Anthropogenic carbon pathways towards the North Atlantic interior revealed by Argo-O₂, neural networks and back-calculations.

Abstract

The subpolar North Atlantic (SPNA) is a region of high anthropogenic CO₂ (C_ant) storage per unit area. Although the average C_ant distribution is well documented in this region, the C_ant pathways towards the ocean interior remain largely unresolved. We used observations from three Argo-O₂ floats spanning 2013-2018 within the SPNA, combined with existing neural networks and back-calculations, to determine the C_ant evolution along the float pathways from a quasi-lagrangian perspective. Our results show that C_ant follows a stepwise deepening along its way through the SPNA. The upper subtropical waters have a stratified C_ant 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_ant footprint (> 55 μmol kg^-1) is mixed down to 1400 and 1800 dbar, respectively, by deep winter convection. As a result, the maximum C_ant concentration is diluted (<45 μmol kg^-1). Our study highlights the role of water mass transformation as a first-order mechanism for C_ant penetration into the ocean. It also demonstrates the potential of Argo-O₂ observations, combined with existing methods, to obtain reliable C_ant estimates, opening ways to study the oceanic C_ant content at high spatio-temporal resolution.