Acidification of the Mediterranean Sea from anthropogenic carbon penetration

Abstract This study presents an estimation of the anthropogenic CO 2 (C ANT) concentrations and acidification (ΔpH=pH 2013 –pH pre-industrial ) in the Mediterranean Sea, based upon hydrographic and carbonate chemistry data collected during the May 2013 MedSeA cruise. The concentrations of C ANTwere calculated using the composite tracer TrOCA. The C ANTdistribution shows that the most invaded waters (>60 µmol kg −1 ) are those of the intermediate and deep layers in the Alboran, Liguro- and Algero-Provencal Sub-basins in the Western basin, and in the Adriatic Sub-basin in the Eastern basin. Whereas the areas containing the lowest C ANTconcentrations are the deep layers of the Eastern basin, especially those of the IonianSub-basin, and those of the northern Tyrrhenian Sub-basin in the Western basin. The acidification level in the Mediterranean Seareflects the excessive increase of atmospheric CO 2 and therefore the invasion of the sea by C ANT. This acidification varies between −0.055 and −0.156 pH unit and it indicates that all Mediterranean Seawaters are already acidified, especially those of the Western basin where ΔpH is rarely less than −0.1 pH unit. Both C ANTconcentrations and acidification levels are closely linked to the presence and history of the different water masses in the intermediate and deep layers of the Mediterranean basins. Despite the high acidification levels, both Mediterranean basinsare still highly supersaturated in calcium carbonate minerals.