Heat of the Moment

Categories: Geology

Heat of the moment: measuring temperature in the rock record

The new method could be used to determine the temperature during periods like ice ages in Earth’s history.
Credit: Sandy Shipley

What was the body temperature of a dinosaur? How cold was it on land when the ice ages began? How much heat does an earthquake generate? Temperature is one of the most significant variables in Earth and environmental sciences – but also one of the most elusive and difficult to measure. Now a new method devised and applied by geochemist John M Eiler of the Division of Geological and Planetary Sciences at the California Institute of Technology in Pasadena, California can measure the temperature in all those situations – and more. He describes how at the Goldschmidt2009 meeting in Davos this week.

The new method depends on using isotopes of common elements such as carbon and oxygen – versions of these elements with atomic nuclei of slightly different weights. The role of these ‘stable isotopes’ has long been recognized because they behave differently in chemical reactions compared to the ordinary atoms. Measuring the proportions of the different isotopes can pick out particular processes such as evaporation. The challenge that Eiler and his team have tackled is measuring the amounts of molecules where two or more atoms have been replaced by their heavier stable isotopes. These multiply-substituted molecules are very rare.

Adult Massospondylus and embryo. By studying biomineralisation in fossils, scientists can actually determine the body temperature of dinosaurs.
Image credit: Robert Reisz, University of Toronto at Mississauga

There’s nothing new in the chemistry, but there is a lot in the measurement and application, according to Eiler. “The initial chemistry was recognized many decades ago, but no one through that you could use them for geochemistry because they were too rare. Five or six years ago, my group and I figured out how to measure the abundances of multiply-substituted molecules at high precision, by subtly modifying gas source mass spectrometers in a way we could have done long ago.” That realization opened the door to a range of applications of the ‘clumped isotope method’, initially based on analysis of carbon dioxide. “It’s as if we look at all the possible substitutions and get a fingerprint from the molecules. Then we can use this to make temperature estimates.”

Carbon (12C) normally has 6 protons and 6 neutrons in its nucleus; 13C is the stable isotope with an extra neutron. Similarly, oxygen (16O) normally has 8 each of protons and neutrons, but the isotope 18O has 2 extra neutrons. If you consider crystals forming in water, as limestone (calcium carbonate, CaCO3) does, the heavier isotope is concentrated in the solid carbonate compared to the water. “If there’s lots of 18O, then it all goes into the solid carbonate rather than in the water”, says Eiler. “But because this is a thermodynamic effect, temperature matters. At very low temperatures, the 18O is all in the carbonate, but as the temperature rises, you get some random scatter that moves it about.” This is what Eiler measures, and it gives the temperature as an independent measure, in away that previous methods do not.

Terrestrial options for early climate. Early earth, snowball, cauldron or temperate? Now, scientists may be able to find out.
Credit: NASA

The simple dependence of the proportion of the clumped isotope molecules on temperature is the reason for the success of this method. Other measures of temperature in geological settings depend on knowing, for example, something about the composition of the water in which a crystal forms. The trouble is, there is rarely any evidence of the water from the geological past. “We’ve plenty of fossil carbon dioxide,” says Eiler, “but show me your fossil water? There pretty much isn’t any.” This new method gives a temperature reading independent of the properties of the long-gone water, putting the temperature record on a sound footing in away that has never before been achieved. It does, however, depend on very precise measurement. “It is analytically challenging,” admits Eiler. “We have to measure changes of 5 parts per million in the relative abundance of the species for every degree Celsius. These are very low abundances and we have to be exquisitely precise. This is a pain, but it’s solvable, by taking measurements over a very long time and using lots and lots of samples. But we’ve demonstrated we can do it. We can do palaeothermometry, we can measure the temperature on land and in lakes in the distant past. We can now start to tackle longstanding debates, such as about the relationship between surface temperature and drivers such as carbon dioxide in the atmosphere. We can delve deep in to the history of the Earth and look at the temperature inland at the start of the ice ages, for example.”

Eiler and his group at Caltech are developing a whole new set of applications for this clever method – which is where the dinosaurs come in. “Because we don’t need to worry about the initial 18O concentration, we can look again at biomineralisation in fossils. The carbonate content of eggshells should indicate the body temperature at which they grew.” In other words, given that dinosaur eggs are known as fossils, they are on the way to measuring the body temperature of dinosaurs. Other applications include major climate variables such as altitude, allowing palaeoclimatologists to distinguish between major climate cooling and localised cooling resulting from uplift of land into higher, cooler parts of the atmosphere. Within the Earth, the new method can show the temperatures that formed oil and gas deposits, and even measure how hot active faults are, where the rocks are heated by friction. And it doesn’t stop at the surface of the Earth; the team have already investigated stratospheric carbon dioxide and discovered a previously-unknown process in the atmosphere. Looking even further afield, examining carbonate minerals in meteorites opens the way to knowing the temperature at which they formed in the cloud of dust and gas that comprised the early solar system – and even to understanding the hydrogeology of asteroids.