Large accumulations of 34S-enriched pyrite in a low-sulfate marine basin: The Sturtian Nanhua Basin, South China

The Cryogenian of the Nanhua Basin (eastern Guizhou Province, South China) consists of a continuous succession of glacial and post-glacial deposits from the Sturtian Glaciation, including diamictite of the Tiesi'ao Formation and Mn-carbonate and black shale of the overlying lower Datangpo Formation. Here, we analyzed the sulfur chemistry of these units with the goal of understanding regional to global changes in the marine sulfur cycle accompanying a Snowball Earth event. The study units are characterized by elevated carbonate-associated sulfate (CAS) delta S-34 (mean +56.0 parts per thousand, range +49.6 to +62.6 parts per thousand) and pyrite delta S-34 compositions (mean +57.5 parts per thousand, range + 48.8 to +66.8 parts per thousand). Both CAS and pyrite delta S-34 show water-depth gradients, with mean values increasing from the shallower Lijiawan area (CAS: +49.3 parts per thousand; pyrite: +44.5 parts per thousand) to the deeper Xixibao (CAS: +57.9 parts per thousand; pyrite: +59.8 parts per thousand) and Gaodi areas (CAS: + 62.4 parts per thousand; pyrite: +61.6 parts per thousand), reflecting a density-stratified water column with limited vertical mixing. Delta S-34 values (i.e., delta S-34(CAS) - delta S-34(py)) range from -6.5 parts per thousand to +8.0 parts per thousand with a mean of + 0.7 parts per thousand. These features, which are similar to those for coeval strata globally, are consistent with low seawater sulfate concentrations, but several additional features of the Nanhua Basin deposits do not conform to existing Cryogenian sulfur-cycle models: (1) high total sulfur contents (mean 2.2 +/- 1.1%), which are difficult to reconcile with low seawater sulfate, and (2) frequent negative Delta S-34 values, which indicate that in situ microbial sulfate reduction (MSR) cannot have been the sole control on pyrite delta S-34. These features point to quantitatively important hydrothermal sulfur inputs to the Nanhua Basin watermass. Based on these considerations, we propose a new sulfur-cycle model for the Sturtian Nanhua Basin in which hydrothermal emissions supplied large amounts of S-34-enriched H2S to the water column. The released H2S was partly precipitated as syngenetic framboidal pyrite and partly oxidized to sulfate that was removed to the sediment as CAS, thus accounting for the unusual combination of high total sulfur concentrations, similar strongly S-34-enriched sulfur-isotopic compositions for CAS and pyrite, and frequent negative Delta S-34 values. As a result of low seawater sulfate concentrations, both delta S-34(CAS) and delta S-34(py) developed water-depth gradients through vertical mixing of strongly S-34-enriched hydrothermal sulfide from deep-graben vents with moderately S-34-enriched sulfate from the global ocean. Our model provides new insights into Sturtian-glacial sulfur cycling processes within a semi-restricted marine basin that are likely to have wider applicability to Neoproterozoic marine systems.