Denitrification under glacial and interglacial conditions: A physical approach

[1] Recent analysis of sedimentary dN records over the last 200,000 years has shown an expansion of water column denitrification zones during warmer periods and a shrinkage of these oxygen-poor regions during glacial periods. Two different mechanisms could be responsible for driving the changes in the denitrification records: variations in local productivity leading to a change in remineralization rates and/or changes in the ocean circulation and ocean temperature resulting in variations of the physical supply of dissolved oxygen. Here we focus on the ‘‘supply’’ mechanism by using a fully coupled atmosphere-ocean-sea ice-land surface scheme model (the University of Victoria Earth System Climate Model) to examine oxygen supply under varying physical conditions while maintaining an invariant biological oxygen utilization scheme. We show that circulation changes can be the cause for the observed changes in dissolved oxygen in the glacial ocean. Lower temperatures and enhanced formation of Antarctic Intermediate Water and North Pacific Intermediate Water during glacial periods increase the physical supply of oxygen and therefore decrease water column denitrification. In our Last Glacial Maximum simulations the change in water characteristics in the eastern tropical Pacific is important enough to reduce denitrification by 46% to 65% compared to present-day conditions, depending on the wind fields used as boundary conditions. The consequences of our findings could be substantial for the near future. With a warming climate, denitrification zones could expand, leading to changes in the biological pump and the flux of N2O into the atmosphere.