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.
