Timing of Medieval Fluvial Aggradation at Bremgarten in the Southern Upper Rhine Graben – a Test for Luminescence Dating

The Holocene fl ood plain of the River Rhine is a complex dynamic sedimentary system. A series of geochronological results for the Bremgarten section including optically stimulated luminescence (OSL) and radiocarbon dating was determined to improve the understanding of part of the Holocene evolution of the River Rhine. The applied single aliquot regenerative (SAR) protocols and the applied experimental studies to fi nd the best luminescence behaviour leave us with confi dence that OSL dating is a suitable method for dating fl uvial sediments from large river systems. Insuffi cient bleaching of the sediments from Bremgarten prior to deposition seems to be not as dramatic as previously thought. OSL and radiocarbon dating results give evidence for a short period of major erosion and re-sedimentation of fl uvial 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 at about AD 1450. Several severe fl oods occurred in Southern Germany between AD 1500 and 1750; all those fl oods correlate to the period of the Little Ice Age, including the destruction of the village of Neuenburg AD 1525. [Die zeitliche Stellung einer mittelalterlichen fluvialen Sedimentakkumulation im südlichen Oberrheingraben am Beispiel Bremgarten – ein Test für Lumineszenz-Datierungen] Kurzfassung: Die holozäne Überfl utungsebene des Rheins ist ein komplexes sedimentäres System. Die geochronologischen Untersuchungen mittels Optisch Stimulierter Lumineszenz (OSL) und Radiokohlenstoff-Datierungsmethode verbessern das Verständnis der holozänen Entwicklung des Rheinsystems und ermöglichen quantitative Studien zu Aggradationsphasen von Flüssen. Die angewandten Single-Aliquot-Regenerierung (SAR)-Protokolle und die angewandten experimentellen Untersuchungen zur Absicherung der Messparameter lassen erkennen, dass die OSL-Datierungstechnik eine geeignete Datierungsmethode für fl uviatile Sedimente aus großen Flusssystemen ist. Eine unvollständige Bleichung der fl uviatilen Sedimente vor der Ablagerung scheint für die Sedimente aus Bremgarten nicht so dramatisch zu sein wie ursprünglich vermutet. OSLund Radiokohlenstoff-Datierungsergebnisse bestätigen eine kurze Periode starker Erosion und Akkumulation im Bereich des Tiefgestades von Bremgarten zwischen 500 und 600 Jahren vor heute. Dieses Zeitfenster korreliert mit dem Beginn der Kleinen Eiszeit um ca. AD 1450. Mehrere schwere Flussüberschwemmungen geschahen in Süddeutschland zwischen AD 1500 und 1750. Diese Überschwemmungsereignisse korrelieren mit dem Zeitfenster der Kleinen Eiszeit, darunter die Zerstörung von Neuenburg im Jahre 1525.


Timing of Medieval Fluvial Aggradation at Bremgarten in the Southern Upper Rhine Graben -a Test for Luminescence Dating 1 Introduction
Investigating terrestrial sediment archives of the past is imperative to learning and understanding the future impact of climate and environmental changes.The Medieval climate optimum and the Little Ice Age are two historically recorded periods during which society was considerably infl uenced by climate and environment change.Fluvial landscapes in modern fl oodplains and lower terraces are particularly sensitive to climate change.Extreme weather events such as the millenium fl ood of the year AD 1342 had a catastrophic impact on Man and the society and indeed caused a signifi cant alteration of landscape, such as up to 15 m deeply incised canyons and reworking of up to 8 m thick fl uvial sediments in southern Germany (BORK 1989).The extent of the fl uvial dynamics and the temporal succession of such events are not yet understood in either large or small rivers.
Extensive research has been carried out in the catchment area of the River Rhine on Pliocene and Pleistocene terrace formation (e.g.BOENIGK & FRECHEN 2006;HAGEDORN & BOENIGK 2008, WESTERHOFF 2008; and references within) demonstrating the complexity of this major fl uvial sediment archive.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 2001).Increased aggradation in the southern Upper Rhine Graben (URG) postdates the fi lling up of the Lake Constance (Bodensee) Basin and occurred synchronous with periods of major ice melting (WESSELS 1998).During these highly dynamic periods, including the glacial overdeepening of Alpine valleys, the peri-alpine basins were deeply eroded.Late Glacial and Holocene fl uvial aggradation and terrace formation are triggered by climate change and tectonic processes as well as human-induced adjustment of river systems (SCHIRMER et al. 2005;BOS et al. 2008;ERKENS et al. 2009;LÄMMERMANN-BARTHEL et al. in press).During the Little Ice Age, signifi -cant erosion and aggradation occurred along the Rhine.
Little is known about the timing of the Holocene fl uvial activity of the River Rhine in the southern Upper Rhine Graben (URG).Historical data is available for major fl ood events during Medieval times.Numerical dating of fl uvial sediments is problematical, as described in detail for the Middle and Lower Rhine area by BOENIGK & FRECHEN (1998, 2006) and the northern Upper Rhine Graben by ERKENS et al. (2009) andFRECHEN et al. (2006).During the past 15 years, much attention has been paid on the investigation of the timing of fl uvial activity in the Rhine system.Fluvial sediments are particularly suitable for the application of optically stimulated luminescence (OSL) dating techniques (BUSSCHERS 2008;JAIN et al. 2003;WALLINGA 2002).In this study, OSL dating on fi ne-sand and radiocarbon dating on wood was carried out on Holocene material from the gravel pit at Bremgarten in the southern part of the Upper Rhine Graben.The aim of this study is to test the suitability of fl uvial sediments for OSL dating and to give a more reliable chronological interpretation for the Medieval fl uvial activity in the southern part of the URG.This is part of an ongoing study investigating fl uvial sediments from the Rhine system in the frame of the Heidelberg Drilling Project.

Geological setting
The Upper Rhine Graben is located between the cities of Basel and Mainz.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 Mainz.The River Rhine fl ows in numerous meanders northwards, which were partly regulated in the years between 1817 and 1874.The maximum subsidence of the Quaternary is located in the Heidelberg Basin near to the city of Heidelberg.BARTZ (1951BARTZ ( , 1967BARTZ ( , 1974) ) MANFRED FRECHEN, DIETRICH ELLWANGER, DANIEL RIMKUS & ASTRID TECHMER   2008) and at Viernheim, is currently aiming to reconstruct in detail the sediment archive of the basin fi ll (Fig. 1) (ELLWANGER et al. 2005).A zone of elevated seismic activity is 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 (HÜTTNER 1991) resulting in 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).NIVIÈRE et al. ( 2008) calculated maximum vertical movements along the faults not exceeding 1.0 mm/yr since the Middle Pleistocene.The current activity is concentrated along the westernmost faults.The coordinates of the section under investigation are H5309115 and R3394285 of the Gauss-Krüger system used in German topographic maps.The gravel pit Bremgarten GmbH is located about 150 m east of the river Rhine and situated in the lowermost part of the Lower Terrace termed "Tiefgestade" in the vicinity of the villages Bremgarten and Hartheim.About 7 m thick sand and gravel successions are exposed including pebbles originating from the Swiss Alps and the Black Forest.The upper part of the sediment succession is coarser than the lower part.A sand lens is covered by not wellsorted gravel about 2 m thick sand-rich.Up to three horizons can be distinguished and make three deposition events likely (Fig. 2, 3).Wood 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 a fi ning up cycle.The trunk has a north-south orientation, which is very likely the transport direction in the river at the time of deposition (Fig. 2).The lower part of the tree has still its bark, whereas the upper part of the trunk is bare of bark, most likely owing to erosion by water and sediment immediately after deposition of the trunk.Sample BRE3 was taken 20 cm above the trunk and sample BRE4 was taken from about 20 cm below the trunk for OSL dating.Samples BRE1 and BRE2 were taken from a sand lens intercalated in light-greyish gravel from the eastern quarry wall in 2003 (Fig. 3).A piece of wood was intercalated in the sand lens, as previously described by LÄMMERMANN-BARTHEL et al. (in press) and used for radiocarbon dating.
Massive Alpine sediment supply into the URG is unlikely for Medieval times, as lake Constance again formed as a sediment trap for the River Rhine (WESSELS 1998).In the geological past, massive Alpine sediment supply probably resulted in a lateral movement of the Rhine over the whole southern URG (LÄMMERMANN-BARTHEL et al. in press).

Luminescence dating
Luminescence dating of aeolian and fl uvial sediments has proved to be successful where radiocarbon and other dating methods are not applicable (FRECHEN et al. 1997;JAIN et al. 2003;LIAN & ROBERTS 2006;RITTENTOUR 2008).Since the late 1980s, luminescence dating of sediments has signifi cantly been improved by the development of more light sensitive techniques such as green or blue optically stimulated luminescence (OSL) or infrared optically stimulated luminescence (IRSL) for monomineralic quartz or feldspar subsamples, respectively (HUNTLEY et al. 1985;HÜTT et al. 1988).A recent review about the lower and upper dating limit is found in WINTLE (2008).
The basic principle of luminescence dating is solid state dosimetry of ionising radiation (WINTLE 1997;AITKEN 1998;PREUSSER et al. 2008).
The quartz signal saturates at lower doses than the feldspar signal.However, the advantage is often offset by the larger dose rates for the feld-   1973).
An important assumption of luminescence dating techniques is that the mineral grains were suffi ciently long exposed to daylight/sunlight prior to deposition to reset the radiometric clock ("zeroing the luminescence signal").
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 are strongly affected by fl uvial sedimentary dynamics including water depth, sediment load, turbulence and turbidity, grain-size and transportation distance (RHODES & POWNALL 1994;FRECHEN 1995;MURRAY et al. 1995;WALLINGA et al. 2000;JAIN et al. 2003).
In many fl uvial environments the probability of complete zeroing of all sediment grains is low.MANFRED FRECHEN, DIETRICH ELLWANGER, DANIEL RIMKUS & ASTRID TECHMER is also attributed to fl ashy seasonal river fl ow (fl oods) associated with high-amplitude precipitation causing input from bank erosion (JAIN et al. 2003).The existence of scatter in equivalent dose determinations is an indication for incomplete zeroing of the OSL signal.Grain-to-grain variations do produce scatter in single aliquot equivalent dose determinations (LAMOTHE et al. 1994;MURRAY & ROBERTS 1997;OLLEY et al. 1999).The detection of partial bleaching was previously investigated by using small single aliquots or single-grain methodology (FUCHS & WAGNER 2003;SINGARAYER et al. 2005;PREUSSER et al. 2007) and different statistical approaches (LEPPER et al. 2000;BAILEY & ARNOLD 2006;ERKENS et al. 2009).Following these studies, it appears to be mandatory to investigate a large number of single grains per sample from fl uvial environments.
The fi rst fl uvial 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 (FRE-CHEN 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 Fig. 5: Preheat plateau, recycling ratio and recuperation for sample BRE3 using quartz extracts and the hot bleach procedure.
Timing of Medieval Fluvial Aggradation at Bremgarten removing the carbonates in 0,1 N hydrochloric acid, organic matter by 30 % hydrogen peroxide and clay particles by sodium oxalate.The sandsized feldspar grains with densities lower than 2.58 g/cm 3 were extracted using heavy liquids and the sand-sized quartz was extracted from the remaining fraction using heavy liquids of 2.62 g/cm 3 and 2.70 g/cm 3 densities.The quartz extracts were etched with 40 % hydrofl uoric acid for 60 minutes to remove feldspars and sieved again with a 100 μm mesh or 150 μm mesh.
The sand-sized quartz or feldspar grains were brought onto 0,9 mm steel discs (aliquots).IR stimulation was applied at the end of the quartz SAR protocol to test whether single aliquots show a contamination with feldspar deriving from inclusions after etching with hydrofl uoric acid.Single aliquot regenerative-dose protocols were carried out for monomineralic quartz and feldspar extracts to determine the De values more precisely (MURRAY & WINTLE 2000;WALL-INGA et al. 2000, 2001).All growth curves were fi tted using a saturating exponential function.
A number of tests were carried out on both quartz and feldspar extracts to investigate the luminescence characteristics and to evaluate the suitability of the applied SAR protocol for the samples under study.

Preheat plateau
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 Abb.9: "Dose recovery test", "Recycling Ratio" und "Recuperation" für Feldspäte der Probe BRE2.
Timing of Medieval Fluvial Aggradation at Bremgarten ing that for most of the samples the aliquots fall within 10 % of unity for the temperature range 200° to 220°C indicating that the SAR sensitivity correction is appropriate for these samples.Furthermore, a hot bleach treatment was used for the quartz extracts.After measurement of the response to the test dose, the aliquots were exposed to the blue diodes for 40 seconds whilst holding them at 280°C in order to reduce any build up of slow components in the OSL signal (Murray & Wintle, 2003).However, the recycling ratios concerning the hot bleach procedure for samples BRE3-quartz and BRE4-quartz fall only within 35 % of unity indicating that the sensitivity correction is problematic although the fi nal OSL age estimates are within 1-standard deviation in agreement with independent age control.The SAR proto-col for feldspar yielded recycling ratios within 10 % of unity for the preheat temperature range between 200° and 240°C.

Dose recovery test
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 confi rms that the applied SAR protocol enables to recover a known laboratory dose (RHODES 2000).After bleaching the aliquots with a dr.hönle solar simulator, the aliquots were given a known laboratory dose close to the De value.The artifi cially dosed aliquots were then treated as "natural" in the SAR protocol (Table 1) to determine the De value (here: Abb.10: "Preheat plateau", "Recycling Ratio" und "Recuperation" für Quarze der Probe BRE1. MANFRED FRECHEN, DIETRICH ELLWANGER, DANIEL RIMKUS & ASTRID TECHMER dose recovery).An example for dose recovery is given for sample BRE2-feldspar using a test dose of 1.0 Gy (Fig. 9).The dose recovery tests yielded the recovering dose values within 5 % from the given dose in the preheat temperature range 200°-220°C.An example for thermal transfer is given in Figure 7. Sample BRE4-quartz seems to be stable for the temperature range between 200° and 220°C but shows an increasing amount of thermal transfer above 220°C.Recuperation is insuffi cient over the whole temperature range for sample BRE4-quartz (Fig. 8), whereas recuperation is less than 10 % and even less than 5 % over the temperature range between 200° and 240°C for sample BRE2-feldspar and BRE1-quartz (Fig. 9, 10).The hot bleach procedure gives a recuperation of >20 % over the temperature range from 200° to 300°C for sample BRE4-quartz.

Single-aliquot regenerative dose protocol
The single-aliquot regenerative (SAR) dose protocol was applied to determine the De values for feldspar and quartz (MURRAY & WINTLE 2000;WALLINGA et al. 2000WALLINGA et al. , 2001;;WINTLE & MURRAY 2000).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 1).The weighted mean De value and the standard deviation were calculated for most of the samples Abb.11: "Preheat plateau", "Recycling Ratio" und "Recuperation" für Quarze der Probe BRE2.
Timing of Medieval Fluvial Aggradation at Bremgarten from 24 aliquots for both feldspar and quartz grains.In general, aliquots were taken into account within a 3-sigma standard deviation and a recycling ratio between 0.8 and 1.2.Most of the dose points for the natural signal plotted between fi rst and second artifi cial dose step.
The hot bleach procedure resulted in two outliers (samples BRE3 and BRE4) with recycling ratios of 0.87.The IRSL signal was measured with a Schott BG39/Corning 7-59 fi lter combination between photomultiplier and feldspar extracts.A Schott U-340 fi lter with a detection window of 290-370 nm was placed for OSL measurements between photomultiplier and quartz extracts.Blue light emitting diodes were used for quartz stimulation.
Equivalent dose determination was carried out using the software Analyst 6.0 (G.A.T. Duller, Aberystwyth, unpublished).Age calculation was applied by the software ADELE (M.Krbetschek, Freiburg, unpublished).

Dosimetry
As the outer shell of the feldspar minerals was not etched by hydrofl uoric acid, alpha effi ciency 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.Tl and 212 Pb for thorium and 40 K for potassium.An average internal potassium content of 12±0.5 % was applied for all feldspar samples (HUNTLEY & BARRIL 1997).Cosmic dose rate was corrected for the altitude and sediment thickness, as described by PRESCOTT & HUTTON (1994).The natural moisture content of the sediment was estimated between 10±2 weight % and 15±5 weight % for the samples. 2 and 3. Uncertainties are given in 1-sigma confi dence interval.The total dose rates range from 2.21±0.22 to 2.53±0.11Gy/ka and 1.60±0.05 to 1.70±0.06Gy/ka for feldspar and quartz, respectively.The D e values range from 0.6 to 1.1 Gy for quartz grains and from 1.6 to 2.0 Gray for feldspar extracts (Table 3), as determined by the SAR protocol (Table 1).Examples for the quartz and feldspar De-distributions of the single aliquot equivalent dose estimates and the radial plots of the same values are shown in fi gs.12-15.They give an impression on the complete resetting of the OSL-signal prior to deposition.The radial plot (Fig. 15) displays an expanded De-distribution for the feldspar sample BRE4 indicating an insuffi cient bleaching of these minerals.In  & WINTLE 2000;WALLINGA et al. 2000WALLINGA et al. , 2001)).
MANFRED FRECHEN, DIETRICH ELLWANGER, DANIEL RIMKUS & ASTRID TECHMER contrast the quartz (Fig. 12) of BRE4 (Fig. 14) shows a nearly typical Gaussian distribution indicating that the mean equivalent dose is an appropriate estimate for the burial dose.OSL age estimates are used for chronostratigraphic interpretation only.IRSL age estimates are used for methodological comparison in this study only.Two samples (BRE1 and BRE2) were taken from a sand lens at a depth of 3.40 m and 3.50 m below surface at the Bremgarten section in 2003.The IRSL age estimates gave 0.9±0.1 ka and 1.0±0.1 ka, whereas OSL age estimates yielded 1.9±0.2ka and 1.4±0.1 ka.It is likely that the OSL age estimates for the two samples from Bremgarten are overestimated owing to the chosen preheat temperature of 260°C causing thermal transfer.Further experiments were unfortunately not possible owing to the lack of material.The samples BRE3 and BRE4 were taken in 2006 at the eastern wall of the gravel pit.Sample BRE3 taken above a trunk, which was dated by radiocarbon, gave an OSL age estimate of 460±170 a, whereas the IRSL age estimate on feldspar extracts yielded 740±270 a.
The hot bleach procedure yielded an OSL age estimate of 370±130 a, slightly age underestimated if compared with the radiocarbon ages on wood from the same profi le.Age underestimation is also indicated by the dose recovery test.Sample BRE4 taken from below the trunk (Fig. 2) yielded OSL age estimates of 690±160 a and 600±190 a (hot bleach), which are in agreement within 1-sigma standard deviation with the results from sample BRE3.The IRSL age estimate is 890±60 a.A trunk of an oak tree was sampled from the fl uvial 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 fi ning-up cycle, whereas the trunk postdates the deposition of the second fi ning-up cycle.The wood is intercalated between the sand layers of samples BRE1 and BRE2.diffi cult to determine, although age overestimation in large river systems seems to be not as dramatical as previously described.The quartz SAR protocol is very likely the best method to apply OSL dating to fl uvial deposits.However, the IRSL age estimates, although not fading corrected, are also in agreement within the 1-sigma standard deviation, if compared to radiocarbon dates and so give also likely true deposition ages.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.
Fig. 3: Picture showing the position of sample BRE1.A second trunk was found 150 m in the north of the profi le under study and sampled for radiocarbon dating.Abb.3: Foto mit der Position der Probe BRE1.Ein weiterer Baumstamm wurde etwa 150 m nördlich dieses Profi ls für eine Radiokohlenstoff-Datierung entnommen.
JAIN et al. (2003),WALLINGA (2002) andSINGA- RAYER et al. (2005)  pointed out that in contrast to aeolian sediment a distribution of values deter-mined for fl uvial deposits usually results in age overestimation, if equivalent doses are measured for samples containing a large number of grains.FUCHS et al. (2005) reported that quartz based equivalent dose (D e ) values yielded distinctly lower results than feldspar based D e values for fl ood sediments giving evidence for better bleaching of quartz minerals extracted from the sediment.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 more likely to be suitable for luminescence dating.Samples taken from sediment of high-energy depositional environments like mass fl ow deposits or sediments from kames, are not suitable for luminescence dating(HÜTT & JUNGNER 1992).Partial bleaching of sediments

Fig. 4 :
Fig. 4: Idealised sketch of the lithological units and the geological interpretation and position of radiocarbon and luminescence samples.Abb.4: Idealisierte Abbildung der Sedimentabfolge in Bremgarten mit geologischer Interpretation und Position der Radiokohlenstoff-und Lumineszenzproben.

Table 2 :
Dosimetric results, as determined by gammaspectrometry (ppm=parts per million; H 2 O = moisture of sediment in weight %).

Table 3 :
Equivalent dose (D e ) values in Gray (Gy) and luminescence age estimates in 1000 years (ka) for quartz and feldspar extracts using SAR protocols.and the occurrence of insuffi cient bleaching prior to deposition can cause problems for the determination of De values, if small aliquots or single-grain techniques are applied (MAYYA et al. 2006).Titanite, monazite and zircon are uranium-and thorium-rich minerals resulting in radioactive hot spots in the sedi-ments and thus giving reason for heterogenous microdosimetry, as previously described by using spatially resolved detection of luminescence (GREILICH & WAGNER 2006).At about 2800 cal BP during the transition from Subboreal to Subatlantic, climate abruptly changed from a relatively warm and continental Timing of Medieval Fluvial Aggradation at Bremgarten ment 427 Fig. 12: Distribution of De values in increased order for quartz extracts of sample BRE4.Abb.12: Verteilung der De-Werte von Probe BRE4 (Quarz) in ansteigender Richtung angeordnet.