CARBON ISOTOPIC COMPOSITION OF HYDROCARBONS IN OCEAN-TRANSPORTED BITUMENS FROM THE COASTLINE OF AUSTRALIA

Bitumens stranding along the coastlines of the Northern Territory, Wes tern Australia, South Australia, Victoria and Tasmania often have biom arker signatures which closely match those of oils of S.E. Asian origi n. A suite of these bitumens was studied to determine the isotopic sig natures of their alkanes and to compare them to similar oils from the Central Sumatra Basin. Saturated hydrocarbons were separated into a fr action containing n-alkanes and isoalkanes and one containing the mult ibranched/cyclic components using adduction into silicalite. Gas chrom atography-isotope ratio mass spectrometry (GC-IRMS) of the n-alkanes r evealed delta(13)C signatures covering a wide range of values from -25 .5 parts per thousand to -29 parts per thousand PDB but generally fall ing between those of the botryococcane-containing Minas and Duri oils from the Central Sumatra Basin. n-Alkanes of the Minas oil are 3-4 par ts per thousand lighter than their Duri counterparts. The isotopic com positions of pristane and phytane cover a similar range, delta(13)C = -24 parts per thousand to -27 parts per thousand. Botryococcane is con sistently heavy with delta(13)C values in the range -11 to -14 parts p er thousand. One bitumen sample with no botryococcane but abundant bic adinanes showed n-alkane delta(13)C values in the range -28 to -29 par ts per thousand, 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 w eathering and biodegradation as minor causes. Instances of low precisi on for replicate analyses (+/- 2 parts per thousand) are caused by the relative abundance of co-eluting isoalkanes. Overall, the isotopic pa tterns of waxy bitumens from the Australian coastline provide independ ent confirmation of their similarity to Central Sumatran lacustrine oi ls. On the other hand, asphaltic bitumens from the southern Australian coastline are isotopically light with n-alkane delta(13)C values in t he range -31.5 to -33 parts per thousand. This distinctive feature may assist identification of their source.