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

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 (δ³⁴S_CAS), pyrite sulfur isotopes (δ³⁴S_PY) and CAS oxygen isotopes (δ¹⁸O_CAS) across the K-O δ¹³C excursion. A 7‰ positive δ³⁴S_CAS anomaly, accompanied by negative shifts in δ³⁴S_PY, δ¹⁸O_CAS, 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 δ³⁴S_CAS 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 δ³⁴S_CAS anomaly in a sulfate-rich Early Mississippian ocean. At the falling limb of the K-O δ¹³C excursion, coupled negative shifts in δ³⁴S_CAS and δ¹⁸O_CAS 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.