Past changes in North Pacific (de)oxygenation – a complex interplay between water mass ventilation and organic matter respiration

The North Pacific basin has undergone large changes in subsurface oxygenation in the past. In general, oxygen-depleted zones increased volumetrically as climate warmed, with the rate of warming playing a critical role in determining the spatial extent of subsurface deoxygenation. The most pronounced deoxygenation episode in the upper ocean occurred midway through the deglaciation, an interval referred to as the Bolling/Allerod (B/A), associated with the reinvigoration of the Atlantic Meridional Overturning Circulation (AMOC). At this time, the upper Indo-Pacific ocean was probably less oxygenated than today. The B/A was characterized by substantial changes in intermediate water circulation, combined with efficient removal of oxygen associated with enhanced remineralization of labile organic matter, as export production increased throughout the subarctic North Pacific. The abrupt decrease in oxygenation affected large swaths of the North Pacific, including shelf environments with detrimental consequences for marine ecosystems.
This contribution will review the available paleoceanographic evidence spanning the last 3 million years and distill the salient constraints that can help better predicting the future evolution of North Pacific (de)oxygenation in the context of anthropogenic climate forcing.