Paper No. 217-2
Presentation Time: 8:30 AM
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 (δ34SCAS‒δ34Spyrite) 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 δ34Spyrite 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.