Occurrence and origin of perylene in Paleogene sediments from the Tasmanian Gateway, Australia

Abstract

Eocene to Lower Oligocene sediments were collected by Ocean Drilling Program Leg 189 at four sites in the Tasmanian Gateway, Australia, and geochemically characterised to investigate the occurrence and controlling factors of perylene. Perylene is enriched in samples from Site 1168, which is located closest to Tasmania, accounting for up to 53.4% of the total quantified aromatic hydrocarbons, with concentrations ranging from 15 ng/g dry weight sediment (dws) to 1700 ng/g dws. However, only trace amounts of perylene (<5 ng/g dws) are present in samples from the other three sites, except for two deep samples from Site 1171, which is furthest from Tasmania, with perylene concentrations of 15 ng/g dws and 95 ng/g dws. The concentrations of perylene increase with burial depth, suggesting its formation from diagenetic processes. Other molecular compositions, including land-plant-derived aromatic hydrocarbons, pyrogenic-related aromatic hydrocarbons and n-alkanes, indicate that the origin of perylene can be attributed to allochthonous organic material associated with terrigenous plants. Contributions from autochthonous organic matter and combustion cannot be validated. Detailed examination of the relationship between the concentrations of perylene and organic matter input and depositional conditions illustrate that both factors are important but not essential. The concentrations of perylene are positively correlated to pristane/phytane ratios, implying that perylene is probably formed under dysoxic rather than highly reducing conditions. Terrigenous-sourced organic matter was detected at all four sites, but perylene is only concentrated at the site closest to Tasmania. It is proposed that the preservation of the precursors of perylene is critical, as these are sensitive to destruction by oxidation. If the precursors of perylene are destroyed during transportation and early sedimentation, no perylene will be formed during diagenesis, no matter how strong the reducing conditions are.