Along an Ancient Coast, Clues to a Global Extinction
Wave-washed sea cliffs along the coasts of western England and Wales are home to spectacular assemblages of rocks and fossils that may hold keys to understanding a sudden global extinction 201.4 million years ago that cleared the way for the rapid evolution of dinosaurs. Paleontologist Paul Olsen and geologist Dennis Kent of Columbia University’s Lamont-Doherty Earth Observatory recently visited with British colleagues and took samples to help determine why so many life forms died off all at once. They are not looking for fossils of the dinosaurs themselves—but rather for what set the stage for them to rise and dominate the earth. In particular, they want to know if these cliffs contain evidence that a large meteorite strike helped kill off competitors. READ THE FULL SCIENTIFIC STORY) Below, Olsen prospecting for samples at low tide near Penarth, Wales.The gentle countryside of Somerset, England, largely unchanged even after many centuries of human habitation, ends abruptly at the Bristol Channel, an estuary that cuts far inland.At low tide, you can walk along a shore of solid rock and inspect sedimentary rocks lain down over eons, documenting a time when this spot lay at the bottom of a shallow sea, just before the dinosaurs’ rise some 200 million years ago. The layers can be read like pages in a book; geologists have been coming here to study them since the 19th century, but Olsen and Kent think modern analysis techniques may reveal more.In Olsen’s words, the layers below the Jurassic—a period called the Triassic–are “crawling with fossils.” Such waters were inhabited by bivalves, eel-like fish, crocodilians and sharks, among other creatures.There is vigorous debate about what suddenly wiped out many Triassic-period creatures, whose fossils disappear at a sharp boundary 201.4 million years ago—the so-called Triassic-Jurassic extinction. Many scientists, including Kent (center) and Olsen (right) lean toward massive volcanic eruptions that alternately cooled and heated the air. But British colleagues have spotted a single layer of rock here that may add a twist.In 2010, scientists working near Rochechouart, western France, dated a large meteorite crater to just before the Triassic-Jurassic extinction. Among other things, it would have created an apocalyptic tsunami—maybe 1,000 feet high. And in fact, these sea cliffs hundreds of miles away contain a similarly dated layer (bottom) in which one giant pile of debris seems to have been washed in all at once—quite unlike layers above and below, which were lain down by slow, steady processes.A meteorite of this size also would have created an earthquake about 100 times more powerful than anything recorded in human time. Along the Triassic-Jurassic boundary, the sea cliffs seem to contain evidence suggesting such an event—sediments that have been shaken, twisted and otherwise contorted, as if by some sudden, enormous force. Olsen and Kent have nicknamed this “the coffin layer,” because it seems to close the lid on so many life forms.University of Oxford geologist Stephen Hesselbo (left) and Olsen carefully document the physical relationship of the coffin layer to other components of the cliffs, in an effort to understand how, or whether, it relates to the extinction.Olsen and Hesselbo carefully chisel out samples from the time of the extinction in order to test for a crucial element: iridium, an element rare in earth’s crust, but abundant in meteorites. If a spike in this element coinciding with the extinction were to turn up, it would add to the case for an extraterrestrial object.Ensuring that the many rock layers are properly dated in relation to one another is crucial, and this is Kent’s specialty. Using a diamond-tipped drill with a hollow bit, he cores out a lower section of Triassic rock, exposed at low tide.Kent will later take such rock cores back to the lab and compare them to known sequences of reversals in earth’s magnetic field—a technique that allows him to determine the rocks’ age.Olsen believes that even if they find evidence of a meteorite here, it was probably too small to cause a worldwide extinction all by itself; there must other parts to the story. For further clues, the team is currently studying sites spread from Morocco to Pennsylvania. On top of the ancient cliffs behind him, signs of humans’ current earthly domination: rental trailers used by Britons seeking a seaside vacation.