The biological carbon pump exerts a strong control on atmospheric CO₂ levels. It includes a range of processes that generate organic carbon in the surface ocean and transport this organic matter from the surface to the deep ocean where it is remineralized and sequestered as inorganic carbon for decades to millennia. While ocean productivity is relatively well observed through a combination of approaches including remote sensing, the magnitude of vertical carbon transport remains poorly constrained by observations and the detailed processes involved are insufficiently understood. In particular, attention to the contribution of small particles has increased in recent years, but previous estimates of the associated vertical carbon flux have ignored remineralization and particle fragmentation. The resulting estimates are likely biased. In this study, we present a method for estimating (1) vertical carbon flux of two different size classes of organic particles and (2) the effect of remineralization and particle fragmentation on mesopelagic flux attenuation using Biogeochemical-Argo profiles of backscattering and dissolved oxygen. We applied this method to observations from the subpolar North Atlantic and found that, on annual timescales, gravitational settling of large organic particles is dominating the vertical flux through the lower mesopelagic zone. However, small particles contribute significantly to the vertical carbon flux at 100 m (around 36% but can be up to 63%), via different mechanisms, and at 600 m (0–25%) since they can be produced by the fragmentation within the mesopelagic zone.
 

BGC-Argo;,Biological carbon pump,fragmentation,remineralization