Using DIC‐δ13C Pair to Constrain Anthropogenic Carbon Increase in the Southeastern Atlantic Ocean Over the Most Recent Decade (2010–2020).

The southeastern Atlantic Ocean is a crucial yet understudied region for the ocean absorption of anthropogenic carbon (Canth). Data from the A12 (2020) and A13.5 (2010) cruises offer an opportunity to examine changes in dissolved inorganic carbon (DIC), its stable isotope (δ13C), and Canth over the past decade within a limited region (1∼3°E, 32∼42°S). For the decade of 2010–2020, Canth invasion was observed from the sea surface down to 1,200 m based on both DIC and δ13C data. The mean Canth increase rate (1.08 ± 0.26 mol m−2 yr−1) during this period accelerated from 0.87 ± 0.05 mol m−2 yr−1 during the previous period (1983/84–2010). The δ13C‐based Canth increase closely matches the DIC‐based estimation below 500 m but is 26% higher in the upper ocean. This discrepancy is likely due to δ13C's longer air‐sea exchange timescale, seasonal variability in the upper ocean, and the chosen ratio of anthropogenically induced changes in δ13C and DIC. Finally, column inventory changes based on the two methods also exhibit very similar mean Canth uptake rates. The paired DIC concentration and stable isotope dataset may enhance our ability to constrain Canth accumulation and its controlling mechanisms in the ocean. Plain Language Summary: The stable carbon isotope signal (δ13C) of oceanic dissolved inorganic carbon (DIC) is a sensitive tracer for the absorption of anthropogenic carbon from the atmosphere. We collected δ13C data from a limited region in the southeastern Atlantic Ocean and used them to examine the anthropogenic carbon changes over the most recent decade. From 2010 to 2020, anthropogenic carbon invasion can be found from the sea surface to a depth of 1,200 m with an accelerated increase rate compared to the period from 1983/84 to 2010. The δ13C‐based estimation of anthropogenic carbon increase matches closely with the DIC‐based estimation below 500 m but is significantly higher in the upper ocean. This discrepancy likely arises from differences in equilibrium timescales, the ratio of the anthropogenic δ13C/DIC change, and different influences by seasonal variability in the upper ocean. Nonetheless, the entire water column inventory changes based on both two methods show very close mean anthropogenic carbon uptake rates. Key Points: Anthropogenic carbon uptake rate estimated from δ13C matches with that from dissolved inorganic carbon except in the surface in the southeastern Atlantic OceanAnthropogenic carbon changes exhibit significant vertical variations depending on water masses and circulationAnthropogenic carbon increase rate during 2010–2020 has accelerated ∼19% from that during 1983/84–2010 in the study region [ABSTRACT FROM AUTHOR]