Lysocline

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The lysocline is a term used in geology, geochemistry and marine biology to denote the depth in the ocean below which the rate of dissolution of calcite increases dramatically.

Shallow marine waters are generally supersaturated in calcite, CaCO₃, so as marine organisms (which often have shells made of calcite or its polymorph, aragonite) die, they will tend to fall downwards without dissolving. As depth (i.e. pressure) increases within the water column, the corresponding calcite saturation of seawater decreases and the shells start to dissolve. The reaction involved, though more complex, can be thought as: CaCO₃(s) + H₂O + CO₂ → Ca²⁺(aq) + 2HCO₃^-(aq). At the lysocline, the rate of dissolution increases dramatically. Below this, there exists a depth known as the carbonate compensation depth (CCD) below which the rate of supply of calcite equals the rate of dissolution, such that no calcite is deposited. This depth is the equivalent of a marine snow-line, and averages about 4,500 meters below sea level.

The depth of the CCD varies as a function of the chemical composition of the seawater and its temperature. Furthermore, it is not constant over time, having been globally much shallower in the Cretaceous through to Eocene. If the atmospheric concentration of carbon dioxide continues to increase, the CCD can be expected to rise, along with the ocean's acidity.