Sulfur Isotope Trends in Archean Microbialite Facies Record Early Oxygen Production and Consumption

Abstract:

The major and minor sulfur isotope composition (δ³⁴S and Δ³³S) of pyrites preserved in ~2.65–2.5 billion-year-old (Ga) microbialites record localized oxygen production and consumption near the mat surface. These trends are preserved in two separate drill cores (GKF01 and BH1-Sacha) transecting the Campbellrand-Malmani carbonate platform (Ghaap Group, Transvaal Supergroup, South Africa; Zerkle et al., 2012; Izon et al., in review). Microbialite pyrites possess positive Δ³³S values, plotting parallel to typical Archean trends (with a Δ³³S/δ³⁴S slope of ~0.9) but enriched in ³⁴S by ~3 to 7‰. We propose that these ³⁴S-enriched pyrites were formed from a residual pool of sulfide that was partially oxidized via molecular oxygen produced by surface mat-dwelling cyanobacteria. Sulfide, carrying the range of Archean Δ³³S values, could have been produced deeper within the microbial mat by the reduction of sulfate and elemental sulfur, then fractionated upon reaction with O₂ produced by oxygenic photosynthesis. Preservation of this positive ³⁴S offset requires that: 1) sulfide was only partially (50­­–80%) consumed by oxidation, meaning H₂S was locally more abundant (or more rapidly produced) than O₂, and 2) the majority of the sulfate produced via oxidation was not immediately reduced to sulfide, implying either that the sulfate pool was much larger than the sulfide pool, or that the sulfate formed near the mat surface was transported and reduced in another part of the system. Contrastingly, older microbialite facies (> 2.7 Ga; Thomazo et al., 2013) appear to lack these observed ³⁴S enrichments. Consequently, the onset of ³⁴S enrichments could mark a shift in mat ecology, from communities dominated by anoxygenic photosynthesizers to cyanobacteria. Here, we test these hypotheses with new spatially resolved mm-scale trends in sulfur isotope measurements from pyritized stromatolites of the Vryburg Formation, sampled in the lower part of the BH1-Sacha core. Millimeter-scale heterogeneities within mat layers will help elucidate the nature of sulfur cycling within the mat, while simultaneously revealing short-term variability in atmospheric Δ³⁶S/Δ³³S signals (e.g., Farquhar et al., 2013).