Clumped isotope geochemistry
Clumped isotope geochemistry builds upon conventional light-element gas-source stable-isotope mass spectrometry. For carbon dioxide liberated from carbonate minerals, as well as measuring cardinal masses 44, 45, and 46 to deduce 13C/12C and 18O/16O ratios, mass 47 (largely 13C18O16O) provides information about the temperature of carbonate formation. The 'clumping' of 13C-18O-16O or deviation (Δ47) in its abundance from a stochastic distribution (attained at high temperature) of this isotopologue has a temperature sensitivity of about 0.004 per ml per °C, at low temperatures. With care, Δ47 can be measured to a precision of±0.01‰, implying a resolution, at Earth-surface conditions, of about ±2 °C. Furthermore, Δ47 is independent of the 18O/16O ratio of the water from which inorganic or biogenic carbonate minerals precipitate. Accordingly, palaeotemperatures can be deduced without knowledge of the past 18O/16O (as δ18O) of the water; indeed, this value is also derived when all three isotopic ratios (m/e 47/44, 46/44, and 45/44) are measured. The technique is finding wide application in palaeoceanography, even to older parts of the geological record (to approximately 500 Ma), and is particularly important in the terrestrial realm for palaeosols and lake carbonates. As phosphatic bones also contain a few percent carbonate, they can also be analysed. One surprising result is that the Δ47 data from some dinosaur bones indicate their warm-bloodedness. The systematics of clumping in non-carbonates such as oxygen (17O-18O and 18O-18O) and the isotopologues of methane, the latter also being temperature-dependent, indicate the expanding field will find wide application in atmospheric, biological, and earth sciences.
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