Carbonate Compensation Depth (CCD)

The Carbonate Compensation Depth (CCD) is a specific depth in the ocean below which calcium carbonate (CaCO₃) dissolves faster than it accumulates, meaning no calcium carbonate sediments can build up on the seafloor.
More precisely, CCD is defined as the ocean depth at which the rate of supply of calcium carbonate from the surface equals the rate of its dissolution in seawater.
At depths shallower than the CCD, calcium carbonate shells and particles can accumulate on the seafloor. But below the CCD, these shells dissolve before they can be buried in the sediments.

How Does CCD Work?

In the upper layers of the ocean, tiny marine organisms called plankton like foraminifera, coccolithophores, and pteropods produce shells or tests made of calcium carbonate (in the forms of calcite or aragonite).
These organisms die and their calcium carbonate shells sink through the water column.

As the shells sink deeper, the pressure increases, temperature decreases, and the concentration of dissolved CO₂ increases.
These changes cause ocean water to become more acidic and under saturated with respect to CaCO₃, which leads to dissolution of the shells.

Saturation Horizon: The depth at which seawater becomes just saturated with CaCO₃. Below this point, dissolution begins.
Lysocline: The depth at which the rate of CaCO₃ dissolution starts to increase rapidly.
CCD: The depth at which 100% of the CaCO₃ dissolves and no accumulation on the seabed occurs.

According to recent research, CCD is expanding it is shifting deeper into the ocean. This means calcium carbonate particles now dissolve at greater depths than before.

The absorption of atmospheric CO₂ by the ocean leads to the formation of carbonic acid (H₂CO₃).
This lowers the pH of seawater and reduces the saturation of calcium carbonate, especially in deeper waters.

As water masses sink in high-latitude regions and move along the ocean floor, they accumulate respired CO₂ from decomposing organic material, further increasing acidity.

Melting polar ice and changes in thermohaline circulation affect ocean chemistry and vertical mixing, possibly influencing CCD.
Rising global temperatures may alter biological productivity, changing the rate at which calcium carbonate is supplied to deep waters.

Less calcium carbonate production at the surface (due to declining plankton populations) means less CaCO₃ reaches the deep sea.

Below the CCD, no carbonate sediments can accumulate.
This affects the marine sediment record, which is used to study Earth's climate history.

Marine organisms that rely on calcium carbonate for their shells such as corals, molluscs, and plankton may find it harder to survive as ocean chemistry becomes more corrosive to CaCO₃.

The oceanic carbon pump, which helps regulate atmospheric CO₂ levels, depends on the efficient deposition of CaCO₃ to the seafloor.
A deepening CCD means less long-term carbon burial, potentially intensifying climate change.

CCD is not constant: It varies by ocean basin and depends on temperature, pressure, CO₂ concentration, and biological productivity.
For example:
- In the North Atlantic, CCD is around 4,500–5,000 meters.
- In the Pacific, where CO₂ concentration is higher in deep water, CCD may be shallower (~4,000 meters).
Recent research indicates that in many parts of the ocean, CCD has deepened by several hundred meters over the past century.