ABSTRACT
Reconstructions of coupled carbon, oxygen, and sulfur cycles rely heavily on sedimentary pyrite sulfur isotope compositions (δ34Spyr). With a model of sediment diagenesis, paired with global datasets of sedimentary parameters, we show that the wide range of δ34Spyr (~100 per mil) in modern marine sediments arises from geographic patterns in the relative rates of diffusion, burial, and microbial reduction of sulfate. By contrast, the microbial sulfur isotope fractionation remains large and relatively uniform. Over Earth history, the effect of increasing seawater sulfate and oxygen concentrations on sulfate and sulfide transport and reaction may explain the corresponding increase observed in the δ34S offset between sulfate and pyrite. More subtle variations may be related to changes in depositional environments associated with sea level fluctuations and supercontinent cycles.
ABSTRACT
A sediment sample, impregnated with asphaltic petroleum and polymetallic sulfide, was dredged from the southern end of Gorda Ridge (the Escanaba Trough) off northern California, within the offshore Exclusive Economic Zone of the United States. The molecular distributions of hydrocarbons in this petroleum show that it was probably derived from terrestrial organic matter in turbidite sediment filling the Escanaba Trough. Hydrothermal activity at the Gorda Ridge spreading center provided the heat for petroleum formation and was the source of fluids for sulfide mineralization.