"We have nothing to indicate another impact," MacLeod said.
But Princeton's Keller remains unconvinced. In an email, she described the new study's claims as "rather hyper-inflated."
Among other things, she said, there are signs of erosion between layers in the core sample, indicating that critical layers might be missing or rearranged. She also challenged the dates ascribed to certain rock layers.
"They do not have a complete record of the K-T extinction," she wrote.
MacLeod, however, believes the features that Keller sees as signs of erosion were actually left in the first hours after the impact, before the arrival of fallout.
The impact, he said, generated a magnitude 12 to 13 earthquake with seismic waves that stirred up the top millimeters of sediment, which later deformed slightly under the weight of the impact layer.
Rather than indicating missing time, he said, "this layer represents a moment in time."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES