万物简史 第218期:把铅撵出去(7)

 　　Rather than restate every one, scientists decided¹ to keep the inaccurate² constant. "Thus," Tim Flannery notes, "every raw radiocarbon date you read today is given as too young by around 3 percent." The problems didn't quite stop there. It was also quickly discovered that carbon-14 samples can be easily contaminated with carbon from other sources—a tiny scrap³ of vegetable matter, for instance, that has been collected with the sample and not noticed.

　　科学家们没有修正每个计算结果，而是决定保留这个不准确的常数。“这样，”提姆·弗兰纳里说，“你只要把今天见到的每一个以放射性碳年代测定法测定的年代减去大约3%。”问题没有完全解决。人们又很快发现，碳－14的样品很容易被别处的碳污染──比如，一小点儿连同样品一起被采集来的而又没有被注意到的植物。

　　铅的危害

　　For younger samples— those under twenty thousand years or so—slight contamination does not always matter so much, but for older samples it can be a serious problem because so few remaining atoms are being counted. In the first instance, to borrow from Flannery, it is like miscounting by a dollar when counting to a thousand; in the second it is more like miscounting by a dollar when you have only two dollars to count.

　　对于年代不大久远的样品来说──年代小于大约2万年的样品──稍有污染并不总是关系很大，而对于年代比较久远的样品来说，这有可能是个严重的问题，因为统计中的剩余原子数实在太少了。借用弗兰纲纳里的话来说，在第一种情况下，就像是1000美元里少数1美元；而在第二种情况下，就像是仅有的2美元里少数了1美元。

　　Libby's method was also based on the assumption that the amount of carbon-14 in the atmosphere, and the rate at which it has been absorbed by living things, has been consistent throughout history. In fact it hasn't been. We now know that the volume of atmospheric⁴ carbon-14 varies depending on how well or not Earth's magnetism⁵ is deflecting⁶ cosmic rays, and that that can vary significantly over time. This means that some carbon-14 dates are more dubious⁷ than others. This is particularly so with dates just around the time that people first came to the Americas, which is one of the reasons the matter is so perennially⁸ in dispute.

　　而且，利比的方法是以如下假设为基础的，即大气里碳－14的含量以及生物吸收这种物质的速度，在整个历史进程中是始终不变的。事实并非如此。我们现在知道，大气里碳－14的数量变化不定，取决于地球的磁场能否有效地改变宇宙射线的方向；在漫长的时间里，变化的幅度可能很大。这意味着，有些以碳－14年代测定法测定的年代要比别的这类年代更无把握。在比较缺少把握的年代当中，有人类首次抵达美洲前后这一段时期的年代。这就是为什么那个问题老是争论不休的原因之一。

1 decided   

adj.决定了的，坚决的；明显的，明确的

参考例句：

• This gave them a decided advantage over their opponents.这使他们比对手具有明显的优势。
• There is a decided difference between British and Chinese way of greeting.英国人和中国人打招呼的方式有很明显的区别。

2 inaccurate   

adj.错误的，不正确的，不准确的

参考例句：

• The book is both inaccurate and exaggerated.这本书不但不准确，而且夸大其词。
• She never knows the right time because her watch is inaccurate.她从来不知道准确的时间因为她的表不准。

3 scrap   

n.碎片；废料；v.废弃，报废

参考例句：

• A man comes round regularly collecting scrap.有个男人定时来收废品。
• Sell that car for scrap.把那辆汽车当残品卖了吧。

4 atmospheric   

adj.大气的，空气的；大气层的；大气所引起的

参考例句：

• Sea surface temperatures and atmospheric circulation are strongly coupled.海洋表面温度与大气环流是密切相关的。
• Clouds return radiant energy to the surface primarily via the atmospheric window.云主要通过大气窗区向地表辐射能量。

5 magnetism   

n.磁性，吸引力，磁学

参考例句：

• We know about magnetism by the way magnets act.我们通过磁铁的作用知道磁性是怎么一回事。
• His success showed his magnetism of courage and devotion.他的成功表现了他的胆量和热诚的魅力。

6 deflecting   

（使）偏斜， （使）偏离， （使）转向( deflect的现在分词 )

参考例句：

• A variety of mechanical surfaces have been employed for deflecting the exhaust jets of solid-propellant rockets. 人们已经用过各种类型的机械控制面来偏转固体推进剂火箭的排气流。
• If she made a leading statement, he was expert deflecting her into more impersonal channels. 只要她一开口，他就会巧妙地把她的话题转到与个人无关的问题上去。

7 dubious   

adj.怀疑的，无把握的；有问题的，靠不住的

参考例句：

• What he said yesterday was dubious.他昨天说的话很含糊。
• He uses some dubious shifts to get money.他用一些可疑的手段去赚钱。

8 perennially   

adv.经常出现地；长期地；持久地；永久地

参考例句：

• He perennially does business abroad. 他常年在国外做生意。 来自辞典例句
• We want to know what is perennially new about the world. 我们想知道世上什么东西永远是新的。 来自互联网