Statistical clumped isotope signatures.

T Röckmann,M E Popa,M E G Hofmann,M C Krol

doi:10.1038/srep31947

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.

Highlights

High precision measurements of molecules containing more than one heavy isotope may provide novel constraints on element cycles in nature
We show here that this referencing convention leads to apparent negative clumped isotope anomalies when the indistinguishable atoms originate from isotopically different populations
The apparent negative statistical clumped isotope signatures that we describe in this paper are fundamentally related to this referencing convention, in particular the choice of the reference ratio iRcl,random that is required to calculate Δi in Eq 3

Summary

OPEN Statistical clumped isotope signatures

Since multiply substituted isotopocules are thermodynamically more stable than single substituted ones, classical isotope theory predicts small but measurable positive clumped isotope anomalies for most molecules under natural conditions[16,17,18,19] These clumped isotope signatures depend on temperature, which is the basis of the new field of clumped isotope thermometry[20]. Yeung et al.[14] attributed the negative Δvalues (see equation 3 for definition) in photosynthetic O2 to different isotopic composition of the two O atoms originating from different sites in the oxygen evolving complex of photosystem II Triggered by this observation we investigated this further and show here that negative clumping anomalies are necessarily expected whenever two or more indistinguishable atoms of the same element but with different isotopic composition combine in a molecule. We demonstrate quantitatively how a certain measured statistical anti-clumping signal can be used to determine the isotopic heterogeneity of indistinguishable atoms in a molecule

Origin of the statistical negative clumped isotope signatures

No effect

Conclusions

Findings

Additional Information