Double jeopardy at the end of the Permian - new evidence for a large impact
The extinction at the end of the Permian was the greatest of all Phanerozoic extinctions. Although it has been widely assumed that it was the massive volcanic eruptions of the Siberian Traps that caused the extinction of some 90% of marine species and 70% of terrestrial species, new evidence for an end-Permian extra-terrestrial impact has been discovered off of the coast of Australia.
Although samples of probable impact-derived material at the Permo-Triassic (P-Tr) boundary have been found at several locations, no evidence of an actual impact site has been previously described. A team led by Luann Becker of UC Santa Barbara has recently discovered some compelling evidence for an impact structure named Bedout ("Bedoo") about 25 km from the Australian northwestern coast (Becker et al., 2004).
The first evidence for an impact structure in the Bedout area was based on petroleum exploration company seismic data described in 1996 by Australian petroleum geologist J. Gorter.
|Interpretation of an Australian Geological Survey seismic profile showing the central uplift (blue line) of the Bedout structure. The entire structure has a diameter of approximately 200 km, which is comparable to that of the K-T Chicxulub crater.|
It is evident that the Bedout structure was exposed on land during parts of the Triassic and Jurassic, and significant erosion may have taken place. (after Becker et al., 2004). The structure has since ben tilted as a result of differential subsidence.
|The Bedout structure has been intersected by
two petroleum exploration holes drilled in 1971 and 1983. Core and cuttings from
depths of over 3000 m in these holes have been examined by
Becker and colleagues, and in both cases strata from the P-Tr boundary rocks include strongly brecciated rock with abundant glass.
The glass has been subsequently altered to chlorite, carbonate and iron
oxides. As shown on the photo to the right, the Bedout breccia is
very similar in character to brecciated rock from the Cretaceous-Tertiary
boundary within the Chicxulub crater of Mexico.
40Ar/39Ar analysis has been used to date feldspar mineral concentrates from the drillholes. Some of the dates are younger than those of the overlying Triassic rocks, and this is attributed to alteration. Two of the least altered samples have dates of 250.1 + 4.5 m.y and 253 + 5 m.y, both of which bracket the P-Tr boundary.
There are several types of mineralogical evidence from the Bedout core samples that are consistent with a large impact event at that location. These include the overall glassy nature of the rock, the presence of feldspars with swallowtail terminations, which is indicative of very rapid crystallization, and the existence of spheroidal glass and extremely siliceous glass, both of which are consistent with impact melting and not with volcanism.
|Thin section from a breccia-zone sample showing feldspar laths (yellow-brown in plane light due to alteration). Under crossed-nicols the feldspars are black at all orientations, indicating that they have been converted into maskelynite (M), a glass that is produced when plagioclase is subjected to pressures of at least 30 GPa. Such presures could only have been produced under impact conditions (after Becker et al., 2004).|
As noted above, several P-Tr boundary sites with evidence of impact material had been found prior to the Bedout discovery. These include shocked quartz from sites in Antarctica and Australia, Fe-Ni-Si and Fe-Ni fragments and spherules at sites in China and Japan (Becker et al, 2004), fullerenes with evidence of extra-terrestrial gases in P-Tr sites in Japan and southern China (Becker et al, 2001), and evidence for a major release of sulphur from the mantle at a site in southern China (Kaiho et al., 2001).
|A shocked quartz grain from the P-Tr boundary at Fraser Park in Australia. The formation of structures of this type within quartz grains is peculiar to impact forces (after Becker et al., 2004).|
Becker et al. (2004) speculate on the relationships between the K-T and (postulated) P-Tr impacts and the coincident intense Deccan and Siberian flood-basalt eruptions. They conclude that the events are probably not related because in both cases the volcanism appears to have started before the impact, the volcanic centres are far away from the impact sites and the energy released from the impacts would not have been sufficient to trigger such eruptions.
Another possibility not put forward by Becker et al. is that the intense Deccan and Siberian basalt eruptions may have stressed populations to such a degree that the impacts then had the effect of decimating a majority of species. An implication is that other impacts of a similar size that were not coincident with flood volcanism might not have been as devastating.
Poreda, R, Hunt, A, Bunch, T and Rampino, M, 2001, Impact event at the
Permian-Triassic boundary: Evidence from extraterrestrial noble gases in
fullerenes, Science, V. 291, p. 530-533.
L, Poreda, R, Basu, A, Pope, K, Harrison, T, Nicholson, C and Iasky, R, 2004,
Bedout: a possible end-Permian impact crater offshore of northwestern Australia,
Science, V. 304, p. 1469-1476.
Kaiho, K and others, 2001, End-Permian catastrophe by a bolide impact: evidence of a gigantic release of sulphur from the mantle, Geology, V. 29, p. 815-819.
Steven Earle, Malaspina University-College, 2004. Return to Earth Science News