Temporal Pattern of Oceanic Anoxia across the Permian-Triassic Boundary

ABSTRACT  In the shallow marine Permian-Triassic boundary (PTB) sections in Ziyun, Guizhou Province, southwestern China, Tesero, Dolomites, Italy, and Selong, Xizang (Tibet), China, the beginning of the end-Permian mass extinction is much earlier than the onset of oceanic anoxia, which occurred nearly synchronously at the bottom of the conodont Hindeodus parvus zone. The mass extinction in Ziyun began around at the end of the conodont Clarkina yini zone. In the deep marine PTB sections in Meishan, Zhejiang Province, southeastern China and Kashmir, Pakistan, the onset of oceanic anoxia is about the same time as or slightly earlier than the beginning of mass extinction. In Meishan, the oceanic anoxia and mass extinction both began at about one or two conodont zones below Hindeodus parvus zone. This shows that, regardless of latitudes, the oceanic anoxia originated in deep-marine areas at the end of Permian and expanded to shallow-marine areas in the earliest Triassic. The anoxic event postdated the beginning of mass extinction in shallow-marine areas and, thus, was not a major cause of end-Permian mass extinction

1. Introduction

The most severe mass extinction in the Phanerozoic occurred at the end of the Permian Period, 251.4 million years ago (Erwin, 1994; Bowring et al., 1998). It killed more than 90% of all marine species, 70% of terrestrial vertebrate genera, most land plants, and 100% reef-building calcisponges (Retallack et al., 1996, 2003; Wu, 2005). The extinction was probably abrupt (Rampino et al., 2000) and may have been caused by bolide impact (Basu et al., 2003), massive flood basalt volcanism (Renne et al., 1995), sea-level and climate changes (Erwin, 1994; Knoll et al., 1996), chemocline upward excursion (Kump et al., 2005), atmospheric oxygen drop (Weidlich et al., 2003), and/or oceanic anoxia (Wignall and Hallam, 1992; Isozaki, 1997; Wignall and Twitchett, 1996). Wignall and others showed that on shallow-marine shelves, both anoxia and extinction began in the latest Permian in the low latitude (Wignall and Twitchett, 2002), but later (late Griesbachian) in the high latitude (Wignall and Newton, 2003). In high-latitude deep-marine areas, however, anoxia and extinction began in the latest Permian (Wignall et al., 2004). They concluded that oceanic anoxia occurred diachronously, and extinction occurred synchronously with and was caused by the anoxia. Their conclusions were debated by others (Retallack, 2004; Twitchett et al., 2004). Our study of Permian-Triassic boundary (PTB) sections in Ziyun, Guizhou Province, southwestern China (Fig. 1) found that mass extinction began much earlier than oceanic anoxia. Our results and reinterpretation of four sections previously studied by other researchers suggest that the PTB oceanic anoxia postdated mass extinction in shallow-marine areas.