Acidification will put 10% of seabed off-limits for creatures with shells

Deep sea coral (NOAA file image). The zone where carbonate-containing life forms like these dissolve is set to expand by about 14 million square miles.

In the deepest parts of the ocean, below 4,000 meters, 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 compensation depth – and it is expanding.

This contrasts with the widely discussed ocean acidification of surface waters due to the ocean absorbing carbon dioxide from the burning of fossil fuels.

But the two are linked: because of rising concentrations of carbon dioxide in the ocean, its pH is decreasing (becoming more acidic), and the deep-sea area in which calcium carbonate dissolves is growing, from the seafloor up.

The transition zone within which calcium carbonate increasingly becomes chemically unstable and begins to dissolve is called the lysocline. Because the ocean seabed is relatively flat, even a rise of the lysocline by a few meters can rapidly lead to large under-saturated (acidic) areas.

Our research showed this zone has already risen by nearly 100 meters since pre-industrial times and will likely rise further by several hundreds of meters this century.

Millions of square kilometers of ocean floor will potentially undergo a rapid transition whereby calcareous sediment will become chemically unstable and dissolve.

Expanding boundaries

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

The carbonate content of seafloor sediments decreases within the lysocline, reaching zero below the carbonate compensation depth (CCD). Above the lysocline is the calcite saturation depth (CSD), with seabed sediments rich in calcium carbonate. Author provided, CC BY-SA

The area below the calcite compensation depth varies greatly between different sectors of the oceans. It already occupies about 41% of the global ocean. Since the industrial revolution, this zone has risen for all parts of the ocean, varying from almost no rise in the western Indian Ocean to more than 300 meters in the northwest Atlantic.

If the calcite compensation depth rises by a further 300 meters, the area of seafloor below it will increase by 10% to occupy 51% of the global ocean.

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Mark John Costello & Peter Townsend Harris, The Conversation, 14 July 2024. Article.

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