Marine redox conditions varied greatly in time and space during the latest Permian-Early Triassic global crisis. Although models of a pervasively anoxic deep ocean have been proposed, reevaluation of oceanic sections suggests a modest shift toward mainly suboxic conditions on the deep seafloor accompanied by strong expansion of the oxygen-minimum zone at intermediate water depths¹. In contrast to the relatively stable redox conditions in the deep ocean, shallow-marine sites were subject to frequent shifts between oxic and euxinic conditions for at least a million years². Such fluctuations resulted from recurrent expansion of a shallow oxygen-minimum zone into the ocean-surface layer in response to several types of environmental perturbations. First, global climates warmed abruptly during this interval, leading to lower dissolved oxygen levels in seawater and a reduced latitudinal temperature gradient that suppressed overturning circulation and deepwater formation. Second, enhanced continental weathering³ led to erosion of soil organic matter and higher nutrient fluxes, stimulating marine primary productivity and the sinking flux of organic matter⁴. This combination of perturbations produced redox conditions that were toxic to marine metazoans, resulting in the greatest mass extinction event of the Phanerozoic. The oceanic cesspool developed during the Permian-Triassic boundary crisis may provide insights concerning the consequences for modern oceans of global warming and nutrient flux changes wrought by human activities.

¹ Algeo, T.J., et al., 2010a. Changes in productivity and redox conditions in the Panthalassic Ocean during the latest Permian. Geology 38, 187-190.

² Algeo, T.J., et al., 2008. Association of ³⁴S-depleted pyrite layers with negative carbonate δ¹³C excursions at the Permian/Triassic boundary. Geochem. Geophys. Geosyst. 9, Q04025.

³ Algeo, T.J., Twitchett, R.J., 2010. Anomalous Early Triassic sediment fluxes due to elevated weathering rates and their biological consequences. Geology 38, 1023-1026.

⁴ Algeo, T.J., et al., 2011. Terrestrial-marine teleconnections in the collapse and rebuilding of Early Triassic marine ecosystems. Palaeogeogr. Palaeoclimatol. Palaeoecol. 308, 1-11.