A 4000-year debris flow record based on amphibious investigations of fan delta activity in Plansee (Austria, Eastern Alps)

Abstract. The frequency of debris flows is hypothesized to have increased in recent decades with enhanced rainstorm activity. Geological evidence test the relationship between climate and flow activity for prehistoric times scarce due incomplete sediment records, complex stratigraphy, insufficient age control, especially Alpine environments. In lacustrine archives, link onshore processes sedimentary record lakes poorly investigated. We present an amphibious characterization alluvial fan deltas a continuous 4000-year from Plansee (Tyrol, Austria), combining light detection ranging (lidar) data, swath bathymetry, core analyses. geomorphic investigation two different developmental stages revealed evolutionary pattern backfilling new channel formation onshore, together active subaqueous progradation on juvenile delta, major deposition, only few, but larger, deposits mature delta. Geomorphic stacked braided lobes, subaquatic landslide deposits, types turbidites cores facilitated process-based event identification, i.e. distinguishing debris-flow-induced or earthquake-induced throughout record. directly correlate lobe-shaped high backscatter signals terrestrial last century. Moreover, turbidite thickness distribution along transect four allows us pinpoint numerous events as being related core, feature more gradual fining upward grain size trend higher TOC (total organic carbon) δ13C values compared turbidites. contains 138 separated into phases similar (df phases). df phase 1 (∼2120 ∼2040 before common era – BCE) reflects second-highest observed frequencies interpreted postseismic landscape response. After long period recurrence intervals without any outstanding increases during 2 (∼2040 BCE ∼1520 CE), there are slightly 3 (∼1520 ∼1920 CE). 4 (∼1920 2018 CE) exhibits drastic increase activity, followed by overall highest whole record, which about 7 than 3. show that matches previously postulated derived aerial photography at triggering controlled short, intense precipitation other mass movement process, we demonstrate records provide unique inventory hazard-relevant over decades, centuries, millennia. presented since start 20th century coincides twofold Northern European Alps and, therefore, provides novel technique systematic understanding non-stationary changing climate.