Multidecadal change in natural carbon dynamics at the interface between Atlantic and Southern Ocean

Although global marine anthropogenic CO₂ inventories have typically highlighted the North Atlantic as being the main feature of interest, southern hemisphere processes also play a key role in the changing marine carbon cycle. The South Subtropical Convergence (SSTC) in the South Atlantic, where low-macronutrient subtropical gyre waters intersect high-macronutrient Antarctic Circumpolar Current waters, is a key location for studying key processes such as the effects of climate change on the biological carbon pump. Here, we present a time series consisting of marine carbonate chemistry measurements from recent expeditions and global marine observations (GLODAPv2.2022 and BGC-ARGO) at 40°S in the Atlantic Ocean. We calculate the rates of change in dissolved inorganic carbon (DIC) and other related variables and use these to disentangle the apparent drivers of DIC change both natural (carbonate pump, C_carb and soft tissue pump, C_soft) and anthropogenic (C_anth). An increase in DIC is observed throughout the water column. The relative contribution of the C_soft and C_anth components to the DIC change varies, however, significantly depending on whether we use nutrients or dissolved oxygen in the analysis. We discuss the causes of this discrepancy, using a water mass analysis to investigate the effect of water mass composition shifts in formation regions, and exploring the impact of reduced oxygenation and increased remineralization in the Southern Ocean, aiming to understand which tracer more accurately represents the changing carbon pump.