INCREASED P DELIVERY VS. P-RECYCLING IN THE APPALACHIAN BASIN

Bottom water anoxia, possibly related to enhanced primary production in the marine realm is often associated with interpretations of the Late Devonian biotic crisis. It has been suggested that variations in nutrient budgets may have been the cause of over-production events, resulting in bottom water anoxia and widespread black shale deposition. The biogeochemical cycle of Phosphorus (P) has been a prime target of inquiry concerning the cause of this major extinction event.

Enhanced P delivery to ocean basins via increased chemical weathering rates mediated by the rise of vascular plants in Devonian time is one hypothesis proposed to explain increased P supply. An alternative explanation invokes recycling of P from sediments to the water column due to the decomposition of organic matter in condensed basinal sequences under anoxic-dysoxic conditions.

In this study, the SEDEX sequential P extraction technique was applied to core samples from the Devonian Appalachian Basin spanning the Upper Kellwasser horizon of the Frasnian-Famennian boundary interval. This interval is characterized by black shales that include two global positive δ13C excursions. Prior analysis of the bulk C/P ratio showed a dramatic increase across the horizon. SEDEX differentiates the mineralogically distinct phases of detrital P from reactive phases that would have been available to primary producers.

Results confirm a significant rise in the ratio of organic C to reactive P, and indicate that the rate of authigenic P burial decreased during this time, consistent with enhanced recycling. There is also a marked rise in concentrations of detrital P preceding the onset of the event. These observations suggest that detrital P influx may have initiated oxygen deficiency via over-production, subsequently shifting the system to a P-recycling eutrophic state. The Devonian Appalachian Basin was a proximal depositional setting, where changes in terrestrial weathering would have directly affected sediment supply. It was also a shallow epieric sea, where fluxes of recycled P from sediments could have significantly influenced surface dwelling primary producers. Application of the SEDEX method has made it possible to identify the signatures of both enhanced weathering and P-recycling processes.