Statistical clumped isotope signatures

Abstract

High precision measurements of molecules containing more than one heavy isotope may provide novel constraints on element cycles in nature. These so-called clumped isotope signatures are reported relative to the random (stochastic) distribution of heavy isotopes over all available isotopocules of a molecule, which is the conventional reference. When multiple indistinguishable atoms of the same element are present in a molecule, this reference is calculated from the bulk (≈average) isotopic composition of the involved atoms. We show here that this referencing convention leads to apparent negative clumped isotope anomalies (anti-clumping) when the indistinguishable atoms originate from isotopically different populations. Such statistical clumped isotope anomalies must occur in any system where two or more indistinguishable atoms of the same element, but with different isotopic composition, combine in a molecule. The size of the anti-clumping signal is closely related to the difference of the initial isotope ratios of the indistinguishable atoms that have combined. Therefore, a measured statistical clumped isotope anomaly, relative to an expected (e.g. thermodynamical) clumped isotope composition, may allow assessment of the heterogeneity of the isotopic pools of atoms that are the substrate for formation of molecules.

Optical clumped isotope thermometry of carbon dioxide

“…At 300 K the ZPE difference factor deviates from unity by 5 parts in 10 6 if Δ ν 0 = 0.001 cm −1 . This implies that an uncertainty of a few ppm – a range that will be amenable to measurements in the near future – is sufficient to determine ZPE differences at the 0.001 cm −1 uncertainty level, irrespective whether the ZPE differences are large, as in the case of the H 2 + D 2 2 HD reaction where Δ ν 0 = 54.867 cm −1 50,51 , or small – as in reaction R1, where calculated values 22,33,34 range from 0.433 to 0.435 cm −1 .…”

“…this will be the predominant clumping mechanism. By definition, statistical combination of two different isotopic reservoirs leads to position-independent (anti-)clumping 48 , 49 . The similar magnitude of isotope fractionation in both reactions R2 and R1 (Fig.…”

H2 clumped isotope measurements at natural isotopic abundances

“…Large ΔDD signals can also occur from mixing H 2 gases with different isotopic compositions, even if, for each of these gases, the ΔDD = 0 . Finally, when forming H 2 from two atom pools with different isotope ratios, a negative ΔDD value can result …”

“…31 Finally, when forming H 2 from two atom pools with different isotope ratios, a negative ΔDD value can result. 32,33 The clumped isotope ratio of molecular hydrogen at natural abundances has never been measured. That is partly because of the very low abundance of the DD molecules.…”

Kapcsolt izotópok (clumped isotopes) a földtudományi kutatásokban

“…Kiemelt cél a hatékonyság és a precizitás növelése, illetve a szükséges mintamennyiség csökkentése (pl. HE et al 2012, BERNASCONI et al 2013, ROSENHEIM et al 2013, CUI & WANG 2014, HU et al 2014, RÖCKMANN et al 2016. A kalibrációs munkák célja az alkalmazhatóság kiterjesztése szélesebb hőmérsékleti tartományokra (pl.…”