Winter Mixing Controls Carbon Sequestration by the Biological Pump in the Subpolar North Atlantic

Abstract Year‐round, full water‐column hydrodynamic and biogeochemical observations at the Ocean Observatories Initiative's Irminger Sea Array provide a unique opportunity to examine the drivers of ocean carbon sequestration via the biological carbon pump in a region with deep winter convection. We use daily resolution oxygen and optical backscatter profiles to quantify carbon remineralization and large particles sinking throughout the water column from 2015 to 2022, during which deep winter convection ranged from 440 to 1,430 dbar. We use calibrated oxygen time series to determine depth‐resolved remineralization rates, finding 6.4 ± 2.0 mol C m −2 yr −1 were exported seasonally from the surface and remineralized at depth. However, 65%–100% of this remineralized carbon was re‐entrained into the mixed layer in the subsequent winter, such that only 0.8 ± 1.2 mol C m −2 yr −1 was sequestered annually below the winter mixing depth. The remineralized carbon sequestered each year depends on winter mixing and carbon transfer efficiency through the thermocline, quantified as remineralization length scale (). Interannual variability in is correlated with the maximum mixing depth of the previous winter, suggesting detrainment of biomass during spring shoaling is an important pathway for export to the mesopelagic. Backscatter profiles reveal a carbon pool of large particles that is efficiently transferred to depth, with sinking particle pulses sequestering ∼0.6–1.5 mol C m −2 yr −1 of carbon that is not captured in our oxygen‐based estimates. These results underscore the importance of sustained multi‐parameter biogeochemical time series data to unravel the intertwined biological and physical drivers of carbon cycling in this highly dynamic region.