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The Ca isotope compositions of 37 late Mesozoic skeletal carbonates, belemnites and brachiopods, from the Tethyan realm were analyzed by thermal (TIMS) and plasma (MC-ICP-MS) ionization mass spectrometry. A poor correlation between δ 44/40 Ca and δ 18 O values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02‰/°C. The δ 44/40 Ca record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater ( δ 44/40 Ca SW ), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite ( α CC–SW ) of ∼ 0.9986. The inferred δ 44/40 Ca SW record, with an average stratigraphic resolution of 1 Ma, shows systematic temporal variation of ∼ 0.5‰ with the Middle/Late Jurassic (∼ 154 Ma) minimum of ∼ 1.4‰ and a subsequent general increase to the Early Cretaceous (∼ 124 Ma) maximum of ∼ 1.9‰. The global nature of the δ 44/40 Ca SW record is supported by identical Ca isotope compositions of coeval (Kimmeridgian) belemnites collected from two distinct paleogeographic regions, the southern (New Zealand) and northern (Germany) margin of the Tethys Ocean. The observed late Mesozoic δ 44/40 Ca SW record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in δ 44/40 Ca SW record can be explained by changes in oceanic input fluxes of Ca that were independent of the carbonate ion fluxes, such as the hydrothermal Ca flux or the release of Ca to the oceans via dolomitization of marine carbonates. |
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