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A Marine Focus
Causes: Impact

Becker et al. reported an anomaly in 3He trapped within fullerenes from rocks at the PTB from Japan and China, suggesting extraterrestrial impact as the cause of the PTB mass extinction. While examining 3He signatures is a useful tool, the stratigraphy of the Sasayama section in Japan is such that this tool is useless: The PTB horizon is missing from this section. The "3He-enriched" samples analyzed have actually come from at least 0.8 meters below the PTB. (Isozaki, 2001)

Another potential flaw in impact theories comes from the analysis of fullerenes. Analyses of the Meishan, China section, a volcanic ash layer called Bed 25, are thought by some to provide evidence for an impact at the boundary in the form of data on the abundance and composition of fullerenes in the boundary clays. Becker et al. report that the fullerenes from the Meishan ash carry extraterrestrial noble gases such as 3He with distinctive 3He/36Ar and 40Ar/36Ar ratios and that this signature is therefore derived from a bolide impact. However, fullerenes may be purely terrestrial (Heymann, 1996) and researchers such as Farley et al. report that they are unable to detect fullerene-hosted extraterrestrial 3He in Meishan material analyzed by Becker et al., nor any in samples of a second Chinese PTB section, and thus find no evidence for an impact. (Farley, 2001)

Figure: Fullerene containing trapped atom