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.
科學(xué)家通過遠古貝殼知曉過去
如果你從美國去新加坡,查看一下旅行手冊就能知道那里什么氣候了。但是如果你需要像古生物學(xué)家或地理學(xué)家那樣了解幾千萬年前新加坡的氣候,你需要參閱另外一種東西。
通過研究貝殼化石的化學(xué)成分,研究人員可以發(fā)現(xiàn)遠古時期的溫度大致如何。
氧元素為發(fā)現(xiàn)遠古時期的溫度提供了鑰匙。氧元素有兩種存在形式-重氧和輕氧。通過測定兩種氧原子在化石中的比例,研究人員可以對這種生物所處環(huán)境的溫度做出較有根據(jù)的推斷。
這兩種氧原子均可發(fā)現(xiàn)于二氧化碳中,后者在大氣和水中都存在。水中二氧化碳含量取決于水的溫度。水溫越低,它可含的二氧化碳越高。
當(dāng)水變暖時,二氧化碳氣體會逸出。由于輕氧原子構(gòu)成的二氧化碳可在較低溫度逸出,使水中留下的多是重氧原子構(gòu)成的二氧化碳。在更高溫度下,由重氧原子構(gòu)成的二氧化碳也開始逸出,使水中所含重氧原子減少。
由于海洋生物使用水中的碳和氧構(gòu)筑它們的殼,因此在較冷氣候下生存的生物的外殼會含有更多的重氧原子。
通過研磨貝殼并測定其各種氧原子的含量,研究人員就可以知道遠古環(huán)境下溫度如何了。