Abstract
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 (Csoft). Using discrete observations from GLODAPv2.2022 and
BGC-Argo at 40°S in the Atlantic Ocean, 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 Csoft were observed. Discrepancies emerged in assessing
Csoft based on the tracers employed: when using AOU, Csoft(AOU) recorded
an increase of +0.20 ± 0.03 μmol kg‑1 yr-1, whereas, using nitrate as
the reference, Csoft(NO3) displayed an increase of +0.85 ± 0.07 μmol
kg-1 yr-1. Nonetheless, our observations at 40°S indicate a significant
intensification of Csoft, which, scaled to the entire ocean, represents
an additional 23% to 35% of organic carbon degradation within the
water column. 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 Csoft 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 challenging the dominance
of anthropogenic sources in global DIC change.