Co-evolving N-Fe redox processes controlled iron minerals in banded iron formation

Abstract

Abstract Biogeochemical cycles in the Precambrian ocean responded to the co-evolution of biosphere (microorganisms) and the physicochemical structure (e.g., redox, temperature) of the ocean, which closely link to the enigma of banded iron formations (BIFs) that primarily triggered by massive Fe(II) oxidation under anoxic-hypoxic condition for two-billon years (~3.8-1.8 Ga). The current Fe(II) oxidation models, however, rarely consider the effects of the evolution of coupled biogeochemical cycles on secular succession (shifting from magnetite to hematite) of dominant iron minerals in BIFs. Here, we investigated the evolution of coupled Fe-N redox processes under the simulated Precambrian ocean conditions, and propose a dynamic model for the origin of iron mineral succession in BIFs: During the early-mid Archean, NO2- was mainly produced by nitrification in the oceans of warm-hot temperatures (>50-60 oC), which favored the primary precipitation of Fe(II)-Fe(III) oxides (magnetite) and silicates (cronstedtite) in the early BIFs. Subsequently, the cooling and oxygenation of paleo-ocean near the GOE promoted the input of both NO2- and NO3-, resulting in co-precipitation of an increasing amount of Fe(III) minerals (goethite and lepidocrocite as precursors of hematite). This dynamic N-Fe coupling model explains the observed secular transition of iron mineral phases in BIF deposition.