Ni isotope fractionation during sorption onto ferrihydrite

A major decline in methanogen populations during the Archaean has been proposed as a driver for the Great Oxidation Event (GOE) (Kasting 2005, Zahnle et al. 2006). Scarcity of oceanic Ni, an essential nutrient for methanogens, was hypothesized to explain decline in methanogen populations (Konhauser et al. 2009). Ni/Fe ratios in banded iron formations (BIFs) demonstrate reduced Ni availability in oceans prior to the GOE, supporting a "Ni famine" for methanogens (Konhaser et al. 2009). A new proxy for methanogen populations, Ni isotopes, may be applied to BIFs in order to test how methanogen populations responded to declining Ni (Cameron et al. 2009). Prior to measuring and interpreting Ni isotope composition of BIFs, a more thorough understanding of Ni isotope systematics is necessary. For instance, sorption of metals onto minerals surfaces can cause isotopic fractionation. Nickel isotopes may fractionate during sorption onto ferrihydrite, the initial iron oxide thought to precipitate during deposition of BIFs (Bjerrum and Canfield 2002). We investigated the fractionation of Ni stable isotopes during sorption to synthetic ferrihydrite via bench-top experiments. Ni isotope compositions of all recovered solutions and solids were measured directly using MC-ICP-MS. An equilibrium fractionation of Delta 60/58Ni = 0.3 ‰ occurred during both sorption and coprecipitation experiments. Additionally, an aging experiment was performed on the ferrihydrite stock to determine the effect of recrystallization on isotope fractionation during sorption. Over the course of 30 days a portion of the ferrihydrite stock recrystallized to a mixture of hematite and goethite. Changes to the ferrihydrite stock mineralogy did not alter isotope fractionation within analytical uncertainty.