Aug 30 2010

Did One or Two Impacts Kill the Dinosaurs?

Note: Late post today. I am covering the in-patient service and more time constrained than usual.

By now most people know that the dinosaurs (now clarified as non-avian dinosaurs), along with 85% of species alive at the time, became extinct 65.5 million years ago as a result of a massive meteor impact. This is almost certainly the impact crater at Chicxulub, which dates to the correct time. In addition, examination of fossils and geological layers centers this extinction event at Chicxulub.

This is referred to at the K-T extinction, referring to the end of the Cretaceous and beginning of the Tertiary periods. However, use of the designation “Tertiary” is being phased out, and the K-T extinction is now being referred to as the K-Pg extinction – for Cretaceous-Paleogene.

While the single impact theory is the current consensus, there are two significant if minority competing theories. One is the Deccan Traps flood basalts – a 200 thousand year long event spanning the K-Pg boundary that involved massive volcanic eruptions, which could have causes extinctions through release of dust and sulfuric aerosols into the atmosphere. While not dead, this hypothesis has not fared well under recent evidence and is supported by only a small minority of paleontologists.

Another theory is that there were multiple impacts, two or more, all around the time of the K-Pg extinction. This is based largely on the presence of other craters that date to the same time period. There is also the Boltysh impact crater in the Ukraine, the Silverpit crater in the North Sea, and the Shiva crater. The thinking is that an asteroid or comet may have broken into multiple pieces which showered the earth over a period of time.

A new study published in Geology provides evidence that suggests that the Boltysh crater occurred 2-5 thousand years before the Chicxulub impact. The study team looked at spikes in fern spores in geological layers in the Boltysh crater. Ferns recover quickly after an impact and quickly colonize the devastated area. Therefore a spike in fern spores is a marker for an impact. What they found is that there is a fern spike in the sediment layer that likely resulted from the Boltysh impact itself. And then, 2-5 thousand years later, this layer was itself devastated resulting in another fern spore spike. They believe this second devastation was due to the Chicxulub impact. (Incidentally, scientists who study pollen and spores are called palynologists.)

This is interesting, if indirect, evidence. It still leaves us with the bulk of the evidence showing that the Chicxulub impact was the major cause of the K-Pg extinction, and probably enough to explain it by itself. The complete extinction of non-avian dinosaurs appears to have occurred right at that time. But it is possible that the ecosystem was being stressed by the Deccan Traps eruptions. It is also possible that one or more smaller impacts also contributed to the extinction event.

This is an interesting refinement to the impact theory, and I am mostly interested in how paleontologists make inferences about what happened in the past. The spore spike is a cool line of evidence, one I did not know about before.

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