The Late Eocene 187Os / 188Os excursion: Chemostratigraphy,cosmic dust flux and the Early Oligocene glaciation |
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| Institution: | 1. Department of Geology and Geophysics, SOEST, University of Hawaii at Manoa, Honolulu, HI 96822, USA;2. Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA;1. LCABIE, U. Pau et Pays de l’Adour, CNRS UMR 525, Hélioparc, 64053 Pau, France;2. Université de Bretagne Occidentale, Institut Européen Universitaire de la Mer, Plouzané, France;3. Dept. of Earth Sciences, University of Oxford, Oxford, UK;4. IFREMER, Centre de Brest, Plouzané, France;5. University of Southern California, Los Angeles, CA 90089, USA;6. Dept. of Oceanography, University of Hawaii, Honolulu, HI 96822, USA;1. UMR CNRS 5276 LGLTPE, Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon, Bâtiment Géode, F-69622 Villeurbanne Cedex, France;2. UMR CNRS 5276 LGLTPE, Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon, 46 Allée d''Italie, F-69364 Lyon Cedex 07, France;3. UMR CNRS 5023 LEHNA, Université Claude Bernard Lyon 1, Bâtiment Forel, F-69622 Villeurbanne Cedex, France;1. Department of Geoscience, Princeton University, Princeton, NJ 08540, United States;2. Department of Earth and Planetary Science, Harvard University, Cambridge, MA 02138, United States;1. Dipartimento di Scienze, Università degli Studi della Basilicata, Campus Macchia Romana, Via dell''Ateneo Lucano 10, 85100 Potenza, Italy;2. Institute of Earth and Planetary Sciences, University College London and Birkbeck, University of London, United Kingdom;3. Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, United Kingdom;4. Dipartimento di Scienze della Terra, dell''Ambiente e delle Risorse (DiSTAR), Università degli Studi di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy |
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| Abstract: | High resolution records (ca. 100 kyr) of Os isotope composition (187Os / 188Os) in bulk sediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene 187Os / 188Os excursion and confirm that the Late Eocene 187Os / 188Os minimum, earlier reported by Ravizza and Peucker-Ehrenbrink Earth Planet. Sci. Lett. 210 (2003) 151–165], is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene 187Os / 188Os minimum can be placed at 34.5 ± 0.1 Ma in the marine records. In addition, two other distinct features of the 187Os / 188Os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. ± 0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and Early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO2. To determine if the hypothesis that enhanced cosmic dust flux in the Late Eocene was a cause for the 187Os / 188Os excursion can be tested by using the paired bulk sediment and leachate Os isotope composition; 187Os / 188Os were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites, and therefore do not yield a robust test of this hypothesis. Comparison of 187Os / 188Os records with high resolution benthic foraminiferal δ18O records across the Eocene–Oligocene transition suggests that 187Os flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decay of ice-sheets and deglacial weathering drove seawater 187Os / 188Os to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine 187Os / 188Os records are obscured by recovery from the Late Eocene 187Os / 188Os excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records. |
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