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Earth Gases May Provide Clues to Evolution of Life

Bijal Trivedi
National Geographic Today
April 5, 2002
 
As far back as the 1880s, miners in North Canada, South Africa, and
Scandinavia have reported mysterious, foul smelling gases that hiss and
spit from crevices deep in mineshafts. Now scientists have discovered
the origin of these gases and have found that these anomalous emissions
may provide a missing link in the evolution of life on Earth.


"Everyone thought the presence of these gases was really weird, because they didn't fit with the geology of the area," said geochemist Barbara Sherwood Lollar, of the University of Toronto in Canada, who led the study that first analyzed these gases.

The gases that Sherwood Lollar's team found—natural hydrocarbons like methane, ethane, propane, and butane, all of which are fossil fuels—are usually formed from processes associated with life. Bacteria in swamps, for example, produce methane, and natural gas is produced from decomposed plants and animals, like dinosaurs that lived millions of years ago. But these particular gases seemed to have no links to life.

Sherwood Lollar's team descended into mineshafts, some up to four kilometers (three miles) deep, and found that the gas was escaping from rocks formed during the Precambrian period—about 4 billion years ago, before life began.

"Barbara [Sherwood Lollar] has found that these gases have a chemical rather than biological origin. And that is very interesting and very significant," said microbial geochemist Philip Bennett, of the University of Texas at Austin.

There has been continuing debate over how hydrocarbons, which are some of the simpler building blocks of life, may have arisen. It is generally accepted that on the early Earth there was carbon dioxide, methane, water and hydrogen. But how these compounds formed simple molecules like ethane, propane and butane has not been shown.

Gases Are Produced Inorganically

Lollar is the first to locate a place on Earth that spews these ancient gases, and to prove that these compounds are being inorganically produced, said Keith Kvenvolden, a senior scientist at the U.S. Geological Survey in Menlo Park, California. Although people had noticed small methane emissions along ocean ridges and suspected the gas has chemical origins, no one had found such a site on land, he added.

"She has really proven it. She observed hydrocarbons in a place where they shouldn't be and used new state-of-the-art methods to prove the gases did not come from [life] processes," said Bennet.

After collecting the gases from these mines, Sherwood Lollar analyzed the very atoms that make up these gases using a new technique called "Continuous Flow Stable Isotope Mass Spectrometry."

Hydrocarbons produced from living organisms are rich in the light forms, or isotopes, of hydrogen and carbon. By contrast, when these same types of hydrocarbons are produced in the laboratory, or in space on meteorites, they are richer in the heavier hydrogen and carbon isotopes.

The chemical signature of the gases from North Canadian mines matched the chemical signatures of hydrocarbons like those from the Murchison meteorite and laboratory produced gases.

These findings, which are published in the April 4 issue of the journal Nature, have broad implications.

Reconsidering Gases as Signature of Life

"Often when methane was found at great depths it was immediately regarded as a signature of life," said Bennett. "Now this must be reconsidered."

More exciting, said Bennett, is that the hydrocarbons provide a food source for life forms living deep within the Earth where there is no light. "I'm itching to go down these mine shafts and see what else might be living down there," he added.

"We have found life as deep as we have cared to look, between three and four kilometers down, and have so far found no limits," said Bennett.

While the prospect of more fuel reserves may be sure to prick up ears, Sherwood Lollar emphasizes, "this is not the answer to the next gas crisis."

"The quantities of gas present are large from a scientific point of view but are certainly not economically viable, and I do not believe that they contribute to major gas deposits."

Producing hydrocarbons through chemical methods is very slow and inefficient compared to conventional natural gas reserves, said Kvenvolden. "It would be a real stretch to consider these energy sources in the future."

Although the new findings provide insight into how and where simple organic molecules formed, both Bennett and Sherwood Lollar are hopeful that life forms discovered in these mines might shed light on early evolution.
 

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