Other
Scientific paper
Mar 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.1391s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 3, pp.1391-1399
Other
5
Scientific paper
An immature, tar-like oil (API~3°, 13 C = -23.6 vs. PDB) from the Miocene Monterey Formation offshore California was selected for a study of carbon isotopic signatures of individual biomarkers. The three principal stereoisomers of 28,30-bisnorhopane (C 28 ) have, within analytical precision, identical carbon isotopic compositions (average 13 C = -32.3 ± 0.4 ) and are considerably depleted in 13 C compared to the whole oil. These 28,30-bisnorhopanes (BNH) differ isotopically from C 29 and C 30 17 (H)-hopanes (-25.8%. and -26.1%.) and C 31 -C 35 extended hopanes ( 13 C = -27.7 ) and suggest different precursors for the C 28 hopanes than for C 29 -C 35 hopanes. The relative depletion of BNH of almost 9 compared to the isotopic composition of the whole oil suggests that these hopanes derive from chemoautotrophic bacteria, possibly not yet identified H 2 S oxidizers, which utilize 13 C-depleted substrates. The C 29 and C 30 hopanes are, within analytical precision, isotopically identical (~ -26 ) and similar to algal-derived compounds, e.g., C 27 steranes (~ -25.9 ), which is consistent with a cyanobacterial source for these hopanes. An archaebacterial biomarker, 1,1'-biphytane ( 13 C = -25.5 ), likely derived from methanogens, is also isotopically similar to C 27 sterane. Norpristane, pristane, and phytane, liberated by desulfurization of the aromatic and polar maltene fractions, show isotopic compositions similar to the same isoprenoids in the free lipids of the bitumen (total range from -24.5 to -27.5 ). This isotopic similarity supports a common origin for the free and sulfur-bound forms of these isoprenoids. This origin could be algal and/or archaebacterial lipids, which both show isotopic compositions within the range of the C 18 -C 20 isoprenoids. Like other marine-derived organic matter, this Monterey oil does not show the strong 13 C depletion typical for methylotroph-derived compounds characteristically found in organic matter of lacustrine origin. This may indicate fundamental differences of methane recycling processes in marine (sulfate-dominated) as compared to lacustrine environments.
Fago Frederick J.
McCaffrey Mark A.
Moldowan Michael J.
Schoell Martin
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