Quantifying the importance of authigenic carbonate in the ancient carbon cycle

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In Earth history research, arguably no element has received more attention than carbon, owing to its dual role as a planetary thermostat and source of atmospheric oxygen. Due to the challenges of direct quantification of mass fluxes, especially in deep time, carbon isotopic measurements (δ13C) of sedimentary phases have emerged as proxies to study the global carbon cycle. Recent work, however, has challenged the traditional paradigm used to interpret δ13C records by suggesting that authigenic carbonate, precipitated from carbon released during anaerobic organic matter degradation, is a quantitatively important third sink for carbon - especially during times of putatively lower O2 concentrations, such as during the Precambrian and transient anoxia in the Phanerozoic.******If this model is correct, then our understanding of the carbon cycle and global redox budgets needs to be redressed at a fundamental level. Testing this model is challenging, however, because authigenic carbonate can be disseminated over a large volume of buried siliciclastic sediment. Thus, rather than viewing geochemical proxies as replacements for empirical quantification of geological mass fluxes, these approaches are necessarily complementary. For example, analyses from Macrostrat, a 4D model of the North American upper crust (macrostrat.org), reveal an end-Devonian crash in shelf carbonate production, with replacement by siliciclastics. This scenario is predicted to produce ideal boundary conditions for the reemergence of an authigenic carbonate sink in an otherwise oxic world.******This proposal outlines a unique and innovative combination of approaches - field work, isotope geochemistry, computational abilities - to assess the strength of the authigenic lever in deep time. Field work that both searches for these end-Devonian carbonates and constrains the composition of seawater chemistry is a first-order test of the model. In the Canadian Front Ranges, end Devonian and early Carboniferous strata are kilometers thick. Work will involve concurrent logging of stratigraphic sections and geochemical sampling of both platformal carbonates and their basinal equivalents. Collected samples will be analyzed for δ13C and trace element abundances at the University of Victoria. The geochemical data developed will be used to model the importance of authigenic carbonate by combining these measurements of sediment composition with the sedimentary mass flux estimates derived from Macrostrat. These efforts will utilize both the newly produced data and data extracted from the literature using machine reading and data mining techniques (geodeepdive.org). Inevitably, this computational approach will reveal data gaps and pose new hypotheses, thereby providing new directions and motivation for further field and laboratory work.