GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 11-12
Presentation Time: 11:15 AM

THE DYNAMIC SULFUR CYCLE ACROSS THE EARLY MISSISSIPPIAN K-O (KINDERHOOKIAN-OSAGEAN) CARBON ISOTOPE EXCURSION


MAHARJAN, Dev K., Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010, JIANG, Ganqing, Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010 and PENG, Yongbo, Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, Ganqing.Jiang@unlv.edu

The Lower Mississippian strata host a large positive δ13C excursion with a magnitude of ≥5‰. This δ13C excursion coincides with global cooling and sea-level fall and predicts significant changes in sulfur fluxes and the inventory of oceanic sulfate. However, sparsely available sulfur isotope (δ34S) data across the K-O δ13C excursion in literature show limited temporal changes, which has been ascribed to the buffering effect of a large marine sulfate reservoir. In contrast, significant δ34S variations have been observed across the other major Phanerozoic positive δ13C excursions, commanding a more comprehensive δ34S dataset for the K-O interval. To fill the data gap and to better understand the coupling of sulfur and carbon cycles at this critical transition, we have analyzed paired carbonate associate sulfate (CAS) sulfur isotopes (δ34SCAS), pyrite sulfur isotopes (δ34SPY) and CAS oxygen isotopes (δ18OCAS) across the K-O δ13C excursion. A 7‰ positive δ34SCAS anomaly, accompanied by negative shifts in δ34SPY, δ18OCAS, and δ13C and increase of ∆34S up to 45‰, is found near the peak of the K-O δ13C excursion. The large magnitude (≥7‰) and short duration of this positive δ34SCAS anomaly requires an unusual pyrite burial event that could happen only if enhanced sulfate reduction and pyrite burial expanded from sediments to ocean water column. The areal and volumetric expansion of sulfate reduction and pyrite burial was likely fused by available organic matter at the peak of the K-O δ13C excursion, during which organic carbon production and burial may have reached maximum and the oxygen minimum zone (OMZ) in the ocean has substantially expanded. Numerical modeling suggests that pyrite burial rates 5–10 times higher than that of the modern ocean are required to produce the observed δ34SCAS anomaly in a sulfate-rich Early Mississippian ocean. At the falling limb of the K-O δ13C excursion, coupled negative shifts in δ34SCAS and δ18OCAS imply increase of sulfide oxidation in the ocean and pyrite-derived riverine sulfate input, in response to global cooling, sea-level fall, and oxygenation resulted from enhanced organic carbon and pyrite burial. Collectively, the integrated carbon, sulfur, and CAS-oxygen isotope data demonstrate a much more dynamic sulfur cycle across the K-O δ13C excursion than previously suggested.