The sulfur isotope signatures of Marinoan deglaciation captured in Neoproterozoic shallow-to-deep cap carbonate from South China

Abstract

We present high-resolution sulfur and carbon isotope records from the cap carbonate of the Doushantuo Formation, South China, a unique suite of depositional facies spanning across a pronounced paleo-oceanic depth gradient. The δ34S of carbonate-associated sulfate (CAS) decreases across a shelf-to-slope depth transect. We propose that the deeper-water slope profile represents syn-glacial sulfur isotope compositions of the seawater sulfates with impact of 34S-depleted hydrothermal sulfur inputs, while the δ34S of the shallow inner-shelf were dominantly controlled by the 34S-enriched terrigenous input. We suggest that the terrigenous sulfur inputs were mainly from weathering and erosion of the pre-Marinoan Datangpo Formation—glacially ground and then freshly exposed during rapid deglaciation. The widespread Datangpo Formation, deposited between the Sturtian and Marinoan glacial intervals, is noted for the unusually high δ34S values of its pyrite.The low sulfate conditions in the ocean allowed for strong local controls and consequent heterogeneities in seawater chemistry. The post-Marinoan ocean was stratified in the immediate wake of glaciation, with anoxic deep waters overlain by an oxic, melt-water layer. During the transgression, the shallow water was disturbed by upwelling. The oxidation of dissolved organic carbon (DOC) from deep water would lead to transiently enhanced 13C depletions but 34S enrichments in the shallow waters.