Large 14C age offsets between the fine fraction and coexisting planktonic foraminifera in shallow Caribbean sediments |
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| Institution: | 1. CEREGE, Aix-Marseille University, CNRS, IRD, Collège de France, Technopôle de l’Arbois, BP80, 13545 Aix-en-Provence, France;2. GEOPS, Univ. Paris-Sud, CNRS, Université Paris-Saclay, Rue du Belvédère, Bât. 504, 91405 Orsay, France;3. Laboratoire de Mesure du Carbone 14 (LMC14), UMS 2572 CNRS, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France;1. Institute of Geography, Fujian Normal University, Fuzhou, 350007, China;2. Key Laboratory for Humid Subtropical Eco-geographical Processes of the Ministry of Education, Fujian Normal University, Fuzhou, 350007, China;3. Institude of Archaeology, Fujian Provincial Museum, Fuzhou, 350001, China;4. Qinghai Institude of Salt Lakes, Chinese Academy of Sciences, Xi''ning, 810008, China;5. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062, China;6. Department of Environment and Geography, Macquarie University, NSW 2109, Australia;1. University of Twente, MIRA-Institute for Biomedical Technology and Technical Medicine, Drienerlolaan 5, 7522 NB, Enschede, The Netherlands;2. Jeroen Bosch Hospital, Henri Dunantstraat 1, 5223 GZ, ''s-Hertogenbosch, The Netherlands;3. Department of Radiology, Jeroen Bosch Hospital, Henri Dunantstraat 1, 5223 GZ, ''s-Hertogenbosch, The Netherlands;1. Department of Geology, Faculty of Sciences, Minufiya University, El-Minufiya, Shibin El Kom, Egypt;2. Department of Palaeontology, Faculty of Mining and Geology, University of Belgrade, Kamenička 6, 11000 Belgrade, Serbia;3. GeoZentrum Nordbayern, Fachgruppe Paläoumwelt der Friedrich-Alexander-Universität, Erlangen-Nürnberg, Loewenichstr. 28, D-91054 Erlangen, Germany;4. Department of Mineralogy, Crystallography, Petrology and Geochemistry, Faculty of Mining and Geology, University of Belgrade, Djušina 7, 11000 Belgrade, Serbia;1. Department of Environmental Studies, University of Maine at Presque Isle, 181 Main Street, Presque Isle, ME 04769 USA;2. Geology and Palaeogeography Unit, Faculty of Geosciences, University of Szczecin, ul. Mickiewicza 18, 70-383 Szczecin, Poland;3. Department of Earth and Atmospheric Sciences, University of Nebraska–Lincoln, Lincoln, NE 68588-0340 USA |
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| Abstract: | Absolute chronologies in paleoceanographic records are often constructed using the 14C dating of coarse fraction foraminifera (>150 μm). However, due to processes such as changes in sediment sources or abundances, sedimentation rates, bioturbation, reworking, the adsorption of modern carbon, etc., several studies conducted in different environmental settings have shown time-lags between records obtained from various granulometric fractions. In this study, we examined temporal phasing between the coarse foraminifera and fine fractions by studying changes in the abundances of δ18O, the 14C ages of the planktonic foraminifera Globigerinoides ruber (G. ruber, 250–350 μm), and the sediment fine fraction (<63 μm) over the last 45 ka in a core obtained from the northern Caribbean Sea. All of the records were found to be in phase during part of the Holocene (at least for the last ≈6 ka). As determined from δ18O records and 14C ages, the fine fraction was younger than G. ruber during the Last Deglaciation (of 1.89 ka). The coupling between bioturbation and changes in the fine fraction, and G. ruber abundances, as tested using a numerical model of the bioturbation record within a mixed-layer depth of 8 cm, was sufficient to explain the results. 14C age discrepancies increased from 5.64 to 8.5 ka during Marine Isotopic Stages (MIS) 2 and 3, respectively. These chronological discrepancies could not be explained by only one process and seemed to result from the interplay between mechanisms: size-differentiated bioturbation (for 1.5 to 2.5 ka), the adsorption of modern atmospheric CO2 (for 3.04 to 5.92 ka), and variations in sedimentological processes that influenced the fine carbonate fraction. However, even if variations in the mineralogical composition of the fine carbonate fraction were identified using scanning-electron microscopy observations, X-ray diffraction measurements, and geochemical analyses (the mol % MgCO3 of magnesian calcite and the Sr/Ca ratio of the bulk fine fraction), they can not account for the observed age differences. The results presented for core MD03-2628 extend beyond this case study because they illustrate the need for a detailed characterization of the various size fractions prior to paleoclimate signal interpretations, especially for chronological studies. |
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| Keywords: | Planktonic foraminifera Fine fraction Caribbean Sea Radiocarbon Bioturbation Modern carbon contamination Sedimentology |
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