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Raman Imaging Identifies Microfossils

Photonics Spectra
May 2001
Daniel S. Burgess

BIRMINGHAM, Ala. -- Micropaleontology has become a viable science only in the last four decades, but it has already enabled researchers to chart the history of life on Earth back 3.5 billion years to its emergence in the Archean.

Unlike macroscopic fossils from the Phanerozoic, however, these remains cannot always be understood by their morphology. In many cases, it may not even be possible to distinguish a microfossil from nonbiogenic minerological structures. Now, an imaging method based on Raman spectroscopy is helping scientists verify that they have found the fossil remains of a microscopic life form.

"It gives, in effect, a map of certain molecular components that may be present -- a fingerprint," explained David G. Agresti, professor of physics at the University of Alabama. He and his colleagues, Thomas J. Wdowiak and Anatoliy B. Kudryavtsev at the university and J. William Schopf of the University of California in Los Angeles, described the Raman imaging of 45-million-, 650-million- and 2.1-billion-year-old microfossils in the Jan. 30 issue of Proceedings of the National Academy of Sciences.

The technique scans a laser probe over the sample, recording the Raman spectra from each spot, some as small as 0.5 x 0.5 µm, depending on the wavelength of the probe. The researchers assign color values to the spectral signals according to the intensity at these "spexels," which they can combine to produce a two-dimensional image of the sample's chemistry. In recent tests, they were able to discern cellular features in the microfossils, such as the cell wall in the xylem of a 45-million-year-old fern from the Clarno formation in Oregon, by the changes in intensity at the 1350- and 1600-cm-1 vibrational bands.

"The new thing is that we're taking Raman imaging to paleontology," Agresti said. "It's the application of a relatively new technique to a new field."

Dreams of a red planet

But use of Raman imaging will not be limited to terrestrial samples. The researchers are working with NASA's Jet Propulsion Laboratory in Pasadena, Calif., Washington University in St. Louis and Cornell University in Ithaca, N.Y., on the development of the Mars Microbeam Raman Spectrometer, a palm-size instrument for inclusion on a rover to be sent to the planet in 2007.

"Raman is a natural field instrument for another world," Wdowiak said. One of its strengths is that it is a general technique, so researchers could use it to identify minerals of interest or sedimentary rocks for return to Earth. "Our objective is to see a Raman spectrometer on Mars and see them used on samples back here."

But the dream is to locate and bring home martian fossils. "My personal feeling is that 3.5 billion to 4 billion years ago Mars could have been habitable to life," Wdowiak said, "even more habitable than the Earth at the time, because Earth was going through the moon-forming event." A rover equipped with a Raman spectrometer may be able to find evidence that life did take hold there.

In the meantime, the team is content to solve terrestrial mysteries. "We are continuing our relationship and collaboration," Agresti said. "We discovered some interesting features in the Clarno sample."


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