<p>The uptake of carbon dioxide (CO<sub>2</sub>) from the atmosphere is changing the ocean&#8217;s chemical state. Such changes, commonly known as ocean acidification, include reduction in pH and the carbonate ion concentration ([CO<sub>3</sub><sup>2-</sup>]), which in turn lowers oceanic saturation states (&#937;) for calcium carbonate (CaCO<sub>3</sub>) minerals. The &#937; values for aragonite (&#8486;<sub>aragonite</sub>; one of the main CaCO<sub>3</sub> minerals formed by marine calcifying organisms) influence the calcification rate and geographic distribution of cold-water corals (CWCs), important for biodiversity. In this work we use high-quality data of inorganic carbon measurements, collected on thirteen cruises along the same track during 1991&#8211;2018, to determine the long-term trends in &#8486;<sub>aragonite</sub> in the Irminger and Iceland Basins of the North Atlantic Ocean, providing the first trends of &#8486;<sub>aragonite</sub> in the deep waters of these basins. The entire water column of both basins showed significant negative &#937;<sub>aragonite</sub> trends between -0.0015 &#177; 0.0002 and -0.0061 &#177; 0.0016 per year. The decrease in &#937;<sub>aragonite</sub> in the intermediate waters, where nearly half of the CWC reefs of the study region are located, caused the &#937;<sub>aragonite</sub> isolines to migrate upwards rapidly at a rate between 6 and 34 m per year. The main driver of the observed decline in &#937;<sub>aragonite</sub> in the Irminger and Iceland Basins was the increase in anthropogenic CO<sub>2</sub>. But this was partially offset by increases in salinity (in Subpolar Mode Water), enhanced ventilation (in upper Labrador Sea Water) and increases in alkalinity (in classical Labrador Sea Water, cLSW; and overflow waters). We also found that water mass aging reinforced the &#937;<sub>aragonite</sub> decrease in cLSW. Based on the observed &#937;<sub>aragonite</sub> trends, we project that the entire water column of the Irminger and Iceland Basins will likely be undersaturated for aragonite when in equilibrium with an atmospheric mole fraction of CO<sub>2</sub> (xCO<sub>2</sub>) of ~860 ppmv, corresponding to climate model projections for the end of the century based on the highest CO<sub>2</sub> emission scenarios. However, intermediate waters will likely be aragonite undersaturated when in equilibrium with an atmospheric xCO<sub>2</sub> of ~600 ppmv, an xCO<sub>2</sub> level slightly above that corresponding to 2 &#186;C warming, thus exposing CWCs inhabiting the intermediate waters to undersaturation for aragonite.</p>
Cold-water corals in the Subpolar North Atlantic Ocean exposed to aragonite undersaturation if Paris 2 ºC is not met