Sulfur Isotope Change across the Early Mississippian K–O (Kinderhookian–Osagean) δ13C Excursion

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Earth and Planetary Science Letters



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Paired carbonate associate sulfate (CAS) sulfur isotopes (SCAS), pyrite sulfur isotopes (SPY) and CAS oxygen isotopes (OCAS) across the Early Mississippian K–O C excursion are documented from two sections of a west-dipping carbonate ramp in the southern Great Basin, western U.S.A. A 4–6‰ positive SCAS anomaly, accompanied by negative shifts in SPY and OCAS, is found within the K–O C excursion. In the section with a broader C excursion, S (SCAS–SPY) increases from 15‰ to 45‰ and Ccarb drops from 7‰ to 4‰ at the same stratigraphic interval. If this SCAS anomaly represents a global phenomenon, the large magnitude (4–6‰) and short duration (shorter than that of C) suggest an unusual pyrite burial event that expanded from sediments to the ocean water column. In this scenario, the areal and volumetric expansion of sulfate reduction and pyrite burial was likely triggered by abundantly available organic matter near the peak of the K–O C excursion, during which organic carbon production and burial may have reached a maximum, thus substantially expanding the oxygen minimum zone (OMZ). Numerical simulations suggest that pyrite burial rates 2.5–5 times higher than that of the modern ocean followed by sulfide oxidation are required to produce the observed SCAS anomaly in a sulfate-rich ([SO4] ≥28 mM) Early Mississippian ocean. Alternatively, the sulfur and CAS oxygen isotope anomalies may record local sulfur cycling in a foreland basin where changes in weathering input and bottom-water redox conditions in response to sea-level fall and cooling resulted in isotope changes. In both scenarios (either local or global), the integrated carbon, sulfur, and CAS-oxygen isotope data suggest a much more dynamic sulfur cycle across the K–O C excursion than has been previously suggested.


Sulfer isotopes; Early Mississippian; Carbon isotope excursion; Great Basin; Western United States





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