COLLABORATIVE RESEARCH: Geochemical and Isotopic Constraints on Mesproterozoic Ocean Chemistry Working Toward a Global Perspective

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9725397 Frank The Mesoproterozoic Eon (1600-1000 Ma) represents a critical period in Earth history from the standpoint of eukaryotic diversification (Knoll, 1991), fundamental transitions in the oxidation state of the Earth's atmosphere and oceans (Des Marais et al., 1992; Canfield and Teske, 1996), and global tectonic reorganization (Dalziel, 1991). Yet despite this obvious significance, the Mesoproterozoic remains one of the most poorly understood and poorly documented intervals in Earth history. For example, in reference to an apparent stability in the Mesoproterozoic marine carbon isotopic record, the era between 1600 Ma and 1000 Ma has been reported as 'the dullest time in Earth's history' (Buick et al., 1995). However, a large positive shift (3-4%) in marine carbonate (13C values has recently been recognized in late Mesoproterozoic (1300 Ma) successions from Siberia, Arctic Canada, Greenland, and the United States. Rather than a short-lived excursion, this shift appears to represent a prolonged and fundamental change that may represent a significant transition in the Earth's global carbon cycle. The timing of this observed transition, between ~1300 and ~1150 Ma, is particularly intriguing in that it occurs in conjunction with the early assembly of the Rodinian supercontinent and a purported rise in the concentration of atmospheric oxygen. The goal of the proposed research is to generate carbon, sulfur and strontium isotopic data for this interval and, thereby, document and begin to address mechanistically the most fundamental (first-order) shifts in the chemistry of the Mesoproterozoic ocean. These objectives will be achieved through a multidisciplinary study that integrates depositional, diagenetic, and geochemical information into a single comprehensive framework. Although no single stratigraphic succession adequately encompasses the interval surrounding the observed carbon isotopic shift, two distinct successions (Society Cliffs Formation, northern Baffin Island; Dismal L akes Group, Coppermine homocline, Northwest Territories) can be effectively spliced to provide coverage across the interval of the observed transition. Each of these successions has been chosen on the basis of accessibility and exposure of stratigraphic units, the presence of a wide variety of facies and lithologies suitable for geochemical analysis, and the abundance of a variety of allochems and marine cements from different depositional environments. Combined, these prerequisites assure recognition of depositional and diagenetic trends that would otherwise hamper the interpretation of geochemical and isotopic data. Such an integrated approach will provide the clearest path toward understanding the complex evolution of the Proterozoic biosphere.