The ocean’s deepest recesses, beyond 13,100 feet (4,000 meters), are characterized by high pressure and low temperatures, conditions that dissolve calcium carbonate – the essential material used by marine organisms to build shells and skeletons. This region, known as the carbonate compensation depth (CCD), is expanding. This phenomenon, while often overshadowed by the widely discussed acidification of surface waters, is intricately linked to it. As the ocean absorbs increasing amounts of carbon dioxide from fossil fuel combustion, its pH drops (becoming more acidic), leading to the upward expansion of the CCD.
The transition zone where calcium carbonate becomes unstable and begins to dissolve is called the lysocline. Because the ocean floor is relatively flat, even a minor rise in the lysocline can rapidly create large acidic areas. Research indicates that this zone has already risen by nearly 100 meters since pre-industrial times and is projected to rise by several hundred meters this century. This will result in millions of square kilometers of ocean floor transitioning into a chemically unstable environment where calcareous sediment will dissolve.
The upper limit of the lysocline transition zone is the calcite saturation depth, where seabed sediments are rich in calcium carbonate and the ocean water is supersaturated with it. The CCD marks its lower limit, below which seabed sediments contain little or no carbonate minerals.
Since the Industrial Revolution, the CCD has risen in all parts of the ocean, ranging from a negligible increase in the western Indian Ocean to over 980 feet (300 meters) in the northwest Atlantic. A further rise of 980 feet could increase the area of seafloor below the CCD by 10%, encompassing 51% of the global ocean.
A recent study has revealed that the CCD acts as a biological boundary, with distinct habitats above and below it. In the northeast Pacific, above the CCD, soft corals, brittle stars, mussels, sea snails, chitons, and bryozoans dominate – all species with calcified shells or skeletons. Below the CCD, sea anemones, sea cucumbers, and octopuses are more prevalent. This acidic habitat already encompasses 54.4 million square miles (141 million square kilometers) of the ocean, and could expand by another 13.5 million square miles (35 million sq/km) if the CCD rises by 980 feet.
Beyond the expansion of the CCD, low-latitude ocean regions are experiencing species loss due to rising temperatures and declining oxygen levels, both consequences of climate change. This double whammy, from the bottom up (rising CCD) and the top down (warming), is shrinking the habitable space for marine species.
The impact of the rising CCD is not uniform. Island nations and countries with large oceanic territories are disproportionately affected, while those with vast continental shelves experience less impact. For example, Bermuda’s exclusive economic zone (EEZ) is predicted to be most affected by a 980-foot rise in the CCD, with 68% of its seabed becoming submerged below the lysocline. In contrast, only 6% of the US EEZ and 0.39% of the Russian EEZ are expected to be impacted.
The fact that 41% of the deep sea is already effectively acidic, with the potential for half to be affected by the end of the century, underscores the gravity of this issue. The recent study demonstrating the effects of the rising CCD on marine life is a stark reminder of the far-reaching consequences of ocean acidification. This silent threat to the ocean’s depths requires immediate and concerted global action to mitigate its impact and ensure the health and sustainability of marine ecosystems.
