万物简史 第487期:多灾多难的生命进程(7)

 　　And how, you may reasonably wonder, can scientists know what oxygen levels were like hundreds of millions of years ago? 你也许有理由想知道，科学家们怎么会知道几亿年以前的氧气浓度？

　　The answer lies in a slightly obscure but ingenious field known as isotope¹ geochemistry. 答案在于同位素地球化学，这是个不大知名而又十分奇妙的领域。

　　The long-ago seas of the Carboniferous and Devonian swarmed² with tiny plankton³ that wrapped themselves inside tiny protective shells. 泥盆纪和石炭纪的古代海洋里生活着大批微小的浮游生物。它们躲在小小的保护性壳里。

　　Then, as now, the plankton created their shells by drawing oxygen from the atmosphere 当时和现在一样，浮游生物从大气里吸收氧气，

　　and combining it with other elements (carbon especially) to form durable⁴ compounds such as calcium⁵ carbonate. 将其与别的元素（尤其是碳）化合，形成了碳酸钙这样的耐久化合物，构筑了自己的壳。

　　It's the same chemical trick that goes on in (and is discussed elsewhere in relation to) the long-term carbon cycle, 在长期碳循环中——这个过程讲起来不大激动人心，但对于把地球变成一个适居的地方却是至关重要的，

　　a process that doesn't make for terribly exciting narrative⁶ but is vital for creating a livable planet. 不停进行的就是这种化学戏法（在介绍长期碳循环的时候，这种戏法已经在别处讨论过）。

　　Eventually in this process all the tiny organisms die and drift to the bottom of the sea, where they are slowly compressed into limestone⁷. 在此过程中，这些微小的生物最后都死了，沉到了海底。慢慢地被压缩成灰岩。

　　Among the tiny atomic structures the plankton take to the grave with them are two very stable isotopes⁸—oxygen-16 and oxygen-18. 在浮游生物带进坟墓的小小原子结构中，有两种非常稳定的同位素——O-16和O-l8。

　　(If you have forgotten what an isotope is, it doesn't matter, though for the record it's an atom with an abnormal number of neutrons⁹.) （要是你忘了什么是同位素，那也不要紧。你只要记住，带有超量中子的原子就是同位素。）

　　This is where the geochemists come in, 地球化学家就利用了这一点，

　　for the isotopes accumulate at different rates depending on how much oxygen or carbon dioxide is in the atmosphere at the time of their creation. 因为同位素以不同的速度积聚，取决于同位素形成之时大气里有多少氧或二氧化碳。

　　By comparing these ancient ratios, the geochemists can cunningly read conditions in the ancient world, 地球化学家把这两种同位素在古代的储存速度进行比较，就可以知道古代世界的情况，

　　oxygen levels, air and ocean temperatures, the extent and timing¹⁰ of ice ages, and much else. 氧气的浓度、空气和海洋的温度、冰期的程度和时间，以及许多别的情况。

1 isotope   

n.同位素

参考例句：

• The isotope ratio is directly used for comparing oils or gases.同位素比率直接用于比较各种石油或天然气。
• How to apply a radio isotope?如何运用放射性同位素？

2 swarmed   

密集( swarm的过去式和过去分词 )； 云集； 成群地移动； 蜜蜂或其他飞行昆虫成群地飞来飞去

参考例句：

• When the bell rang, the children swarmed out of the school. 铃声一响，孩子们蜂拥而出离开了学校。
• When the rain started the crowd swarmed back into the hotel. 雨一开始下，人群就蜂拥回了旅社。

3 plankton   

n.浮游生物

参考例句：

• Plankton is at the bottom of the marine food chain.浮游生物处于海洋食物链的最底层。
• The plankton in the sea feeds many kinds of animals. 海的浮游生物成为很多种动物的食物。

4 durable   

adj.持久的，耐久的

参考例句：

• This raincoat is made of very durable material.这件雨衣是用非常耐用的料子做的。
• They frequently require more major durable purchases.他们经常需要购买耐用消费品。

5 calcium   

n.钙(化学符号Ca)

参考例句：

• We need calcium to make bones.我们需要钙来壮骨。
• Calcium is found most abundantly in milk.奶含钙最丰富。

6 narrative   

n.叙述，故事；adj.叙事的，故事体的

参考例句：

• He was a writer of great narrative power.他是一位颇有记述能力的作家。
• Neither author was very strong on narrative.两个作者都不是很善于讲故事。

7 limestone   

n.石灰石

参考例句：

• Limestone is often used in building construction.石灰岩常用于建筑。
• Cement is made from limestone.水泥是由石灰石制成的。

8 isotopes   

n.同位素；同位素( isotope的名词复数 )

参考例句：

• the many isotopes of carbon 碳的诸多同位素
• Tritium is one of the mildest radioactive isotopes. 氚是最和缓的放射性同位素之一。 来自《简明英汉词典》

9 neutrons   

n.中子( neutron的名词复数 )

参考例句：

• The neutrons and protons form the core of the atom. 中子和质子构成了原子核。 来自《简明英汉词典》
• When an atom of U235 is split,several neutrons are set free. 一个铀235原子分裂时，释放出几个中子。 来自《简明英汉词典》

10 timing   

n.时间安排，时间选择

参考例句：

• The timing of the meeting is not convenient.会议的时间安排不合适。
• The timing of our statement is very opportune.我们发表声明选择的时机很恰当。