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      Abrupt global-ocean anoxia during the Late Ordovician–early Silurian detected using uranium isotopes of marine carbonates

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          Significance

          The Late Ordovician mass extinction (LOME) terminated one of the greatest biodiversity radiations in Earth history eliminating ∼85% of marine animals, and it is coincident with the first major glaciation of the Phanerozoic. To evaluate LOME origins, we use uranium isotopes from marine limestones as a proxy for global-ocean redox conditions. Our results provide evidence of an abrupt global-ocean anoxic event coincident with the LOME onset and its continuation after the biologic recovery, through peak glaciation, and the following early Silurian deglaciation. These results also provide evidence for widespread ocean anoxia initiating and continuing during icehouse conditions.

          Abstract

          Widespread marine anoxia is hypothesized as the trigger for the second pulse of the Late Ordovician (Hirnantian) mass extinction based on lithologic and geochemical proxies that record local bottom waters or porewaters. We test the anoxia hypothesis using δ 238U values of marine limestones as a global seawater redox proxy. The δ 238U trends at Anticosti Island, Canada, document an abrupt late Hirnantian ∼0.3‰ negative shift continuing through the early Silurian indicating more reducing seawater conditions. The lack of observed anoxic facies and no covariance among δ 238U values and other local redox proxies suggests that the δ 238U trends represent a global-ocean redox record. The Hirnantian ocean anoxic event (HOAE) onset is coincident with the extinction pulse indicating its importance in triggering it. Anoxia initiated during high sea levels before peak Hirnantian glaciation, and continued into the subsequent lowstand and early Silurian deglacial eustatic rise, implying that major climatic and eustatic changes had little effect on global-ocean redox conditions. The HOAE occurred during a global δ 13C positive excursion, but lasted longer indicating that controls on the C budget were partially decoupled from global-ocean redox trends. U cycle modeling suggests that there was a ∼15% increase in anoxic seafloor area and ∼80% of seawater U was sequestered into anoxic sediments during the HOAE. Unlike other ocean anoxic events (OAE), the HOAE occurred during peak and waning icehouse conditions rather than during greenhouse climates. We interpret that anoxia was driven by global cooling, which reorganized thermohaline circulation, decreased deep-ocean ventilation, enhanced nutrient fluxes, stimulated productivity, which lead to expanded oxygen minimum zones.

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          Most cited references53

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          Geochemistry of oceanic anoxic events

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            Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period

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              Uranium in the oceans: Where it goes and why

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                Author and article information

                Journal
                Proc Natl Acad Sci U S A
                Proc. Natl. Acad. Sci. U.S.A
                pnas
                pnas
                PNAS
                Proceedings of the National Academy of Sciences of the United States of America
                National Academy of Sciences
                0027-8424
                1091-6490
                5 June 2018
                21 May 2018
                21 May 2018
                : 115
                : 23
                : 5896-5901
                Affiliations
                [1] aEarth and Planetary Sciences, University of New Mexico , Albuquerque, NM 87131;
                [2] bDepartment of Geology and Geophysics, Louisiana State University , Baton Rouge, LA 70803;
                [3] cGeography, Earth and Environmental Sciences, University of Birmingham , Birmingham B15 2TT, United Kingdom;
                [4] dEarth and Environmental Sciences, University of Ottawa , Ottawa ON K1N 6N5, Canada
                Author notes
                1To whom correspondence should be addressed. Email: dolomite@ 123456unm.edu .

                Edited by Edward A. Boyle, Massachusetts Institute of Technology, Cambridge, MA, and approved April 10, 2018 (received for review February 21, 2018)

                Author contributions: M.E. designed research; R.B., M.E., J.R.W., and A.D. performed research; R.B., M.E., J.R.W., V.P., and A.D. analyzed data; and R.B., M.E., J.R.W., V.P., A.D., and Y.A. wrote the paper.

                Author information
                http://orcid.org/0000-0001-7861-8108
                Article
                201802438
                10.1073/pnas.1802438115
                6003337
                29784792
                38f27a90-877b-468d-ae5e-67887b11fafe
                Copyright © 2018 the Author(s). Published by PNAS.

                This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

                History
                Page count
                Pages: 6
                Funding
                Funded by: National Science Foundation (NSF) 100000001
                Award ID: 17-536
                Award Recipient : Maya Elrick
                Funded by: RCUK | Natural Environment Research Council (NERC) 501100000270
                Award ID: IMF518/0514
                Award Recipient : James R. Wheeley
                Categories
                Physical Sciences
                Earth, Atmospheric, and Planetary Sciences

                late ordovician extinction,u isotopes,seawater redox,ocean anoxic event,glaciation

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