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Geochemistry
Rocky debate over early life

In 1996, geologists reported in Nature the presence of carbon that indicated that there was once biological activity in 3.85-billion-year-old rocks on Akilia Island, in southwest Greenland — pushing back the date of the earliest evidence for life on Earth by several hundred million years. But some subsequent research has cast doubt on the evidence, and a new study, using the same rocks, has been unable to replicate the Akilia finding, calling into question the rocks’ place in the study of ancient life.

Nearly a decade ago, Stephen Mojzsis, then a graduate student at Scripps Institution of Oceanography in La Jolla, Calif., and colleagues from the University of California, Los Angeles, measured the ratio of carbon-13 to carbon-12 in graphite inclusions in grains of the mineral apatite in a rock from Akilia. They found the ratio to be low, and because organisms prefer to use carbon-12, such a light isotope ratio is a tantalizing suggestion of past life.

Now, a research group led by Aivo Lepland of the Geological Survey of Norway, Trondheim, reports in the January Geology that its attempts to replicate the carbon analysis failed to turn up any carbon. “The surprising results were we couldn’t see any,” says Scripps geochemist Gustaf Arrhenius, co-author on the paper. “We failed to find any such graphite or carbon.” As Mojzsis’ former adviser at Scripps, Arrhenius co-authored the original 1996 study, but, he says, “in science, you always have to change your mind when you find new evidence.”

In recent years, researchers at conferences reported failures to turn up graphite inclusions in samples from the same Akilia outcrop; however, the new study was unable to locate graphite in the same rock sample that was used in the 1996 paper.

Mojzsis, now at the University of Colorado, Boulder, says that he is repeating his analysis and using additional methods to confirm his results, and that he objects to the methods used by Lepland and colleagues. “The problem with this report is that the technique that they used, optical microscopy and backscatter electron imaging, would not allow them to see graphite in apatite because it’s a surface analysis technique,” Mojzsis says.

Both the Nature and Geology studies involved optical microscopy, using transmitted light, and scanning electron microscopy to view sections of the mineral apatite. In the optical work, “when you look through the microscope at transparent crystals, you see all the way through,” Arrhenius says. “And neither in the always-transparent crystals, nor in sections through them, could we see any such inclusions.”

“There are really two problems here,” says Heinrich Holland, a geochemist at Harvard University who is familiar with the debate. “One is whether graphite is really present, and the second is what it means.”

Since publication of the 1996 study, scientists have raised questions about the age of the rock, whether the carbon is organic or inorganic in origin, and whether the rock was sedimentary or igneous prior to undergoing extensive and repeated metamorphism. Although at the time it was thought that only biological processes could produce such a light carbon isotope signature, it is now known that some inorganic processes can as well.

Additionally, the sedimentary nature of the rock is important because igneous rocks would be unable to preserve organic carbon. Reporting in the Dec. 17 Science, Nicolas Dauphas of the University of Chicago presents iron isotope evidence suggesting that the Akilia rocks were indeed metamorphosed sedimentary banded iron formations. This analysis contradicts a 2002 study by George Washington University geochemist Christopher Fedo and Martin Whitehouse of the Swedish Museum of Natural History, both co-authors of the current Geology paper, that suggested the rocks were igneous (see Geotimes, July 2002). Whether or not the Akilia rocks are sedimentary, however, may now be immaterial, as researchers say that without carbon, the rocks are irrelevant to the study of early life.

“This has to be sorted out,” Lepland says. “This is really a fundamental claim and has to be on a solid basis.”

If the Akilia claim to the earliest evidence of life does not stand, the distinction goes to nearby Isua, Greenland, where graphite with biologic signatures have been found in 3.8-billion-year-old rocks. The Isua rocks are much less severely metamorphosed, and the larger formation makes it more amenable to geologic interpretation.

A team led by another 1996 co-author, Allen Nutman of the Australian National University, Canberra, is currently trying to replicate the study as well. Holland says the debate could be settled within the next few months: “These are perfectly straightforward questions, and I think that they can be answered.”

Sara Pratt
Geotimes contributing writer

Link:
"Dispute over first rocks fit for life," Geotimes, July 2002

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