Fossil plants from the Eocene London Clay: the use of pyrite textures to determine the mechanism of pyritization

Pyritized twigs and roots from the Eocene London Clay of SE England were studied to gain a better understanding of the process of pyritization by investigating pyrite textures in relation to cell type and quality of preservation. Highly polished sections and fractured surfaces taken from 124 specimens were examined using optical microscope and SEM, the latter equipped to map pyrite and carbon. Pyrite textures include microcrystalline, framboidal, massive polycrystalline, and subhedral or euhedral forms. The highest fidelity of preservation is always associated with microcrystalline pyrite precipitation on wall surfaces with subsequent infilling of cells with framboids or polyhedra preventing compression during burial but contributing nothing to actual ultrastructural preservation. Ultrastructurally, parenchymatous cell walls are coalified, whereas microcrystalline pyrite plus coalified material were observed within lignified cell walls. In all, four stages of pyritization are documented. Observations are interpreted in the light of recent experiments on pyritization of living material and the chemistry of pyrite formation in anoxic environments involving an aqueous, and hence mobile, FeS cluster complex as a precursor. The complexity of the fossilization process is reflected in the presence of different textures in adjacent cells of the same tissue. This demonstrates the development of isolated chemical microenvironments as pH and Eh vary in response to decay, and mineralization and pyrite overgrowths within a cell indicate local microenvironmental changes through time.