The Calcium Isotope Composition of Modern Seawater Determined by Thermal Ionisation Mass Spectrometry |
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Authors: | Anne-Désirée Schmitt Guido Bracke Peter Stille Bernard Kiefel |
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Institution: | Centre de Géochimie de la Surface (CNRS) - EOST, 1 rue Blessig, F-67084 Strasbourg Cedex |
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Abstract: | The Ca isotopic composition of modern seawater has been determined using a 43Ca-48Ca double spike, which was calibrated using a 42Ca/44Ca seawater ratio of 0.30587 ± 0.00026. This ratio was determined from a total evaporation experiment in which the ion beam was measured from the beginning to the end of the emission. With integration of the peak intensities, the fractionation effects can be minimised, since total evaporation of the reservoir cancels out the effect of vapour enrichment in the light isotopes. This experiment avoids the gravimetric uncertainty inherent in the double spike calibration. This calibration allows the precise redetermination of the seawater isotopic composition of Ca. A mean 40Ca/44Ca ratio for two Atlantic water samples of 45.143 (2smean= 0.003) was found. The good reproducibility of the Ca isotope ratios in present seawater and the very strong isotopic homogeneity of Ca in the oceans illustrate the advantage of using seawater as the common standard, with the advantage of decreasing interlaboratory bias. |
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Keywords: | calcium isotopic composition seawater mass spectrometry total evaporation Ca standard |
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