"The sample we found strongly supports the single impact hypothesis," said lead researcher Ken MacLeod of the University of Missouri-Columbia.

    Geological evidence reveals that about 65 million years ago a giant meteorite about six miles wide smashed into the Yucatan Peninsula close to the current Mexican town of Chicxulub. That impact sent dust high into the atmosphere where it locked the sun's light for decades or centuries.

    It also triggered volcanic eruptions, massive earthquakes and tsunamis.

    Plants and animals began to die because of the dark skies, temperatures dropped dramatically, and white-hot debris fell back to Earth igniting wildfires all over the globe. The smoke from the fires combined with rain clouds to create a scalding acid downpour.

    Many scientists believe the combined calamities killed off most of the life on Earth, including dinosaurs, in the so-called K-T extinction event .

    A small team of scientists, however, have argued that a single meteorite was not enough to end the dinosaurs' reign, and that the Yucatan impact occurred 300,000 years too early. The biggest proponent of this alternative scenario is Gerta Keller of Princeton University.

    Keller thinks that the Chicxulub impact, combined with volcanoes in India and global warming, only upset the ecological balance, causing many species to shrink in size. But these things weren't enough to trigger a mass extinction, she believes. Instead, Keller speculates that a second, currently unidentified meteor crashed sometime after Chicxulub.

    But a new examination of sediments taken from the Demerara Rise in the Atlantic Ocean casts fresh doubt on Keller's minority view.

    Located some 3,000 miles from the Yucatan Peninsula, the Demerara Rise is considered an intermediate distance from the impact site. Interpretation of samples collected from locations close to the crater are complicated by factors such as waves, earthquakes and landslides that were triggered by the impact and which shuffle the sediment layers. Samples from farther away, meanwhile, receive little impact debris and are much less helpful in recreating events.

    The Demerara Rise sample thus provides an unusually clear picture of the events at the time of the mass extinction that claimed the dinosaurs. Analysis revealed a unique layer composed of impact-related material, but none above or below that layer.

    The Demerara Rise sediment, therefore, shows "no support for multiple impacts or other stresses leading up to or following the deposition of material from the impact," MacLeod said.

    The team's findings are detailed in an online version of the Geological Society of America Bulletin.

    (Agencies)