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Stable Ni Isotope Fractionation in Systems Relevant to Banded Iron- Formations

Haleigh Howe, Lev Spivak-Birndorf, Derrick Newkirk, and Laura Wasylenki
Indiana University Bloomington

An important event in the evolution of life was the rise of atmospheric oxygen during the Proterozoic. Preceding the rise in O2 was a decline in atmospheric methane concentrations, likely due to decline of methanogenic Archaea. Based on Ni concentrations in banded iron-formations (BIF), Konhauser et al. (2009) hypothesized that mantle cooling during the Archaean reduced the amount of Ni present in igneous rocks and in oceans, causing a Ni shortage for methanogens. Methanogens use Ni for cofactor F430, a catalyst during methanogenesis. To confirm Konhauser’s hypothesis, a proxy for methanogen productivity is necessary, in order to determine whether a decline in methanogen populations correlated with the observed decrease in Ni in rocks from the Archaean.

Ni isotope ratios recorded in BIF may provide evidence of a decline in methane production. Cameron et al. (2009) have shown that methanogens preferentially assimilate light Ni isotopes. Thus Ni isotopes in BIF have potential use as biomarkers for methanogenesis. During BIF deposition ferrihydrite was the dominant Fe oxide precipitating. We investigated the relationship between Ni isotopes in solution and Ni associated with ferrihydrite.

Experiments involving adsorption of aqueous Ni onto surfaces of synthetic ferrihydrite were conducted. Preliminary results indicate that light isotopes are preferentially associated with ferrihydrite, with an average fractionation of 0.4‰ in terms of δ60/58 Ni. Future experiments will investigate whether the observed isotope fractionations reflect kinetics or equilibrium, thus determining whether or not BIF provide a straightforward record of Ni isotopes of the water mass.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013