HR: 13:45h
AN: PP42C-01 INVITED     [PDF]
TI: On the Origin of Mesozoic Oceanic Anoxic Events (OAEs): An Overview
AU: * Arthur, M A
EM: arthur@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AB: The search for commonality in the nature and origin of "black shale" units that characterize Jurassic-Cretaceous "Oceanic Anoxic Events" has not come to fruition. Indeed, it would appear that OAEs differ from one another in duration, distribution, organic carbon contents, and carbon isotope signatures. In addition, they differ in origin as interpreted from faunal, floral and stable isotope data. Considerable uncertainty remains regarding the relative roles of productivity (carbon flux) and mechanisms of preservation (low dissolved oxygen at the sediment/water interface, high sedimentation rate, or high mineral surface area) during OAEs in general. For example, it is difficult to separate anoxia from high surface-water productivity as a cause for enhanced organic matter preservation, in part because oxygen depletion in deeper water masses is a response to high fertility and organic carbon flux. Mineral surface area recently has been proposed as the only control on organic carbon contents, but this seems doubtful. There are, however, interesting patterns that bear further examination. OAEs, and black shales in general, typically form during transgressive episodes. For epicontinental black shales, transgression creates conditions that favor nutrient trapping in relatively isolated basins; these nutrients originate either from fluvial sources or are transported into epicontinental seas from adjacent ocean basins with well-developed, nutrient-rich oxygen minimum zones. It is not clear how transgression induces more global, open-ocean OAEs. They may be a response to several factors related to the cause(s) of the transgressions, including changes in deepwater overturn rates induced by increasing expanse of shallow shelf regions or opening of oceanic gateways, and/or by overall higher nutrient fluxes from weathering brought about by warmer, wetter climates related to times of greater outgassing resulting from increased volcanism. In some cases, enhanced oceanic stratification may have result in anoxia and a preservational OAE (e.g., OAE 1d), whereas most OAEs appear to involve enhanced mixing and nutrient fluxes (e.g., OAE 1b and OAE 2). There has been more excitement recently regarding decomposition of methane hydrates as a possible trigger for OAEs producing sudden warming and consumption of deep-water oxygen. Although significant negative carbon isotope excursions accompany the Toarcian and Aptian (OAE 1a) OAEs, the isotopic patterns can be explained by volcanic events. There is evidence that increased volcanism precedes or accompanies some OAEs. It is clear that more high-quality, globally distributed geochemical and biotic records are required to resolve the OAE problem.
DE: 3022 Marine sediments--processes and transport
DE: 3030 Micropaleontology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting