C T and Alk are measurable quantities. If they are known then the system is completely defined. It reflects the stoichiometry of three important processes. There are two crystalline forms of calcium carbonate, calcite and aragonite.
Calcite is the more stable and more common of the two.
Gridded data files of carbonate ion distribution , and the difference between the observed distribution and calcite solubility are available along with information on how they were constructed from David Archer, University of Chicago. The surface waters are supersaturated and the deep waters understaturated. Aragonite becomes undersaturated at a shallower level than calcite, i.e., calcite is the stable phase at these temperatures and pressures. The answer is not as simple as equating the saturation horizon with the boundary between carbonate and non-carbonate bearing sediments. We next consider the processes controlling the position of these two horizons.