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

Abstract

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 (δ34Spy) 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 34S-enriched H2S supplied from sulfidic (H2S rich) seawater. Instructed by a novel sulfur cycling model, we propose that the emission of 34S-depleted volatile organosulfur compounds (VOSC) that were generated via sulfide methylation may have contributed to the formation of 34S-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 the VOSC cycling is an important component of the sulfur cycle in Proterozoic oceans.

Pyrite Sulfur Isotope Systematics Suggest Low Marine Sulfate Levels across the Ediacaran–Cambrian Transition

“…Variation in δ 34 S py is closely related to changes in marine redox conditions and seawater sulfate concentrations [43,[54][55][56][57][58][59]. For example, the δ 34 S py from the oxic and high-sulfate Phanerozoic ocean commonly shows a low and negative value, whereas the δ 34 S py from the anoxic and low-sulfate Proterozoic ocean is predominantly characterized by a relatively high and positive value [28,34,53].…”

“…Several models are commonly used to measure the transportation, concentration, and diagenetic profiles of sulfate in sediments, such as the diffusion-bioturbation-irrigation model, the diagenetic model, and the one-dimensional diffusion-advection-reaction model (1D-DAR) [39][40][41]. The 1D-DAR model can be helpful in identifying the controlling factors in bulk sample δ 34 S py records and in estimating sulfate levels in ancient clastic rocks under non-sulfidic water conditions [41][42][43][44]. This study analyzed the succession in the Sichuan Basin, South China (the Dengying Formation at the Heyu section), and marine sulfate concentrations were modeled during the terminal Ediacaran period using the 1D-DAR model.…”

Deglacial volcanism and reoxygenation in the aftermath of the Sturtian Snowball Earth

“…1E). The superheavy δ 34 S py from the nonglacial interlude has been interpreted to result from microbial sulfate reduction (MSR) under low-sulfate conditions, TSR, or sulfide methylation (12)(13)(14)(15)73). However, previous models are based on δ 34 S py data only and do not address the Δ 33 S py and overlying light δ 34 S py .…”

“…Thus, multiple sulfur isotopes hold promise for tracking major changes in oxygenation through time. Published δ 34 S values of pyrite (δ 34 S py ) from the Cryogenian nonglacial interlude show uniformly superheavy values, but their origins are unknown (12)(13)(14)(15)(16)(17). In addition, volcanism emits Hg to the Earth's surface via direct magmatic degassing and by intrusive contact metamorphism with organic-rich rocks (18).…”