The analysis of mass 47 isotopologues of CO_2 (mainly ^(13)C^(18)O^(16)O) is established as a constraint on sources and sinks of environmental CO_2, complementary to δ^(13)C and δ^(18)O constraints, and forms the basis of the carbonate ‘clumped isotope’ thermometer. This measurement is commonly reported using the Δ_(47) value — a measure of the enrichment of
doubly substituted CO_2 relative to a stochastic isotopic distribution. Values of Δ_(47) approach 0 (a random distribution) at high temperatures (≥ several hundred degrees C), and increase with decreasing temperature, to ~1 ‰ at 25 °C
