A seasonal 450-year record of flood layers archived in varved lake sediments from Ammersee (Southern Germany)

Lakes can be utilized as long-term observatories of environmental and climate changes in the human habitat because they contain long and continuous sediment records of several thousands of years. Especially from annually laminated lake records even down to seasonal information can be obtained. Such long time series of high-resolution data ideally complement multi-scale observational data as obtained within the TERENO observatories in order to achieve a comprehensive mechanistic understanding of climate and environmental change. Here we present a case study of two short varved sediment cores from Lake Ammersee (Southern Germany), which have been studied with a combination of micro-facies analyses, high resolution element scanning (μ-XRF) and stable oxygen isotope analyses. The main results are (1) a precise and independent chronology obtained by counting of calcite varves using a petrographic microscope and (2) the identification of short-term fluxes of detrital matter into the lake caused by extreme precipitation and flooding events. The position of individual flood layers within the annual deposition cycle additionally allows reconstructing the season of its deposition. The record of flood layers of the last 80 years has been compared with observed runoff data of the main inflowing river (Ammer) and local precipitation data, which confirmed the interpretation of detrital layers as flood layers. Our data shows significantly more spring and summer flood layers during intervals of colder climate for the period before ca AD 1900. This apparent link between decadal-scale temperature variations and flood layer frequency disappears during the last ca 100 years, most probably because of intensified human activities in the catchment of the inflowing river Ammer.