Constraints on the coevolution of oxic and sulfidic ocean iron sinks from Archean–Paleoproterozoic iron isotope records

The drivers of Fe isotope variations of Archean–Paleoproterozoic pyrite have been debated since discovery of δ56Fe values (per mil shifts in 56Fe/54Fe ratios versus iron isotopic reference material [IRMM-014]) as low as −3.5‰ in pyrites predating the Great Oxygenation Event (GOE) at ca. 2330 Ma. These values were taken as evidence that extensive removal of high-δ56Fe Fe3+ oxides during partial oxidation of upwelled Fe2+-rich waters occurred in the early oceans. However, low pyrite δ56Fe can also reflect kinetic isotopic shifts during pyrite formation. Compiled δ56Fe records of oxic (iron formation) and sulfidic (pyrite) sinks of Fe negatively covary before the GOE, contrary to expectations that Fe2+ oxidation would drive the δ56Fe values of these sinks on parallel trends as oxidation progressed. Positive covariation of pyrite δ56Fe with the dispersion of sedimentary sulfide δ34S fractionation through time suggests that sulfur availability during pyritization at least partially drove trends in negative δ56Fe fractionation before the GOE. The δ56Fe records of pyrite and iron formation suggest that oxic and sulfidic Fe sinks grew in concert before and during the GOE.