There's an acidic zone 13,000 feet beneath the ocean surface — and it's getting bigger

An underwater photo of corals and tropical fish on the ocean floor

(Image credit: stock_colors via Getty Images)

In the deepest parts of the ocean, below 13,100 feet (4,000 metres), 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.

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 metres can rapidly lead to large under-saturated (acidic) areas.

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

Millions of square kilometres 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.

A diagram showing how carbonate content changes in the ocean — 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. (Image credit: Mark John Costello and Peter Townsend Harris, 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 980 feet (300 m) in the northwest Atlantic.

If the calcite compensation depth rises by a further 980 feet, the area of seafloor below it will increase by 10% to occupy 51% of the global ocean.

A diagram showing how CCD has changed in the ocean — These maps show the changes in area of ocean exposed to corrosive bottom waters in 17 different regions. The pre-industrial CCD is dark blue and areas above the lysocline are light blue. Map A shows the present day and map B shows a lysocline rise of 300 metres. (Image credit: Mark John Costello and Peter Townsend Harris, CC BY-SA)

Distinct habitats

For the first time, a recent study showed the calcite compensation depth is a biological boundary with distinct habitats above and below it. In the northeast Pacific, the most abundant seabed organisms above the calcite compensation depth are soft corals, brittle stars, mussels, sea snails, chitons and bryozoans, all of which have calcified shells or skeletons.

However, below the calcite compensation depth, sea anemones, sea cucumbers and octopus are more abundant. This under-saturated (more acidic) habitat already limits life in 54.4 million square miles (141 million square kilometres) of the ocean and could expand by another 13.5 million square miles (35 million sq/km) if the calcite compensation depth were to rise by 980 feet.

In addition to the expansion of the calcite compensation depth, parts of the ocean in low latitudes are losing species because the water is getting too warm and oxygen levels are declining, both also due to climate change.

Thus, the most liveable habitat space for marine species is shrinking from the bottom (rising calcite compensation depth) and the top (warming).

Expanding boundaries

Distinct habitats

Island nations most affected