Abstract
Sulfur mass- independent fractionation (S-MIF) preserved in Archean sedimentary pyrite isinterpreted to reflect atmospheric chemistry. Small ranges in Δ33S that expanded into largerfractionations leading up to the Great Oxygenation Event (GOE) 2.45 to 2.2 Ga aredisproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. Thesepatterns of S-MIF attenuation and enhancement may differ from the timing and magnitude ofminor sulfur isotope fractionations reported from other cratons, thus obscuring local for globalsulfur cycling dynamics. By expanding the Δ33S record to include the relatively underrepresentedSão Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIFpreservation prior to the GOE. In an early Neoarchean sequence (2763–2730 Ma) from the Riodas 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 methaneoxidation. 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 enhancepreservation of S-MIF input from atmospheric sources of elemental sulfur. The sequenceculminates 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‰ to0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in thecontext of other sequences worldwide, the Rio das Velhas helps differentiate the influences ofglobal atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes.Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimatedrole in pre-GOE sulfur minor isotope records.