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A compilation of δ44/40Ca (δ44/40Ca) data sets of different calcium reference materials is presented, based on measurements in three different laboratories (Institute of Geological Sciences, Bern; Centre de Géochimie de la Surface, Strasbourg; GEOMAR, Kiel) to support the establishment of a calcium isotope reference standard. Samples include a series of international and internal Ca reference materials, including NIST SRM 915a, seawater, two calcium carbonates and a CaF2 reference sample. The deviations in δ44/40Ca for selected pairs of reference samples have been defined and are consistent within statistical uncertainties in all three laboratories. Emphasis has been placed on characterising both NIST SRM 915a as an internationally available high purity Ca reference sample and seawater as representative of an important and widely available geological reservoir. The difference between δ44/40Ca of NIST SRM 915a and seawater is defined as ‐1.88 O.O4%o (δ44/42CaNISTSRM915a/Sw= ‐0.94 0.07%o). The conversion of values referenced to NIST SRM 915a to seawater can be described by the simplified equation δ44/40CaSa/Sw=δ44/40CaSa/NIST SRM 915a ‐ 1.88 (δ44/42CaSa/Sw=δ44/42CaSa/NIST SRM 915a ‐ 0.94). We propose the use of NIST SRM 915a as general Ca isotope reference standard, with seawater being defined as the major reservoir with respect to oceanographic studies.

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