Coupled nitrogen and iron redox cycling in the Paleoarchean: Insights from nitrogen isotope signatures of the ca. 3.25 Ga Mapepe Formation, Barberton Greenstone Belt

Earth’s surficial environments and biosphere have co-evolved through geological time. Constraining past biological activity is thus crucial for understanding Earth’s evolution. However, the nature of microbial metabolic processes and their evolution in the Archean ocean remain unclear. Here, we investigated the carbon and nitrogen cycles in the Paleoarchean ocean using δ¹³C_org and δ¹⁵N values of shales from the ca. 3.25 Ga Mapepe Formation of the Fig Tree Group in the Barberton Greenstone Belt, South Africa. Hydrogen, carbon, and nitrogen contents of kerogen extracts were also measured, along with δ¹⁵N_kerogen values, to assess the potential effects of metamorphism on the isotope data. The molar H/C ratios suggest the studied shales have been metamorphosed at the greenschist to amphibolite facies. The δ¹⁵N_bulk values are consistently positive throughout the studied section, with a notable increase to as high as +8.5‰ in the stratigraphic interval between 57 m and 63 m. Within this high-δ¹⁵N_bulk interval, total organic carbon contents and δ¹⁵N_kerogen values increase to approximately 4 wt.% and +6‰, respectively, while δ¹³C_org values decrease by ∼5‰. Petrographic observations indicate the iron is present mainly as iron oxides rather than sulfides in the studied shales, consistent with deposition under sulfate-poor ferruginous conditions. The Fe/Al ratios exhibit a positive correlation with δ¹⁵N_kerogen values (R² = 0.49) and a negative correlation with δ¹³C_org values (R² = 0.37). These observations suggest anaerobic oxidation of ammonium and methane coupled with iron reduction. Since ammonium oxidation can occur via both biotic and abiotic pathways, further investigations are needed to clarify the role of organisms in the coupled nitrogen and iron cycling during the Paleoarchean.