The Oceans Were Not Globally Anoxic During the Permian Extinction

Molybdenum isotopic evidence for oxic marine conditions during the latest Permian extinction

Authors:

1. Bernadette C. Proemse (a)
2. Stephen E. Grasby (a,b)
3. M.E. Wieser (c)
4. B. Mayer (a)
5. B. Beauchamp (a)

Affiliations:

a. Department of Geoscience, University of Calgary, 2500 University Drive, Calgary, Alberta T2N 1N4, Canada

b. Geological Survey of Canada, 3303 33rd Street NW, Calgary, Alberta T2L 2A7, Canada

c. Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada

Abstract:

The latest Permian extinction (LPE), ca. 252 Ma, represents the most severe extinction event in Earth's history. The cause is still debated, but widespread marine anoxic to euxinic (H2S rich) conditions, from deep to shallow water environments, are commonly suggested. As a proxy for marine oxygen levels, we analyzed δ98/95Mo of two LPE sections that represent a gradient in water depth on the northwest margin of Pangea. Results from deep-water slope environments show a large shift in δ98/95Mo values from −2.02‰ to +2.23‰ at the extinction horizon, consistent with onset of euxinic conditions. In contrast, sub-storm wave base shelf environments show little change in the molybdenum isotopic composition (−1.34‰ to +0.05‰), indicating ongoing oxic conditions across the LPE. These results indicate that areas of the continental shelf of northwest Pangea underwent mass extinction under oxic conditions throughout the LPE event, and that shallow-water anoxia was therefore not a global phenomenon.