Extinctions Due To Exogenous Sources

Evidence for impact from the geologic boundary between the Cretaceous and Tertiary periods (the so called K/T boundary) is the Chicxulub crater in the Yucatan Peninsula in Mexico, and the global distribution of an anomalous iridium (Ir) layer (Keller and Stinnesbeck, 2000). In addition, impact ejecta such as pressure-shocked mineral grains support the meteorite impact hypothesis. Perhaps the better discussed evidence for the K-T impact at 65 Ma (million years ago) was the Ir-abundance coinciding with the geologic evidence of mass extinction. In spite of the very abundance of Ir in well-studied meteorites, the Ir-rich deposit may alternatively be interpreted as volcanic ejecta. In other words, the possibility remains that the layer could instead have been produced by volcanic Ir-rich eruptions.

The Permian period gave way to the Triassic at about 251 Ma. At that time the Earth experienced its greatest mass extinction known to us. Ninety percent of all marine species, including the trilobites, disappeared, while on land pervasive extinctions opened the way for the rise of the dinosaurs. But despite the magnitude of mass extinction its cause is a source of controversy (Kerr, 2001).

A new analysis of rock that marks the Permian-Triassic (P-T) extinction now suggests that it was caused by the hypervelocity impact of an asteroid or comet similar to the one thought to have led to the extinction of dinosaurs at the K-T boundary (Kaiho et al., 2001). There is some evidence for some catastrophic event that gave rise to the P-T extinction. Paleontologic evidence seems to suggest that a single event may have been responsible for the P-T transition. One such possibility shall be discussed in the next section.

Noble gases such as helium and argon apparently were trapped in molecular cages of carbon (fullerenes). This hypothesis follows the extraction of the gases from rocks at the P-T transition (Becker et al., 2001). Analyses of these gases show that their isotopic compositions are analogous to those found in meteorites, and are not typical of the Earth-bound abundances. This is some evidence that a major impact may have delivered the noble gases to Earth at the time close to the period when the extinctions did take place. Indeed, this suggestion provides an indicator for a P-T impact that is analogous to the earlier theory of the impact at the K-T boundary, an event that we saw above to have been supported by the Ir-data.

Fullerenes are also candidates for indicators of impact. Previous work by others showed that they are present in rock at the K-T boundary (Heymann et al., 1994). Together these findings suggested that fullerenes are the product of the high pressures and temperature generated in the collision and are impact markers like iridium. That prompted Becker and her colleagues to look for the compounds in rock at the P-T boundary at the in South China, and in southwest Japan and reported the detection of fullerenes in boundary rock, but not in similar rock a few centimetres to meters above, or below the boundary. However, it should be kept in mind that fullerenes can be produced by, for instance, forest fires.

In the case of the K-T mass extinction shocked quartz was detected, (i.e., crystals containing distinctive lamellae made only in the extreme pressures of large impacts). Shocked quartz has not been identified with the same certainty at the P-T transition, but the noble gas indicators may offer additional evidence. Fullerenes can trap gas atoms. When the gases trapped in fullerenes from P-T-boundary rocks was analyzed (Becker et al., 2001), it was found that the abundance of helium-3 was significantly enhanced above what it was immediately above or immediately below the boundary. The ratio of helium-3 to helium-4 was typical of meteorites. Besides the ratio of argon-40 to argon-36 in boundary fullerenes is likewise analogous to that of meteorites.

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