OXYGEN-ISOTOPE EXCHANGE BETWEEN CARBONIC-ACID, BICARBONATE, CARBONATE, AND WATER - A REEXAMINATION OF THE DATA OF MCCREA (1950) AND AN EXPRESSION FOR THE OVERALL PARTITIONING OF OXYGEN ISOTOPES BETWEEN THE CARBONATE SPECIES AND WATER

An examination of MCCREA'S work (1950) shows that the O-18/O-16 ratio of rapidly precipitated calcium carbonate depends upon the pH value of the solution and discloses that the oxygen isotope equilibrium fractionation between the sum of the dissolved carbonate species and water is linearly related to the equilibrium between the carbonate species. This relation permits the calculation of fractionation factors for the individual carbonate species and the expression of an overall fractionation factor in terms of individual factors and the oxygen proportions of the carbonate species. The fractionation factors a(H2CO3-H2O) = 1.0396, a(HCO3--H2O) = 1.0346, and a(CO32--H2O) = 1.0183 (25-degrees-C) compare well to recent data.