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

[摘要] During four austral summers (December to January) from 2006 to 2010, weinvestigated the surface-water carbonate system and its controls in thewestern Antarctic Ocean. Measurements of total alkalinity (A_T),pH and total inorganic carbon (C_T) were investigated incombination with high-frequency measurements on sea-surface temperature(SST), salinity and Chl a. In all parameters we found large interannualvariability due to differences in sea-ice concentration, physical processesand primary production. The main result from our observations suggests thatprimary production was the major control on the calcium carbonate saturationstate (Ω) in austral summer for all years. This was mainly reflectedin the covariance of pH and Chl a. In the sea-ice-covered parts of thestudy area, pH and Ω were generally low, coinciding with low Chl aconcentrations. The lowest pH in situ and lowest aragonite saturation(Ω_Ar ~ 1.0) were observed in December 2007 in thecoastal Amundsen and Ross seas near marine outflowing glaciers. These lowΩand high pH values were likely influenced by freshwater dilution.Comparing 2007 and 2010, the largest Ω_Ar difference wasfound in the eastern Ross Sea, where Ω_Ar was about 1.2 unitslower in 2007 than in 2010. This was mainly explained by differences in Chla (i.e primary production). In 2010 the surface water along the Ross Seashelf was the warmest and most saline, indicating upwelling of nutrient andCO₂-rich sub-surface water, likely promoting primary production leadingto high Ω and pH. Results from multivariate analysis agree with ourobservations showing that changes in Chl a had the largest influence on theΩ_Ar variability. The future changes of Ω_Arwere estimated using reported rates of the oceanic uptake of anthropogenicCO₂, combined with our data on total alkalinity, SST and salinity(summer situation). Our study suggests that the Amundsen Sea will becomeundersaturated with regard to aragonite about 40 yr sooner than predicted bymodels.

[学科分类] 地球化学与岩石