Isostatic forcing of fluvial systems in lowland areas? A case study through improved optical dating of Late Pleistocene deposits of the River Rhine (The Netherlands).

Keywords: Quaternary Geology, Isostasy, Fluvial response, River Rhine, Optical dating Tectonic movements in pro-glacial lowland environments are often associated with crustal response to glacial and hydrostatic loading and unloading of the region. These isostatic movements are likely to affect fluvial systems, but lack of suitable dating methods has so far obstructed investigations. As a direct consequence, isostatic influences are ignored in sequence-stratigraphic models. We propose to apply novel optical dating methods to investigate the impact of isostatic movements on fluvial systems in pro-glacial lowland areas. The River Rhine in The Netherlands is chosen as a case study because: i) Late-Quaternary deposits of the River Rhine are well documented, and the river’s response to climate and sea-level changes is relatively well understood. ii) The Rhine has had two principal drainage routes through The Netherlands since the retreat of the Saalian (Oxygen Isotope Stage 6) Fennoscandian ice sheet; switching between these routes is likely related to isostatic tilting. The project will concentrate on Rhine deposits in and around the IJsselvalley, a glacially scoured basin formed during the Saalian (OIS 6) ice advance. Multidisciplinary investigations of the sedimentary fill of the IJsselbasin will include sedimentary and microfossil analysis. We will develop new optical dating methods for accurate dating of deposits older than 100.000 years. These methods will be tested through the dating of fluvial deposits formed during the Eemian (OIS 5e) interglacial, identified by the pollen content of associated clay and peat layers and U-Th dating of peat. We will apply the newly developed optical dating methods to establish the periods of occupation of the two drainage routes by the River Rhine during the past two glacial-interglacial cycles. Combining field data on the timing of avulsion of the Rhine with isostatic models for the area will allow us to establish whether the drainage route of this continental-scale fluvial system is isostatically controlled.