Pyrite sulfur isotope heterogeneity across the Nanhua Basin records intensified biogeochemical cycling on continental slopes during Marinoan deglaciation

The termination of the Marinoan Snowball Earth (∼635 Ma) reorganized ocean circulation and biogeochemical cycling, but the spatial recovery of marine primary productivity remains poorly constrained. Here we present a basinal scale sulfur isotope (δ 34 S) 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 basin environments. Stratigraphically weighted δ 34 S values of disseminated pyrite reveal pronounced lateral heterogeneity, with the highest values and the greatest dispersion on continental slopes (weighted mean 28.3‰; 95% CI, 24.3–33.0‰), compared with shallow shelf (13.0‰) and basinal environments (15.7‰). Petrographic observations and trace element data indicate syndepositional to early diagenetic pyrite formation, whereas crystal scale δ 34 S homogeneity in disseminated pyrite, limited isotope variation from nodule rims to cores, and numerical modeling show that porewater sulfate reduction alone cannot account for the observed combination of heavy δ 34 S values and abundant pyrite. Combined with widespread pyritization, these observations indicate that most H 2 S incorporated into pyrite originated in euxinic bottom waters, while further pyrite growth and local isotope modification occurred near the sediment water interface and in shallow porewaters. The δ 34 S maximum on continental slopes records enhanced local sulfate consumption. The corresponding demand for metabolizable organic matter indicates that continental slopes were focal zones of organic matter export and remineralization during Marinoan deglaciation. Periodic upwelling and mixing intensified by topography likely concentrated regenerated nutrients along continental margins. Meltwater dilution limited nutrient retention in shallow waters, while persistent stratification restricted nutrient exchange in distal basins. Continental margin circulation therefore played a key role in organizing marine biogeochemical recovery after Snowball Earth.