UNUSUAL CARBON-ISOTOPE COMPOSITIONS OF BIOMARKER HYDROCARBONS IN A PERMIAN TASMANITE

The Permian oil shale sample of this study is from a deposit in Tasmania in which the only recognizable structures are large, thick-walled, unicellular specimens of the green alga Tasmanites. The fossils are so densely packed that this shale is termed tasmanite. The carbon isotopic composition of bulk kerogen carbon (deltaC C-13(org) = -16.6 parts per thousand, vs. PDB) is unusually enriched in C-13 compared to marine Permian organic carbon (-25 to -30 parts per thousand). This C-13 enrichment suggests specific environmental conditions (CO2 draw down) and/or physiological specialties (cell size and growth rate) of Tasmanites which, in modern environments, are known to cause C-13 enrichment in marine phytoplankton. Isotope signatures of extractable organic species, unlike in normal immature oil shales, are considerably enriched in C-13 compared to the kerogen. The bitumen (-10.9 parts per thousand) is composed predominantly of extended saturated C18-C25 tricyclic terpane hydrocarbons (-9.9 to -12.2 parts per thousand total range, with an average of -10.8 parts per thousand). These tricyclics are not only found as free lipids in the extract but also in pyrolysates with similar isotopic signatures and are therefore likely biosynthesis products of Tasmanites. The monoaromatic and diaromatic tricyclic terpane hydrocarbons (C-17-C19) have similar isotopic signatures (-9.3 to -12.8 parts per thousand total range, with an average of -10.6 parts per thousand). Two monoaromatic tetracyclic hydrocarbons (C23 and C24), a quantitatively minor but biogenetically interesting group, average at -10.2 parts per thousand. In contrast, pristane and phytane, as well as n-alkanes (- 18 to -22 parts per thousand), are depleted in C-13 compared to kerogen carbon. The isotopic similarity of all the cyclic terpanes in this sample, together with their occurrence in the free lipids and kerogen, suggests that they have a common origin and are biosynthesis products of the marine unicellular green algae Tasmanites. The tetracyclic terpanes, therefore, are not des-A-oleananes derived from land plants but are possibly monoaromatic des-A-gammaceranes or other des-A-triterpenoids derived from marine sources. The unusual enrichment in C-13 in the cyclic hydrocarbons is hypothesized to result from the special growth conditions of the algae. Isoprenoids and n-alkanes are also likely biosynthesis products of Tasmanites, possibly during the spore formation stage when physiological and environmental conditions were different than during planktonic biosynthesis.