ON THE INFLUENCE OF EDDY VISCOSITY AND VERTICAL STRATIFICATION ON SUBSURFACE DISSOLVED OXYGEN CONCENTRATION IN RESPONSE TO GREENHOUSE GAS FORCING DURING THE PALEOZOIC

The Permian-Triassic Boundary (P-Tr, ~251.5 Ma) marks the largest mass extinction of the Phanerozoic, with a reduction of marine family diversity of 60% and an extinction of marine organisms of 90%, and is characterized by large oscillatory excursions of carbon isotopes, high greenhouse gas levels, and wide-spread anoxia linked to ocean stratification. In this paper, we follow the approach of Munk (1966) to estimate the response of vertical eddy viscosity to greenhouse gas forcing from long-term equilibrium climate simulations performed with comprehensive climate models. Terms in the equation of motion are computed from long-term climate simulations for the present, the Paleocene Eocene Thermal Maximum, and the End-Permian, and near-steady state is assumed. For this approximation, the vertical velocities times the vertical density gradient estimated from the comprehensive climate simulations is in equilibrium with the eddy viscosity terms, thus allowing us to compute the eddy viscosity coefficient. We demonstrate that with increase in greenhouse gas forcing, and associated vertical stratification, the eddy viscosity coefficient will be significantly reduced, leading to a decline in dissolved oxygen levels and promoting mass extinction in subsurface layers.