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.

In addition, the carbon isotope composition of the carbonaceous matter was found to be 3.5% to 2% less carbon-13 to carbon-12 compared with a standard value found in plain rock, consistent with fixation by the biological Calvin cycle process, Tice said. Organic matter formed in hydrothermal vents can also have that composition, but tends to be present among other compositions as well. "They tend to be all over the map, so finding them, particularly in this range, is also really consistent with origins from biotic processes."

"It's not claims of an abiotic origin that have to be dispelled, but instead claims for an abiotic origin which have to strain for any shred of credibility in the face of the evidence summarized by Tice and Lowe," John M. Hayes of Woods Hole Oceanographic Institute, told The Scientist.

The Buck Reef Chert is "stuffed" with features characteristic of the environments that Tice and Lowe describe, said Hayes, who was not involved in the study. "Claims of an abiotic origin are respectable for units that can be related to hydrothermal settings, but they can't begin to account for all the features of these 3.5-billion-year-old sediments."

But Martin Brasier, a proponent of the hydrothermal origin of organic matter 3.4 billion years ago, remained skeptical. "I'm not convinced," he told The Scientist. Donald Lowe recently published a paper stating that the ocean temperature was about 60ºC or higher at that time–in other words, that the whole ocean was hydrothermal–in direct conflict with their current hypothesis, he said.

Tice replied that this was a semantic argument: "hydrothermal" refers to such things as vents—which Brasier himself suggests to be the source of the organic material—not the ambient ocean temperature. "Having a hot ocean is fine, but it doesn't get you any closer to those metals," Tice said.

In addition, Brasier claimed that Tice and Lowe had but a single line of geochemical evidence for organic life—inadequate in his view. Tice did not feel additional geochemical evidence was required to show life.

Brasier also said that Tice and Lowe had but a single line of geochemical evidence for organic life – which he felt was not enough. He also said that the authors "rather mischievously" show a single picture of two parallel lines in a piece of rock and call it a microfossil. Brasier has previously criticized controversial work by J.W. Schopf from the University of California at Los Angeles.

"Brasier goes very close to saying that Schopf is a hoaxer," said Hayes, "which is taking things way too far in my opinion."