Neoarchean atmospheric chemistry and the preservation of S-MIF in sediments from the São Francisco Craton

Abstract

Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ33S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE; 2.45–2.2 Ga) are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ33S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763–2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ13Corg (<−30‰) and dampened Δ33S (0.4‰ to −0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ13Corg (−33.4‰ to −19.2‰) and expanded Δ33S (2.3‰ ± 0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ13Corg (−30‰ to −25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ33S (−0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.