Secular variations of δ13Corg, δ34S, Δ33S and δ15N through geological times show large shifts around 2.7 Ga. These shifts were mostly reported from independent sediment sections, making it difficult to integrate the paleoenvironment for this key period in the evolution of the Earth. In this study, we measured isotopic ratios and elemental contents of organic and carbonate C, sulfide S and bulk N in the same samples from a pristine drill core retrieved from the Tumbiana Formation (2.73 Ga) in the Western Australia Craton, which only experienced low-grade metamorphism (< 300°C).
Organic δ13C values vary from –43 to –34‰, reflecting variable contribution of 13C-depleted methanotrophic biomass to organic matter. Sulfide δ34S show small variation from –5,8 to 2.7‰, suggesting that the 2.73 Ga environment of the Tumbiana Formation was sulfate-limited and oxygen-poor. Mass-Independent-Fractionation of S isotopes show significant anomalies ranging between -0.24 and 1.64‰, indicating anoxic environmental conditions. A positive correlation between the 12C-enrichment of organic matter and increasing MIF-S was observed, maybe due to the decrease in atmospheric methane concentration as a result of biotic methane assimilation, which enhanced the atmospheric MIF-S production. Sedimentary N show strong 15N enrichment with δ15N values ranging between 8.6 and 50.4‰. The 15N-enrichment correlates with the 12C-enrichment of organic matter. I propose here a process of the oxidation of ammonium to nitrite to explain these high δ15N values.
This thesis reports, for the first time, integrated C, N and S isotopes measurements in the same sedimentary formation at 2.73 Ga. It shows that δ15N and δ13C strong excursions are synchronous with the onset of Δ33S increase while δ34S variations remain minor. These integrated isotopes approach facilitate to identify different types of metabolisms with respect to environmental conditions. Our results suggest that anaerobic methane-oxidation bacteria, sulfureta communities, ammonium oxidizing bacteria and ANAMMOX could be dominant in the 2.73 Tumbiana paleo-environment. Because oxidant was needed for these oxidation processes, the isotope features from this study may imply that a stepwise increase in the oceanic oxidation degree could exist 400 Ma before the Great Oxidation Event.