Origin of pollen and spores in surface lake sediments: Comparison of modern palynomorph assemblages in moss cushions, surface soils and surface lake sediments

Abstract

Studies of modern pollen deposition based on analyses of recent lake sediments, moss cushions or pollen traps are commonly used to assist with the interpretation of fossil lake sediment sequences. However, lake basins often derive their pollen and spores (palynomorphs) from both aerial deposition and inwash, and therefore soil palynomorphs are also important. Here, the palynomorph content of moss cushions and catchment soils are compared with those derived from recent lake sediments to establish how well the sediments reflect the contemporary vegetation, and to identify different sources and transport routes of pollen and spores to lake sediments. Bracken ( Pteridium esculentum) and tree fern ( Dicksonia spp. and Cyathea spp.) spores are given particular attention in this study, as they are important indicators in the fossil record of natural and anthropogenic vegetation disturbance. Palynomorph corrosion is also quantified to identify the proportion of inwashed and contemporary palynomorphs reaching the lake sediments. We found that moss cushions, soils and surface lake sediments have different palynomorph spectra, with mosses providing the most accurate representation of the contemporary vegetation. In contrast, the palynomorphs in soils are mainly corroded and dominated by the most resistant types. The surface lake sediments contained a mix of the pollen and spore types found in both the moss and soil sample pollen spectra, but were heavily influenced by reworked pollen and spores inwashed from catchment soils. Reworked palynomorphs and over-representation of robust pollen and spore types can have the potential to distort the accuracy of the fossil record in vegetation reconstructions, and affect the accuracy of quantitative pollen/climate reconstructions where modern pollen training sets are used to develop transfer functions. For best results in such cases, careful consideration should be made when selecting surface sampling sites for modern pollen training sets.