Detrital layers marking flood events in recent sediments of Lago Maggiore (N. Italy)and their comparison with instrumental data

1. A succession of 20 detrital layers was detected in five short cores from the Pallanza Basin in the western part of Lago Maggiore (Italy) by combining thin-section analyses and high-resolutionmicro-X-ray fluorescence (l-XRF) scanning techniques. The detrital layers range in thickness from 0.6 to 17.4 mm and appear most distinct in the upper 20-25 cm of each core, where eutrophication since the early 1960s resulted in the deposition of a dark, organic sediment matrix. 2. The age-depth model of previously dated cores was transferred by precise intra-basin correlation of distinct marker layers, thus providing a reliable chronology for the 20 detrital layers covering the time period 1965-2006. 3. All detrital layers are related to regional floods as recorded by short-term lake level rises and peaks in discharge of the River Toce, the main tributary to the Pallanza Basin. 4. Detailed intra-basin correlation of detrital layers allows us to distinguish river run-off events from local erosion, as well as evaluate the relation between detrital layer thickness and flood amplitude. Massive clay accumulation on top of the thickest detrital layers might have affected lake ecology by attenuating light and influencing metabolic activity. 5. In the clastic-dominated sediments deposited before 1965, detrital layers are less clearly discernible because of the predominantly clastic pelagic sediment matrix. The combination of thinsection and l-XRF techniques, however, shows the potential to establish even longer flood layer time series from Lago Maggiore sediments.