Impacts theory of mass extinctions
Catastrophes of global extent, which accompanied or were followed by mass extinction of species, could have been caused in history by the impacts of cosmic bodies, asteroids or comets. The impact of a sufficiently large body leads to mechanical and thermal destruction of an extensive territory and, in many cases, to the massive transport of dust particles into the atmosphere, causing prolonged changes in the light and temperature regime all over the planet. For example, the impact of a large meteorite causes destructive pressure waves, enormous tsunami, forest fires and subsequent acid rain or darkness as a result of dust and soot that destroy extensive ecosystems far away from the actual site of impact. It is not clear whether these phenomena lead to global warming or global cooling; it probably depends on whether cooling occurs as a consequence of shading of the surface of the Earth or warming as a result of the greenhouse effect. Study of impact craters on the Earth and Moon has confirmed that impacts of sufficiently large bodies on the surface of the Earth occur sufficiently frequently for this phenomenon alone to explain most of the mass extinctions distinguished in the paleontological record. It can be estimated that, approximately once every 400 thousand years, a body falls to the surface of the Earth and forms a crater at the site of impact with a diameter of 20 km and, once every 50 million years, a body falls forming a crater with a diameter of 100 km. If the frequency of formation of the individual size categories of craters is compared with the intervals between mass extinctions of a certain intensity (Fig. XXII.4), it follows that the impact of a body forming a crater with a diameter of 24 km should cause the extinction of approximately 5 % of species once every million years and the impact of a body forming a crater with a diameter of 60 km should cause the extinction of approximately 50 % of species once every 88 million years (Raup 1992). These figures follow from a certain unrealistic overestimation of the assumption that all the mass extinctions are the result of the impact of cosmic bodies. Nonetheless, it seems quite realistic, either from the aspect of the frequency of impacts of cosmic bodies and also from the viewpoint of their probable effect on biodiversity (Jetsu & Pelt 2000; Sepkoski 1989).
At the present time, most impact craters are under the surface of the sea and the craters that are located on the continents have been destroyed to a major degree by erosion. Nonetheless, a number of craters have been discovered whose age corresponds very well with the dating of a mass extinction. The Manson crater in Iowa is an example. This crater has a diameter of 32 km and is 65 million years old, the same age as the mass extinction that occurred at the end of the Mesozoic and beginning of the Tertiary. The Chicxulub crater, which is of the same age, has a diameter of approximately 300 km and is buried in the Yucatan Peninsula on the coast of Mexico (Sharpton & Marin 1997; Schuraytz et al. 1996). Possibly more than 50% of species became extinct at the end of the Mesozoic and beginning of the Tertiary, and this extinction apparently affected all types of ecosystems over the whole surface of the Earth (Raup 1994). Almost all of the big five extinctions and a great many less extensive mass extinctions are now related to an impact crater of the corresponding age.
In addition to impact craters, there are also further indications of the connection between mass extinction and the impact of a cosmic body. The most convincing of them is the occurrence of the iridium anomaly, i.e. the occurrence of layers with iridium concentrations that are elevated by more than an order of magnitude; this element occurs in only trace amounts on the Earth, while it is far more frequent in some cosmic bodies. The best known iridium anomaly was described in 1980 by the Nobel-prize winner L. W. Alvarez and his coworkers at the boundary between the Mesozoic and Tertiary (the KT-boundary) (Alvarez et al. 1980) It is interesting that this team was originally looking for evidence for the hypothesis that mass extinctions do not, in actual fact, exist and that the discontinuities in the compositions of the fauna and flora between some layers are a result of the fact that sediments were not deposited for a long time at the particular site. Only a minimum of iridium remained in the surface rocks of the Earth after cooling of the planet so that practically all of this element found in sediments is derived from meteorites or comets. Alvarez assumed that, even at times when no classical sediments were deposited, iridium was nonetheless constantly deposited so that the iridium concentration would have to be elevated in the layers of sediments that were deposited abnormally slowly. However, the measured increase in the iridium content at the boundary between the Mesozoic and Tertiary was 2-3 orders of magnitude and was thus so great that it could not be explained by gradual constant accumulation of this element. The period for which classical sediments were lacking would have to have been too long. The only reasonable explanation of the iridium anomaly seems to be a sudden, short-term increase in the amount of iridium that fell over the entire surface of the Earth during its collision with a cosmic body, most probably the core of a comet, approximately 65 million years ago.
Over time, it was found that the iridium anomaly and other geological indications for the impact of a cosmic body (chock minerals, microtektites) also accompany some other periods of mass extinction, for example the anomaly in the Frasnian period in the Devonian, the Callovian in the Jurassic, the Cenomanian in the Cretaceous and two anomalies in the Tertiary (Rampino & Haggerty 1996; Rampino, Haggerty, & Pagano 1997). Some of these indications can be found in layers of the same age even at very distant locations, while other similar anomalies tend to have a local character.