"Rapid expansion of oceanic anoxia immediately before the end-Permian mass extinction" by Gregory A. Brennecka, Achim D. Herrmann, Thomas J. Algeo & Ariel D. Anbar, PNAS 108(43)

Proceedings of the National Academy of Sciences, Vol. 108, No. 43 (October 25, 2011) 17631-17634; doi: 10.1073/pnas.1106039108

Rapid expansion of oceanic anoxia immediately before the end-Permian mass extinction

1. Gregory A. Brennecka^a,*, 
2. Achim D. Herrmann^a,^b, 
3. Thomas J. Algeo^c and
4. Ariel D. Anbar^a,^d

1. ^aSchool of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404;
2. ^bBarrett, the Honors College, Arizona State University, Tempe, AZ 85287-1612;
3. ^cDepartment of Geology, University of Cincinnati, Cincinnati, OH 45221-0013; and
4. ^dDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1404

1. Edited by Donald E. Canfield, University of Southern Denmark, Odense M., Denmark, and approved September 11, 2011 (received for review April 18, 2011).

Abstract

Periods of oceanic anoxia have had a major influence on the evolutionary history of Earth and are often contemporaneous with mass extinction events. Changes in global (as opposed to local) redox conditions can be potentially evaluated using U system proxies. The intensity and timing of oceanic redox changes associated with the end-Permian extinction horizon (EH) were assessed from variations in²³⁸U/²³⁵U (δ²³⁸U) and Th/U ratios in a carbonate section at Dawen in southern China. The EH is characterized by shifts toward lower δ²³⁸U values (from -0.37‰ to -0.65‰), indicative of an expansion of oceanic anoxia, and higher Th/U ratios (from 0.06 to 0.42), indicative of drawdown of U concentrations in seawater. Using a mass balance model, we estimate that this isotopic shift represents a sixfold increase in the flux of U to anoxic facies, implying a corresponding increase in the extent of oceanic anoxia. The intensification of oceanic anoxia coincided with, or slightly preceded, the EH and persisted for an interval of at least 40,000 to 50,000 y following the EH. These findings challenge previous hypotheses of an extended period of whole-ocean anoxia prior to the end-Permian extinction.

• *Correspondence e-mail: brennecka@asu.edu
• http://www.pnas.org/content/108/43/17631