Methanotrophs regulated atmospheric sulfur isotope anomalies during the Mesoarchean (Tumbiana Formation, Western Australia)

Abstract

The 2.73 billion year old Tumbiana Formation of the Fortescue Group in the Pilbara Craton (Western Australia) preserves one of the best records of the Fortescue negative carbon isotope excursion with organic δ 13C values as low as − 60‰. The prevailing environmental conditions at this biological excursion are not well understood. Here, we present an integrated carbon ( 12C, 13C) and multiple sulfur ( 32S, 33S, 34S, 36S) isotopes study of a pristine diamond drill core through the Tumbiana Formation collected at Meentheena by the Pilbara Drilling Project. Our results show centimeter- to meter-scale heterogeneities in sulfur and carbon contents and isotope compositions. Organic δ 13C values vary at the meter scale and display a bimodal distribution with peaks at about − 44 and − 34‰. This bimodal distribution is interpreted to reflect variations in the contribution of methanotrophic metabolism to the biomass. The δ 34S V-CDT values of sedimentary pyrite vary from − 5.73 to 2.75‰. Sulfur isotopes, sulfur contents, and S/C ratios are variable, suggesting a complex local biotic sulfur cycling. Sedimentary pyrite preserves mass-independent-fractionation of sulfur (MIF-S) with Δ 33S V-CDT ranging between − 0.25 and 1.64‰ (average 0.47‰) corresponding to the characteristic dampened Mesoarchean anomalies. Positive correlation between sulfide MIF-S and a 13C-depletion of organic matter argues for a causal link between methanotrophs activity and the preservation of MIF-S anomalies in the rock record.