Southern Ocean Carbon Export Revealed by Backscatter and Oxygen Measurements From BGC‐Argo Floats.

The Southern Ocean (south of 30°S) contributes significantly to global ocean carbon uptake through the solubility, physical and biological pumps. Many studies have estimated carbon export to the deep ocean, but very few have attempted a basin‐scale perspective, or accounted for the sea‐ice zone (SIZ). In this study, we use an extensive array of BGC‐Argo floats to improve previous estimates of carbon export across basins and frontal zones, specifically including the SIZ. Using a new method involving changes in particulate organic carbon and dissolved oxygen along the mesopelagic layer, we find that the total Southern Ocean carbon export from 2014 to 2022 is 2.69 ± 1.23 PgC y−1. The polar Antarctic zone contributes the most (41%) with 1.09 ± 0.46 PgC y−1. Conversely, the SIZ contributes the least (8%) with 0.21 ± 0.09 PgC y−1 and displays a strong shallow respiration in the upper 200 m. However, the SIZ contribution can increase up to 14% depending on the depth range investigated. We also consider vertical turbulent fluxes, which can be neglected at depth but are important near the surface. Our work provides a complementary approach to previous studies and is relevant for work that focuses on evaluating the biogeochemical impacts of changes in Antarctic sea‐ice extent. Refining estimates of carbon export and understanding its drivers ultimately impacts our comprehension of climate variability at the global ocean scale. Plain Language Summary: Phytoplankton take up atmospheric carbon dioxide through photosynthesis and help sequester carbon to the deep ocean. Using robotic floats equipped with biogeochemical and physical sensors, we developed a new method to quantify the carbon export from the surface to the deep ocean across the Southern Ocean. We highlight that the Southern Ocean exports 2.69 billion tonnes of carbon per year, which is about a quarter of the mean global ocean carbon export. The sea‐ice zone, an area mostly overlooked due to a historical lack of observations, is responsible for 8% of the Southern Ocean carbon export, but this estimate is very sensitive to the depth range considered. Our results suggest that the Southern Ocean carbon export is spatially highly variable and seasonally sea‐ice covered regions could play a significant role in the global carbon cycle. Key Points: We investigate carbon export in the Southern Ocean using a new method based on sinking particulate organic carbon and oxygen drawdownWe estimate a total basin‐integrated carbon export of 2.69 ± 1.23 PgC y−1, where the polar Antarctic zone (PAZ) is the largest contributor (41%)The sea‐ice zone (SIZ) contributes 8% to Southern Ocean carbon export, and most of the carbon produced in this zone is respired in the upper 200 m [ABSTRACT FROM AUTHOR]