Drivers of the Formation of Mesopelagic Particle Layers and Deep Export Events in the Hypoxic Northern Benguela from Glider, Ship and Model Data

Hypoxic oxygen minimum zones (OMZs) are found below the surface of about 27% of the ocean, but the mechanisms regulating the organic carbon cycle within these waters are far from understood. Here we study the link between sub-monthly oxygen variability and mesopelagic particle layers observed in the hypoxic northern Benguela Upwelling System using a synthesis of ship data, four months of high-resolution glider data and coupled model output. Glider data show that mesopelagic hypoxia, including intermittent hypoxic events, are associated with secondary deep maxima of both large and small particle concentrations, which are found in the depth range of 250-500 m. These deep particle layers also give rise to deep export events driven by large particles that extend from 500 m to below 1000 m depth. Both sediment resuspension from the continental shelf and biological activity within the OMZ can drive the formation of these deep layers. Model data indicates that lateral currents can transport sediment from the shelf to the glider location, a process that can supply suspended particles. Ship data shows diurnal vertical migration (DVM) to the OMZ by nekton and high zooplankton biomass during day and night within the hypoxic zone. Both can contribute to the formation of these mesopelagic particle layers and to the deep export events by supplying and transforming organic carbon at depth. Our results provide indications on how to improve model representations of mesopelagic carbon fluxes and highlight the need to better understand the role of hypoxic OMZs in the global carbon cycle.