By bombarding fossilized leaves with X-rays brighter than a million suns, researchers have created a chemical map that displays the atomic arrangement of the various elements in the ancient plants.
The fossil leaves are about 50 million years old, and they come from extinct elm, poplar, and plane trees found in the Eocene-aged Green River Formation in the western U.S. “We know that plant chemistry can be preserved over hundreds of millions of years -- this preserved chemistry powers our society today in the form of fossil fuels,” lead author Nicholas Edwards of University of Manchester explains in a press release. But that’s just the combustible part. “Until now no one has completed this type of study of the other biochemical components of fossil plants, such as metals.”
Plants harness light energy using transition metals. And what Edwards and colleagues have done is show what metals were present, and where, within extremely old plants. The method could help scientists understand how the physics of life has developed over long periods of time.
The x-rays were produced by synchrotron particle accelerators. Previous work has shown that synchrotrons can tease out extra information from fossils -- specifically, pigmentation in ancient animals. “With this study, we wanted to use the same techniques to see whether we could extract a similar level of biochemical information from a completely different part of the tree of life,” Edwards says.
First, they tested the chemistry of the specimen to make sure the fossil material was derived directly from the living plant -- and not degraded or replaced during the fossilization process, perhaps by the rock where it was entombed. By making sure the chemistry of the fossils were sourced from the leaves themselves (and not the surrounding environment), they were able to decipher their original chemical makeup.
Then, by combining the powers of two synchrotron facilities -- Stanford Synchrotron Radiation Lightsource and Diamond Light Source -- the researchers created detailed images that show how the distribution of copper, zinc, and nickel in the fossil leaves was almost identical to that in modern leaves. Each element was concentrated in distinct structures -- like veins and leaf edges -- and the way the organic metals and sulfur compounds attached to other elements was very similar to what we see in modern leaves and plant matter.
The copper may have been used as a “natural biocide,” study author Phil Manning of Manchester says, “slowing down the usual microbial breakdown that would destroy delicate leaf tissues.” This property of copper is utilized today in wood preservatives for garden fences.
“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 the leaf,” study author Manchester’s Roy Wogelius says. “The chemistry of these fossil tubes remarkably still matches that of the leaf on which the caterpillars fed.”
Image: P. wyomingensis, specimen BHI-3113 via University of Manchester and the Royal Society of Chemistry