EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Toward a quantitative framework for assessing the global severity of Oceanic Anoxic Events

Matthew Clarkson1, Timothy Lenton2, Claudine Stirling3,4, Alexander Dickson5, and Derek Vance1
Matthew Clarkson et al.
  • 1ETH Zurich, Geochemistry and Petrology, Earth Sciences, Switzerland (
  • 2Global Systems Institute, University of Exeter, Exeter EX4 4QE, UK
  • 3Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand
  • 4Centre for Trace Element Analysis, University of Otago, PO Box 56, Dunedin, New Zealand
  • 5Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, UK

Oceanic anoxia is a common response to past climate perturbations and often invoked as a direct cause of mass extinctions and faunal turnover events. During the Phanerozoic, there are numerous events that show qualitatively similar expressions of de-oxygenation, including ‘black shale’ development and distinct geochemical disturbances on global or local scales. These perturbations can be broadly grouped together as Oceanic Anoxic Events (OAEs), but their similarities, and differences, require greater quantification.

Advances in geochemistry over the last decade mean we are approaching a point where such a quantified comparison is possible. In particular, uranium isotopes (δ238U) have become established as an important tool for estimating the global extent of seafloor anoxia, overcoming the geographic limitations of relying on local proxy records. Typically, records from oxic marine carbonates, that are thought to track seawater trends, show negative U isotope excursions that reflect the preferential removal of isotopically heavy 238U into anoxic sediments. Here we present a compilation of δ238U datasets for a series of past climate perturbations, including the Permo-Triassic mass extinction, mid-Cretaceous OAE 2 and the PETM. In combination with a dynamic biogeochemical model, we explore the use of such datasets as a quantitative framework for comparing the ‘severity’ of OAEs. We highlight the strengths and weaknesses of the U isotope approach and outline important guidelines for considering δ238U records and the temporal relationship to other proxy datasets, such as δ13C and temperature.

How to cite: Clarkson, M., Lenton, T., Stirling, C., Dickson, A., and Vance, D.: Toward a quantitative framework for assessing the global severity of Oceanic Anoxic Events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2730,, 2021.

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