Calcium-isotope fractionation in selected modern and ancient marine carbonates

Thomas Steuber, Dieter Buhl

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The calcium-isotope composition (δ44/42Ca) 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 Ca2+aq-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 δ44/42Ca 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 44Ca in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the δ44/42Ca 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 δ18O values and Mg concentrations. The reconstruction of secular variations in the δ44/42Ca value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modern and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the δ44/42Ca values of Cretaceous skeletal calcite suggest that the δ44/42Ca value of Cretaceous seawater was 0.3-0.4‰ lower than that of the modern ocean.

Original languageBritish English
Pages (from-to)5507-5521
Number of pages15
JournalGeochimica et Cosmochimica Acta
Issue number22
StatePublished - 15 Nov 2006


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