The carbonate equilibrium depth (the region where high pressure and low temperature create conditions acidic enough to dissolve shells and skeletons) could comprise half of the world’s oceans by the end of the century. In the deepest parts of the ocean, below 4,000 meters (13,100 feet), the combination of high pressure and low temperature creates conditions that dissolve calcium carbonate, the material marine animals use to make their shells.

This zone is known as the carbonate stabilization depth and is expanding. This contradicts the much-discussed acidification of surface ocean waters due to the ocean absorbing carbon dioxide from the burning of fossil fuels.

But these two aspects are interrelated: Due to the increasing concentration of carbon dioxide in the ocean, its pH decreases (becomes more acidic) and the deep-sea area where calcium carbonate dissolves, starting from the seabed, increases.

The transition zone where calcium carbonate becomes increasingly chemically unstable and begins to dissolve is called a lysocline. Because the ocean floor is relatively flat, lysocline rise of even a few meters can quickly lead to large, undersaturated (acidic) areas.

Ours research showed that this region has risen by about 100 meters since pre-industrial times and is likely to rise several hundred meters more this century. Millions of square kilometers of ocean floor are potentially undergoing rapid transition, causing calcareous sediment to become chemically unstable and dissolve.

Expansion of borders

The upper limit of the lysocline transition zone is known as the calcite saturation depth, above which the seafloor sediments are rich in calcium carbonate and the ocean water is supersaturated with it. The calcite equilibration depth is the lower limit below which the seafloor sediments contain little or no carbonate minerals.

The area below the calcite stabilization depth varies greatly across different sectors of the oceans. It currently covers about 41% of the world’s oceans. Since the Industrial Revolution, this area has risen throughout the ocean, from almost no rise in the western Indian Ocean to over 980 ft (300 m) in the northwestern Atlantic. If the calcite stabilization depth were to increase by another 980 ft, the seafloor area below it would increase by 10%, covering 51% of the world’s oceans.

Great places to live

For the first time, a recent study has shown that the depth of calcite compensation is a biological boundary with distinct habitats above and below it. In the northeastern Pacific, the most common seafloor organisms above the calcite compensation depth are soft corals, brittle stars, mussels, limpets, chitons, and bryozoans, all of which have calcified shells or skeletons.

But below the calcite compensation depth, sea anemones, sea cucumbers, and octopuses are more common. This unsaturated (more acidic) habitat currently limits life in 54.4 million square miles (141 million square kilometers) of ocean and could increase by another 13.5 million square miles (35 million square kilometers) if the depth of calcite compensation were increased by 980 feet.

In addition to increasing depth of calcite compensation, some parts of the ocean at lower latitudes are losing species because the water is getting too warm, and oxygen levels decreaseAlso due to climate change. Thus the most habitable area for marine life is reduced from below (increasing the calcite compensation depth) and from above (warming).

Island countries suffered the most

Extraordinary economic zones Some countries will suffer more than others. In general, oceanic and island countries will suffer greater losses, while countries with larger continental shelves will suffer proportionately less.

Bermuda’s EEZ is expected to be most affected by the increase in calcite offset depth to 980 feet above current levels, with 68% of the country’s seabed falling below the lysocline. In comparison, only 6% of the U.S. GEO and 0.39% of the Russian GEO are expected to be affected.

From a global perspective, it’s astonishing that 41% of the deep sea is actually acidic, that it will be half that by the end of the century, and the first study showing the impact of marine life was published last year.