Anaerobic photoferrotrophy and hydrothermal influence in the Mesoarchean Girar banded iron formation: insights from micropaleontology and geochemistry

Banded Iron Formations (BIFs) serve as critical archives of early biosignatures; however, uncertainties remain regarding the biological affinities, metabolic pathways, and their role in Earth's early oxidation history. This study investigates the Mesoarchean Girar BIF (2850 ± 26 Ma) from the Bundelkhand craton using an integrated micropaleontological and geochemical approach to address these questions. Well-preserved spheroidal microfossils, morphologically similar to Huroniospora , are documented. The organic carbon isotopic signatures (δ 13 C-org ranging from -27.8‰ to -35.2‰) indicate a biological origin mainly linked to anoxygenic photosynthesizers, methanogenic archaea, or methanotrophic bacteria, and to some extent cyanobacterial activity. These observations suggest that Fe 2+ oxidation within the Girar-Baraitha BIF was predominantly driven by anaerobic photoferrotrophs, rather than abiotic processes. Geochemical proxies, including Sm/Yb versus Eu/Sm ratios, point to hydrothermal fluid and seawater mixing in proportions of 1:1000 to 1:100, supported by a pronounced positive Eu/Eu* (> 2), indicating substantial hydrothermal Fe input. Collectively, these findings provide evidence that anaerobic photosynthesis, particularly photoferrotrophy, played a central role in iron deposition in the Girar-Baraitha BIF. Abiotic oxidation, while present, was likely a secondary process facilitated by localized oxidative conditions. This study contributes to a more nuanced understanding of early Earth's biogeochemical cycles, microbial ecosystems, and the mechanisms driving surface oxidation and iron formation during the Mesoarchean. • Oldest Huroniospora Record – Girar BIF hosts the oldest known Huroniospora, evidencing early microbial activity. • Microbial Fe Oxidation – Anaerobic photoferrotrophs dominated Fe 2+ oxidation, with minor abiotic oxidation in suboxic waters. • Hydrothermal Influence – Sm/Yb, Eu/Sm ratios confirm hydrothermal-seawater mixing, driving Fe deposition in Girar BIF. • Limited Oxygenic Photosynthesis – Microbial mediation in Fe deposition suggests anaerobic pathways, not oxygenic photosynthesis, shaped early BIFs.