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Akilia, a small island off the coast of southwest Greenland, contains an outcrop of metamorphic rocks that are rich in pyroxene and quartz and interpreted to be highly altered, iron-rich quartzite sediments. At 3.85 billion years old, they are also believed to be the oldest rocks on Earth. Some researchers also say these rocks host what may be the earliest evidence of life on Earth: carbon isotope signatures suggestive of biological activity at that time.
But this idea remains under discussion, with other researchers questioning whether the rocks are even capable of hosting any evidence of life. During a "Hot Topics" session at the annual meeting of the Geological Society of America in Denver last week, geologists Stephen Mojzsis from the University of Colorado-Boulder and Christopher Fedo from George Washington University, drew on a debate they and their co-authors also have in the Nov. 1 Science.
Akilia's geology includes supracrustal rocks that represent remnants of the surface geology of early Earth. However, their intense deformation from subsequent metamorphic processes has complicated understanding of their origins. The complex structural and tectonic geology has also fueled debate.
"I find it disappointing that after six years, the basic facts surrounding the carbonaceous matter in Akilia rocks are still under debate," says John Valley, a geochemist at the University of Wisconsin-Madison and researcher in Precambrian geology. "How can there be disagreement on what the carbon looks like or whether this famous rock is 35 percent or 70 percent quartz?"
On one side of the debate, Mojzsis says the rock interpreted to host the carbon isotopes is an iron-rich quartzite, originally a sedimentary rock precipitated from seawater. The ratios of carbon isotopes within the quartzites, he says, are evidence of life because they have values consistent with biological processes. On the other side, Fedo suggests that the rock is igneous in origin and that its carbon isotope signatures are evidence of inorganic alteration processes (Geotimes, July 2002).
Primary evidence used to demonstrate each side of the argument is derived from rock geochemistry and structural mapping. Mojzsis and his colleagues highlight the quartz-rich samples over data for the rocks that have more mafic minerals such as pyroxene, assuming the quartz-rich rocks are the most pristine and thus reflect original composition. Fedo and Whitehouse highlight the more mafic samples, suggesting that they, instead, are truer to the rock's original composition. "We see an originally mafic-ultramafic igneous rock shot with vein quartz -- and would have had other things like carbonate minerals too -- and highly deformed. Steve [Mojzsis] sees a pure quartz rock with younger veins of pyroxene," Fedo says.
"To demonstrate the point, we took the rare earth and trace element data of Fedo and [Martin J.] Whitehouse and simply re-plotted all of it," Mojzsis says. "Our plots support an opposite conclusion to theirs." He says that Fedo and Whitehouse, his co-author at the Swedish Museum of Natural History in Stockholm, did not point out that their most quartz-rich rocks have chemical signatures inconsistent with ultramafic igneous rocks generated from within Earth's mantle, but rather have signatures consistent with sedimentary iron formations from Isua, Greenland. Fedo defends his original plots by suggesting that the most quartz-rich rocks are also the most altered and thus naturally plot away from their original compositional field. In turn, he questions why Mojzsis and his colleagues' interpretations for the approximately five-meter-thick sequence of rocks under debate are based primarily on a single rock sample that has been published at different times as representing different mineralogies and quartz contents.
"You can think of the current debate this way: Picture these rocks as a peanut-butter and jelly sandwich that you take in your hand and mash up then put away in a Tupperware," Mojzsis says. "When you pull this out a few days later you see peanut butter, jelly and bread. Fedo and Whitehouse only focused on the bread and are asserting that the peanut butter and jelly are somehow formed from the bread." He adds that the mapping done by Fedo and Whitehouse is not nearly accurate or detailed enough to account for all the components of the supracrustal package or to discriminate between more altered and less altered sections of the outcrop. Along with Clark Friend from Oxford Brookes University, in Britain, and Allen Nutman and Victoria Bennett from the Australian National University, Canberra, he says that sampling pyroxene-veined sections, which he says do not represent the geochemistry of the quartz-rich sections, has skewed Fedo and Whitehouse's interpretations.
Fedo defends their mapping of the pyroxene-rich sections, maintaining that they are integral to the original make-up of the rock. Stephen Moorbath, a geochemist from Oxford, agrees with Fedo and Whitehouse. "I have been to this locality twice in recent years, and collected many rock samples there. I find it astonishing that these rocks were ever identified as BIF-type [banded iron formation] sediments in the first place. There is simply no evidence for this in the field."
Valley adds that another reason the debate continues is because of how data are presented. "I think the answer is an over-reliance on high visibility letters-style journals," he says, referring to how data are often omitted such that they cannot be evaluated by others when presented in the short formats specified by scientific journals.
Although Akilia's remoteness makes the island difficult to access and study
-- and thus to re-evaluate its structural and regional geology together -- Mojzsis
also stresses the importance of reporting all the known data, both geochemical
and structural. "This debate is an interesting and important exercise to
be doing. The only information available to us about the early Earth is the
geological evidence, which can be complicated and confusing because it is so
old; but it is essential to report all the data because this makes it useful
to the scientific community." For the time being, whether these rocks are
capable of hosting the earliest evidence for life on Earth remains a subject
of debate.
Salma Monani
Geotimes Contributing Writer
Read a related news note about
rocks for early life from the July 2002 issue.
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