Stable isotope profiles (Ca,O, C) through modern brachiopod shells of T. septentrionalis and G. vitreus: Implications for calcium isotope paleo-ocean chemistry |
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| Authors: | Katja von Allmen Thomas F Nägler Thomas Pettke Dorothee Hippler Erika Griesshaber Alan Logan Anton Eisenhauer Elias Samankassou |
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| Institution: | 1. Department of Earth Sciences, Memorial University of Newfoundland, St. John''s, NL, A1B 3X5, Canada;2. Department of Earth Sciences, Brock University, St. Catharines, ON L2S 3A1, Canada;3. Institute of Biology, University of Iceland, Askja, Sturlugata 7, 101 Reykjavík, Iceland;1. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing, Jiangsu 210008, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing 210008, China;4. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;5. State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation, College of Geoscience, Chengdu University of Technology, Chengdu 610059, China;1. Ruhr-University Bochum, Institute of Geology, Mineralogy and Geophysics, Universitätsstraße 150, D-44801 Bochum, Germany;2. Graz University of Technology, Institute of Applied Geosciences, Rachbauerstraße 12, 8010 Graz, Austria;3. Géosciences Environnement Toulouse, CNRS, UMR 5563, Avenue Edouard Belin 14, 31400 Toulouse, France;4. GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstraße 1-3, Kiel, Germany;1. Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Milano 20133, Italy;2. Department of Earth Sciences, Brock University, St. Catharines, Ontario L2S 3A1, Canada |
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| Abstract: | Brachiopod shells are widely used as an archive to reconstruct elemental and isotopic composition of seawater. Studies, focused on oxygen and carbon isotopes over the last decades, are increasingly extending to the emerging calcium isotope system. To date, only little attention has been paid to test the reliability of fossil brachiopods on their modern counterparts.In this context, the present study investigates two modern brachiopods, Terebratulina septentrionalis (eastern Canada, 5–30 m depth, 7.1 °C seasonal temperature variation, two-layer shell) and Gryphus vitreus (northern Mediterranean, 200 m depth, constant all-year round temperature, three-layer shell). Both species were sampled along the ontogenetic growth direction and calcium, oxygen, and carbon isotopes as well as elemental concentration were measured. Calcium isotopes were analyzed on TIMS. The elemental composition was analyzed by LA-ICP-MS and ICP-AES.The results indicate an intra-specimen δ44/40Ca variation ranging from 0.16 to 0.33‰, pointing to a fairly homogenous distribution of calcium isotopes in brachiopod shells. However, in the light of the suggested 0.7‰ increase in calcium isotopes over the Phanerozoic such intra-specimen variations constrain ocean reconstruction. δ44/40Ca values of T. septentrionalis do not seem to be affected by growth rate. Calcium isotopic values of G. vitreus are heavy in the central part of the shell and trend towards lighter values in peripheral areas approaching the maximum isotopic composition of T. septentrionalis. The maximum inter-species δ44/40Ca difference of 0.62‰ between T. septentrionalis and G. vitreus indicates that care should be taken when using different taxa, species with different strontium content or brachiopods with specialized shell structure, such as G. vitreus, for ocean water reconstruction in terms of Ca isotopic composition. T. septentrionalis may record Ca isotopic fractionation related to seasonal seawater temperature variations in its shell but this is difficult to resolve at the current analytical precision. Average δ18O-derived temperatures of the two investigated species are close to on-site measured temperatures. |
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