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COMMENTARY

Mass of life and catastrophic flood volcanism

Michael R. Rampino1 Department of Biology and Environmental Studies Program, New York University, New York, NY 10003

xtinctions have played an im- million cubic kilometers over less than 1 evidence of a crisis among calcareous portant role in the history of life million , in most cases) (4). Two organisms in the (11). E by clearing out niches and foster- other major mass extinctions have been The most likely source of greenhouse ing adaptive radiations. Major correlated with flood basalt episodes: the gases may be rapid release from reactions mass extinctions involving 70% to more end-Cretaceous event (65 Mya) with the between igneous intrusions accompa- than 90% of extant species occurred at Deccan of India and the end- nying the flows and surrounding sedi- least five times during the last 540 million event (251 Mya) with the ments. For example, Svensen et al. (12) years. The discovery by Alvarez et al. (1) Siberian basalts. However, the Deccan proposed that the PETM and associated that the end-Cretaceous (65 Mya) mass eruptions now are known to have started negative carbon-isotope excursion coincided with evidence for the before the end-Cretaceous mass resulted from explosive release of 13C- impact of an asteroid or comet ∼10 km extinction/, and the Siberian depleted methane from intrusion of con- in diameter focused interest in the causes flows still are only roughly correlated current basaltic sill complexes into of the other mass extinctions. It was ex- with the end-Permian die off (4). organic-rich sediments. Further support pected that evidence of a similar impact for this idea comes from the presence of might be found at other mass extinction Flood basalt episodes unusual igneous rocks produced by events. Such evidence, however, has been melting of sediments in contact with the slow in coming (2). At the same time, may be major causes North Atlantic intrusions (13). More episodic massive continental flood basalt recently, a similar model has been eruptions were suggested as another of climatic and suggested for release of greenhouse possible cause of mass extinctions (3, 4). gases from the eruption of the Siberian This connection is illustrated by a study biologic change. and Karoo basalts, where intrusions are by Whiteside et al. (5) that provides evi- accompanied by pipes of highly fractured dence that the eruption of the Central rock that indicate explosive release of Atlantic magmatic province (CAMP) ba- Lesser extinctions and paleoclimatic events thermogenic gases from the intruded salts, with a preserved volume greater than are correlated with the 55-Mya North sediments (14, 15). 1 × 106 km3 and covering more than Atlantic basalts (with the Paleocene- 7 × 106 km2, coincided with the end- Eocene Thermal Maximum or PETM) and Catastrophes Triassic (ETE) (201.4 the 183-Mya Karoo basalts (with an Early Whatever the ultimate cause of the extinc- Mya) on land and in the oceans. warming and extinction event). tions and climatic perturbations, the results The report by Whiteside et al. (5) To determine a cause-and-effect relation- of Whiteside et al. (5) provide a convincing presents carbon-isotope results obtained ship, what we need now are tightly con- link between the ETE and the CAMP ba- from leaf wax n-alkanes, wood, and total strained stratigraphic studies similar to that salts. The recognition that catastrophic organic carbon from two nonmarine sec- of Whiteside et al. (5) linking the events such as large impacts or flood basalt fl tions from the Newark and Hartford ows to the records of the extinctions and episodes may be major causes of climatic and Basins in the eastern United States, which other environmental perturbations in biologic change represents a sea change in include the CAMP basalts and which are marine and nonmarine sections. the geological sciences. James Hutton tightly constrained by magnetic reversals, (1726–1797) is said to have discovered orbital cycles, and pollen studies. The Causes of Extinction deep time—the almost unimaginable length correlation utilizes the levels of the ETE What is the mechanism causing extinc- of geologic time—and Charles Lyell and coincident carbon-isotope excursion tion? Climatic cooling from volcanic aer- (1797–1875) interpreted deep time as ac- and the Hettangian-Sinemurian boundary osols in the upper atmosphere has been commodating the idea that the directly 1.8 Mya later, which bracket the CAMP suggested, as has warming resulting from observable slow and steady geological episode. The sections are calibrated at magmatic carbon dioxide emissions. processes working over the long ages might high (20-ky) precision. These data are However, magmatic emissions of CAMP explain great geological and biological matched to orbitally forced carbon-isotope carbon dioxide probably were too small to changes. By contrast, natural events of data from the marine St Audrie’s Bay, have affected the greatly, and the various kinds in the real world tend to UK section, showing that the sharp initial long-term cooling from aerosols is very follow an inverse-power law relationship negative carbon-isotope shift and ex- uncertain (8). The initial negative δ13C between frequency F and magnitude M D tinction horizon are synchronous in ma- isotopic excursions in the Newark, Hart- so that F =1/M ,whereD is positive (see, rine and nonmarine sections. The oldest ford, and St Audrie’s Bay sections suggest for example, refs. 16, 17). Thus, small- CAMP basalts in the Newark and Hart- a massive input of 13C-depleted methane magnitude events (e.g., earthquakes, vol- ford Basins slightly postdate the extinction coincident with the onset of CAMP, and canic eruptions, impacts) tend to happen horizon (by ∼20 ky), but in similar Mo- the duration of the initial carbon-isotope much more frequently than potentially roccan sections the basalts may be simul- excursion is estimated at only 20–40 ky. taneous with the extinction horizon (6, 7). [The longer period of 13C-depleted values As precise radiometric ages have during the CAMP extrusion might be the Author contributions: M.R.R. wrote the paper. become available, it has been determined result of the biotic crisis itself (9).] A car- The author declares no conflict of interest. that flood basalt episodes are brief and bon dioxide super-greenhouse is sup- See companion article 10.1073/pnas.1001706107. severe (with peak output of more than 1 ported by paleobotanical studies (10) and 1E-mail: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1002478107 PNAS Early Edition | 1of2 Downloaded by guest on October 2, 2021 catastrophic large-magnitude events. The infrequently; in fact, tens to hundreds of such as large-body impacts and flood basalt reasons are variable, but in general, a millions of years could elapse between the volcanism will happen from time to time probabilistic relationship exists between greatest events. The significance of deep (perhaps quite “often” compared with the the magnitude and frequency of events. time is that, even though we expect length of geological time), and the results Thus, the notion of deep time must take extremely large events only very infre- of these very energetic events should be an into account the fact that the events with the quently, the long geologic time scale virtually important aspect of the geologic and greatest magnitude should happen very guarantees that potential catastrophes biologic records.

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