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42426-G2
Use of Paleobiology to Investigate the Preservation of Plant Biochemistry in Coal and Organically Preserved Fossils
In the second and final year of this grant, significant progress has been made on the primary goals of the grant regarding geochemical analysis of the tree lycopods that were a primary contributor to Carboniferous coal. Completion of this project is expected shortly after the end of the grant term and this work has opened important and unexpected new directions of research regarding the role of included organic matter in influencing the mineral fabric of the fossils.
Lignin may have been a surprisingly modest proportion of the original biochemical source of Carboniferous coals because the single largest contribution would not have been wood, but rather the cortical tissues of now extinct tree lycopods for which the original biochemistry is unknown. During the first year of the grant, synchrotron beam time was secured on a scanning transmission X-ray microscope for X-ray Absorption Near-Edge Structure (XANES) spectroscopy in order to provide spatially constrained organic geochemical comparisons of these fossil tissues and cell types. However, it was found that the need for demineralization, which leads to essentially one dimensional spatial information, strips away too much of the anatomical context for such complicated tissues. [The beam time freed up by this hurdle has been applied to other questions regarding early terrestrial life as discussed in the first progress report.] In the second year, application has been made of an X-ray photoelectron emission microscope which provides the same XANES data without requiring X-ray transmission, thereby allowing direct analysis of mineralized fossil surfaces. This new approach so far has allowed both partial interpretation of tree lycopod biochemistry and suggested new insights into the role of that biochemistry for the mineralization process.
X-PEEM analysis of fossil thick sections has shown little similarity between the organic chemistry of the scant wood possessed by tree lycopods and that of their thick cortical tissues, which supplied most of the plant's structural support. This suggests these cortical tissues were not lignified, but rather that their thick dense cell walls were impregnated with alternative compounds, perhaps such as a suberin. Constraining what that alternative biochemistry might have been must await broader comparisons with other plants from the same coal beds; this is planned for the next round of synchrotron beam time scheduled for October immediately after this report is due. However, even without knowing a specific biochemical source, simply the indication that the source was not lignin will have important implications for our understanding of the processes of geochemical alteration resulting in coal—processes for which lignin is often assumed as a starting point.
Carbonate permineralized fossils like those used here are traditionally studied using demineralized material in which the exposed organic matter on the surface of acid etched fossils is stuck to acetate sheets. As a result, carbonate mineralized plant fossils have not been widely studied in a direct fashion for 50 years or more. Our X-PEEM analysis of Carboniferous lycopods shows extreme differences in carbonate crystal size and shape between woody tissue, cortical tissue, and abiotic voids infilled with carbonate, suggesting the specific biochemical composition of different types of cell walls likely played important roles in crystal nucleation, growth, or inhibition and may, with further study, provide an independent proxy for original biochemical composition that would be an easily available tool for paleontologists.
Additionally, work regarding the bizarre Devonian fossil Prototaxites, has appeared in print during this second year of the grant and garnered a great deal of public and media attention. Prototaxites consists of tree-like trunks up to eight meters long constructed only of interwoven tubes less than 50 microns in diameter. In the 150 years since its first discovery, this fossil has been variously thought to be a conifer, a fungus, a red, green, or brown algae, or a lichen. Prototaxites specimens from five localities differ from contemporaneous vascular plants by exhibiting a carbon isotopic range, within and between localities, of up to 13‰ d13C. Pyrolysis-gas chromatography/mass spectrometry highlights compositional differences between Prototaxites and co-occurring plant fossils and supports interpretation of isotopic distinctions as biological rather than diagenetic in origin. Such a large isotopic range is difficult to reconcile with an autotrophic metabolism, suggesting instead that Prototaxites was a heterotroph that lived on isotopically heterogeneous substrates: in this context, a fungus. Light isotopic values of Prototaxites approximate those of vascular plants from the same localities; in contrast, heavy extremes appear to reflect consumption of primary producers with carbon-concentrating mechanisms, such as cryptobiotic soil crusts or, possibly, bryophytes. Prototaxites biogeochemistry, thus, suggests that a biologically heterogeneous mosaic of primary producers characterized land surfaces well into the vascular plant era.
