Sediment transfer and recycling in segmented sedimentary systems: An example from the Aare river (Switzerland)

Environmental signals produced by changes in climate or tectonic regime are transferred and modified through sedimentary systems from source to sink. Sediment (and therefore signal) transfer in segmented systems is interrupted through sediment storage in e.g. lacustrine sinks. The Alpine orogen is a mountain chain characterized by high denudation rates that sources important rivers such as the Rhine river. This study focuses on the Aare river, a tributary to the Rhine, that originates in the Swiss High Alps. The course of the present-day Aare river is controlled by an overdeepened valley carved during the Pleistocene glaciations, and segmented by several lakes. We present a provenance dataset based on detrital garnet geochemistry from modern river sands, Pleistocene deposits from drill core samples and river terraces, a Pliocene river terrace, and the Miocene Molasse bedrock. We show that before the Pleistocene, i.e. before valley incision, detritus was transferred directly from the High Alps to the Rhine river plain. After the incision in the Pleistocene, the Aare river recycled the incised local Molasse bedrock. Today, the Aare river is in turn recycling those Pleistocene river terraces as well as the local bedrock. First-cycle detritus from the High Alps is instead stored in man-made and natural lakes. Our data shows that the “erosional engine” of sedimentary systems changes drastically in response to geomorphic reorganizations, and that provenance analysis is a prerequisite to locate the sediment source, as well as the origin of any environmental signal produced in that source.