CARBON ISOTOPIC FRACTIONATION IN SYNTHETIC ARAGONITE AND CALCITE - EFFECTS OF TEMPERATURE AND PRECIPITATION RATE

An open-system chemostat technique was employed to study temperature and precipitation rate effects on carbon isotopic fractionation during the inoganic precipitation of aragonite and calcite from solutions held at constant chemical and isotopic composition. Calcito-bicarbonate enrichment factors (epsilon(el-HCO3)) average 1.0 +/- 0.2 parts per thousand and are essentially constant throughout the range of temperatures (10, 25, and 40-degrees-C) and precipitation rates (10(2.6) to 10(4.8)-mu-mol m-2 h-1) studied. Calcite-CO2(g) enrichment factors (epsilon(el-CO2)) are temperature sensitive and are described by the equation epsilon(el-CO2) = 11.98 (+/- 0.13) - 0.12(+/- 0.01). T(degrees-C). Aragonite-bicarbonate enrichment factors (epsilon(ar-HCO3)) average 2.7 +/- 0.6 parts per thousand for temperatures of 10, 25, and 40-degrees-C and for rates from 10(2.3) to 10(4.1)-mu-mol m-1 h-1 epsilon(ar-HCO3) data are independent of temperature and display no precipitation rate effect at 10-degrees-C, a small negative relationship to rate at 25-degrees-C, and a small positive relationship to rate at 40-degrees-C. The relative weakness of the effect coupled with the lack of consistency in the trends suggests that epsilon(ar-HCO3BAR) is independent of rate for most geologic processes. Aragonite-CO2(g) enrichment factors (epsilon(ar-CO2)) are temperature-sensitive and are described by the equation epsilon(ar-CO2) = 13.88(+/- 0.16) - 0.13(+/- 0.01). T(degrees-C). Aragonite-calcite fractionation (epsilon(ar-el)) is 1.7 +/- 0.4 parts per thousand and is independent of temperature from 10 to 40-degrees-C. 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.