Effects of aridity and vegetation on plant-wax δD in modern lake sediments

We analyzed the deuterium composition of individual plant-waxes in lake sediments from 28 watersheds that span a range of precipitation D/H, vegetation types and climates. The apparent isotopic fractionation (epsilon(a)) between plant-wax n-alkanes and precipitation differs with watershed ecosystem type and structure, and decreases with increasing regional aridity as measured by enrichment of H-2 and O-18 associated with evaporation of lake waters. The most negative epsilon(a) values represent signatures least affected by aridity; these values were -125 +/- 5 parts per thousand for tropical evergreen and dry forests, -130 parts per thousand for a temperate broadleaf forest, -120 +/- 9 parts per thousand for the high-altitude tropical paramo (herbs, shrubs and grasses), and -98 +/- 6 parts per thousand for North American montane gymnosperm forests. Minimum epsilon(a) values reflect ecosystem-dependent differences in leaf water enrichment and soil evaporation. Slopes of lipid/lake water isotopic enrichments differ slightly with ecosystem structure (i.e. open shrublands versus forests) and overall are quite small (slopes = 0-2), indicating low sensitivity of lipid delta D variations to aridity compared with coexisting lake waters. This finding provides an approach for reconstructing ancient precipitation signatures based on plant-wax delta D measurements and independent proxies for lake water changes with regional aridity. To illustrate this approach, we employ paired plant-wax delta D and carbonate-delta O-18 measurements on lake sediments to estimate the isotopic composition of Miocene precipitation on the Tibetan plateau. (C) 2010 Elsevier Ltd. All rights reserved.