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Concept
Clumped isotopes
Résumé
Clumped isotopes are heavy isotopes that are bonded to other heavy isotopes. The relative abundance of clumped isotopes (and multiply-substituted isotopologues) in molecules such as methane, nitrous oxide, and carbonate is an area of active investigation. The carbonate clumped-isotope thermometer, or "13C–18O order/disorder carbonate thermometer", is a new approach for paleoclimate reconstruction, based on the temperature dependence of the clumping of 13C and 18O into bonds within the carbonate mineral lattice. This approach has the advantage that the 18O ratio in water is not necessary (different from the δ18O approach), but for precise paleotemperature estimation, it also needs very large and uncontaminated samples, long analytical runs, and extensive replication. Commonly used sample sources for paleoclimatological work include corals, otoliths, gastropods, tufa, bivalves, and foraminifera. Results are usually expressed as Δ47 (said as "cap 47"), which is the deviation of the ratio of isotopologues of CO2 with a molecular weight of 47 to those with a weight of 44 from the ratio expected if they were randomly distributed.
Molecules made up of elements with multiple isotopes can vary in their isotopic composition, these different mass molecules are called isotopologues. Isotopologues such as 12C18O17O, contain multiple heavy isotopes of oxygen substituting for the more common 16O, and are termed multiply-substituted isotopologues. The multiply-substituted isotopologue 13C18O16O contains a bond between two of these heavier isotopes (13C and 18O), which is a "clumped" isotope bond.
The abundance of masses for a given molecule (e.g. CO2) can be predicted using the relative abundance of isotopes of its constituent atoms (13C/12C, 18O/16O and 17O/16O). The relative abundance of each isotopologue (e.g. mass-47 CO2) is proportional to the relative abundance of each isotopic species.
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