Unusual carbon isotope compositions of biomarker hydrocarbons in a Permian tasmanite

Details Unusual carbon isotope compositions of biomarker hydrocarbons in a Permian tasmanite Unusual carbon isotope compositions of biomarker hydrocarbons in a Permian tasmanite

Sep 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.4205s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 17, pp.4205-4211

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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 ( C 13 C org = -16.6%., vs. PDB) is unusually enriched in 13 C compared to marine Permian organic carbon (-25 to -30%.). This 13 C enrichment suggests specific environmental conditions (CO 2 draw down) and/or physiological specialties (cell size and growth rate) of Tasmanites which, in modern environments, are known to cause 13 C enrichment in marine phytoplankton. Isotope signatures of extractable organic species, unlike in normal immature oil shales, are considerably enriched in 13 C compared to the kerogen. The bitumen (-10.9%.) is composed predominantly of extended saturated C 18 -C 25 tricyclic terpane hydrocarbons (-9.9 to -12.2%. total range, with an average of -10.8%.). 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 -C 19 ) have similar isotopic signatures (-9.3 to -12.8%. total range, with an average of -10.6%.). Two monoaromatic tetracyclic hydrocarbons (C 23 and C 24 ), a quantitatively minor but biogenetically interesting group, average at -10.2%.. In contrast, pristane and phytane, as well as n -alkanes (-18 to -22%.), are depleted in 13 C 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 13 C 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.

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