Lignin biomarkers and pollen in postglacial sediments of an Alaskan lake

We analyzed a 12,000-yr sediment core from Wien Lake, central Alaska, for a suite of phenolic products from CuO oxidation of lignin polymers and compa red their composition with pollen data from the same core to assess lignin phenols as sedimentary biomarkers. Inferences of the gross taxonomic origin of sediment organic matter from lignin-phenol composition agree with veget ational reconstructions based on pollen assemblages. In particular, the rat ios of syringyl to vanillyl phenols are consistently higher before 6500 yr BP, when angiosperms dominated or codominated the regional vegetation, than after 6500 yr BP, when gymnosperms dominated. However, the ratios of cinna myl (p-coumaric and ferulic acids) to total vanillyl phenols (C/V) do not s how patterns expected from the abundance of woody plants. C/V ratios are pa rticularly high (0.7-0.85) after 6500 yr BP when pollen spectra suggest clo sed boreal forests dominated by Picea, and the stratigraphic patterns are s trikingly similar between C/V and Picea pollen concentrations. CuO oxidatio n of modern pollen of P. glauca and P. mariana yields exceptionally high am ounts of cinnamyl phenols (8.90 and 6.41 mg/100 mg OC for P. glauca and P. mariana, respectively). In particular, p-coumaric acid is obtained in large amounts (8.87 and 6.41 mg/100 mg OC for P. glauca and P. mariana, respecti vely) versus vanillyl phenols (0.25 and 0.49 mg/100 mg OC for P. glauca and P. mariana, respectively) and ferulic acid (0.03 and 0.00 mg/100 mg OC for P. glauca and P. mariana, respectively). Thus lignin phenols derived from fossil Picea pollen preserved in sediments likely drive the C/V profile of the Wien Lake core. These data imply that if Picea pollen concentrations ar e sufficiently high, the amount of nonwoody tissue in sediments may be gros sly overestimated when the lignin composition of gymnosperm needles is used as the end member of nonwoody tissues. Given that pollen grains are among the most resistant components of sedimentary organic matter and that p-coum aric acid is labile, it is important to consider pollen as a nonwoody tissu e type when lignin biomarkers are used to determine the sources of vascular -plant material in sediments. Copyright (C) 1999 Elsevier Science Ltd.