An 8.6-million-year record of plant wax n-alkane distributions from Lake Baikal, Russia. Plant wax n-alkanes are a useful indicator of paleovegetation change due to the tendency of different plant growth forms to produce different chain length n-alkanes. In Siberia and Mongolia, the C27 n-alkane often dominates the distribution of leaf waxes in conifer needles, woody shrubs, and Late Quaternary sedimentary horizons associated with taiga (Russian boreal forest) pollen assemblages while the C31 n-alkane often dominates herbaceous vegetation and Late Quaternary sediments with steppe pollen distributions. These observations are consistent with studies of surficial sedimentary n-alkane distributions elsewhere, including boreal environments in Canada analogous to the Lake Baikal region.

Methods:All sediments analyzed for biomarkers were freeze dried and weighed prior to homogenization. Lipids were extracted by an accelerated solvent extractor (ASE) 350 using 2:1 dichloromethane: methanol (c.f. Auderset et al., 2020; Powers et al., 2010). Oven temperature was set to 100°C, rinse volume to 150%, purge time to 120 seconds, heating time to 5 minutes, and with 4 10-minute static cycles. Total lipid extracts were spiked with 100 µL of a general recovery standard (20.5 ng/µL 5α-androstane, 20.5 ng/µL stearyl stearate, 20.5 ng/µL 1,1'-binapthyl, 20.5 ng/µL cis-11-eicosenoic acid, 20.5 ng/µL 19-methyleicosenoic acid, 20.5 ng/µL C20:1 Δ11-eicosenol, and 20.5 ng/µL 5α-androstan-3β-ol), desulfurized by sequential addition of activated copper wire, and evaporated under N2 prior to elution on a 0.5 g dry-packed LC-NH2 column to separate neutral (4 mL 2:1 dichloromethane: isopropanol), acid (4 mL 4% acetic acid in diethyl ether), and polar (4 mL methanol) fractions. Neutral and polar fractions were recombined and again dried under N2 prior to elution on a wet-packed silica gel (0.5 g, 60 Å, 70–230 mesh, Millipore) column with 3 mL hexanes (apolar / aliphatic), 4 mL dichloromethane (semi-polar / aromatic), and 4 mL methanol (polar / alcohols). The apolar / aliphatic fraction containing n-alkanes were quantified by gas chromatography-mass spectrometry (GC-MS) at the University of California, Santa Cruz, on an Agilent 7890A GC 5975B MS with split/splitless injector and HP-5 column (30 m length, 250 µm i.d., 0.25 µm phase thickness). 1 µL of sample was injected at an inlet temperature of 320°C and oven temperature of 60°C. Oven temperature was held at 60°C for 1.5 minutes, ramped to 150°C at 15°C/min and then to 320°C at 4°C/min and held for 10 minutes. The MSD ion source was held at 350°C with an electron energy of 70 eV and quadrupole temperature at 150°C. Samples were analyzed in fill scan mode, scanning from 50–550 dalton cycling at ~3 scans/s. Peaks were integrated on the Agilent Chemstation software by extracting ion peak areas for the n-alkanes (m/z 57) and the 5α-androstane (m/z 245) internal recovery standard. n-alkane concentrations were determined via external calibration of n-alkane response factors relative to the recovery standard 5α-androstane every 10 sample injections. Further funding information: PR 1414/1-1 Deutsche Forchungsgemeinshaft Priority Program "ICDP" 1006 Geological Society of America Continental Drilling Science Division Graduate Student Grant 13282-21* Sigma Xi Grants in Aid of Research G20211001-101