Observations of Acidification in the Weddell Sea on a Decadal Time Scale

The amount of anthropogenic CO2 (Cant) that entered the Weddell Sea between 1992 and 2008 is assessed using the extended multiple linear regression method (eMLR). Two multiple linear regressions for total dissolved inorganic carbon (CT) were conducted independently for two data sets from 1992 and 2008. Subtracting these two relations leads to an estimate of anthropogenic carbon (ΔCT) accumulated in the considered time span assuming that the underlying natural correlations between the input parameters and CT do not change with time. In the Warm Deep Water (WDW) and in the Weddell Sea Bottom Water (WSBW) ΔCT values are insignificant, whereas values as high as 8~μmol kg-1 are observed at the shelf. ΔCT concentrations in the surface layer vary with latitude between 2 and 11~μmol kg-1. Weak intrusion of anthropogenic CO2 into Weddell Sea Deep Water (WSDW) was demonstrated, ΔCT yields 1.5 - 2~μmol kg-1. That more Cant is found in the WSDW than in the WSBW is surprising, but can be explained by the more intense ventilation of the WSDW from east of the Weddell Gyre. The invasion of Cant provokes a shift in the equilibria of the carbonate system, resulting in acidification and reduction of CO32-. The mean decrease of pH in the upper 200~m layer is 0.016. Further effects are decrease of the calcite and aragonite saturation states. Our results indicate a slower decrease of aragonite of surface waters in the Weddell Sea than recent model-based estimates.