If you were traveling from the United States to Singapore, a guide book could tell you what sort of climate to expect. But if, like paleontologists and geologists, you needed to know what the weather was like in Singapore tens of millions of years ago, you’d have to consult a different source.

By examining the chemistry of shell fossils, researchers can discover what temperatures were like in ancient climates.

Oxygen provides the key to finding out about ancient temperatures. Oxygen comes in two different forms — heavy and light. By determining the ratio of the two different types of oxygen in the fossils, researchers can make an educated guess at the temperature of the environment the animals lived in.

Both types of oxygen are found in carbon dioxide, which is found both in the atmosphere and in water. The amount of carbon dioxide water contains depends on the temperature of the water. Cooler water contains more carbon dioxide.

But as the water warms, carbon dioxide gas escapes. Carbon dioxide made of the lighter oxygen escapes at cooler temperatures, leaving more carbon dioxide made of the heavy oxygen in the water. At warmer temperatures, carbon dioxide made of heavy oxygen begins to escape, leaving less heavy oxygen in the water.

Since animals living in the ocean use the carbon and oxygen that remains in the water to build their shells, the shells of animals that lived in a cool climate contain more heavy oxygen.

By grinding up the shells and determining how much of each type of oxygen they contain, researchers can learn what temperatures were like in ancient environments.

科学家通过远古贝壳知晓过去

如果你从美国去新加坡，查看一下旅行手册就能知道那里什么气候了。但是如果你需要像古生物学家或地理学家那样了解几千万年前新加坡的气候，你需要参阅另外一种东西。

通过研究贝壳化石的化学成分，研究人员可以发现远古时期的温度大致如何。

氧元素为发现远古时期的温度提供了钥匙。氧元素有两种存在形式-重氧和轻氧。通过测定两种氧原子在化石中的比例，研究人员可以对这种生物所处环境的温度做出较有根据的推断。

这两种氧原子均可发现于二氧化碳中，后者在大气和水中都存在。水中二氧化碳含量取决于水的温度。水温越低，它可含的二氧化碳越高。

当水变暖时，二氧化碳气体会逸出。由于轻氧原子构成的二氧化碳可在较低温度逸出，使水中留下的多是重氧原子构成的二氧化碳。在更高温度下，由重氧原子构成的二氧化碳也开始逸出，使水中所含重氧原子减少。

由于海洋生物使用水中的碳和氧构筑它们的壳，因此在较冷气候下生存的生物的外壳会含有更多的重氧原子。

通过研磨贝壳并测定其各种氧原子的含量，研究人员就可以知道远古环境下温度如何了。