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

A range of well-preserved Neoproterozoic acanthomorph and sphaeromorph acri tarchs from the Australian Centralian Superbasin have been individually ana lysed by combined microscopic and chemical methods. The acanthomorph acrita rchs generally exhibit two or more cell wall categories and have fine lamin ations with no obvious trilaminar sheath structure, whereas the sphaeromorp h acritarchs are characterised by a single cell wall structure with no dist inct laminations. The significant organic carbon content of the acritarchs is evident by the EDS analysis. Petrological indices (Thermal Alteration In dex less than or equal to 2.8, and strong yellow fluorescence) suggest a lo w thermal maturity for both coexisting acritarch groups. The biomarker comp osition of the enclosing-rock is similarly indicative of low thermal maturi ty, although most of the rock extract has probably originated from other so urces including the lipid fraction of the acritarchs' cell membrane and cyt oplasm rather than the preserved cell walls. In contrast, however, very lim ited chemical data was obtained directly from the morphologically-preserved component of the acritarchs by micro-FTIR and pyrolysis GC-MS analyses, an d 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-Ram an spectroscopy. A unique ultrastructural and molecular entity of the studi ed Neoproterozoic acritarchs is indicated although a distinct fibrillar mul tilayered wall and a condensed polyaromatic macromolecular structure sugges t a possible link between acanthomorph taxa and dinoflagellates. The contra sting 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.