The carbonate compensation depth (CCD) is an oceanic zone where high pressure and low temperatures make conditions so acidic that they dissolve shells and skeletons. By the end of the century, this zone could encompass half of the global ocean. At depths below 13,100 feet (4,000 meters), the combination of high pressure and low temperature causes calcium carbonate, the material that marine animals use to build their shells, to dissolve.
This expanding zone is known as the carbonate compensation depth.
This phenomenon differs from the more widely discussed surface ocean acidification, which occurs due to the ocean absorbing carbon dioxide from fossil fuel emissions. However, the two processes are interconnected: rising carbon dioxide levels are decreasing the ocean’s pH (making it more acidic), leading to an expansion of the deep-sea area where calcium carbonate dissolves, moving upwards from the seafloor.
The transition zone where calcium carbonate becomes increasingly unstable and starts to dissolve is called the lysocline. Since the ocean seabed is relatively flat, even a small rise in the lysocline can result in the rapid expansion of large under-saturated (acidic) areas.
Our research indicates that this zone has already risen by nearly 100 meters since pre-industrial times and is expected to rise by several hundred meters more this century.
As a result, millions of square kilometers of ocean floor could undergo a rapid transition, causing calcareous sediments to become chemically unstable and dissolve.
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