Ni isotope fractionation during sorption to ferrihydrite: Implications for Ni in banded iron formations

A major decline in marine methanogen populations during the Late Archaean Eon has been proposed as a driver for the Great Oxidation Event (GOE) at ~2.4Ga. Konhauser et al. (2009, Nature 458, 750-753) recently hypothesized that a dramatic decrease between 2.7 and 2.4Ga in the concentration of dissolved Ni, an essential nutrient for methanogens, may have been responsible for the decline. To test that hypothesis, Ni stable isotopes, a novel proxy for methanogen Ni limitation, will be applied to the Late Archaean-Paleoproterozoic rock record in the future, possibly through analysis of banded iron formations (BIFs). In order to facilitate interpretation of those data, we investigated the fractionation of Ni stable isotopes during sorption to synthetic ferrihydrite, which was likely the primary precipitating phase during BIF deposition, via bench-top experiments. The observed fractionations for adsorption and coprecipitation experiments were indistinguishable and averaged δ^60/58Ni_{dissolved-sorbed}=+0.35±0.10‰ (1 sd; light Ni sorbed). Parallel, linear trends in isotope compositions of dissolved and sorbed Ni as a function of fraction of Ni sorbed indicate an equilibrium isotope effect with rapid and continuous exchange between dissolved and sorbed Ni. Additionally, we observed fractionation of δ^60/58Ni_{dissolved-sorbed}=+0.23±0.07‰ (1 sd) in experiments in which ferrihydrite had been aged for up to 30days and had partially transformed to goethite and hematite; thus the fractionations in all of our experiments are the same within uncertainties. Although our simple system differs from Archaean seawater, our results suggest that ferrihydrite precipitating to form BIFs may always have been offset by a constant fractionation from coexisting dissolved Ni in seawater.