Nonequilibrium clumped isotope signals in microbial methane

Abstract

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 O 2 , which can contain 16 O, 17 O, and 18 O 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

The geochemistry of methane isotopologues

“…Experimental setups that allow rigorous accounting of carbon budgets and biomass density may allow for quantitative models of isotopologue systematics, similar to those created for δ 13 C (Templeton et al, 2006;Nihous, 2008; Nihous, 2010), to be used in evaluating the potential effects of diffusion of methane to and through cells. Our data thus also encourages consideration of 3Comparisons of the fractionation factor for D/H equilibrium between CH4(g) and H2O(l) derived from the calibrations of different studies reveal a substantial range in estimates (up to 30‰ at 30-37°C, see Wang et al, 2015). This is mainly due to uncertainty in extrapolations of experimental calibrations of H2(g)/H2O(g) at >200°C to lower temperatures.…”

“…5.1) that shows several insights into the origin and fate of the methane stable isotopologues in the environment. As of the time of writing, methane clumped isotoplogue data have appeared in at least a dozen articles Stolper et al, 2014a;Stolper et al, 2014b;Inagaki et al, 2015;Stolper et al, 2015;Wang et al, 2015;Douglas et al, 2016;Lopes et al, 2016;Wang et al, 2016;Young et al, 2016;Whitehill et al, 2017;Young et al, 2017). The ∆ 13 CH 3 D data were shown to be independent of and complementary to δ 13 C and δD.…”

“…Carbon-and hydrogen-isotope ratios encode signals related to the sources of C and H, respectively, as well as isotopic fractionations incurred during the synthesis of methane. Complementary to such information, measurement of methane clumped isotopologues provides an independent estimate of the temperature at which the C-H bonds in methane were formed or last equilibrated (Stolper et al, 2014a;Wang et al, 2015). Constraining the temperatures at which methane synthesis occurs within oceanic crust has direct implications for the distribution and availability of reduced carbon substrates and energy sources that may support a deep biosphere, as well as for the transfer of mantle-derived carbon to the Earth's surface.…”

“…Furthermore, clumped isotopologue data provide another dimension for probing kinetic and equilibrium isotope effects and for constraining isotope exchange processes in natural settings (e.g., Eiler and Schauble, 2004;Yeung et al, 2012;Yeung, 2016 has an equilibrium constant K that varies between~1.007 at 0°C to 1.000 at temperatures approaching infinity (at which isotopes are randomly distributed amongst all possible isotopologues, i.e., the stochastic et al, 2015). The nonequilibrium isotope signatures were attributed to intrinsic clumped isotopologue effects expressed during biological methanogenesis under conditions of low reversibility (Stolper et al, 2015;Wang et al, 2015). Using inferences based on δ 13 C and δD data, methane oxidation was excluded as a significant origin of the nonequilibrium isotope signals .…”

“…The initial isotope values were chosen for illustrative purposes only and do not represent any particular natural sample; however, the chosen values are typical of modern microbial methane generated in wetland and lake sediments. Following Wang et al (2015), the gray field in panel (a) represents the temperature range within which microbial life has been shown to occur , and the gray fields in panel (b) represent empirical methane source fields suggested by Whiticar (1999). mass transport and bioavailable methane levels when evaluating methane isotope data in field settings where oxidation may be occurring.…”