Dynamics of oceanic iron prior to the Great Oxygenation Event

Abstract

We report Fe isotope compositions in banded iron formations (BIF) from three cores from the pre-GOE Transvaal Supergroup, South Africa, and one core from the pre-GOE Joffre Member of the Hamersley Group, Australia. The low abundances of detrital elements such as Al, Ti, Sc, and V suggest that these BIF were deposited in distal positions with respect to Precambrian continents, while the very low P abundances are incompatible with strong biological productivity at these localities. A combination of U–Pb chronology and cobalt accumulation rates is used to establish a high-resolution time scale and deduce chemical fluxes. The e-folding time of δ56Fe variations up stratigraphy is used to determine Fe oceanic residence times and Fe concentrations as well as the dissolved carbonate content of Early Proterozoic seas. Iron oceanic residence times increased from 0.2 to 2.3 Ma during the time interval between 2521 and 2394 Ma covered by the present cores, translating into ocean Fe concentrations increasing from 6.4 to 37 mmolkg−1. Massive BIF precipitation was triggered by release of CO2 into the atmosphere and subsequent surges of alkalinity into the ocean due to the weathering of large subaerial volcanic systems. We argue that a suitable electron acceptor for Fe2+ oxidation to magnetite is the inorganic conversion of CO2 (or dissolved inorganic carbon) to CH4. In the process, H+ is produced, which is reinjected into oceanic hydrothermal systems liberating Fe2+. The couple Fe2+-magnetite may, in the Archean, have played the same buffering role as the couple Ca2+-calcite plays today. Massive injection of methane into the atmosphere would accompany BIF deposition and make the early Earth similar to modern Titan. Therefore, although biological processes may have assisted iron oxidation and precipitation, they are not a prerequisite for BIF deposition.