SECULAR VARIATION IN THE STABLE SULFUR ISOTOPE COMPOSITION OF ORGANIC MATTER

Secular changes in the stable sulfur isotope composition of seawater sulfate have previously been inferred from that of evaporite minerals and from sulfate in carbonate shell matrices. The stable sulfur isotope compositions of marine sulfides follow a similar secular trend to that of sulfate, although depleted in ³⁴S owing to fractionation via bacterial sulfate reduction. Few if any studies of secular changes in the stable sulfur isotope composition of organic matter have been undertaken. This is in part because of the complexity of diagenetic pathways for the incorporation of sulfur into organic matter subsequent to biosynthesis. However, given the range for the stable sulfur isotope composition of seawater sulfate (and resulting sulfide) during the Phanerozoic (approximately 25 to 30 per mil), it is hypothesized that the magnitude of this isotopic variability will be reflected by the stable sulfur isotope composition of organic matter, irrespective of diagenesis. To test this hypothesis, the stable sulfur isotope compositions of nondegraded crude oils (containing 1 to 3 wt %S) ranging in source rock age from Neoproterozoic to Neogene were determined. The source rocks were all marine carbonates/marls of low to moderate maturity. The secular changes for the stable sulfur isotope compositions of the oils tracked the sulfate curve, although more depleted in ³⁴S, owing to the reduced sulfur incorporated into the source rock protokerogen. Similar to the sulfate curve, oils from the Neoproterozoic/Cambrian were most enriched in ³⁴S, becoming progressively depleted in ³⁴S by the Permian, and then increasing in their percentage of ³⁴S towards the Neogene. Oils formed from source rock depositional environments exhibiting photic zone euxinia based on the presence of abundant aryl isoprenoids, biomarkers for green sulfur bacteria, were more depleted in ³⁴S than oils of similar ages from phytoplankton/bacterial-dominated sources.