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  • Raman Imaging Probes Ancient Rocks

Photonics Spectra
May 2002
Daniel S. Burgess

The microscopic structures discovered in the 3.5 billion-year-old rocks of Western Australia known as the Apex cherts, are among the earliest known signs of life on Earth. Or are they?

The cherts were reported in 1993 by J. William Schopf of the University of California in Los Angeles. Because morphology alone does not necessarily distinguish a fossil from a nonbiogenic mineralogical feature, he and his colleagues at the University of Alabama in Birmingham have re-examined the cherts using an imaging technique based on Raman spectroscopy that generates two-dimensional chemical maps of a sample.

Researchers are debating whether the features in the 3.5 billion-year-old Apex cherts are microfossils. In the Raman imaging technique used to study the structures, the user first defines a region of interest in the optical image (top), which is overlaid with a checkerboard pattern for local spectroscopic analysis. In this case, the squares are 500 x 500 nm. The intensity of a selected Raman band (here, the 1350-cm–1 "D" band) is indicated in gray scale for each square (middle) and smoothed with software to produce a chemical map of the region (bottom). Courtesy of David G. Agresti, University of Alabama in Birmingham.

The structures -- like accepted microfossils that the team examined from 770 million, 2.1 billion and 3.4 billion years ago -- display peaks at the 1350- and 1600-cm–1 Raman bands characteristic of biogenic carbonaceous material. The chemical maps also fit the optical images of the structures, indicating that the signal does not arise from the bulk.

A research group from the UK and Australia, however, is not convinced. Led by Martin D. Brasier of Oxford University in the UK, the researchers propose that only after exhausting all possible nonbiological explanations should one declare a structure a fossil. They suggest that the geochemical evidence is not conclusive of life.

Their study, which also included Raman spectroscopy, found that the structures display carbon isotopic ratios indicative of biogenesis. But based on digital image analysis using software from Syncroscopy Ltd. of Cambridge, UK, that generated montages of the specimens from images at different focal depths, they question that the morphology of the structures is indicative of fossilized microbes. Moreover, their new examination of the site where the samples were found suggests that a hydrothermal vent formed the cherts at temperatures of up to 350 °C. They claim it is unlikely that even the thermophilic archeans known today could have existed there.

They propose instead that Fischer-Tropsch-type synthesis was responsible. To support this claim, the researchers cite a report that appeared last year of hydrocarbons formed this way by vents at the mid-Atlantic ridge.

In Fischer-Tropsch, Brasier explained, a metal catalyst exposed to CO2 or CO generates hydrocarbons, and the products can become isotopically light as the process advances. In the Apex cherts, the researchers suggest, these hydrocarbons then wrapped around quartz microcrystals, giving them the appearance of cellular structure.

"We feel we should draw attention to the alternative possibilities of inorganic synthesis," Brasier said, noting that researchers largely have assumed that isotopically light carbon denotes biogenesis. "Our aim is to keep the debate about early life both honest and open."

Thomas J. Wdowiak of the University of Alabama does not believe Fischer-Tropsch is a viable explanation. "Fischer-Tropsch synthesis is a construct of chemical engineering," he said, that had been offered in the 1960s to account for carbonaceous meteorites but that subsequently was rejected. "It has never been shown to have occurred in a terrestrial environment."

Schopf agrees. "Kerogen -- the particulate organic matter in rocks of which the fossils are composed -- of nonbiological origin has never been identified in the geological record," he said. He finds it problematic that Brasier's team does not leave a means to distinguish biogenic from nonbiogenic material, if neither morphology nor chemistry is considered admissible.

In itself, the hydrothermal origin of the cherts does not suggest that the structures are not microbial remains, Wdowiak said, though they are unlikely to be photosynthetic cyanobacteria as first proposed. He noted that microfossils from the 2.1 billion-year-old Gunflint Formation were preserved in such an environment and that archeans are widespread at today's deep-sea vents and hot springs.

The geological map helps to address the Early Sun Problem and to explain the existence of microfossils from this era, he added. Astrophysicists theorize that the sun was only 70 percent as luminous 3.5 billion years ago as it is today, making it an unlikely source of energy for organisms at the time.

"Hydrothermal activity back then was necessary for life to exist," he said.

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