3.4-billion-year-old controversy
Evidence for life 3400 million years ago, but hydrothermal proponents still
don't agree
By Cathy Holding

Carbonaceous compounds found in sedimentary rocks were laid down by mats of
photosynthetic organisms living in shallow seas 3.4 billion years ago,
according to a paper in Nature this week. But the article does little to
quell the debate over whether the origin of these compounds is biological,
or simply the result of chemical reactions at the time.

Michael M. Tice and Donald R. Lowe, from Stanford University, California,
believe their paper demonstrates unequivocally that organic matter found in
the Buck Reef Chert in South Africa-one of the oldest sedimentary areas in
the world-was the result of activity of living organisms, and not abiotic
hydrothermal processes.

"Chert is a microcrystalline form of quartz," explained Tice,
"and in the
pictures, all the white stuff you see is chert. All the black stuff you see
is carbonaceous mineral which is organic matter produced by organisms and
which has since been heated to such a degree and for a long enough time that
it's now approaching graphite."

Tice examined very thin sections of chert under the microscope and
catalogued all the various shapes and structures, some of which looked like
they could have been microbial mats, he said. "The mat is the stringy,
drapey stuff that's lying on top of the big carbonaceous chunk."

But controversy over the limited value of shapes in sections led Tice and
Lowe to collect further evidence. They found laminations only in rocks
deposited in underwater depths between 15 and 200 meters, which in the
modern ocean is about the depth that light can penetrate. "So it looked like
these mats were being formed only where light was available and that's
something that is far more consistent with origins from microbes or
something living rather than just hydrothermal systems," Tice told The
Scientist.

Full Text at The Scientist
http://www.biomedcentral.com/news/20040930/01

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