Holocene fan systems in the Alpine Rhine basin as records of sedimentary pulses during a warming climate

We explore the mechanisms leading to the formation of the Disentis and Sedrun fans, which represent the largest and thickest Quaternary fan systems in the headwater area of the Alpine Rhine. These fans are several km 2 in size and underlain by tens of meters-thick, non-consolidated breccias. The sedimentary fabric of these deposits is matrix-supported, suggesting deposition by debris flows. In addition, concentrations of in-situ 10 Be in detrital quartz minerals that are embedded in these deposits reveal high paleo-denudation rates of nearly 2 mm yr −1 . These rates are substantially higher than the mean denudation rates <1 mm yr −1 that we measured for the headwaters of the Alpine Rhine. This confirms the view where the fan deposits at Disentis and Sedrun record a time interval when material supply occurred through sedimentary pulses. In addition, regional studies reveal that the construction of the investigated fan systems postdates the formation of moraines that were dated to the Egesen Stadial during the Younger Dryas. We thus propose that the accumulation of the sedimentary material at Disentis and Sedrun most likely occurred during the same epoch as the Flims rock avalanche. This was the largest mass failure event in the European Alps and resulted in the accumulation of tens of meter-thick, non-consolidated breccias on the valley floor some kilometers farther downstream. The result was the formation of a lake upstream of the rock avalanche deposits and the rise of the base level on the valley floor. We thus envisage an interpretation where the higher base level together with the release of large volumes of sediment caused the construction of the fans at Disentis and Sedrun. Once the lake was breached and the hillslope sediment reservoirs were exhausted, the Alpine Rhine started to incise into the previously deposited material, thereby leaving distinct cut levels at the toes of these fans. We acknowledge that we lack precise numerical ages for the time period when the fan systems at Disentis and Sedrun were constructed. Nevertheless, based on the available information, we conclude that the fan sediments in Alpine valleys serve as suitable archives for improving our understanding of the environmental response to climate warming. • We present 10Be-based paleo-denudation rates recorded by terrace deposits in the Alpine Rhine. • The results show that these sediments, which accumulated after the change from the cold period of Egesen Stadial to the Boreal warm period, record the occurrence of sedimentary pulses. • The accumulation of the sedimentary material at Disentis and Sedrun most likely occurred at the same time as the Flims rock avalanche, the largest mass failure event in the European Alps. • Based on our research, we consider that terrace sediments and breccia deposits in Alpine valleys as suitable archives for improving our understanding of the environmental response to climate warming.