Particulate Organic Carbon Flux in the Subpolar North Atlantic as Informed by Bio-Optical Data from the Ocean Observatories Initiative

Abstract

The biological carbon pump in the North Atlantic Ocean is powered by the annual spring phytoplankton bloom. These primary producers use inorganic carbon in the surface oceans and convert it into organic carbon, a fraction of which is exported out of the surface mixed layer and sequestered at depth. Determining the rate of carbon flux below the maximum winter mixed layer depth, driving sequestration on annual or longer timescales, is critical to understanding the North Atlantic carbon cycle.To constrain daily-to-annual scale changes in carbon export in the subpolar North Atlantic, I analyzed seven years of daily optical backscatter depth profiles (200-2600 m) collected from the subsurface profiler mooring at the Ocean Observatories Initiative (OOI)’s Global Irminger Sea Array from September 2014 to May 2021. This is the longest-running time series of daily, year-round optical backscatter profiles that has been collected in this region, providing novel opportunities to assess seasonal and interannual variations in particulate organic carbon (POC) flux to depth. This analysis, focused on large particles and aggregates identified from optical backscatter spikes, shows annual pulses of sinking particles initiating in May to June during each year of our seven-year time series, consistent with these export pulses being driven by organic matter production during the spring phytoplankton bloom. These pulses of particles sink through the water column at rates ranging from 10 and 30 meters per day, and though particle concentration attenuates through the water column due to remineralization, coherent large particle pulses generally extend deeper than 1500 m, the deepest maximum annual mixed layer depth over this period. Although deep winter mixing in this region requires sinking particles to penetrate much deeper than in other parts of the ocean to be sequestered long-term, pulses of large particles consistently penetrate to below even the deepest annual mixed layer depths in the region, highlighting the importance of these large particle pulses to carbon sequestration at depth in the subpolar North Atlantic.