Ocean Acidification and Long‐Term Changes in the Carbonate System Properties of the South Atlantic Ocean

Abstract

The wind‐driven part of the South Atlantic Ocean is primarily ventilated through central and intermediate water formation. Through the water mass formation processes, anthropogenic carbon (Cant) is introduced into the ocean's interior which in turn makes the South Atlantic region vulnerable to ocean acidification. Cant and the accompanying acidification effects have been estimated for individual sections in the region since the 1980s but a comprehensive synthesis for the entire basin is still lacking. Here, we quantified the Cant accumulation rates and examined the changes in the carbonate system properties for the South Atlantic using a modified extended multiple linear regression method applied to five hydrographic sections and data from the GLODAPv2.2021 product. From 1989 to 2019, a mean Cant column inventory change of 0.94 ± 0.39 mol C m−2 yr−1 was found. Cant accumulation rates of 0.89 ± 0.33 μmol kg−1 yr−1 and 0.30 ± 0.29 μmol kg−1 yr−1 were observed in central and intermediate waters, accompanied by acidification rates of −0.0020 ± 0.0007 pH units yr−1 and −0.0009 ± 0.0009 pH units yr−1, respectively. Furthermore, increased remineralization was observed in intermediate waters, amplifying the acidification of this water mass, especially at the African coast along 25°S. This increase in remineralization is likely related to circulation changes and increased biological activity nearshore. Assuming no changes in the observed trends, South Atlantic intermediate waters will become unsaturated with respect to aragonite in ∼30 years, while the central water of the eastern margins will become unsaturated in ∼10 years.

The Changing Biological Carbon Pump of the South Atlantic Ocean

“…This is especially true along the AAIW from 50°S to 30°S where increased AOU was observed at a mean rate of AOU change of 0.23 ± 0.68 μmol kg −1 yr −1 south of 30°S which concurs with our results (Fig. 3; Piñango et al, 2022). This is consistent with the deoxygenation trend reported by Santos et al (2016) for the AAIW in the South Atlantic subtropical gyre from 1960 to 2015, which was −0.18 ± 0.04 μmol kg−1 yr−1.…”

“…3h; Fine et al, 2017;Tanhua et al, 2017;Waugh et al, 2013. This suggests that the apparent increase in remineralization within the BCP as inferred from DIC, AOU, and NO 3 measurements, is primarily due to the enhanced advection of older deep waters to the surface. Piñango et al (2022) also reported increased organic matter remineralization in the AAIW likely indicating deoxygenation. This is due to an enhanced flux of organic matter, which increases microbial oxygen consumption, and enhanced ventilation.…”

“…Only data with a quality control deemed "Good" (Flag = 2) were used. This analysis considered the full depth of the water column, with an emphasis on the central and intermediate waters during the discussion as most change has been observed in these waters (Piñango et al, 2022).…”

Advancing Ocean Sustainability for Climate-Resilient Change

“…Addressing Ocean Decade Challenge 7 (expand the global ocean observing system) and Challenge 8 (create a digital representation of the ocean), GO-SHIP contributes to global monitoring, mapping, and identification of observational gaps The recent re-occupation of the A13.5 transect (Ghana to Bouvet Island, February-March 2024), contributed to a universally coordinated network of continuous hydrographic sections, thereby enhancing the broader global ocean/climate observing system. The western margin and southern limit of the South Atlantic Ocean have been significantly impacted by anthropogenic carbon (C ant ) uptake, as shown by a C ant column inventory change in the region that was higher than the global mean storage rate [41]. Predictions have warned that aragonite unsaturation will occur in the next two decades along the eastern margin if anthropogenic changes maintain the present trend and have recommended more extensive monitoring in the eastern margin of the South Atlantic to identify the extent to which ecosystems health, marine life, and societies will be affected in the future.…”