The 100-km-diameter Popigai structure in Siberia is probably the fifth largest impact crater on Earth. Its age has been disputed — it could be anything from 5 to 65 million years (Myr) old. But this controversy appears to be finally settled: on page 365of this issue1, Bottomley et al. report ages of 35.7±0.2 Myr for rocks from the Popigai crater that were melted by the impact. That means that 35.5 million years ago, in the space of only a few hundred thousand years — or perhaps much less — two large asteroids or comets struck the Earth, one in Siberia and another just off the west coast of North America.
The Late Eocene age for Popigai opens up intriguing questions about how two large impacts in quick succession might have affected the Earth. The period from the Late Eocene to the Eocene/Oligocene boundary (which is dated at 33.7±0.5 Myr ago) is marked by the most severe mass extinction since the dinosaurs disappeared 65 Myr ago. The climate also worsened considerably in the Late Eocene, with cooling oceans and the first appearance of ice sheets in Antarctica.
Could these changes be the result of two almost coeval large impact events? We now know1 that the Popigai crater is the same age as impact indicators (iridium and shocked quartz) found below the Eocene/Oligocene boundary in the Massignano section, Italy2,3,4, which are dated radiometrically using volcanic ash layers at 35.7±0.4 Myr old2,3. However, compositional evidence linking the Popigai crater and the Massignano ejecta is lacking; more work on this topic is urgently required. But the age coincidence is remarkable.
Excitement about the Popigai date grows in the light of the discovery a few years ago of a contemporaneous impact event on the other side of the world: the 85-km-diameter Chesapeake Bay impact structure, off the Virginia shore5,6. The Chesapeake Bay crater had already been dated at 35.5 to 35.2 Myr old6. It is probably the source of the huge field of tektites (characteristic glassy nodules produced by melting of the target rock in an impact event) strewn mainly over Georgia, as indicated by the chemical similarities between the glass and the Chesapeake Bay target rocks7,8. Similarly, the crater appears to be the source of microtektites and layers of excess iridium found on Barbados and in several ocean-floor drilling cores (such as DSDP 612), and dated at 35.4±0.6 and 35.5± 0.3 Myr old9,10, respectively.
So, within the error bars, the Popigai and Chesapeake Bay craters are the same age; at most, they occurred a few hundred thousand years apart. The tremendous energy released by these two successive huge impacts may have induced the long-lasting perturbation of the global environment which led to the terminal Eocene cataclysm and global changes.
We finally have a second solidly based case study, after the Cretaceous/Tertiary (K/T) boundary, of the Earth's response to large impacts. In both cases, the location, size and age of the impact craters are known, there is a reasonable though not yet complete record of global or regional ejecta layers, and a detailed fossil succession is present in coeval, often continuous sediments from which a climatic and oceanographic record can be extracted.
In the case of the K/T boundary, 65 Myr ago, the coincidence and possible cause-effect relationship between the large impact which formed the 180-310-km-diameter Chicxulub crater in Yucatán, and the mass extinction of organisms, is striking. In the light of the K/T event, a rigorous assessment is needed of the fossil and climate record at small timescale across the Late Eocene. A concerted and open-minded effort11 from palaeontologists, sedimentologists, geochemists and impact researchers should determine the role played by the Popigai and Chesapeake Bay impacts on the biological and climatic evolution of the Earth in the Late Eocene.
These two almost coeval impacts shed new light on the random behaviour of the terrestrial impact flux and the size-frequency distribution of impactors as a function of time. According to Bottomley et al.1, “the expected repeat rate of impact structures of this size is of the order of ten million years”. So the question arises: how important are the statistics of small numbers for the effectiveness of impact-induced disturbances in the evolution of the Earth, and the related mass extinction events and climate changes in the past 600 million years?
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