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

Abstract

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) δ34S (mean +56.0‰, range +49.6 to +62.6‰) and pyrite δ34S compositions (mean +57.5‰, range +48.8 to +66.8‰). Both CAS and pyrite δ34S show water-depth gradients, with mean values increasing from the shallower Lijiawan area (CAS: +49.3‰; pyrite: +44.5‰) to the deeper Xixibao (CAS: +57.9‰; pyrite: +59.8‰) and Gaodi areas (CAS: +62.4‰; pyrite: +61.6‰), reflecting a density-stratified water column with limited vertical mixing. △34S values (i.e., δ34SCAS – δ34Spy) range from −6.5‰ to +8.0‰ with a mean of +0.7‰. 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 △34S values, which indicate that in situ microbial sulfate reduction (MSR) cannot have been the sole control on pyrite δ34S. 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 34S-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 34S-enriched sulfur-isotopic compositions for CAS and pyrite, and frequent negative △34S values. As a result of low seawater sulfate concentrations, both δ34SCAS and δ34Spy developed water-depth gradients through vertical mixing of strongly 34S-enriched hydrothermal sulfide from deep-graben vents with moderately 34S-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.