Vertical Carbon Export During a Phytoplankton Bloom in the Chukchi Sea: Physical Setting and Frontal Subduction

In order to quantify pelagic-benthic coupling on high-latitude shelves, it is imperative to identify the different physical mechanisms by which phytoplankton are exported to the sediments. In June-July 2023, a field program documented the evolution of an under-ice phytoplankton bloom on the northeast Chukchi shelf. Here, we use in situ data from the cruise, a simple numerical model, historical water column data, and ocean reanalysis fields to characterize the physical setting and describe the dynamically driven vertical export of chlorophyll associated with the bloom. A water mass front separating cold, high-nutrient winter water in the north and warmer summer waters to the south-roughly coincident with the ice edge-supported a baroclinic jet which is part of the Central Channel flow branch that veers eastward toward Barrow Canyon. A plume of high chlorophyll fluorescence extending from the near-surface bloom in the winter water downwards along the front was measured throughout the cruise. Using a passive tracer to represent phytoplankton in the model, it was demonstrated that the plume is the result of subduction due to baroclinic instability of the frontal jet. This process, in concert with the gravitational sinking, pumps the chlorophyll downwards an order of magnitude faster than gravitational sinking alone. Particle tracking using the ocean reanalysis fields reveals that a substantial portion of the chlorophyll away from the front is advected off of the northeast Chukchi shelf before reaching the bottom. This highlights the importance of the frontal subduction process for delivering carbon to the sea floor.