Sulfurisation of lipids in a marine-influenced lignite

Compelling evidence is presented for the process of lipid sulfurisation in humic coal-forming environments. The production of reduced inorganic sulfides by sulfate-reducing bacteria during a marine transgression, which occurred during early diagenesis, enabled the selective sequestration of functionalised lipids in the polar and asphaltene fractions from the Eocene, marine-influenced Heartbreak Ridge lignite deposit, southeast Western Australia. Nickel boride desulfurisation experiments conducted on these fractions released small but significant quantities of sulfur-bound hydrocarbons. These comprised mostly higher plant triterpanes, C29 steranes and extended 17β(H),21β(H)-hopanes, linked by one sulfur atom at, or close to, functionalised sites in the original natural product precursors. These sulfurised lipids mostly derive from the same carbon sources as the free hydrocarbon lipids, the exception being the sulfurised extended hopanoids, which may be partially derived from a different bacterial source compared to that of the free hopanoids. These results indicate that the selectivity and nature of steroid and hopanoid vulcanisation in coal-forming mires is akin to that observed in other sedimentary environments. However, the diversity of higher plant triterpanes that can be sulfurised in marine transgressed coals is greater than that typically reported in immature terrestrial coals. This preservation mechanism explains the formation of the structurally related biomarkers in more mature sulfur-rich humic coals