Anthropogenic carbon pathways towards the North Atlantic interior revealed by Argo-O, neural networks and back-calculations.

The subpolar North Atlantic (SPNA) is a region of high anthropogenic CO (C) storage per unit area. Although the average C distribution is well documented in this region, the C pathways towards the ocean interior remain largely unresolved. We used observations from three Argo-O floats spanning 2013-2018 within the SPNA, combined with existing neural networks and back-calculations, to determine the C evolution along the float pathways from a quasi-lagrangian perspective. Our results show that C follows a stepwise deepening along its way through the SPNA. The upper subtropical waters have a stratified C distribution that homogenizes within the winter mixed layer by Subpolar Mode Water formation in the Iceland Basin. In the Irminger and Labrador Basins, the high-C footprint (> 55 μmol kg) is mixed down to 1400 and 1800 dbar, respectively, by deep winter convection. As a result, the maximum C concentration is diluted (<45 μmol kg). Our study highlights the role of water mass transformation as a first-order mechanism for C penetration into the ocean. It also demonstrates the potential of Argo-O observations, combined with existing methods, to obtain reliable C estimates, opening ways to study the oceanic C content at high spatio-temporal resolution.