Biological affinities of Neoproterozoic acritarchs from Australia: microscopic and chemical characterisation

A range of well-preserved Neoproterozoic acanthomorph and sphaeromorph acritarchs from the Australian Centralian Superbasin have been individually analysed by combined microscopic and chemical methods. The acanthomorph acritarchs generally exhibit two or more cell wall categories and have fine laminations with no obvious trilaminar sheath structure, whereas the sphaeromorph acritarchs are characterised by a single cell wall structure with no distinct laminations. The significant organic carbon content of the acritarchs is evident by the EDS analysis. Petrological indices (Thermal Alteration Index less than or equal to 2.8, and strong yellow fluorescence) suggest a low thermal maturity for both coexisting acritarch groups. The biomarker composition of the enclosing-rock is similarly indicative of low thermal maturity, although most of the rock extract has probably originated from other sources including the lipid fraction of the acritarchs' cell membrane and cytoplasm rather than the preserved cell walls. In contrast, however, very limited chemical data was obtained directly from the morphologically-preserved component of the acritarchs by micro-FTIR and pyrolysis GC-MS analyses, and these results are consistent with a polyaromatic biomacromolecule of high recalcitrance. A large signal attributable to significant carbon-ordering-a characteristic of polyaromatic structures-was detected by laser micro-Raman spectroscopy. A unique ultrastructural and molecular entity of the studied Neoproterozoic acritarchs is indicated although a distinct fibrillar multilayered wall and a condensed polyaromatic macromolecular structure suggest a possible link between acanthomorph taxa and dinoflagellates. The contrasting maturity values indicated by the respective petrographic and chemical data highlight the need for a more rigorous correlation of these different maturity parameters. (C) 2000 Elsevier Science Ltd. All rights reserved.