A geochemist goes à la recherche des climats perdus.

As a young postdoc at the California Institute of Technology (Caltech) in Pasadena I remember glancing through the 1952 logbook of a gas mass spectrometer while the machine readied my samples. In the book, Sam Epstein, one of the founders of modern geochemistry, had scribbled numbers representing the first attempt to determine past temperatures from oxygen-isotope abundances in fossils.

Since Epstein's measurements, the abundance of oxygen-18 in the carbonate skeletons of fossil sea creatures has become a broadly used indicator of past ocean temperatures. Such data are key to understanding modern climate change. But the usefulness of 18O in 'palaeothermometry' is limited by problems including variations in oxygen-isotope levels in sea water and in the way different organisms take up the isotopes.

Recently, a group at Caltech proposed a measurement that may work better. As before, the carbonates are broken down into carbon dioxide for analysis. Instead of looking only for molecules containing 18O, the Caltech team measures the abundance of molecules that contain both 18O and the uncommon carbon isotope, carbon-13. The excess of this species over what would be expected through random combination of carbon and oxygen atoms indicates the temperature at which the carbonate formed.

Early tests of this 'clumped' thermometer on corals and fish ear bones were promising (P. Ghosh et al. Geochim. Cosmochim. Acta 70, 1439–1456; 2006; and Geochim. Cosmochim. Acta 71, 2736–2744; 2007). Since then, the method has provided a new record of ocean temperature during the Palaeozoic era, which began 543 million years ago (R. E. Came et al. Nature 449, 198–201; 2007).

I believe that clumped isotope thermometry is going to be a valuable new tool for palaeoenvironmental studies.

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