Ella Hughes

and 10 more

The isotopic composition of organic sulfur (δ34Sorg) is a potential recorder of past biogeochemical conditions that has, thus far, received relatively little attention in comparison to the pyrite sulfur isotope record (δ34Spyr). This study presents continuous organic and pyrite δ34S records from three basins of the organic-rich Miocene Monterey Formation, deposited over a similar time interval of c.14.5-6 Ma but under varying depositional conditions. In the San Joaquin basin, δ34Sorg and δ34Spyr average 0‰ and -4‰ respectively and maintain a relatively constant pyrite-organic sulfur isotopic offset of c. 5‰. The Santa Maria Basin exhibits δ34Sorg values that are >10‰ higher than in coeval San Joaquin basin intervals, with average δ34Sorg of c. 24‰ in the upper siliceous member the highest yet reported for marine organic sulfur and roughly 2-3‰ higher than Miocene seawater sulfate. δ34Spyr is consistently c. 12‰ depleted in comparison to organic sulfur in the lower phosphatic member of the Santa Maria Basin, but an abrupt enrichment in both δ34Spyr and δ34Sorg coincident with a sharp lithostratigraphic transition at c. 11 Ma reduces this offset to <4‰ for much of the upper siliceous shales. The Santa Barbara Basin shows a sulfur isotope record intermediate between the San Joaquin and Santa Maria Basins, with average δ34Spyr and δ34Sorg of 3‰ and 12‰ respectively, and relatively consistent c. 10‰ pyrite-organic isotope offset. Records for all three basins demonstrate a close correlation between coeval δ34Spyr and δ34Sorg values which we attribute to derivation from an equivalent, or at least similar, source of sedimentary or water column sulfide. However, marked offset in the isotopic composition of coexisting pyrite and organic sulfur, of variable magnitude within and between basins, implies some contrast in the diagenetic processes underlying sulfur incorporation into the two phases. We argue that the prominent δ34Spyr and δ34Sorg isotopic differences between broadly coeval basin sections are largely the result of differences in sedimentation regime and the associated balance of iron and sulfide supply during diagenesis. A likely factor of additional importance to this iron-sulfide balance is basin-specific sedimentary and water column redox. These findings illustrate the importance of determining independent constraints on the nature of a sedimentary system before conclusions are made relating the sulfur isotope composition of sedimentary species to paleoenvironmental conditions. Additionally, we suggest that records of δ34Spyr have a strong dependence on interaction with organic sulfur during formation, and thus that existing δ34Spyr records are more effectively interpreted in combination with δ34Sorg records.