INVESTIGATIONS OF FUSAIN TRANSITION FOSSILS FROM THE LOWER CARBONIFEROUS - COMPARISONS WITH MODERN PARTIALLY CHARRED WOOD

Plant fossils showing vitrinite-semifusinite(-fusinite) transitions and permineralised-semifusinite transitions were collected from Lower Carboniferous locations within the British Isles. Several were obtained from East Kirkton Quarry, Bathgate, Scotland, and one fusinite-semifusinite-vitrinite-semifusinite-fusinite transition fossil was collected at Muckros Head, County Donegal, Ireland. The Muckros Head fossil was found in a limestone sea-cliff face, where it had been weathered proud of the rock matrix, thus enabling a detailed examination and interpretation of the macroscopic appearance of the specimen. The distinctive 'sandwich-like' cross-section of the fossil is interpreted as being caused by the burial and coalification of a partially charred plant stem, with recent exposure and weathering resulting in the loss of its 'shattered' edges. Under S.E.M. and reflected light microscopy, non-permineralised transition fossils are seen to grade from structureless vitrinite into compressed and ''bogen-structured'' semifusinite, and occasionally continue through to structured uncompressed fusinite; with corresponding increases in reflectance. These gradations parallel the reflectances seen in modern, partially charred stems. Stable carbon isotopic analysis of the transition fossils (deltaC-13= - 17.19 parts per thousand to - 19.26 parts per thousand; - 22.46 parts per thousand to - 24.54 parts per thousand; - 24.47 parts per thousand to - 26.20 parts per thousand), show a significant difference between the vitrinite and semifusinite (-fusinite), with the semifusinite (-fusinite) isotopically heavier. The various techniques employed to investigate these transition fossils all indicate that they represent coalified, partially charred stems. Correlation of the properties and characteristics of the semifusinite(-fusinite) in transition fossils, with the properties and characteristics of isolated semifusinite and fusinite fragments collected from rocks of the Lower Carboniferous through to the Tertiary, supports the view that most of these isolated fragments also represent fossil charred wood and charcoal.