Transport of carbonate ions towards the deep North Atlantic Ocean has fallen by more than 40% since the Industrial Revolution, posing a potential threat to cold-water corals, reports a paper published online this week in Nature.
Atmospheric CO2 levels have increased from 280 to 400 parts per million since the Industrial Revolution. About 30% of this anthropogenic CO2 has been captured by the world’s oceans, increasing ocean acidity and decreasing carbonate ion concentrations. Deep cold-water corals, such as Lophelia pertusa, are vulnerable to ocean acidification, which can make it difficult for them to maintain their calcium carbonate skeletons.
Fiz Perez and colleagues show that the present rate of transport of acidified waters to the deep Atlantic could cause the ‘aragonite saturation horizon’ — below which concentrations of the calcium carbonate aragonite are greatly reduced — to shoal, or move to shallower depths, by 1,000 to 1,700 metres in the North Atlantic within three decades. Such shoaling exposes corals to corrosive waters.
The authors find that transport of carbonate ions by a system of currents known as the Atlantic meridional overturning circulation (AMOC) towards the deep ocean has decreased by 44% since preindustrial times. Furthermore, doubling of atmospheric anthropogenic CO2 levels, which could occur within three decades according to a ‘business-as-usual’ climate scenario, could reduce transport of carbonate ions to a quarter of preindustrial levels, which could severely endanger deep cold-water coral habitats. Moreover, the AMOC would export this acidified deep water southwards, spreading corrosive waters to the global deep ocean.
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Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean. Fiz F. Perez, Marcos Fontela, Maribel I. García-Ibáñez, Herlé Mercier, Anton Velo, Pascale Lherminier, Patricia Zunino, Mercedes de la Paz, Fernando Alonso-Pérez, Elisa F. Guallart & Xose A. Padin, Nature, Published online:12 February 2018, doi:10.1038/nature25493.