Mass extinctions of life and catastrophic flood basalt volcanism.

E xtinctions have played an important role in the history of life by clearing out niches and fostering adaptive radiations. Major mass extinctions involving 70% to more than 90% of extant species occurred at least five times during the last 540 million years. The discovery by Alvarez et al. (1) that the end-Cretaceous (65 Mya) mass extinction coincided with evidence for the impact of an asteroid or comet ∼10 km in diameter focused interest in the causes of the other mass extinctions. It was expected that evidence of a similar impact might be found at other mass extinction events. Such evidence, however, has been slow in coming (2). At the same time, episodic massive continental flood basalt eruptions were suggested as another possible cause of mass extinctions (3, 4). This connection is illustrated by a study by Whiteside et al. (5) that provides evidence that the eruption of the Central Atlantic magmatic province (CAMP) basalts, with a preserved volume greater than 1 × 10 km and covering more than 7 × 10 km, coincided with the endTriassic extinction event (ETE) (201.4 Mya) on land and in the oceans. The report by Whiteside et al. (5) presents carbon-isotope results obtained from leaf wax n-alkanes, wood, and total organic carbon from two nonmarine sections from the Newark and Hartford Basins in the eastern United States, which include the CAMP basalts and which are tightly constrained by magnetic reversals, orbital cycles, and pollen studies. The correlation utilizes the levels of the ETE and coincident carbon-isotope excursion and the Hettangian-Sinemurian boundary 1.8 Mya later, which bracket the CAMP episode. The sections are calibrated at high (20-ky) precision. These data are matched to orbitally forced carbon-isotope data from the marine St Audrie’s Bay, UK section, showing that the sharp initial negative carbon-isotope shift and extinction horizon are synchronous in marine and nonmarine sections. The oldest CAMP basalts in the Newark and Hartford Basins slightly postdate the extinction horizon (by ∼20 ky), but in similar Moroccan sections the basalts may be simultaneous with the extinction horizon (6, 7). As precise radiometric ages have become available, it has been determined that flood basalt episodes are brief and severe (with peak output of more than 1 million cubic kilometers over less than 1 million years, in most cases) (4). Two other major mass extinctions have been correlated with flood basalt episodes: the end-Cretaceous event (65 Mya) with the Deccan basalts of India and the endPermian event (251 Mya) with the Siberian basalts. However, the Deccan eruptions now are known to have started before the end-Cretaceous mass extinction/impact event, and the Siberian flows still are only roughly correlated with the end-Permian die off (4).