Early diagenetic transformation of organic matter in a marine-influenced lignite

Abstract

The polar lipid and macromolecular compositions of three samples from the marine-influenced Heartbreak Ridge lignite deposit (Bremer Basin, Western Australia) were investigated for chemical changes wrought during early diagenesis. The results of Rock-Eval, microscope Fourier transform infrared spectroscopic and thermal desorption/pyrolysis-gas chromatography–mass spectrometric analyses indicate that the composition of the lignite and its extract are typical of immature Type III organic matter, and thus contain a high proportion of aromatic and oxygen-containing functionalities. Down-seam variation in the proportions of these functionalities, together with changes in the carbon preference index (CPI) of the pyrolysate n-alkenes and the wax indices of straight chain n-alkyl pyrolysis products, is consistent with more pronounced degradation of the lowermost lignite horizon. This profile is also reflected in the diagenetic products of higher plant triterpenoids, identified in the lignite extracts by gas chromatography–mass spectrometry. The extent of aromatisation, demethylation, ring opening and defunctionalisation is recorded through this depth profile by the compositions of the free, sulfurised and oxygenated higher plant triterpenoids. In contrast, the extended hopanoid composition is invariant in all three lignite samples. Oxygen-containing lipids are implicated as the primary source of the sulfurised hydrocarbons, although not necessarily via direct sulfurisation of the identified oxygenated lipids, but through the formation of more reactive intermediate species. These results suggest that the onset of marine incursion and associated sulfur fixation inhibits and, through preservation, illuminates the typical biogeochemical transformation of lipids and biopolymers in coal-forming environments.