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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