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Mathieu Daëron
mathieu@daeron.fr
CNRS / LSCE

Clumped isotopes

“Clumped isotope” geochemistry is concerned with the thermodynamic propensity of rare, heavy isotopes (2H, 13C, 18O...) to bond preferentially with each other rather than with light isotopes (1H, 12C, 16O...). In a given geochemical sample, this “clumping” manifests as a statistical overabundance of multiply substituted isotopologues (2H2H, 16O13C18O...) relative to an ideal “stochastic” state of the sample material, corresponding to a purely random distribution of all isotopes. In natural materials, deviations from this stochastic distributions generally remain small, within 1% (relative), and are influenced by several types of processes including thermodynamically-controlled reactions, kinetic fractionations, diffusion-limited transport processes, and mixing of samples with different bulk isotopic compositions. A relatively painless introduction to these issues can be found in J. Eiler's review paper [1].

A significant number of people from various research institutions have recently contributed to analytical developments aiming to precisely measure the degree of clumping in geochemical samples, and applied this type of measurements to the study of various natural systems. To date, the most mature sub-field within clumped isotope geochemistry concerns CO2 with a nominal cardinal mass of 47, comprising mostly 16O13C18O. Not only does this constitute a new, promising tracer of bio/geological processes affecting CO2, but it may also be used to quantify the clumping of 13C18O bonds in carbonate minerals, providing a new quantitative method for carbonate paleothermometry which does not require any information on the δ18O of ancient waters. This approach has for example been used to reconstruct Cenozoic paleotemperatures from marine and terrestrial settings, to characterize bio-calcification mechanisms [6], or to study the thermal histories of aqueously altered meteorites.

The field of clumped isotope studies is rapidly evolving. If you would like to know more about it, and to keep up to date with the latest exciting developments, you can browse the Clumpy Wiki, which is a collaborative website dedicated to clumped isotopes.

[1] J. Eiler. “Clumped-isotope” geochemistry—The study of naturally-occurring, multiply-substituted isotopologues. Earth and Planetary Science Letters (2007), 262:309–327.

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