A Flashback on the Dawn of the Meteorite Impact/Extinction Theory

Abstract

A long history preceded the publication of two papers (Alvarez et al. 1980; Smit and Hertogen 1980) on a hypothesis that was to change the way we think about mass extinctions. The idea of a ma− jor impact ending the reign of the dinosaurs had been launched earlier, albeit without a scrap of evidence. Many other hypotheses had been put forward and had fared no better by lack of supporting data. De Laubenfels (1956) introduced “one more hypothesis”, a meteorite impact, and suggested that the heat flash had killed off the dinosaurs. The problem with this and other hypotheses (i.e., diseases, egg predation, pituitary gland anomalies, oversize, over− specialisation, magnetic reversal, sea level changes, etc.) is that they account for only small group of (generally) terrestrial verte− brates, but that they do not explain the simultaneous extinction of marine life. De Laubenfels’s impact idea was inspired by the proximity in 1937 and 1941 of the 1km−sized asteroid Hermes, at only 1,6 times the Earth−Moon distance. He even estimated the frequency of large, 10 km−sized planetesimal collisions correctly: about 2–3 such impacts during the Phanerozoic, a number subsequently con− firmed by better observations and statistics (Dachille 1977; Grieve et al. 1995). Nobel laureate Harold Urey suggested in 1973 that geologists should be on the lookout for (micro)tektites at geological bound− aries, especially at the Cretaceous–Paleogene (K/Pg) boundary, but failed to find them (Urey 1973). Christensen et al. (1973) ana− lysed in detail the Stevns Klint K/Pg boundary clay by Atomic Absorption Spectroscopy (AAS) and found anomalously high concentrations of Cr and Ni. However, further than a comparison with anaerobic black shale enrichments they did not go, although the enrichments were much greater than those in black shales. In the 1960s we learned from Professor H.A. Brouwer (per− sonal communication 1969) that the craters on the moon might be volcanic. In our petrology classes we were taught that the Sudbury igneous body was a volcanic lopolith (Lowman 1992) and O’Keefe (1976) believed that tektites could be produced by lunar volcanoes. The Apollo lunar missions changed all that and generated detailed research of impacts and meteorites. Urey (1957), Shoemaker (1963) and others convincingly demon− strated that both the Moon and the Earth were saturated with im− pact craters, but that those on the Earth were largely eroded away. Impacts slowly came to be seen as “a matter of fact” in the geological record. In the early 1960s, in particular in the Chicago Fermi labora− tories, the analytical technique of Neutron Activation (INAA) (developed by George de Hevesy) was used for the detection of many trace elements, in particular iridium. Barker and Anders (1968) and Crocket and Kuo (1979) used this technique to esti− mate the accretion rate of cosmic matter. They could apply this estimate because it had previously been discovered that iridium in cosmic matter (meteorites) was orders of magnitude more abundant than in terrestrial crustal materials (~500 versus 0.02 ng/g). Iridium is a special element in INAA. It has a relatively large “neutron capture cross section”, which means that Ir ab− sorbs easily a neutron to become the radioactive isotope Ir. This Ir decays with a half life of 74 days emitting two easily identifiable gamma rays of 316 and 468 kev energy. Therefore, among the Platinum Group Elements (PGE), it is by far the easi− est to identify.