How fast do mass extinctions occur

Do mass extinctions occur within days, weeks, months, or years? Or do they represent clusters of independent extinction episodes occurring over hundreds of millennia? It is difficult to read rates of extinction directly from the fossil record (Benton 1994). The existing evidence gives a mixed message about the rate at which mass extinctions occurred: some evidence points to protracted extinction episodes; other evidence suggests sudden extinction. An immediate problem poses itself: how sudden is sudden.? Some researchers think that sudden means sudden - a year or so (McLaren 1988). The discovery of a marker horizon at the Cretaceous-Tertiary boundary gave strong support to this view of suddenness: here was a process that may be able to stress the biosphere within days (Alvarez et al. 1980). Some geologists took the marker horizon as concrete evidence that the terminal Cretaceous extinction event was indeed geologically instantaneous and caused by an asteroid's colliding with the Earth. The association of impact-event signatures within boundary layer sediments lends much weight to the view that impacts did occur at the same time that the boundary-layer clay formed. Some boundary clays contain organic chemicals with a composition highly suggestive of a cosmic origin (e.g. Zhao and Bada 1989), and some contain glass spherules of probable impact origin (e.g. Claeys and Casier 1994). To be sure, some geochemical signatures do change suddenly at boundary events. An example is the carbon-isotope ratio at Permo-Triassic boundary (e.g. Wang et al. 1994; Rampino and Caldeira 2005). That does not mean that the impacts were necessarily the primary cause of the extinctions; they might simply have been the knockout blow.

The fossil record almost invariably does record long-lasting extinction episodes, especially for small marine animals. Investigations of many boundary sites show that mass extinctions occurred in a series of discrete steps spread over a few million years (stepwise extinction), and not in an instant. Mass extinctions during Late Ordovician, Late Devonian, and Late Permian times were long affairs in which tropical marine biotas, including stenothermal calcareous algae, declined greatly, and reef communities were decimated (Stanley 1988a, 1988b). In the classic Permo-Triassic boundary section in southern China, a rich collection of fossils and several datable ash bands enable a detailed study of extinctions. Jin Yugan and colleagues (2000) identified 333 species belonging to 15 marine fossil groups (including microscopic foraminifera, fusulinids, and radiolarians; rugose corals, bryozoans, brachiopods, bivalves, cephalopods, gastropods, trilobites, conodonts, fish, and algae). Of these, 161 species became extinct during a period of 4 million years before the close of the Permian (Figure 7.9). Extinction rates in particular beds amounted to 33 per cent or less. Immediately below the Permo-Triassic boundary, at the contact of beds 24 and 25 (extinction level B in the diagram), most of the remaining species disappeared, giving a huge rate of loss of 94 per cent at that level. Some species survived the 1 million years to extinction level C, but died out stepwise during that timeslice (Figure 7.9). Similarly, biostratigraphic analyses of Italian Permo-Triassic sections, which record an essentially continuous transition from the Palaeozoic to the Mesozoic, suggest that the extinctions took place within about 1 m in the lowermost Tesero horizon (Rampino et al. 2002). Based on estimated average sedimentation rates, this translates to less than 10,000 years, with faunal turnover occurring in less than 8,000 years.

The Late Triassic witnessed significant biotic decline, and the apparent mass extinction event at the Triassic-Jurassic boundary seems to be largely a consequence of stage-level correlation (Tanner et al. 2004). The most prominent faunal groups of the marine realm caught up in the end-Triassic extinction, including ammonoids, bivalves, and conodonts, seem to have experienced a gradual to stepwise extinction throughout the Norian, particularly during the middle to upper Norian, and Rhaetian. However, the terrestrial record of tetrapod and floral extinctions at the time is less clear, and may reflect substantial regional effects, rather than global events (Tanner et al. 2004).

Figure 7.9 The extinction of life at the end of the Permian in southern China. Stratigraphic ranges of fossil species (indicated by vertical grey lines) from the latest Permian to the Early Triassic in the Meishan sections projected onto the composite section. Species numbers are shown on the j^-axis. Fossil range scaled to time. Faunal change appears gradual except around 251.4 million years ago. A, B, and C indicate three previously proposed extinction levels. Source: Reprinted with permission from Y. G. Jin et nl. (2000) Pattern of marine mass extinction near the Permian-Triassic boundary in South China. Science 289, 432-5. Copyright © 2000 AAAS.

Different species

Figure 7.9 The extinction of life at the end of the Permian in southern China. Stratigraphic ranges of fossil species (indicated by vertical grey lines) from the latest Permian to the Early Triassic in the Meishan sections projected onto the composite section. Species numbers are shown on the j^-axis. Fossil range scaled to time. Faunal change appears gradual except around 251.4 million years ago. A, B, and C indicate three previously proposed extinction levels. Source: Reprinted with permission from Y. G. Jin et nl. (2000) Pattern of marine mass extinction near the Permian-Triassic boundary in South China. Science 289, 432-5. Copyright © 2000 AAAS.

The detailed pattern of Late Cretaceous extinctions also suggests a relatively gradual extinction-rate increase for many groups of organisms, followed by a catastrophe lasting a few tens of thousands of years. In the marine realm, the extinction of planktonic foraminiferal species spanned 300,000 years below, and some 200,000 to 300,000 years above, the Cretaceous-Tertiary boundary (Keller 1989; see also Keller et al. 1993). The dinosaurs might have suffered a gradual extinction (Williams 1994), but the Hell Creek Formation in eastern Montana and western North Dakota is strongly suggestive of a sudden extinction at the very end of the Cretaceous period (Sheehan et al. 1991; Fastovsky and Sheehan 2005).

To some extent, the rapidity of mass extinctions is a question of perspective. The timing of Pleistocene megafaunal extinctions varied on different continents (Barnosky et al. 2004). In northern Europe, Siberia, and Alaska, two pulses of extinction occurred, the first from about 50,000-25,000 years ago, and the second from about 12,000-9,000 years ago. In central North America, the megafaunal extinctions occurred from 11,500-10,000 years ago, in South America 15,000-10,000 years ago, in Africa 50,000-10,000 years ago, and in Australia 72,000-44,000 year ago. Does this spread of extinction dates count as a sudden loss? Does it suggest a single cause? Fossil hunters 50 million years from now would presumably rate the Pleistocene mass extinctions as 'geological instantaneous' and conclude that a one-off calamitous event caused it, unless of course modern libraries have somehow survived to help them solve the mystery.

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