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Biomarker accumulation rates in nine different time slices in three cores on and at the foot of a submarine high in the northern Arabian Sea (the Murray Ridge) were measured to investigate the influence of oxygen exposure time on the preservation of biomarker signals in the sedimentary record. All three sites experienced the same history of surface water productivity and sediment supply but had different bottom-water redox conditions due to their different positions (in, just below, well below) relative to the present location of the intense oxygen minimum zone (OMZ). Past variations in the intensity and position of the OMZ, known from a wide variety of proxies (TOC content, distribution and abundance of planktonic and benthic foraminifera and pteropods, trace metals, and δ 15 N), enabled specific biomarker (i.e., n -alkanes, steroids, alkenones, alkyldiols, C 26 fatty acid, loliolide, biphytane diols, and archaeal tetraether lipids) accumulation rates at contrasting oxygen exposure times to be compared. The results indicate that these accumulation rates can vary by more than an order of magnitude for marine biomarkers. In addition, there are significant differences in the degree of oxic degradation of different types of biomarkers: Terrestrial n -alkanes are much more resistant than alkenones and n -alkyl diols, which are more refractory than steroids and biphytane diols. These differences in degree of oxic degradation indicate that biomarker distributions will change on increasing exposure to oxygen. These findings have a significant impact on the application of biomarkers to sedimentary settings in which oxygen exposure time is likely to change significantly.

The 300 m section of the Miocene Monterey Formation outcropping at Shell Beach (Pismo basin; ca. 15- 11 Ma) is composed of calcareous phosphatic ( 15.1- 14.5 Ma) and siliceous facies ( 14.5- 11 .O Ma). An objective of this paper is to document lateral paleoenvironmental changes in the Miocene Monterey Formation by comparing the Shell Beach (SB) profile with the Naples Beach (NB) section in the Santa Barbara-Ventura basin (Schouten et al., 1997) which is -80 km to the south. Eight samples (one sample representing, on average, a time period of ca. 2000 y ) from this section were analyzed for variations of extractable biomarkers and their carbon isotopic signatures as indicators for paleoenviron- mental change during the Miocene. Saturated hydrocarbons present include 28,30dinorhopane, phytane, n-alkanes (C,,-C3r), lycopane, and 17/?,21p(H)-homohopane. The biomarkers released after desulfur- ization of the polar fractions predominantly consist of phytane, 2,6,10,14-tetramethyl-7-( 3-methylpen- tyl)pentadecane, C,,-C3, n-alkanes, regular 5a- and SD-steranes, dinosteranes, and (22R)-17&21@(H)- pentakishomohopane. Steranes have similar carbon isotopic compositions (-25 to -27%0) throughout the section and are isotopically similar at both sites, indicating laterally similar and vertically stable environmental conditions for algae living in the upper part of the photic zone. Free and S-bound n- alkanes at SB mainly originate from marine organisms and not from terrestrial sources as in the NB section. S-bound pentakishomohopane is ca. l-4%0 depleted compared to the steranes and is thought to be derived from the deeper water dwelling cyanobacteria. These findings are consistent with stable carbon isotopic data obtained for these compounds from Middle Miocene Monterey sediments at Naples Beach and indicates similar environmental conditions in the depositional environments of the Santa Barbara-Ventura and the Pismo basin. S-bound highly branched isoprenoids have, at both sites, different isotopic compositions indicating the presence of different diatom species, special growth conditions, or different bloom periods in the Pismo basin. The carbon isotopic composition of 28,30dinorhopane shifts to more depleted values up section, suggesting that the dinorhopane-synthesizing organism or organisms live on C02, which is isotopically influenced by methane production and oxidation. The Cs, hopane is enriched by l-4%0 in 13C compared to other hopanes and steranes. Specific bacteria, possibly heterotrophs, may have been the organisms producing this compound. Copyright 0 1997 Elsevier Science Ltd

n-Alkanes, highly branched isoprenoids, monomethylalkanes (MMAs), dimethylalkanes (DMAs), and trimethylalkanes (TMAs) are the most abundant components in the hydrocarbon fractions of extracts of four modern and two Holocene cyanobacterial mats (t1500 and 5110 ± 170 y bp) collected in Abu Dhabi (United Arab Emirates). The homologous families of MMAs, DMAs, and TMAs were identified by comparison of mass spectral and relative retention time data with published data. DMAs were also identified by synthesis of authentic standards, 3,9-dimethyltricosane, 5,9-dimethyltricosane, and 11,15-dimethylheptacosane. MMAs, DMAs, and TMAs of the cyanobacterial mats can be separated into two groups on the basis of their distribution patterns and structures. MMAs and DMAs in the C16-C22 range are characterized by methyl substituents mainly located at C-6 (or ω6) and C-7 (or ω7) and are derived from cyanobacteria. They are relatively abundant components in the modern cyanobacterial mats, but with increasing age of the mats they become much less abundant. On the contrary MMAs, DMAs, and TMAs in the C24-C45 range are exclusively found in the Holocene cyanobacterial mats. Their longest chains mainly contain an odd number of carbon atoms and they always carry the methyl substituents at odd numbered carbon atoms. The similarity in composition of this very specific group of branched alkanes with that encountered in insect epicuticular waxes suggests that these sedimentary hydrocarbons originate from insects, which probably grazed on the cyanobacterial mats.

The aliphatic hydrocarbon fraction of an immature Monterey bitumen contains a high relative abundance of C27-C29 Δ 4 , Δ 5 , Δ 8(14) 5α - and Δ 14 5α -sterenes, each having a very similar carbon number distribution. This is the first report of abundant Δ 8(14) 5α - and Δ 14 5α - sterenes in an immature sediment. The Δ4 and Δ5 sterenes appear to originate directly from Δ 5 sterols. For the Δ 8(14) 5α - and Δ 14 5α sterenes we propose an origin from Δ 7 5α -sterols which may be derived from a microbial transformation of Δ 5 sterols. The presence of 5α- and 5β-steranes in an unusual ratio of ca. 55:45 suggests the formation of a high abundance of 5β-stanols via the known microbial reduction pathway of δ 5 sterols.

Surface sediments (0–25 cm) from Ace Lake (eastern Antarctica), a saline euxinic lake, were analyzed to study the early incorporation of reduced inorganic sulfur species into organic matter. The apolar fractions were shown to consist predominantly of dimeric (poly)sulfide linked C 27 -C 29 steroids. These steroid moieties were identified by GC-MS analysis of the apolar fractions after cleavage of polysulfide linkages using MeLi and MeI and after desulfurisation. The polar fractions contained the oligomeric analogues. The S-bound steroids are most likely formed by sulfur incorporation into steroidal ketones formed from Δ 5 sterols by biohydrogenation by anaerobic bacteria. The concentrations of these sulfurised steroids increased with depth in the sediment. The sulfurisation reaction is completed in 1000–3000 years. Despite a wide range of functionalised lipids present in these sediments that are potentially available for sulfurisation, there is a very strong preference for the incorporation of sulfur into steroidal compounds. A predominance of sulfurised C 27 steroids contrasted with the distribution of free sterols, which showed a strong predominance of C 29 sterols. This indicates that the incorporation of sulfur is biased towards C 27 sterols. The results demonstrate that intermolecular sulfurisation of organic matter can occur in surface sediments at low temperatures and in the absence of light.

The bitumen of the Jurf ed Darawish Oil Shale has been analysed for organic sulphur compounds (OSC). A number of OSC are reported for the first time: several C 28 8, C 37 and C 38 2,5-dialkylthiolanes and -thiophenes and 2,6-di- n -alkylthianes, and C 19 branched thiophenes possessing the 9-methyloctadecane carbon skeleton. A number of these compounds were identified by synthesis of authentic standards. All the OSC compound classes mentioned exhibit structural isomer distributions dominated by a limited number of all theoretically possible isomers. This provides direct evidence for the formation of these OSC by abiogenic sulphur incorporation into functionalised lipids at the early stages of diagenesis. Precursors for the OSC identified are suggested. From these observations and from data on the occurrence of other OSC and of sulphur in high molecular weight substances a general model for the incorporation of sulphur into organic matter is proposed. Sulphur incorporation into precursors with double bonds (or other reactive functionalities) will lead to formation of OSC and sulphur-rich high molecular weight substances. Only precursors with two double bonds in favourable position for intramolecular addition of intermediate thiols can yield low molecular weight OSC. Double bond isomerisations by a sequence of H 2 S addition and elimination reactions may play a role in this respect.

The organic sulphur compounds (OSC) present in sixteen immature samples (both crude oils and bitumens) from different geographical locations and of different ages representing different palaeoenvironments have been analysed by GC-MS. In all samples OSC (thiolanes, thianes, thiophenes and benzo[b]thiophenes) with structures related to well-known geologically occurring hydrocarbons (n-alkanes, isoprenoid alkanes, steranes, triterpanes) occur, although the relative amounts and distribution patterns of the various OSC classes vary considerably. This variation is interpreted as a result of different sources of organic matter and different degrees of thermal maturation. The palaeoenvironments of the samples were anoxic and H2S, produced by sulphate-reducing bacteria, probably has exceeded the input of reactive iron minerals. These conditions resulted in a surplus of free H2S, which reacted with organic matter, leading to the formation of OSC. The distributions of OSC may be useful as molecular indicators for the assessment of sources of organic matter, palaeoenvironment and thermal maturity and for oil-oil and oil-source rock correlation studies. The distributions of the C20 isoprenoid thiophenes in combination with those of the methylated 2-methyl-2-(4,8,12-trimethyltridecyl) chromans can be used to discriminate non-hypersaline from hypersaline palaeoenvironments.

Detailed compositional data for the sterols isolated from a surface, Unit I, sediment from the Black Sea are reported. A procedure based on digitonin precipitation has been used to separate the more abundant free sterols from those occurring in esterified forms. Saponification of the solvent extracted sediment residue liberated only a small quantity of residual bound sterols in contrast to studies of other sediments. 4-Methylsterols are much more abundant than 4-desmethylsterols in both the free and esterified sterol fractions which we attribute to a major dinoflagellate input, as in deeper Unit II sediment. The desmethylsterol fraction appears to derive from a variety of sources including dinoflagellates, coccolithophores, diatoms, terrigenous detritus and perhaps invertebrates. 5α(H)-Stanols are particularly abundant in the free sterol fraction. An analysis of the stanol/stenol ratios suggests that the 4-desmethyl-5α(H)-stanols are the result of specific microbial reductions of Δ5-sterols and/or the reflection of a contribution of stanol containing source organisms.