ROLE OF ENHANCED NUTRIENT RECYCLING AND EUTROPHICATION IN DEVELOPMENT OF DEVONIAN ORGANIC CARBON-RICH DEPOSITS: DATA AND MODELING RESULTS FROM THE APPALACHIAN BASIN

Analysis of integrated sedimentologic and biogeochemical data from two cores penetrating Eilfelian through Famennian strata suggest the need to reevaluate models for the origin of Devonian black shales in the Appalachian basin. The data indicate that few of these black shales were deposited under anoxic-sulfidic conditions, that all are characterized by sedimentary condensation and decoupling of C:N:P, and that many show positive excursions in the d13C of organic matter (OM). Based on these and other observations, a new model for excess OM burial is developed. The key factor in this model is establishment and breakdown of seasonal thermoclines, which alternatively allowed build-up of nitrate and phosphate during anoxic-suboxic OM degradation, and their subsequent mixing to surface waters, where they fueled a "eutrophication pump." Trends in detrital proxies suggest the initiation of anoxic-suboxic decomposition was caused by siliciclastic sediment starvation, which increased OM concentrations in surface sediments leading to oxygen depletion and a shift to nitrate and sulfate reduction. Conversely, cessation of black shale deposition is correlated with increased flux of terrigneous material, suggesting that dilution shut down the nutrient pump. In order to further test this model, a new paleogeographic-paleobathymetric map was used to define basin area and volume for which calculation of N and P flux could be conducted. Using modern observations of the rates of N and P release from organic matter under oxygen-deficient conditions, a simple model is developed to test the relative potential of hypothesized nutrient release. The result is compared to modern estimates of riverine nutrient flux. Given that some recent studies have suggested that terrigenous rather than recycled nutrients account for enhanced carbon burial in the Late Devonian, this comparison adds an important dimension to the debate. The talk will review geochemical data supporting the eutrophication model, as well as the modeling results.