Early diagenesis of bacteriohopanoids in Recent sediments of Lake Pollen, Norway

The distribution of free bio- and geohopanoids were studied in a short (46 cm) core from Recent sediments of Lake Pollen, Norway. Bacteriohopanepolyols are abundant components of the sediments (totalling up to 321 μg/g dry sediment; 7485 μg/g TOC), being dominated by C-32,33,34,35 tetrafunctionalised hopanoids. Significantly, bacteriohopanetetrol only accounts for between 7 to 62% of these (24% on average), indicating that composite bacteriohopanoids are more abundant sedimentary constituents, and thus more important precursors of geohopanoids than previously recognised. Downcore profiles of bio- and geohopanoid abundance show maxima at ca. 25 to 30 cm, with a marked decrease in abundance above this depth which coincides with a well-documented change in environment from fjord to isolated lake. A second maximum in biohopanoid abundance occurs at ca. 5 to 12 cm depth. Specific biohopanoids were affected to different extents by the change in environment, suggesting that they have, at least partly, different sources. Our data thus indicate that biohopanoids, and their geohopanoid products, may be sensitive to changes in depositional environment and that they have potential to act as markers for environmental conditions.