Isoprenoid thiophene , distribution

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 large variation in the distribution patterns of C20 isoprenoid thiophenes has been encountered upon analysis of sediments and oils ranging from normal marine saline to hypersaline (16). Figure 8 shows that in samples representing a normal marine salinity environment (Monterey Shale-Naples and -El Capitan Beach, Jurf ed Darawish-45) isoprenoid thiophenes VI and VII are dominant whereas in samples from hypersaline palaeoenvironments (Sicily seep oil-E2, Rozel Point oil) isoprenoid thiophene V and the so-called midchain isoprenoid thiophenes (I-IV) are relatively abundant. C20 isoprenoid bithiophenes (VIII-X) only occur when the midchain isoprenoid thiophenes are relatively abundant (Figure 9). 

Figure 8. C q isoprenoid tiophenes as indicators of palaeosalinity structures, definition of the isoprenoid thiophene ratio (ITR) and some typical distributions (based on peak heights in m/z 308 mass chromatograms) in sediments and petroleums. Description of the samples is given elsewhere (16).

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. 

Alkylthiophenes with isoprenoid carbon skeletons. Striking variations in the distributions of the isoprenoid thiophenes with depth are observed (Figure 13). In most of the samples from this oil shale the C2q, C2 and C40 isomers represent ca. 80 % of the total amount of isoprenoid thiophenes. Depth profiles of these major isoprenoid thiophenes will be discussed in detail. 

The variation in the distribution of the various isoprenoid C20 thiophenes throughout the sequence is exemplified by the isoprenoid thiophene ratio (ITR Figure 14a). This ratio is defined as depicted in... 

Figure 13. Carbon number distribution patterns of major isoprenoid thiophenes for the Jurf ed Darawish oil shale samples indicated.

Gas chromatography - mass spectrometry data of the "aromatic hydrocarbon" fractions of nearly 100 Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment samples have been re-examined for the occurrence of organic sulfur compounds (OSC). Approximately 70% of the samples contain OSC with varying distribution patterns, although isoprenoid thiophenes are invariably present. 

In Mesozoic samples, mostly Cretaceous and Jurassic black shales, the abundance and variance of OSC is generally low. A few samples of Jurassic age show a distribution pattern of C20 isoprenoid thiophenes which may indicate increased salinity during deposition. A C20 isoprenoid thiolane is the most abundant compound in black shales from the Falkland Plateau, whereas at other locations the C35 thienylhopane dominates. In one case two C33 mid-chain 2,5-dialkylthiophenes were identified. 

Figure 5. Partial mass chromatograms of m/z 308, showing the distribution of C20 isoprenoid thiophenes. Numbers refer to Figure 3.

The distribution of thiophenes in the microbial mat and in the paleosoil of mangrove are noticeably different. In the microbial mat, isoprenoid, branched, midchain and n-alkylthiophenes occur mainly between C14 and C21 and are dominated by the C20 thiophenic isoprenoids. In the paleosoil of mangrove, if some of the isoprenoids and branched thiophenes are present, the sulfur compounds are dominated by the C28-C30 n-alkylthiophenes and by a presumed C30 n-alkylthiolane thiol, and in the extract by a C29 sterane-thiol. These differences are likely to be a reflection of the type of molecules in which sulfur might have been incorporated. 

---