I

Age (million years)

is the biggest mass extinction event of all and saw the demise of as many as 95 per cent of all species on Earth. A key question is what could have brought about such a huge loss of life. Evidence collected over the last decade reveals a model of global change in which normal feedback processes failed to cope, and the chemical and temperature balance of the atmosphere and oceans fell into catastrophic breakdown (Benton and Twitchett 2003; Rampino and Caldeira 2005). A similar collapse of the biosphere occurred in the other big mass extinctions and in some of the lesser ones.

Bolide impacts

Disaster in the biosphere wrought by an impact event is a popular explanation of mass extinction in some quarters (see Koeberl and Macleod 2002). The ramifications of a large-body impact are many and various - there are many paths to impact-induced mass extinction (Figure 7.4; see also Toon et al. 1994).

Since at least the seventeenth century, scientists have realized that a close encounter, or indeed a collision, between the Earth and a comet would have disastrous consequences. However, the idea was not entertained fully until the 1940s and 1950s. In 1942, Nininger wrote that the collision between the Earth and planetoids offers an adequate explanation for the successive revolutions of movements in the Earth's crust, and for the sudden extinction of biota over large areas as revealed by the fossil record (cf. p. 34). These ideas sparked little sober attention because, interesting though they were as speculations, they could not be tested. However, an improved understanding of the pattern of mass extinctions in the fossil record, and particularly the discovery of signs of post-impact fallout in the stratigraph-ical column, have led to a much fuller appreciation of the process of hypervelocity impact and its probable effects on ecosystems.

A large bolide impact would swiftly destroy regional faunas and floras and set in train climactic changes that, over months and years, would traumatize communities worldwide (Table 7.3). The bolide travelling at hypervelocity through the atmosphere and smiting the ground causes primary damage within minutes or hours of the impact. The extent of primary damage inflicted within the first hours depends on the size of the bolide. The 'lethal radius', in which all life is exterminated (apart perhaps from a few lucky individuals who happened to be in caves or deep burrows at the time) depends on bolide size. Within it, a blast wave creates enormous air pressures that at their peak would destroy forests and kill animals. Particularly vulnerable would be the large land vertebrates with a small ratio of strength to weight, a fact that has been used to explain the selective extinction of large dinosaurs at the close of the Cretaceous period. A wave of intense heat would also radiate from the site of impact, killing all exposed organisms, and triggering wildfires within the

Figure 7.3 (a) Proportion of genus extinction. The shaded section highlights the Cambrian and Early Ordovician band of very high extinction proportions. (b) Natural logarithms of the proportion of genus extinction. Time slices of predominantly high or low proportions define 'stratigraphic neighbourhoods'. The dashed line shows the natural log of 40 per cent genus extinction. (c) Proportions of genus origination and genus extinction. Black shaded shows intervals where extinction exceeds origination; pale grey shading shows intervals where origination outstrips extinction. Source: Reprinted by permission from R. K. Bambach, A. H. Knoll and S. C. Wang (2004) Origination, extinction, and mass depletions of marine diversity. Paleobiology, 30, 522-42. Copyright © The Paleontological Society.

Table 7.1 Possible causes of mass extinctions.

Ultimate cause

Proximate cause

Possible effects and examples

Cosmic causes

Single large impacts

Comet storms Radiation from supernovae

Large solar flares

Geological causes Geomagnetic reversals (with spin rate changes) Continental drift

Volcanism

Sea-level change

Arctic spill-over (release of cold fresh or brackishwater from an isolated Arctic Ocean

Shock-waves, heat-waves, wildfires, impact winters (shutting down of photosynthesis), super-acid rain, toxic oceans, superwaves and superfloods (oceanic impact) Same as above Direct exposure to cosmic rays and X-rays Ozone destruction and exposure the excessive amounts of ultraviolet solar radiation Exposure to large does of ultraviolet radiation, X-rays and photons

Ozone depletion

Increased flux of cosmic rays

Climate change:

Glaciations when continents encroach upon the poles

Aridity increase when continents move into low latitudes

Cold conditions (possible volcanic winter), acid rain, and reduced alkalinity of oceans, resulting from release of sulphur volatiles. Toxic trace elements. Climatic change from release of ash and carbon dioxide)

Loss of habitat

Ocean temperature falls by about 10°C

Atmospheric cooling and drought

Grand global dying:

Cretaceous-Tertiary event (L. W. Alvarez et al. 1980), but possibly in a stepwise manner (Smit et al. 1994) Stepwise extinction events: Cenomanian-Turonian, Eocene-Oligocene (Donovan 1987)

Sterilizes and kills organisms, causes mutations - selective mass extinctions (exposed animals, including shallow-water aquatic forms, but not plant life): possibly any event (Schindewolf 1963; Terry and Tucker 1968) Mass extinctions: events during magnetic reversals (Reid et al. 1978) and sporadic faunal extinctions (Hauglustaine and Gérard 1990)

Mass extinction: Late Ordovician, Late Devonian, Late Permian, Late Cretaceous (Whyte 1977) Global cooling: Late Ordovician, Late Devonian, Late Permian marine events associated with encroachment of landmasses on poles (Stanley 1988a, b) Extinctions because species find themselves in inhospitable climatic zones: many land plants died as India drifted northwards (Knoll 1984) Stepwise mass extinctions: end Cretaceous flood basalt eruptions (McLean 1981, 1985; Officer et al. 1987)

Mass extinctions of susceptible species (e.g. marine reptiles): Cretaceous-Tertiary event (Bardet 1994) Mass extinctions in marine ecosystems: Late Cretaceous event (Thierstein and Berger 1978)

Mass extinctions land: change of vegetation with drastic effect on large reptiles (Gartner and McGuirk 1979)

Table 7.1 Continued.

Ultimate cause

Proximate cause

Possible effects and examples

Salinity changes

Reduced salinity

Mass extinctions in marine realm:

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