Causes of mass extinctions

Abstract

Both bolide impacts and  ood basalts have been suggested as the cause of major perturbations in the history of the biosphere, and the external (cosmic) and internal (Earth-bound) factors of mass extinctions were usually presented as competing paradigms (‘impactor’ contra ‘volcanist’ debate: Glen 1994, Frankel 1999). We present below some new evidence from the literature that both these ideas are not conclusive in their pure form and that both groups of factors (terrestrial and extraterrestrial) might have been interrelated, with some extinctions at least being the result of a cumulative effect of volcanism and impact. In a recent review of extinction problematics, Olsen (1999) presented the new data on the giant igneous event across the Triassic–Jurassic (T–J) boundary, and considered a bolide impact as a ‘viable alternative’ for a factor responsible for the T–J biotic turnover. On the other hand, many authors see both the Earthbound (volcanism, sea-level change, oceanic anoxia, etc.) and extraterrestrial mechanisms as jointly participating in a catastrophic environmental disturbance on a global scale; an impact is discussed and modelled as a trigger for paroxysmal magmatism (e.g. Boslough et al. 1996; Jones 2000). From this standpoint, even the end-Permian Siberian Traps might be induced by a violent impact (Frankel 1999, p. 155; see also new evidence from extraterrestrial noble gases in fullerenes in Becker et al. 2001). With respect to the best-studied extinction, at the Cretaceous– Tertiary (K–Pg) boundary, authors of the impact hypothesis (Alvarez et al. 1984) have already envisaged the ‘terminalCretaceous extinctions on two time scales: a slow decline unrelated to the impact and a sharp truncation synchronous with and probably caused by the impact’. As emphasized by MacLeod (1998), the K–Pg multicausal scenario is suggested by (1) the high number of bio-crises known to have occurred prior to the onset of impact debris, (2) diverse ecological characters and differing fates of known survivors, and (3) the group-speciŽ c biogeography of the biotic change. This opinion is supported more and more now by independent data on high-resolution biotic and climatic patterns (e.g. Li & Keller 1998; Zinsmeister 1998; but see Cadee 1999). Of major signiŽ cance is the recognition of the relative timing of impact and the Deccan Trap volcanism, due to identiŽ cation of an Ir-enriched K–Pg boundary layer sandwiched between basalt  ows (see summary in Courtillot et al. 2000). The presence of three lava  ows below this layer makes it clear that the overall brief period of volcanism (three chrons only) had already been active when the ejecta layer was deposited and the impact of the K–Pg bolide evidently did not trigger the Deccan volcanism, which started 0.5– 0.1 m.y. earlier. In fact, this observation can be expected in light of the current understanding of impact cratering processes and a minimal probability of impact-induced volcanism (Melosh 2000). Although some authors believe the K–Pg extinction could have resulted from environmental stress even without signiŽ cant extraterrestrial addition (e.g. Li & Keller 1998), others (Sutherland 1996; Chatterjee 1997; Hallam & Wignall 1997; Courtillot 1999; Keller 2000) emphasize contemporaneity of endogenous and exogenous events precisely at the boundary. Notably, many workers believe the mantle plume event is not a terrestrial alternative for the end-Cretaceous extraterrestrial catastrophe scenario. A combination of longer-term abnormal hot-spot/hotline deep-plume volcanism, catastrophically Ž nalized in possibly multiple K–Pg impact episodes (see Keller 2000), is favoured in Sutherland’s (1996) integrative model. The end-Cretaceous global crisis is seen as a cumulative effect of gradually induced extinction pressures, in response to a prolonged vanishing of the Aptian to Maastrichtian greenhouse period, promoted by the mantle superplume activity (Pele hypothesis of Landis et al. 1996). Similar conclusions are being formulated for some other Phanerozoic mass extinctions (see Walliser 1996; Hallam & Wignall 1997):