Modern pollen assemblages in topsoil and surface sediments of the Xingyun Lake catchment, central Yunnan Plateau, China, and their implications for interpretation of the fossil pollen record

Abstract

Modern pollen data from topsoils and surface lake sediments are commonly used as modern analogues and as climate calibration data for reconstructing vegetation and climate history from fossil pollen profiles. However, the potential differences and biases in pollen assemblages from these two sample types, as well as the processes of pollen sedimentation in lake basins, have received relatively little attention. Here we present the results of the analysis of modern pollen assemblages in 10 surface sediment samples and 25 topsoil samples from Xingyun Lake and its catchment, a shallow alpine lake site in the central Yunnan Plateau, southwest China. The modern pollen assemblages from both types of samples are mainly composed of Pinus, evergreen and deciduous oaks, Alnus, Poaceae, Artemisia, Brassicaceae and Amaranthaceae, and are consistent with the modern vegetation. Although the pollen assemblages of topsoil samples vary with the type of modern local vegetation, in general they are representative in term of the composition of the main pollen taxa, with some significant differences in the case of some vegetation types. With regard to the surface lake sediment samples, the pollen assemblages are relatively constant with no significant changes with sample location and water depth. The main pollen taxa are Pinus (around 40%), Alnus (around 18%) and Poaceae (around 11%), with relatively high percentages of evergreen and deciduous oaks, Betula (2–4%), and low percentages of Ulmus, Rosaceae, Corylus, Cupressaceae, Euphorbiaceae, Myrica and Elaeagnaceae. This composition is a good representation of the main types of catchment vegetation. The relative uniformity of the pollen assemblages from the lake surface samples may partly result from the effect of currents in redistributing the pollen input to the lake by rivers and wind. Therefore, we suggest that it is better to use pollen data from surface lake sediments, rather than from topsoils from the lake catchment, for interpreting fossil pollen assemblages of lake sediment cores. Our results indicate that the pollen assemblages in a sediment core from any part of Xingyun Lake are likely to provide a reliable record of the regional vegetation history and thus climatic change. Our modern pollen results, together with modern pollen data from other surface lake sediment samples along a large altitudinal gradient from Yunnan to southwest Sichuan Province, were used to reanalyze fossil pollen assemblages during the Last Glaciation period at Xingyun Lake. The results show that the vertical vegetation belts decreased in altitude from late Marine Isotope Stage 3 (late MIS3) to the Last Glacial Maximum (LGM) and then increased during the Last Deglaciation.