Unusual occurrence of alkylphenanthrenes in upper Eocene to Oligocene sediments from the western margin of Tasmania, Australia

• Published in: Marine Geology
• Main Authors: Li, Zhongxuan, Huang, Haiping, George, Simon C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2022
• Subjects: Alkylphenanthrenes; Biomarker; Eocene; Southern Ocean; Tasmanian Gateway
• Online Access: https://researchers.mq.edu.au/en/publications/33606a03-d88a-4010-b6b5-970d0f44297d; https://doi.org/10.1016/j.margeo.2022.106859; http://www.scopus.com/inward/record.url?scp=85134246586&partnerID=8YFLogxK; http://purl.org/au-research/grants/arc/LE160100067; http://purl.org/au-research/grants/arc/LE210100028

The occurrence and distribution of alkylphenanthrenes in upper Eocene–Oligocene sediments from Ocean Drilling Program Leg 189 (Sites 1168, 1170, 1171 and 1172) that were drilled off the coast of Tasmania have been examined by gas chromatography–mass spectrometry. Samples are thermally immature and contain predominantly terrigenous organic matter. Peculiar alkylphenanthrene distributions occur in four samples from Site 1168 at depths from 748 m to 762 m, with methylphenanthrene (MP) isomers showing complicated and irregular distributions. A dominance of 3-MP, 9-MP and 1-MP occurs in different samples, while 2-MP is systematically low in all the samples. Concentrations of alkylphenanthrenes and various other aromatic hydrocarbons generally increase with burial depth at Site 1168, likely indicating diagenetic formation. Commonly used source input and thermal maturity parameters derive inconsistent and confusing interpretations for all four sites. Maturity does not appear to be a decisive factor that influences the distributions of methylphenanthrene and dimethylphenanthrene (DMP) isomers and the degree of alkylation. The direct decomposition of retene to form an abnormal enrichment of 1-MP in one sample from Site 1168 is also difficult to envisage. The low 1,7-DMP/1,3- + 3,9- + 2,10- + 3,10-DMPs and ∑alkylphenanthrenes/∑alkylnaphthalenes ratios, coupled with the predominance of hopanoids over steroids, point to microbial input of 3-MP and 1-MP as a plausible interpretation in certain samples, while the dominance of 9-MP throughout the sedimentary column reflects local biomass input formed in the marine environment. Input of combustion-derived alkylphenanthrenes is unlikely to be the sole reason to cause unusually high phenanthrene/∑methylphenanthrenes ratios in the samples from the four sites. Oxidation during deposition and the immaturity of the sediments at an early diagenesis stage are likely to be an alternative interpretation.