Constraints on the coevolution of oxic and sulfidic ocean iron sinks from archean-paleoproterozoic iron isotope records

Abstract

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.