Int J Earth Sci (Geol Rundsch) (2010) 99:1955–1974 DOI 10.1007/s00531-009-0482-9

ORIGINAL PAPER

Late Pleistocene fluvial dynamics in the Hochrhein Valley and in the Upper Rhine Graben: chronological frame Manfred Frechen Æ Dietrich Ellwanger Æ Matthias Hinderer Æ Jo¨rg La¨mmermann-Barthel Æ Inge Neeb Æ Astrid Techmer

Received: 1 December 2008 / Accepted: 25 August 2009 / Published online: 13 September 2009 Ó Springer-Verlag 2009

Abstract During the Pleistocene, the Rhine glacier system acted as a major south–north erosion and transport medium from the Swiss Alps into the Upper Rhine Graben, which has been the main sediment sink forming low angle debris fans. Only some aggradation resulted in the formation of terraces. Optically stimulated luminescence (OSL) and radiocarbon dating have been applied to set up a more reliable chronological frame of Late Pleistocene and Holocene fluvial activity in the western Hochrhein Valley and in the southern part of the Upper Rhine Graben. The stratigraphically oldest deposits exposed, a braided-river facies, yielded OSL age estimates ranging from 59.6 ± 6.2 to 33.1 ± 3.0 ka. The data set does not enable to distinguish between a linear age increase triggered by a continuous autocyclical aggradation or two (or more) age clusters, for example around 35 ka and around 55 ka, triggered by climate change, including stadial and interstadial periods (sensu Dansgaard–Oeschger cycles). The braided river facies is discontinuously (hiatus) covered by coarse-grained gravel-rich sediments deposited most likely during a single event or short-time period of major melt

M. Frechen (&)
A. Techmer Leibniz Institute for Applied Geophysics (LIAG), Section S3: Geochronology and Isotope Hydrology, Stilleweg 2, 30655 Hannover, Germany e-mail: [email protected] D. Ellwanger
I. Neeb Regierungspra¨sidium Freiberg, Landesamt fu¨r Geologie, Rohstoffe und Bergbau Baden-Wu¨rttemberg, Albertstr. 5, 79104 Freiburg, Germany M. Hinderer
J. La¨mmermann-Barthel Institut fu¨r Angewandte Geowissenschaften, Technische Universita¨t Darmstadt, Schnittspahnstr. 9, 64287 Darmstadt, Germany

water discharge postdating the Last Glacial Maximum. OSL age estimates of fluvial and aeolian sediments from the above coarse-grained sediment layer are between 16.4 ± 0.8 and 10.6 ± 0.5 ka, and make a correlation with the Late Glacial period very likely. The youngest fluvial aggradation period correlates to the beginning of the Little Ice Age, as confirmed by OSL and radiocarbon ages. Keywords Upper Rhine Graben
River
Aggradation
Chronology
Quaternary

Introduction The River Rhine is one of the few fluvial systems that connect the areas of the Alpine and the Scandinavian glaciation providing one of the most complete terrestrial sediment archives for the late Cenozoic. The fluvial record includes at least 15 different Pliocene and Pleistocene terraces based on the correlation between Lower Rhine embayment and Middle Rhine area (Boenigk and Frechen 2006). The formation of these fluvial terraces is significantly influenced by climatic and tectonic processes. The Upper Rhine Graben (URG) has acted as a main sediment sink since its origin in the late Eocene (Ziegler 1990; Cloetingh et al. 2005). During the Quaternary, the URG has been affected by increased tectonic subsidence creating accommodation space, which has been subsequently filled by sediments from the Alpine source area and to a minor degree from the margin mountains of Vosges and Black Forest. The Rhine glacier system, including the Walensee Rhine Glacier, the Reuss Glacier and the Aare Glacier, and an eastern transfluence from the Rhoˆne Glacier (Fig. 1) acted as the main south–north erosion and transport medium from the Swiss Alps into the URG,

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Fig. 1 Map showing the working area in southern Germany, including a map of the European Cenozoic Rift System (modified after Ellwanger et al. 2003). LGM Last Glacial Maximum; Riss penultimate glacial deposits; MEG most extensive glaciation; LGT Last Glacial Termination

respectively. Flood events discharged large amounts of the sediments from the inner Alpine valleys and basin landscapes through the Hochrhein Valley into the URG forming low angle alluvial fans (Ellwanger et al. 2003).

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The large increase in terrigenous sediment supply during the Quaternary is a result of continuous disequilibrium between weathering, erosion, sediment transport and aggradation triggered by climatic perturbations (Hinderer

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2001). Increased aggradation in the southern URG postdates the filling up of the Lake Constance (Bodensee) Basin and occurred synchronous with periods of major ice melting. During these highly dynamic periods, the perialpine basins were deeply eroded, including the glacial overdeepening of Alpine valleys. These processes resulted in a hiatus at the base of the fluvial sequences in the southern URG and in the deposition of block-rich and gravel-rich sediments. The hiatus and these coarse-grained deposits are termed event layer in the German literature (Ellwanger et al. 2003). Quantitative data on the changes of sediment fluxes of the Rhine system for the Quaternary time period are absent owing to the lack of reliable chronological data for the Rhine system downstream from Lake Constance (Hinderer 2001, 2003; Ellwanger et al. 2003; Peters and van Balen 2007). Absolute dating of fluvial sediments is problematical, as described in detail for the Middle and Lower Rhine area by Boenigk and Frechen (2006). Radiocarbon dating of organic-rich deposits is often not applicable to fluvial sediments owing to the lack of in situ organic material and the limited age range of about 50 ka (Geyh 2005). Uranium-series dating is applied only in a few cases to interstadial and interglacial peat layers intercalated in fluvial or aeolian deposits (Frechen et al. 2006). 40Ar/39Ar dating provides a few age constraints for tephra layers intercalated in fluvial sediments (van den Bogaard et al. 1988; Boenigk and Frechen 1998). Palaeomagnetic data are particularly useful for reconstructing the history of polarity changes in the earth’s magnetic field, and can provide indirectly chronological constraints for fluvial sediments, as described for the fluvial records in the Middle Rhine area and in the Lower Rhine embayment (Boenigk and Frechen 2006). Cosmogenic nuclides like 10Be in quartz and 3He in hornblende and pyroxene are used for exposure dating of fluvial terraces (Mercier et al. 1999; Schaller et al. 2002; Ruszkiczay-Ru¨diger 2007), but have not been applied to the Rhine terraces so far. Optically stimulated luminescence (OSL) dating methods have been widely used to determine the deposition age of aeolian sediments (Frechen and Dodonov 1998; Frechen 1999). Fluvial sediments are particularly suitable for the application of OSL dating techniques (Busschers et al. 2005; Jain et al. 2003; Wallinga 2002). In this study, OSL dating on fine sand was carried out from several gravel pits in the Hochrhein Valley near Rheinfelden and Waldshut-Tiengen and in the southern part of the Upper Rhine Graben between Basel and Freiburg. The samples were taken from sand layers intercalating the gravel successions from below and above the event layer, which is coeval with the youngest major discontinuity correlating very likely to the transition from the Upper Pleniglacial to the Late Glacial.

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The aim of this study is to set up a more reliable chronological frame for the Late Pleistocene and Holocene fluvial activity in the Hochrhein area and in the southern part of the URG and to give evidence for aggradation periods during that time period. This is part of an ongoing study investigating fluvial sediments from the Rhine system.

Geological setting In the study area, the River Rhine (German: Rhein) is divided into three geographical regions: the Alpenrhein, the Hochrhein and the Upper Rhine (German: Oberrhein) (Figs. 1, 2). The Alpenrhein area and the Hochrhein area are located between the city of Reichenau and Lake Constance, and between Lake Constance and the city of Basel, respectively (Fig. 2). During the last glaciation, fluvial melt water floods of the Rhine glacier and glacier tongues in the Swiss Midlands brought much unsorted material from the perialpine area into the URG. During the ice-transgression, the Hochrhein area has acted as a barrier between the inner Alpine basins and the main sediment sink of the southern URG. After the major deglaciation periods, Lake Constance Basin was ice-free, and has acted as a sediment trap resulting in a reduced discharge and sediment load of the River Rhine further downstream (Hinderer 2001; Ellwanger et al. 2003). The Upper Rhine Graben is located between the cities of Basel and Bingen. Its extension has a length of about 300 km and a width of up to 36 km and about 20 km in the southern part and in the northern part, respectively. The altitude of the River Rhine decreases from about 280 m above sea level (asl) in the south near Basel to about 80 m asl in the north near Bingen. The River Rhine flows in numerous meanders northwards, which were partly regulated in the years between 1817 and 1874. The URG is part of the Cenozoic European Rift system (Ziegler 1990; Cloetingh et al. 2005), which extends from the North Sea coast of the Netherlands via the URG to the western Mediterranean. The northerly striking URG bifurcates northward into the northwest trending Roer Valley Graben in the Lower Rhine Embayment and the north-easterly trending Hessian Grabens transecting the Rhenish Massif (Cloetingh et al. 2005). During the Late Eocene, the URG began to subside. Cloetingh et al. (2005) relate the northerly directed compressional stresses to the collision interaction of the Pyrenees and the Alps with their foreland. The present main tectonic stress field has a NW–SE direction, which is angular to the present course of the URG resulting in a sinistral offset and an increased subsidence in parts of the northern and southern part of the URG, the Heidelberg Basin and the Geiswasser Basin,

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Fig. 2 Map showing the location of the sections under study and the thickness of Quaternary sediments and the position of boreholes of the Heidelberg Drilling Project in the URG modified after Hagedorn (2004) and Bartz (1974)

respectively (Fig. 2). The maximum subsidence of the Quaternary is located in the Heidelberg Basin near to the city of Heidelberg. Bartz (1951, 1974) reported a thickness of more than 300 m for the Quaternary sequence and about 680 m for the Pliocene record. The Heidelberg Drilling Project, including the sediment cores from Heidelberg-

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UniNord, Ludwigshafen-Parkinsel and Viernheim, is currently aiming to reconstruct the sediment archive of the basin fill more precisely (Fig. 2) (Ellwanger et al. 2005). A second area of increased subsidence is located in the Geiswasser basin southwest of the Kaiserstuhl located in the southern part of the URG. Differences in altitude

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between the surface of the Lower Terrace and the floodplain are partly caused by the ongoing tectonic and seismic activity with lateral movements of about 0.5 mm/a (Bartz 1967) and modern uplift rates up to 1 mm/a (Demoulin et al. 1998). A zone of elevated seismic activity is also evidenced by the historical earthquakes of Basel Anno Domini (AD) 1021 and AD 1356. In the southern URG, tectonically active faults have caused offsets of up to 20 m during the Holocene (Hu¨ttner 1991). There is a distinct terrace step east of the River Rhine. The higher level is called ‘‘Hochgestade’’ and the lower part between the step and the river is called ‘‘Tiefgestade’’. The difference in altitude between ‘‘Hochgestade’’ and ‘‘Tiefgestade’’ reaches several metres, and has formed by a tectonically active fault propagating parallel to the present river course (Bram et al. 2005). The Pleistocene sequence of the URG consists of sand and gravel units as well as dune sands and loess deposited as a result of fluvial and aeolian activity, respectively. The surface of the floodplain consists of reworked sand and gravel, often covered by alluvial clay and flood loam deposits of Holocene age. The floodplain has acted as a sediment source for deflation, as evidenced by loess deposits, cover sands and dunes up to 20-m thick (Zo¨ller and Lo¨scher 1999; Lang et al. 2003; Frechen et al. 2003, 2007), although a finer fraction may have been derived from more distant sources. A coeval increased fluvial and aeolian activity is likely. In the southern part of the URG the sediments are coarser than in the northern part. Local rivers laterally supply sediments from the Vosges and Black Forest into the URG, where the sediments mix up with Alpine materials. Furthermore, the petrography of the sediments changes from south to north owing to increased sorting and mixture with local material from the margins of the Upper Rhine Graben (Hagedorn 2004; Hagedorn and Boenigk 2008). A correlation of the Quaternary sediment successions from south to north is problematical owing to lesser subsidence in the area around the city of Karlsruhe resulting in a reduced thickness of Quaternary sediments (Fig. 2). In the southern part of the URG, the Quaternary sediment succession is subdivided lithologically into three units, namely the Iffezheim Formation, the Breisgau Formation and the Neuenburg Formation (from the oldest to the youngest). Near Bremgarten, the Neuenburg Formation consists of 52 m mainly coarse gravel of Alpine origin, including pebbles from the Black Forest (Bram et al. 2005). The upper part of the Neuenburg Formation contains a prominent coarse sediment layer, which forms a marker horizon along the Hochrhein valley and can be easily identified in wells of the southern Upper Rhine Graben. Sedimentological and stratigraphical data point to a single event or short-time period of strongly increased melt water

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discharge, which deposited this coarse layer. This coarse sediment layer is termed event layer in the literature (Ellwanger et al. 2003). The sections at Rheinheim, Herten and Wyhlen are located in the Hochrhein Valley between Lake Constance and Basel. Six samples were taken from below the event layer, which is coeval with the main last glacial discontinuity correlating to the transition of the Last Glacial Maximum (LGM) to the Late Glacial, from sections located in the Hochrhein Valley. The sediments correlate stratigraphically to a lower terrace. The gravel pits at Hupfer, Knobel and Bremgarten are situated between Basel and Freiburg in the southern part of the URG (Fig. 2). The Rheinheim section is located east of Waldshut-Tiengen upstream from the confluence of River Rhine and River Aare in the Hochrhein Valley. This part of the study area is located in a valley-widening forming a small sediment basin. Upstream and downstream from the valleywidening, an entrenched valley has formed. The Rhine glacier catchment was the source area for the fluvial sediments deposited at the Rheinheim section. The sediment successions include a hiatus covered by coarse-grained deposits (event layer) on top of braided-river sediments. The event layer has a vale-like morphology thinning out to the valley sides. The basin facies is coarse-grained, including blocks, gravel and sand, whereas the marginal facies is more fine-grained, including sand and gravel. Three samples were taken from sand lenses (scour-pools) intercalated in the gravel correlating to the braided-river facies (Figs. 3, 4). The uppermost sample was taken from a sand layer intercalated in the coarse-grained sediments covering the hiatus of the event layer. The Herten section is situated on a lower terrace of the River Rhine in the Hochrhein Valley west of Rheinfelden. In the gravel pit, the event layer is exposed covering deposits of a braided-river facies. The coarse-grained sediments of the event layer were discontinuously

Fig. 3 Picture showing part of the Rheinheim section, including the position of samples RHE2 and RHE3 taken from sand lenses intercalated in the fluvial succession

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Fig. 4 Picture of the upper part of the Rheinheim section, including position of sample RHE4 taken from a sand lens intercalated in the gravel succession

deposited, similar like at the Rheinheim section. At the Herten section, the coarse-grained sediments of the event layer were not accumulated horizontally like a terrace surface but gently inclining to both valley sides, which is also the case at the Rheinheim section and at the Wyhlen section. Coarse-grained, unsorted and layered sediments of the event layer alternate with gravel and partly sand-filled scour-pools. Two samples were collected from sandy scour-pool fills below the event layer (Fig. 5). The gravel pit Wyhlen (Holcim) is situated in the middle part of the Hochrhein Valley near to Grenzach-Wyhlen. The source area of the fluvial sediment includes the Swiss Midlands with the catchment areas of the Reuss glacier, the Aare glacier and the Rhoˆne glacier. The section is located about 5 km south-west of the Herten section, and its sediment succession is similar to the one from Herten. The former gravel pit Wyhlen is one of the most detailed exposures, including sediments of a braided-river facies, a hiatus and coarse-grained deposits (event layer), covering the time span of the LGM and Late Glacial. It is very likely that this event layer is coeval with the one described from the Herten section. In the URG, this stratigraphic marker

Fig. 5 Picture of part of the Herten section, including position of sample HER1 and HER2 taken from sand lenses intercalated in the gravel deposits

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horizon is found in drillings owing to increased subsidence rates. The coarse-grained sediments of the event layer are covered by sandy loess. One sample was collected from the braided-river facies taken from below the event layer, and two samples from sandy loess postdating the event layer. The aim of the sampling at the Wyhlen section was to provide an age range sandwiching the event layer. The gravel pit Wyhlen became protected as a geological monument in 2006. The Hupfer section is located in the gravel pit of Haltingen-Holcim northwest of Weil am Rhein about 5 km downstream from the city of Basel in the southernmost part of the URG. The exposed fluvial succession correlates morphologically to the upper part of the lower terrace covering the last glacial main discontinuity, coeval to the event layer. A coarse sediment layer (‘‘event layer’’) is intercalated in sediments of the lower part of the lower terrace in Alsace, as evidenced by sedimentological results from sediment cores [archives of Service Ge´ologique Re´gional Alsace (BRGM) and Landesamt fu¨r Geologie, Rohstoffe und Bergbau Baden-Wu¨rttemberg im Regierungspra¨sidium Freiburg (LGRB)]. The sediment structure consists of extensive amalgamation of scour-pool architecture. The scour-pools were partly filled by sand, which was sampled. Fluvial sediments about 20-m thick comprise the ‘‘Hochgestade’’ and the ‘‘Tiefgestade’’ and correlate to an ‘‘upper’’ lower terrace. Near to the base of the gravel body at the Hupfer section, a large block of Helvetic limestone was exposed. It is likely that this Helvetic limestone several metre in diameter was transported by ice-drift. Three samples were collected between 5 and 20 m below surface correlating to the sediment succession predating the LGM. The Knobel section is located near to the village of Bremgarten and situated on the ‘‘Tiefgestade’’. The morphological situation resembles a terrace stair. There is a maximum difference in altitude of about 5 m between ‘‘Hochgestade’’ and ‘‘Tiefgestade’’, both correlating with the lower terrace, in the study area. The event layer (hiatus) is intercalated in the fluvial successions about 9 m below surface, as evidenced by drilling results. The fluvial sediments from above this event layer have a smaller grain-size than those of the event layer. Four samples were collected from sand lenses intercalated in the fluvial succession between 1 and 4 m below surface. The geological estimates make a Late Glacial deposition age likely for the sediments under study. The section at Bremgarten is situated in a gravel pit close to the River Rhine. The event layer is intercalated in the sediment succession between 15 and 20 m below surface, as shown from drillings, and covered by a fluvial sediment succession showing two fining up cycles of lower magnitude. The latter deposits are less coarse-grained than

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the sediments of the event layer. A third fining-up cycle is exposed in the wall of the gravel pit. Two samples were taken from sand lenses intercalated in light-greyish gravel from the eastern wall in 2003. Wood from a trunk was collected for radiocarbon dating from the western wall about 3 m below surface. In 2006, two further trunks were exposed in the south wall and sampled for radiocarbon dating. The two trunks were located at the top of a sand layer which correlates to the base of the third fining up cycle. At the bottom site of the two trunks, the bark is still present, whereas the top site of the two trunks is eroded owing to fluvial processes. The stratigraphical position of these sediments taken from the Bremgarten section is younger than those taken from the Knobel section, whereas the altitude of Bremgarten section is lower than at the Knobel section. These sediments correlate to the Upper Neuenburg Formation (Ellwanger et al. 2003) and correlate with fan deposits coeval with the last glacial and/or postdating the last glacial.

Luminescence dating Luminescence dating of aeolian, fluvio-aeolian and fluvial sediments has proved to be successful where radiocarbon and other dating methods are not applicable (Frechen et al. 1997, 2001; Jain et al. 2003; Lian and Roberts 2006). Since the late 1980s, luminescence dating of sediments has significantly been improved by the development of more light sensitive techniques such as blue OSL or infrared optically stimulated luminescence (IRSL) for monomineralic quartz or feldspar, respectively (Huntley et al. 1985; Hu¨tt et al. 1988). The basic principle of luminescence dating is solid state dosimetry of ionising radiation (Aitken 1998; Bo¨tterJensen et al. 2003; Wintle 1997). Luminescence is the light emitted from crystals such as quartz, feldspar or zircon when they are stimulated with heat or light after receiving a natural or artificial radiation dose. As a result of natural radiation in sediments, the number of electrons lodged at traps and caused by crystal lattice defects increases with time and dose until all traps are filled and saturation is reached. The equivalent dose is a measure of the past radiation energy absorbed in natural dosimeters like quartz and feldspar minerals and, in combination with the dose rate, which is the rate of radiation absorbed per unit time, yields the time elapsed since the last exposure to sunlight. The quartz signal saturates earlier than the feldspars signal. Prescott and Robertson (1997) report about quartz, which becomes non-linear at about 30 Gray (Gy) and usually saturates before 150 Gy indicating an upper saturation limit for equivalent dose calculations in the region of

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100–200 Gy. The saturation level of the quartz grains has to be determined for each source area. Feldspars tend to have a significant higher saturation dose than quartz implying the theoretical possibility to date older samples up to 1 Ma (Aitken 1998). However, the advantage is often offset by the larger dose rates for the feldspars owing to high potassium content within the feldspar mineral grains and athermal signal instability owing to anomalous fading (Wintle 1973). An important assumption of luminescence dating techniques is that the mineral grains were sufficiently long enough exposed to daylight/sunlight prior to deposition to reset the radiometric clock (‘‘zeroing the luminescence signals’’). The level of zeroing depends on the exposure time of the mineral grains to light, the available light intensity and the light spectrum. These parameters strongly depend on fluvial sedimentary dynamics, including water depth, sediment load, turbulence and turbidity, grain-size and transportation distance (Rhodes and Pownall 1994; Frechen 1995; Murray et al. 1995; Wallinga et al. 2000; Jain et al. 2003). In many fluvial environments the probability of complete zeroing of all sediment grains is low. Jain et al. (2003), Wallinga (2002) and Singarayer et al. (2005) pointed out that in contrast to aeolian sediment a distribution of values determined for fluvial deposits usually results in age overestimation, if equivalent doses are measured for samples containing a large number of grains. However, in large river systems the zeroing has been found to be more complete or nearly complete owing to the very likely multiple recycling of sediment during repeated phases of deposition and erosion (Wallinga 2002). Most bleaching does occur when the grains are close to the water surface, where the light intensity is greater and the light spectrum is more complete. Sediment from overbank deposits and scour pools are suitable for luminescence dating. Samples taken from sediment of high-energy depositional environments like mass flow deposits or sediments from kames are not suitable for luminescence dating (Hu¨tt and Jungner 1992). Partial bleaching of sediments is also attributed to flashy seasonal river flow (floods) associated with high-amplitude precipitation causing input from bank erosion (Jain et al. 2003). It is difficult to determine the degree of zeroing of the OSL signal. The existence of scatter in equivalent dose determinations is an indication for incomplete zeroing of the OSL signal. Grainto-grain variations do produce scatter in single aliquot equivalent dose determinations (Lamothe et al. 1994; Murray and Roberts 1997; Olley et al. 1999). The detection of partial bleaching was previously investigated by using small single aliquots or single-grain methodology (Singarayer et al. 2005; Preusser et al. 2007) and different statistical approaches (Lepper et al. 2000; Bailey and Arnold 2006). Following these studies, it seems to be

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mandatory to investigate a large number of single grains per sample from fluvial environments, which is part of an ongoing challenging investigation. The first fluvial sediments from Germany, which were investigated by a dating approach combining IRSL, OSL and thermoluminescence (TL), were taken from sand layers of the lower terrace from the River Emscher (Frechen 1995). TL dating results showed large uncertainties most likely owing to incomplete bleaching prior to deposition; IRSL dating of potassium-rich feldspars showed a better agreement with the geological age estimates. Frechen (1995) suggested to investigate 100–200-lm potassiumrich feldspar specimen to test the bleaching behaviour for different transmission wavelength, which was later extended for fluvial and glaciofluvial sediments from the Swiss Midlands and Northern Germany by Preusser (1999). In the latter study, the glaciofluvial sediments from northern Germany yielded IRSL age overestimation most likely owing to insufficient zeroing prior to deposition. However, IRSL dating of fluvial sediments from the Swiss Midlands showed good agreement with independent age control (Preusser 1999; Preusser et al. 2003). Recent advances in instrumentation and measurement protocols have played a key role in enabling the dating of fluvial sediments. OSL dating of quartz has been significantly improved by single-aliquot regenerative (SAR) protocols (Murray and Wintle 2000; Wallinga et al. 2001). Sensitivity changes are monitored and corrected in the SAR protocol. This procedure allows the determination of equivalent dose by interpolation with a precision of about 5%. Feldspar IRSL dating can be troubled by anomalous fading (Wintle 1973) causing age underestimation. Anomalous fading is an athermal signal loss following irradiation and reflecting the instability of some of the electron traps. Huntley and Lamothe (2001) pointed out that it is essential to determine the fading rate precisely and carried out fading experiments to correct the measured IRSL age estimates. However, age correction is problematic and a reliable procedure is still under discussion (Auclair et al. 2003; Lamothe et al., 2003; Huot and Lamothe 2003). There is a close relation between fading rate and geological provenance. Huntley and Lamothe (2001) determined fading rates for North American sediments ranging from 2 to 10% per decade (decade means a factor of 10 in time since irradiation). These fading corrections are restricted to the low-dose linear portion of the dose response and are not expected to be applicable to samples older than *20–50 ka. Fading corrections with a potential to be applicable over a much wider range are under discussion (Lamothe et al. 2003). Preusser et al. (2003) reported that long-term fading has apparently no effect on potassium-rich feldspar specimen from the Swiss Alpine Foreland.

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Natural ionising radiation includes alpha, beta and gamma irradiation released by the radioactive decay of the unstable isotopes of the 238U, 235U and 232Th decay chains, 40 K and to a minor content 87Rb, 14C and cosmic radiation. The dose rate depends on a number of external factors, some of them are even variable during burial time, like moisture, sediment thickness, grain-size, geochemical weathering of minerals or salts, mobilisation of clay minerals and radioactive equilibria/disequilibria of the decay chains and internal factors like the amount of 40K in potassium-rich feldspars.

Experimental details and luminescence characteristics Eighteen samples were taken in light-tight metallic cylinders from sand lenses (16) intercalated in gravel-rich fluvial deposits and fluvio-aeolian sand/sandy loess (2) covering gravel deposits. The outer light-exposed part of the sediment was removed under subdued light at both ends of the cylinder. The sample preparation included sieving to separate the 100–200-lm or the 150–200-lm grain-size fractions, followed by removing the carbonates in 0.1 N hydrochloric acid, organic matter by 30% hydrogen peroxide and clay particles by sodium oxalate. The sand-sized feldspar with densities lower than 2.58 g/cm3 was extracted using heavy liquids. The sand-sized quartz was extracted from the remaining fraction using heavy liquids of 2.62 and 2.70 g/cm3 densities. The quartz fraction was etched with 40% hydrofluoric acid for 60 min to remove feldspars and sieved again with a 100- or 150-lm mesh. Grain-sizes, the selected minerals and the protocols to determine the equivalent dose are given in Tables 1–3. The sand-sized quartz or feldspar grains were brought onto 0.8-mm steel discs (aliquots). IR short shine was applied for normalisation following the multiple aliquot additive dose (MAAD) protocol for feldspar. IR stimulation was applied at the end of the SAR protocol to test whether the single aliquots (quartz) show a contamination with feldspar deriving from feldspar inclusions after etching with hydrofluoric acid. The equivalent dose (De) values were determined for potassium-rich feldspar by the MAAD protocol, increasing the natural luminescence stepwise by additional irradiations Table 1 Multiple-aliquot additive dose (MAAD) protocol for feldspar grains 1. IR short shine of 0.4 s for normalisation 2. Irradiation of natural ? dose aliquots 3. Delay of [40 days between irradiation and further treatment 4. Preheat of all aliquots at 230°C for 1 min 5. Measurement of IR decay curves

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Table 2 Single aliquot regenerative (SAR) protocol, as applied for both quartz (Qz) (OSL stimulation) and feldspar (Fsp) grains (IRSL stimulation) (modified after Murray and Wintle 2000; Wallinga et al. 2000, 2001) (beta irradiation 1 s equals 0.172 Gy) 1. Preheat of natural 2. IRSL decay of naturals for 300 s at 50°C/OSL decay for 40 s at 125°C 3. Test dose (10 s) 4. Preheat of test dose at 210°C for 10 s (Fsp), and 210°C for 0 s (Qz) 5. Measurement of test dose (IRSL decay for 300 s at 50°C for Fsp/OSL decay for 40 s at 125°C for Qz) 6. Regenerative dose 7. Preheat of regenerative dose for 10 s at 290°C (Fsp), 240°C or 260°C (Qz) 8. IRSL decay of regenerative dose for 300 s at 50°C/OSL decay for 40 s at 125°C 9. Test dose for 10 s 10. Preheat of test dose at 210°C for 10 s (Fsp), and 160°C or 210°C for 0 s (Qz) 11. IRSL decay for 300 s at 50°C/OSL decay for 40 s at 125°C 12. Repeat steps 6–11 for R2, R3, zero point and recycling point, etc.

of increasing dose (Table 1). The natural signal was subsequently compared with the artificial radiation signal of irradiated subsamples of known doses. Single aliquot regenerative-dose protocols (SAR) were carried out for monomineralic studies of quartz extracts and feldspar extracts to determine the De values more precisely (Murray and Wintle 2000; Wallinga et al. 2000, 2001). All growth curves were fitted using a saturating exponential function. Feldspar multiple aliquot additive dose (MAAD) protocol The MAAD protocol was applied involving 48 aliquots for measurements of the dose response curve resulting in the determination of one equivalent dose estimate, which is thought to be representative for the sample. The samples were irradiated by a 90Sr/90Y beta source (dose rate 0.172 Gy/s at the time of irradiation) in seven dose steps with five discs each and a maximum radiation dose of 720 Gy. Three aliquots were bleached for 3 h by a Dr Ho¨nle solar simulator and used for background subtraction. All discs were stored at room temperature for at least 40 days after irradiation. The irradiated samples were preheated for 1 min at 230°C before infrared stimulation. A Schott BG39/Corning 7-59 filter combination (transmission window 320–440 nm) was placed between photomultiplier and aliquots for IRSL measurements. Optical stimulation was generated by a 1.0-W infrared laser diode with a wavelength of 830 nm. The equivalent dose was obtained by calculating the 1–10-s integral of the IRSL decay curves. Single-aliquot regenerative (SAR) protocol for quartz and feldspar A number of tests were carried out on both quartz and feldspar minerals to investigate the luminescence

characteristics and to evaluate the suitability of the applied SAR protocols for the samples under study. To determine appropriate preheat conditions for the determination of De values and to avoid thermal transfer effects, the variation of equivalent dose with preheat temperature was measured for both feldspar and quartz extracts. A plateau of De values against preheat temperature (SAR quartz) was determined using 10-s preheats at 20°C intervals from 200 to 300°C (cp. Murray et al. 1997). Results were obtained on eight samples, which had De values ranging from 0.8 to 51.5 Gy, for examples of preheat plateau see Fig. 6. The older samples did not show a trend for De values to give a plateau at higher temperatures. A preheat temperature of 240°C for 10 s was used for natural and regenerative doses for quartz extracts, except for samples BRE1 and BRE2, which were preheated at 260°C (Table 2). Changes in the measured luminescence intensity with dose are known to occur in quartz and feldspar during single aliquot measurement procedures. Therefore, a small test dose is given to each aliquot after the measurement of the natural or regenerated signal to determine sensitivity changes. The sensitivity correction is calculated by dividing the natural or regenerated IRSL or OSL intensity (Lx) by the test dose IRSL or OSL intensity (Tx). The recycling ratio (Rx/R1) is a test for the effectiveness of the sensitivity correction and determined by repeating the first dose point in the growth curve at the end of the measured cycle. The recycling ratio should be consistent with unity. Murray and Wintle (2000) suggested that aliquots should be rejected if recycling ratios are outside 10% of unity. However, Murray and Wintle (2000) pointed out that the recycling ratio only tests whether sensitivity changes following laboratory measurements have been accurately corrected for. In this study, most aliquots fall within 10% of unity. Figure 7 shows a plot of recycling ratios for 24 single aliquots of samples HUP3 and HUP1 showing a range from 0.81 to

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Fig. 7 Examples for recycling ratios of samples HUP3 (upper) and HUP1 (lower) using quartz extracts. The recycling ratios range from 0.81 to 1.29 (n = 24) and from 0.78 to 1.11 (n = 24) for sample HUP3 (upper) and HUP1 (lower), respectively

Fig. 6 Preheat plateau for samples HER1, WYL2 and BRE2: De values, as determined for quartz by the SAR protocol applying 10 s preheat at 20°C intervals ranging from 200 to 300°C. Equivalent dose is given in seconds (1 s equals 0.172 Gy)

1.29 and from 0.78 to 1.11, respectively, indicating that the SAR sensitivity correction is appropriate for these samples. In this study a recycling ratio of 1.0 ± 0.2 was accepted. A successful dose recovery test indicates that the SAR protocol produces internally consistent results for a sample and evaluates the creditability of the equivalent dose measured from a natural sample and so confirms that the applied SAR protocol enables to recover a known laboratory dose (Rhodes 2000). After bleaching the aliquots with the Dr. Ho¨nle solar simulator, a known laboratory dose close to the De value was given. The artificially dosed aliquots were then treated as ‘‘natural’’ in the SAR protocol (Table 2) to determine the De value (here: dose recovery). Examples for dose recovery tests are given for sample HUP3 (feldspar) using a test dose of 105 Gy (Fig. 8), and samples WYL2 and KNO4 (quartz) using a given dose of 17.5 Gy (Fig. 9). Most of the dose recovery tests yielded the recovering dose values within 10% from the given dose. A comparison of De values as measured by the feldspar SAR protocol and the feldspar MAAD protocol yielded

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excellent agreement within the 1-sigma standard deviation (Table 5). Examples of growth curves are presented in Fig. 12, as determined by the MAAD protocol for feldspar specimen. The single-aliquot regenerative (SAR) dose protocol was applied to determine the De values for feldspar and quartz (Murray and Wintle 2000; Wallinga et al. 2000, 2001; Wintle and Murray 2000). To determine the De values, a dose–response curve with typically three dose points is measured on a single aliquot by repeated irradiations, preheats and IRSL/OSL measurements. Sensitivity changes occurring due to laboratory heat treatment are monitored after each OSL measurement and corrected (Table 2). The weighted mean De value and the error of the weighted mean were calculated for most of the samples from 24 aliquots for both feldspar and quartz grains (Figs. 10, 11). Aliquots were taken into account, when the natural signal was between first and second artificial dose step. The IRSL signal was measured with a Schott BG39/ Corning 7-59 filter combination between photomultiplier and feldspar specimen. A Schott U-340 filter with a detection window of 290–370 nm was placed for OSL measurements between photomultiplier and quartz specimen. Blue light emitting diodes were used for quartz stimulation.

Int J Earth Sci (Geol Rundsch) (2010) 99:1955–1974

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Fig. 8 Dose recovery test with a given dose of 105 Gy for sample HUP3 using potassium-rich feldspar specimen; a preheat temperature between 220 and 290°C seems to be appropriate for the SAR protocol under application

Equivalent dose determination was carried out using the software Analyst 6.0 (G.A.T. Duller, Aberystwyth).

Fig. 9 Dose recovery test with a given dose of 17.5 Gy for samples WYL2 (upper) and KNO4 (lower) using quartz extracts; a preheat temperature between 200 and 260°C seems to be appropriate for the SAR protocol under application

Dosimetry As the outer shell of the feldspar minerals was not etched by hydrofluoric acid (cp. Zander 2000), alpha efficiency was estimated to a mean value of 0.2 ± 0.1 for all feldspar samples (Duller 1994). Dose rates for all samples were calculated from potassium, uranium and thorium contents, as measured by gamma spectrometry in the laboratory, assuming radioactive equilibrium for the decay chains (Tables 3, 4). An average internal potassium content of 12 ± 0.5% was applied for all feldspar samples (Huntley and Barril 1997). Cosmic dose rate was corrected for the altitude and sediment thickness, as described by Prescott and Hutton (1994). The natural moisture content of the sediment was estimated between 10 ± 2% and 25 ± 5% for all samples depending on the depth below surface and the position of the present water table.

Results

Fig. 10 De distribution (n = 24) for sample HUP3 resulting in a weighted mean De value of 108.0 ± 1.8 Gy for feldspar specimen

Dosimetric results, including the data of all measured radioisotopes, De values, IRSL and OSL age estimates are shown in Tables 3, 4 and 5. Uncertainties are given in 1-sigma confidence interval.

The total dose rates range from 1.89 to 3.36 Gy/ka and from 1.12 and 2.38 Gy/ka for feldspar and quartz, respectively. The sediments downstream from Basel have a higher uranium and thorium content giving evidence for a

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Fig. 11 De distribution (n = 24) for sample HUP3 resulting in a weighted mean De value of 57.6 ± 2.0 Gy for quartz extracts

different sediment source. Uranium- and thorium-rich heavy minerals like titanite and monazite are typical for sediments from the crystalline basement of the Vosges and Black Forest (Hagedorn and Boenigk 2008). Radioactive disequilibrium is not a problem for the sediments under

investigations. A compilation of measured radioisotopes is presented in Table 4. The De values for feldspar, as determined by the MAAD and SAR protocol, range from 2.2 to 141 Gy and from 2.1 Gy to 151 Gy, respectively, indicating a slight underestimation of those determined by the SAR protocol (Table 5). Evidence for anomalous fading is given by the comparison of IRSL and OSL age estimates. OSL gives significantly higher age estimates than those determined by IRSL. The largest IRSL age underestimation was determined for the samples collected from the Bremgarten section, the Knobel section and the Wyhlen section. The De values range from 2.4 to 60.7 Gy for quartz grains (Table 5), as determined by the SAR protocol (Murray and Wintle 2000). OSL age estimates are used for chronostratigraphic interpretation only. IRSL age estimates are used as minimum ages, if OSL age estimates are not available. At the Rheinheim section, three samples were taken from sand lenses intercalated into the gravel succession between 5 and 10 m below the coarse-grained sediment of the event layer, which is only discontinuously exposed in this section. The IRSL age estimates of the three lower samples range from 61.7 ± 5.8 to 44.6 ± 3.5 ka. The quartz grains of sample RHE1 gave an OSL age estimate of 36.7 ± 2.9 ka underestimating the IRSL age estimate by about 22%. Most of the aliquots of samples RHE2, RHE3 and RHE4 yielded recycling ratios with negative values,

Table 3 Dosimetric results, as determined by gammaspectrometry Sample

LUM

Depth (m)

Grain size (lm)

Uranium (ppm)

Thorium (ppm)

Potassium (%)

Cosmic (lGy/a)

H2O (%)

Dose rate1 (Gy/ka)

Dose rate2 (Gy/ka)

HER1

50

3.00

100–200

1.33 ± 0.06

2.87 ± 0.06

1.11 ± 0.02

130 ± 13

25 ± 5

1.95 ± 0.17

1.26 ± 0.09

HER2

49

2.50

100–200

1.15 ± 0.03

3.24 ± 0.07

1.18 ± 0.03

135 ± 13.5

25 ± 5

2.01 ± 0.18

1.31 ± 0.10

WYL1

52

1.50

150–200

1.41 ± 0.04

3.68 ± 0.07

1.20 ± 0.02

160 ± 16

10 ± 2

2.54 ± 0.12

1.71 ± 0.06

WYL2

51

1.80

100–200

0.85 ± 0.04

3.46 ± 0.07

1.09 ± 0.02

145 ± 14.5

10 ± 2

2.20 ± 0.15

1.47 ± 0.06

WYL3

53

5.30

150–200

1.12 ± 0.02

2.93 ± 0.05

0.86 ± 0.02

90 ± 9

25 ± 5

1.77 ± 0.12

1.02 ± 0.07 1.16 ± 0.08

RHE1

57

*5.50

150–200

1.22 ± 0.03

2.76 ± 0.06

0.95 ± 0.02

80 ± 8

20 ± 5

1.92 ± 0.13

RHE2

56

12.50

150–200

1.53 ± 0.04

3.30 ± 0.08

0.83 ± 0.03

25 ± 2.5

20 ± 5

1.89 ± 0.13

–

RHE3

55

6.00

100–200

1.34 ± 0.04

2.83 ± 0.05

0.93 ± 0.02

75 ± 7.5

20 ± 5

1.87 ± 0.17

–

RHE4

54

3.50

150–200

1.46 ± 0.04

2.92 ± 0.06

0.84 ± 0.02

120 ± 12

20 ± 5

1.94 ± 0.13

KNO1

58

3.00

150–200

1.60 ± 0.06

4.16 ± 0.06

1.38 ± 0.02

130 ± 13

10 ± 2

2.76 ± 0.12

1.90 ± 0.06

–

KNO2

59

3.60

150–200

1.40 ± 0.04

3.99 ± 0.08

1.51 ± 0.03

120 ± 12

10 ± 2

2.80 ± 0.12

1.95 ± 0.06

KNO3 KNO4

61 60

1.60 1.70

100–200 100–200

2.71 ± 0.04 2.74 ± 0.07

7.76 ± 0.09 7.31 ± 0.12

1.37 ± 0.03 1.42 ± 0.04

145 ± 14.5 145 ± 14.5

10 ± 2 10 ± 2

3.36 ± 0.20 3.28 ± 0.17

2.35 ± 0.08 2.38 ± 0.09

BRE1

63

3.50

150–200

0.90 ± 0.03

3.08 ± 0.06

1.29 ± 0.02

120 ± 12

10 ± 2

2.39 ± 0.10

1.60 ± 0.05

BRE2

62

3.40

150–200

1.17 ± 0.03

3.95 ± 0.08

1.28 ± 0.03

120 ± 12

10 ± 2

2.53 ± 0.11

1.70 ± 0.06

HUP1

66

8.00

150–200

1.46 ± 0.03

4.05 ± 0.06

0.90 ± 0.02

55 ± 5.5

25 ± 5

1.91 ± 0.13

1.12 ± 0.10

HUP2

65

7.50

100–200

1.28 ± 0.03

3.92 ± 0.08

1.24 ± 0.03

50 ± 5

25 ± 5

2.04 ± 0.18

1.32 ± 0.10

HUP3

64

4.50

200–250

1.40 ± 0.04

4.36 ± 0.08

1.45 ± 0.03

90 ± 9

25 ± 5

2.47 ± 0.18

1.54 ± 0.10

ppm parts per million

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Table 4 Data of radioisotopes, as determined by gammaspectrometry Sample

LUM

Uranium Th-234 (Bq/kg)

Thorium Bi-214 (Bq/kg)

Pb-214 (Bq/kg)

Pb-210 (Bq/kg)

Ac-228 (Bq/kg)

Tl-208 (Bq/kg)

Pb-212 (Bq/kg)

HER 1

50

14.21 ± 2.71

16.73 ± 1.14

16.71 ± 1.11

16.60 ± 2.32

13.78 ± 1.43

12.86 ± 1.34

11.54 ± 0.25

HER 2

49

14.13 ± 3.13

15.51 ± 0.89

13.77 ± 0.53

14.83 ± 1.62

13.55 ± 1.36

13.50 ± 1.27

13.10 ± 0.30

WYL 1

52

16.50 ± 2.97

17.00 ± 0.89

17.08 ± 0.71

27.60 ± 4.65

18.69 ± 1.29

14.73 ± 0.70

14.76 ± 0.32

WYL 2

51

12.79 ± 2.27

14.11 ± 1.16

8.91 ± 0.61

20.06 ± 2.82

16.72 ± 1.48

14.25 ± 1.11

13.91 ± 0.30

WYL 3

53

13.17 ± 2.07

13.78 ± 0.40

14.19 ± 0.48

15.20 ± 2.28

13.67 ± 1.32

12.41 ± 1.17

11.81 ± 0.23

RHE 1

57

13.31 ± 2.62

14.41 ± 0.50

16.34 ± 0.60

16.89 ± 2.23

12.82 ± 0.74

11.72 ± 0.62

10.87 ± 0.27

RHE 2 RHE 3

56 55

20.32 ± 3.46 15.19 ± 2.83

19.24 ± 0.94 17.34 ± 0.80

18.78 ± 0.63 16.30 ± 0.59

22.27 ± 3.00 18.17 ± 2.91

14.94 ± 1.27 12.72 ± 0.78

13.50 ± 1.58 10.53 ± 0.53

13.27 ± 0.32 11.56 ± 0.25

RHE 4

54

16.17 ± 2.35

19.10 ± 1.01

17.86 ± 0.58

22.55 ± 3.63

16.15 ± 1.18

12.51 ± 0.96

11.58 ± 0.27

KNO 1

58

18.45 ± 2.77

19.13 ± 0.91

21.10 ± 1.25

21.57 ± 2.90

18.31 ± 1.19

17.45 ± 1.00

16.74 ± 0.28

KNO 2

59

17.54 ± 3.15

17.74 ± 0.89

17.09 ± 0.67

22.70 ± 3.39

20.10 ± 1.55

16.63 ± 1.53

16.00 ± 0.33

KNO 3

61

26.56 ± 8.15

32.81 ± 1.86

34.26 ± 1.01

39.87 ± 4.11

32.13 ± 2.35

28.72 ± 2.06

29.59 ± 0.51

KNO 4

60

32.33 ± 3.95

33.26 ± 0.71

34.87 ± 0.82

30.05 ± 1.94

35.64 ± 1.70

30.85 ± 0.90

31.36 ± 0.43

BRE 1

63

10.75 ± 2.75

10.91 ± 0.41

11.88 ± 0.61

11.45 ± 1.83

14.59 ± 1.16

21.28 ± 3.03

12.33 ± 0.25

BRE 2

62

9.18 ± 3.06

14.72 ± 1.15

14.44 ± 0.45

19.22 ± 2.67

18.82 ± 1.82

13.57 ± 2.43

15.00 ± 0.34

16.79 ± 0.92

HUP 1

66

17.17 ± 2.30

17.03 ± 1.01

18.24 ± 0.50

22.52 ± 2.33

18.04 ± 1.18

HUP 2

65

15.58 ± 2.81

15.29 ± 0.55

16.47 ± 0.63

18.54 ± 1.68

17.96 ± 1.15

HUP 3

64

14.20 ± 3.74

17.91 ± 0.93

16.94 ± 0.65

25.42 ± 3.25

24.62 ± 1.51

16.32 ± 0.27 15.75 ± 0.32

15.33 ± 1.94

17.42 ± 0.35

Table 5 Equivalent dose (De) values in Gray (Gy) and luminescence age estimates in 1,000 years (ka) Sample

LUM

Delay (days)

Equivalent dose in Gray (Gy) MAAD-Fsp

HER1

50

43

SAR-Fsp

75.0 ± 6.5

74.2 ± 1.2

SAR ratio

OSL age estimate in 1,000 years

SAR-Qz

Qz/Fsp

MAAD-Fsp

SAR-Fsp

SAR-Qz

58.7 ± 3.0

1.23

38.1 ± 4.7

37.7 ± 3.4

46.4 ± 4.1

HER2

49

45

66.0 ± 5.7

123.4 ± 2.5

48.7 ± 9.0

0.61

32.9 ± 4.0

61.5 ± 5.5

37.3 ± 7.4

WYL1

52

40

21.0 ± 5.9

14.8 ± 0.2

18.0 ± 0.5

1.82

8.28 ± 2.36

5.83 ± 0.28

10.6 ± 0.5

WYL2

51

41

–

15.3 ± 0.3

24.2 ± 0.8

2.35

WYL3

53

40

73.6 ± 2.7

72.6 ± 1.8

60.7 ± 4.5

1.45

42.4 ± 1.6

RHE1

57

47

85.6 ± 3.6

90.4 ± 1.5

RHE2

56

42

119.0 ± 4.7

104.1 ± 1.8

–

– 41.7 ± 3.2

6.97 ± 0.50

16.4 ± 0.8

41.1 ± 3.0

59.6 ± 6.2 36.7 ± 2.9

0.78

44.6 ± 3.5

47.1 ± 3.2

–

63.1 ± 5.1

55.2 ± 4.0

– –

RHE3

55

41

115.7 ± 2.8

110.5 ± 1.9

–

–

61.7 ± 5.8

59.0 ± 5.4

RHE4

54

41

140.8 ± 2.0

150.5 ± 3.2

–

–

72.4 ± 4.9

77.4 ± 5.4

KNO1

58

51

26.5 ± 2.2

24.5 ± 0.4

1.79

9.60 ± 0.90

8.88 ± 0.41

8.66 ± 0.90

30.2 ± 1.0

KNO2

59

48

24.2 ± 2.3

24.4 ± 0.4

29.5 ± 1.0

1.73

KNO3

60

49

–

22.4 ± 0.4

31.2 ± 1.1

1.99

22.6 ± 0.4

27.5 ± 0.9

1.68

2.1 ± 0.1

3.0 ± 0.3

2.16

KNO4

61

56

24.6 ± 2.3

BRE1

62

49

ca.2.7

BRE2 HUP1

63 66

50 47

ca.2.2 75.1 ± 4.7

2.4 ± 0.1 61.9 ± 1.0

2.4 ± 0.1 37.2 ± 1.1

1.47 1.02

HUP2

65

48

80.0 ± 3.0

61.8 ± 1.2

47.8 ± 1.4

HUP3

64

50

110.5 ± 3.3

108.0 ± 1.8

57.6 ± 2.0

inconsistent with unity, thus these OSL dates were rejected. IRSL and OSL results correlate with a Middle Pleniglacial deposition age of the fluvial deposits from below the event

– 7.50 ± 0.80 –

– 15.9 ± 0.7

8.73 ± 0.39

15.1 ± 0.7

6.67 ± 0.42

13.3 ± 0.7

6.89 ± 0.37

11.6 ± 0.6

0.88 ± 0.06

1.9 ± 0.2

– 39.3 ± 3.7

0.95 ± 0.06 32.4 ± 2.3

1.4 ± 0.1 33.1 ± 3.0

1.20

39.1 ± 3.8

30.2 ± 2.7

36.2 ± 3.0

0.63

44.7 ± 3.4

43.7 ± 3.2

37.5 ± 3.1

layer (Fig. 13). The uppermost sample was collected from outwash sands. These deposits were most likely transported by suspension in a highly dynamic fluvial system,

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including the transport of blocks. The outwash sands gave significantly higher IRSL age estimates of 72.4 ± 4.9 and 77.4 ± 5.4 ka, which are stratigraphically inconsistent most likely owing to insufficient bleaching prior to deposition. At the Herten section, two samples taken from below the event layer gave IRSL age estimates of 61.5 ± 5.5 and 32.9 ± 4.0 ka. The quartz extracts yielded OSL age estimates of 46.4 ± 4.1 and 37.3 ± 7.4 ka (Fig. 13), thus correlating to Middle Pleniglacial deposition ages. At the Wyhlen section, three samples were taken between 1.50 and 5.30 m below surface. The OSL age estimates of the two upper samples taken from aeolian or fluvio-aeolian deposits range from 16.4 ± 0.8 to 10.6 ± 0.5 ka. The lowermost sample from the Wyhlen section, taken from a sand lens intercalated in coarsegrained sediments about 5.30 m below surface, yielded an OSL age estimate of 59.6 ± 6.2 ka. A hiatus is most likely to occur between the fluvial and fluvio-aeolian sediments (Fig. 13). The Hupfer (Haltingen-Holcim) section, the Knobel section and the Bremgarten section are located downstream from the city of Basel, making an input of local igneous and metamorphic material from the Vosges and Black Forest very likely. Three samples were taken at the Hupfer section at 4.50, 7.50 and 8.00 m below surface. The quartz grains gave OSL age estimates between 36.2 ± 3.0 and 27.5 ± 3.1 ka (Fig. 13). A deposition age of the fluvial sediment during the Middle Pleniglacial is most likely. Four samples were taken between 1.60 and 3.60 m below surface at the Knobel section. OSL age estimates between 15.9 ± 0.7 and 11.6 ± 0.6 ka were determined. The IRSL age estimates underestimate the OSL ages up to about 100% indicating a significant amount of anomalous fading for these samples. A period of fluvial aggradation during the last Late Glacial is most likely. Two samples were taken from a sand lens at a depth of 3.40 and 3.50 m below surface at the Bremgarten section. The IRSL age estimates gave 0.9 ± 0.1 and 1.0 ± 0.1 ka, whereas OSL age estimates yielded 1.9 ± 0.2 and 1.4 ± 0.1 ka. It is likely that the OSL age estimates for the two samples from Bremgarten are slightly overestimated owing to the chosen preheat temperature of 260°C causing thermal transfer. A trunk of an oak tree was sampled from the fluvial deposits at a depth of 3.00 m below surface from the Bremgarten section and studied by radiocarbon dating. The sand most likely correlates to the top of the second fining-up cycle, whereas the trunk postdates the deposition of the second fining-up cycle. The wood gave a conventional radiocarbon age of 510 ± 45 BP (Hv 25350) resulting in a calibrated age of AD 1330–1440. The chronological results are stratigraphically consistent and in agreement with geological estimates.

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Discussion In this study, we are confident that most of the quartz grains yielded reliable results and uncertainties for De values up to 60 Gy. The reason for the odd behaviour of quartz grains from the Rheinheim section are probably related to the presence of quartz from igneous and metamorphic rocks from the Alps and the Black Forest. Similar problems were previously noted for sediments from the Alpine Foreland (Klasen et al. 2006). Feldspar grains up to 150 Gy are about in the linear range of the growth curve (Fig. 12). Anomalous fading is very likely the reason for significant age underestimation of the feldspar grains compared to the results of the quartz grains. Similar results, including the methodological problems of anomalous fading, were obtained for aeolian sand from the Mackenzie delta in Northern Canada (Huntley and Lamothe 2001; Murton et al. 2007). Both IRSL and OSL dating yielded very likely age overestimation for the fluvial sands from the Bremgarten section, if compared with radiocarbon data although the OSL samples were taken from stratigraphically older horizons. A similar behaviour was described for samples from historically deposited sediments from the RhineMeuse system in the Netherlands (Wallinga et al. 2001). The likely reason for this age overestimation is insufficient bleaching prior to deposition. In this study, the De values were determined from large aliquots, which are not suitable for detecting poor bleaching. However, the results are in stratigraphic order, and small offsets for the youngest samples indicate that insufficient bleaching seems to be not dramatic, as suggested for the samples from Bremgarten, for which independent age control is available (Frechen et al. 2008; La¨mmermann-Barthel et al. 2009). It is still under discussion whether changes in sensitivity during the measurement of the natural signal or contamination of the quartz fast component OSL signal by a less stable slow component are responsible for the IRSL age overestimation/OSL age underestimation. It remains risky to extrapolate the results of Holocene fluvial sediments to those fluvially transported and deposited during the Wu¨rmian (last) glaciation. However, the fluvial transport mode of the sediment grains was very likely similar. Wallinga et al. (2001) found that the feldspar from older samples significantly underestimates the geological expected age and the quartz OSL age estimates and suggested that IRSL age estimates are erroneously young for the entire age range, partly caused by changes in trapping probability due to the applied preheating procedure or anomalous fading. The studied feldspars from the Hochrhein Valley and the URG give evidence for an IRSL age underestimation of up to 100%, if compared with

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Fig. 12 Growth curves of samples HER1 (upper), WYL3 (middle) and KNO3 (lower) and De values, as determined for potassium-rich feldspars using the MAAD protocol

quartz results and apparently confirm the observations of Wallinga et al. (2001). In this study, OSL age estimates are used for the chronostratigraphic interpretation only. The OSL data set is stratigraphically consistent for most of the samples strengthening our approach in sampling overbank deposits from intercalated sand lenses (Figs. 13, 14). In the Hochrhein Valley at the Rheinheim section and at the Wyhlen section, fluvial sediments more than 15-m thick were deposited between 59.6 ± 6.2 and 36.7 ± 2.9 ka. During that time period, interstadial conditions occurred in the Swiss Midlands, as evidenced by lignite horizons of the Gossau-Interstadial-Complex, which correlates to MIS 3

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(Fig. 15) (Geyh and Schlu¨chter 1998; Preusser et al. 2003; Frechen et al. 2006). During that time period, fluvio-aeolian coversands were deposited along the Meuse River in Belgium and the Netherlands (Frechen et al. 2001; Frechen and van den Berg 2002) and loess intercalated by weak soil horizons accumulated in the URG (Zo¨ller and Lo¨scher 1999; Lang et al. 2003; Bibus et al. 2007; Frechen et al. 2007). Busschers et al. (2005, 2007) described mediumgrained fluvial sediments from the River Rhine in the west-central Netherlands and determined quartz OSL age estimates ranging from 40 to 30 ka (cores Zegveld, Leidschendam and Wassenaar). At the Herten section and at the Hupfer section (Fig. 2), OSL age estimates are between 46.4 ± 4.1 and 37.5 ± 3.1 ka resulting in a correlation of these sediments with MIS 3. Interstadial remains have not been found from that period in the study area. A radiocarbon age of 32,500 ± 620 BP (Hv 12531) was determined for wood pieces intercalated in fluvial deposits between 11 and 15 m below surface from the Groß-Rohrheim section located in the northern part of the URG. The radiocarbon age makes a correlation with OIS 3 likely (von Koenigswald and Beug 1988; Frechen et al. 2006). In the southern part of the URG, the Pleniglacial fan deposits have been reworked partly in a braided river system. Climate-induced changes in fluvial activity led to a distinct stepwise progradation of coarse sediments downstream, which takes place in form of alluvial fans resulting in incision and reworking of an alluvial fan. OSL age estimates of intercalated sand lenses of the upper 4 m of the sediment successions from the Knobel section and the upper 2 m of the deposits from the Wyhlen section yielded Late Glacial and Early Holocene deposition ages (Fig. 13; Table 5). In the Luthern valley in Switzerland, gravel deposits yielded IRSL age estimates ranging from 13 ± 1 to 15 ± 3 ka (Preusser et al. 2001), but fading correction was not applied. In the Middle Rhine area, Choi et al. (2006) investigated modern and known age Pleniglacial and Late Glacial fluvial sands from the older and younger Lower Terrace of the River Rhine near Bad Breisig (cp. Boenigk and Frechen 2006). Some of the age ranges given by OSL overestimated the geological interpretation by a few thousand years, most likely owing to insufficient bleaching. Choi et al. (2006) explained that the scatter of the De values not only relates to insufficient bleaching but also to microdosimetry. Heterogenous radiation fields, including an unequal distribution of radioisotopes in the sediment and the occurrence of insufficient bleaching prior to deposition, can make problems for the determination of De values, if small aliquots or single-grain techniques are applied (Mayya et al. 2006). Titanite and monazite are uranium- and thorium-rich minerals resulting in radioactive hot spots in the sediments, and thus giving reason for heterogenous microdosimetry, as previously described

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Fig. 13 Lithological sketch including sample position and OSL age estimates in ka (1 ka = 1,000 years). The correlation line gives the base (or hiatus) of the event horizon. The description of the sediment succession is found in the text

Fig. 14 Idealised sketch of accumulation periods and luminescence age results (IRSL age estimates in grey squares; OSL age estimates in black squares). A calibrated radiocarbon date (BRE 14C) is plotted as a diamond yielding an age of AD 1330–1440. Note that the samples from different sections were grouped with increasing depth below surface

by using spatially resolved detection of luminescence (Greilich and Wagner 2006). The uppermost 1–2 m of the fluvial sediment successions from the Wyhlen section and from the Knobel section accumulated between 11.6 ± 0.6 and 10.6 ± 0.5 ka. These OSL age estimates indicate an erosion of the ‘‘Hochgestade’’ material and re-deposition of these eroded sediments north of the Kaiserstuhl-Colmar line. The lowering of the base level of the Rhine course led to an increased erosion of the Younger Dryas fan. In the Middle and Lower Rhine area, incision of the Rhine took place prior to the deposition of the older lower terrace (Boenigk and Frechen 2006). High mass accumulation rates of dust were likely to occur in the transition between the Pleniglacial and the Late Glacial period (Frechen et al. 2003, 2007; Bibus et al. 2007). Early summer floods were able to transport blocks; whereas a dry surface and dry flood plain remained during the rest of the year enabling an extended deflation of silt-sized and

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sand-sized material. There seems to be some correlation between increased mass accumulation rates in the loess record (silt and fine sand, dust) and periods of increased aggradation in the fluvial record (sand, gravel). In the study area, the major period of dust accumulation took place during the last Pleniglacial and the Late Glacial (Frechen et al. 2003, 2007). Major fluvial aggradation periods of sand and gravel occurred in the Hochrhein and URG during the Late Glacial and the Middle Pleniglacial, as determined by OSL age estimates in this study. However, due to highly dynamic transport processes, as evidenced by poorly sorted coarse components including Helvetic limestone up to large block (car) size in fine-grained material (suspension), most of the fluvial deposits are not suitable for OSL dating, and so the chronological frame for the fluvial sediment successions remains discontinuous. During the Pleniglacial and Late Glacial period, the sediments from below the older lower terrace of the River Rhine were deposited under a braided-river system in the Middle Rhine area and the Lower Rhine Embayment, prior to the eruption of the Laacher See independently dated to about 12.9 ka by 40Ar/39Ar dating, radiocarbon dating, luminescence dating and varve counting, as summarised in Boenigk and Frechen (2006). The older lower terrace most likely covers older terrace sediments but cannot be distinguished from older deposits owing to the lack of an appropriate dating method. Cordier et al. (2005, 2006) found evidence for a third ‘‘Lower Terrace’’ along the river Moselle, as the main tributary of the Rhine in the Middle Rhine area, which most likely accumulated during the early Pleniglacial. The older lower terrace most likely correlates to the Last Pleniglacial and/or Late Glacial. The sediments from below the younger lower terrace of the River Rhine were deposited after the cataclysmic eruption of Laacher See, as evidenced by pumice intermingled in the gravel, and correlate with the Younger Dryas (Litt et al. 2008). The surface of the younger lower terrace is about 2 m lower than the surface of the older lower terrace. Fluvio-aeolian sediments from Belgium and the Netherlands gave average

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Fig. 15 Comparison of an idealised sequence of the fluvial record from the Hochrhein Valley and the Southern URG with the an idealised sequence from the Swiss Midlands, Gossau section and Zell section as summarised in Frechen et al. (2006). The luminescence age estimates are given in ka, whereas the 14C age is given in years before present (BP)

IRSL age estimates of 15.8 ± 1.8 and 10.3 ± 1.0 ka (without fading corrections and thus very likely age underestimated) and average OSL age estimates of 15.8 ± 1.8 and 12.5 ± 1.1 ka for the Younger and Older Coversands, respectively (Bateman and van Huissteden 1999; Frechen et al. 2001). These luminescence age constraints were confirmed by calibrated radiocarbon ages ranging from 16,800 to 10,200 BP (Bateman and van Huissteden 1999). The older and younger coversands most likely correlate with the sediments of the older lower terrace. Busschers et al. (2007) performed quartz OSL dating on fluvial deposits from the Rhine-Meuse fluvial system in the Netherlands resulting in a more reliable chronological frame for the sedimentary evolution of this large fluvial system in the Netherlands for the time period of the penultimate and last interglacial/glacial cycles. Furthermore, Busschers et al. (2007) presented quartz OSL data ranging from 24 to 14 ka (Delft II core) for fluvial deposits from the River Rhine in the west-central Netherlands, which were deposited coevally with the older Coversands. These coarse-grained sediments have a thickness of about 10 m and correlate with the older lower terrace indicating a Pleniglacial/Late Glacial deposition age. The younger drift sands from the Netherlands were deposited between 10 and 7 ka and most likely correlate to an Early Holocene aggradation period. It becomes obvious that the Holocene fluvial history of the Rhine system is a result of a complicated interaction of neotectonics, discharge variations, within channel sedimentation and human impact in the Netherlands and elsewhere.

Luminescence and radiocarbon dating results give evidence for a short period of major erosion and re-sedimentation of fluvial sediments from the ‘‘Tiefgestade’’ at the Bremgarten section between 500 and 600 years before present. This time period correlates with the beginning of the Little Ice Age lasting from about AD 1450 to 1850 (Bork 1989). Furthermore, an extreme weather event resulting in a millennium flood is described for the year AD 1342. This flood caused a significant alteration of landscape such as up to 15 m deeply incised canyons and reworking of 6–8-m thick fluvial sediments as debris flows in southern Germany (Bork, 1989). The weather anomaly of AD 1342 could be very likely the reason for the fluvial dynamics, including the aggradation of fluvial sediments more than 3-m thick in the southern URG. A serious historical earthquake around the city of Basel happened in the year AD 1356. It remains unlikely that earthquakes could have triggered a flood causing erosion and accumulation of about 3.50 m of gravel at the Bremgarten section. In the Netherlands, the youngest drift sands from the Maas River yielded a radiocarbon age of \1,000 BP and an OSL age estimate of 0.6 ± 0.1 ka (Bateman and van Huissteden 1999), and so might be correlated with the remobilised fluvial sediments from the southern URG. The present data set leaves us uncomfortably with questions about the linkage of the aggradation periods with climate forcing of the Rhine system in southern Germany. The results contradict the cold climate origin of terrace sediments in the southern Upper Rhine Graben and open up new challenging questions. Methodological problem such

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as the degree of bleaching during fluvial transport is still difficult to determine, although age overestimation in large river systems seems to be normally small. The quartz SAR protocol is very likely the best method to apply OSL dating for last glacial fluvial deposits. This is part of an ongoing study investigating the timing of periods of increased aggradation in the Rhine system highlighting the response of a Central European large river system situated in a tectonically active region to environmental and climate change.

Conclusion The chronology of the Last Glacial and Holocene deposits was studied by feldspar IRSL and quartz OSL dating. The OSL age estimates are in excellent agreement with the stratigraphical concept of sedimentary dynamics in the southern Upper Rhine Graben. All samples from below the coarse-grained sediment of the event layer, alternatively the discontinuity, yielded OSL age estimates older than 20 ka, whereas all samples from above the event layer gave OSL age estimates younger than 20 ka. However, the complicated puzzle of the complex Hochrhein terrace landscape and the sediment successions along the URG requires more precise and reliable dating to support our stratigraphic approach, which falsify the traditional terrace stratigraphy. Dating of deposition ages is the only means to correlate the patchy distributed sedimentary unit, which does not show any significant changes in petrography for this time period. The validity of SAR protocols in OSL dating was demonstrated by the application to known-age samples, but for samples pre-dating the Holocene further evidence is required to prove the accuracy and reliability of OSL dating for fluvial sediments. Acknowledgments This study was funded by the Deutsche Forschungsgemeinschaft (DFG) (HI 643/2-3), which is appreciated. Sabine Mogwitz, Petra Posimowski and Sonja Riemenschneider are thanked for their excellent technical support and, last but not least, Juliane Herrmann for the art-work.

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