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Abstract

A Quaternary interglacial lake sediment record from the Piànico-Sèllere Basin (northern Italy) consists of biochemical calcite varves with intercalated detrital layers. At the end of the Piànico Interglacial, continuous varve formation was replaced by predominantly detrital sedimentation. However, 427 varve-years before this shift, an abrupt increase in the frequency and thickness of detrital layers occurred. Microfacies analyses reveal a total of 152 detrital layers, ranging from 0·2 to 20·15 mm in thickness, deposited during the last 896 years of the Piànico Interglacial. Three microfacies types are distinguished: (i) graded layers, (ii) non-graded silt layers, and (iii) matrix-supported layers. The position of detrital layers within an individual varve provides additional information on the season in which they have been deposited. Microfacies analyses in combination with varve counting further enabled precise varve-to-varve correlation of the detrital layers for two sediment sections cropping out ca 130 m apart. The detailed intra-basin correlation allows the source regions of detrital layers to be inferred. Moreover, micro-erosion at sub-millimetre scale has been established. Of the described facies types, only the accumulation of summer and spring graded and non-graded silt layers abruptly increased before the end of interglacial varve formation whereas non-graded winter silt and matrix-supported layers are randomly distributed over the entire study period. Heavy rainfalls are assumed to have triggered spring and summer graded layers, so that the occurrence of these layers is thought to be a proxy for extreme precipitation events in the past.