Long-term alkalinity trends in the Baltic Sea and their implications for CO2-induced acidification

Anthropogenic CO2 emissions currently decrease open ocean pH, but on multi-millennial time scales intensified continental weathering is expected to contribute to increasing oceanic alkalinity (A(T)) and thus mitigate the acidification signal. The Baltic Sea is an ideal study site for such A(T) dynamics, due to its direct link to terrestrial processes, short water residence time and long history of A(T) measurements dating back to the early 20(th) century. We compiled an extensive A(T) data set that revealed the highest data quality and coverage for the past two decades. Within that period, surface water A(T) levels increased throughout the Baltic Sea. The rates of change were highest in the low-saline, northern areas and decreased gradually toward constant levels in the North Sea. The A(T) increase observed in the Central Baltic Sea (+3.4 mu mol kg(-1) yr(-1)) and the Gulf of Bothnia (+7 mu mol kg(-1) yr(-1)) has compensated CO2-induced acidification by almost 50% and 100%, respectively. Further, the A(T) trends enhanced the CO2 storage capacity and stabilized the CaCO3 saturation state of the Baltic Sea over the past two decades. We discuss the attribution of the A(T) trends to potential changes in precipitation patterns, continental weathering driven by acidic rain and increasing atmospheric CO2, agricultural liming and internal A(T) sources.