COLLABORATIVE RESEARCH: Organic Carbon Burial, Anoxia, and Ecological-Evolutionary Events in the Appalachian Basin During the Middle to Late Devonian (Eifelian-Famennian)

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9725441 Sageman The widespread but episodic deposition of organic carbon-rich facies (ORF) in marine basins during Earth history has been recognized as an important problem in the geological sciences. Water column stratification and consequent anoxia has commonly been the favored hypothesis to account for ancient ORF deposition, but in recent years alternate hypotheses have emerged that highlight the importance of coupled biogeochemical and sedimentological processes (e.g., high productivity, stratigraphic condensation), and even suggest that water column anoxia may not have been as prevalent as once thought. Yet oxygen deficiency, whether as a cause or consequence of organic carbon burial, has been postulated as a potential driving force for evolutionary change and extinction during the Phanerozoic. For example, it was recently implicated as a possible cause of overturn events punctuating distinct Silurian and Devonian benthic faunas of the Appalachian basin. These benthic faunas are termed ecological-evolutionary subunits. The pattern of long-term stability and punctuated change at the community level is called coordinated stasis, and many of the identified faunal overturns are coincident with ORF deposits. Although coordinated stasis has rapidly become a forefront of research in evolutionary paleoecology, very few studies have integrated anallyses of faunal changes with independent assessments of coeval environmental change, and none have focused on the role of oxidation-reduction chemistry at or near the sediment-water interface (redox dynamics). What are the spatial and temporal relationships between organic matter burial events and oxygen deficiency? How does oxygen depletion relate to evolutionary change within a community, a basin, or an ecosystem? This proposal outlines a plan to re-evaluate hypotheses for the origin of ORF deposits in the Middle to Late Devonian Appalachian basin (late Eifelian through early Famennian), and to use the data collected for this purpose to i nvestigate the causes of faunal stasis and overturn. A preliminary data set of geochemical analyses from core samples has been generated (Fig. 3, Table 2) that illustrates why previous hypotheses for the origin of ORF in the Appalchian basin need to be reconsidered. This data set has provided a foundation for the development of working hypotheses relating ORF deposition and coordinated stasis, as well as a trial for selection of the best paleoenvironmental parameters to employ in the study. The two year project will focus on two cores from west central New York that will be analyzed for sedimentologic and ichnologic trends and sampled for geochemical analyses. Three types of geochemical data sensitive to redox dynamics will be collected (carbon isotopes; abundance of C, Fe, S, and P; and concentrationof trace metals), and the results will be compared to a compilation of published and unpublished data on macrofaunal trends within and around the Appalachian basin. Tests for the statistical covariance of redox sensitive proxies and faunas will allow the fundamental hypothesis, that oxygen depletion controls faunal turnover, to be confirmed or refuted. Alternate mechanisms, such as species-area effects related to relative sea level changes, will also be considered. Past global climate and environmental changes have been linked to variability in ocean circulation and biogeochemical processes/cycles. This study is directed at assessing the linkage between physical, chemical, and biological processes within an ancient marine system, and the results will provide important constraints for currently evolving ideas about past global change. The research will make a significant contribution to the understanding of organic carbon accumulation processes in the evolution of epicontinental foreland basins, and of the relationship between carbon cycling, redox history, and ecological-evolutionary events in the rock record.