A large proportion of deep sea water is undersaturated with respect to both calcite and aragonite. This conclusion is based upon laboratory studies of the. effects of temperature and pressure on carbonate equilibria in sea water combined with shipboard measurements of water properties as well as upon actual measurements of dissolution rates of calcium carbonate samples held at various depths. Undersaturation is caused by biological production of CO2 at shallower depths and by the effect of increased pressure on the solubility of CaCO, at greater depths. The so-called compensation depth below which CaCO3 disappears from deep sea sediments does not simply represent a downward change from supersaturated to undersaturated water. This is proven by the presence of undersaturated water above the compensation depth and by the fact that the compensation depth may be located thousands of meters below depths where CaCO<sub>3</sub> shows definite signs of dissolution.
About 1000 metres above the compensation depth, the rate of dissolution shows an abrupt downward increase. This zone, called the lysocline, has been predicted from laboratory measurements of dissolution rate as a function of the degree of undersaturation of sea water. The sudden rate increase can be explained by a change in the mechanism of dissolution at the surface of calcite. Dissolution at all depths is too slow to be controlled by transport of material from particle surfaces by either diffusion, turbulence, or changes in water velocity. This is due to the inhibiting effects of ions and/or organic molecules adsorbed onto the carbonate surfaces from the surrounding sea water.
Surface sea water, in contrast to deep water, is generally supersaturated with respect to both calcite and aragonite. However, inorganic precipitation rarely occurs due to the inhibiting effects of Mg++, phosphate, organic matter and possibly other substances both as dissolved species and as adsorbed coatings on mineral grains.