Basin-Scale Sulfur Isotope Heterogeneity Reveals Slope-Centered Productivity Hotspots During Marinoan Deglaciation

The termination of the Marinoan Snowball Earth (~635 Ma) marks a major reorganization of the ocean-atmosphere, yet the basin-scale pattern of primary productivity recovery during deglaciation remains poorly constrained. Here we present a basin-scale sulfur isotope (δ34S) dataset from the upper Nantuo Formation of South China, based on disseminated pyrite and coeval pyrite nodules from 19 sections spanning shallow shelf, continental slope, and deep basin environments. Stratigraphically weighted δ34S values of disseminated pyrite reveal pronounced lateral heterogeneity, with the highest values and the greatest variance on continental slopes (weighted mean 28.3‰), compared with shallow water (13.0‰) and distal basinal environments (15.7‰). Petrographic observations and trace element data indicate syndepositional to early diagenetic pyrite formation, but micron-scale isotopic homogeneity within individual pyrite crystals and nodules, together with numerical modeling, show that porewater sulfate reduction alone cannot account for the observed combination of heavy δ34S values and abundant pyrite. Instead, the isotopic record primarily reflects sulfide supplied from an overlying euxinic water column. We interpret the slope-centered δ34S maximum as evidence for intensified water-column microbial sulfate reduction driven by enhanced organic matter export, identifying continental slopes as localized productivity hotspots during Marinoan deglaciation. This nonmonotonic shelf-to-slope-to-basin pattern is most consistent with preferential nutrient resupply to intermediate depth settings along continental margins, likely promoted by upwelling and topography-enhanced mixing, whereas shallow settings were influenced by meltwater dilution and distal basins remained isolated by persistent stratification. These results show that circulation-driven nutrient delivery exerted a first-order control on the spatial recovery of marine biogeochemical cycling in the aftermath of Snowball Earth.