Methane is a potent greenhouse gas, second only to carbon dioxide in its capacity to trap heat in Earth’s atmosphere for a long time. The gas can originate from lakes and swamps, natural-gas pipelines, deep-sea vents, and livestock. Understanding the sources of methane, and how the gas is formed, could give scientists a better understanding of its role in warming the planet.
Now a research team led by scientists at MIT and including colleagues from the Woods Hole Oceanographic Institution, the University of Toronto, and elsewhere has developed an instrument that can rapidly and precisely analyse samples of environmental methane to determine how the gas was formed.
The approach, called tunable infrared laser direct absorption spectroscopy, detects the ratio of methane isotopes, which can provide a “fingerprint” to differentiate between two common origins: microbial, in which microorganisms, typically living in wetlands or the guts of animals, produce methane as a metabolic byproduct; or thermogenic, in which organic matter, buried deep within the Earth, decays to methane at high temperatures.
The researchers used the technique to analyse methane samples from settings including lakes, swamps, groundwater, deep-sea vents, and the guts of cows, as well as methane generated by microbes in the lab.
“We are interested in the question, ‘Where does methane come from?’” says Shuhei Ono, an assistant professor of geochemistry in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “If we can partition how much is from cows, natural gas, and other sources, we can more reliably strategise what to do about global warming.”
Ono and his colleagues, including first author and graduate student David Wang, publish their results this week in the journal Science.
Locking in on methane’s frequency
Methane is a molecule composed of one carbon atom linked to four hydrogen atoms. Carbon can come as one of two isotopes (carbon-12 or carbon-13); hydrogen can also take two forms, including as deuterium – an isotope of hydrogen with one extra neutron.
The authors looked for a very rare molecule of doubly isotope-substituted methane, known as 13CH3D – a molecule with both an atom of carbon-13 and a deuterium atom. Detecting 13CH3D was crucial, the researchers reasoned, as it may be a signal of the temperature at which methane formed – essential for determining whether methane is microbial or thermogenic in origin.
Last year, Ono and colleagues, working with scientists from Aerodyne Research, built an instrument to detect 13CH3D. The technique uses infrared spectroscopy to detect specific frequencies corresponding to minute motions within molecules of methane; different frequencies correspond to different isotopes. This spectroscopic approach, which is fundamentally different from the classical mass spectrometric methods being developed by others, has the advantage of portability, allowing its potential deployment in field locations.