大灭绝 PBS NOVA Last Extinction [720p] [HDTV]

资源信息：

中文名: 大灭绝
英文名: PBS NOVA Last Extinction
别名: 最后的猛犸
资源格式: HDTV
版本: [720p]&
发行时间: 2009年03月31日
地区: 美国
语言: 英语
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概述:

15万年前，北美大陆就像一个大号的塞伦盖蒂国家公园（东非坦桑尼亚的，好想去看野生狮子不是动物园的），到处奔跑着巨大到难以想象的野生动物；然而到了距今1万年的冰河时代末期，约15至35种巨型生物突然灭绝了。在本集中，NOVA将跟随几名著名科学家一同寻找一种假说的证据；这个可能解释这场生物物种突然而普遍灭绝的惊人假说是——在约1.29万年前一颗掠过地球大气层的彗星解体并撞击了北美大陆——彗星撞击假说。

由 科学时报 于 2009-4-12 10:28 发表
流星雨湮没猛犸 科学家心存疑虑
3月31日的傍晚，在美国卡本代尔市南伊利诺伊大学的校园中，12名大学教员和学生正围坐在一起享用一顿丰盛的“猛犸烤肉”，接下来他们将观看美国公共电视台（PBS）的《新星》（NOVA）节目。即将播出的是一个很受关注且争议颇多的节目 ——一次灾难性的天体碰撞导致生活在北美洲的猛犸和其他许多大型动物在距今12900年前灭绝。真的是这样吗？
“ 这只是《新星》的一次演出。”这次烧烤的主人、地质学家Nicholas Pinter表示，“它很有趣，这里有很多很棒的动物，但是在聚会上，很多人都对碰撞的故事提出了怀疑。”事实上，之前在格陵兰岛首次发现的新证据为真的存在这样一个来自宇宙的杀手提供了更多的支持。
由24名研究人员组成的松散团体曾在2007年10月出版的美国《国家科学院院刊》（PNAS）上发表文章，提出了上述天体碰撞假设。他们在距今12900年前——大约是猛犸灭绝的时间——形成的一个薄沉积层中发现了6个被认为是彗星或小行星高速碰撞后留下的痕迹。然而这些证据并不足以完全支撑起这个故事。
如今，纳米金刚石的发现又重新让这一假说活了起来。研究人员在今年1月份出版的美国《科学》杂志上报告说，他们在遍布北美洲和欧洲的具有12900年历史的相同沉积层中找到了一些纳米尺度大小的金刚石。他们断定，这些纳米金刚石只有可能来自于天体碰撞。
在《新星》节目中，电视观众看到奥罗诺市缅因大学的冰川学家Paul Mayewski和同事，正在挖掘格陵兰岛上与发现纳米金刚石的沉积层同时代的冰川。最终他们在冰川中也发现了纳米金刚石。Mayewski在电视节目中表示：“它们绝对是证明当时曾经下过流星雨的关键。”然而有评论家指出，那些纳米金刚石会随着微小陨石而不断飘落到地球表面，并通过某种方式浓缩在沉积层中，例如被冲刷到湖泊中。但是Mayewski和他的同事表示，在冰川中发现纳米金刚石——这里不会发生上述过程——反驳了这些评论家。
观看过这期节目的碰撞科学家在享受乐趣的同时扔不免心存疑虑。美国华盛顿哥伦比亚特区史密森国家自然历史博物馆的矿物学家Bevan French表示：“我仍然不相信纳米金刚石本身代表了来自地球以外的物质。”French说，矿物学家还需要证实这些纳米颗粒确实是金刚石。一旦得到证实，还需要有人搞清它们是否来自于陨石以外的其他地方。French强调：“尽管我并不相信这是真的，但我还是会继续关注下去。”
（群芳 译自www.science.com，4月2日）
《科学时报》 (2009-4-3 A3 国际)

以下是官方网站摘要：
http://www.pbs.org/wgbh/nova/clovis/
TV Program Description
Original PBS Broadcast Date: March 31, 2009
Fifteen thousand years ago, North America was like the Serengeti on steroids, with mega-creatures roaming a continent teeming with incredible wildlife. But then, in a blip of geologic time, between 15 and 35 magnificent large types of animals went extinct. In this television exclusive, NOVA joins forces with prominent scientists to test a startling hypothesis that may finally explain these sudden and widespread extinctions—that a comet broke apart in the atmosphere and devastated North America 12,900 years ago. (See a video clip about the kind of damage a comet airburst could do.)
The program uses stunning computer animations to show what the continent may have been like thousands of years ago, with herds of woolly mammoths, hulking saber-toothed cats, giant ground sloths, and armadillo-like glyptodonts. Also on hand were the first well-documented humans in North America, known as the Clovis people. (Or were they the first? See our interactive map showing much earlier sites.)
The conjectured comet crash was practically yesterday compared to the dinosaur-killing asteroid of 65 million years ago, which humans were not around to see. But the Clovis people would have been there to witness this particular disaster unfold. While their remarkable stone-tool culture vanishes from the record at this point, we have no way of knowing how the comet strike affected these prehistoric Americans. (For examples of the striking Clovis culture, see Extraordinary Artifacts and Stone Age Toolkit.)
Whatever the culprit, something definitely happened to cause rapid extinctions across North America. One longstanding view is that the Clovis hunters arrived from Asia to find the North American big game easy prey to their sophisticated weaponry and hunting techniques. Within a few centuries they had wiped out the most vulnerable species. But geologist James Kennett of the University of California, Santa Barbara, argues that while comparable “overkills” occurred on many islands, the idea that bands of Stone Age hunters could annihilate a continent’s-worth of big animals is absurd.
Another long-established theory proposes that a sudden, sharp flip back to ice-age climate conditions was responsible for the die-off. Evidence shows, however, that the big animals had survived similar episodes of severe climate stress before. A third idea, that a virulent disease or diseases wiped out the megafauna, has also been advanced. Now the comet-impact hypothesis introduces a fourth possible culprit for the extinctions. (In End of the Big Beasts, hear the strong opinions of a spokesman for each of the four camps, including Jim Kennett.)
Proposed by Kennett and others, the comet-impact hypothesis has been bolstered by unusual discoveries in a distinctive soil layer known as the “black mat,” found at more than 50 sites across North America. The most intriguing clue is the presence of nanodiamonds, tiny gems believed to be forged only under the enormous heat and pressure of an extraterrestrial impact.
To test the hypothesis, NOVA brings leading climate scientist Paul Mayewski of the University of Maine and other experts to Greenland to hunt for such cosmic materials trapped in a narrow layer of ice associated with the time of the supposed impact. Mayewski likens the search to “worse than looking for a needle in a haystack.” Amazingly, they find the layer and its inclusions.
In addition to Kennett and Mayewski, “Last Extinction” features independent geologist Allen West, archeologist Vance Haynes of the University of Arizona, and physicist Mark Boslough of Sandia National Laboratories, an outspoken skeptic of the comet-impact scenario. (Follow the decades-long rumpus over the cause of the die-offs, in The Extinction Debate.)
One of the most moving moments in the program comes when Kennett sees evidence for his hypothesis materialize before his eyes on an electron microscope display. “Exciting isn’t really the word,” he says, choking up. “It’s an experience you usually don’t have much in your scientific career. Moments of intense discovery are very emotional for scientists.”
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