Carbonate Compensation Depth (CCD)

Limestone, thin section, polarised LM
Thin section of a Nummulitic limestone. The large objects are the remains of large foraminifera, Nummulites, which are embedded in a fine-grained matrix of calcareous remains of smaller, planktonic organisms. PASIEKA / Getty Images

Carbonate Compensation Depth, abbreviated as CCD, refers to the specific depth of the ocean at which calcium carbonate minerals dissolve in the water quicker than they can accumulate.

The bottom of the sea is covered with fine-grained sediment made of several different ingredients. You can find mineral particles from land and outer space, particles from hydrothermal "black smokers" and the remains of microscopic living organisms, otherwise known as plankton.

Plankton are plants and animals so small that they float their whole lives until they die.

Many plankton species build shells for themselves by chemically extracting mineral material, either calcium carbonate (CaCO3) or silica (SiO2), from the seawater. Carbonate compensation depth, of course, only refers to the former; more on silica later. 

When CaCO3-shelled organisms die, their skeletal remains begin sinking towards the bottom of the ocean. This creates a calcareous ooze that can, under pressure from the overlying water, form limestone or chalk. Not everything that sinks in the sea reaches the bottom, however, because the chemistry of ocean water changes with depth. 

Surface water, where most plankton live, is safe for shells made from calcium carbonate, whether that compound takes the form of calcite or aragonite. These minerals are almost insoluble there. But the deep water is colder and under high pressure, and both of these physical factors increase the water's power to dissolve CaCO3.

More important than these is a chemical factor, the level of carbon dioxide (CO2) in the water. Deep water collects CO2 because it's made by deep-sea creatures, from bacteria to fish, as they eat the falling bodies of plankton and use them for food. High CO2 levels make the water more acidic.

The depth where all three of these effects show their might, where CaCO3 starts to dissolve rapidly, is called the lysocline.

As you go down through this depth, seafloor mud starts to lose its CaCO3 content—it is less and less calcareous. The depth at which CaCO3 completely disappears, where its sedimentation is equaled by its dissolution, is the compensation depth.

A few details here: calcite resists dissolution a little better than aragonite, so the compensation depths are slightly different for the two minerals. As far as geology goes, the important thing is that CaCO3 disappears, so the deeper of the two, calcite compensation depth or CCD, is the significant one.

"CCD" can sometimes mean "carbonate compensation depth" or even "calcium carbonate compensation depth," but "calcite" is usually the safer choice on a final exam. Some studies do focus on aragonite, though, and they may use the abbreviation ACD for "aragonite compensation depth."

In today's oceans, the CCD is between 4 and 5 kilometers deep. It is deeper in places where new water from the surface can flush away the CO2-rich deep water, and shallower where lots of dead plankton build up the CO2. What it means for geology is that the presence or absence of CaCO3 in a rock—the degree to which it can be called limestone—can tell you something about where it spent its time as a sediment.

Or conversely, the rises and falls in CaCO3 content as you go up or down section in a rock sequence can tell you something about changes in the ocean in the geologic past.

I mentioned silica earlier, the other material that plankton use for their shells. There is no compensation depth for silica, although silica does dissolve to some extent with water depth. Silica-rich seafloor mud is what turns into chert. And there are rarer plankton species that make their shells of celestite, or strontium carbonate (SrSO4). That mineral always dissolves immediately upon the death of the organism.

Edited by Brooks Mitchell