The depth in the sea at which the rate of dissolution of solid calcium carbonate equals the rate of supply. Surface ocean waters are usually saturated with calcium carbonate, so calcareous materials are not dissolved. At mid-depths the lower temperature and higher CO₂ content of sea water cause slow dissolution of calcareous material. Below about 4500 m, waters are rich in dissolved CO₂ and able to dissolve calcium carbonate readily. Carbonate-rich sediments are common in waters less than 3500 m depth, but are completely absent below about 6000 m. See also calcareous ooze.