Bed 3 yields C. deflecta (Mei, 1996; Yao and Li, 1987,) and correlates with beds 26 to 27b of Meishan section. Bed 4 yields H. parvus (Mei, 1996). Wignall et al. (Yin et al., 2001) suggested that the extinction began in the middle part of Bed 4. Retallack (Wignall and Newton, 2003) and Twitchett et al. (2004), however, suggested that Wignall et al. may have mis-interpreted the post-extinction biota as pre-extinction biota. The abundance of stenotropic organisms in Bed 1 and their absence in Bed 3 suggest that mass extinction began at the end of Bed 1, before the onset of anoxia in Bed 4.  

2.4 High-Latitude Deep-marine Section at Guryul, Pakistan

The PTB section at Guryul Ravine, Kashmir, Pakistan was located at a paleolatitude similar to Selong. The uppermost Permian consists of deep-marine (bathyal) facies (Wignall et al., 2005; Kapoor, 1996). The section is lithologically subdivided into four beds (Kapoor, 1996; Twitchett, 1999; Fig. 4B). The basal Bed 1 consists of shale interbedded with limestone. The limestone contains a moderately diverse biota of Permian-type brachiopods, bivalves, foraminifers, gastropods, ammonoids, and conodonts. The overlying beds 2, 3, and 4 consist also of shale interbedded with limestone. Limestones in beds 2 and 3 contain ammonoids and conodonts, and those in Bed 4 ammonoids, conodonts, and bivalves.

The change from the diverse biota in Bed 1 to the simple biota in beds 2 and 3 signifies onset of extinction. Pyrite framboids in the upper part of Bed 1 and beds 2 and 3 (Wignall et al., 2005) indicate an anoxic environment. Bed 3 yields H. parvus. Mass extinction and anoxia started around the same time in this section. However, the onset of anoxia predated that in the shallow-marine Selong section.

Our analysis of five PTB sections, deposited in shallow and deep-marine environments in high and low paleoaltitudes, provides insights on the timing of oceanic anoxia and mass extinction. First, the onset of anoxia was diachronous, early (at the end of Clarkina yini conodont zone) in deep-marine area and later (at the beginning of H. parvus zone) in shallow-marine area. This confirms previous findings (Twitchett, 1999; Isozaki, 1997) that oceanic anoxia probably originated in deep-marine environment and then expanded to shallow-marine environment. The duration of anoxic water expansion can be approximated as the duration from the end of Clarkina yini zone to the beginning of H. parvus zone, which is less than 0.7 million years (Bowring et al., 1998; Rampino et al., 2000).

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