POST-STURTIAN SULFUR ISOTOPE ANOMALIES IN THE NANHUA BASIN, SOUTH CHINA RELATED TO UPWARD H2S MIGRATION

The Neoproterozoic Datangpo Formation was deposited in the Nanhua Basin of South China during the interglacial interval between the Sturtian and Marinoan glaciations (~663–654 Ma). A series of manganese deposits was precipitated at the base of the Datangpo Formation following the Sturtian Glaciation, with much of the Mn thought to derive from hydrothermal sources based on several types of discriminant plots, e.g., Y/P₂O₅–Zr/Cr, Fe/Ti–Al/(Al+Fe+Mn), and SiO₂–Al₂O₃ crossplots (Yu et al., 2016). The carbonate-associated sulfate (CAS) δ³⁴S values of the manganese layers are high, ranging from +45.6 ‰ to +67.5 ‰ (mean ~+58 ‰), indicating that seawater sulfate within the Nanhua Basin was highly ³⁴S-enriched. Early diagenetic pyrite framboids formed within the Mn layers are also characterized by high δ³⁴S values, ranging from +47.5 ‰ to +70.7 ‰ (mean ~+60 ‰). The △³⁴S (δ³⁴S_CAS‒δ³⁴S_pyrite) values range from ‒11.6 ‰ to +9.3 ‰ (mean ‒1.8 ‰), and 62% of △³⁴S values are negative. Although microbial sulfate reduction (MSR) in a closed porewater system can result in small positive △³⁴S values, the large number of negative △³⁴S values in our dataset indicates that in situ MSR cannot have been the sole control on δ³⁴S_pyrite values in the Nanhua Basin. Noteworthy is that organic carbon (2.9–9.2 %, mean 5.9 %) and total sulfur (1.0–4.2 %, mean 2.1 %) exhibit a negative correlation (r = ‒0.48, p(α)＜0.05, n=29). In other marine units, this pattern has been linked to vertical migration of H₂S out of OC-rich beds, where it is generated in larger quantities, and into OC-poor beds, where it is fixed by greater availability of reactive Fe. Based on these analyses, we propose that the upward H₂S migration played an important role in sulfur cycling in the Neoproterozoic Nanhua Basin, which could provide a new explanation different from traditional ones for the high δ³⁴S values. Seawater sulfate in the Nanhua Basin may have been gradually enriched in ³⁴S through long-term operation of the MSR process with continuous upward flux of isotopically heavy H₂S, a part of which would have migrated out of the sediment into the overlying water column to be re-oxidized to isotopically heavy sulfate. Large-scale upward migration of H₂S within the sediment can account for the observation that pyrite δ³⁴S is somewhat heavier than CAS δ³⁴S.