THE OXYGEN MINIMUM ZONE AS A KEY CONTROL ON EXCURSIONS, RECOVERY, AND ENVIRONMENTAL CHANGE FOLLOWING THE END-PERMIAN EXTINCTION (Invited Presentation)

Reduced intensity of bioturbation coupled with increased prevalence of black shales has long indicated that recovery from the end-Permian mass extinction was delayed or interrupted over an interval of several million years. Subsequent identification of large excursions in the carbon and sulfur isotope records from Lower Triassic strata strengthened the evidence for a link between the rebuilding of marine animal ecosystems and the stabilization of global biogeochemical cycles. More recently, the identification of steep paleo-depth gradients in the carbon isotope composition of marine limestones suggests the presence of a shallow and intense oxygen minimum zone during Early Triassic time. Further, the documentation of a protracted negative excursion in the uranium isotope composition of shallow-marine carbonates confirms that this expansion of shallow anoxic bottom waters was geographically widespread, reaching more than twenty times its pre-extinction value. Recent model results from cGENIE, an Earth system model of intermediate complexity, show how shallow-marine anoxia can be explained by a shift toward cyanobacterial dominance of primary production and warming of surface ocean waters. In addition, a box model of the coupled geological carbon and phosphorus cycles illustrates how shallow marine anoxia can make the global biogeochemical cycles of carbon and sulfur particularly sensitive to the extent of continental flooding. We hypothesize that the depth of the oxygen minimum zone was an important factor modulating environmental and biological recovery from the end-Permian extinction. The same changes in the position and extent of the oxygen minimum zone that appear to account for the pattern of recovery from this biotic catastrophe may also explain the Phanerozoic stabilization of the geological carbon cycle and the associated decline in extinction rates documented in the fossil record of marine animals.