Sulfur isotope change across the Early Mississippian K–O (Kinderhookian–Osagean) δ13C excursion

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© 2018 Paired carbonate associate sulfate (CAS) sulfur isotopes (δ34SCAS), pyrite sulfur isotopes (δ34SPY) and CAS oxygen isotopes (δ18OCAS) across the Early Mississippian K–O δ13C 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 δ34SCAS anomaly, accompanied by negative shifts in δ34SPY and δ18OCAS is found within the K–O δ13C excursion. In the section with a broader δ13C excursion, Δ34S (Δ34S=δ34SCAS–δ34SPY) increases from 15‰ to 45‰ and δ13Ccarb drops from 7‰ to 4‰ at the same stratigraphic interval. If this δ34SCAS anomaly represents a global phenomenon, the large magnitude (4–6‰) and short duration (shorter than that of δ13C) 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 δ13C 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 δ34SCAS 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 δ13C excursion than has been previously suggested.

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Earth and Planetary Science Letters

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