Conclusions

Of the ten well-dated episodes of major flood-basalt eruptions or LIPs, three are closely correlated with major mass extinctions (end-Permian, end-Triassic, and end-Cretaceous) and four with minor ones (late Permian, early Jurassic, mid Creta ceous, and early Palaeogene). There is also a strong correlation with episodes of oceanic anoxia or dysoxia, although in only four instances is anoxia regarded as the proximate cause of the marine extinctions (end-Permian, early Toarcian, end-Cenomanian, and late Palaeocene).

So although Courtillot was over-optimistic in calling the correlation between LIPs and mass-extinction episodes 'almost perfect' it is indeed good. The correlation is best for the four events in the middle part of the Phanerozoic, from the Emeis-han to the Karroo-Ferrar. On the evidence of the radiometric dates, the onset of the eruptions or the interval preceding them is associated with the most damaging environmental change. One such change may be widespread uplift. The links between volcanism and extinctions after the Jurassic appear to be much more tenuous. There appears to be no simple correlation between the volume of the eruptive rocks and the intensity of extinctions. The rapidity of the eruptions may possibly be a more significant factor. The failure to find evidence for cooling events triggered by volcanism may be due to the short residence time of sulphate aerosols in the atmosphere (Fig. 8.2). Episodes of global warming, on the other hand, show a good correlation, which is best for the end-Permian, mid-Cretaceous, and early Palaeogene events, but less certain for those of the end-Triassic and early Toarcian. If the eruption of the Deccan Traps had little to do with the end-Cretaceous mass extinction, as is argued here, it is a remarkable coincidence in time. The lack of LIPs older than the Permian is puzzling. It could perhaps relate to the plate-tectonic regime in the earlier Phanerozoic, with a lack of major rifting episodes and continents tending to converge on each other rather than split apart.

We can now integrate the knowledge acquired from reviewing the various likely causes of mass extinctions to see if any general pattern begins to emerge, and we can also consider the subsequent recovery of the organic world after the extinction episodes.

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