Dissolved Oxygen in the Oceans: An Examination of the Late Ordovician and the Near Future Using an Earth System Climate Model

One of the largest ecosystem controls in the oceans is the presence of dissolved oxygen. As oxygen levels fall, both micro- and macroorganisms face shrinking habitats and potential mortality. There have been several periods in Earth history where oxygen levels have fallen to anoxic (dissolved O2 concentration < 10 µmol L-1) or hypoxic (< 60 µmol L-1) levels in certain ocean basins or within inland seas and some of these events could potentially be linked to mass extinction events. Several hypotheses exist regarding the depletion of oxygen, the spread of hypoxia-anoxia, and why the low oxygen events occur at certain points in the geologic record, including rapid climate warming, enhanced nutrient inputs, and modifications to the surface biological pump. Unfortunately, there is little agreement on which of these potential hypotheses caused individual events and what might impact the oxygenation of our oceans in the future. This dissertation will test hypotheses related to deep ocean oxygen using the University of Victoria Earth System Climate Model. The first set of experiments feature Late Ordovician winds and paleogeography and test the impacts of atmospheric CO2 and O2, ocean bottom topography, and nutrient loadings on deep ocean oxygen concentrations. The second set of experiments is also within the Late Ordovician, but tests the impacts of remineralization rates, detrital sinking velocities, and ocean surface albedo on ocean oxygenation. The final set of experiments tests the impacts of a warming climate on the oxygenation of near-future oceans, in addition to the impacts of detrital sinking velocities and ocean surface albedo.For the Late Ordovician, the factors most favorable for the spread of anoxia are reduced atmospheric O2, increased loadings of nitrate, and a reduction in ocean surface albedo. Climatic factors (namely, increased CO2) played little role in the spread of anoxia or the depletion of oxygen in these experiments. Similarly, phosphate, enhanced remineralization rates, and slower sinking velocities do little to diminish the dissolved oxygen, but can actually work to increase oxygen below the surface layer, potentially based on where the remineralization is occurring due to these modifications.The future oceans will be impacted by warming; the largest reductions in ocean oxygen can be attributed to decreased dissolution of atmospheric oxygen in warmer surface waters. Shrinking sizes of particles (done here by reducing particle velocities) may actually work to counteract some of the reductions in ocean oxygen due to climate warming, as fewer particles fall into the deep ocean, reducing deep remineralization and uptake of oxygen.