Pristane/phytane ratios, indicator

A number of selected molecular parameters obtained from analysis of immature crude oils and sediment extracts are evaluated as indicators of palaeosalinity. The nature of these parameters is discussed taking into account the role of intermolecular and intramolecular incorporation of sulfur into specific functionalized lipids. Specific distribution patterns of methylated chromans and C20 isoprenoid thiophenes and the relative abundance of gammacerane are excellent indicators for palaeosalinity, whilst other parameters such as 14< (H),17a(H)/140(H),170(H) -sterane ratios, the pristane/phytane ratio, the even-over-odd carbon number predominance of n-alkanes and the relative abundance of C35 hopanes and/or hopenes may indicate palaeohypersalinity but are affected by environmental factors other than hypersalinity and by diagenesis. 

A more critical evaluation of the above mentioned ratios and phenomena reveals the usefulness of the various palaeosalinity indicators. Distribution patterns of methylated chromans and the relative abundance of gammacerane are not influenced by sulfur incorporation reactions and may directly reflect species distributions in the palaeoenvironment. To some extent this holds for 14a(H),17a(H)/140(H),170(H)-steraneratios as well, although incorporation of sulfur may influence this ratio and original A7/A5-sterol ratios do not always correlate with hypersaline environments. The isoprenoid thiophene ratio is highly useful as a palaeosalinity indicator since the distribution of the C20 isoprenoid thiophenes directly reflects the distribution of their substrates. The other parameters (pristane/phytane ratio, odd-over-even carbon number predominance of n-alkanes, relative abundance of C35 hopanes and/or hopenes) should be used with caution because they obviously depend on the quenching by sulfur of specific lipids, a process which is not restricted to hypersaline environments. 

Bitumens, were separated by chromatography, urea clathration and 5A molecular sieve occlusion before and after analyses of many of the aliphatic sub-fractions by GC and gas chromatography-mass spectrometry (GC-MS). Experimental details are noted in a previous publication (16) in which the distribution of cyclic alkanes in two lacustrine deposits of Devonian (N.E. Scotland) and Permian (Autun, France) age, (the D and C series samples) were discussed, Chromatographic separation into aliphatic, aromatic and polar compounds of the bitumens extracted from the shales gave the results shown in Table VI. Carbon Preference Indices and pristane/phytane ratios were measured in this work space limitations precluded... 

Most of the alphatic fractions were separated into normal and branched alkane fractions after the removal of unsaturated hydrocarbon. Most of these subfractions were analysed by GO to give fingerprint chromatograms. These analyses were used to provide data that allowed carbon preference indices, pristane/phytane ratios etc. to be measured. Also, the relative amounts of (17 H, 21/ H 17/3 H, 21 oCH and 17 H, 21(3 H) hopanes, steranes, 4-methyl steranes, carotenoid and other related compounds were assessed from gas chromatograms and reconstructed ion chromatograms. A large number of Kovats Retention Indices (0V-101) were measured and tabulated for these compounds also (10). 

Gas Chromatography. Whereas N.M.R. effectively senses the f mctional groups present in a mixture, differences between retention times enable G.C. to identify individual members (provided they are moderately volatile) of a homologous series such as the n-alkanes. Positive identification requires calibration by co-injection with authentic samples (here n-alkanes) or coupling of M.S. with, preferably, a capillary G.C. column. Although G.C. stick diagrams of n-alkane distributions have been used for comparing extracts from fossil fuels, we advocate acyclic-isoprenoid-hydrocarbon distributions, and particularly the pristane/phytane ratio, as more reliable indicators of fossil fuel maturation 6). 

In Table I all the Ci8 phytane ratios are between 0.8 and 2.3, as would be expected for a petroleum source. Pristane iphytane ratios are (with one exception) substantially greater than one, indicating a planktonic source as well. 

Hughes, W. B., Holba, A. G. Dzou, L. I. P. 1995. The ratios of dibenzothiophene to phenathrene and pristane to phytane as indicators of deposi-tional environment and lithology of petroleum source rocks. Geochimica et Cosmochimica Acta, 59, 3581-3598. 

Many reports have been made on the presence of pristane and phytane. The values of the ratio pristane/phytane given by Ledet and Laseter (1974) for hydrocarbons from the air/sea interface, varying from 1.5 to 2.3, are close to those found for petroleum (1.5—2.5 after Blumer and Sass, 1972) and are consequently indicative of anthropogenic pollution. 

The presence of pristane, phytane and their ratios (ratios no. 3-5) indicate a petrochemical input. 

Further work is necessary to better understand and apply the obvious information present. For example, one wonders whether the high phytane/pristane ratio is the result of relatively high amounts of phytenes and/or diphytanylethers present in (halophilic) archaebacteria and is not at all an indication for anoxicity (46,47). 

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