Evolution of the oceanic calcium cycle during the late Mesozoic: Evidence from δ44/40Ca of marine skeletal carbonates |
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| Affiliation: | 1. University of Ottawa and Ottawa-Carleton Geoscience Centre, Ottawa, Canada K1N 6N5,ON;2. Institut für Geologie, Mineralogie und Geophysik, Ruhr Universität, Bochum 44801, Germany;3. Carleton University and Ottawa-Carleton Geoscience Centre, Ottawa, Canada K1S 5B6,ON;1. Deep & Early Earth Processes (DEEP) Research Group, Department of Geology, University of Johannesburg, 2006 Auckland Park, South Africa;2. Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, 14473 Potsdam, Germany;3. Institut für Mineralogie, Westfälische Wilhelms-Universität, Corrensstrasse 24, 48149 Münster, Germany;4. Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen, Denmark;5. School of Geosciences, University of the Witwatersrand, 2050 Johannesburg, South Africa;6. Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, T6G 2E3 Alberta, Canada;7. Centre for Earth Evolution and Dynamics, University of Oslo, 0316 Oslo, Norway;8. Geodynamics Division, Geological Survey of Norway, 7491 Trondheim, Norway;1. Department of Earth Sciences, Durham University, Durham DH1 3LE, UK;2. Department of Earth and Planetary Sciences, Northwestern University, Technological Institute, 2145 N. Sheridan Road, Evanston, IL 60208, USA;1. GEOMAR, Helmholtz-Zentrum für Ozeanforschung Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany;2. Department of Geochemistry, Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic;3. Department of Environmental Geosciences, Czech University of Life Sciences, Kamýcká 129, 165 21 Prague 6, Czech Republic;4. Geological Institute, Slovak Academy of Sciences, Dubravska cesta 9, 84005 Bratislava, Slovak Republic;5. Ottawa-Carleton Geoscience Center, University of Ottawa, Ottawa ON K1N 6N5, Canada;1. Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA;2. Berkeley Geochronology Center, Berkeley, CA 94709, USA;3. State Key Lab of Isotope Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, PR China;4. Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA;5. Geochemistry Department, Earth Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94270, USA;6. Department of Geological Engineering, Middle East Technical University, Ankara 06531, Turkey;7. State Key Laboratory of Geobiology and Environmental Geology, China University of Geosciences, Wuhan, Hubei 430074, PR China |
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| Abstract: | 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 δ44/40Ca and δ18O values of belemnites suggests only a weak temperature dependency for the Ca isotope composition of belemnites, likely less than 0.02‰/°C. The δ44/40Ca record of belemnites was therefore used to reconstruct the Ca isotope composition of paleo-seawater (δ44/40CaSW), based on an experimentally determined fractionation factor between seawater Ca and belemnite calcite (αCC–SW) of ∼ 0.9986. The inferred δ44/40CaSW 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 δ44/40CaSW 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 δ44/40CaSW record was simulated using a simple Ca isotope mass balance model, and the results indicate that the variation in δ44/40CaSW 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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