50 / 2025-03-29 10:37:24

Mechanisms for the formation of rearranged hopanes: insights from the Sydney Basin, Australia

Diahopane,Hopanoid,Sydney Basin,Biomarker,Rearranged hopane

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4e: Microbial Lipid Biomarkers and Their Isotopes in Environment, Biology, and Evolution

Rearranged hopanes are a class of biomarkers with the same carbon skeleton as regular hopanes, but with a rearranged methyl group (or groups) in the ring (e.g. Moldowan et al., 1991). It was proposed that rearranged hopanes form from bacteriohopanoid precursors by clay-mediated acidic catalysis during early diagenesis in oxic or suboxic depositional environments. Other factors that have been reported to contribute to high abundances of rearranged hopanes include high thermal maturity, terrigenous organic matter inputs, and the presence of saline water during deposition (e.g., Jiang et al., 2018). In the Permian of the Sydney Basin three series of rearranged hopanes were identified in outcrop and core samples by multiple reaction monitoring gas chromatography–mass spectrometry: 18α(H)-neohopanes, 17α(H)-diahopanes, and the early-eluting 9,15-dimethyl-25,27-bisnorhopane homologues. There is a high relative abundance of rearranged hopanes (C₃₀ 17α(H)-diahopane/C₃₀ αβ hopane ratio [C₃₀*/C₃₀αβ] = 2.3–6.9) in outcrops of the early Permian Wandrawandian Siltstone in the Sydney Basin, which was deposited on the continental slope (Baydjanova and George, 2019). This formation has extensive evidence of slumping and soft sediment deformation, and this may have resulted in enhanced diagenetic and catalytic rearrangement reactions, leading to the elevated levels of rearranged hopanes. However, diasteranes are not highly abundant compared to steranes in the Wandrawandian Siltstone, suggesting different formation mechanisms for diasteranes compared to diahopanes (Baydjanova and George, 2019).

More recently, a study of outcrop samples from the late Permian–early Triassic has shown very variable C₃₀*/C₃₀αβ ratios, including the highest ever recorded in the Tongarra Coal (late Permian Illawarra Coal Measures; 77–93; Kampoli and George, 2025). The Bulli Coal, just below the Permian–Triassic mass extinction, has C₃₀*/C₃₀αβ ratios of 12–32, and mudstones close to these coals have ratios of 1–33. The extensive and sometimes near quantitative alteration of 17α(H)-hopanes to rearranged hopanes was not due to high thermal maturity, depositional water salinity, or slumping, and may have been enhanced by the high abundance of terrigenous organic matter, deposited under fresh water sub-oxic conditions. It is also not related to oxidation by surface weathering processes that could have happened after uplift and erosion, based on the presence of abundant n-alkanes and low molecular weight aromatic compounds in the samples, and by high C₃₀*/C₃₀αβ ratios (3–24) in the late Permian in the Duncans Creek-1 borehole in the central part of the Sydney Basin, and in the Metropolitan Coal from a deep mine (430 m; C₃₀*/C₃₀αβ ratio = 4.8; Ahmed et al., 2009). A key driver is suggested to be extensive volcanic activity, for which there is strong evidence in the three Tongarra coal seams which are separated by laterally-continuous pale-coloured ash-fall tuffs. In addition to facilitating hopane rearrangement, volcanic activity likely provided high temperatures and increased nutrient supply that triggered cyanobacterial activity, also resulting in the formation of monomethylalkanes (Kampoli and George, 2025). Several fundamental questions remain unanswered. Did the volcanic activity alter the pH of the depositional environment? Was the presence of specific chemical elements, such as potassium or aluminium, a contributing factor? Does the type of volcanism, felsic or mafic, make a difference? Or were other specific factors related to volcanism involved?