Mass extinction － Day of reckoning The Economist April 6th 2019 恐竜絶滅
Stony evidence of the hellfire that drove dinosaurs to extinction
When, in 1980, Luis Alvarez, a physicist, and his son Walter, a geologist, made public their theory that the dinosaurs were killed by a massive asteroid strike, it came as a curveball to paleontologists, who believed dinosaurs had gradually died out through other means. The father-and-son team from the University of California, Berkeley, argued that evidence of the catastrophe was hiding in plain sight, the world over, as a thin layer of sediment enriched in iridium, a metal commonly found in asteroids but rare on Earth. They pointed out that no dinosaurs, with the exception of birds, were ever found beyond this critical layer and suggested a devastating impact was responsible.
The only piece of the puzzle that has been missing is evidence of what actually happened when the asteroid struck. Now, almost 40 years later, an American fossil bed is revealing details of the raging hellstorm that followed just minutes after the asteroid impact, and eventually drove the dinosaurs to extinction.
Under most circumstances, fossils form when animals die in places like river deltas where fine sediment slowly covers up their bones and ultimately encases them in rock. Not so at the aptly named Hell Creek formation of Tanis in North Dakota. Here. Robert DePalma, a PhD student at the University of Kansas, and a team of colleagues that includes Walter Alvarez are reporting the discovery of a 1.3-meter-thick sedimentary layer that was catastrophically dumped in a single day.
The layer is loaded with the bodies of marine and freshwater fish. This alone struck Mr. DePalma as odd since Hell Creek is not known for the preservation of brackish ecosystems where such animals could mingle. But what proved truly unnerving was the fact that all of the bodies were intact, faced the same direction and were scattered among felled tree trunks. That hinted at a sudden surge of water: the streamlined shape of fish means they automatically orient themselves with their heads pointing into a current of fast-moving water. That the bodies were all intact suggests that they were rapidly buried. Moreover, only the most powerful of currents can knock trees down, so the assemblage must have formed during a single devastating event.
Wedged between a 66m-year-old layer of Cretaceous sediment, and another dating from the subsequent Tertiary period, when mammals came to dominate Earth, the Hell Creek fossils are in the perfect position to record the moments that immediately followed the asteroid impact.
Supporting this, spheres of what was once molten glass and fragments of quartz generated under exceptionally high pressures and blasted into the air are scattered throughout the site.
Some of it was lodged inside the gills of fossilised fish. Presumably, they sucked it in with their last desperate gasps. The bottom layer of the site contains burrows that appear to have been dug by mammals and are filled with coarse sand brought in over land at great speed, the signs of which are seen in the ripples left in the sand. Dusting the top of the formation is an ominous layer of iridium.
Other fossil finds, yet to be confirmed, include fish impaled on spines of one another, wasp nests, flooded ant hills, ancient primates and the leaves of plants probably related to the modern banana tree. The team are studying these but their findings have yet to be peer-reviewed and so are not included in the discovery’s scientific announcement, which was published by proceedings of the National Academy of Sciences last week.
What is clear already from the confirmed evidence is the sequence of events that unfolded in the minutes and hours after the asteroid hit. It struck the Mexican coast, sending enormous volumes of gas and molten material into atmosphere, and igniting a firestorm that would have engulfed much of the planet. Its impact crater, located beneath the Yucatan peninsula and the southern Gulf of Mexico, has been a focus of scientific interest for many years. Undoubtedly, this would have created an enormous tsunami, but Mr DePalma suspects that the Tanis fossils, located thousands of kilometers to the north, were killed by a different phenomenon, triggered by the impact: a seiche wave.
Also known as standing waves, seiche waves form in large bodies of water that are either steadily blown by strong winds or shaken by tremors. Mr DePalma and his colleagues propose that the asteroid impact shook Earth so forcefully that seiche waves as tall as 100 meters rose up in every large body of water across the planet, including the shallow sea near Tanis.
Further fossil evidence will be needed to prove the theory, but if Mr DePalma is correct then the inferno initiated by the impact was made worse by devastating walls of water everywhere. No wonder the dinosaurs threw in the towel.
Some of it was lodged inside the gills of fossilised fish. Presumably, they sucked it in with their last desperate gasps. The bottom layer of the site contains burrows that appear to have been dug by mammals and are filled with coarse sand brought in over land at great speed, the signs of which are seen in the ripples left in the sand. Dusting the top of the formation is an ominous layer of iridium.
Other fossil finds, yet to be confirmed, include fish impaled on spines of one another, wasp nests, flooded ant hills, ancient primates and the leaves of plants probably related to the modern banana tree. The team are studying these but their findings have yet to be peer-reviewed and so are not included in the discovery’s scientific announcement, which was published by proceedings of the National Academy of Sciences last week.
What is clear already from the confirmed evidence is the sequence of events that unfolded in the minutes and hours after the asteroid hit. It struck the Mexican coast, sending enormous volumes of gas and molten material into atmosphere, and igniting a firestorm that would have engulfed much of the planet. Its impact crater, located beneath the Yucatan peninsula and the southern Gulf of Mexico, has been a focus of scientific interest for many years. Undoubtedly, this would have created an enormous tsunami, but Mr DePalma suspects that the Tanis fossils, located thousands of kilometers to the north, were killed by a different phenomenon, triggered by the impact: a seiche wave.
Also known as standing waves, seiche waves form in large bodies of water that are either steadily blown by strong winds or shaken by tremors. Mr DePalma and his colleagues propose that the asteroid impact shook Earth so forcefully that seiche waves as tall as 100 meters rose up in every large body of water across the planet, including the shallow sea near Tanis.
Further fossil evidence will be needed to prove the theory, but if Mr DePalma is correct then the inferno initiated by the impact was made worse by devastating walls of water everywhere. No wonder the dinosaurs threw in the towel.
