An extraterrestrial impact at the Permian-Triassic boundary?

Abstract

Becker et al. (1) presented geochemical evidence that suggests that the largest mass extinction in Earth history, at the Permian-Triassic boundary (PTB) 250 million years ago (Ma), coincided with an extraterrestrial impact comparable in size to the one that likely caused the end-Cretaceous extinctions 65 Ma (2). Although Becker et al. analyzed material from sections in Hungary, Japan, and China, the Hungarian section yielded no extraterrestrial signature, and their identification of the PTB in the Japanese section is questioned in the accompanying comment by Isozaki (below). Thus, only their analyses of the Chinese section provide hitherto uncontested evidence for an impact at the boundary—in the form of data on the abundance and composition of fullerenes in the “boundary clay,” a volcanic ash layer called Bed 25 at Meishan, China (3). Although fullerenes may be purely terrestrial [see, e.g., (4)], Becker et al. report that the fullerenes from the Meishan ash carry extraterrestrial noble gases in the cage structure, rich in ^(3)He and with distinctive ^(3)He/^(36)Ar and ^(40)Ar/^(36)Ar ratios, and that this signature therefore derived from a bolide impact. Here, we report that we are able to detect fullerene-hosted extraterrestrial ^(3)He neither in aliquots of the same Meishan material analyzed by Beckeret al., nor any in samples of a second Chinese PTB section, and that we thus find no evidence for an impact.