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Transformational Instrumentation Differentiates Abiotic vs. Biotic Methane
Mysteries associated with methane’s origins and evolution have intrigued the scientific community for decades. Hunting for elusive clues, scientific detectives focused their efforts on seeking reliable signatures of abiotic versus biotic sources of methane. Innovative technology for “fingerprinting” methane is enabling investigators to decipher complicated “crime” scenes at field sites around the world — revolutionizing our understanding of methane.
The Deep Carbon Observatory helped launch a new era of high-tech sleuthing when Young et al. (2016) developed a new instrument, the Panorama mass spectrometer, to make the first resolved measurements of two rare forms of methane . These measurements can provide unprecedented insights into the origins and evolution of methane, achieving one of DCO’s decadal goals.
The Panorama mass spectrometer at the University of California Los Angeles, USA, during a meeting of the DCO Deep Energy Community.
In a groundbreaking new paper (Young et al., 2017), Edward Young (University of California, Los Angeles) and 23 co-authors from 14 institutions in 8 countries unequivocally demonstrate Panorama’s proof of concept . They report the first resolved measurements of two rare forms of methane in gases collected from a wide range of geologic settings across the globe. They sampled sources of putative abiotic methane gas together with thermogenic gases and gases thought to have a significant biogenic component. According to Edward Young, co-chair of DCO’s Deep Energy Community, “It is now possible to identify sources of methane in unparalleled ways with unparalleled precision.”
The locations of methane samples analyzed as part of this survey (Google Earth).
Methane molecules (CH4) incorporating a single rare isotope, such as 13C or 2H (deuterium, D) have been studied for decades. Methane molecules that are multiply substituted with rare isotopes, also known as “clumped isotopes,” are exceedingly rare but can serve as powerful tracers. Recently, a doubly substituted species of methane, 13CH3D, was measured in natural methane samples by mass spectrometry and adsorption spectroscopy. Building on these important accomplishments, Young et al. (2017) report resolved measurements of two doubly substituted species of methane, 13CH3D and 12CD2H2, at natural abundances in a variety of methane gases produced naturally and in the laboratory. “The use of two multiply-substituted isotopic species provides insights into the provenance of methane gases, the physical chemical pathways of methane formation, and subsequent processing,” according to Young et al. (2017).
The Panorama mass spectrometer was designed to overcome the technical challenges of measuring rare molecular species of almost identical molecular weight. It is capable of resolving the two, doubly substituted mass-18 isotopologues of methane, 13CH3D and 12CD2H2. If thermodynamic equilibrium is achieved, then the two parameters D13CH3D and D12CD2H2 can serve as two independent, intra-molecular thermometers. Concordant temperatures are reliable for thermometry.
If thermodynamic equilibrium is not achieved, then temperatures are unavailable from these data. However, resolved measurements of 13CH3D and 12CD2H2 provide novel information about the formation mechanism of the methane gas and the presence or absence of multiple sources or sinks. “In particular, disequilibrium isotopologue ratios may provide the means for differentiating between methane produced by abiotic synthesis vs. biological processes,” according to Young et al. (2017). Disequilibrium isotopologue ratios can reveal methane formation mechanisms and serve as tracers that provide insights into mixing, diffusion, kinetics, quantum tunneling, and other processes.
Natural methane samples are broadly categorized by Young et al. (2017) into three groups in D12CD2H2 vs. D13CH3D space: (1) samples exhibiting isotopic bond ordering equilibrium, (2) samples with clear negative digressions from equilibrium, which can be produced by abiotic reactions, and (3) samples with clear positive excursions from equilibrium.
Natural samples exhibiting isotopic bond ordering equilibrium provide concordant temperatures that do not always match previous hypotheses based on other methods, underscoring the importance of reliable thermometry based on the methane molecules themselves.
For samples exhibiting isotopic bond ordering disequilibrium, the data provide important information about the history and formation mechanism of the gas. Deficits in 12CD2H2 compared with equilibrium values in CH4 gas produced be made by abiotic reactions; large deficits may point towards a quantum tunneling origin. Positive excursions from equilibrium can be produced by mixing of gases or by fractionation of the bulk isotopic composition of the gas without bond reformation.
Isotopic values can change over time as a gas evolves. For example, Young et al. (2017) conclude, “Isotopologue signatures of abiotic methane production can be erased by infiltration of microbial communities, and Δ12CH2D2 values are a key tracer of microbial recycling.”
The study of multiple multiply substituted isotopologues represents a major step towards developing a new branch of isotope chemistry. Recent instrumentation breakthroughs are opening vast frontiers for new research, with applications to such topics as energy resources, greenhouse gases in the atmosphere, and the discovery of methane in the atmosphere of Mars.
1. Young ED, Rumble D, Freedman P, Mills M (2016) A large-radius high-mass-resolution multiple-collector isotope ratio mass spectrometer for analysis of rare isotopologues of O2, N2, CH4 and other gases. International Journal of Mass Spectrometry 401:1-10
2. Young ED, Kohl IE, Sherwood Lollar B, Etiope G, Rumble D, Li S, Haghnegahdar MA, Schauble EA, McCain KA, Foustoukos DI, Sutclife C, Warr O, Ballentine CJ, Onstott TC, Hosgormez H, Neubeck A, Marques JM, Pérez-Rodríguez I, Rowe AR, LaRowe DE, Magnabosco C, Yeung LY, Asha JL, Bryndzia LT (2017) The relative abundances of resolved 12CH2D2 and 13CH3D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases. Geochimica et Cosmochimica Acta 203:235-264