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The term “rare isotopologue” refers to molecules composed of more than one rare isotope. The potential significance of these molecules has been understood for decades (Richet et al. 1977) but has been underutilized. Their usefulness lies in the fact that their distribution is an intra-phase or intra-molecular isotope signal that can be read without reference to any other species. Rare isotopologues can be used to determine the temperature of formation of methane gases and the mixing and transport of these gases after formation. Indeed, it is fully expect that development of rare isotopologues of CH4 as tracers will transform our ability to “source” methane across the globe. In this regard, these studies addressed a decadal goal of the Deep Energy Community, and that is to answer the question: how do we differentiate between biotic and abiotic carbon compounds?
Tunable Laser for Clumped Methane Isotopologue Measurements
The MIT group led by Prof. Shuhei Ono2 has developed a tunable laser infrared direct absorption spectroscopy (TILDAS) instrument to measure abundance of doubly substituted methane from natural samples. Deep Energy funding support was essential in developing some key instrumentation for the project, including the constructions of inlet system for TILDAS as well as a sample preconcentration and purification manifold. This allowed us to measure a range of methane samples to gain a first order understanding of methane clumped isotopologue systematics.
The paper describing the method and calibration of the instrument was published early 2014 in Analytical Chemistry (Ono et al., 2014) and the discovery from the initial set of data was published in the journal Science in March 2015 (Wang et al., 2015). Several DCO scientists were involved and co-authored for the papers (e.g., B. Sherwood-Lollar, K-U Hinrichs, J. Seewald). Wang et al. (2015) demonstrated the wealth of new information gained by the clumped methane isotopologue about the generation mechanisms of methane in the environment. They reported distinct isotopologue signals from methane from two serpentinization sites in California (The Cedars, and CROMO), indicating the diverse origins of methane in serpentinization sites.
The MIT group is continuing their analysis of samples from a range of setting, e.g., methane from serpentinization, cattles. They are also currently focusing on some experiments to study isotopologue fractionations for the formation and destruction of methane. During the summer 2014 and January 2015, Danielle Gruen (a graduate student at MIT), has visited Martin Könneke and Kai Hinrichs at MARUM in Bremen to culture methanogens under different substrate (acetate, ethanol, and CO2-H2). Preliminary data showed distinct methane isotopologue signals for methane produced by different pathways. They are also working on methane oxidation by microbes as well as atmospheric process. Construction of a device to trap and concentration atmospheric methane is currently underway. These effort will be used to gain the systematic and mechanistic understanding of clumped methane isotopologue fractionations in nature and this will be used to identify the source(s) and sink(s) of nature from the environment.
Clumped Isotopes of Methane from Hydrothermal Vents
The Caltech group led by Prof. John Eiler established through study of vent fluids from submarine mid ocean ridge sites that methane emitted in those fluids is characterized by clumped isotope temperatures that are comparable to independent estimates of vent fluid temperature (~300 ˚C in these cases), and substantially hotter than most other natural methane sources (excepting the artificially high apparent temperatures of some biogenic methane from culture or natural surface waters). These results are consistent with methane from such environments forming at or near equilibrium with respect to its internal isotopic distribution, and with a relatively high blocking temperature for the methane clumped isotope thermometer (at least in cases where quenching to surface temperatures is rapid). Their findings for serpentinization-related gases from Chimera, Turkey are analytically clear — they found four separately collected samples yield apparent temperatures of 220±4 ˚C (analytically indistinguishable). However, common interpretations of the Chimera deposit argue that methane is being released from active serpentinization occurring in the relatively shallow sup surface at temperatures of 80 ˚C or less. Two plausible explanations are possible: (1) Methane formed by serpentinization is controlled by kinetic isotope effects that mimic higher temperature equilibrium states of isotopic ordering. This is imaginable, but would make these gases unusual exceptions to a general pattern we have noted in over 100 other natural gases: methane from sub-surface conditions nearly always seems to reflect formation in equilibrium at temperatures that are quite plausible for the local setting. Or (2) the methane at Chimera was generated at higher than expected temperatures, either deep in the modern sub-surface or (more plausibly) earlier in the history of hydrothermal alternation, obduction and metamorphism of the serpentinites (i.e., it is being released from the rocks now but was generated earlier in their geologic histories).
Development and Implementation of the Panorama Mass Spectrometer
The purpose of this project led by Prof. Ed Young3 at UCLA was to construct a gas-source isotope ratio mass spectrometer with sufficient mass resolving power to measure the relative abundances of rare isotopic species of methane gas. The science goal was to provide a new isotopic tool for sourcing methane. In particular, the new instrument was designed to measure the relative abundances of both 13CH3D and 12CH2D2, two very stable but very rare doubly-substituted isotopologues with the same cardinal mass of 18 atomic mass units, allowing us to move beyond temperature of formation as the sole information to be obtained from 13C-D bond ordering. Studies made possible with this novel instrument should provide new information about the mode of formation, presence of mixing, and importance of migration of natural CH4 gas. Results will facilitate sourcing fugitive methane as well as helping to specify the origin of gas where provenance is uncertain. The project was funded by the U.S. National Science Foundation, the U.S. Department of Energy, The Sloan Foundation, UCLA, The Carnegie Institution of Washington, and Shell.
The new mass spectrometer, called the Panorama, was delivered to UCLA in early March of 2015. It is currently functioning beyond expectations in the purpose-built laboratory at UCLA. This instrument, the largest of its kind in the world, was developed for our laboratory at UCLA and is unique. The magnetic sector has a radius of 800 mm that, when combined with typical slit widths and aberrations yields an operational mass resolving power of ~ 45,000, more than 40 times that of conventional gas-source isotope ratio mass spectrometers and three times the operational value for the next highest resolution instrument of its kind. Sensitivity is provided by an ion counter on the central channel. With this instrument we measure D13CH3D and DCH2D2, where D here refers to per mil departures from the stochastic relative abundances of these rare species, with virtually no mass spectrometric interferences and no corrections.
The UCLA team has run numerous tests on the instrument that verify its accuracy and precision, including comparisons of bulk D/H and 13C/12C relative to other methods for measuring these isotope ratios in methane gas, with excellent agreement. It is important to recognize that these measurements are made on the methane gas itself as the analyte. Previous methods require converting the CH4 to CO2 for carbon isotope ratio analysis and H2 for D/H. In addition, the internal precision we obtain for D/H is ~ 0.01 per mil, an extraordinary value compared with other methods.
As a lead into the full application of the Panoram instrument, the UCLA group explored the clumped isotope systematics in O2. 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, they demonstrated that biological oxygen (O2) cycling drives the clumped-isotope composition of O2 away from isotopic equilibrium. Their 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. This work was published in Science (April 24, 2015; Yeung et al. p. 431-434).
2 S. Ono received support from the DCO Instrumentation fund which supported development of his instrument.
3 E. Young received partial support from the DCO Instrumentation fund for development of his Panorama instrument.