The Evolution of Vascular Plant Cell Wall Chemistry and the Preservation of Fossil Organic Matter as Illuminated by X-Ray Spectromicroscopy

One of the largest potential sources of geochemical information is one that is frequently overlooked: the anatomy of organically preserved fossils. Permineralized plant fossils can provide striking subcellular detail and simply the identification of cell types with visual light microscopy can indicate original biochemical composition before the first geochemical analysis is performed. Not every fossil has a closely related living counterpart, but original composition of more enigmatic fossils can be constrained by comparisons to the better-understood tissues of other fossils from the same locality using spatially constrained analyses of fossil organic chemistry. This strategy is possible using synchrotron-based X-ray spectromicroscopy with which submicron-scale imaging over a range of X-ray frequencies provides information concerning the relative abundance of different organic bonds using X-ray Absorption Near Edge Spectroscopy. X-ray absorption can be either measured directly by transmittance through an ultrathin section of a demineralized sample or indirectly be measuring the photoelectrons emitted upon X-ray absorbance by an undemineralized fossil thick section, with each technique having its own advantages and limitations. These techniques have been applied to the study of the evolution of cell wall chemistry and proliferation of cell types among the vascular plants and to the study of the permineralization process and the nature of organic preservation. Much as an organic matrix influences precipitation in biominerals, ongoing XANES studies suggest crystal fabric in calcite permineralized fossils may reflect the original organic composition of the tissue and, thus, may provide another avenue for exploring organic preservation and the physiology of fossil organisms.