Plume winter scenario for biosphere catastrophe: The Permo-triassic boundary case

Abstract

The Permo-Triassic boundary (P-TB) interval is unique in having experienced three unusual geologic phenomena on a global scale that had never occurred in the rest of the Phanerozoic, viz., 1) the greatest mass extinction, 2) development of superanoxia (nearly 20 million year-long anoxia), and 3) initial breakup of the supercontinent Pangea. The so-called end- Paleozoic mass extinction was double-phased, one at the Guadalupian-Lopingian boundary (G-LB; ca. 260 Ma) and the other at the Changhsingian/Induan boundary (P-TB sensu stricto; ca. 252 Ma). The coincidence in timing between the G-LB mass extinction, the onset of superanoxia, and the probable initial rifting of Pangea strongly suggests their causal link. This article reviews current understanding of the end-Paleozoic event, and proposes a possible scenario to explain these three unique geological phenomena in a cause-effect context. Violent volcanism is favored by many on the basis of the apparent coincidence in timing between the two Late Permian mass extinctions and the volcanism of continental flood basalts (CFB). However, the only available material-based evidence for putative violent volcanism is the felsic tuffs in the Upper Permian in South China and Japan that appear inconsistent with the CFB link. The end-Paleozoic global environmental change was likely triggered by the episodic activity of a mantle plume, not through basaltic but rhyo-dacitic volcanism. The felsic nature of the volcanism suggests not only highly explosive eruption but also extensive delivery of air-borne ash. Alkaline magmatism derived directly from a mantle plume, may have played an important role both in the G-LB and P-TB extinction-relevant environmental turmoils. The "Plume Winter" scenario is proposed here to explain the unique geological phenomena of the end-Paleozoic event. When a plume head hits the bottom of a pre-existing continental lithosphere, alkaline volcanism of felsic to intermediate composition, often accompanying kimberlite/carbonatite, occurs in a highly explosive manner prior to a voluminous CFB eruption in a later stage. The violent volcanism directly causes 1) extensive ash fall, 2) formation of a dust/aerosol-screen in the stratosphere, and 3) acid rain, and these are followed by severe destruction of the photosynthesis-supported global food web both on land and in sea through 4) less insolation and 5) low temperature.Asharp decline or cessation of the primary production may have triggered cascaded extinctions of marine and terrestrial heterotrophic animals. In the aftermath, increase in atmospheric CO2 by volcanism drives global warming, prolonged anoxia, and delayed biotic recovery. "Plume Winter" initiated by the uprising African superplume may have caused the end-Paleozoic double-phased mass extinction and the superanoxia. The G-LB event appears to have been more important than that at P-TB, because everything geologically significant started at that time, viz. the initial breakup of Pangea, mass extinction, and superanoxia. This is also supported by the remarkable turning point of the Phanerozoic in the global sea level, initial 87Sr/86Sr ratio of seawater, and geomagnetic field behavior. The major re-organization of the mantle plume structure from the Pacific to the Atlantic regimes corresponds to the largest turning point in the Phanerozoic history of life from the Paleozoic to the Mesozoic-Modern regime. © 2007 Springer.