International 357 (2015) 110e124

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Quaternary International

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Luminescence dating of glaciofluvial deposits linked to the penultimate glaciation in the Eastern

* Lukas Bickel a, , Christopher Lüthgens a, Johanna Lomax b, Markus Fiebig a a Institute of Applied Geology, Department of Civil Engineering and Natural Hazards, University of Natural Resources and Life Sciences, Peter Jordan-Straße 70, 1190 Vienna, Austria b Department of Geography, Justus-Liebig-University Giessen, Senckenbergstraße 1, 35390 Giessen, Germany article info abstract

Article history: During the penultimate glaciation vast areas of the Alps were glaciated, with piedmont glaciers pro- Available online 6 November 2014 truding into the foreland. In the easternmost part of the northward draining valleys of the Alps, the glaciers did not reach the foreland, but formed valley glaciers confined by the mountainous terrain. This Keywords: also applies to the Ybbs valley, where samples for luminescence dating out of glaciofluvial gravel ac- Penultimate glaciation cumulations were taken at three locations along the present day river course. In a highly dynamic Deglaciation depositional environment, such as a glacier-fed river system, incomplete resetting of the luminescence Luminescence dating signal is possible, in particular when transport distances are short. In such cases, quartz usually is the Eastern Alps Glaciofluvial deposits preferred mineral over feldspar, especially if dose rates are low and may theoretically allow obtaining quartz ages beyond 150 ka. Because previous research has shown, and as corroborated within this study, quartz from the research area exhibits analytical problems in the high age range. Therefore luminescence properties of coarse grain (100e200 mm) quartz and in addition K-rich feldspar were investigated with the aim to reconstruct the chronology of the glacial processes within the Ybbs catchment area. Issues of incomplete bleaching were pIRIR225 encountered and addressed by comparing quartz OSL, fading cor- rected K feldspar IR50 and pIRIR225 to identify reliable ages. Depositional ages based on quartz OSL and feldspar pIRIR225 signals revealed deposition of ice marginal kame terraces and glaciofluvial foreland terraces during late to middle MIS 6. In combination with results from previous studies, we could reconstruct the valley evolution during the . Newly gained luminescence ages of the deglaciation in the easternmost Alps coincide with OSL dated deglaciation events in the Western Alps, indicating that climatic change along the north side of the Alps happened simultaneously. © 2014 The Authors. Published by Elsevier Ltd and INQUA. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).

1. Introduction morphologically distinguishable gravel levels in the German NAF, the quadriglacial system was amended by three additional glacials The Alpine region and its foreland played a major role in the (, Donau, Haslach) by Eberl (1930), Schaefer (1953), and investigation of Quaternary glacial and paleo-climatic processes Schreiner and Haag (1982). This morphostratigraphic model is still since the beginning of the 19th century (Agassiz, 1841). At the used in some alpine areas, but has been fine-tuned and amended beginning of the 20th century, Penck and Brückner (1909) devel- since then, especially in terms of the chronostratigraphic position oped the model of the “glacial series” in the German part of the of the deposits. However, clear genetic relations in terms of the northern Alpine Foreland (NAF), a concept that describes the ge- glacial series often can be ambiguous due to the complete lack of netic connection of basal till, terminal moraines and adjoining sedimentary remains and the often only poor preservation, espe- outwash gravels. From the vertical succession of four gravel terrace cially those of the oldest glacials which underwent several cycles of levels, they developed a morphostratigraphic model which attri- severe geomorphological changes during subsequent glaciations butes a to each of these geological units (old to and . young: Günz, Mindel, Riss, Würm). Based on three elevated, In this study we investigated the optically stimulated lumines- cence (OSL) and infrared stimulated luminescence (IRSL) properties of glaciofluvial quartz (Q) and potassium rich feldspar (KFs) deposited during the penultimate alpine glaciation along the * Corresponding author. eastern alpine Ybbs River (Fig. 1C). For the first time luminescence E-mail address: [email protected] (L. Bickel). http://dx.doi.org/10.1016/j.quaint.2014.10.013 1040-6182/© 2014 The Authors. Published by Elsevier Ltd and INQUA. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). L. Bickel et al. / Quaternary International 357 (2015) 110e124 111 ages from glaciofluvial sediments attributed to the penultimate foreland of the Swiss Alps (Fiebig, 2011). The type locality of sedi- glaciation from the eastern part of the alpine realm are presented. ments of the Riss glaciation, which is correlated with the penulti- mate glaciation, is located in the Riss valley near Biberach in 1.1. Penultimate glaciation Southern Germany (Penck and Brückner, 1909). Here, the deposits of the Rissian Rhine glacier record a total of three individual glacial Depending on regional lithostratigraphy, the penultimate advances (Older-, Middle- and Younger-Riss; Ellwanger et al., 2011). glaciation is termed the late Saalian glaciation in northern Europe In connection with these terminal moraines of the Riss, two gravel “ ” “ (Ehlers et al., 2011b), Illinosian glaciation in (Curry terrace levels can be found: The Upper High Terrace ( Obere ” “ ” “ et al., 2011), Guxiang glaciation in the Qinghai-Tibetan plateau Hochterrasse , Middle Riss) and the Lower High Terrace ( Untere ” “ ” (Shangzhe et al., 2011), and the Riss glaciation in the classic strat- Hochterrasse , Younger Riss), with the Upper High Terrace igraphic approach in the Alps (Van Husen and Reitner, 2011a) e all gravels deposited unconformably above gravels and basal till of the attributed to the late Middle (Cohen and Gibbard, Older Riss (Schreiner, 1989). A trichotomy of the Riss moraines is 2011). The most common chronostratigraphic interpretation is a also observed in the Bavarian and Upper Austrian part of the NAF time equivalence with Marine Isotope (MIS) 6, however, this including two distinct terrace levels (Van Husen, 1977; Kohl, 1999; is only rarely based on numerical dating. An important step to- Doppler et al., 2011). wards a time constraint of the penultimate alpine glaciation was In terms of stratigraphy, the difference between recent ap- presented by Drescher-Schneider (2000), who could place a sedi- proaches in the Rhine glacier area (Ellwanger et al., 2011) and the mentary sequence starting with a basal till to MIS 6e3 based on stratigraphic approach in the Eastern Alps (e.g. Van Husen and palynological findings. During the largest extent of the penultimate Reitner, 2011a) is the timing and quantity of reconstructed gla- alpine glaciation, ice advanced into the NAF, where it formed cials. While the German stratigraphic chart (Stratigraphic Table of piedmont-style glacier lobes in vast areas (Schlüchter, 1986; Ehlers Germany (2002)) subdivides the Riss into two discrete glacials et al., 2011a). At that time, glacier extent gradually decreased from separated by a full (Ellwanger et al., 1995; Miara et al., € west to east, with the most extensive piedmont glaciations in the 1996; Rogner, 2008), Ellwanger et al. (2011) propose a new

Fig. 1. A: Overview of the Alpine and Northern European ice extent during the penultimate glaciation (Ehlers and Gibbard, 2004) B: Ice extent of the Eastern Alps during the penultimate glaciation characterized by piedmont glaciers extending into the Swiss, German and Austrian NAF. Glacial cover of the easternmost part was not as extensive and glaciers often terminated within the mountainous area (e.g. Ybbs valley). ILL … Isar-Loisach glacier lobe, IL … Inn glacier lobe, SL … Salzach glacier lobe C: Study area. Quaternary units are redrawn according to Ruttner and Schnabel (1988), Krenmayr et al. (2006) and Untersweg et al. (2012). Dashed lines represent borders of major geological units (NCA ¼ Northern Calcareous Alps of the Austro-Alpine mega unit, PEN ¼ Penninic and Helvetic Units, MOL ¼ Tertiary molasse, BM ¼ Bohemian Massive). Investigate sites indicated by stars, MAU ¼ Mauer, OHO ¼ Oberhombach,€ TOI ¼ Tonibauer/Hochau. D: Detailed view of the inner alpine surroundings of sampling site TOI. Dashed rectangle in figure C indicates the position of the close up. 112 L. Bickel et al. / Quaternary International 357 (2015) 110e124 stratigraphy for the Rhine glacier area in which the Riss is attrib- Also, better bleaching conditions are reported for coarse grained uted to only one glacial and an additional older glacial (Hosskirch) samples (Olley et al., 1998; Vandenberghe et al., 2007; Rittenour, is introduced. However Van Husen and Reitner (2011a) proposed a 2008). To maximize the chance of dating well bleached grains stratigraphic model for the Eastern Alps which attributes all Riss and minimizing the effects of signal averaging, we investigated the deposits to one glaciation correlated with MIS 6 defined by oscil- luminescence signals from small aliquots of coarse grained quartz lations on a stadialeinterstadial scale. A recent overview of the and feldspar (100e200 mm). Apart from putting a numerical regional stratigraphic interpretations was given in EandG Quater- luminescence time constraint on the sedimentary processes of the nary Science Journal Vol. 60 No. 2e3 and a brief stratigraphic penultimate glaciation, we explored the luminescence properties of overview is given in Supplementary table S2. During the maximum these sediments to assess their suitability for luminescence dating extent of the penultimate glaciation, the Ybbs valley glacier was purposes. one of the easternmost north draining valley glaciers with a connection to the inner-alpine glacier network (Van Husen, 2000) 2. Selected sites and geological setting e even though the glacier was bound to the inner-alpine realm it did not extend into the foreland to form a piedmont style glacier The petrographic composition of the catchment area is pre- lobe. The extent of mountain glaciers in general depends on several dominantly made up of carbonate rocks (limestone and dolostone) factors, most of all changes in temperature and precipitation (Owen with only minor input of siliciclastic material (mostly sandstones). et al., 2009). In contrast to the vast piedmont glaciers of the NAF Crystalline rocks are completely absent in the River Ybbs catch- (e.g. Rhine glacier, Salzach glacier) originating in the highest ment. Deposits of the alpine glaciations are characterized by mountain ranges of the Alps, glacier systems within lower lying extensive gravel accumulations that form gravel terraces in the mountain ranges (e.g. the easternmost Alps) are likely to react foreland, deposited in braided river environments. In the case of the earlier and more sensitive to changes in climatic factors. Therefore foreland terraces of the penultimate glaciation in the Ybbs valley, a insights on the timing of glacial and proglacial processes of the connection to a terminal moraine system (c.f. glacial series) is not eastern Alpine glacier systems may allow a reconstruction of pa- given, as terminal moraines of this time are morphologically leoclimatic conditions during the penultimate glaciation. indiscernible and not exposed. This makes independent numerical chronologies even more important to establish reliable strati- 1.2. Recent dating approaches of terrace gravels in the Eastern Alps graphic models of such deposits. The sedimentary environment is and its foreland characterized by heavily gravel-dominated glaciofluvial deposits, with uniformly sorted sandy layers only occurring very scarcely. Stratigraphic correlations strongly based on morphological in- Caused by the high hydrodynamic depositional setting that also dicators of terrestrial archives (e.g. foreland terraces) may be creates erosive unconformities, mostly lenticular sand bodies can erroneous because of asynchronous formation and/or fragmented be found. These lenses often hardly meet the required sedimentary conservation of such features. Therefore it is of utmost importance layer thickness (~30e50 cm, Preusser et al., 2008) that is desired to to establish reliable numerical chronologies of such terrestrial ar- guarantee a homogeneous radiation field for a luminescence sam- chives. Few luminescence ages from loess covering High Terrace ple. Proximal deposits in this area tend to be formed in a braided gravels have been obtained (Rogner€ et al., 1988; Miara et al., 1996; river environment dominated by gravel bar deposits. As Thrasher Rentzel et al., 2009). However, these ages only provide minimums et al. (2009) pointed out, bleaching probability for sand-sized for the formation of the gravel terraces. The same is true for an grains in this kind of environment are relatively low. Increased important age constraint of High Terrace gravels using U/Th dating probability for bleaching may exist in shallow, calm water for bar- of cemented gravel (Terhorst et al., 2002). Direct evidence provided top and back-bar sand deposits. Field investigations revealed only by luminescence ages of the terraces in the NAF is very sparse (e.g. three accessible sites (one inner alpine, two located in the NAF) Megies, 2006; Klasen, 2008) and only one High Terrace age of the attributed to the penultimate glaciation (Fischer, 1962, 1963, 1996; Austrian NAF has been published. The age (Infrared stimulated Nagl, 1968, 1970; Untersweg et al., 2012) which meet the pre- luminescence, K-feldspar, not corrected for fading) places the for- requisites for OSL dating as described beforehand. These sites will mation of the High Terrace formed by Salzach and Inn glacier melt be described in the following sections using lithofacies codes c.f. waters around 170 ± 20 ka (Megies, 2006). Previously, the only Miall (2006) and Thrasher et al. (2009). comprehensive luminescence dating study of glaciofluvial sedi- ments was carried out by Klasen (2008, see also Klasen et al. 2007) 2.1. Inner alpine site: Hochau in the NAF of Southern Germany. This study identified analytical problems associated with dating sediments from this area (anom- The site “Hochau” (UTM 33N E491021 N5297650) is located alous fading, incomplete bleaching, age underestimation of quartz), within Mesozoic units of the Northern Calcareous Alps in the alpine and thus a robust chronology of the terraces could not be estab- part of the Ybbs valley (Fig. 1). The exposed Quaternary sediment lished. However, these studies revealed important insights into succession exposes one discernable thick gravel body with bleaching processes of quartz and feldspar (Klasen et al., 2006), the increasing irregular surface morphology and gradually decreasing fading characteristics of feldspar and the signal composition of thickness towards the valley floor. The unit mainly consists of quartz OSL (Klasen, 2008). The available luminescence ages of the horizontally bedded, grain supported, coarse gravel and cobble in a High Terrace deposits were tentatively correlated with MIS 6 and 8 coarse sandy matrix (Fig. 2A and B) of mostly carbonate origin. in southern Germany and Switzerland (Klasen, 2008; Rentzel et al., Discordantly cut sand lenses and discontinuous sand layers are 2009). With respect to the Austrian NAF, various studies primarily scarce and irregularly scattered within the outcrop. Two samples focus on the chronostratigraphy of loess sequences (Preusser and (TOI1, TOI2) were taken from two isolated coarse sand lenses with Fiebig, 2011; Terhorst et al., 2011; Thiel et al., 2011a, 2011c; Van planar cross stratification (Sp), interpreted as proximal bar-growth Husen and Reitner, 2011b). deposits. Pure, coarse grained gravel open-frameworks are a com- In fine-grain luminescence dating approaches (usually using the mon sedimentary feature. This grain size variability indicates a 4e11 mm grain size fraction) for fluvial and glaciofluvial samples it hydrologically highly variable depositional environment. Sediment may be problematic to identify incomplete bleaching (Schielein and subsidence structures (e.g. extensional faults; see Fig. 2A lower part Lomax, 2013) because of signal averaging effects (Rhodes, 2007). and Fig. 3A) are common and interpreted to be caused by post- L. Bickel et al. / Quaternary International 357 (2015) 110e124 113

Fig. 2. Lithological sections of the study sites. Sampling spots with sample codes are indicated with bold arrows. Facies codes cf. Miall (2006) and Benn and Evans (1998).Sh… horizontal laminated sand, Sp … planar cross-bedded sand, Sl … low angle (<15) cross-bedded sands; Gp … planar cross-bedded gravel, Gh … clast supported, horizontally, poorly stratified gravel, P/L … fine grained paleosol/loess loam sequence, Dmm … massive diamicton. sedimentary meltout of buried dead ice, indicating deposition of (MAU1e3) were taken. Thin seams of fine gravel separate the beds. the sediments close to the former ice margin (kame terrace). On top This mostly horizontally stratified sand sequence (Sh) is interpreted of the gravel deposits lies a 5 m thick cover that consists of a deeply as aggradational deposits of a slow flowing water body with weathered, unsorted diamicton that was previously interpreted as intermitting periods of increased discharge (c.f. Miall, 2006). An solifluction sediment by Untersweg et al. (2012). additional sample was taken from a confined sand lens showing planar cross-bedding (Sp) located within the overlying gravels (MAU4). 2.2. Sites of the alpine foreland: Oberhombach€ and Mauer

The two foreland sites are located in the Upper Ybbs valley, 2.3. Ybbs valley during the penultimate glaciation 20e25 km to the south-west of today's Ybbs River confluence with the . Both sites are correlated to the High Terrace, a Pleistocene sedimentary remnants preceding the penultimate deposit of the penultimate glaciation (Fischer,1963,1996; Schnabel, glaciation are scarce in the Ybbs valley. Isolated occurrences of 2002; Krenmayr et al., 2006). Fischer (1996) found a change in elevated coarse gravel sediments indicate that the pre-penultimate drainage direction after deposition of the High Terrace sediments glaciation valley floor could have been considerably higher (Fig. 4A; when the Ybbs abandoned a side channel (Fig. 1, Zauchbach) and Untersweg et al., 2012). These deposits were first thought to be in the discharge shifted more or less towards the modern day river genetic relation to the Hochau sediments as glaciofluvial remnants course. At the outcrop of Oberhombach€ (Fig. 1, OHO, UTM 33N of an earlier stadial during the penultimate glaciation (Nagl, 1968, E489893 N5324284), sediments of the abandoned channel are 1970) but they were later reinterpreted as possible remnants of exposed. 15.5 m of coarse gravels are disclosed in horizontally to an even older valley floor (Untersweg et al., 2012). Sedimentary sub-horizontally strata (Fig. 2C), where only one layer consists of paleocurrent indicators, base slope, and lithology of these gravels medium-sized sand suitable for luminescence sampling (Fig. 3B, are interpreted as indicators of a westward discharge directed from OHO1). the Ybbs valley into the Enns valley (Nagl, 1970; Untersweg et al., Four samples were taken at Mauer (Fig. 3C, MAU1e4, UTM 33N 2012). No terminal moraines in the foreland from glaciations E484591, N5325948) which is located 5 km to the northwest of the older than the penultimate glaciation have been found in the Ybbs Oberhombach€ site (Fig. 1). The outcrop is dominated by horizon- glacier area (van Husen, 1981) but supposedly older glacial erratics tally bedded, coarse gravel deposits, which are separated from the can be found (Ruttner and Schnabel, 1988). During the ice advance top lying loess loam and carbonate luvisol by a thin, lime-free of the penultimate glaciation, a sidearm of the Enns glacier blocked weathering horizon attributed to an interglacial period (Fischer, the discharge to the west and forced the Ybbs to change its flow 1996, Fig. 2D). Below this horizon there are prominent, contin- direction northward (Nagl, 1970) towards the Danube, much like uous horizontal beds of fine-to medium-grained sands, partly cross the modern day river course (Fig. 4B). There are no terminal mo- laminated. Here, three samples for luminescence analysis raines that mark the maximum extent of the penultimate glaciation 114 L. Bickel et al. / Quaternary International 357 (2015) 110e124

Fig. 3. Photographs of the investigated glaciofluvial deposits. A: Inner alpine ice marginal glaciofluvial gravel deposit at Hochau with sedimentary settling structures caused by melting dead ice (white arrows and white dotted lines). B: Horizontally stratified coarse gravel deposit at the Oberhombach€ gravel terrace. An intercalated sand lens (dotted line) was sampled for OSL measurements (OHO1, black arrow). C: Mauer sampling site. Bottom: continuous horizontally bedded sand layers. Top: Horizontally stratified coarse gravels with small interbedded sand lenses (white arrows). The bottom part of the section indicates a relatively quiet depositional environment with well sorted and stratified sediments compared to the overlying unsorted gravelly sediments which indicate an environment dominated by strong hydrodynamics. Four samples were taken from this sedimentary succession (black arrows, numbers correspond to sample name, i.e. 1 / MAU1). in the Ybbs valley. This might be owed to the very narrow valley This would result in an irregular surface morphology when sedi- morphology where sediment relics are only spared from erosion if ment covered dead ice is melting. An ice marginal depositional they are located in sheltered positions (e.g. Hochau). Nagl (1968, environment is also strongly supported by sedimentary evidence 1970) interpreted the Hochau deposits as proximal terrace sedi- (settling structures, Fig. 3) recorded in the course of this study. ments and attributed the height difference between the surface Simultaneous deposition of large quantities of loose gravel material levels (up to 30 m) to two different glaciation stages, where the would take place in the foreland (sites MAU and OHO; Fig. 4D). higher unit was in direct connection to the ice margin of the Ybbs glacier and the lower unit was the terrace connected to outwash 3. Equipment and methods gravels of a younger glaciation which allegedly terminated about 6 km upstream from Hochau. Recent investigations (Untersweg 3.1. Sampling and sample treatment et al., 2012) present a new hypothesis for the valley evolution of the Ybbs valley during the penultimate glaciation: newly found Samples for equivalent dose determination were collected by lateral moraine deposits infer that the glacier's most extensive ice forcing steel tubes into unconsolidated sand layers and sealing the advance reached at least 8 km further downstream from Hochau. tubes' ends with aluminum foil. Samples for determination of the According to their interpretation, the sediments previously attrib- environmental dose rate were taken from the direct sedimentary uted to two different glaciation events by Nagl (1968, 1970; e.g. surroundings of the luminescence sample (few centimeters radially Hochau) are actually ice marginal remnants (kame terraces) of a around the sampling spot). The light exposed ends of the lumi- long single valley glaciation which were deposited onto and later- nescence sample were later discarded in the laboratory under ally to a glacier decaying into individual dead ice bodies (Fig. 4C). subdued red light conditions. The samples were dried and sieved to L. Bickel et al. / Quaternary International 357 (2015) 110e124 115

Fig. 4. Time sketch of Ybbs valley development. A: Situation before the penultimate glaciation: The Ybbs was flowing to the east which is documented by sedimentary evidence of Ybbs valley provenience in the Enns valley. B: Ice advance during the penultimate glaciation. The connection to the east was blocked by an advancing Enns glacier, damming up a lake which drained northward. C: During the deglaciation phase the glacier melted and disintegrated into several individual dead ice bodies. Sediment was deposited next to and onto these glacier remnants (TOI). D: Quasi-synchronous onset of the deposition of melt water sediments due to massive supply of loose sediment in the foreland of the retreating Ybbs glacier. extract the grain size fraction of 100e200 mm. The treatment with NAF). For quartz, the influence of alpha particles was not accounted 10% hydrochloric acid and 15% hydrogen peroxide removed car- for, as the rim affected by alpha particle damage of coarse grained bonates and organic matter, Sodium oxalate was added to defloc- quartz was removed by etching (8 ± 2 mm) with 40% HF for culate clay minerals. Potassium rich feldspar and quartz were 30e40 min. For KFs, a mean internal potassium content of concentrated by mineral gravity separation using LST fastfloat (at a 12.5 ± 0.5% was assumed (cf. Huntley and Baril,1997). Today's water density of 2.58 g/cm3 and 2.68 g/cm3). The quartz concentrate was content was determined by drying the sample at 80 C which etched with 40% hydrofluoric acid for 30e40 min to remove the resulted in a measured mean water content of 5 ± 2%. Due to outer layer of the grains affected by alpha radiation and to remove mining activity at all investigated sites these values are regarded as residual plagioclase. The separated quartz was then treated with minimum water content because of increased drainage at the 15% hydrochloric acid to remove newly formed fluorides. Finally, gravel pit walls. For this reason, dose rate calculations with a the etched concentrates were sieved (100e200 mm) to remove significantly higher water content of 15 ± 10% is assumed to smaller grain fragments that resulted from the etching process and represent the average natural conditions, covering a range from may potentially contain feldspar remnants. Feldspar contamination almost dry to almost saturated conditions. This is in the range of was additionally monitored analytically during the measuring water content values reported by other studies dating coarse grain process. material from similar settings (Klasen et al., 2007; Pawley et al., 2010). Dose rate calculations were performed using the ADELE software (Kulig, 2005) using the dose rate conversion factors by 3.2. Dosimetry Adamiec and Aitken (1998). Radionuclide concentrations, water content, and effective environmental dose rates are given in Table 1. Environmental dose rates were calculated from measurements of the relevant radionuclides (40K, 232Th, 238U) that contribute to the dose rate using a Canberra high resolution-high purity Germanium gamma detector (40% n-type). ~1000 g of sample were Table 1 Results of radionuclide analysis and dose rate calculation. Dr_eff ¼ effective dose stored in sealed Marinelli beakers for a minimum of two weeks to rate, caused by cosmic and environmental radiation. ensure secular Rn equilibrium before measurement. The influence 1 of cosmic radiation was calculated according to Prescott and Hutton Sample K [%] ± Th [ppm] ± U [ppm] ± Dr_eff [Gy ka ] (1994). Because of mining activity that removed the sedimentary TOR1 0.85 0.02 1.25 0.03 4.43 0.13 1.31 cover, the influence of cosmic radiation in sample TOI2 had to be APP1 1.04 0.02 9.58 0.26 2.97 0.06 2.86 reconstructed by extrapolating the original burial depth using APP2 1.11 0.02 3.28 0.07 10.54 0.28 3.15 DES1 0.37 0.01 2.00 0.07 1.12 0.03 0.69 elevation data of nearby original undisturbed terrain surfaces. Ra- DES2 0.27 0.01 1.38 0.05 1.33 0.03 0.63 diation damage caused in non-etched feldspar crystals by a-parti- DES3 1.45 0.03 12.78 0.34 3.53 0.08 3.04 cles was accounted for by using an assumed mean alpha efficiency UNT1 0.74 0.02 2.42 0.08 1.27 0.03 0.85 (a-value) of 0.07 ± 0.02. This is based on values used by Klasen UNT2 0.70 0.00 2.50 0.04 1.09 0.02 1.07 (2008) for coarse grain feldspar in a similar setting (German (continued on next page) 116 L. Bickel et al. / Quaternary International 357 (2015) 110e124

Table 1 (continued ) 6 mm KFs aliquots of sample MAU4 to direct sunlight over three Sample K [%] ± Th [ppm] ± U [ppm] ± Dr_eff [Gy ka 1] days of fair weather conditions in July 2013 in Vienna (Austria) with variable exposure times. Aliquots were exposed in following steps: HOR1 0.31 0.01 1.46 0.06 1.40 0.03 0.71 HOR2 0.42 0.01 1.72 0.06 1.23 0.03 0.78 10 s, 30 s, 1 min, 2 min, 5 min, 15 min, 30 min, 1 h, 2 h, 3 h, 5 h, 1 day/ ORO1 0.69 0.02 3.30 0.10 1.54 0.04 1.17 night cycle, 2 day/night cycles, and 3 day/night cycles. Residual ORO2 0.77 0.02 4.51 0.14 1.90 0.04 1.41 doses were determined using the pIRIR-protocol that was used for SRN1 0.12 0.00 0.45 0.02 1.93 0.04 0.76 ED determination (see above). Mean global solar radiation for the SRN2 0.11 0.00 0.44 0.02 1.93 0.04 0.62 experiment site in July is ~166 kWh/m2 (ZAMG, 2002). Bleaching DRF1 0.16 0.00 0.70 0.03 3.06 0.06 0.89 DRF2 1.64 0.04 13.58 0.35 4.10 0.08 3.22 properties for quartz were not investigated, as the OSL signal of DRF3 0.34 0.01 1.26 0.05 2.36 0.05 0.92 quartz resets much faster than IR50 and pIRIR225 signals (Murray DRF4 0.91 0.02 1.69 0.04 4.09 0.12 1.42 et al., 2012; Starnberger et al., 2013). MAU1 0.40 0.02 1.93 0.05 3.81 0.03 1.25 Additionally, fading tests were carried out on twelve 6 mm ali- MAU2 0.14 0.03 0.63 0.06 4.03 0.03 1.05 MAU3 0.23 0.02 0.82 0.04 4.38 0.02 1.16 quots of coarse grained potassium rich feldspars for one sample per MAU4 0.17 0.03 0.69 0.06 4.55 0.03 1.16 site (MAU4, OHO1, TOI1) following the approach described by OHO1 0.24 0.02 0.91 0.05 3.77 0.03 1.10 Auclair et al. (2003). Storage times after irradiation ranged between TOI1 0.10 0.03 0.44 0.06 1.97 0.03 0.60 10 min and 29 h. TOI2 0.16 0.02 0.60 0.05 1.98 0.03 0.58 Unfortunately, some quartz samples from the NAF show an unfavorable OSL signal behavior for dating purposes (Klasen, 2008) because their relative medium component contribution in the re- 3.3. Luminescence measurements generated signal is higher compared to the natural signal. Furthermore, a thermal instability of the medium component is The luminescence was measured in two automated Risø TL/OSL- observed when the sample is thermally stressed. To evaluate the DA-20 luminescence readers (Bøtter-Jensen et al., 2000, 2003; influence of a potentially thermally unstable medium component, 90 90 Thomsen et al., 2006), each equipped with a Sr/ Y beta source pulse annealing experiments were carried out (cf. Fan et al., 2011) delivering a dose rate of ~0.10 Gy/s and ~0.13 Gy/s, respectively. on single aliquots of samples OHO1, TOI1, TOI2 and MAU2 with 2 Quartz was stimulated by a set of blue LEDs (470 nm, ~41 mW cm increasing preheat temperatures (200e400 Cin20C steps, 10 s at @ 100% power) and detected by a photomultiplier tube (PMT) each temperature; sensitivity correction: test dose 20 Gy, cut heat through an ultraviolet transmitting Hoya U340 (7.5 mm) filter. An 200 C). array of IR emitting diodes was used to stimulate feldspar lumi- Aliquots passing the following SAR quality criteria were used for nescence which was detected by a PMT using a 410/30 interference Q and KFs in this study: filter (2 mm). Before De measurements, we conducted combined dose recov- ery preheat plateau tests (DPT) on artificially irradiated (~155 Gy) 1 mm aliquots for KFs and 4 mm aliquots for Q to assess the reproducibility of a known dose at incremental rising preheat temperatures using the SAR protocol. The aliquots were bleached with the stimulation LEDs of the luminescence reader (blue LEDs for quartz, IR diodes for KFs). Dose recovery preheat plateau tests were performed on MAU2 and MAU4 for quartz andMAU2, MAU3 and MAU4 for KFs using three aliquots per preheat step (KFs: 210e290 C for 10 s and a 50 C/225 C pIRIR protocol; Q: 200e260 C for 10 s, test dose cutheat 20 C lower, measured at 125 C with a standard SAR protocol). Because of low yield in both Q and KFs after the separation process, dose recovery tests for some samples were measured with just one preheat temperature that was inferred from dose recovery preheat plateau tests from MAU2, MAU3,MAU4, and OHO1 (Q 240/220 C, KFs 250 C). For the equivalent dose (De) measurements small aliquots of Q and KFs with 1 or 2 mm diameter (approximately 60 grains and 150 grains respectively) were fixed with silicone spray onto 9.7 mm steel discs. We used standard single-aliquot regenerative-dose (SAR) protocols (Wintle and Murray, 2006) for dose determination of OSL of quartz at a stimulation temperature of 125 C for 40 s. Feldspar post-IR-IR (pIRIR225) doses were determined by an elevated temperature IR stimulation (225 C for 100 s) following Buylaert et al. (2009). The IR50 signal at a stimulation temperature of 50 C for 100 s of this protocol was also used for De determination (IR50). Signal integrals for quartz OSL were chosen following Cunningham and Wallinga (2010) who use early background sub- traction (0e0.4 s signal integral, 0.4e1.4 s background integral). For feldspar IRSL, the first 2 s of the luminescence signal were used and the average background was derived from the last 20 s of each measurement. Fig. 5. Results of dose recovery experiments (DRT). 3 aliquots per temperature step were measured. Error bars represent 1s standard error. Solid grey line represents unity The bleaching characteristics of the natural IR50 and pIRIR225 between given dose and recovered dose. Dashed grey lines represent ±10% of unity, signals were investigated by exposing triple sets of natural which is the maximum deviation from unity allowed in this study. L. Bickel et al. / Quaternary International 357 (2015) 110e124 117

Recycling ratio 0.9e1.1 between given and recovered dose was found at a preheat tem- Max. test dose error 10% perature of 250 C(Fig. 5B and C) which was used for all KFs Max. recuperation 5% samples in this study. Signal more than 3 sigma above background The dose recovery tests for quartz were carried out on 6 mm aliquots. Best dose recovery ratios were found for preheat tem- Additionally, three “soft rejection criteria” were used for the peratures of 240 C(Fig. 5A). The dose recovery tests for both selection of quartz OSL signals that we incorporated in the De mineral types did yield acceptable results (at 250 C for KFs, 240 C evaluation of each aliquot. First, the initial part of the natural and for Q) in terms of recycling ratio (<10%) and signal recuperation regenerated decay curves must show a steep slope to background (<5%). level, indicating a fast component dominated signal. Secondly, ali- quots showing substantial change in decay curve geometry be- tween natural and regenerated signals in the SAR sequence were 4.2. Dose saturation properties discarded because of possible change in signal component composition. Finally, all aliquots emitting a measurable IRSL signal Three natural aliquots of each sample were exposed to a high response after short beta irradiation (20 Gy) and subsequent pre- dose (1200 Gy) during the course of a SAR sequence to assess the heat must be discarded owing to possible feldspar contamination. saturation behavior of the individual signals. By fitting a single Given errors in this study are standard errors of the arithmetic saturating exponential growth curve to the regenerated points, the mean (1s) except for calculated central ages (central age model, 2D0 value could be deduced (Fig. 6). 2D0 values define the upper Galbraith et al., 1999) where the standard errors of the model are confidence level, where De's are still ~15% below laboratory satu- provided. ration (Wintle and Murray, 2006). Saturation thresholds for all samples in this study were determined using the 2D0 approach 4. Luminescence properties with a high laboratory dose (results given in Table 2). While the OSL signal was in saturation after a 1200 Gy irradiation, the IR50 signal 4.1. Dose recovery preheat plateaus of KFs still did show potential growth (Fig. 6). The pIRIR225 signal did show signs of saturation at 1200 Gy which is also expressed in The recovered and given doses from all feldspar samples show considerably smaller 2D0 values compared to those of the IR50 very good accordance from 210 to 270 C preheat temperature signal. In any case, equivalent doses used for age determination (Fig. 5B and C). Above 270 C, the recovered doses for MAU1 and were well below the 2D0 thresholds.

Table 2 Summary of OSL and IRSL data. The reliability of ages is based on methodological implications (e.g. age overlap of different signals) intra-section stratigraphic relations and comparison with ages from the same geologic unit (e.g. comparison MAU and OHO). Fading Corrected IR50 ages are not considered for age interpretation and are only discussed as possible minimum ages.

a b c d e Sample Location Signal Mineral ntotal npassed g-value De [Gy] OD [%] 2D0mean [Gy] Age [ka] MAU1 Mauer OSL Q 72 10 e 145 ± 18 40% 257 ± 83 116 ± 17 IR50_corr KFs 38 34 2.89 ± 0.93 292 ± 58 22% 1234 ± 117 156 ± 31 pIRIR225 KFs 30 21 e 291 ± 19 24% 566 ± 147 156 ± 16 MAU2 Mauer OSL Q 94 19 e 146 ± 8 15% 262 ± 56 140 ± 16 IR50_corr KFs 44 41 2.89 ± 0.93 222 ± 9 21% 1205 ± 242 134 ± 21 pIRIR225 KFs 29 20 e 282 ± 27 40% 770 ± 133 170 ± 23 MAU3 Mauer OSL Q 56 20 e 147 ± 11 28% 348 ± 26 127 ± 14 IR50_corr KFs 55 39 2.89 ± 0.93 306 ± 16.0 28% 931 ± 106 171 ± 27 pIRIR225 KFs 49 18 e 317.0 ± 28 33% 521 ± 27 178 ± 22 MAU4 Mauer OSL Q 60 21 e 149 ± 10 24% 253 ± 92 127 ± 14 IR50_corr KFs 39 31 2.89 ± 0.93 211 ± 20 27% 1258 ± 158 117 ± 18 pIRIR225 KFs 29 17 0.95 ± 0.47 247 ± 10 4% 844 ± 79 137 ± 13 OHO1 Oberhombach€ OSL Q 32 20 e 158 ± 6 3% 225 ± 37 128 ± 14 IR50_corr KFs 33 32 2.98 ± 0.59 337 ± 39 27% 1170 ± 214 218 ± 30 pIRIR225 KFs 29 20 0.88 ± 0.41 544 ± 24 10% 846 ± 73 325 ± 33 TOI1 Hochau/Tonibauer OSL Q 39 20 e 85 ± 2 5% 283 ± 67 142 ± 15 IR50_corr KFs 34 33 3.27 ± 0.44 151 ± 10 11% 937 ± 109 128 ± 6 pIRIR225 KFs 20 20 0.80 ± 0.49 178 ± 7 9% 740 ± 69 150 ± 14 TOI2 Hochau/Tonibauer OSL Q 30 22 e 75 ± 3 7% 194 ± 21 129 ± 14 IR50_corr KFs 30 28 3.27 ± 0.44 176 ± 31 30% 1066 ± 202 158 ± 20 pIRIR225 KFs 26 23 e 310 ± 18 23% 828 ± 93 279 ± 26

a Number of aliquots investigated. b Number of aliquots which passed the quality criteria. c Representative g-values calculated for one sample per site (MAU4, OHO1, TOI1). d 2D0 mean values derived from three artificially high dosed aliquots for each sample. e Error: 1 sigma; ages assumed to be the most reliable are printed in bold.

MAU2 deviate more than 10% of the given dose and imply that 4.3. Residual doses of pIRIR225 and IR50 signals sensitivity changes in the first measurement cycle are not appro- priately corrected for at these temperatures. This result also implies The bleaching experiment conducted for the infrared stimulated that the high temperature pIRIR290 protocol (e.g. Thiel et al., 2011b) signals of sample MAU-4 shows that the IR50 signal depletes faster cannot be applied to these samples. In summary, good correlation in direct sunlight compared to the pIRIR225 signal (Fig. 7). After 10 s 118 L. Bickel et al. / Quaternary International 357 (2015) 110e124

Fig. 6. Representative dose response curves for Q OSL, KFs IR50 and KFs pIRIR225 of sample MAU4. Arrows indicate the mean 2D0 value for the respective signal.

of sunlight exposition IR50 De values dropped to 54 ± 2% of the IR50 > 100 Gy, pIRIR225 > 150 Gy), no residual dose correction was mean unbleached natural De (154 ± 9 Gy) while the pIRIR225 value applied. only lost 6 ± 4% of its initial dose and started to drop below 50% at exposure times of 60 s and longer. After 2 h of exposure to direct 4.4. Fading of KFs signals sunlight, IR50 values levelled at a relatively stable residual plateau of <1% (~1.3 ± 0.2 Gy) of the mean natural IR50 D Similar e. Samples investigated for fading (TOI1, OHO1, MAU4; 1 sample bleaching characteristics for low temperature IR stimulated feld- per location) show mean fading rates (g-value) for the IR50 signal spar were reported by Godfrey-Smith et al. (1988), and in recent between 2.9% and 3.2% overlapping within one standard deviation further investigated by different authors (Buylaert et al., (Fig. 8). Even though most of the measured D values plot on the 2012; Li et al., 2014). In contrast, pIRIR225 D 's were still around e e exponential part rather than the linear part of the growth curve, 4.4 ± 0.2% (10.9 ± 0.5 Gy) of the paleodose (247 ± 10 Gy) after two IR50 ages were corrected using the method of Huntley and hours of sunlight exposure. Even after exposure times of 3 complete Lamothe (2001). There are reports, that age underestimation after day/night cycles, D values still indicate a slight downward trend, e fading correction following Huntley and Lamothe (2001) for the but within D values of 1.0 ± 0.2% of the initial level. Based on these e exponential part of the dose response curve is indeed significant results, samples OHO1 and TOI1 were exposed to direct sunlight but not necessarily severe (Auclair et al., 2007) However, age un- under the same perquisites (time, location) as MAU4 for three day/ derestimation of the true depositional age cannot be quantified and night cycles to estimate the dimension of unbleachable residual ruled out completely; therefore all presented fading corrected IR50 doses of the post-IR signal (cf. Buylaert et al., 2011; Reimann et al., ages are interpreted as minimum ages. The fading rates are rather 2011; Thiel et al., 2011a,b,c). MAU4, OHO1 and TOI1 show residual uniform throughout the whole study area and are not expected to doses below 1 Gy for the IR50 signal and below 4 Gy for the change for feldspars from the same sedimentary unit. Therefore the pIRIR225 signal. As these small residual doses have a negligible calculated g-values for TOI1 and MAU4 were used for remaining influence on the paleodose of old samples (expected D e samples of the same site (TOI2 and MAU1e3). Results of the correction are discussed in Section 5. The observed pIRIR225 fading rates were considerably lower compared to IR50 signals with g-values averaging at 0.9%. The different fading rates of both IR signals in this study are in agree- ment with previously published studies (Thomsen et al., 2008; Buylaert et al., 2009; Thiel et al., 2011b). Various studies regard such low pIRIR225 values as laboratory artifacts (Buylaert et al., 2011; Thiel et al., 2011b) and hence these are not corrected for here.

4.5. Thermal stability of quartz

OSL signal data from pulse annealing experiments was imported in an R software environment and deconvoluted using the “Lumi- nescence” package (Kreutzer et al., 2012) using a function with multiple, overlapping first order exponentials to split the OSL signal into its individual components (Jain et al., 2003). The experiments (Fig. 9) revealed a decrease of the medium component contribution in the regenerated signal with increasing preheat temperatures. The decrease is evident for TOI1 and OHO1 from the first increased preheat temperature step and upward, while the medium component OSL of MAU4 started to decrease significantly at 240 C and higher. While the bulk signal influence of the medium component for OHO1 and TOI1 is relatively low (around 20% of the initial 0.1 s) it can contribute to almost 40% of the initial signal Fig. 7. Resetting characteristics for IR50 (open circles) and pIRIR225 (solid circles) intensity for the investigated aliquots of sample MAU4. Similar signals of KFs sample MAU4 under natural sunlight exposure. Data points represent residual dose recovered after a certain time of exposure to sunlight (error 1s standard behavior was also observed for sample MAU2. When present, a error). thermally unstable medium component in the initial part of the L. Bickel et al. / Quaternary International 357 (2015) 110e124 119

Fig. 8. Results of laboratory fading tests for one sample of each investigated site. IR50 and pIRIR225 fading rates are rather uniform (~3%/decade and ~0.9%/decade).

Fig. 9. Results of the pulse annealing experiment of sample a) TOI1 and b) MAU4. Datapoints represent the normalized, sensitivity corrected OSL signal with 1 s standard error. The fast component is stable up to a preheat temperature of 280 C for both samples. In contrast, the medium component has a gradual decreased signal intensity from 220 C and higher for TOI1 and 240 C and higher for MAU4. signal may lead to dose underestimation due to differential Wallinga, 2002). In this case it is appropriate to apply the central behavior of the natural and regenerated signals (Steffen et al., age model (CAM, Galbraith et al., 1999) to estimate the accurate 2009). To minimize the influence of a thermally unstable medium paleodose of a set of aliquots. The calculated central doses are component in the course of De measurements, quartz OSL signals 85 ± 2 Gy for TOI1 and 75 ± 3 Gy for TOI2 with low overdispersion were carefully selected according to OSL decay rate and were of 5% and 7% (Table 2). The resulting ages of 142 ± 15 ka (TOI1) and evaluated using an early background approach (Cunningham and 129 ± 14 ka (TOI2) overlap within 1 sigma (Fig. 11). Applying the Wallinga, 2010). On the other hand, the fast component shows a CAM to the IR50 distribution gives ages of 90 ± 8 ka (TOI1) and relatively stable plateau up to temperatures of 280 C and signifi- 95 ± 11 ka (TOI2). When this is corrected for fading the ages in- cantly decreases at temperatures at and above 300 C for MAU4 and crease to 128 ± 6 ka and 158 ± 20 ka respectively (ODTOI1_IR50 ¼ 11%, TOI1. OHO1 shows the first significant drop in signal intensity for ODTOI2_IR50 ¼ 30%). CAM ages for the pIRIR225 De values result in the fast component at a temperature of 320 C. In summary, for the 150 ± 14 ka for TOI1 and 279 ± 26 ka for TOI2 (ODTOI1_pIRIR225 ¼ 7%, sedimentary catchment area we assume that the fast component in ODTOI2_pIRIR225 ¼ 23%). As the three different signals have different our samples is stable up to a temperature level of at least 280 C. intrinsic bleaching properties, an offset in ages calculated with the same statistical approach is a strong indicator for incomplete 5. Equivalent dose distribution and age calculations bleaching (Murray et al., 2012; Kars et al., 2014). Quartz, fading corrected IR50 and pIRIR225 ages for TOI1 all overlap within 1 5.1. Inner-alpine deposits (TOI) sigma standard error. The same can be observed for the OSL and fading corrected IR50 ages of TOI2 but the pIRIR225 age shows a significant offset from the ages gained from the quartz OSL signal. Both quartz samples show a narrow, uniform De distribution (Fig. 10B) even though the gained natural signals mostly intersect We attribute this behavior to incomplete bleaching of the post-IR the dose response curve in the non-linear part. This is somewhat signal prior to deposition for sample TOI2. In contrast, TOI1 unexpected, as dose distributions from old samples derived from seems to be well bleached which is supported by overlapping ages the non-linear part of the saturating growth curve often result in derived from all three evaluated signals. Resulting De values and asymmetric distributions (Murray and Funder, 2003). In contrast, ages are given in Table 2. the distribution of IR50 De values is broader. When comparing De values of IR50 we found that sample TOI1 has a symmetrical dis- 5.2. Sediments of the alpine foreland (OHO and MAU) tribution with values that span a range of ~100 Gy whereas the range of De values of the same signal is doubled in TOI2 (over OHO1 De distributions (Fig. 10A) of quartz are similarly narrow 200 Gy). De values are scattered even wider for pIRIR225 signals of shaped like sample TOI2, whereas IR50 and pIRIR225 De‘s cover a both samples; the largest span was found in sample TOI2, where relatively wide De range (IR50: 132e517 Gy, pIRIR225: De's covers a range of over 300 Gy. Quasi-Gaussian distribution of 390e774 Gy). Compared to the quartz CAM age of 148 ± 16 ka, the all quartz samples indicate that quartz is bleached completely (e.g. fading corrected IR50 age of 218 ± 30 ka and a pIRIR225 age of 120 L. Bickel et al. / Quaternary International 357 (2015) 110e124

Fig. 10. Cumulative dose distribution and kernel density plots of two representative samples for OSL, IR50 and pIRIR225 signals. For dose distribution of other samples see supplement S1. Note the different scales of the x-axis in figure AþB. While De's for OHO1 (A) range from 110 Gy to almost 800 Gy, De's of TOI1 (B) range from 70 to around 250 Gy. Inset (right top) shows KDE for TOI1 scaled to x-axis of A. Apart from obvious differences of effective dose rate between both sampling sites, also differences in bleaching may account for this discrepancy.

6. Discussion

6.1. Methodological implications

Even though short transport distances are to be expected for the ice marginal deposits of Hochau (TOI), incomplete resetting does not pose a problem for the quartz OSL signal in both samples. All investigated signals (OSL, IR50, pIRIR225) for TOI1 overlap within 1 sigma. We interpret overlapping OSL and pIRIR ages on the one hand as signs of complete resetting prior to deposition and on the other hand as indication of negligible influence of an eventual thermally unstable component in the OSL signal and athermal fading in the pIRIR signal on the calculated age. The quartz age of TOI2 is consistent with the quartz age of TOI1. Incomplete bleaching of TOI2's pIRIR225 signal seems to be an issue as the age calculation

Fig. 11. Comparison of CAM ages for OSL, IR50 (corrected and uncorrected) and using this signal overestimates the OSL and IR50 ages by ~100 ky. pIRIR225 signals. Even though perquisites were taken to sample deposits with an increased probability of complete resetting (see 2.1 and 2.2), sam- ples from similar depositional environments do not necessarily show the same bleaching condition which once more confirms that 325 ± 33 ka are considerably higher. We interpret these offsets bleaching is a heterogenous process (Murray and Olley, 2002). Gaar between quartz, IR50 and pIRIR225 ages to be caused by incom- et al. (2013) found similar discrepancies in bleaching conditions in plete bleaching. Because quartz OSL signals are bleached more two samples of one and the same glaciofluvial deposit in the NAF of rapidly in nature compared to both investigated IR signals (KFs), the Switzerland. quartz age from OHO1 is regarded to be the most reliable (Fig. 10A). Post IR as well as fading corrected IR50 ages from Oberhombach€ Although differences in calculated ages indicate incomplete reset- (OHO) seem to be vastly overestimated when they are compared to ting of the luminescence signal, overdispersion values are low in luminescence dating results of the nearby Mauer site. In addition, particular for pIRIR225 (ODOHO1_pIRIR225 ¼ 10%). the geomorphological position of the Oberhombach€ site and results Ages of the Mauer site were calculated using the central age of previous field studies (Nagl, 1968, 1970; Fischer, 1996) suggest model as well. The samples which were taken out of the continuous contemporary deposition at the Oberhombach€ and Mauer sites. horizontal sand layers (MAU1e3, Figs. 2 and 3) gave a mean CAM Unbleachable residual doses are ruled out as a reason for pIRIR225 quartz age of 128 ± 12 ka. Of all measured aliquots, only 13% passed overestimation and incomplete bleaching seems to be the most the rejection criteria. 64% of aliquots were discarded because of likely cause. Reports from other parts of the Eastern Alps (Klasen failure to meet set SAR quality criteria (low signal to noise ratio, et al., 2007; Klasen, 2008) indicate that partial bleaching is a recycling ratio, etc.) and additionally 23% of measurements were common problem among this type of sediments. In contrast, in our discarded due to suspicious signal behavior (slow signal decay, study incomplete bleaching is mainly a problem (if present) that change in decay curve geometry, feldspar contamination, see 3.4) inherits feldspar signals and quartz seems to be relatively well regardless of passing the SAR error threshold. MAU4 yielded an OSL bleached throughout. age of 134 ± 15 ka. All quartz ages for this site overlap within their In contrast to the samples from Mauer, the Oberhombach€ respective 1 sigma error margins. This also applies for the calcu- sample received additional sediment input from a short tributary lated feldspar pIRIR225 ages, which overlap in a 1 sigma range with river (Fig. 1, Zauchbach) that erodes almost exclusively siliciclastic central values between 137 and 178 ka (Fig. 11). No significant material from the Flysch zone (mainly clay- and sandstones). The offsets between pIRIR225 and OSL ages that hint towards incom- distance between the modern day Zauchbach spring and the plete bleaching or influence of a thermally unstable medium assumed confluence with the Paleo-Ybbs River is only ~5 km. This component as 2 ages overlap within 1 sigma and all ages overlap increases the chance of poorly bleached sediment input from a within 2 sigma. After correcting IR50 Des for fading, the ages agree tributary river. Low overdispersions are often regarded as in- with pIRIR225 ages within uncertainties but are only regarded as dicators for complete resetting of the natural signal (Olley et al., potential minimum sedimentary ages. Resulting ages and equiva- 2004; Bateman et al., 2008; Neudorf et al., 2012). Although the lent doses are shown in Table 2. pIRIR225 age of OHO1 shows low overdispersion values (~10%), it L. Bickel et al. / Quaternary International 357 (2015) 110e124 121 overestimates the quartz age by ~200 ka. Wallinga (2002) indi- showing a simultaneous or at most a slightly temporally delimited cated, that broad but symmetric distributions (thus low over- sedimentation (Table 2, Fig. 4D). As the gravel terrace of dispersion) also can show incomplete bleaching if the count of Oberhombach€ was not incised by later Ybbs River activity (e.g. poorly bleached grains is high. Therefore overdispersions calcu- during the last glacial cycle), the change of river course must have lated from the Galbraith et al. (1999) central age model can only be happened after the deposition of the High Terrace sediments. regarded as vague indicators of incomplete bleaching in this study. Unbleachable residual doses are ruled out as a significant contrib- 6.3. The “Riss” glaciation utor to De overestimation of pIRIR signals, as they only represent <1% of the total equivalent dose and therefore can be neglected. According to Nagl's (1968, 1970) model, TOI1 is supposed to In recent years, evidence has been found that high dosed quartz correspond to the older Riss and TOI2 to the younger Riss. However, samples measured by SAR may exhibit systematic age underesti- the luminescence ages of both samples in combination with sedi- mation (Murray et al., 2007; Lowick and Preusser, 2011; Timar- mentary evidence of deposition onto ice, strongly favor the concept Gabor and Wintle, 2013). The quartz ages presented in this study of a single kame terrace accumulation during a final retreat phase of are unlikely to be underestimated due to high dosage, as all ages the penultimate glaciation. If there had been a two-phase Riss from different samples agree within error even though some glaciation, with an older, more extensive glacier advance (Nagl, samples are relatively high dosed (e.g. OHO1: 158 Gy) and others 1970), the blocking of drainage to the west by the Enns valley show a relatively low equivalent dose (e.g. TOI2: 75 Gy). glacier would have caused the Ybbs to redirect north and therefore also deposit its coarse bedload to the north in two phases. However, 6.2. Valley evolution in the light of luminescence ages as only a single High Terrace level is present in the Ybbs catchment area, this scenario seems unlikely. Terrace accumulation ages The luminescence ages strongly support the hypothesis of (Table 2) of the Mauer and Oberhombach€ terrace also indicate Untersweg et al. (2012) that the sedimentary remnants of the deposition towards the end of MIS 6 when large amounts of loose penultimate glaciation indeed did form during a single glaciation sediment was available in the forefront of the retreating glacier. event. Calculated quartz OSL ages for the TOI samples (Table 2) The OSL ages presented in this study coincide with the Beringen indicate that ice decay and associated deposition of the kame glaciation of the Western Alps which was dated to 180e130 ka terrace happened in closing MIS 6. A synoptic lateral profile sketch (Preusser and Schlüchter, 2004; Dehnert et al., 2010; Preusser et al., is displayed in Fig. 12, showing the genetic relationship of the 2011) and later subdivided into two separate ice advances at investigated sites. ~185 ka and ~140 ka (Dehnert et al., 2012). Fig. 13 shows a com- U/Th dating on calcite cements of the topmost gravel deposits at parison of numerical dating results discussed in this section. Hochau revealed that polyphase cementation took place at Luminescence ages obtained in this study do not point towards a 56.8 ± 2 ka and 79.7 ± 2ka(Untersweg et al., 2012). Calcite cements two-phased Riss glaciation in the Ybbs valley, but sediments from delta sediments deposited in front of the Ybbs glacier ~8 km deposited during a hypothetical first glacier advance could have downstream from the Hochau sediments yielded U/Th cementation been eroded or be so far unrecognized as such. A transitional ages of 56 ± 2 and 115 ± 2ka(Untersweg et al., 2012). Such U/Th sequence from high energy glaciofluvial gravels of the penultimate cementation ages from a glaciofluvial sedimentary environment glaciation to low energy lacustrine fine sedimentation attributed to have to be considered as post-depositional ages and support the the last interglacial in the Western Alps is documented in the site of luminescence ages presented in this study. Thalgut (Schlüchter, 1989). OSL dates of the sandy transitional zone There is weak geomorphological indication (slight altitudinal (Preusser and Schlüchter, 2004) revealed a deposition at c. 130 ka, offset of terrace levels) that the OHO sediments could have been which correlates well with the dated ice decay in the Ybbs valley, deposited before the MAU terrace sediments (Fischer, 1996). probably indicating that climatic amelioration happened simulta- However, chronological differentiation of the depositional age be- neously in the Western and Eastern Alps. Definitive evidence for tween the supposedly younger Mauer gravel terrace and the older the presence of full interglacial climatic conditions in the Eastern Oberhombach€ gravel terrace is not possible by means of lumines- Alps is given in the U/Th-dated speleothem records of the high- cence dating because of overlapping ages. pIRIR225 and quartz OSL lying Spannagel Cave (Austria) which indicate ice free conditions ages from the Mauer site all indicate deposition in relatively short around 125 ka (Spotl€ and Mangini, 2006; Spotl€ et al., 2006). As ice succession, with no significant age hiatus in between. Calculated in the Ybbs valley was still present around 135 ka near Hochau ages indicate that the accumulation of the gravel terraces in the (TOI1e2) a rapid transition to interglacial conditions is very likely alpine foreland of the Ybbs valley most likely happened during (i.e. Termination II, Drysdale et al., 2009). In contrast, U/Th ages of middle to late MIS 6 stage and that the terraces of Mauer and calcite cements of glaciofluvial gravels of the Ybbs Valley only allow Oberhombach€ can be attributed to the same depositional unit, precise determination of the formation of the cement, but only permit minimum estimates of the timing of the initial deposition process. Compared to the northern European glaciation, the timing of the ice decay of the Ybbs glacier chronologically correlates with the final stage of the Saalian glaciation in northern Germany (“Warthe stage”, Litt et al., 2007) where quartz derived from glaciofluvial sandur sediments was dated by luminescence methods between 150 and 130 ka (Lüthgens et al., 2010) and OSL ages of Danish fluvial and glaciofluvial sediments deposited above a till layer indicate that deglaciation during the Warthe stage started as early as ~165 ka (Houmark-Nielsen, 2007). Although the luminescence ages presented in this study are able to place the Riss glaciation in the middle to late MIS 6, the ages are fl Fig. 12. Synoptic lateral profile sketch of the Ybbs valley. Elevations and distances of still af icted with uncertainties covering many thousands of years. the illustration are not to scale. While luminescence ages permit detailed comparison of 122 L. Bickel et al. / Quaternary International 357 (2015) 110e124

Fig. 13. OSL ages from the Ybbs valley in comparison with numerically dated terrestrial records of the greater alpine region. Benthic d18O stack and (Lisiecki and Raymo, 2005) are shown on top of the illustration for orientation purposes. A: Mean OSL age of the glaciofluvial deposits of the Ybbs valley (MAU1e4, OHO1, TOI1e2). B: U/Th ages of calcite cement generations of glaciofluvial deposits of the Ybbs valley. C: Mean OSL ages of fluviolacustrine sediments of the Thalgut site (THG1e3) in Switzerland. The authors interpret these sediments to represent the transition between the penultimate glaciation and the last interglacial (Preusser and Schlüchter, 2004). D: Mean OSL ages of lacustrine samples of the Niederweningen drill core NW09 (Dehnert et al., 2012). The dated sediments indicate a cold climate, proximal lacustrine environment with two ice advances between 185 ka and 140 ka. E: U/Th dated speleothem growth phases of the Eastern Alpine Spannagel Cave (Austria, Spotl€ and Mangini, 2007). No growth was recorded between ~130 and ~170 ka, indicating conditions within the cave which prevented speleothem growth (e.g. temperatures below freezing). sedimentary/climatic archives on a millennial scale for the last thoughts on the manuscript. This project was funded by the Aus- glaciation, the results of old deposits (>120 ka) are inherently trian Science Fund (FWF): P23138. limited to a lower resolution. Nonetheless, luminescence dating of old sediments can provide valuable information about the timing of Appendix A. Supplementary data depositional phases, even more when combined and compared with different dating approaches (e.g. U/Th dating). Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quaint.2014.10.013. 7. Conclusions

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