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ABSTRACT

The physicochemical parameters needed for the calculation of the degree of saturation of calcium carbonate in deep sea from the chemical data measured at ambient barometric pressures are reviewed. When the alkalinity and total carbon dioxide concentrations are used to calculate total CO3= species, the result is not sensitively affected by the choice of the available sets of the apparent dissociation constants for carbonic and boric acids in sea water determined by Lyman (1957), Hansson (1973a), and Mehrbach and others (1974). The apparent solubility products of calcite in seawater determined by Ingle and others (1973) at 1 bar total pressure are preferred over those determined by McIntyre (1965), because of the possible disequilibrium conditions existing in McIntyre’s experiments. An inconsistency of an order of 10 cm3/mole for the partial molal volumes of Ca++ and/or CO3= are present among the available data. This would cause an uncertainty of about 15 percent in the degree of saturation calculated for a pressure of 500 bars (or a 5,000 meter depth). The degree of saturation of calcite in the Atlantic and Pacific Oceans has been calculated from about 4,000 sets of the alkalinity and total CO2 measurements obtained during GEOSECS. It has been found that the calcite compensation depths obtained by Berger and Winterer (1974) coincide with the level of 75 and 65 percent undersaturation respectively in the Atlantic and the Pacific Oceans with an exception of the southern extreme of those oceans. In the southern extreme (~ 60° S), the calcite compensation depths become shallower coinciding with a level of 90 percent undersaturation. On the basis of the rates of dissolution of calcite in seawater determined by Morse and Berner (1972) and the calculated pH values at in situ pressure and temperature conditions, the dissolution rates for calcite at the calcite compensation depths have been estimated. General agreement between the dissolution rates thus obtained and the carbonate productivity in the surface water suggests that the calcite compensation depth in the oceans is controlled by the biological productivity of carbonates in the surface water and the degree of undersaturation of calcite in seawater which, in turn, regulates the rate of dissolution. Specific areas of studies needed to be investigated are summarized.

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