Contrasting drivers and trends of ocean acidification in the subarctic Atlantic

The processes of warming, anthropogenic CO₂ (C_anth) accumulation, decreasing pH_T (increasing [H⁺]_T; concentration in total scale) and calcium carbonate saturation in the subarctic zone of the North Atlantic are unequivocal in the time-series measurements of the Iceland (IS-TS, 1985-2003) and Irminger Sea (IRM-TS, 1983-2013) stations. Both stations show high rates of C_anth accumulation with different rates of warming, salinification and stratification linked to regional circulation and dynamics. At the IS-TS, advected and stratified waters of Arctic origin drive a strong increase in [H⁺]_T, in the surface layer, which is nearly halved in the deep layer (44.7 ± 3.6 and 25.5 ± 1.0 pmol kg^-1 yr^-1, respectively). In contrast, the weak stratification at the IRM-TS allows warming, salinification and C_anth uptake to reach the deep layer. The acidification trends are even stronger in the deep layer than in the surface layer (44.2 ± 1.0 pmol kg^-1 yr^-1 and 32.6 ± 3.4 pmol kg^-1 yr^-1 of [H⁺]_T, respectively). The driver analysis detects that warming contributes up to 50% to the increase in [H⁺]_T at the IRM-TS but has a small positive effect on calcium carbonate saturation. The C_anth increase is the main driver of the observed acidification, but it is partially dampened by the northward advection of water with a relatively low natural CO₂ content.