Calcium isotope fractionation in calcite and aragonite

Calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite. Precipitation temperatures ranged from 0 to 28 degrees C. Calcium isotope fractionation shows a temperature dependence in accordance with previous observations: 1000 (.) ln(alpha(cc)) = -1.4 + 0.021 (.) T (degrees C) for calcite and 1000 (.) ln(alpha(ar)) = -1.9 + 0.017 (.) T (degrees C) for aragonite. Within uncertainty the temperature slopes are identical for the two polymorphs. However, at all temperatures calcium isotopes are more fractionated in aragonite than in calcite. The offset in delta Ca-44/40 is about 0.6 parts per thousand. The underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals, or to different Ca reaction behavior at the solid-liquid interface. Recently, the observed temperature dependence of the Ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting (Lemarchand et al., 2004). We show that this mechanism can in principle also be applied to CaCO3 precipitation in natural environments in normal marine settings. Following this model, Ca isotope fractionation in marine Ca carbonates is primarily controlled by precipitation rates. On the other hand the larger Ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates. The rate control model of Ca isotope fractionation predicts a strong dependence of the Ca isotopic composition of carbonates on ambient CO32- concentration. While this model is in general accordance with our observations 3 in marine carbonates, cultured specimens of the planktic foraminifer Orbulina universa show no dependence of Ca-isotope fractionation on the ambient CO32- concentration. The latter observation implies that the 3 carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water. Copyright (c) 2005 Elsevier Ltd.