Scientists say they're homing in on the precise date of both the extinction of the dinosaurs 66 million years ago, and the massive impact that's believed by many to have caused it.
The dates are so close, they say, that the comet or asteroid, if not wholly responsible for the global extinction, must certainly at least have dealt the dinosaurs their death blow.
"The impact was clearly the final straw that pushed Earth past the tipping point," says Paul Renne, BGC director and UC Berkeley professor in residence of earth and planetary science. "We have shown that these events are synchronous to within a gnat's eyebrow, and therefore the impact clearly played a major role in extinctions, but it probably wasn't just the impact."
The extinction was characterized by the almost overnight disappearance from the fossil record of land-based dinosaurs and many ocean creatures. The new date for the impact - 66,038,000 years ago - is the same within error limits as the date of the extinction.
The extinction of the dinosaurs was first linked to a comet or asteroid impact in 1980, with the finger pointed at a 110-mile-wide crater in the Caribbean off the Yucatan coast of Mexico. Called Chicxulub, the crater is thought to have been created by an object six miles across that spread glassy spheres or tektites, shocked quartz and a layer of iridium-enriched dust around the world.
Renne's team first recalibrated and improved the existing dating method, known as the argon-argon technique, and then used it to analyze volcanic ash from the Hell Creek area in Montana.
They also gathered previously dated tektites from Haiti and analyzed them using the same technique. The new extinction and impact dates are precise to within 11,000 years, they say.
But while the impact appears to have happened at the same time as the extinction, this doesn't mean that it was the sole cause. Dramatic climate variation over the previous million years, including long and uncharacteristic cold snaps, probably brought many creatures to the brink of extinction - and the impact tipped them over the edge.
One cause of the climate variability could have been a sustained series of volcanic eruptions in India that produced the extensive Deccan Traps. Renne plans to re-date those volcanic rocks to get a more precise measure of their duration and onset relative to the dinosaur extinction.
"These precursory phenomena made the global ecosystem much more sensitive to even relatively small triggers, so that what otherwise might have been a fairly minor effect shifted the ecosystem into a new state," he says. "The impact was the coup de grace."