The Changing Biological Carbon Pump of the South Atlantic Ocean

Global marine anthropogenic CO₂ inventories have traditionally emphasized the North Atlantic's role in the carbon cycle, while Southern hemisphere processes are less understood. The South Subtropical Convergence (SSTC) in the South Atlantic, a juncture of distinct nutrient-rich waters, offers a valuable study area for discerning the potential impacts of climate change on the ocean's biological carbon pump (C_soft). Using discrete observations from GLODAPv2.2022 and BGC-Argo at 40°S in the Atlantic Ocean from 1972 to 2023, an increase in dissolved inorganic carbon (DIC) of +1.44 ± 0.11 μmol kg⁻¹ yr⁻¹ in surface waters was observed. While anthropogenic CO₂ played a role, variations in the contribution of C_soft were observed. Discrepancies emerged in assessing C_soft based on the tracers employed: when using AOU, C_soft(AOU) recorded an increase of +0.20 ± 0.03 μmol kg⁻¹ yr⁻¹, while using nitrate as the reference, C_soft(NO3) displayed an increase of +0.85 ± 0.07 μmol kg⁻¹ yr⁻¹. Key processes such as water mass composition shifts, changes in oxygenation, remineralization in the Southern Ocean, and the challenges they pose in accurately representing the evolving C_soft are discussed. These findings highlight that while global studies primarily attribute DIC increase to anthropogenic CO₂, observations at 40°S reveal an intensified biological carbon pump, showing that regional DIC changes are more complex than previously thought and emphasizing the need for better parameterizations to compute the BCP in the marine carbon budget.