Seasonal sedimentation in the Pliocene Villarroya Lake (N Spain) inferred from pollen analysis

Abstract

High-resolution pollen analysis was performed on a short (8.7 cm thick) section with 94 varve couplets (made up of dark and light laminae) from a lacustrine facies of the Pliocene Villarroya Basin, Spain. The aim of the present study was (i) to verify the seasonality of the sedimentation and (ii) to determine if pollen data record the short-term climatic fluctuations related to variations in laminae thickness and colour. The samples displayed high pollen concentrations and very good taxonomic diversity. The composition of the pollen spectra indicated that the vegetation structure was closely linked to relief: (i) a regional open herbaceous vegetation, (ii) a local marshy and riparian environment, (iii) deciduous mixed forest at mid-altitudes, and (iv) coniferous mixed forest in the surrounding mountains at higher altitudes. In the 48 distinct couplets where the dark and light laminae have been separately sampled, the pollen content showed higher concentrations of early flowering trees in the dark ones, whereas late-flowering plants were more abundant in the light ones. Total pollen concentrations in the 48 couplets were higher in most of their dark layers. These results point to a seasonal layering. According to present-day flowering times, the dark laminae represent sedimentation in winter/spring and the light laminae sedimentation in late summer/autumn. The correlation between the AP (Arboreal Pollen)–NAP (Non-Arboreal Pollen) percentages and lamina thickness linked lamina thickness with the highest values of AP, indicating warmer periods. Variations in the ecological groups enabled us to differentiate eight pollen cycles related to short-term climatic fluctuations. These alternations seem to be more associated with seasonality in precipitation than with temperature oscillations and could be correlated with the NAO/ENSO-like and sunspot-like cycles.