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Two Lab-on-Chip sensors, one measuring nitrate + nitrite (here after nitrate) and one measuring silicic acid (here after silicate), were deployed on the Ocean Observing Initiative (OOI) Southern Ocean Array surface mooring at a depth of approximately 12m on the near surface instrument frame in the southeast Pacific Ocean (-54 N, -89W). The nitrate sensor operated as expected for the full deployment period (6/12/2018 to 19/1/2020), collecting daily measurements. The silicate sensor operated as expected for almost ten months (until 1/10/2019), collecting up to four measurements per day. The OOI surface mooring was deployed in December 2018 on research cruise DY096 and recovered in January 2020 on research cruise DY112. The sensors and associated research cruises (DY096 and DY112) were supported by the Natural Environment Research Council (NERC) RoSES Carbon Uptake and Seasonal Trends in Antarctic Remineralisation Depth (CUSTARD) project. This material is based upon work supported by the Ocean Observatories Initiative, which is a major facility fully funded by the National Science Foundation (NSF). 

Abstract The ocean contributes to regulating atmospheric CO₂levels via the biological carbon pump (BCP). One critical aspect of the BCP is the depth at which sinking particulate organic carbon (POC) remineralizes in the mesopelagic zone (200–1,000 m). In the Southern Ocean, the circulation is such that the products generated from POC remineralization may have drastically different fates depending on (a) the latitude at which sinking particulate material is produced and (b) the depth at which its remineralization occurs. Here, we assess latitudinal and depth variations of POC export marine aggregate abundance and composition in the Southeast Pacific sector of the Southern Ocean. We show changes in flux attenuation depth horizons in the upper mesopelagic in the subantarctic zone. These correspond to rapid particle accumulation below the depth of the euphotic zone followed by abrupt export. We believe that such rapid changes may be linked to diatom life cycles, including resting cell and spore formation and resulting changes in particle sinking velocities rather than attenuation due to heterotrophic degradation or solubilization in the upper mesopelagic zone. We further discuss the occurrence of such features in the Southern Ocean and at the global scale. Our results highlight the importance of alternative flux attenuation processes, such as sudden changes in particles sinking velocities, in explaining variability in organic carbon sequestration by the ocean's BCP.