Polyfunctionalised bio- and geohopanoids in the Eocene Cobham Lignite

Abstract

We investigated the bacteriohopanepolyol (BHP) distribution in the Cobham Lignite sequence (SE England) deposited across the Palaeocene–Eocene boundary, including part of the Palaeocene–Eocene Thermal Maximum (PETM) as shown previously by a negative carbon isotope excursion (CIE). A variety of BHPs were identified, including the commonly occurring and non-source specific biohopanoid, bacteriohopanetetrol (BHT), and 32,35-anhydroBHT which was the most abundant polyfunctionalised geohopanoid in the majority of samples. BHPs with a terminal amine functionality, diagnostic biomarkers for methanotrophic bacteria, were found throughout the sequence, with similar distributions in both the lower laminated and upper blocky lignite except that 35-aminobacteriohopanepentol (aminopentol), indicative of Type I methanotrophs (gammaproteobacteria), was generally more abundant in the upper section within the CIE. The diagenetic fate of these compounds is currently poorly constrained; however, we also identified the recently reported N-containing transformation product, anhydroaminotriol, and several tentatively assigned novel N-containing structures potentially containing ketone functionalities. Although present throughout the section, there was a sharp peak in the occurrence of these novel compounds, which correlated with the onset of the CIE and highly isotopically depleted hopanes in the upper part of the laminated lignite, both also correlating well with aminopentol peak abundance. The significant abundance of these compounds suggests that 35-aminoBHPs have their own specific diagenetic pathway, potentially providing an alternative method allowing methanotroph activity to be traced in older samples even if the original biohopanoid markers are no longer present.At this time we cannot preclude the possibility that some or all of these BHPs have been produced by more recent subsurface activity, post deposition of the lignite; however, this would not be expected to generate the observed stratigraphic variability and we suggest that unprecedented observations of a range of highly functionalised biohopanoids in samples of this age could significantly extend the window of their known occurrence.