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What opinion does the professor express when he discusses nanodiamonds?
What opinion does the professor express when he discusses nanodiamonds?
admin
2018-08-16
45
问题
What opinion does the professor express when he discusses nanodiamonds?
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 rewind for a moment. 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, perhaps space.
I mean that 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 YOB, 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 especially 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 YOB was laid down, well, not a single one. You may be wondering, 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 YOB 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. In order 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. In actual fact, 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 YOB? 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 YOB. 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. This means that 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.
选项
A、Researchers should get a better understanding of their origin.
B、Researchers should focus on magnetic particles instead of nanodiamonds.
C、Their presence is not strong proof of a meteor impact.
D、They indicate that a geological sample has been contaminated.
答案
C
解析
态度题。线索词为教授所说: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.如原文所述,nanodiamonds(纳米金刚石)的来源并非仅局限于流星撞击,所以它的存在不能有力地证明动物灭绝是由流星撞击地球导致的。A选项错误,因为教授未提及研究人员应更好地去了解纳米金刚石的起源。况且,文中研究人员已经很全面地解释了它的起源。B和D选项错误原因一致,原文未提及研究人员应专注于研究磁性颗粒而不是纳米金刚石,也未提及地质样本已被污染。
转载请注明原文地址:https://kaotiyun.com/show/gjfO777K
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