History of Research
Timing of the Traps and the Permo-Triassic boundary

Relative timing of the Traps and the P-Tr boundary at Meishan, China

40Ar/39Ar ages of sanidine feldspars from tuffs above (249.25±0.14 Ma, Reichow et al., 2009) and below (249.8±0.15 Ma, Renne et al., 1995) the Permo-Triassic boundary at Meishan, China suggest an age for the boundary of just less than 250 Ma. The age difference between these two tuff layers bracketing the P-Tr boundary is 0.58±0.21 myr, assuming no interlaboratory bias. This age is (within error) the same as most published 40Ar/39Ar ages of the basalts from Siberia

Several authors have proposed that the Siberian volcanism is synchronous with the P-Tr boundary and associated mass extinction (e.g., Campbell et al., 1992; Renne et al., 1995; Kamo et al., 1996, 2003; Reichow et al. 2002, 2009) and consequently inferred that the volcanism was responsible for the end Permian climatic changes. Figure 2 illustrates the relative difference between the timing of volcanism and the mass extinction, with negative values describing ages older than the reference age of Bed 25.

Figure Caption. Relative difference between the timing of Siberian Traps volcanism and the Permo-Triassic mass extinction. Age differences are calculated between Siberian basalts and gabbros, and the 40Ar/39Ar age of volcanic ash Bed 25 (Renne et al., 1995, recalculated to Fish Canyon Sanidine 28.02 Ma) with the peak extinction of 94% occurring at the base of this unit. Modified after Reichow et al. (2009).

The U/Pb (zircon) age of the Meishan and Shangsi tuffs is slightly older than the equivalent 40Ar/39Ar sanidine age. Taken separately, the two dating techniques - Ar-Ar and U-Pb - give internally consistent ages that suggest that the Traps are contemporaneous with the Permo-Triassic boundary at Meishan. The Ar-Ar method suggests that the boundary is approximately 250 million years old, whereas the U/Pb method suggests that the boundary is slightly older, at about 252 Ma. Which of these two absolute dates is correct, and the reasons for the discrepancy, are matters of debate (e.g. Min et al., 2000, Kuiper et al., 2008). The crucial point is that both techniques show that the Traps are, within the errors of the technique, contemporaneous with the age of the P-Tr boundary.