Carbonate equilibria in the open ocean and their bearing on the interpretation of ancient carbonate rocks

Abstract

A model of steady-state equilibrium deposition of carbonates in the open sea, where exchange of material between the sediment and the water is permitted, is studied from the phase-equilibrium point of view. Allowing for the possibility of arbitrary variations in temperature, pressure and activity of CO 2 in the system, in general only one-carbonate deposits, either calcite or dolomite, should result. The contact between calcite beds and dolomite beds should be sharp although the two carbonates should be in mutual equilibrium. Changes in hydrographie conditions may result in onlap-offlap relations in the deposits and apparent interbedding of the strata. Experimental results in the literature on the solubility of dolomite in water are not all internally consistent, although they do indicate that, under atmospheric conditions, the solution relations are sensitive to the value of the CO 2 activity. Increases in total pressure (depth) favour the precipitation of dolomite. Available thermochemical data indicate that dolomite may be the carbonate phase in equilibrium with many sea-water compositions. Modern carbonate deposits are either calcite or aragonite at the surface, but may change over, abruptly, to dolomite at depth. Two-carbonate deposits, where carefully studied, show evidence of incomplete reaction. The prevalence of one-carbonate deposits thus accord with theoretical considerations although the nature of the surface deposit is at variance with thermochemical data. In the sedimentary record, carbonate rocks with nearly equal amounts of calcite and dolomite are not uncommon; these beds cannot be due to direct chemical precipitation in the open sea.