CARBON ISOTOPIC FRACTIONATION IN SYNTHETIC ARAGONITE AND CALCITE - EFFECTS OF TEMPERATURE AND PRECIPITATION RATE
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An open-system chemostat technique was employed to study temperature and precipitation rate effects on carbon isotopic fractionation during the inorganic precipitation of aragonite and calcite from solutions held at constant chemical and isotopic composition. Calcite-bicarbonate enrichment factors ({lunate}cl-HCO-3) average 1.0 0.2%. and are essentially constant throughout the range of temperatures (10, 25, and 40C) and precipitation rates (1026 to 104.8 mol m-2 h-1) studied. Calcite-CO2(g) enrichment factors ({lunate}cl-CO2) are temperature sensitive and are described by the equation {lunate}cl-CO2 = 11.98(0.13) -0.12(0.01) T(C). Aragonite-bicarbonate enrichment factors ({lunate}ar-HCO-3) average 2.7 0.6%. for temperatures of 10, 25, and 40C and for rates from 102.3 to 104.1 mol m-2 h-1 {lunate}ar-HCO-3 data are independent of temperature and display no precipitation rate effect at 10C, a small negative relationship to rate at 25C, and a small positive relationship to rate at 40C. The relative weakness of the effect coupled with the lack of consistency in the trends suggests that {lunate}ar-HCO-3 is independent of rate for most geologic processes. Aragonite-CO2(g) enrichment factors ({lunate}ar-CO2) are temperature-sensitive and are described by the equation {lunate}ar-CO2 = 13.88(0.16) -0.13(0.01) T(C). Aragonite-calcite fractionation ({lunate}ar-cl) is 1.7 0.4%. and is independent of temperature from 10 to 40C. While the enrichment factors obtained here agree with those of some previous studies, temperature and rate effects do not. Our results, which benefit from better control on precipitate mineralogy and precipitation rate, are probably the most accurate to date. 1992.
