How do Biogeochemical Tracers Leave the Surface Mixed Layer? : Anthropogenic Carbon Subduction and Ocean De-oxygenation

Physical processes by which dissolved biogeochemical tracers such as carbon, nutrients or oxygen cross the base of the mixed layer are still poorly understood. Here, we use an ocean biogeochemical model to quantify these terms, and hence separate the respective role of advective, diffusive, and entrainment / detrainment processes in the different regions of the world ocean. We apply this framework to (1) the natural carbon cycle and gross fluxes of dissolved carbon across the mixed layer base, to (2) the pathways of anthropogenic carbon subduction over the last decades, and to (3) ocean de-oxygenation in the coming decades. We find that physical fluxes of dissolved inorganic carbon (DIC) across the mixed layer base are two orders of magnitude larger than any other carbon fluxes and consist into two large counter-balancing fluxes of 275.5 PgC/y for obduction (back into the mixed layer) and -264.5 PgC/y for subduction (leaving the mixed layer). We also find that, whereas advection drives the spatial patterns of subduction / obduction of both natural and anthropogenic carbon, vertical diffusion is a primary mechanism of anthropogenic carbon subduction (contributing 65% of total subduction), despite very low local fluxes. Finally, using simulations with an Earth System Model over the 21st century, we show that, in addition to a solubility-driven component, the decrease of dissolved oxygen in the subsurface waters of the world ocean is explained by the combination of several subduction mechanisms.