Archaeologist Robert A. DePalma, working in Southcentral North Dakota, has found the best evidence yet that the dinosaur killer, the huge meteor that struck the Yucatan Peninsula, leaving the Chicxulub crater, did indeed kill the dinosaurs. He has found, entombed in the rocks of the Hell Creek Formation, the archaeological equivalent of a smoking gun.
At the end of the Cretaceous, 65.76 million years ago, much of the middle of the United States was part of the Western Interior Seaway (WIS), a neck of water extending from the proto-Gulf of Mexico to North Dakota. A river – long since vanished – ran into the WIS near the present-day North/South Dakora boundary. Something, on the long ago day, caused a massive wave to flow up that fossil river, reversing its normal flow, depositing debris, burying fish and ammonites, leaves and entire trees. DePalma has explored the fossilized remains of that catastrophe. A long article in New Yorker back in March of this year give a good lay treatment and captures the importance in the article’s title: “The Day the Dinosaurs Died.” If you want a more scholarly treatment, The Proceedings of the National Academy of Sciences has DePalma et al’s article available on-line.
What makes this discovery so exciting is that it preserved, in the order they happened, the fall of ejecta from the meteor strike, then the debris from the floods, and then the capping layer or iridium-rich ash. The ejecta, the “splash” of material thrown up by the meteor strike, is composed of tiny glass spheres, melted globules from the heat of the strike. Those glass sphere, 0.3 to 1.4 mm in diameter, are embedded in the gills of the fossilized fish, demonstrating that the ejecta arrived first.
The flood happened next, too soon to have been a tsunami – and the meteor strike has to have generated a truly fearsome tsunami – but more likely a series of seiche waves. We see seiche waves on a smaller scale after every major earthquake. The ML 9 Great Alaska Earthquake of 1964 sloshed swimming pools in Louisiana. The meteor strike that created the Chicxulub crater is projected to have generated a ML 10 – 11 earthquake; orders of magnitude greater than any human-recorded earthquake. The seiche waves from such an event would have been astonishing. Those waves forced the saltwater of the WIS up into the river channel, mixing ocean and freshwater species in a mass graveyard.
And after the seiche waves, the dust generated by the meteor strike settled out of the atmosphere, creating the distinctive KPg Layer, the famous worldwide deposit of iridium-rich ash discovered by the Alvarezes back in 1980.1 The time at which the ejecta would have arrived and the time at which the materials of the KPg Layer arrived can be calculated, and together they bracket the time the seiche waves must have occurred. The site DePalma has discovered has allowed scientists, for the first time, to identify the exact sequence and timing of events following the arrival of the dinosaur killer.
Not the date it happened, although argon dating has narrowed that window a bit. The dinosaurs (except for a family of flying dinosaurs that later evolved into birds) were wiped off the earth 65.76 million years ago, plus or minus 150,000 years. But whatever date it happened, we now know exactly what happened next, which is pretty remarkable for events so far in the past. Read both articles, if you can. It’s worth the effort.
It’s a truly amazing discovery, a kind of geologic Rosetta Stone for the Chicxulub. WC expects that further research is gong to reveal much more.
- The KPg layer used to have a different name. It was the Cretaceous-Tertiary Boundary or K-T Boundary. When science renamed the Tertiary the Paleogene, the layer got renamed, too. Geologists. What can you expect? ↩