Methane is a potent greenhouse gas that traps heat about 20 times more effectively than carbon dioxide.
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 including scientists at the Massachusetts Institute of Technology, 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 method detects the ratio of methane isotopes, which can provide a “fingerprint” to differentiate between two common origins: microbial, in which microorganisms, such as might live in 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 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.”
The group noticed something surprising and unexpected in some samples. For example, based on the isotope ratios they detected in cow rumen, they calculated that this methane formed at 400 degrees Celsius — impossible, as cow stomachs are typically about 40 C. They observed similar incongruences in samples from lakes and swamps. The isotope ratios, they reasoned, must not be a perfect indicator of temperature.
Instead, study author David Wang and his colleagues identified a relationship between a feature of the bonds linking carbon and hydrogen in methane molecules — a quality they deemed “clumpiness” — and the rate at which methane was produced: The clumpier the bond, the slower the rate of methanogenesis.
“Cow guts produce methane at very high rates — up to 500 liters a day per cow. They’re giant methane fermenters, and they prefer to make less-clumped methane, compared to geologic processes, which happen very slowly,” Wang says. “We’re measuring a degree of clumpiness of the carbon and hydrogen isotopes that helps us get an idea of how fast the methane formed.”
Wang added “Now we have a baseline that we can use to explore how methane forms in environments on Earth and beyond”.
The study is published this week in the journal Science.
The ocean floor is teeming with methane, the natural gas that fuels our homes. According relatively modest changes in global ocean temperatures or sea level could trigger a massive release of oceanic methane. If a rise in water temperatures passes a certain threshold, sizable methane hydrate deposits could decompose rapidly and release a large quantity of heat-trapping gas back into the atmosphere.
Photos: Danielle Gruen (edited by Jose-Luis Olivares/MIT); MIT
When Seafloor Meets Ocean, the Chemistry Is Amazing, Oceanus Magazine, Vol. 42, No. 2, Apr. 2004