经济学人下载:大灭绝 陨石很小不过危害却很致命

Science and technology
科学技术

Mass extinctions
大灭绝

Small but deadly
陨石很小不过危害却很致命

The biggest extinction in history was probably caused by a space rock that changed the climate
历史上规模最大的一次物种灭绝，也许是由于陨石坠落导致的气候变化而造成

AS EVERY schoolchild knows, the dinosaurs were wiped out in an instant, when a rock from outer space hit what is now southern Mexico.
就像每个小学生知道的那样，当一块天外陨石坠落到现今的墨西哥南部地区之后，恐龙就那在一瞬间遭受了灭顶之灾。

That happened 66m years ago, at the end of the Cretaceous period.
这一切就发生在6600万年前的白垩纪末期。

Well-informed schoolchildren also know that this mass extinction was neither unique nor the biggest.
一些博学的小学生还知道这次物种大灭绝既不是历史上唯一的一次，也不是历史上最大的一次。

The geological record speaks of four others since animal life became complex at the beginning of the Cambrian period 541m years ago.
地质学档案记录了5亿4100万年前寒武纪物种多样化以来的另外四次物种大灭绝。

What neither these clever schoolchildren nor anyone else knows, however, is whether these extinctions had similar causes.
有些历史不论是那些聪明的小学生还是别的人都还不知道，比如，导致这些灭绝的原因是否相似？

But evidence is accumulating that the biggest extinction of all, 252.3m years back, at the end of the Permian period, was indeed also triggered by an impact.
不过有证据不断地证明发生在2亿5230万年前二叠纪时期的那次大灭绝也是因为陨石坠落而造成的。

Nevertheless, though the trigger was the same, the details are significantly different, according to of the University of Western Australia.
然而，按照西澳大利亚大学的埃里克·托瓦尔教授的说法，虽然两次大灭绝引发的原因是一致的，但是其中细节却大不相同。

When the dinosaurs vanished they were accompanied by more than 70% of the other animal species on Earth.
当时和恐龙一起灭绝的还有地球上超过70%的物种。

At the end of the Permian, the extinction figure was more than 80%.
在二叠纪末期，那场大灭绝带走了超过80%的物种。

And just as the Cretaceous slate-clearing permitted the rise of a hitherto obscure group called the mammals,
就如同白垩纪大灭绝成就了一个迄今为止都未被研究透彻的物种—哺乳类动物的崛起，

so the Permian clearance permitted the rise of the reptiles,
二叠纪大灭绝也成就了爬行类生物的崛起，

one branch of which turned into Tyrannosaurus, Diplodocus and all the other names familiar from childhood.
它们的一支最后演化成了如今小学生们口中的暴龙、梁龙以及其它各种叫龙的动物。

The idea that an impact caused the Permian extinction has been around for a while.
导致二叠纪大灭绝的原因长期以来都没有一个定论。

As at the end of the Permian, as at the end of the Cretaceous, huge volcanic eruptions had been going on for hundreds of thousands of years.
在二叠纪和白垩纪末期，大量火山持续爆发了数百到数千年。

These may have weakened the world's ecosystems, making them vulnerable to an external shock.
这可能破坏了地球的生态系统，使其难以抵御外来的破坏。

But the abruptness of both extinctions indicates that the coup de grace was administered by something else,
但是两次大灭绝来得如此突然，这说明了实施致命一击”的另有其人，

and in the case of the Permian some fragments of meteorite of the correct age, found in rock in Antarctica, suggest that, as with the Cretaceous, that something was an asteroid or a comet.
而且在南极洲的岩石里发现的一些陨星碎片与二叠纪的年代相吻合，这说明了地球在二叠纪与在白垩纪一样，遭受了小行星或是彗星的袭击。

What was missing from the story, though, was a suitable crater.
只不过这个故事里还缺一个部分—一个合适的陨石坑。

Fracking hell!
如同置身地狱一般的裂解反应

Last year Dr Tohver and his colleagues thought they might have found it.
去年，托瓦尔博士和他的同事表示他们也许找到这个陨石坑了。

They redated a hole that straddles the border of the states of Mato Grosso and Goias in Brazil, called the Araguainha crater, to 254.7m years, with a margin of error of plus or minus 2.5m years.
他们重测了一个位于位于巴西马托·格罗索州和戈雅斯州交界的洞穴—阿拉瓜尼亚陨石坑，并确定其形成年代为2亿5470万年前，年代误差在正负250万年左右。

Previous estimates had suggested Araguainha was 10m years younger,
早期研究所估计的阿拉瓜尼亚陨石坑的形成时间要较这个数据晚1000万年，

but Dr Tohver has put it within geological spitting distance of the extinction date, which itself has a margin of error of plus or minus 200,000 years.
不过托瓦尔博士把这个时间差归结于大灭绝发生的具体时间本身就有一个范围，大概是正负20万年的误差。

Which would all be fine and dandy, except most people think Araguainha is too small to be the culprit. It is a mere 40km across.
除开大部分人觉得阿拉瓜尼亚实在太小，不足以成为元凶以外，这对大灭绝来说会是个很好的解释。

The Chicxulub crater in Mexico, which did for the dinosaurs, is 180km in diameter, and it may have been paired with an even bigger impact in the Indian Ocean.
导致恐龙灭绝的墨西哥希克苏鲁伯陨石坑，其直径有180公里，而且它可能与同时期在印度洋产生的另一个更大的碰撞共同作用，从而导致白垩纪大灭绝。

Dr Tohver, however, has an answer to this criticism.
面对质疑，托瓦尔博士有着合理的解释。

His latest paper, just published inPalaeogeography, Palaeoclimatology, Palaeoecology, describes the rock in the area in which Araguainha resides.
他最近在古地理学，古气候学，古生态学”杂志上发表的一片文章详细描述了阿拉瓜尼亚附近的岩石性质。

After an extensive geological survey, he and his team discovered that a sizeable amount of this rock is oil shale.
经过了广泛的地质学研究调查后，他和他的团队发现这些岩石很多都是油页岩，

Any hydrocarbons in the crater would certainly have been vaporised.
这说明所有的碳氢化合物已经被汽化。

More intriguingly, the researchers calculate that the impact would have generated thousands of earthquakes of up to magnitude 9.9 for hundreds of kilometres around.
更有趣的是，研究人员通过计算发现撞击所产生的能量足以在附近数百公里的范围内产生几千次9.9级地震。

In effect, it would have been the biggest fracking operation in history, releasing oil and gas from the shattered rock in prodigious quantities.
从效果上来说，这也许是历史上最大的裂解工程，它将数量惊人的石油和天然气从岩石中释放出来。

The upshot, Dr Tohver believes, would have been a huge burp of methane into the atmosphere.
托瓦尔博士相信，这导致了巨量的甲烷被排向大气层。

Since methane is a powerful greenhouse gas, that burp would have resulted in instant global warming, making things too hot for much of the planet's animal life.
而甲烷是很强的温室效应气体，它就使得全球气温瞬时上升，过高的气温危及了许多物种的生存。

Presto! The Permian mass extinction is explained.
是的！这就是二叠纪物种大灭绝的解释。

Determining whether this was really what happened will take a lot more digging, of course.
当然，要确认这是不是造成这一切的真实原因还有待更多的探索研究。

Even now, there are those who think the formation of the Chicxulub crater was a coincidence, and that what did for the dinosaurs was actually the volcanoes, so Dr Tohver will have to work hard to convince the sceptics.
直到今天还有人认为希克苏鲁伯陨石坑的形成只不过是个巧合，真正让恐龙消亡的原因是火山爆发，所以托瓦尔博士还需要加把劲来证明他的假设。

If he does, though, he will have proved himself a great geological detective, for he will have been responsible for solving one of the biggest puzzles in palaeontology.
如果他做到了，他将证明他是一个伟大的地质学发现者，因为他解开了古生物学中的一个最重要的谜题。