Schedule May 21, 2018
Carbon export in the ocean: gravitational, mixed-layer & subduction pumps
Laure Resplandy (Princeton)

Carbon dioxide in the atmosphere is rising from fossil fuel emissions, trapping heat and warming the planet. The ocean plays an essential role removing carbon from the atmosphere by absorbing and exporting it to the deep ocean. However, it is unclear how the rise in human- induced carbon affects this process. Carbon export occurs as part of the biological pump, in which tiny marine organisms (phytoplankton) use sunlight to perform photosynthesis, producing particles containing carbon that sink to the seafloor. This occurs by three mechanisms: the gravitational, mixed layer, and subduction pumps. The gravitational pump describes carbon sinking to the seafloor due to its weight. The second involves the seasonal depth change of the mixed layer (the upper ocean layer), which becomes deep in the winter and shallow in the spring, causing carbon to be entrained below. The third is the subduction pump, which occurs when physical forcing such as wind causes vertical motion that actively pushes the carbon to the deep on shorter time scales. It is known that the gravitational and mixed layer pumps dominate carbon export, but the role of the subduction pump is unclear. This study uses a numerical model of ocean circulation coupled with a model representing biology to explore effects of the subduction pump. They find that subduction produces high variability of carbon export throughout the year, but has a very small net impact because downward motion is compensated by upward motion. They note that small-scale physical processes have little impact on the subduction pump, but may play an important role in the mixed layer pump.

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