An extraterrestrial impact at the Permian-Triassic boundary?

Becker et al. (1) presented geochemical evidence that suggests that the largest mass extinction in Earth history, at the PermianTriassic 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 He and with distinctive He/Ar and Ar/Ar ratios, and that this signature therefore derived from a bolide impact. Here, we report that we are able to detect fullerene-hosted extraterrestrial He neither in aliquots of the same Meishan material analyzed by Becker et al., nor any in samples of a second Chinese PTB section, and that we thus find no evidence for an impact. Becker et al. reported helium in bulk rock and in fullerenes extracted from Meishan Bed 25 following acid demineralization. Their two aliquots of bulk rock yielded 0.43 and 0.58 pcc/g (10 cc g at standard temperature and pressure) of He. From 40 g of rock, Becker et al. extracted 14 mg of fullerene that yielded very high He concentrations, implying that fullerenehosted helium accounted for at least 0.052 pcc/g of the He in Bed 25; this number could be higher, because Becker et al. provided no indication of fullerene extraction efficiency. We first analyzed 15 aliquots of bulk rock from Bed 25, provided by S. Bowring to be representative of the material he supplied to Becker et al. Samples were initially dried in an oven for 2 hours at ;90 to 100 °C to drive off adsorbed water. Based on stepped-heating results on fullerenes (1), no He would have been lost during sample drying. We then gently powdered 150 g of rock by hand with a mortar and pestle and thoroughly homogenized the sample. Ten aliquots (;350 mg each) were drawn from this homogenized powder; the remaining five aliquots were taken from several different clumps of the material to assess spatial heterogeneity. Samples were fused under vacuum at 1400°C following procedures reported earlier (5), except that the acetic acid step, designed to remove CaCO3, was not used on these carbonatepoor rocks. None of these samples yielded a significant amount of He (Fig. 1): The mean of the 15 runs was 0.005 pcc/g, and the maximum for any single aliquot was only 0.01 pcc/g. We obtained similar results from six samples of the stratigraphically equivalent bed at Shangsi, China (also provided by Bowring). Hence, we obtained He concentrations from bulk rock samples that were a factor of 45 to 150 lower than those reported by Becker et al. To ensure that we were quantitatively extracting all the He at 1400°C, we outgassed a single sample at 1800°C after fusion at 1400°C; no additional He was released. We then demineralized a 16 g aliquot of Meishan Bed 25, following the same HFBF3 digestion procedure (6 ) used by Becker et al. This residue contained only 0.003 pcc of He per gram of starting material. Because the demineralized residue does not contain significant He, fullerenehosted He within this residue cannot be significant either, so we did not isolate fullerene for noble gas analysis. This experiment places an upper limit on the fullerene-hosted He in Bed 25 that is a factor of 15 lower than the concentration reported by Becker et al. (1). The helium we obtained from Bed 25 samples is reasonable for a 250-million-year-old volcanic ash bed. Large inter-aliquot variability in He content and the survival of most He through HF demineralization (Fig. 1) suggest that accessory zircons, known to exist in Bed 25 (3), control the distribution of this isotope. The He concentration and He/He ratio (average ,0.003 RA) of Bed 25 are lower than we obtained from several hundred deep-sea carbonate sediments [see, e.g., (5)] and are at the low end of the range expected for purely terrestrial radioactive decay processes (7). The dearth of He from interplanetary dust particles (IDPs)—not to be confused with a fullerenehosted impact signature—is not surprising, because Bed 25 is a volcanic ash and was likely deposited quickly. We thus find no evidence for the impact-derived He reported by Becker et al. Our analytical technique for He is as sensitive and precise [see details in (5)] as that used by Becker et al., so the discrepancy between our results and theirs is probably not analytical in origin. Sample heterogeneity is also an unlikely explanation: Although Becker et al. found substantial He in all three aliquots they analyzed (a total of 41 g of rock), we were unsuccessful in detecting extraterrestrial He in any of our 22 aliquots (150 g of homogenized Bed 25 in 10 aliquots, 1.5 g of spatially distributed spot samples in five aliquots, and 16 g of demineralized rock in one aliquot from Meshian, as well as 2 g of rock in six aliquots from three samples of the Shangsi P-Tr boundary bed). Without confirmation of fullerene-hostFig. 1. He isotope data for Chinese PTB samples. Filled symbols, Becker et al. (1); open symbols, this study. T E C H N I C A L C O M M E N T S