Analyses of the gases dissolved in water trapped in ancient minerals suggest that methane-generating microbes have been around almost 3.5 billion years, more than 700 million years longer than previous geologic evidence had indicated. Because methane prevents the loss of heat from Earth, the gas generated by those microbes could explain how the planet kept warm during the Archaean era even though the sun then produced less than three-fourths the radiation that it does today.
Methane is a minor constituent of Earth’s atmosphere, today making up only about 1.8 parts per million of air. There are three major sources of atmospheric methane: some types of microbes that live in oxygen-poor environments, the heat-induced degradation of organic matter trapped in sediments, and the chemical reactions of simple inorganic compounds such as carbon dioxide and hydrogen.
Although methane produced via one method is chemically indistinguishable from that produced by the others, the ratio of carbon isotopes found in a sample of methane provides a clue to its source, says Yuichiro Ueno, a geochemist at the Tokyo Institute of Technology in Yokohama, Japan. Methane from biological sources contains less carbon-13 than does methane from nonbiological sources.
Ueno and his colleagues analyzed samples of transparent quartz taken from the Dresser formation in Western Australia. Radioactive dating of those samples, as well as of the volcanic layer deposited directly atop them, suggests that the Dresser quartz formed between 3.49 billion and 3.46 billion years ago, says Ueno. When the quartz crystallized, it trapped tiny droplets of water.
Spectral analyses revealed minuscule amounts of methane dissolved in those droplets. The researchers then ground up small samples of fluid-bearing quartz and analyzed the methane that was released.
Because the escaping methane contained much less carbon-13 than is normally found in atmospheric methane, the carbon in the gas probably had a biologic origin, say the researchers. The dearth of propane and other long-chain hydrocarbon gases coming from the droplets indicates that the methane didn’t derive from the thermal degradation of organic matter. Therefore, the researchers propose in the March 23 Nature that the methane trapped in the quartz must have been produced by microbes.
Ueno and his colleagues “have probably uncovered the oldest-known samples of biologically produced gas,” says Don E. Canfield, a biogeochemist at the University of Southern Denmark in Odense.
“It’s wonderful news if they’ve measured a preserved sample [of methane],” says James F. Kasting, a geochemist at Pennsylvania State University in University Park. Most theories about Earth’s early atmosphere presume concentrations of methane more than 500 times as great as those in today’s atmosphere, he notes.
Previous studies had identified microorganisms that used sulfate to fuel their metabolism 3.5 billion years ago, but those microbes produced no methane. The new research adds a methane producer to the mix of microbes known to be active then, says Canfield.