Apatite and magnetite as probes into dissimilatory iron reduction in banded iron formations

Abstract

Banded iron formations (BIFs) are ancient sedimentary deposits that provide critical insights into the evolution of Earth's early atmosphere, hydrosphere, and biosphere. This study focuses on the Ouyang BIF in the North China Craton (NCC), highlighting the substantial impact of dissimilatory Fe(III) reduction (DIR) on both the remobilization of iron from the primary iron precipitates and subsequent authigenic mineralization during diagenesis. This biological process is manifested in the light iron isotopes in magnetite (δ⁵⁶Fe values down to −2.1 ± 0.02 ‰) and distinct geochemical signatures in associated apatite. Specifically, the elevated Mn/U ratios (ranging from 467 to 1210) in apatite indicates redox cycling, while elevated concentrations of Sr (ranging from 15,453 to 20,466 ppm), and OH (33 % to 58 % in molar) in apatite are characteristic of biomineralization processes during BIF formation. The relationship observed between Sm/Nd ratios (and Nd isotopes) in apatite and δ⁵⁶Fe values in magnetite further suggests that iron was sourced to the Wuyang BIFs from both hydrothermal fluids and a benthic iron flux. Notably, the benthic Fe flux likely resulted from the DIR of ferric oxyhydroxide minerals in nearshore sediments, highlighting the essential role of continental iron and DIR in transporting Fe from the continents to BIF depositional centers. This study underscores the critical influence of microbial activity in iron cycling within ancient oceans. It further showcases how DIR was recorded by apatite and magnetite, offering insights into past DIR bacteria during BIF deposition.