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Biological signatures in clumped isotopes of O2

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Science  24 Apr 2015:
Vol. 348, Issue 6233, pp. 431-434
DOI: 10.1126/science.aaa6284

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What controls clumped isotopes?

Stable isotopes of a molecule can clump together in several combinations, depending on their mass. Even for simple molecules such as O2, which can contain 16O, 17O, and 18O in various combinations, clumped isotopes can potentially reveal the temperatures at which molecules form. Away from equilibrium, however, the pattern of clumped isotopes may reflect a complex array of processes. Using high-resolution gas-phase mass spectrometry, Yeung et al. found that biological factors influence the clumped isotope signature of oxygen produced during photosynthesis (see the Perspective by Passey). Similarly, Wang et al. showed that away from equilibrium, kinetic effects causing isotope clumping can lead to overestimation of the temperature at which microbially produced methane forms.

Science, this issue p. 431; p. 428; see also p. 394

Abstract

The abundances of molecules containing more than one rare isotope have been applied broadly to determine formation temperatures of natural materials. These applications of “clumped” isotopes rely on the assumption that isotope-exchange equilibrium is reached, or at least approached, during the formation of those materials. In a closed-system terrarium experiment, we demonstrate that biological oxygen (O2) cycling drives the clumped-isotope composition of O2 away from isotopic equilibrium. Our model of the system suggests that unique biological signatures are present in clumped isotopes of O2—and not formation temperatures. Photosynthetic O2 is depleted in 18O18O and 17O18O relative to a stochastic distribution of isotopes, unlike at equilibrium, where heavy-isotope pairs are enriched. Similar signatures may be widespread in nature, offering new tracers of biological and geochemical cycling.

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