THE SULFUR CYCLE ACROSS THE EARLY MISSISSIPPIAN POSITIVE δ13C EXCURSION

The Early Mississippian strata host a large carbon isotope (δ¹³C) excursion with a magnitude of ≥ 5‰ that has been documented from numerous sections across the globe. This δ¹³C excursion, informally named as the TICE (Tournaisian carbon isotope excursion), coincides with global cooling and sea-level fall and predicts significant changes in weathering fluxes and the inventory of oceanic sulfate. However, sparsely available sulfate sulfur isotope data (δ³⁴S_CAS) across the TICE in literature seem not to be responsive to the perturbation of the carbon cycle. To better understand the coupling of sulfur and carbon cycles at this critical greenhouse-icehouse transition, we have analyzed paired sulfur isotopes of CAS (δ³⁴S_CAS) and pyrite (δ³⁴S_pyr) from two Early Mississippian sections in the southern Great Basin of western USA. The two sections represent peritidal and outer shelf environments of a west-dipping carbonate platform. In both sections, middle–late Kinderhookian strata show stable δ³⁴S_CAS and δ³⁴S_pyr and constant sulfur isotope fractionations (D³⁴S = δ³⁴S_CAS - δ³⁴S_pyr), suggesting that the sulfur cycle was “decoupled” from the carbon cycle at the rising limb of the δ¹³C excursion. A significant increase of sulfur isotope fractionation (up to 45‰) occurs near the peak of the δ¹³C excursion in the outer shelf section, followed by a decrease in both δ³⁴S_CAS and δ³⁴S_pyr at the falling limb of the δ¹³C excursion. The increase of sulfur isotope fractionation at the peak of the δ¹³C excursion suggest maximum primary production and organic carbon burial that may have led to expansion of anoxia/euxinia and pyrite burial. The decrease of δ³⁴S_CAS and δ³⁴S_pyr at the falling limb of the δ¹³C excursion suggests a significant increase of pyrite-derived sulfate input in response to global cooling and oxygenation, which is an anticipated outcome of enhanced organic carbon and pyrite burial associated with the δ¹³C excursion. Collectively, the data suggest that the Early Mississippian sulfur and carbon cycles were coupled. The seemly “decoupled” δ³⁴S and δ¹³C at the rising limb of the TICE may record a delayed response of sulfur to the carbon cycle due to the much longer resident time of sulfur in the ocean.