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

The molecular distribution and the carbon-isotopic composition (delta(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 bee n inferred for the odd carbon-numbered long-chain (> C-27) n-alkanes, on th e 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-C-27 alkane relative to n-C-31 homologue is observed across the late Pleistocene glacial-Holocene interglacial climate change, perhaps indicativ e of the climate-induced vegetational change previously deduced from palyno logical analyses. Compound-specific isotope analyses indicate remarkably un iform delta(13)C values in the range of -31.0 to -33.5 parts per thousand f or the leaf-wax C-27-C-33 n-alkanes in the entire cored sequence. Such an i sotopic compositional range is characteristic for n-alkanes biosynthesized by plants utilizing the C-3 photosynthetic pathway. Our data suggest that t he observed C-13-enrichment in the bulk organic matter in the glacial age s ediments, relative to delta(13)C values of total organic carbon in the Holo cene section, is therefore unlikely to be attributed to an expansion of C-4 -type vegetation in the Baikal watershed during the late Pleistocene glacia l interval. (C) 2000 Elsevier Science Ltd. All rights reserved.