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What is the Calcite Compensation Depth (CCD) and what is its significance for carbonate deposits?

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The Calcite Compensation Depth is the depth below which all calcite minerals are dissolved. The solubility of calcite depends on both pressure and temperature. Under high pressure, low temperature conditions calcite is most soluble, and in deep parts of the ocean only siliceous deposits are found.

Typical values for the CCD are between 3000 and 4000 m, but since the CCD also depends on the amount of calcite in the water it can occur as shallow as 2500 m (in parts of the Pacific Ocean).

Taking all this together it becomes clear carbonate deposits can form and/or be preserved only above the CCD.

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The Calcite Compensation Depth (CCD) is the depth at which the dissolution of calcite outpaces the accumulation of calcitic sediments. This is distinct from the Aragonite Compensation Depth (ACD), which is similar to the CCD in that it is the depth at which aragonite sediments dissolve faster than they can accumulate. The ACD is generally shallower than the CCD.

Further below the CCD, where the dissolution of opal outpaces the accumulation rate of opaline sediments we have the, you guessed it, Opal Compensation Depth (OPC). These depth boundaries are all a function of pressure and temperature; carbonaceous materials being more soluble at low temperatures and high pressures. Thus, deep cold seawater can hold more calcite and aragonite and will dissolve it faster than those sediments are able to accumulate. There is no net accumulation of these sediments below their relevant compensation depth.

The sediments we are referring to are biogenic carbonate (and silica/opal, I'll go over that in a second) tests, or skeletal and shell materials, made by planktonic, nektonic, and benthonic organisms floating around in the water column. These sorts of sediments generally only become important away from the continental edge, where they are no longer diluted by terrigenous sediments. I'll say that in a different way in case it isn't clear: there is a gradational pattern of sedimentation on the seafloor as you move away from continents, further out into ocean proper. When fine sands and silts are no longer being carried out and deposited onto the seafloor, then these biogenic sediments become relatively enriched because they are really the only thing accumulating (along with some red clays) and form carbonaceous and siliceous oozes (unconsolidated sediments that are >30% biogenic material). Above the ACD aragonitic, calcitic, and siliceous sediments dominate. Further out, and/or, deeper in the ocean, we hit the ACD and those aragonitic sediments are winnowed out. Now we only have calcitic and siliceous sediments so we're making carbonaceous oozes generally. Even further/deeper and we hit the CCD, the calcite seds are removed and now we're only accumulating siliceous sediments; we're making siliceous oozes. Again, we go deeper, and we hit the OCD. Now only red clays are accumulating. This is an idealized gradational scale, changes in topography can bring the seafloor back up above one of these boundaries, for example at a mid ocean ridge. Or maybe the continental shelf drops off too quickly and those terrigenous continent derived sediments are not winnowed out before we cross the CCD. In that case we would never form carbonaceous oozes.

Because these depths are related to the solubility of the minerals, they can be raised or depressed by increasing the amount of that mineral we are pumping into the system. In other words, if you have a big boom in primary productivity somewhere over the pacific and plankton are growing like crazy, the CCD is depressed, often by as much as 500m. This is because all of that calcite floating down is saturating those cold bottom waters, so we have to move even deeper to reach a point where the water is able to hold more calcite and dissolution outpaces accumulation again.

The depth of the CCD is also higher at the poles/rises towards northern (colder) latitudes and sinks towards the warmer equatorial waters.

Hope that helps.

here is a good resource if you'd like to learn more or rake in those details: https://www.sciencedirect.com/science/article/pii/B9780444530004000044

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