Timing of Permian-Triassic Anoxia

Abstract

Yukio Isozaki outlines thresholds in deepening oceanic anoxia and subsequent oxygenation across the Permian-Triassic boundary (Reports, 11 Apr., p. 235). He implies that mass extinction at this time was a product of oceanic stagnation many millions of years in the making. There is, however, continuing uncertainty about the position of the Permian-Triassic boundary in the Japanese section studied. That section lacks critical ammonites and conodonts, or even geochemical markers, such as the change in carbon-13 isotopic lightening, that characterize the boundary elsewhere (1, 2). The Permian-Triassic boundary could have equally preceded or coincided with the onset of “climax superanoxia,” rather than postdated it, as proposed in Isozaki's stratigraphic section. If “climax superanoxia” postdated the Permian-Triassic boundary, as in the best documented sections elsewhere (1), then there are alternative hypotheses to long-term descent into “superanoxia” and recovery. Instead, Late Permian anoxia could have been related to an extinction event during the Late Permian (end-Guadalupian extinction) (3), with “climax superanoxia” and delayed recovery forced by the greatest of all mass extinctions at the end of the Permian (end-Changxingian extinction) (1). Oceanic anoxia could then have been a consequence of death, decay, and boom-or-bust population cycles (4) forced by other agencies (5), such as voluminous volcanic eruptions (6), impact of an unusually large bolide (7), or both (8).