Lysocline

The lysocline is 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 and 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 of as: CaCO₃(s) + H₂O + CO₂ → Ca(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. Hence, the lysocline and CCD are not equivalent. The lysocline and compensation depth occur at greater depths in the Atlantic (5000-6000 m) than in the Pacific (4000 - 5000 m), and at greater depths in equatorial regions than in polar regions.