Assessing the role of methanogens in driving earth oxygenation using Ni isotopes

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The first substantial and long-lasting accumulation of oxygen in the atmosphere and surface waters of the oceans ~2.4 billion years ago was arguably one of the three most important turning points in the history of life, but exactly why and how this transformation took place remains poorly understood. One phenomenon that likely drove or at least stabilized the transition to a more oxidizing state was collapse of methanogens, a dominant form of life throughout much of the Archean Eon that sustained global anoxia through production of abundant methane. The objective of this project is to test the recent hypothesis of Konhauser et al. (2009; Nature 458, 750), who attributed the decline of methanogens to a shortage of nickel, an essential trace nutrient for these organisms. These authors argued that biogenic methane production might have waned well before the rise of oxygen, because oceanic Ni concentrations decreased substantially between 2.7 and 2.5 Ga. The recent discovery that modern methanogens strongly fractionate stable isotopes of Ni suggests that Ni limitation of methanogens may have left an isotopic fingerprint in the sedimentary record. The aim of this study is to analyze Ni isotope compositions of the same samples in which Konhauser et al. documented the Ni concentration decline, to see if an isotopic shift reflecting the onset of Ni limitation of methanogens is evident. Samples will be extracted from the hematite-rich layers of banded iron formations by microdrill, and isotopic compositions will be measured by MC-ICP-MS. Investigator will focus in particular on relationships between Ni concentrations and Ni isotopes for samples with ages between 2.8 and 2.3 billion years, the interval over which methanogens may or may not have assimilated increasingly large proportions of available dissolved Ni in seawater.