Michael R. Rampino

Earth & Environmental Science Program, New York University, New York, NY 10003 <mrr1@nyu.edu>
and NASA, Goddard Institute for Space Studies, New York, NY 10025 <mrampino@giss.nasa.gov>

A New Paradigm? Since 1980, a new geological paradigm has been developing regarding the effects of catastrophic impacts on the earth (1). The major mass extinction that marks the Cretaceous-Tertiary (K-T) boundary (65 Ma) coincided with the impact of a comet or asteroid ~10 km in diameter, which created the ~180 km diameter Chicxulub impact structure in the Yucatán region of Mexico, and a global ejecta fallout layer. This large impact (energy equivalent to ~10⁸ Mt of TNT) is calculated to have caused a global catastrophe (1).

The net result of 20 years of research on impacts and mass extinction boundaries suggests that impacts provide a promising explanation for extinction events in the fossil record. At the same time, some workers are still suggesting that: a) some extinction episodes could be artifacts of variations in the preservation of sedimentary rocks; or b) that flood basalt eruptions, with ages overlapping those of the extinctions, are the primary cause of the die-offs.

Astronomical and planetary cratering records, however, provide a quantitative framework for the terrestrial cratering rate, so that, in an ~100 Ma period, the Earth should be hit by one ~10-km diameter (~10²⁴ J, 10⁸ Mt) object (most likely a comet), and several asteroids or comets larger than a few km in diameter (~10²³ J, 10⁷ Mt TNT). A >10⁸ Mt or "first-order" impact event would produce a more severe and widespread environmental disaster than an ~10⁷ Mt "second-order" event (2).

Using the first-order K-T impact as a standard, these results lead to the prediction of ~5 major mass extinctions, and about 25 ± 5 less severe pulses of extinction during the last 540 Ma resulting from large impacts. The independent paleontological record of extinctions for that interval shows 5 major mass extinctions and ~20 less severe extinction pulses, in agreement with the estimates for impact-induced extinctions.

A “kill curve” relationship between mass extinctions and impacts of various magnitudes can be compared with data representing all of the largest known impact craters with well-defined ages that overlap the ages of mass-extinction boundaries (2). The observed data agree with the predicted curves within the errors of the geologic data, supporting at least a first-order relationship between large impacts and mass extinctions. Furthermore, a good statistical correlation exists between the rate of formation of large craters and extinction events over the last 300 Ma, and smaller craters have ages that tend to match times of stratigraphic stage boundaries defined by lesser faunal changes.

Impacts and Extinction Events: The Evidence. Considering the coincidence of the K-T mass extinction and the Chicxulub impact, the predicted environmental effects of large impacts, and the statistics of expected number of impacts, it is not unreasonable to hypothesize that a one-to-one correlation of large impacts and mass extinctions might exist. The geologic test of the hypothesis consists of impact ejecta layers at times of mass extinctions, and dated craters that match extinction episodes.

TABLE 1. STRATIGRAPHIC EVIDENCE OF IMPACT DEBRIS AT OR NEAR EXTINCTION EVENTS (Various sources)

Age	Evidence
Pliocene (2.3 Ma)	Impact melt debris
Late Eocene (35 Ma)	Microtektites (multiple),tektites , microspherules, shocked quartz
Cretaceous-Tertiary (65 Ma)	Microtektites, tektites, shocked minerals, stishovite, Ni-rich spinels, and Ir
Jurassic-Cretaceous (~142-144 Ma)	Shocked quartz, Ir
Late Triassic (~201-214 Ma)	Shocked quartz (multiple?), Ir
Late Devonian (~368-365 Ma)	Microtektites (multiple), and Ir

Although the K-T event is considered by many workers to be the only mass extinction convincingly tied to a large impact, impact materials have been reported in stratigraphic horizons close to the times of several other recorded extinction pulses (Table 1, for refs., see 1 and 2). The possibility of impactor showers spread out over several million years time may explain the occurrence of multiple impacts at extinction episodes (Table 2).

TABLE 2. DATED IMPACT CRATERS AND ASSOCIATED EXTINCTIONS

Extinction	% Species	Crater	Diameter (km)	Age (Ma)
Late Eocene	30	Popigai Chesapeake	100 90	35.7±0.8 35.2±0.3
K-T	76	Chicxulub Boltysh	180	65.2±0.4 65.17±0.64
J-K	42	Morokweng Mølnir Gosses Bluff	100? 40 22	145±0.8 142.6±2.6 142.5±0.8
Late Triassic	75 or 42	Manicouagan Rochechouart	100 23	214±1 214±8
Late Devonian	60	Siljan Rochechouart	52 46	368±1 ~360

Recent and Pending Evidence for Impacts and Extinctions. I will review recent studies that suggest: 1) multiple layers of impact spherules at the Frasnian-Famennian extinction boundary, 2) multiple impacts in the Late Triassic, 3) impact ejecta, tsunami deposits and multiple impacts at the Jurassic-Cretaceous boundary, 4) a lack of typical impact signature for the Permian-Triassic boundary, and 5) evidence for regional destruction from smaller impacts.

References

1. Rampino, M.R. et al. 1997. New York Acad. Sci 822, 403-431.

2. Rampino, M.R. 2002. Geol. Soc. Amer. Spec. Pap. 356, 667-678.