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

Abstract

The southeastern Atlantic Ocean is a crucial yet understudied region for the ocean absorption of anthropogenic carbon (C_anth). 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 C_anth over the past decade within a limited region (1∼3°E, 32∼42°S). For the decade of 2010–2020, C_anth invasion was observed from the sea surface down to 1,200 m based on both DIC and δ¹³C data. The mean C_anth 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 C_anth 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 C_anth uptake rates. The paired DIC concentration and stable isotope dataset may enhance our ability to constrain C_anth accumulation and its controlling mechanisms in the ocean.