247 / 2024-09-12 16:31:56

Vertical carbon export via frontal dynamics during an under-ice phytoplankton bloom in the Chukchi Sea

Chukchi Sea,Phytoplankton sinking rates,baroclinic instability

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. 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.