Potential for accurate and precise radiocarbon ages in deglacial-age lacustrine carbonates |
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| Institution: | 1. Grup de Mineralogia Aplicada i Medi Ambient, Facultat de Geologia, Universitat de Barcelona, C/Martí i Franquès s/n, 08028 Barcelona, Spain;2. Grup d''Hidrologia Subterrània (GHS), Departamentd''Enginyeria del Terreny, Cartogràfica i Geofìsica, Universitat Politècnica de Catalunya—BarcelonaTech, JordiGirona 1-3, Mòdul D-2, 08034 Barcelona, Spain;3. Grup d''Hidrologia Subterrània (GHS), Instituto de Diagnóstico Ambiental y Estudios del Agua, IDAEA-CSIC, C/Jordi Girona, 18, 08028 Barcelona, Spain;4. Hydrogeology Group, Institute for Regional Development (IRD), University of Castilla-La Mancha (UCLM), Campus Universitario de Albacete, 02071 Albacete, Spain;1. Zuckerberg Institute for Water Research, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer 84990, Israel;2. Swiss Institute for Dryland Environmental and Energy Research, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990, Israel |
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| Abstract: | Lacustrine carbonate deposits (tufa) record variations in terrestrial hydrology and are preserved in many now-arid regions of the world, but are challenging to date with precision and accuracy. Many contain detritus and/or unsupported thorium (Th) that degrades or prevents the measurement of precise uranium-series (U-series) ages, and radiocarbon ages are frequently affected by both the reservoir (hard-water) effect and contamination with younger atmospheric carbon. The usual method of testing the accuracy of carbonate 14C measurements, comparison with U/Th ages or organic carbon dates, does not separate the reservoir and modern contamination effects, allowing for only relatively imprecise age estimates in samples undatable by U/Th.We have separated the modern contamination problem and the reservoir effect using a step dissolution technique on a variety of carbonate materials from Mono Lake, California, a long-lived closed-basin lake sensitive to regional precipitation variability. New dissolution experiments focus on the deglacial sediments of the Wilson Creek Formation, which preserve ostracodes and fans and mounds of thinolite, a cold-water, hydrated calcium carbonate (CaCO3.6H2O). Stepped-dissolution ages of thinolite crystals and dense calcites increased by 500–1000 years over the bulk age, and produced a plateau of analytically indistinguishable ages, indicating nearly complete removal of modern carbon. Repeated experiments on ostracodes from a single sample showed an increase of >3500 years over the bulk age and >6500 years from first to last step, but ages increased up to the last ～5% of the CO2 evolved, without forming a plateau. This may be due to the extremely large surface area-to-volume ratio of ostracodes, and inhomogeneous dissolution of the hundreds of individual shells required for the experiments. Further experiments are planned to test the effects of modern carbon on tufa, gastropods, and other shells, with the goal of systematically testing the precision and reliability of chronologies for dramatic changes in lake level in arid regions of the world. |
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