Calcium Carbonate Compensation Depth
The calcium carbonate compensation depth (CCD) is the particular depth at any locality at which the rate of dissolution of calcium carbonate equals the rate of supply of calcium carbonate to the seafloor, so that no net accumulation of carbonate takes place. [1] Above this depth, carbonate particles can accumulate on the seafloor; below it, dissolution outruns supply and no carbonate sediment is preserved. The concept is analogous to the snowline of mountain ranges: just as snow only persists above a certain altitude, carbonate only accumulates above a certain depth.
Why CaCO₃ Solubility Increases with Depth
Much of the warm surface water of the modern ocean is supersaturated with calcium carbonate, but this condition changes rapidly with depth. [1] The degree of calcium carbonate saturation drops off abruptly in waters below the surface layer, and at depths greater than a few hundred meters seawater is undersaturated. [1]
Several factors drive the undersaturation at depth. CO2 production increases toward the ocean floor owing to the respiration of benthonic organisms and the oxidation of organic matter on the seafloor. [1] Colder water at depth can contain more dissolved CO2 than warmer surface waters, and both the decrease in temperature and the increase in hydrostatic pressure with depth cause an increase in the solubility of calcium carbonate and thus the corrosiveness of seawater. [1] These factors compound each other: the deeper the water, the colder it is, the more CO2 it holds, the higher the pressure — all effects pointing in the same direction of increased dissolution.
The Lysocline and the Abrupt Transition
The rate of dissolution does not increase in a linear fashion with depth. Experiments in which calcite spheres were suspended on deep-sea moorings demonstrated only slight corrosion of the spheres above a depth of about 3500 m, but solution of the spheres increased abruptly at that depth. [1] This abrupt transition — the lysocline — is the zone where corrosion accelerates before the true CCD is reached. Below the CCD itself, effectively all carbonate dissolves before it can accumulate.
Modern Depth Values
In different parts of the modern ocean the CCD ranges from about 3500 to 5500 m, reflecting differences in rates of CaCO3 production in surface waters and variations in the factors controlling carbonate saturation. [1] The average depth of the calcite CCD in today’s ocean is about 4500 m; the aragonite CCD is shallower than 2000 m — much closer to the surface — because aragonite is less stable than calcite and dissolves more readily. [1] The average water depth of the modern ocean is about 3800 m, and water depth ranges to slightly more than 11,000 m. [1] This means a significant fraction of the modern ocean floor lies below the calcite CCD, explaining why large areas of abyssal seafloor are carpeted by pelagic clay or siliceous ooze rather than calcareous ooze.
Seafloor Zonation
White carbonate oozes cover elevated areas of the seafloor above the CCD in areas where biogenic oozes are accumulating, but they give way to brown or gray pelagic clays or siliceous oozes below. [1] This zonation is a direct function of carbonate preservation: mid-ocean ridges and seamounts that rise above the CCD can accumulate thick calcareous ooze sequences, while the surrounding abyssal plains below the CCD preserve no carbonate at all.
Because calcium carbonate secretion is confined mainly to very shallow water areas of the ocean and to the supersaturated surface waters above the CCD, and because dissolution prevails in deeper undersaturated waters, carbonate production and accumulation are strongly decoupled: organisms throughout the surface ocean can produce carbonate particles, but only those sinking onto seafloor shallower than the CCD are preserved. [1]
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs
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