Cracking the superheavy pyrite enigma: possible roles of volatile organosulfur compound emission.

The global deposition of superheavy pyrite (pyrite isotopically heavier than coeval seawater sulfate in the Neoproterozoic Era and particularly in the Cryogenian Period) defies explanation using the canonical marine sulfur cycle system. Here we report petrographic and sulfur isotopic data (δS) of superheavy pyrite from the Cryogenian Datangpo Formation (660-650 Ma) in South China. Our data indicate a syndepositional/early diagenetic origin of the Datangpo superheavy pyrite, with S-enriched HS supplied from sulfidic (HS rich) seawater. Instructed by a novel sulfur-cycling model, we propose that the emission of S-depleted volatile organosulfur compounds (VOSC) that were generated via sulfide methylation may have contributed to the formation of S-enriched sulfidic seawater and superheavy pyrite. The global emission of VOSC may be attributed to enhanced organic matter production after the Sturtian glaciation in the context of widespread sulfidic conditions. These findings demonstrate that VOSC cycling is an important component of the sulfur cycle in Proterozoic oceans.