SULFUR ISOTOPE RECORDS IN NEOARCHEAN CARBONATES: IMPLICATIONS FOR THE EARLY PRECAMBRIAN SULFUR CYCLE

Abstract

Mass-independent fractionation of sulfur isotopes found in Early Precambrian records is the main evidence supporting an oxygen-poor atmosphere before ~2.4 Ga when ancient sulfur cycling was different than today. In previous studies, shale facies formed in deep-water environments have been the main target that were used to constraint the ancient sulfur cycle using sulfur isotopes, even though, among sedimentary Neoarchean strata, carbonate rocks are found to be more abundant. In order to follow previous observations and reveal processes operating in shallow water environments, I conducted a series of systematic studies of Neoarchean carbonate archives. Elemental and isotope measurements of sulfur and carbon in carbonate (and some shale) facies were obtained from multiple cores drilled through ~2.7 to 2.5 Ga successions of South Africa (GKF01, GKP01, and BH1-Sacha), Western Australia (AIDP-2, AIDP-3, BB, PR, RP, and RG) and Brazil (GDR-117). This study demonstrates that carbonate facies preserve distinctive MIF-S compositions relative to shale facies. Drilled pyrites in carbonate formations mostly preserved negative Δ33S values suggesting that the major sulfur source to shallow environments was atmospheric sulfate that also was isotopically redistributed through microbial sulfate reduction producing δ34S > 35‰ isotope fractionation. Atmospheric sulfate was the main source for seawater sulfate making its concentration in the Neoarchean ocean of less than 10µM/l. At this low concentration, reservoir effects would be pronounced leading to the formation of carbonate associated pyrites with highly positive δ34S compositions ranging to > +30‰. The bulk pyrites in most carbonate formations from South African and Western Australian cores possess small positive Δ33S signals (<+3.0‰) suggesting the incorporation of 20-35% of photolytic elemental sulfur. Photolytic sulfate with Δ36S/Δ33S deviations found in macroscopic pyrites with negative Δ33S from the Carawine Formation provide evidence for changes in atmospheric reactions during periods of an organic hazy atmosphere. My study of Δ36S/Δ33S in contemporaneous Jeerinah shale indicates the possible temporal decoupling of the MIF-S signal on a basinal scale implying heterogeneous haze structure. Integration of sulfur and carbon isotopes measured in carbonate facies suggests that sulfur-metabolizing microbes such as sulfur phototrophs and sulfate reducers were actively recycling these elements in shallow marine environments.