thus providing independent confirmation of this change in seawater chemistry over geologic time.