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Stephen J. Mojzsis (University of Colorado)

When May 18, 2011
from 02:00 to 04:00
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Trace metal abundances in banded iron-formation sulfides track secular changes in microbial community structure with progressive oxygenation

Secular changes in surface redox chemistry, coupled with biotic innovations, marked the Archean-Proterozoic transition at first by small increments of atmospheric O2 at ca. 2.5 Ga, followed by irreversible increases ~2.4 Ga in the Great Oxidation Event (GOE). Mixed valence Fe-oxides in banded iron-formations (BIF) document decreases in molar Ni/Fe ratios through the GOE; the resulting “Ni-famine” may have robbed methanogens of a necessary metal co-factor that stymied methane output. Diminished production of methane – an effective reductant and greenhouse gas – enabled progressive oxidation driven by cyanobacteria and contributed to the initiation of a glacial epoch. Sedimentary sulfides preserve trace-metal records of seawater (Ni, Co, Zn, etc.) as well as redox chemistry (e.g. CH4/CO2) via mass-dependent (MDF) and mass-independent (MIF) sulfur isotope fractionations. Here, I present new data that combines sulfur isotopes and trace metal concentrations in sulfides from the Paleoproterozoic Turee Creek Group of Western Australia and which overlap in time with the GOE. These data are part of larger database of sulfur and trace elements assembled from sedimentary sulfides which span the Archean and Paleoproterozoic. Results indicate drops in sulfide Ni/Fe corroborating the Ni famine hypothesis and decreased Co/Fe at about 2.4 Ga; these changes are also reflected in whole-rock BIF and shale Ni and Co concentrations. I postulate that like Ni, diminished marine Co is due to both the global diminution of ultramafic (komatiitic) volcanism and metal scavenging by greatly enhanced ferro-ferric-oxyhydroxide precipitation with the advent of oxygenic photosynthesis in the late Archean. I propose a “Co-famine” at that time for cyanobacteria, for which Co is a necessary element. The time required for cyanobacteria to adapt to low-Co by developing Co ligands may have played a hitherto unrecognized role in the apparent ~100 Myr lag between the “whiff” of oxygen and the GOE. Molar Zn/Fe ratios also drop off in the sulfide record, but not until at ca. 2 Ga, which Ipropose is a response to the gradual shift from a ferrous to sulfidic ocean following oxidation.

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