TY - JOUR
T1 - Calcium-isotope fractionation in selected modern and ancient marine carbonates
AU - Steuber, Thomas
AU - Buhl, Dieter
N1 - Funding Information:
We thank Adrian Immenhauser and Detlev K. Richter for providing sample material and important information about the petrography of samples. We thank Markus Rauch for providing data on the chemical and isotopic composition of rudist bivalves. Constructive comments of Clark Johnson, two anonymous reviewers, and of associate editor Timothy W. Lyons improved the manuscript and are gratefully acknowledged. This work was funded by Deutsche Forschungsgemeinschaft and the European Science Foundation as part of the ’Euromargins’ program (Ste 670/9, Ve 112/20).
PY - 2006/11/15
Y1 - 2006/11/15
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=33750518221&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2006.08.028
DO - 10.1016/j.gca.2006.08.028
M3 - Article
AN - SCOPUS:33750518221
SN - 0016-7037
VL - 70
SP - 5507
EP - 5521
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 22
ER -