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Project Progress

The major goal of my participation in this grant was to identify possible impact events at extinction boundaries in the geologic record, using ³He as a tracer (Farley 2001).

Earlier in this grant we (PI’s Ward and Farley and UW graduate student Garrison) completed a study (Farley et al. 2005) of the Permian/Triassic Boundary at Opal Creek, Canada, and showed that a) there is no evidence for any extraterrestrial ³He at this boundary, and b) the absence of extraterrestrial ³He is fully expected for sections (like this one) that have been evenly mildly metamorphosed. An important corollary of the latter is that it is very surprising that previous workers (Poreda et al. 2003) found extraterrestrial ³He at the Graphite Peak, Antarctica section. Despite having experienced elevated temperatures from extensive local and regional volcanism, these workers suggest ³He is preserved in both fullerenes and cosmic dust. We propose an alternative explanation: the ³He is a late-stage contaminant added to the rocks after metamorphism.

Ultimately this work provides no evidence against an impact at the Permian/Triassic boundary, but it does refute evidence in favor of impact.

In this year of the grant we moved forward on two fronts.

1. Meishan Permian/Triassic Blind Sampling

In the summer of 2005 Frank Kyte and several others (including Luann Becker) undertook a very detailed resampling of the best-studied Permian/Triassic boundary in the world, at Meishan, China. Care was taken to sample every single bed in the vicinity of the boundary. In addition all unusual bedding planes (thin clay seams) were also sampled. Very large samples were acquired, carefully homogenized, and distributed blindly to about 15 different investigators. I analyzed all 19 of the consortium samples for He concentration and isotopic composition. I detected no extraterrestrial ³He in any of the samples. These results are consistent with our earlier work (Farley et al. 2001) but again are contrary to the published results of Becker et al. (Becker et al. 2001).

The consortium will meet sometime this winter to discuss the results and prepare a publication.

2. Triassic/Jurassic Boundary

Previous evidence for an impact at the Triassic/Jurassic Boundary includes shocked quartz (Bice et al. 1992) and a small iridium anomaly (Olsen et al. 2002). To further investigate the possible role of impact at this time we (Ward and Farley) undertook a detailed study of He concentration and isotopic composition across the T/J boundary in Muller Canyon, in the New York Mountains of Nevada. The results are shown in Figure 1.

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The major features of this data are 1) the clear presence of elevated ³He concentrations (i.e., >0.05 on this plot), 2) the spike in ³He at 0 meters, i.e., a little before the extinction boundary; and 3) the very steady decline in ³He through the latest Triassic.

The ³He/⁴He ratios of the most ³He-rich samples indicate the presence of ³He from a source other than radioactive decay. There are two possibilities: extraterrestrial helium and cosmic-ray spallogenic helium. The latter seems somewhat implausible given the very systematic pattern in Figure 1 along with the expected poor retentivity of ³He in bulk matter of this fine-grained mudstone. However it cannot be ruled out.

Analyses of the magnetic fraction and the thermal release spectrum of the ³He were undertaken on several different samples from this section, but the results proved ambiguous.

Currently we believe the only way to resolve this issue is to resample the section, especially the Triassic part, in such a way that cosmic-ray exposure can be eliminated. In practice this will involve digging into the section more than ~60 cm to expose non-surficial rock. Ward and Farley plan to undertake this work in the fall as a last project under this grant.