Molecular-isotopic stratigraphy of long-chain n-alkanes in Lake Baikal Holocene and glacial age sediments

The molecular distribution and the carbon-isotopic composition (δ13C) 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 δ13C values in the range of −31.0 to −33.5‰ for the leaf-wax C27–C33 n-alkanes 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 13C-enrichment in the bulk organic matter in the glacial age sediments, relative to δ13C 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.