TOEFL Listening: ETS-TOEFL听力机经 - _Q51VH3BF3Y9DJ19Q$

Lecture: Mass extinction: Narrator: Listen to part of a lecture in an Earth science class. Professor: Now, one of the things I like to do from time to time in this class is look at how knowledge we've gained from studying Earth's geology has been applied to questions outside our field. Take the mass extinction that occurred around 13,000 years ago when most of the giant mammal species of North America vanished from the geological record. Creatures like wholly mammoths, saber-toothed cats, giant sloths, beavers, and camels. So, what caused these animals to suddenly disappear? One possible answer lies at the start of a period of sudden climate change called the Younger Dryas. Let me back up a minute. Just before this extinction, Earth was coming out of a long ice age. Glaciers were beginning to recede, but then temperatures in North America suddenly plummeted again, setting off this frigid thousand year period known as the Younger Dryas. North America became so cold, that glaciers started expanding again and one theory is that this sudden change in climate would have made it difficult for these large beasts to survive. Of course there's been other climate changes, some even more extreme and longer lasting, yet with no evidence that they triggered any extinction events, so maybe not the strongest theory, which means we need to look elsewhere, maybe space. I mean, we're all familiar with how impact events can affect life on Earth, like it's now generally agreed that a meteor triggered the extinction of the dinosaurs. Of course, since that theory's been widely accepted it's tempting to look to space to explain all extinction events. A large meteor crashing into Earth would scatter cosmic debris and cause massive firestorms and that's the theory proposed recently as an explanation for the large mammal extinction. Researchers who support this theory claim to have found evidence of a meteor impact at a site in the state of Arizona in a layer of sediment called the Younger Dryas Boundary. The Younger Dryas Boundary, or YDB, is a very thin layer of sediment that was laid down across North America at the beginning of the Younger Dryas period and what's particularly significant about this 13,000 year old layer is that under it, we find lots of fossils of these large mammals, but above it, that is after the YDB was laid down, well, not a single one. So, what happened here? Although the researchers suspect a meteor event, they didn't find any evidence of an impact crater, but they say they did find evidence of several types of particles commonly associated with meteor impacts, including nanodiamonds and high concentrations of certain magnetic particles. Nanodiamonds are small particles that can either originate in space or be formed in the extreme pressure of an intense explosion like the impact of a meteor for example. The magnetic particles can also come from space and the ones found in the YDB are very similar to particles associated with other meteor impacts. Recently however, a new group of researchers tried to replicate these findings and while they also found nanodiamonds and high concentrations of magnetic particles at the Arizona site, these researchers wondered whether the presence of such particles might be accounted for in other ways. So, to test whether the magnetic particles were unique to the YDB, they analyzed dirt from the rooftop of a researcher's house and in fact, they found magnetic particles in the rooftop sample too. Turns out that these particles are not just the result of impact events. They can have many origins, including the ash from nearby coal burning, electrical power plants or even cosmic dust that originates in space and then falls to Earth. Ok, but then how to explain the elevated concentrations of the magnetic particles in the sample from the YDB. Well, these samples had come from a riverbed where rainwater would have carried and deposited the particles and in fact, additional samples taken from outside the riverbed contained only normal concentrations of the magnetic particles in the YDB. As for the nanodiamonds, while a group of meteorite or comet fragments colliding with Earth would certainly provide the high temperatures and pressures needed to create such nanodiamonds, there might be other ways to explain their existence. It turns out, they're also commonly found in cosmic dust, which could explain their presence not only in the YDB, but also in the rooftop sample. So, if there's no strong evidence for a meteor impact or for a sudden climate change, then we need some other explanation for the disappearance of so many large, North American mammal species during the Younger Dryas.