Calcium-isotope fractionation in selected modern and ancient marine carbonates

The calcium-isotope composition (delta Ca-44/42) was analyzed in modern, Cretaceous and Carboniferous marine skeletal carbonates as well as in bioclasts. non-skeletal components, and diagenetic cements of Cretaceous and Carboniferous limestones. In order to gain insight in Ca-aq(2+)-CaCO3-isotope fractionation mechanisms in marine carbonates, splits of samples were analyzed for Sr, Mg, Fe, and Mn concentrations and for their oxygen and carbon isotopic composition. Biological carbonates generally have lower delta Ca-44/42 values than inorganic marine cements, and there appears to be no fractionation between seawater and marine inorganic calcite. A kinetic isotope effect related to precipitation rate is considered to control the overall discrimination against Ca-44 in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the delta Ca-44/42 values with Sr concentrations in Cretaceous limestones that contain biological carbonates at various stages of marine diagenetic alteration. No significant temperature dependence of Ca-isotope fractionation was found in shells of Cretaceous rudist bivalves that have recorded large seasonal temperature variations as derived from 8180 values and Mg concentrations. The reconstruction of secular variations in the delta Ca-44/42 value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modem and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the delta Ca-44/42 values of Cretaceous skeletal calcite suggest that the delta Ca-44/42 value of Cretaceous seawater was 0.3-0.4 parts per thousand lower than that of the modern ocean. (c) 2006 Elsevier Inc. All rights reserved.