Ocean acidification state in western Antarctic surface waters: drivers and interannual variability

Each December during four years from 2006 to 2010, the surface water carbonate system was measured and investigated in the Amundsen Sea and Ross Sea, western Antarctica as part of the Oden Southern Ocean expeditions (OSO). The I/B Oden started in Punta Arenas in Chile and sailed southwest, passing through different regimes such as, the marginal/seasonal ice zone, fronts, coastal shelves, and polynyas. Discrete surface water was sampled underway for analysis of total alkalinity (AT), total dissolved inorganic carbon (CT) and pH. Two of these parameters were used together with sea-surface temperature (SST), and salinity to obtain a full description of the surface water carbonate system, including pH in situ and calcium carbonate saturation state of aragonite (ΩAr) and calcite (ΩCa). Multivariate analysis was used to investigate interannual variability and the major controls (sea-ice concentration, SST, salinity and chlorophyll a) on the variability in the carbonate system and Ω. This analysis showed that SST and chlorophyll a were the major drivers of the Ω variability in both the Amundsen and Ross seas. In 2007, the sea-ice edge was located further south and the area of the open polynya was relatively small compared to 2010. We found the lowest pH in situ (7.932) and Ω = 1 values in the sea-ice zone and in the coastal Amundsen Sea, nearby marine out flowing glaciers. In 2010, the sea-ice coverage was the largest and the areas of the open polynyas were the largest for the whole period. This year we found the lowest salinity and AT, coinciding with highest chl a. This implies that the highest ΩAr in 2010 was likely an effect of biological CO2 drawdown, which out-competed the dilution of carbonate ion concentration due to large melt water volumes. We predict and discuss future Ω values, using our data and reported rates of oceanic uptake of anthropogenic CO2, suggesting that the Amundsen Sea will become undersaturated with regard to aragonite about 20 yr sooner than predicted by models.

Mattsdotter Björk M., Fransson A. & Chierici M., 2013. Ocean acidification state in western Antarctic surface waters: drivers and interannual variability. Biogeosciences Discussions 10: 7879-7916. Article.

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