Evolution of the oceanic calcium cycle during the late Mesozoic:: Evidence from δ44/40Ca of marine skeletal carbonates

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 delta Ca-44/40 and delta O-18 values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02 parts per thousand/degrees C. The delta Ca-44/40 record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater (delta Ca-44/40(SW)), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite (alpha(CC-SW)) of similar to 0.9986. The inferred delta Ca-44/40(SW) record, with an average stratigraphic resolution of 1 Ma, shows systematic temporal variation of similar to 0.5 parts per thousand with the Middle/Late Jurassic (similar to 1.54 Ma) minimum of similar to 1.4 parts per thousand and a subsequent general increase to the Early Cretaceous (similar to 124 Ma) maximum of similar to 1.9 parts per thousand. The global nature of the delta Ca-44/40(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 delta Ca-44/40(SW) record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in delta Ca-44/40(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. (c) 2006 Elsevier B.V. All rights reserved.