Two-thirds of all living species on Earth, including the dinosaurs, were suddenly wiped out 65 million years ago, at the Cretaceous/Tertiary (K/T) boundary. Although the occurrence of an extraterrestrial impact is widely accepted, the nature of the impactor and the global biological and climatic repercussions from this impact are debated. Possible candidates for the impactor are a single asteroid/comet, or a member of a comet shower. An extraterrestrial impact would have severely perturbed the terrestrial ecosystem and climate by injecting large quantities of dust and climatically active gases into the upper atmosphere. An alternate hypothesis to explain the biotic calamity invokes voluminous volcanism. Recent work suggests that most of the Deccan Traps flood basalts were erupted in a <1 Myr interval coincident with the K/T boundary. The global environmental effects from extensive volcanism could be similar to a large impact, but the timescale of the two processes would be different. The perturbation on climate and ecosystems from an impact would be essentially instantaneous, but the effects from volcanism would be spread over at least a few hundred thousand years. Therefore resolving these hypotheses requires estimating the duration of the K/T event. Unfortunately most chronological techniques do not have sufficient resolution to date geologically ‘instantaneous’ events

The Earth is constantly accreting fine-grained cosmic dust, which are deposited in sediments. Helium-3 (³He), the rare isotope of helium, is a tracer of such cosmic dust, and does not record the arrival of a single large impactor. Measurements of ³He in sediments reveal a near-constancy in the influx of cosmic dust across the K/T boundary. Hence, the flux of ³He can be used to compute the sediment accumulation rate and the total duration of the K/T event. Our results show that the mass-extinction at the K/T boundary was indeed catastrophic and rule out volcanism and sea level changes as major players. Following the severe biotic catastrophe, life rebounded and ecosystems and food chains were restored in less than 10000 years. We have further constrained the deposition time of the Ir-rich layer to < 60 years. With this kind of a time resolution it may now be possible to investigate in detail some of the climatic and biological events occurring in the first few thousand years after the impact.

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While the exact mechanism(s) responsible for the mass extinction at the K-T are yet to be identified, it is an interesting intellectual excersize to speculate what role impacts may have played in shaping the biology of our the planet. Howerver, other than the K-T extinction, evidence that extraterrestrial impacts have led to mass-extinction events are largely lacking. Either impacts are not important and the dinosaurs just got unlucky, or the evidence is out there and we have to figure out how to extract it from the geologic record. Recently, Luann Becker and colleagues have suggested that the extinction at the Permo-Triassic boundary, the largest of the mass-extinction events, may have been driven by impacts but clear evidene for such an impact has yet to be found.

Artists impressions of the K/T impact. (Pictures courtesy of NASA)