Uncovering a 50-million-year-old mystery
When prehistoric organisms conspire with time and sediment to preserve a piece of nature’s skeleton, it’s something to gawk at: Fossils are lifted from their thousand-, million- or even billion-year respites to meet the public’s observing eyes and the investigative techniques of scientists. And although plants’ chemistry can be preserved over hundreds of millions of years, their biochemical beginnings have been largely unstudied – until now.
Researchers at the University of Manchester, Diamond Light Source and the Stanford Synchrotron Radiation Lightsource have used a synchrotron particle accelerator to expose the biochemistry of a 50-million-year-old fossil plant. Dug from the Eocene epoch’s Green River Formation in Colorado, Wyoming and Utah, the leaf fossil was bombarded with x-rays to extract biochemical information.
“We needed to test the chemistry of the fossil plants to see if the fossil material was derived directly from the living organisms, or degraded and replaced by the fossilization process,” said Dr. Nicholas Edwards, lead author of the study and postdoctoral researcher at the University of Manchester.
By combining the capabilities of two synchrotron facilities, the team produced detailed images of both living and fossilized leaves, showing the various chemical elements within them.
A composite image of a 50-million-year-old leaf fossil in natural light, left, and an x-ray false-color image, right. Copper is represented as red, zinc as green and blue as nickel (image width ~17 cm). The metals correlate with the leaf’s original biological structures. Courtesy of Nicholas Edwards; data collected at Stanford Synchrotron Radiation Lightsource, operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.
“Part of what I do involves detailed measurements of the physics of how plants actually harness light energy using transition metals,” said Dr. Uwe Bergmann, team physicist. “Here, we are able to show what metals were present, and where, within extremely old plants – and this just may let us understand, eventually, how the complicated physics of life has developed over long periods of time.”
The distribution of copper, zinc and nickel in the fossils was found to be almost identical to that in modern leaves. Each element was concentrated in distinct biological structures, such as the leaves’ veins and edges.
“The type of chemical mapping and the ability to determine the atomic arrangement of biologically important elements … can only be accomplished by using a synchrotron particle accelerator,” said Dr. Roy Wogelius, professor of geochemistry at the University of Manchester. “In one beautiful specimen, the leaf has been partially eaten by prehistoric caterpillars – just as modern caterpillars feed – and their feeding tubes are preserved on that leaf. The chemistry of these fossil tubes remarkably still matches that of the leaf on which the caterpillars fed.”
The data suggests that the chemistry of the fossil leaves is not wholly sourced from the surrounding environment, as previously suggested, but actually represents the living leaves. The chemical distributions show how ancient plants might have regulated their metal inventories.
“There is a sharp contrast in the chemistry of the fossils from that of the rock in which they are entombed – this is true for both the trace metals and the organic compounds,” said Dr. Bart van Dongen, a geochemist at the University of Manchester. “The organic part of the chemistry clearly shows a plant-derived component.”
These results demonstrate how trace metals within a plant participate in a range of geochemical reactions during the decay of tissue and the formation of stable molecules in sedimentary earth. These reactions promote the leaf’s preservation and transition into a fossil. However, the leaf’s fossilization process had even more in-house help from one specific element.
“We think that copper may have aided preservation by acting as a natural biocide, slowing down the usual microbial breakdown that would destroy delicate leaf tissues,” said Dr. Phil Manning, co-author and a paleontologist at the University of Manchester. “This property of copper is used today in the same wood preservatives that you paint on your garden fence before winter approaches.”
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