SIMULATING LATE ORDOVICIAN DEEP OCEAN O2 WITH AN EARTH SYSTEM CLIMATE MODEL: FIRST RESULTS

The geological record provides several lines of evidence that point to the occurrence of widespread and long lasting deep ocean anoxia during the Late Ordovician, between about 460-440 million years ago (ma). While a series of potential causes have been proposed, there is still large uncertainty regarding how the low oxygen levels came about. Here we use the University of Victoria Earth System Climate Model (UVic ESCM) with Late Ordovician paleogeography to verify the impacts of paleogeography, bottom topography, nutrient loading, and atmospheric concentrations of O₂ and CO₂ on deep ocean oxygen concentration during the period of interest. Preliminary results are based on 10 simulations covering the following parameter space: CO₂ concentrations of 1120 to 3360 ppmv (4x to 12x pre-industrial), atmospheric O₂ ranging from 8% to 12% by volume, oceanic PO₄ and NO₃ loadings from present day to double present day, reductions in wind speed of 50% and 30% (winds are provided as a boundary condition in the UVic ESCM), and two sets of bottom topography. For most simulations the deep ocean remains well ventilated. Small pockets of bottom anoxia – here defined as concentrations <10 μmol L^-1 – are found in most simulations, however these cases are generally restricted to certain paleogeographic ocean basins, especially in the Panthalassic Ocean northwest of Laurentia, and the Paleotethys Ocean off of the west coast of Gondwana. More widespread anoxia is simulated with higher concentrations of oceanic NO₃ and lower concentrations of atmospheric O₂.