Abstract
The net uptake of carbon dioxide (CO 2 ) from the atmosphere is changing the ocean's chemical state. Such changes, commonly known as ocean acidification, include a reduction in pH and the carbonate ion concentration ([CO 3 2− ]), which in turn lowers oceanic saturation states (Ω) for calcium carbonate (CaCO 3 ) minerals. The Ω values for aragonite (Ω aragonite ; one of the main CaCO 3 minerals formed by marine calcifying organisms) influence the calcification rate and geographic distribution of cold-water corals (CWCs), important for biodiversity. Here, high-quality measurements, collected on thirteen cruises along the same track during 1991–2018, are used to determine the long-term changes in Ω aragonite in the Irminger and Iceland Basins of the North Atlantic Ocean, providing the first trends of Ω aragonite in the deep waters of these basins. The entire water column of both basins showed significant negative Ω aragonite trends between −0.0014 ± 0.0002 and −0.0052 ± 0.0007 per year. The decrease in Ω aragonite in the intermediate waters, where nearly half of the CWC reefs of the study region are located, caused the Ω aragonite isolines to rapidly migrate upwards at a rate between 6 and 34 m per year. The main driver of the decline in Ω aragonite in the Irminger and Iceland Basins was the increase in anthropogenic CO 2 . 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 Ω aragonite decrease in cLSW. Based on these Ω aragonite trends over the last three decades, 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 2 (xCO 2 ) of ~880 ppmv, corresponding to climate model projections for the end of the century based on the highest CO 2 emission scenarios. However, intermediate waters will likely be aragonite undersaturated when in equilibrium with an atmospheric xCO 2 exceeding ~630 ppmv, an xCO 2 level slightly above that corresponding to 2 °C global warming, thus exposing CWCs inhabiting the intermediate waters to undersaturation for aragonite. • Aragonite saturation state significantly decreased for 1991–2018. • Anthropogenic CO 2 uptake is the main driver for the observed decrease. • At cold-water corals (CWC) depths, the aragonite isolines shoaled at 6–34 m yr −1 . • Projection of future change in aragonite saturation state based on observed change. • CWC reefs exposed to undersaturation at atmospheric CO 2 levels causing 2 °C warming.
Highlights
The global ocean has absorbed ~30% of the anthropogenic CO2 (i.e., Cant; CO2 from human activities) released into the atmosphere since the industrial revolution (Friedlingstein et al, 2019; Gruber et al, 2019) and will likely sequester ~85% of Cant emissions on the time scales of several thousands of years (Archer et al, 2009)
Intermediate waters will likely be aragonite undersaturated when in equilibrium with an atmospheric xCO2 exceeding ~630 ppmv, an xCO2 level slightly above that corresponding to 2 ◦C global warming, exposing cold-water corals (CWCs) inhabiting the intermediate waters to undersaturation for aragonite
The gradual reduction over time of Ωaragonite in the Subpolar North Atlantic Ocean was assessed from observations in the Irminger and Iceland Basins spanning the last three decades (1991–2018)
Summary
The global ocean has absorbed ~30% of the anthropogenic CO2 (i.e., Cant; CO2 from human activities) released into the atmosphere since the industrial revolution (Friedlingstein et al, 2019; Gruber et al, 2019) and will likely sequester ~85% of Cant emissions on the time scales of several thousands of years (Archer et al, 2009). By the end of the century, further uptake of Cant is projected to shoal the North Atlantic Ocean ASH depth by more than 2000 m under the IS92a ‘business-asusual’ scenario (Orr et al, 2005) and reduce the pH by more than 0.2 units over 21% of the seafloor area below 500 m depth in the North Atlantic sector based on RCP8.5 (Gehlen et al, 2014) Such shoaling of the ASH and pH reduction will alter the chemical environment of in termediate and deep waters and expose the majority of the CWC reefs to undersaturated (potentially corrosive) waters for aragonite. We use the observed Ωaragonite trends to estimate the expected changes in Ωaragonite for future increases in atmospheric CO2 Those projections are used to infer when deep CWC communities inhabiting the Subpolar North Atlantic Ocean would be exposed to waters under saturated for aragonite
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