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Sulfidic anoxia in the oceans during the Late Ordovician mass extinctions – insights from molybdenum and uranium isotopic global redox proxies

Dahl, Tais W.; Hammarlund, Emma U.; Rasmussen, Christian Mac Ørum; Bond, David P.G.; Canfield, Donald E.


Tais W. Dahl

Emma U. Hammarlund

Christian Mac Ørum Rasmussen

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David Bond
Palaeoenvironmental Scientist and Schools Liason Officer

Donald E. Canfield


The Late Ordovician Mass Extinction wiped out 85% of animal species in two phases (LOME1 and LOME2). The kill mechanisms for the extinction phases are debated, but deteriorating climate and the expansion of marine anoxia appear to have been important factors. Nevertheless, the spatial extent and intensity of marine anoxia and its temporal relationship with the extinctions are not well understood. Here, we review existing global paleoredox proxy data based on molybdenum (Mo) and uranium (U) isotopes from four paleocontinents combined with new Mo isotope data from Dob's Linn, Scotland. Individually, these sedimentary records demonstrate significant redox fluctuations, but our coupled dynamic oceanic mass balance model for the evolution of the marine Mo and U cycles reveals that globally expansive ocean anoxia is best constrained by δ238U in carbonates from Anticosti Island that record expansive anoxia during LOME2. In addition, we consider periodic sulfidic anoxia developing in well-ventilated parts of the shallow oceans (e.g. during warmer periods with greater solar insolation) to have produced temporarily high seawater δ98Mo values during LOME1 in accordance with trends to high values observed in the sedimentary records. In this view, oceanic oxygen loss had a causal role during both extinction phases in the Late Ordovician.


Dahl, T. W., Hammarlund, E. U., Rasmussen, C. M. Ø., Bond, D. P., & Canfield, D. E. (2021). Sulfidic anoxia in the oceans during the Late Ordovician mass extinctions – insights from molybdenum and uranium isotopic global redox proxies. Earth-Science Reviews, 220, Article 103748.

Journal Article Type Article
Acceptance Date Jul 20, 2021
Online Publication Date Jul 24, 2021
Publication Date Sep 1, 2021
Deposit Date Jul 23, 2021
Publicly Available Date Aug 19, 2021
Journal Earth-Science Reviews
Print ISSN 0012-8252
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 220
Article Number 103748
Keywords Paleoenvironment; Geochemistry; Global redox proxies; Hirnantian; Stable isotope fractionation
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Copyright Statement
Crown Copyright © 2021 Published by Elsevier B.V. This is an open access article under the CC BY license (

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