Mud time line clarifies dinosaurs' demise.

A 16-inch core of mud tells the clearest story yet of how life on Earth suffered after a comet or meteor slammed into the planet about 65 million years ago, reports a team of oceanographers. Scientists drilled the sample from the ocean bed about 320 kilometers east of Jacksonville, Fla., earlier this year and reported their findings last week in Washington, D.C.

"This is the most significant discovery in geosciences in 20 years," says Robert W. Corell of the National Science Foundation in Arlington, Va., the primary funder of the expedition. Corell says that the sample ends the debate about what killed the dinosaurs.

That debate began in 1980, when scientists from the University of California, Berkeley discovered evidence that a chunk of rock the size of Manhattan slammed into Earth at the end of the Cretaceous period. At that time, an estimated 70 percent of Earth's species went extinct (see p. S20). The theory that the impact caused the mass extinction gained momentum after researchers discovered a 200-km-wide crater buried beneath the tip of Mexico's Yucatan Peninsula (SN: 3/5/94, p. 156).

The new mud core displays the entire time line of the catastrophic event with a clarity never seen before, says Richard D. Norris, a geologist at the Woods Hole (Mass.) Oceanographic Institution and a codirector of the recent expedition, which was part of the Ocean Drilling Program. Earlier samples showed only some of the layers, often damaged by erosion, currents, or waves.

"We've had the pieces of the puzzle for a long time," Norris says. "This puts all the pieces together in one package."

From the thickness of the fossil-poor layer above the impact debris in the new core, Norris estimates that 5,000 years passed before life recovered. Although excited about the find, Gerta Keller, a paleontologist at Princeton University, says Corell's evaluation contains "a lot of hyperbole." Until researchers perform chemical tests on the sample, she says, the findings will add little to scientists' understanding.

Virgil L. Sharpton, a geologist with the Lunar and Planetary Institute in Houston, agrees that the finding's importance may be "overstated" but adds that it may clarify events following the impact. Overall, Sharpton calls the findings more evolutionary than revolutionary. "They will answer a few things and cause a few more to be asked."