Physical pathways for carbon transfers between the surface mixed layer and the ocean interior

Although they are key components of the surface ocean carbon budget, physical processes inducing carbon fluxes across the mixed-layer base, i.e., subduction and obduction, have received much less attention than biological processes. Using a global model analysis of the preindustrial ocean, physical carbon fluxes are quantified and compared to the other carbon fluxes in and out of the surface mixed layer, i.e., air-sea CO₂gas exchange and sedimentation of biogenic material. Model-based carbon obduction and subduction are evaluated against independent data-based estimates to the extent that was possible. We find that climatological physical fluxes of dissolved inorganic carbon (DIC) are two orders of magnitude larger than the other carbon fluxes and vary over the globe at smaller spatial scale. At temperate latitudes, the subduction of DIC and to a much lesser extent (<10%) the sinking of particles maintain CO₂undersaturation, whereas DIC is obducted back to the surface in the tropical band (75%) and Southern Ocean (25%). At the global scale, these two large counter-balancing fluxes of DIC amount to +275.5 PgC yr^-1 for the supply by obduction and -264.5 PgC yr^-1 for the removal by subduction which is ∼ 3 to 5 times larger than previous estimates. Moreover, we find that subduction of organic carbon (dissolved and particulate) represents ∼ 20% of the total export of organic carbon: at the global scale, we evaluate that of the 11 PgC yr-1 of organic material lost from the surface every year, 2.1 PgC yr^-1 is lost through subduction of organic carbon. Our results emphasize the strong sensitivity of the oceanic carbon cycle to changes in mixed-layer depth, ocean currents, and wind. Key Points Global physical DIC fluxes across the mixed-layer are +275/ -264 PgC/yr DIC physical fluxes are 50-100 times larger than sedimentation and CO2 flux Organic C physical flux is 30% of total organic C export from the surface