Monoaromatic, Diaromatic, Triaromatic, and Tetraaromatic Hopanes in Kukersite Shale and Their Stable Carbon Isotopic Composition

Abstract

Abundant monoaromatic, diaromatic, triaromatic, and tetraaromatic hopanes as well as benzohopanes occurred in the Ordovician Estonia Kukersite shale. They were present in sufficient concentrations for reliable stable carbon isotopic composition measurements. There was almost no difference between steroids and monoaromatic steranes, but 13C enrichments of 2‰–4‰ were observed in aromatized hopanes, compared with hopanoids. Regular hopanes (C27–C35) generally are proposed to be sourced from the bacteriohopanetetraols (C35), while aromatic hopanes were derived from the diplopterol and/or diploptenes (both C30). Hopanoids were aromatized via a progressive pathway from the D-ring to the A-ring with increasing aromaticity, with successive losses of four angular methyl groups on the A-, B-, C-, and D-rings, as well as one of the gem-dimethyl groups at C-4 (i.e., C-23 and C-24). Thus, different hopanoid precursors with their own δ13C values as well as the successive losses of up to five carbon atoms in the aromati...