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 delta Fe-56 values (per mil shifts in Fe-56/Fe-54 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-delta Fe-56 Fe3+ oxides during partial oxidation of upwelled Fe2+-rich waters occurred in the early oceans. However, low pyrite delta Fe-56 can also reflect kinetic isotopic shifts during pyrite formation. Compiled delta Fe-56 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 delta Fe-56 values of these sinks on parallel trends as oxidation progressed. Positive covariation of pyrite delta Fe-56 with the dispersion of sedimentary sulfide delta S-34 fractionation through time suggests that sulfur availability during pyritization at least partially drove trends in negative delta Fe-56 fractionation before the GOE. The delta Fe-56 records of pyrite and iron formation suggest that oxic and sulfidic Fe sinks grew in concert before and during the GOE.