Late Pleistocene evolution of the Rhine-Meuse system in the southern North Sea basin: imprints of climate change, sea-level oscillation and glacio-isostacy

Abstract

High-resolution continuous core material, geophysical measurements, and hundreds of archived core descriptions enabled to identify 13 Late Pleistocene Rhine-Meuse sedimentary units in the infill of the southern part of the North Sea basin (the Netherlands, northwestern Europe). This sediment record and a large set of Optical Stimulated Luminescence dates, ¹⁴C dates and biostratigraphical data, allowed to establish detailed relationships between climate change, sea-level oscillation, glaciation history and the sedimentary development of the Rhine fluvial system during the last glacial cycle (Marine Isotope Stages 5e-2, Eemian-Weichselian). A well-preserved Eemian sediment record was encountered as the infill of a Late Saalian (MIS6) subglacial basin. Part of this record reflects groundwater rise controlled (fine-grained) sedimentation as a result of postglacial (early) Eemian sea-level rise. It shows strong analogy to developments known from the Holocene Rhine-Meuse delta. Outside of the glacial depressions near coastal deposits are only fragmentarily preserved. The Early Glacial Rhine sediment record is dominated by organic debris and peat layers, marking landscape stability and low fluvial activity. Part of this record may have been formed under near coastal conditions. Significant amounts of reworked marine biomarkers in the lag-deposits of Early Pleniglacial (MIS4) fluvial systems indicate that this period is characterized by extensive reworking of older (MIS5) near-coastal sediments. Despite the marked Early Pleniglacial climatic cooling, input of new sediment from the drainage basin was relatively low, a feature that is related to the presence of regolith protective relic soil complexes in the basin. During the early Middle Pleniglacial, a major Rhine avulsion indicates the system was in an aggrading mode and that sediment supply into the lower reaches of the Rhine had strongly increased. This increase in sediment supply coincided with the timing of major climate cooling that occurred from ∼50 to 45 ka onwards. The increase in sediment supply is related to final breakup of the soil complexes in the drainage basin. After ∼24 ka, a strong input of coarse-grained gravelly sediments was observed which indicates a strong increase in physical weathering processes and periglacial-controlled supply of bedload sediment in the catchment. A time delay between climate change (∼30 ka) and channel belt aggradation (<24 ka), is explained as a result of transport path length between source and sink and/or effects of higher continental runoff rates after 22 ka. The Late Middle Pleniglacial, Late Pleniglacial and Lateglacial Rhine-Meuse record testifies for strong influence of glacio-isostatic-controlled differential upwarping of the study area. Glacio-isostatic-controlled forebulge upwarping and lateral valley tilting is shown to have deflected Rhine-Meuse channel belts after 35 ka. Glacio-isostatic upwarping is seen as the main cause for strong incision during the first phase of the Late Pleniglacial (30–24 ka). At later stage glacio-isostatic-controlled incision was overruled due to high climate-controlled sediment input from the catchment and probably initial glacio-isostatic subsidence. Migration of channel belts towards the direction of the former centre of glacio-isostatic uplift indicates that glacio-isostacy influenced Rhine-Meuse paleogeography until far into the Lateglacial.

Introduction

Rivers are transfer systems between areas of sediment production and sediment storage and therefore have an important role in long-term sediment re-organization across the Earth's surface. Insights into the sensitivity of the fluvial exchange system under external forcings are essential for understanding present and past landscape stability, for improvement of knowledge on basin-marginal sediment records and for predicting future change. Late Pleistocene fluvial records in northwestern Europe constitute important archives for improving the understanding of the response of fluvial systems to external forcing. Well-dated ice-core, marine and terrestrial records (Guiot et al., 1989; Sidall et al., 2003; NGRIP Members, 2004; Ménot et al., 2006) show that the rivers from this period experienced periodical, high amplitude changes in climate and sea-level while the Scandinavian and British ice-masses expanded and disintegrated multiple times (Clark et al., 2004; Mangerud, 2004; Svendssen et al., 2004). A long history of fluvial response to external forcing is registered in the record of the Rhine river system in northwestern Europe (Fig. 1). Its Late Pleistocene deposits are relatively well preserved and within dating range of several techniques enabling correlation between records of response and independently dated (proxy) records of forcing. The Rhine is a ∼1300 km long, rain- and snowmelt-fed river system that drains 185,000 km² of the central and northwest European foreland towards the North Sea basin, which saw the southern limit of Scandinavian and British ice multiple times in the Middle and Late Pleistocene.

During the Late Pleistocene, considerable quantities of primarily coarse-grained sediment were produced in the upland areas of the catchment that includes the Alps, Vosges, Black Forest and uplands of the Rhenish Shield (Fig. 1). A major part of that sediment was trapped in the south-eastern part of the North Sea sedimentary basin (Fig. 1). It forms a record consisting of 10–25 m of primarily coarse-grained gravely sands in the subsurface of the Netherlands (Fig. 2, Kreftenheye Formation, cf. Zonneveld, 1947; Doppert et al., 1975; Westerhoff et al., 2003), that within the North Sea basin is well-preserved owing to burial by the Holocene coastal prism (e.g. Berendsen and Stouthamer, 2000) and owing to the fact that Late Pleistocene fluvial units are much wider than Holocene (deltaic) channel belts in the area, that only locally caused erosion.

This record reflects the infill of the southern North Sea basin of the last interglacial–glacial cycle and is well preserved due to background tectonic subsidence, relatively unconstrained valley widths and occurrence of deeply scoured glacial basins that were formed during the Penultimate (Saalian) glaciation (Fig. 2). The record was formed under strongly varying climate (Fig. 3) including periods of full interglacial conditions (Eemian) and times of alternating stadial–interstadial conditions (Weichselian) with superimposed rapid oscillations at shifts between colder and warmer states (Voelker, 2002).

Eustatic sea-level oscillations of tens of meters in amplitude occurred repeatedly during the Late Pleistocene, but were interspersed with periods of relative sea-level stability (Fig. 3). The ultimate low stand was over 100 m below modern MSL some 25–20 ka BP. Sea-level oscillation led to repetitive extension and truncation of the river Rhine across the North Sea floor and beyond through the Dover Strait (Bridgland and D’Olier, 1995; Gibbard, 1995; Antoine et al., 2003; Fig. 1). In addition, the lower reaches of the Rhine experienced significant glacio-isostasy-driven crustal warping (forebulge buildup and collapse) because of its particular distance to centers of ice-sheet formation in Scandinavia, the Baltic and Britain (Mörner, 1979; Peltier, 1994, Peltier, 2004; Lambeck, 1995; Kiden et al., 2002; Lambeck et al., 2006).

Recently, a series of publications have presented new data on the sedimentary buildup and chronology of the Late Pleistocene Rhine system (Törnqvist et al., 2000, Törnqvist et al., 2003; Wallinga et al., 2004; Busschers et al., 2005). These papers documented the first applications of OSL-dating on the Rhine sediments, carried out in a limited study area in the west-central Netherlands. Their discussion sections focused on the role of sea-level change and related sequence stratigraphic interpretations (Törnqvist et al., 2000, Törnqvist et al., 2003) and later also on evaluating the relative roles of coeval forcing factors during the last interglacial–glacial cycle (Wallinga et al., 2004; Busschers et al., 2005). The major part of the Late Pleistocene fluvial deposits, however, remained to be studied in similar detail. Such work was a necessity to validate the many ideas on the bearings of the Rhine-record, and the accumulated data presented in this paper first allows doing so.

The present paper evaluates the imprints of northwest European climate-change, sea-level oscillations and glaciation history on the sedimentary record of the Rhine fluvial system in the southern part of the North Sea basin. We focus on the Rhine record of the last glacial cycle (Eemian-Weichselian-MIS5e-2, cf. Bassinot et al., 1994) that was deposited in the Netherlands and adjacent offshore area. We make use of new continuous core–material and numerous digitally archived core descriptions from the DINO-database of TNO Built Environment and Geosciences, Geological Survey of the Netherlands (TNO, 2006). This data is used for facies analysis (grain-size, sedimentary structures and mineralogy) and construction of geological transects and palaeogeographical maps. Chronological control is obtained by 94 quartz OSL-dates (optically stimulated luminescence) and by biostratigraphy (pollen zonation schemes). This enabled well-constrained reconstruction of sedimentation in the downstream reaches of the Late Pleistocene Rhine valley(s) and allows for explaining sediment composition variations, shifting loci of sediment preservation, variation in channel belt geometry, bounding surface formation and palaeo-geographical evolution by causal linkage to records of external forcing.