7), 5[alpha], 14[alpha] and 17[alpha](H) steranes
are present in the [C.sub.27]-[C.sub.29] range.
Batterman, "PAHs (polycyclic aromatic hydrocarbons), nitro-PAHs, and hopane and sterane
biomarkers in sediments of southern Lake Michigan, USA," Science of the Total Environment, vol.
Compared with other suites of the source rocks, P3d source rocks show relatively high [C.sub.27]/[summation][C.sub.27]-[C.sub.29] regular steranes
ratios (Table 3).
The slightly higher concentrations of [C.sub.29] monoaromatic steranes
(49%), compared with [C.sub.28] (22%) and [C.sub.27] (29%) monoaromatic steranes
, suggests a mixed input of terrestrial and marine organic matter.
In addition, biomarker features can reflect the oil maturity, such as [T.sub.s]/([T.sub.s] + [T.sub.m]), [C.sub.27] ~ [C.sub.29]-diasteranes/steranes, the content of low molecular weight steranes
, tricyclic terpanes, and homohopanes .
and Moldowan J.M.: "Application of steranes
, terpanes and monoaromatic to the maturation, migration and source of crude oils".
Yet the samples also held small quantities of hopanes, a class of organic chemicals produced by cyanobacteria and some other bacteria, as well as steranes
, which are produced only by eukaryotes.
CHANGES IN MOLECULAR COMPOSITION AND LEVELS OF AEROBIC BIODEGRADATION LEVEL DESCRIPTION 0 paraffinic crude oil not biodegraded 1 [C.sub.1] to [C.sub.15] n-alkanes depleted minor 2 90% of [C.sub.1] to [C.sub.35] n-alkanes light removed 3 alkylcyclohexanes and alkylbenzenes removed, moderate isoalkanes (including isoprenoids) reduced 4 isoprenoids and methylnaphthalenes removed, moderate aromatics depleted in order of increasing molecular weight and alkyl substitution 5 [C.sub.14]-[C.sub.16] bicyclic alkanes extensive removed 6 smaller steranes
removed, larger steranes
heavy altered, 25-norhopanes formed 7 all steranes
removed, 25-norhopanes more heavy abundant 8 hopanes altered very heavy 9 hopanes removed, disteranes altered, severe
JAHNKE: Regarding the timing of the origin of eukaryotes, there is some very excellent evidence in the organic molecular record for steranes
in the 1500 to 1600 million year range.
The presence of a homologous series of [C.sub.27]-[C.sub.29] regular steranes
was observed in crude oil and source rock samples.
N-alkanes, isoprenoid alkanes, Steranes
, and Terpanes are mainly used as saturated hydrocarbon biomarker compounds.
The regular steranes
are dominated by [C.sub.29] and the [C.sub.27] > [C.sub.28] regular steranes