Middle and Upper Devonian Black Shales: Testing the Productivity-Anoxia Feedback and Land Plant-Weathering Rate Hypotheses

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Middle and Upper Devonian Black Shales: Testing the Productivity-Anoxia Feedback and Land Plant-Weathering Rate HypothesesThomas Algeo (University of Cincinnati), Ariel Anbar (Arizona State University), Robert Creaser (University of Alberta), Peter Sauer (Indiana University), and Lorenz Schwark (University of Cologne)EAR-0618003ABSTRACTThe Middle to Late Devonian was characterized by profound changes in marine and terrestrial biotas (including the "Frasnian/Famennian mass extinction"), weathering processes and soil development, global climate, and the exogenic C-S cycle. Inconclusive evidence for major bolide impacts and a wealth of data documenting long-term changes in marine nutrient cycles favor intrinsic processes based on oceanographic and/or weathering rate controls. Two important hypotheses based on such mechanisms are the productivity-anoxia feedback (Ingall et al., 1993) and land-plant weathering rate (Algeo et al., 1995, 2001) models. These models envision similar environmental conditions in Devonian epicratonic seas (eutrophic surface waters, elevated primary productivity, and benthic oxygen depletion) yet different, but not mutually exclusive, controls thereon. The productivity-anoxia feedback model invokes enhanced microbial recycling of nutrients within the water column in combination with hypolimnial denitrification, seasonal water-column overturn, and N limitation of primary productivity, whereas the land plant-weathering rate model invokes elevated fluxes of weathering-derived nutrients as a consequence of the spread of land plants and intensified pedogenesis. PIs propose to test these models within key horizons (the North American equivalents of the Kellwasser and Hangenberg events) in drillcores from the Appalachian, Illinois, and Alberta basins using integrated chemostratigraphic datasets that will include elemental concentration data, TOC-TIC, organic d13C and d15N, whole-rock 187Os/188Os and d97/95Mo, HI-OI, and biomarkers. PIs will make their data available to the larger scientific community by importing them into the CHRONOS System and PaleoStrat. The broader impacts of this project are varied and include public outreach, mentoring of undergraduate and graduate students, development of research synergies among a diverse group of geoscience professionals, and the potential for results of broad scientific significance. The PIs are committed to advancing science education in the public schools, to building "bridges" (especially for women and underrepresented minorities) to college-level science programs, and to training the next generation of scientists.