Geologists are able to tell you the exact history of the waxing and waning of glaciers over the past five million years because microscopic creatures in the ocean have been unwittingly recording this dance in their shells. Their shells are made from the carbon and oxygen found in seawater. As glaciers form, seawater is removed from the ocean and trapped on land, resulting in subtle changes in the chemistry of the ocean. These changes are recorded in the shells, which create a detailed history as they pile up on the ocean floor.
For decades paleoclimatologists have used the records of seashells to reconstruct either the volume of glacial ice trapped on land or the temperature history of the ocean, providing a beautifully detailed picture of climate over the past 5 million years. These approaches, however, are limited by the fact that the scientist must know the exact chemistry of the water that the shells were formed in to calculate temperature, or the exact temperature at which they formed to calculate the chemistry of the water. This fact has confounded hundreds of studies about the history of our planet. A revolutionary new method has solved that problem. It also has its sights set on topics as diverse as the biology of dinosaurs and the evolution of man.
These methods work because some molecules of the same element have different masses. These variants are known as isotopes. Many of these isotopes are unstable, meaning they break down into other elements while releasing harmful radiation. Just as important to geologists, though, are the stable ones – atoms that exist for eternity with a fixed number of protons and neutrons. Carbon has two stable isotopes, one with an atomic mass of 12, and one with an atomic mass of 13. Similarly, oxygen as three: masses 16, 17, and 18. In both cases the light isotopes are common, while the heavier ones are exceedingly rare.
Because of these variations in mass, nature treats the heavy isotopes slightly differently than the light ones. When the shells of ocean creatures are formed, they form with a fixed ratio of heavier and lighter isotopes, leaving hints about environmental conditions at that time. These ratios are trapped in carbonate, a molecule that contains one carbon and three oxygen atoms and is the primary building block of seashells (among many other things).
Relative changes in the temperature of the ocean at the time the shell formed can be calculated using these ratios. This is because at colder temperatures oxygen 18 and oxygen 16 behave more similarly than at warm temperatures, when increased energy makes oxygen 16 more likely to react and form carbonate than oxygen 18. Shells that form under colder temperatures, then, will have more oxygen 18 than at warm temperatures.
Information like this is critical if we wish to understand how our climate system operates, and what changes humanity will face as our planet continues to warm. But actual temperature values (i.e. degrees Celsius) would be even more useful.
Using oxygen isotope ratios to calculate absolute temperature is problematic, though. This ratio is also affected by the amount of oxygen 18 and oxygen 16 in the water to begin with. Ice prefers to form from oxygen 16. Therefore as more ice is trapped on land, more oxygen 16 is stripped from the ocean water. This results in oceans with more oxygen 18 in glacial times. This is the principle that is employed when scientists study shells to reconstruct the history of ice ages, but it means that more than one process can affect the oxygen isotope ratio in shells.
Because fluctuations in this ratio can be driven by both temperature and the original isotopic composition of the seawater, a researcher could not simply take, for example, a 65 million year old seashell and tell you the temperature of the water was when it formed. Absolute temperature values are almost impossible to calculate from oxygen isotopes in older carbonate samples.
That was before 2006, before the term ‘clumped-isotope’ entered the geologic lexicon and revolutionized the use of isotopes to study temperature. Using a new method known as ‘clumped-isotope paleothermometry,’ a researcher could indeed pick up a 65 million year old seashell and tell you the temperature in which it was formed without knowing anything else about it.
While the technique represents a complex and technical scientific achievement, the premise behind the method is fairly straightforward. When a carbonate molecule forms with more than one heavy isotope, the bond holding that molecule together is stronger than if it formed with only the common light isotopes. Before a carbonate mineral is formed and locked in place as a bone, shell, or rock, the carbon and oxygen atoms dance around, repeatedly switching partners. Because of this dance, you might expect a random distribution of light isotope to light isotope bonds (i.e. carbon 12 bonded to oxygen 16) and heavy-to-heavy bonds (i.e carbon 13 to oxygen 18).
The beauty lies in the fact that this is not the case. Because heavy-to-heavy bonds are stronger, they last just a little bit longer than the other arrangements. This is especially true in cold conditions, when there is less energy to break bonds to begin with. The higher the temperature in this sea of isotopes, the more chaotic the dance becomes. With more chaos, the ability for heavy-to-heavy bonds to remain together is reduced, and eventually removed, yielding the random distribution of bonds one might expect. The result is simple: carbonate formed in cool conditions will have more molecules with more than one heavy isotope, whereas carbonate formed in warm conditions will have fewer.
The process by which heavy isotopes join together is referred to as ‘clumping,’ and with the advent of new and highly sophisticated laboratory equipment, scientists can measure the degree to which it has occurred in carbonate. Years of experiments have related the amount of clumping in carbonate directly to temperature. Best of all, the starting composition of the water that formed the carbonate is irrelevant. If it formed at the same temperature, a researcher will get the same value whether large amounts of oxygen 16 were removed from the ocean by ice or not.
Ocean temperatures are not the only questions that can be addressed using clumped-isotopes, though.
Drs. Benjamin Passey and Naomi Levin at Johns Hopkins University, for example, are interested in human evolution. They wanted to tackle an old but important question: was the time period that led to the emergence of hominids cooler than the present in key anthropologic sites in Africa? Some researchers have said yes, but many have suggested that it was significantly warmer than present.
Many theories of human evolution depend on knowing what the environment was like at this time, so Passey and Levin decided to apply clumped-isotope methods to fossilized African soil (soil commonly contains carbonate minerals). They found that temperatures over the past 5 million years have been either the same as, or warmer than, the present. This limits, in their view, any hypothesis relating the evolution of human traits to those that can be explained by similar or warmer temperature to the present.
Jumping back many millions of years, Dr. Robert Eagle at Caltech wanted to know more about the metabolism of sauropod dinosaurs, massive creatures like the famous Brachiosaurus. A long-standing debate amongst paleontologists is whether these creatures were cold-blooded, deriving their energy from the environment like modern day reptiles, or if they possessed some form of endothermy, maintaining their body heat internally as mammals and birds do today.
Because the bone and teeth of vertebrates are composed of bioapatite, a carbonate mineral, Eagle decided to use clumped isotopes to tackle this question. Previous work on modern animals has shown that the temperatures derived from teeth are representative of body temperatures. So in a 2011 paper, he looked at the temperatures recorded in the fossilized teeth of sauropods. He determined that the temperature at which the bioapatite in their teeth were forming was much higher than those for modern reptiles, similar to mammals, but lower than birds. This ruled out a cold-blooded dinosaur, and posed new questions about dinosaur biology.
Clumped-isotope paleothermometry is still in its infancy, but it is rapidly expanding. 2006 was the first year any paper used the term “clumped-isotope.” In 2011, 19 papers did, and many more are on the horizon.
Still many kinks need to be worked out. Methods need to be standardized and conclusions need to be scrutinized. Undoubtedly a period of time will come, as is the case with most scientific developments, where researchers identify more and more problems, adding a dose of reality to optimism. For now, though, the slight preference for some isotopes to stay bonded together is ushering in a new world of possibilities for earth scientists. This is the beginning of something big.