Space debris may have been blamed wrongly for the demise of dinosaurs, reports Dick Ahlstrom.
Scientists may have to find an alternative cause for the mass extinctions that brought an end to the age of the dinosaurs. New research shows that the most popular "smoking gun" theory, an asteroid impact in Mexico 65 million years ago, happened 300,000 years too soon to have done down T-rex and his Late Cretaceous fellows.
The latest findings in this ongoing controversy were published last week in the US Proceedings of the National Academy of Sciences. They show that the Chicxulub crater on the Yucatan peninsula - seen as the most likely site of the asteroid impact responsible for the demise of the dinosaurs - predates the extinction.
The actual point of extinction is identified in the geological record as a thin, distinct band of material known as the K-T (Cretaceous-Tertiary) boundary.
It is rich in iridium, an element common to asteroids, and the K-T boundary has been detected in rock sites right around the world. Whatever event formed the boundary, it was something that influenced things across the entire planet.
The Chicxulub crater in Yucatan, Mexico has served as the "smoking gun" site since the 1990s, Prof Gerta Keller and colleagues point out in their PNAS report.
The crater measures between 180 and 240 kilometres across, indicating an impactor of colossal size, probably surpassing the one-kilometre-diameter planet-killer threshold needed to cause huge species extinction.
"These observations made a convincing case for Chicxulub as the long-sought K-T boundary impact crater and the cause for the end-Cretaceous mass extinction," the authors say.
Even so, doubts persisted about the crater's age and size, and about other geological clues the impactor left behind. To help resolve the issue, the International Continental Scientific Drilling Program drilled a new core within the Chicxulub crater, core Yaxcopoil-1 or Yax-1, taken from a site 40 kilometres south-west of Merida, Mexico. Yax-1 samples reached down to 1,511 metres below the surface, bringing up long rock cores.
On display from Yax-1 were clear signs of the distinctive geological layer formed after the asteroid struck, made up of melted then resolidified rock and known as "impact breccia".
Unfortunately for those who favour the Chicxulub extinction theory the researchers also identified telling geological deposits that show Chicxulub came too soon to be a source for the K-T boundary.
The researchers found a 50-centimetre layer of limestones above the impact breccia that had been laid down over hundreds of thousands of years.
These limestones contained "Late Maastrichtian microfossils", signs of life occupying the site after Chicxulub and before the K-T boundary which provides the next layer along the Yax-1 core.
There were two possible reasons for this, the authors suggest, "backwash and crater infill after the impact event, or the Chicxulub impact predates the K-T boundary and hence was not the cause of the end-Cretaceous mass extinction as commonly believed".
To get their answers they studied the Yax-1 core between the critical 794.65 to 793.85 metres interval "at high resolution" using a range of technologies. Their results suggest a "quiet" underwater environment, not a once-off backwash of material. There were also plentiful signs of past life.
"Bioturbation at these horizons and the K-T boundary indicate an ocean floor colonised by invertebrates," the authors state.
The evidence increasingly points to a pre-K-T boundary for the Chicxulub impactor, the authors conclude, with a likely date 300,000 years before the event that formed the K-T itself.
"The Late Maastrichtian Chicxulub impact coincided with major Deccan volcanism, greenhouse warming 65.4 to 65.2 million years ago and a gradual decrease in species diversity during the last 700,000 years before the K-T boundary."
Together these environmental changes would have caused "major biotic stress", particularly the increased volcanism.
By the time the K-T impactor arrived, the late-Cretaceous species were just right for a mass extinction.
"This finding suggests that the K-T boundary impact (and volcanism) may have been the straw that broke the camel's back, rather than the catastrophic kill of a healthy thriving community," the authors say.
The location of the K-T impactor "remains unknown" they conclude, but the Shiva crater in India is a possible candidate.