DO URANIUM ISOTOPES OF MARINE LIMESTONES PROVIDE EVIDENCE FOR SEAWATER ANOXIA AS A COMMON DRIVER FOR PHANEROZOIC MASS EXTINCTIONS?

Widespread marine anoxia has been implicated in 4 of the ‘big 5’ Phanerozoic mass extinction events. Evidence for anoxia as the main extinction driver has been based on lithologic, biologic, or geochemical tracers which record local redox conditions. To test the hypothesis that global marine anoxia coincided with mass extinction events, we analyzed U isotopes (δ²³⁸U) from marine limestones across 3 of the big 5 extinctions.

The Late Ordovician mass extinction was studied at Anticosti Island, Canada, the Late Devonian Kellwasser (KW) extinction events at Devil’s Gate, Nevada, and the end-Permian extinction at Daxiakou, South China. δ²³⁸U trends from the Late Ordovician signal a rapid negative shift (more reducing seawater conditions) coincident with the onset of the second extinction pulse. These low values continue through the peak Hirnantian glaciation and into the early Silurian deglaciation indicating that major glaciation and associated eustasy had little effect on seawater anoxia. δ²³⁸U trends from the Late Devonian record a gradual negative isotopic shift coincident with the lower KW event, and higher isotopic values (more oxic conditions) during the upper KW event. These trends suggest that anoxia was involved in the lower KW, but that anoxia as the driver for the upper and larger KW extinction was limited to epeiric seas rather than global Late Devonian oceans. δ²³⁸U trends during the Middle Permian through Early Trissic indicate more oxic Permian conditions followed by a large, abrupt, negative shift coincident with the end-Permian extinction event and continuing into the Early Triassic; this supports previous Tethyan δ²³⁸U redox records.

These data, combined with recent δ²³⁸U results from the Triassic-Jurassic mass extinction, highlight the value of U isotopes in marine limestones as a globally integrated seawater redox proxy and that extensive seawater anoxia was a common factor for 4 of the big 5 Phanerozoic mass extinctions.