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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