Biological control on iron phyllosilicate authigenesis and Archean ocean chemistry

Hydrous iron phyllosilicates, predominantly greenalite, is one of the earliest precursor phases of Archean (~3.2–2.5 Ga) banded iron formations (BIFs). While most geochemical models attribute early diagenetic formation of greenalite to hydrothermally-induced pore water alkalinity or alkaline oceanic water, we propose a biotic mechanism based on biogeochemical evidence from late-Archean BIFs of the Dharwar Craton, South India. Fe-(and greenalite)-rich microbands exhibit higher total organic carbon contents (TOC=0.14±0.08%, n=11) than Fe-(and greenalite)-poor cherty microbands (TOC=0.04±0.01%, n=5). Presence of relic ferrihydrite within greenalite-bearing mineral clusters suggest formation of Fe ²⁺ -rich greenalite from a Fe ³⁺ -bearing ferrihydrite precursor. Associated negative δ ¹³ C signatures of organic matter (δ ¹³ C _Org = ‒26.75±3.93‰, n=17) and inorganic bulk carbonates (δ ¹³ C _Carb = ‒5.87±0.32‰, n=6) collectively indicate Fe-reducing microbes or organic necro-mass (preserved in authigenic sediment) reduced the reactive ferrihydrites, and in the process produced isotopically ‘light’ CO ₂ (and HCO ₃ ^‒ ) through oxidation of the biomass, eventually leading to a rise in alkalinity. This biologically-driven alkalinity pump, combined with late-Archean seawater chemistry, led to the co-precipitation of greenalite and bulk inorganic carbonates (Mg-rich calcite, calcite, and siderite) with anomalously ‘light’ δ ¹³ C _Carb signatures.