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

The occurrence and distribution of alkylnaphthalenes in ten sediment samples from the upper Eocene–Oligocene in Site 1168 of Ocean Drilling Program Leg 189 in the western margin of Tasmania, Australia have been examined by gas chromatography–mass spectrometry. The samples are thermally immature and contain predominantly terrigenous organic matter. Concentrations of alkylnaphthalenes generally increase with burial depth. Peculiar isomer distributions occur in some samples near the Eocene–Oligocene boundary at 748–762 m, and in the deepest sample at 850 m. Three isomers, 1,3,6-trimethylnaphthalene (TMN), 1,3,6,7-tetramethylnaphthalene (TeMN), and 1,2,3,6,7-pentamethylnaphthalene (PMN), have abnormally high abundance relative to total alkylnaphthalenes. The relative abundance of 2,6- + 2,7-dimethylnaphthalene (DMN) is also higher than in other samples, whereas there is a low relative abundance of 1,6-DMN, 1,2,5-TMN, 1,2,5,6-TeMN, and 1,2,3,5,6-PMN in these samples. While 2,6- + 2,7-DMN, 1,3,6-TMN, 1,3,6,7-TeMN and 1,2,3,6,7-PMN are thermally more stable than 1,6-DMN, 1,2,5-TMN, 1,2,5,6-TeMN and 1,2,3,5,6-PMN, these isomers are unlikely to have been inherited from thermally mature strata, because other more thermally stable isomers such as 1,3,7-TMN, 1,3,5,7-TeMN and 1,2,3,5,7-PMN have not been enriched in a similar way in these immature samples. It is also unlikely that selective biodegradation of other alkylnaphthalene isomers resulted in relative concentration of 2,6- + 2,7-DMN, 1,3,6-TMN, 1,3,6,7-TeMN, and 1,2,3,6,7-PMN, as these compounds have not been demonstrated to have higher ability to resist biodegradation, and no consistent supportive evidence could be obtained from other organic geochemical parameters. Relative depletion of terrigenous-sourced 1,6-DMN, 1,2,5-TMN, 1,2,5,6-TeMN, and 1,2,3,5,6-PMN and algal-derived 2,3,6-TMN point to microorganisms as the most plausible precursors for the occurrence of these peculiar alkylnaphthalene isomers in the studied samples. 1,3,6-TMN, 1,3,6,7-TeMN and 1,2,3,6,7-PMN are structurally and genetically connected, and are considered to be the diagenetic products of drimane-type sesquiterpenes or hopanes from bacteria. More investigation of the diagenetic pathways of alkylnaphthalenes are required, because the identification of a microbial origin of alkylnaphthalene isomers may provide deep insights into the geochemical application of parameters based upon alkylnaphthalenes, such as thermal maturity parameters.