High-precision 40Ar/39Ar dating of pleistocene tuffs and temporal anchoring of the Matuyama-Brunhes boundary |
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| Institution: | 1. Scottish Universities Environmental Research Centre, Isotope Geosciences Unit, Rankine Avenue, East Kilbride, Scotland, G75 0QF, UK;2. Department of Earth & Environmental Science, School of Geography & Geosciences, University of St Andrews, St Andrews, KY16 9AJ, UK;3. Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA, 94709, USA;4. Department of Earth and Planetary Science, University of California, Berkeley, CA, 94720, USA;5. Research Laboratory for Archaeology and the History of Art, University of Oxford, Oxford, OX1 3QY, UK;6. Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research Office KR111, NASA Johnson Space Center, Houston, TX, 77058, USA;7. Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zurich, Clausiusstrasse 25, 8092, Zürich, Switzerland;8. Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, SY23 3DB, UK |
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| Abstract: | High-precision 40Ar/39Ar ages for a series of proximal tuffs from the Toba super-volcano in Indonesia, and the Bishop Tuff and Lava Creek Tuff B in North America have been obtained. Core from Ocean Drilling Project Site 758 in the eastern equatorial Indian Ocean contains discrete tephra layers that we have geochemically correlated to the Young Toba Tuff (73.7 ± 0.3 ka), Middle Toba Tuff (502 ± 0.7 ka) and two eruptions (OTTA and OTTB) related to the Old Toba Tuff (792.4 ± 0.5 and 785.6 ± 0.7 ka, respectively) (40Ar/39Ar data reported as full external precision, 1 sigma). Within ODP 758 Termination IX is coincident with OTTB and hence this age tightly constrains the transition from Marine Isotope Stage 19–20 for the Indian Ocean. The core also preserves the location of the Australasian tektites, and the Matuyama-Brunhes boundary with Bayesian age-depth models used to determine the ages of these events, c. 786 and c. 784 ka, respectively. In North America, the Bishop Tuff (766.6 ± 0.4 ka) and Lava Creek Tuff B (627.0 ± 1.5 ka) have quantifiable stratigraphic relationships to the Matuyama-Brunhes boundary. Linear age-depth extrapolation, allowing for uncertainties associated with potential hiatuses in five different terrestrial sections, defines a geomagnetic reversal age of 789 ± 6 ka. Considering our data with respect to the previously published age data for the Matuyama-Brunhes boundary of Sagnotti et al. (2014), we suggest at the level of temporal resolution currently attainable using radioisotopic dating the last reversal of Earths geomagnetic field was isochronous. An overall Matuyama-Brunhes reversal age of 783.4 ± 0.6 ka is calculated, which allowing for inherent uncertainties in the astronomical dating approach, is indistinguishable from the LR04 stack age (780 ± 5 ka) for the geomagnetic boundary. Our high-precision age is 10 ± 2 ka older than the Matuyama-Brunhes boundary age of 773 ± 1 ka, as reported previously by Channell et al. (2010) for Atlantic Ocean records. As ODP 758 features in the LR04 marine stack, the high-precision 40Ar/39Ar ages determined here, as well as the Matuyama-Brunhes boundary age, can be used as temporally accurate and precise anchors for the Pleistocene time scale. |
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| Keywords: | Matuyama-Brunhes Geomagnetic Toba Bishop tuff Orbital tuning Australasian tektite |
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