The answer lies in a slightly obscure but ingenious field known as isotope 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 swarmed with tiny plankton 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 durable compounds such as calcium 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 narrative 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 limestone. 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 isotopes—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 neutrons.) （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 timing 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。