The influence of winter ventilation on the ocean’s biological carbon pump
The ocean is the ultimate carbon sink on human time scales of decades to centuries, and has absorbed ~40% of anthropogenic carbon emissions to date. The biological pump – in which a fraction of photosynthetically-fixed organic carbon is exported from the surface to the deep ocean – plays a key role in enabling the ocean to absorb carbon dioxide from the atmosphere. However, in regions that experience deep winter mixing, much of the organic carbon exported from the seasonally-stratified surface ocean can be ventilated back to the atmosphere during winter rather than being sequestered on annual or longer time scales. In this talk, I will present both observational and model-based evidence of the importance of winter ventilation for the ocean’s biological carbon pump. Observations using both ship-of-opportunity geochemical tracer sampling in the North Pacific and time-series autonomous sensor measurements of dissolved oxygen in the subpolar North Atlantic show that in these high-latitude regions which experience deep winter mixing, seasonally-exported carbon is respired in the thermocline and ventilated during winter. Analysis of global earth system model output shows that this phenomenon is globally important: studies of the biological pump which do not account for winter ventilation of respired carbon can overestimate the global rate, efficiency, and spatial variability of biological carbon export, due to the pronounced influence of winter ventilation in high-latitude regions. This mechanistic coupling between physical and biological carbon cycling processes illustrates the importance of accounting for winter ventilation in future observational and model-based studies of the biological pump and its role in the overall ocean carbon sink.