Earth's Oldest Fossils Reverse Course
|Chert is a chemical precipitate formed by groundwater.
Credit: Georgia Southwestern State University
In paleontology, as in real estate, location is everything. The rocks which enclose a fossil are a prime source of information on the age and history of that fossil.
The importance of location has resurfaced in a spirited debate over microscopic structures found in ancient greenstone beds from Western Australia. (Greenstones are ancient sedimentary and volcanic rocks that have been heated and deformed through the ages; the ones in question were apparently deposited at the bottom of an ocean.) The disputed structures are either the oldest fossils on Earth or deceptive geologic features that happen to contain carbon and, according to some, to resemble fossilized bacteria.
The Western Australian greenstones, together with similar rocks from Greenland and South Africa, are some of the oldest rocks on Earth. In 1993, J. William Schopf, a professor of paleobiology at the University of California at Los Angeles, published an article interpreting microscopic structures found in 3.465-billion-year-old Western Australian greenstones as the fossils of 11 species of bacteria.
Specifically, the fascinating fossils were found in vertical intrusions called chert dikes that cut through the greenstone rocks. Chert is a fine-grained sedimentary stone similar to flint. Since dikes, by definition, crosscut existing rock bodies, they must be younger than those rocks, but these dikes are almost as old as the greenstones.
|"We have examined dozens and dozens of dikes, and in every one where we can demonstrate which way the material moved, it moved down, not up. Therefore this material is not hydrothermal." -Donald Lowe
Credit: Stanford University
Schopf's findings were controversial, since his purported fossils were more than 1 billion years older than the oldest unquestioned bacterial fossils. If the structures were verified as fossil microorganisms, scientists could firmly establish that, by around 3.5 billion years ago, life had already established a firm foothold on Earth.
Some of the most convincing contrary evidence came from Martin Brasier, professor of paleobiology at Oxford University, and colleagues from the Johnson Space Center in Houston and the Carnegie Institutioni of Washington. Brasier, in collaboration with Marti van Kranendonk, of the Geological Survey of Western Australia, who has mapped the region extensively, concluded that the chert dikes had precipitated from hot "hydrothermal" fluids that were moving upward through the crust just under the ocean floor.
The situation was akin to the "black smokers" found at the ocean-bottom today, where mineral-rich fluids emerge from the ocean floor at 300 degrees C (572 degrees F) and up. If that was the case when the cherts formed, Brasier argues, the fluid was far too hot for life. That, he says, is one of eight reasons why the suspicious structures are not fossils.
In February, however, the possibility that chert dikes could contain fossils got a boost from a talk at a general meeting of the NASA Astrobiology Institute. Stanford University Professor of Geological Science Donald Lowe presented results of a long-term study of chert dikes in South Africa. Lowe said the Barberton Greenstone Belt, which is about the same age as the Western Australia greenstones, formed from sediment that flowed down from the ocean floor and settled into cracks in rocks below. "We have examined dozens and dozens of dikes, and in every one where we can demonstrate which way the material moved, it moved down, not up. Therefore this material is not hydrothermal."
To support his claim, Lowe pointed to spherules in the dikes. These tiny spherical grains contain isotopes indicating an extraterrestrial origin, and were apparently formed by asteroid impacts. A period of heavy impacts from 3.47 to 3.22 billion years ago left a number of spherule layers, 10 to 30 cm (4 to 12 inches) thick, on the surface around the globe, Lowe says. One of these layers supplied spherules to the sediments that later flowed into cracks in South African greenstones. The result was chert dikes containing telltale spherules.
Because spherules are absent from rock below the chert dikes, Lowe says, the logical conclusion is that the dikes were emplaced from above, not formed by upwelling hydrothermal fluids.
|A "black smoker" in the East Pacific, discovered by the submersible Alvin.
Credit: PBS Online
Two factors limit the applicability of Lowe's "location, location, location" interpretation to the debate over ancient life. First, Lowe did not look for microfossils. "It's an incredibly tedious job, and I've not undertaken any systematic search for microfossils," he says. And second, Lowe looked in South Africa, not in Australia. However, Lowe, who is an expert on the early terrestrial surface, says, "Chert dikes are very common in both greenstone terrains, and it would be unusual if they had a radically different origin."
Brasier writes, via email, that he and his colleagues "welcome this fresh debate, [but that]while Lowe may be correct about some of the South African dikes, he appears to be mistaken about the Apex chert in Western Australia. We did indeed consider the possibility" that they were derived from overlying sedimentary rocks, "before coming to our hydrothermal conclusion." The presence in chert of chemical compounds typical of hydrothermal fluids supports the hydrothermal hypothesis, he said.
But Paul Knauth, a geologist at Arizona State University, who has seen both the Australian and South African cherts, says, "I think Lowe is probably on the correct track. Evidence for the alternative view, that the Australian chert breccias [sedimentary rock containing chunks of rock bonded together by smaller grains of sediment] were hydrothermal vents on the sea floor, is incredibly weak. Lowe is one of the few experienced sedimentologists to have worked on Archean strata, so his interpretations should be considered very seriously." The Archean was roughly 3.8 billion to 2.5 billion years ago.
Lowe's view of the dikes represents a reversal of a recent trend in geological analysis. Since the discovery of black smokers, hydrothermal vents at mid-oceanic ridges, "there has been a major movement over the last 10 to 15 years, toward viewing early Earth as dominated by hydrothermal processes," he says. "Our experience is that hydrothermal processes were not as significant as many would have us believe."
If the chert dikes indeed formed from downward-flowing sediment rather than precipitated from hot fluids, they could be an excellent location for fossil-hunters. Sediment collects traces of organisms from the ocean floor. While forming into rock, it traps and protects organisms that can become fossils.
Still, the issue is likely to remain contentious. Interpreting such ancient geology, says Lowe, amounts to "reading a book in a language we don't understand yet." To make the point he asks his students to consider a book written in Chinese. "They could take a Chinese class and learn a little bit of Chinese, but if two students translated one sentence, it would come out differently. It's the same way with the rock record. It's complicated, and very competent people understand the language in different ways."
However, now that he's "read a part of the book" on South African cherts, Lowe would like to extend his studies to the greenstones of Western Australia.
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