FIELD AND EXPERIMENTAL EVIDENCE FOR OUT-OF-PHASE FLUVIAL RESPONSES TO RAPID CLIMATE CHANGE

Although numerous investigations, particularly theoretical studies, have demonstrated that nonlinear, complex responses to allogenic forcing are likely to occur in sediment-dispersal systems, relatively simple combinations of external controls are commonly invoked to interpret fluvial morphodynamics over geologic timescales. Here, a case is presented of fluvial longitudinal-profile adjustment of the northwest European Rhine system under conditions of rapid climate and sea-level change associated with the last deglaciation. Previous interpretations proposed a fairly simple interaction of these two controls, where downstream aggradation due to relative sea-level rise occurred coeval with upstream incision due to climate-controlled reduced sediment supply. It is hypothesized that this type of response could also be triggered exclusively by climate change, where aggradation in the downstream reaches is the result of a propagating sediment wave, fed by upstream incision, that exceeded sediment transport capacity and was still en route seaward by the time a subsequent, rapid climate and sea-level change disrupted the prevailing boundary conditions. Experimental modeling of fluvial longitudinal-profile evolution demonstrates that a rapid, 25% increase in discharge (with constant sediment supply) leads to coeval upstream incision and downstream aggradation, similar to the inferences made for the Rhine. Such out-of-phase responses considerably complicate the interpretation of the fluvial stratigraphic record, and highlight the need for a holistic approach that considers the entire fluvial system, from source to sink. Also, a more comprehensive understanding is needed as to how common such out-of-phase responses are in the Late Quaternary geologic record.