Changes in landscape evolution patterns during the Mid-Pleistocene Revolution

River terraces in the Alpine Foreland are sedimentary archives that have recorded paleoenvironmental changes during the Quaternary. In the Swiss Alpine Foreland, Quaternary terraces represent a succession of proximal glaciofluvial gravels deposited by meltwater during repeated glaciations of the Alps and the foreland and lying unconformably on Tertiary molasse or Mesozoic carbonate bedrock. The morphostratigraphy of the terrace gravels has a reversed stratigraphic relationship such that older deposits occur at higher elevations than younger deposits. The aim of this study is to reconstruct the landscape evolution of the northern Alpine Foreland since the early and middle Pleistocene. We focus on seven different sites where we establish their chronology by applying cosmogenic depth-profile and isochron burial dating with 10Be, 26Al and 36Cl. Additionally we perform detailed investigations of clast fabrics, petrographic compositions and clast morphometries to identify the sediment source areas and interpret their transport mechanisms and depositional environments. Finally, coupling the reconstructed chronologies with interpolated height differences between the bedrock underlying the Quaternary deposits and the bedrock beneath the modern river allows estimating post-depositional bedrock incision rates. Analyses show that a first gravel accumulation occurred at around 2 Ma, with most of the sediments originating from the northern Central Alps or being reworked from the Miocene Molasse conglomerates. At that time, the Alpine Rhine was draining through Lake Constance into the Danube River and eastwards into the Black Sea. Long-term incision rates into bedrock are ≤ 100 m/Ma. A second phase of gravel accumulation was observed at around 1 Ma, coinciding with the Mid-Pleistocene Revolution. Results from provenance studies showed that these sediments were derived either from the northern central and eastern Alps or were reworked from the Molasse in the Alpine Foreland. During the second accumulation phase, the Alpine Rhine was already redirected westwards into the Upper Rhine Valley from where it drained northwards into the North Sea. The reorganisation of the drainage network in the foreland probably led to an acceleration in bedrock incision rates with estimated values between 100 and 350 m/Ma.