Articles | Volume 12, issue 5
Research article
05 Mar 2015
Research article |  | 05 Mar 2015

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

L. A. Salt, S. M. A. C. van Heuven, M. E. Claus, E. M. Jones, and H. J. W. de Baar

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 (ΔCant). Application of the extended multi-linear regression (eMLR) method shows that the ΔCant 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 Cant, 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 ΔCant, 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 Cant, 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.

Short summary
The increase in anthropogenic atmospheric carbon dioxide is mitigated by uptake by the world ocean, which alters the pH of the water. In the South Atlantic we find the highest rates of acidification relative to increase in anthropogenic carbon (Cant) found in Subantarctic Mode Water and Antarctic Intermediate Water. The moderate rates of increase in Cant combined with low buffering capacities, due to low salinity and alkalinity values, have caused rapid acidification in the Subantarctic Zone.
Final-revised paper