Carbon isotopic composition of hydrocarbons in ocean-transported bitumens from the coastline of Australia

Abstract

Bitumens stranding along the coastlines of the Northern Territory, Western Australia, South Australia, Victoria and Tasmania often have biomarker signatures which closely match those of oils of S.E. Asian origin. A suite of these bitumens was studied to determine the isotopic signatures of their alkanes and to compare them to similar oils from the Central Sumatra Basin. Saturated hydrocarbons were separated into a fraction containing n-alkanes and isoalkanes and one containing the multibranched/cyclic components using adduction into silicalite. Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) of the n-alkanes revealed δ 13C signatures covering a wide range of values from −25.5‰ to −29‰ PDB but generally falling between those of the botryococcane-containing Minas and Duri oils from the Central Sumatra Basin. n-Alkanes of the Minas oil are 3–4‰ lighter than their Duri counterparts. The isotopic compositions of pristane and phytane cover a similar range, δ 13C = −24‰ to −27‰. Botryococcane is consistently heavy with δ 13C values in the range −11 to −14‰. One bitumen sample with no botryococcane but abundant bicadinanes showed n-alkane δ 13C values in the range −28 to −29‰, at the light extreme for waxy bitumens in our sample suite. Sample to sample differences in carbon isotopic signatures of n-alkanes are attributed mainly to variations in primary source, with weathering and biodegradation as minor causes. Instances of low precision for replicate analyses (± 2‰) are caused by the relative abundance of co-eluting isoalkanes. Overall, the isotopic patterns of waxy bitumens from the Australian coastline provide independent confirmation of their similarity to Central Sumatran lacustrine oils. On the other hand, asphaltic bitumens from the southern Australian coastline are isotopically light with n-alkane δ 13C values in the range −31.5 to −33‰. This distinctive feature may assist identification of their source.