Methane is one hot gas. It's a prominent component of natural gas, an important atmospheric gas, and a product of both biology and chemical reactions. Its presence was recently confirmed in the atmosphere of Mars by NASA's Curiosity Rover and it has made the news both as a critical greenhouse gas and as a groundwater contaminant resulting from fracking. Yet, while methane seems to be everywhere, many questions remain about the reactions that produce and consume this high-energy compound.
"Deciphering the many pathways by which methane is produced is one of the holy grails of organic geochemistry," said University of Toronto's Barbara Sherwood Lollar, one of the authors of a paper being published tomorrow in Science Express that reports a breakthrough in methane identification.
The new approach adds Tunable Infrared Laser Direct Adsorption Spectroscopy (TILDAS) to the set of instruments that can help identify the temperature at which methane is formed as well as provide details on the environment in which methane-producing microbes thrive.
Historically, the origins of methane have been determined by using a form of geochemical fingerprinting called stable isotope signatures. Researchers around the world measure how many methane molecules contain not only the most abundant light carbon-12 isotope, but the heavier carbon-13. This ratio of heavy to light carbon isotopes allows scientists to decipher whether methane is produced by biological processes, by the heating of organic matter or by chemical reactions in the crust.
"However, there has always been a limitation," said Sherwood Lollar. "Even when coupled with measurements of the light and heavy hydrogen isotopes in the methane molecule, significant overlaps exist in the signatures - like fingerprints that are too similar to allow reliable identification."
While one in 100 methane molecules contain a carbon-13 isotope, and one in 10,000 contain deuterium, the heavy isotope of hydrogen, there is a fingerprint tracer that is rarer still - methane molecules that contain both the heavy carbon-13 and deuterium isotopes at the same time. These so-called "clumped" isotopes have proven to be uniquely sensitive to the processes that form methane, provided they can be measured precisely enough.
The paper's lead author, David Wang, a graduate student at the Massachusetts Institute of Technology, likens the rarity of these clumps to "finding 2,000 people in the entire United States with its population of 320 million and being able to say precisely if there are 1999 or 2001." The study shows that when this level of precision is achieved, as it is with TILDAS, information can be gleaned about the temperature at which methane forms, as well as about the environment in which methane-producing microbes are living.
"Whether in natural gas deposits or deep groundwater that may be analogous to the Mars subsurface, or in living microbial systems such as cows, our paper shows how information from the elusive clumped isotopes is transforming our ability to track down the details of methane formation," said Sherwood Lollar.
Authors include graduate student Danielle Gruen and senior author Shuhei Ono, assistant professor of biogeochemistry, both of MIT, and a team of international colleagues.
Barbara Sherwood Lollar
Department of Earth Sciences
University of Toronto
Communications, Arts & Science
University of Toronto
Department of Biogeochemistry
Massachusetts Institute of Technology