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We characterized the compositions of organic compounds in a Cheremushka bog sediment core (deposited over the last 35 kyr), located at the eastern coast of Lake Baikal, to obtain basic information about the terrestrial organic matter (OM) which contributed to Lake Baikal sediments. The bog sediment was analyzed for the molecular composition of n -alkanes, lignin phenols and n -C 24 to C 30 alkanoic acids, as well as the carbon isotopic composition of plant wax derived n -C 27 to C 33 alkanes. Concentrations of lignin phenols [vanillyl (V) plus syringyl (S) phenols] normalized to total organic carbon (TOC) in the Holocene are twice those for the last glacial maximum (LGM), while concentrations of TOC-normalized n -C 24 to C 30 alkanoic acids do not change markedly in this period. Thus, the ratio of lignin phenols to n -C 24 to C 30 alkanoic acids increases from the LGM to the Holocene. This result is essentially consistent with pollen analysis indicating an expansion of woody plants in the Holocene and a prevailing herb-abundant environment for the LGM. The δ 13 C values of n -C 27 to C 33 alkanes (e.g. −29‰ to −33‰ for C 31 ) indicate the presence of C 3 -dominant plants throughout the core. The contribution of terrestrial OM to Lake Baikal sediments was estimated using the biomarkers, on the assumption that the OM in the bog sediments is a representative of the terrestrial OM around the lake. Hence, the estimation using lignin phenol or n -C 24 to C 30 alkanoic acid parameters indicates that 11–24% of the TOC in the Academician Ridge sediments is land-derived for both the Holocene and the LGM, which is similar to the estimates from C/N values of bulk OM. However, the estimates for terrestrial OM using the n -C 27 to C 33 alkane parameter are generally higher than those using lignin phenol or n -C 24 to C 30 alkanoic acid parameters. The difference is thought to be associated with the difference in source and behavior of these biomarkers.

The concentrations of the [a] and [c] isomers of benzocarbazoles in a number of crude oils and source rock bitumens were obtained by “in house methods” and from reported published data. The changes in benzocarbazole concentrations and the benzocarbazole [a]/([a]+[c]) ratio relating to source maturity, facies and migration are described. With increasing source maturity in North Sea oils and source rocks, up to 0.85% vitrinite reflectance equivalent, benzocarbazole concentrations increase due to generation followed by a decrease above 0.85% vitrinite reflectance equivalent due to expulsion/dilution by newly generated hydrocarbons. Comparison of benzocarbazole concentrations between reservoired petroleum and related source petroleum revealed a concentration bias in favour of source petroleum where concentrations of benzocarbazoles reached up to 10 times that found in equivalent reservoired petroleum. The differences in benzocarbazole concentrations between related oils and source rocks indicate that the major fractionation is introduced during primary migration and expulsion, because when secondary migration is limited, as in the Miller field, we see extensive benzocarbazole fractionation between reservoired petroleum and associated source petroleum. The relationships between source rock TOC content and petroleum benzocarbazole concentrations and [a]/([a]+[c]) ratios indicate that source organic matter exerts a strong influence on benzocarbazole distributions. The introduction of a reservoir source fractionation parameter (RSFP), defined as the ratio between the concentrations of benzo[a]-+benzo[c]carbazole in reservoired petroleum versus the concentration of benzo[a]-+benzo[c]carbazole in source petroleum, enables description of the relationship between reservoired petroleum and source petroleum from different petroleum systems. Petroleum deriving from clastic source rocks demonstrated high levels of benzocarbazole fractionation between reservoired petroleum and source petroleum during primary and secondary migration compared to petroleum deriving from carbonate systems. The RSFP parameter may also be used to examine fractionation effects during migration between different compound types. The more polar benzocarbazoles are fractionated more strongly than dibenzothiophene and phenanthrene between reservoired petroleum and source petroleum. Whereas sometimes source maturity signals do dominate the benzocarbazole signals in migrated oils, in other circumstances, often associated with clastic source rocks, no clear maturity signal is transferred to migrated oils.

The molecular distribution and the carbon-isotopic composition (d 13 C) of n-alkanes extracted from a Lake Baikal core spanning the last 20 kyr of sediment accumulation have been investigated. A terrestrial origin has been inferred for the odd carbon-numbered long-chain (>C27) n-alkanes, on the basis of the observed high CPI27-33 values (range: 8.7‐ 10.8) typical of n-alkanes derived from leaf waxes of higher plants. A shift in the abundance of n-C27 alkane relative to n-C31 homologue is observed across the late Pleistocene glacial‐Holocene interglacial climate change, perhaps indicative of the climate-induced vegetational change previously deduced from palynological analyses. Compound-specific isotope analyses indicate remarkably uniform d 13 C values in the range ofˇ31.0 toˇ33.5% for the leaf-wax C27‐C33 nalkanes in the entire cored sequence. Such an isotopic compositional range is characteristic for n-alkanes biosynthesized by plants utilizing the C3 photosynthetic pathway. Our data suggest that the observed 13 C-enrichment in the bulk organic matter in the glacial age sediments, relative to d 13 C values of total organic carbon in the Holocene section, is therefore unlikely to be attributed to an expansion of C4-type vegetation in the Baikal watershed during the late Pleistocene glacial interval. # 2000 Elsevier Science Ltd. All rights reserved.