Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean

Abstract. Observations along the southwestern Atlantic WOCE A17 line made during the Dutch GEOTRACES-NL programme (2010–2011) were compared with historical data from 1994 to quantify the changes in the anthropogenic component of the total pool of dissolved inorganic carbon (ΔC_ant). Application of the extended multi-linear regression (eMLR) method shows that the ΔC_ant from 1994 to 2011 has largely remained confined to the upper 1000 dbar. The greatest changes occur in the upper 200 dbar in the Subantarctic Zone (SAZ), where a maximum increase of 37 μmol kg⁻¹ is found. South Atlantic Central Water (SACW) experienced the highest rate of increase in C_ant, at 0.99 ± 0.14 μmol kg⁻¹ yr⁻¹, resulting in a maximum rate of decrease in pH of 0.0016 yr⁻¹. The highest rates of acidification relative to ΔC_ant, however, were found in Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The low buffering capacity of SAMW and AAIW combined with their relatively high rates of C_ant, increase of 0.53 ± 0.11 and 0.36 ± 0.06 μmol kg⁻¹ yr⁻¹, respectively, has lead to rapid acidification in the SAZ, and will continue to do so whilst simultaneously reducing the chemical buffering capacity of this significant CO₂ sink.