Scholarly article on topic 'Luminescence Dating of Fluvial Deposits from the Weser Valley, Germany'

Luminescence Dating of Fluvial Deposits from the Weser Valley, Germany Academic research paper on "Earth and related environmental sciences"

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Academic research paper on topic "Luminescence Dating of Fluvial Deposits from the Weser Valley, Germany"


GEOCHRONOMETRIA 42 (2015): 126-138 DOI 10.1515/geochr-2015-0015

Available online at http://www.degruyter.eom/view/j/geochr




'Institut für Geologie, Leibniz Universität Hannover, Callinstraße 30, D-30167 Hannover, Germany 2Leibniz Institute for Applied Geophysics (LIAG), Stilleweg 2, D-30655 Hannover, Germany

Received 25 October 2014 Accepted 25 May 2015

Abstract: Luminescence dating was applied on coarse-grained monomineralic potassium-rich feldspar and polymineralic fine-grained minerals of five samples derived from fluvial deposits of the River Weser in northwestern Germany. We used a pulsed infrared stimulated luminescence (IRSL) single aliquot regenerative (SAR) dose protocol with an IR stimulation at 50°C for 400 s (50 ^s on-time and 200 ^s off-time). In order to obtain a stable luminescence signal, only off-time IRSL signal was recorded. Performance tests gave solid results. Anomalous fading was intended to be reduced by using the pulsed IRSL signal measured at 50°C (IR50), but fading correction was in most cases necessary due to moderate fading rates. Fading uncorrected and corrected pulsed IR50 ages revealed two major fluvial aggradation phases during the Late Pleistocene, namely during marine isotope stage (MIS) 5d (100 ± 5 ka) and from late MIS 5b to MIS 4 (77 ± 6 ka to 68 ± 5 ka). The obtained luminescence ages are consistent with previous 230Th/U dating results from underlying interglacial deposits of the same pit, which are correlated with MIS 7c to early MIS 6.

Keywords: pulsed infrared stimulated luminescence, fluvial deposits, independent age control, Late Pleistocene, Weser valley, northern Germany.


Optically stimulated luminescence (OSL) dating was applied to fluvial deposits in order to give insights into

the timing of fluvial aggradation and degradation (e.g.

Wallinga, 2002; Busschers et at., 2008; Cordier et at., 2010; Lauer et at., 2010). The major difficulty in dating sediments by means of luminescence is mainly caused by the occurrence of insufficient bleaching of the luminescence signal, which is considered a great challenge for especially fluvial deposits (e.g. Murray et at., 1995; Gemmell, 1997; Olley et at., 1999; Stokes et at., 2001). In a fluvial environment, insufficient bleaching can be

Corresponding author: J. Roskosch e-mail:

caused by different environmental conditions, such as water depth, transport distance, and the mode of transport. In the water column, sunlight is being attenuated and therefore generally hampers the probability for the transported minerals to be sufficiently bleached. Furthermore, rapid erosion and transport due to storm, highdischarge and flooding events may also limit the time needed for resetting the luminescence signal (cf. Wallinga, 2002; Jain et al., 2004; Rittenour, 2008). However, luminescence dating of fluvial deposits has been successfully applied in many case studies (Lewis et al., 2001; Wallinga et al., 2001; Rittenour et al., 2005; Briant et al., 2006; Choi et al., 2007; Busschers et al., 2008; Frechen et al., 2008, 2010; Krbetschek et al., 2008; Lauer et al., 2010, 2011). Lauer et al. (2011) compared the quartz and feldspar luminescence ages from fluvial sand samples from the River Rhine intercalated with the Laacher See tephra (12.9 ka). Both quartz and feldspar

ISSN 1897-1695 (online), 1733-8387 (print)

© 2015 J. Roskosch et at. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

ages agreed perfectly with the independent tephra age, suggesting that insufficient bleaching, if any, might not be a problem for Pleistocene samples.

In order to check if the problem related to insufficient bleaching exists, one can perform measurements of multiple luminescence signals with different bleachabilities and compare the obtained results with each other. Such comparison is normally done using quartz and feldspar signals (e.g. Murray et al., 2012). The use of quartz minerals for luminescence measurements is often restricted to younger deposits (<70 ka; e.g. Fuchs and Lang, 2001; Lewis et al., 2001; Wallinga, 2002; Briant et al., 2006; Busschers et al., 2008) due to the lower saturation level of quartz (about 100-200 Gy). The quartz luminescence signal is much more light-sensitive, thus faster to bleach than the feldspar luminescence signal, but feldspar minerals allow for dating comparably older (fluvial) sediments (e.g. Krbetschek et al., 2008; Lauer et al., 2011) due to the higher saturation limit of the luminescence signal. Yet, feldspar minerals may suffer from a certain signal loss over time, referred to as anomalous fading (Wintle, 1973; Aitken, 1985; Spooner, 1994). When the quartz OSL signal cannot be used, equivalent doses or ages obtained from the infrared stimulated luminescence (IRSL) signal measured at low temperatures and the post-IR IRSL signal has also been used for comparison to evaluate the bleaching degree of a sample (Buylaert et al., 2013).

However, in order to identify the limits of different dating methods, including their uncertainties, and to calibrate the chronological framework, independent age control can be substantially helpful. Independent age control can be provided e.g. by additional radiocarbon (14C) dating (e.g., Thomas et al., 2006; Frechen et al., 2008; Murray et al., 2012), electron spin resonance dating (ESR; e.g., Molodkov, 2012; Zhao et al., 2012), amino acid racemization (AAR; e.g., Novothny et al., 2009) or uranium-thorium (230U/Th) dating (this study) of (i) the sediment itself or of (ii) the under- and/or overlying deposits, depending on the availability of appropriate dating material (e.g. organic matter in case of 14C dating). Given the fact that results of all applied dating methods are consistent with each other, the accuracy and reliability of the performed dating technique(s) can be proven.

In this study, we present new feldspar luminescence ages of fluvial deposits in northwestern Germany, which are supported by independent age control based on 230U/Th dating of underlying interglacial deposits. The obtained luminescence ages are of great importance as they shed new light on the previously established Middle to Late Pleistocene depositional model of the studied area.


The study area is located in the southern Weser valley in northwestern Germany (Fig. 1A) and is characterised by up to 530 m high mountain ridges of the Central German Uplands (Fig. 1B). Here, the folded Variscan basement is unconformably overlain by Lower Permian red beds ('Rotliegend'), Upper Permian marine evaporites and carbonates ('Zechstein'), Lower Triassic sandstones ('Buntsandstein') and Middle Triassic shallow marine sediments ('Muschelkalk') (Lepper and Mengeling, 1990; Lepper, 1991). From the late Cretaceous to the Neogene, these sediments experienced uplift, which led to a subsequent incision of the River Weser that formed its isoclinal valley between the Buntsandstein anticlinal at its east and the steep cuestas of the outcropping Lower Muschelkalk at its west during the subsequent Neogene to Late Pleistocene (Grupe, 1912, 1929; Lepper, 1991).

The Nachtigall pit is located at the western flank of the Buntsandstein anticlinal about 5 km southwest of Holzminden (Fig. 1B). The lowermost part of the sedimentary record, probably comprising Middle Pleistocene (Saalian) fluvial deposits of the River Weser (e.g. Rohde, 1989; Rohde et al., 2012), is not exposed in the studied Nachtigall pit but is assumed to occur at an altitude range from about 70-80 m a.s.l. (Rohde et al., 2012). Generally, the term "terrace" is geomorphologically defined as and associated with those deposits preserved above the present floodplain. In this paper, the terms "Older and Younger Middle Terraces and "Lower Terraces" are used on a geochronological basis, referring to those fluvial

Fig. 1. (A) Map of northern Central Europe focusing on northern Germany. The black box marks the study area. The hill-shaded relief model is based on SRTM data. (B) Close-up view of the study area of Weser valley with location of the Nachtigall pit. The hill-shaded relief model (DEM5) is based on data from the Landesamt für Geoinfor-mation und Landesvermessung Niedersachsen (LGLN).

deposits that are considered to have been accumulated during the Middle and Late Pleistocene. Older and Younger Middle Terrace deposits are both considered to have been accumulated after the retreat of the Elsterian glaciation and prior to the advance of the Saalian Drenthe ice sheets (Middle Pleistocene), namely during the early Saalian (Older Middle Terrace) and during the late Saali-an (Younger Middle Terrace). Deposition of the Lower Terrace is linked to the Weichselian glaciation (Late Pleistocene) (Rohde et at., 2012).

The unexposed fluvial deposits are referred to as Older Middle Terrace deposits and are overlain by 13-25 m thick fine-grained interglacial limnic and fen peat of the so-called Nachtigall-Complex. The Nachtigall-Complex ranges over an altitude of about 80-96 m a.s.l. (Rohde et at., 2012; this study). The interglacial deposits are uncon-formably overlain by 8 m thick coarse-grained fluvial sediments, occurring over an altitude range of 96-104 m

a.s.l., deposited by a braided river system (Winsemann et at., 2015) (Figs. 2A and 2B). These fluvial deposits are referred to as Younger Middle Terrace deposits (e.g. Rohde, 1989; Kleinmann et at., 2011; Waas et at., 2011; Rohde et at., 2012).

In the western part of the pit, the lowermost 5 m of the braided river deposits (96-101 m a.s.l.) consist of gravel sheet deposits, which are overlain by up to 1 m thick fine-grained overbank deposits, consisting mainly of ripple cross-laminated and planar-parallel laminated silt and silty sand. These overbank deposits, which are intercalated with up to 0.4 m thick gravel sheet deposits, are truncated and overlain by about 2 m thick gravel sheet deposits (Winsemann et at., 2015) (Fig. 2A). The fluvial deposits in the western and eastern parts of the Nachtigall pit are separated by a major (erosional) bounding surface, characterised by a vertical erosion of about 9 m (Fig. 2B).

Fig. 2. (A) Photo panel and (B) line drawing of the Nachtigall pit. Covered and unexposed deposits are grey, fine-grained interglacial limnic and fen peat deposits of the Nachtigall-Complex occur over an altitude range of about 80-96 m a.s.l. (dark grey) and are overlain by braided river deposits, occurring over an altitude range of about 90-104 m a.s.l. (white), and by loess (light grey). Two major erosional bounding surfaces are indicated (black lines). The lowermost bounding surface separates the interglacial from the overlying fluvial deposits. The uppermost bounding surface of about 9 m separates the western from the eastern fluvial deposits. In the west, braided river deposits (96-104 m a.s.l.) are characterised by gravel sheet deposits, overlain by fine-grained overbank deposits (sample NG5), which are again overlain by gravel sheet deposits. In the east, braided river deposits (about 90-103 m a.s.l.) are characterised by channel belt deposits, lateral and downstream macroforms, and sandy bedforms (samples NG1 to NG4) which are truncated and overlain by gravel sheet deposits (Winsemann et al., in review). Luminescence samples are indicated (black circles). Note that dimensions may be distorted due to panorama view. (C) Schematic outline of the Nachtigall pit. Locations of the photo panel ol Fig. 2A and of the core drilling (black star) referred to in Waas et al. (2011) and Kleinmann et al. (2011) are marked. The 230U/Th samples by Waas et al. (2011) were taken from the core about 175 m northwest from sample NG1.

The fluvial sediments in the eastern part of the pit are at least 15 m thick and consist of channel belt and overbank deposits of a gravelly to sandy braided river system (Winsemann et al., 2015). Exposed fluvial deposits occur over an altitude range of about 90-103 m a.s.l. (Figs. 2A and 2B). Here, the lowermost part is characterised by about 2 m thick channel-fill deposits, passing upwards into lateral and downstream accretion macroforms as well as sandy bedforms, comprising planar-parallel stratified, planar or trough cross-stratified or ripple cross-laminated medium- to fine-grained sand. These deposits are truncated and overlain by about 4 m thick gravel sheets (Winsemann et al., 2015) (Fig. 2A). Locally, deposits are overlain by fine-grained floodplain deposits and draped by loess. The floodplain area of River Weser is expected to comprise Late Pleistocene (Weichselian) fluvial deposits (cf. Rohde et al., 2012). For further detailed information on the sedimentology of the Nachtigall deposits and the large-scale depositional architecture, which is being reconstructed from the outcrop section and digital elevation models, see Winsemann et al. (2015).

Previous research

Reconstruction of the fluvial terrace architecture of the River Weser is largely based on lithostratigraphy and morphology (Rohde, 1983, 1989, 1994). Up to 11 terrace levels were mapped, recording about 170 m of fluvial incision during the Pleistocene (Fromm, 1989; Rohde 1989, 1994).

The Nachtigall pit, which has long been exploited for brick production, must be considered as a key section because its interglacial sediments were intensely analysed especially by means of palynology in order to correlate the deposits with other interglacial successions in Germany and France (e.g. Kleinmann et al., 2011). Studies dealing with the deposits of the Nachtigall pit go back to the 19th century and focused on the interglacial sediments (e.g. Dechen, 1884; Carthaus, 1886; Koken, 1901). The interglacial deposits exposed were allocated either to the Holsteinian (based on pollen analysis; Grupe, 1929) or to the Eemian (based on their stratigraphic position related to the Middle Terrace deposits; Siegert, 1912, 1921; Soergel, 1927, 1939). Much later, Mangelsdorf (1981) performed detailed palynological analysis on the interglacial deposits and proposed a late Cromerian age (Bilshau-sen/Rhume interglacial). Later pollen analysis of the interglacial sediments of the Nachtigall pit did not support such a late Cromerian age but tentatively pointed to a Saalian deposition (Lepper, 1998). Recently, 230U/Th dating and palynological studies on the interglacial limnic sediments support this finding and refer to a deposition during MIS 7c to early MIS 6 (227 -8 ka to 177 ± 8 ka; Kleinmann et al., 2011; Waas et al., 2011). Based on these ages and stratigraphic relations, the underlying fluvial deposits were assumed to have been deposited during MIS 8 and are referred to as Older Middle Terrace

deposits (Kleinmann et al., 2011; Rohde et al., 2012), whereas the overlying fluvial deposits were interpreted to have been deposited during MIS 6 (Kleinmann et al., 2011; Waas et al., 2011; Rohde et al., 2012) and form part of the so-called Middle Terraces that accumulated prior to the Saalian Drenthe glaciation.

So far, much research has been carried out on a lithostratigraphical and palynological basis. However, robust numerical ages only exist for the interglacial deposits and the 230U/Th ages published by Waas et al. (2011) only provide maximum ages for the overlying fluvial deposits. Reliable luminescence ages for the overlying fluvial sediments are still missing, thus hamper the establishment of a chronological framework for these deposits.


Sampling and preparation

Five luminescence samples were taken in 2012 from the fluvial sediments of the Nachtigall pit (Figs. 2A and 2B). Samples NG1, NG2, NG3 and NG4 were taken from sandy bedform deposits from the eastern part of the Nachtigall pit, while sample NG5 was taken from overbank deposits from the westernmost part of the Nachtigall pit (Figs. 2A and 2B). The 230U/Th ages determined by Waas et al. (2011) were derived from interglacial deposits about 175 m northwest of sample NG1 (Fig. 2C).

Sampling and preparation was performed as described in Roskosch et al. (2015). For luminescence measurements, both monomineralic coarse-grained (150-200 ^m) potassium-rich feldspar minerals and polymineralic finegrained (4-11 ^m) minerals were used (Table 1). For coarse-grained minerals, small-sized (2.5 mm) aliquots with about 100-120 grains were created by mounting coarse-grained minerals on 9.8 mm stainless steel discs using silicone spray as an adhesive. Fine-grained minerals (>105 grains; Fuchs et al., 2005, 2013) were mounted on 9.8 mm aluminum discs from a suspension in acetone.

Sample preparation and luminescence measurements were performed at the Leibniz Institute for Applied Geophysics (Hannover, Germany). For luminescence measurements, an automated Rise TL/OSL reader (DA-20) with a calibrated 90Sr/90Y beta source (1.48 GBq = 40 mCi) was used (Better-Jensen et al., 2010). Feldspar

Table 1. Basic information on fluvial samples that were taken for luminescence dating using feldspar minerals.

_ . . . ... Depth Altitude Grain

Sam- Lab Longitudes Latitudes .

° h c a c CI70

NG1 2665 09°24'11.16" 51°48'30.83" 9.50 94.00 150-200

NG2 2666 09°24'11.16" 51°48'30.83" 9.00 94.50 150-200

NG3 2667 09°24'10.90" 51°48'31.90" 6.70 96.80 150-200

NG4 2668 09°24'09.13" 51°48'31.49" 6.30 98.70 150-200

NG5 2828 09°24'07.90" 51°48'31.69" 2.50 99.00 4-11

signals were stimulated by pulsing by IR light-emitting diodes (LED) either using an external pulsing box (Thomsen et al., 2008a) or a pulsed stimulation attachment (Lapp et al., 2009). A Schott BG39/Corning 7-59 filter combination was used and the feldspar signals were detected in the blue-violet (320-460 nm) during the offperiods of each pulse cycle, with a delay of 5 ^s after the LED pulses switched off.

Equivalent dose and dose rate determination

For equivalent dose (De) determination, 10 aliquots per sample were measured using a pulsed IRSL single aliquot regenerative (SAR) dose protocol (Table 2). A preheat at 250°C for 60 s was used, followed by a pulsed IR stimulation at 50°C for 400 s with 50 ^s on-time and 200 ^s off-time. Only off-time signal was recorded because it was found to give a stable luminescence signal (Tsukamoto et al., 2006). The pulsed IRSL signal at 50°C (IR50) was chosen over the elevated temperature post-IR IRSL signal (pIRIR; Thomsen et al., 2008b) because it appears to be more sensitive to light. Comparison of both pulsed IR50 and pIRIR290 results showed that pIRIR290 De values were generally higher by about 100-150 Gy than the pulsed IR50 ones (see Fig. 3B in Roskosch et al., 2015). Jain et al. (2015) compared the residual dose obtained from a modern beach sample using continuous wave (CW) IR50, pulsed IR50, pIRIR225 and pIRIR290 signals and a much larger residual dose of ~10 Gy was obtained from the pIRIR290 signal than all the other signals (less than 2 Gy). This was probably caused by the hard to bleach nature of the pIRIR290 signal, as has been supported by results of a bleaching study performed by Kars et al. (2014). Based on the above mentioned findings, we focused on the pulsed IR50 signal for De determination of the fluvial sediments of this study.

The net feldspar luminescence signal was then calculated from the middle part of the decay curve (21-60 s) after subtracting a late background of the last 50 s (see Roskosch et al., 2015). The initial part of the decay curve (0-20 s) was actually reported to give considerably higher fading rates (up to 4.42 ± 0.46%), whereas the middle part was found to show only negligible anomalous fading (see Roskosch et al., 2015).

Aliquots were accepted when they passed for following criteria: recycling ratio limit within 10% of unity; maximal test dose error 10%; signal intensity larger than 3 sigma above background. We assumed a measurement error of ±2.0%. In order to calculate De values, dose response curves were fitted using a single-saturating exponential function.

For dose rate determination, the radionuclide concentration of uranium (238U), thorium (232Th) and potassium (40K) was determined by high-resolution gamma spec-trometry. For coarse-grained feldspar minerals, an internal potassium content of 12.5 ± 0.5% was assumed (Huntley and Baril, 1997). The a-value was set to 0.15 ± 0.05 for monomineralic coarse-grains (Balescu

Table 2. Pulsed IRSL SAR protocol for feldspar measurements.

Run Treatment_

1 Dose

2 Preheat, 60 s @ 250°C

3 Pulsed IR stimulation, 400 s @ 50°C

4 Test dose

5 Preheat, 60 s @ 250°C

6 Pulsed IR stimulation, 400 s @ 50°C

7 Pulsed IR stimulation, 100 s @ 200°C

8 Return to step 1_

Fig. 3. Results of dose recovery and recycling ratio tests.

and Lamothe, 1994) and 0.08 ± 0.02 for polymineralic fine-grains (Lang et al., 2003), respectively. Cosmic radiation was corrected for altitude and sediment thickness after Prescott and Hutton (1994). Water content was measured using samples from the direct surroundings of the luminescence samples, and was 7% (NG4), 9% (NG3, NG5), 10% (NG1) and 11% (NG2). Based on these values, the overall water content was then set to an average value of 10 ± 5% for both the coarse-grained braided river and the fine-grained overbank deposits. Dosimetry results are provided in Table 3.

Performance tests

Dose recovery experiments on three aliquots of each sample were performed prior to De measurements to check for the suitability of the applied SAR protocol under laboratory conditions. Within the Rise TL/OSL reader, aliquots were bleached by IR diodes and then given a similar beta dose that was close to the natural expected one (271 Gy for samples NG1, NG2, NG3 and nG4; 401 Gy for sample NG5). Afterwards, the same

Table 3. Dosimetry results, dose rates and total dose rate of coarse-grained monomineralic potassium-rich feldspar and polymineralic fine-grained minerals. The a value was 0.15 ± 0.05 for monomineralic coarse-grains (Balescu and Lamothe, 1994) and 0.08 ± 0.02 for polymineralic fine-grains (cf. Lang et al., 2003). The average water content for all samples was 10 ± 5%.

Dosimetry Dose rates Total dose

Sample Uranium Thorium Potassium Da Dß Dy Dinternal Dcosmic rate

(ppm) (ppm) (%) (mGy/a) (mGy/a) (mGy/a) (mGy/a) (mGy/a) (mGy/a)

NG1 1.20 ± 0.01 4.91 ± 0.03 1.97 ± 0.01 0.10 ± 0.06 1.49 ± 0.06 0.71 ± 0.05 0.69 ± 0.09 0.05 ± 0.01 3.04 ± 0.13

NG2 1.73 ± 0.01 7.04 ± 0.03 2.08 ± 0.01 0.14 ± 0.07 1.65 ± 0.06 0.86 ± 0.05 0.69 ± 0.09 0.06 ± 0.01 3.39 ± 0.14

NG3 2.16 ± 0.01 8.98 ± 0.03 2.57 ± 0.01 0.17 ± 0.07 2.04 ± 0.06 1.07 ± 0.05 0.69 ± 0.09 0.08 ± 0.01 4.06 ± 0.14

NG4 2.43 ± 0.02 8.03 ± 0.03 2.23 ± 0.01 0.17 ± 0.07 1.83 ± 0.06 0.99 ± 0.05 0.69 ± 0 09 0.08 ± 0.01 3.77 ± 0.14

NG5 2.65 ± 0.02 11.04 ± 0.04 2.49 ± 0.01 0.85 ± 0.18 2.33 ± 0.09 1.25 ± 0.09 - 0.16 ± 0.02 4.57 ± 0.22

SAR protocol was applied to check if the given dose could be accurately recovered.

Recycling ratio tests were conducted by applying the same dose twice (namely at the beginning and at the end of the measurement). A recycling ratio value that is within 10% of unity (0.9-1.1; Wallinga et al., 2000) indicates that sensitivity changes which might occur during measurement were successfully corrected. Dose recovery and recycling ratios are presented in Table 4 and Fig. 3.

Recuperation was calculated from a zero-dose point in order to check if thermally-transferred charge from light-insensitive traps to the luminescence traps occurred. A recuperation level of <5% of the natural signal is acceptable (Wallinga et al., 2000). Recuperation values are presented in Table 4.

Fading tests and age calculations

Feldspar minerals have been observed to show an instability of the luminescence signal, which is also known as anomalous fading (Wintle, 1973; Aitken, 1985; Spooner, 1994). This signal loss over time results in (significantly) lower, thus severely underestimated IRSL

ages. Huntley and Lamothe (2001) proposed a fading correction model, which was applied to three aliquots of each of our samples to obtain fading rates (g-values). Based on Thiel et al. (2011) and Buylaert et al. (2012), g-values below the threshold of ~1.5% per decade were considered to be laboratory artefacts, thus samples with g-values above this threshold called for fading corrections. For comparison, we calculated g-values for the pIRIR225 and pIRIR290 signals of sample NG5 (n = 3). In both cases, g-values were above the threshold of 1.5% per decade. At the same time, they were higher than the pulsed IR50 g-value of sample NG5 of 0.6 ± 0.2% per decade, namely 2.8 ± 0.2% per decade (pIRIR225) and 2.0 ± 0.3% per decade (pIRIR290; Table 4). This additional test proved that the use of the pulsed IR50 signal did not only benefit from a more stable and faster to bleach signal but also used that part of the signal that showed comparably less fading at least for this sample. Fading rates, fading uncorrected and corrected pulsed IR50, pI-RIR225 and pIRIR290 ages are shown in Table 4.

Final ages were calculated taking into account the mean pulsed IR50 De values of all accepted aliquots

Table 4. Results of luminescence measurements using the (A) pulsed IR50 signal, (B) the pIRIR225 signal, and (C) the pIRIR290 signal, including number of measured aliquots (nm) and number of aliquots taken for age calculation (nc), mean recycling ratios, dose recovery ratios, mean recuperation, total dose rates, fading rates (g-values), mean De values, and fading uncorrected and fading corrected ages. Final ages are written in bold.

(A) pulsed IR50 nm/nc Mean recycling ratio Dose recovery ratio Mean recuperation (%) Total dose rate (mGy/a) g-value (% per decade) Mean pulsed IR50 De (Gy) Uncorr. pulsed IR50 age (ka) Corr. pulsed IR50 age (ka)

NG1 10/10 1.04 ± 0.04 0.95 ± 0.00 5.3 3.04 ± 0.13 2.5 ± 0.1 177 ± 3 58 ± 3 73 ± 3

NG2 10/10 1.03 ± 0.04 0.95 ± 0.00 5.1 3.39 ± 0.14 2.7 ± 0.4 202 ± 4 59 ± 3 77 ± 6

NG3 10/10 1.03 ± 0.04 0.94 ± 0.04 4.6 4.06 ± 0.14 2.1 ± 0.4 227 ± 3 56 ± 2 68 ± 5

NG4 10/10 1.03 ± 0.04 0.94 ± 0.02 4.8 3.77 ± 0.14 2.6 ± 0.2 216 ± 2 57 ± 2 73 ± 4

NG5 10/09 1.02 ± 0.04 1.01 ± 0.01 2.8 4.57 ± 0.22 0.6 ± 0.2 456 ± 5 100 ± 5 105 ± 6

(B) pIRIR225 nm/nc Mean recycling ratio Dose recovery ratio Mean recuperation (%) Total dose rate (mGy/a) g-value (% per decade) Mean IR225 De (Gy) Uncorr. IR225 age (ka) Corr. IR225 age (ka)

NG5 6/6 1.02 ± 0.06 - 1.74 4.57 ± 0.22 2.8 ± 0.2 412 ± 5 90 ± 4 119 ± 7

(C) pIRIR290 nm/nc Mean recycling ratio Dose recovery ratio Mean recuperation (%) Total dose rate (mGy/a) g-value (% per decade) Mean IR290 De (Gy) Uncorr. IR290 age (ka) Corr. IR290 age (ka)

NG5 6/6 1.02 ± 0.06 - 2.19 4.57 ± 0.22 2.0 ± 0.3 474 ± 27 103 ± 7 125 ± 12

(Table 4). The age error of an uncorrected pulsed IR50 age was calculated by taking the 1-sigma standard error of the obtained De value. The age error of a corrected pulsed IR50 age was calculated by adding the uncorrected age error to the mean age error.


For all luminescence samples, dose response curves and frequency-De histograms as well as radial plots were

100 150 200 Dose (Gy)

Dose (Gy)

mean FSP De = 202 Gy

100 150 200 Dose (Gy)

250 300

❖ natural De o recycling ratio

200 300 400 Dose (Gy)

mean FSP De = 227 Gy

100 150 200 Dose (Gy)

250 300

200 300 400 Dose (Gy)

mean FSP De - 216 Gy

U_ J 210-

50 100 150 200 250 300 Dose (Gy)

200 300 400 Dose (Gy)

mean FSP De - 456 Gy

Fig. 4. Representative frequency-De histograms (top) including mean feldspar De values (solid line), dose response curves (bottom) and decay curves (inset) of luminescence samples NG1, NG2, NG3, NG4 and NG5.

200 300 400 Dose (Gy)

Relative Error (%) 36 18 12 9

Relative Error (%) 36 18 12 9

36 18 12

Relative Error (%) 36 18 12 9

Fig. 5. Radial plots of all samples. Measured aliquots are all within the 2-sigma-level of the mean De values.

£ 2 -I » ■

Relative Error {%) 36 18 12 £

created based on the accepted aliquots (Figs. 4 and 5). Dose response curves are characterised by single saturating exponential growth. For the frequency-De histograms, bin widths are close to the median of De values as suggested by Lepper et al. (2000). Frequency-De histograms are characterised by very narrow and tight De distributions (Fig. 4) and radial plots are characterised by De values which are all within the 2-sigma range of the mean De value (Fig. 5).

Results of dose recovery and recycling ratio tests are all satisfying and in the acceptable range of 10% of unity (0.9-1.1; Fig. 3; Wallinga et al., 2000). Dose recovery ratios range between 0.94 ± 0.04 (NG3) to 1.01 ± 0.01 (NG5; Table 4). These results indicate that the applied SAR protocol is able to reliably recover a given dose, creating consistent De values. Recycling ratios range between 1.02 ± 0.04 (NG5) and 1.04 ± 0.04 (NG1; Table 4). Sensitivity changes that might occur during measurements were successfully corrected by the chosen SAR protocol. Recuperation values were all <5% of the natural signal (Table 4) and therefore in an acceptable range (Wallinga et al., 2000).

The obtained g-values for all samples were between 2.1 ± 0.4% per decade (NG3) and 2.7 ± 0.04% per decade (NG2) for monomineralic coarse-grains and 0.6 ± 0.2% per decade (NG5) for polymineralic fine-grains. Due to

their higher g-values, the determined fading uncorrected pulsed IR50 ages of the coarse-grained samples NG1 to NG4 needed a subsequent fading correction (Thiel et al., 2011; Buylaert et al., 2012). Fading uncorrected and corrected pulsed IR50 ages are presented in Table 4. Dose rate results gave values ranging from 3.04 ± 0.13 mGy/a (NG1) to 4.57 ± 0.22 mGy/a (NG5; Table 3).

Final depositional ages point to two major deposition-al phases. Sample NG5 gave a fading uncorrected pulsed IR50 age of 100 ± 5 ka, indicating a Late Pleistocene (Early Weichselian) deposition, correlating with MIS 5d. Samples NG1 to NG4 gave fading corrected pulsed IR50 ages ranging from 77 ± 6 ka (NG2) to 68 ± 5 ka (NG3), pointing to a Late Pleistocene (Early Weichselian to Early Pleniglacial) deposition which can be correlated with late MIS 5b to MIS 4. These ages reveal a chronological gap of about 12 ka between the Late Pleistocene MIS 5d and MIS 5b to MIS 4 fluvial sediments which seems to coincide with the major (erosional) bounding surface of about 9 m, separating the western and older from the eastern and younger fluvial sediments (Figs. 2A and 2B). Interpretation of the large-scale terrace architecture led to the assumption that the fluvial deposits display laterally attached terraces (Winsemann et al., 2015), which form when either both rates of fluvial aggradation and degradation are balanced or the generation of accommodation is low (Archer et al., 2011).


Luminescence results: reliable and robust?

Since feldspar minerals are known to suffer from anomalous fading, it is recommended to use only those parts of the IRSL signal which are less fading-dependent (e.g., Thiel et al., 2011). We followed the approach by Roskosch et al. (2015) who stated that the middle part of the decay curve of the pulsed IR50 signal is characterised by a more stable, thus less fading-dependent luminescence signal when compared to other (parts of the) signals. The results of the additionally applied fading test of sample NG5 using the pulsed IR50, pIRIR225 and pIRIR290 signals suggests that the pulsed IR50 signal is more stable than the pIRIR225 and pIRIR290 signal (Table 4), confirming the use of the pulsed IR50 signal. However, the applied fading tests for the other four samples indicated that some effect of anomalous fading was still present within our samples and fading correction seemed to be necessary for most of the samples. Table 4 shows that fading un-corrected pulsed IR50 ages underestimated the fading corrected pulsed IR50 ages by up to about 18 ka (NG2). So far, correction models for older samples (e.g. Lamothe et al., 2003; Kars et al., 2008) have not been tested on an accurate basis. Huntley and Lamothe (2001) strongly advise against using their correction model for (comparably) older deposits because their model is just applicable to the 'linear' part of the decay curve, thus (comparably) younger sediments. However, we followed the promising studies of Buylaert et al. (2011) and Roskosch et al. (2015), who successfully generated fading corrected ages of Middle Pleistocene (Elsterian, Saalian, Eemian) sediments. Consequently, we believe that the effect of age underestimation based on the occurrence of anomalous fading was minimized as far as possible by both using a more stable luminescence signal (Tsukamoto et al., 2006) and applying a suitable fading correction model (Huntley and Lamothe, 2001).

Age overestimation is commonly linked to the occurrence of insufficient bleaching of the luminescence signal prior to deposition. We additionally performed bleaching tests for the CW IR50 (obtained as a part of the pIRIR225 sequence) and pIRIR225 signals and the pulsed IR50 signals of samples NG2 and NG5. Natural aliquots of both samples were bleached in a Honle SOL2 solar simulator for different bleaching durations between 0 and 6 hours and the remaining sensitivity-corrected signal intensity was plotted against the natural signal intensity. The results clearly demonstrate that the pulsed IR50 signal is much faster to bleach (sample NG5) or bleaches in a similar way (sample NG2) as the pIRIR225 signal, although this signal is harder to bleach than the CW IR50 signal (Fig. 6). Taking the remaining signal after 30 minutes bleaching as an example, the pulsed IR50 signal for both samples bleached to ~4-6% of the natural, whereas the pIRIR225 signal has 6-11% remaining signal. Since the pulsed IR50 signal is considered to be much

more light-sensitive than other elevated temperature pI-RIR signals (e.g. Jain et al., 2015; Roskosch et al., 2015) but has not been used widely so far (e.g. Roskosch et al., 2015), our objective was to use this stable, less fading-dependent and faster bleachable signal in order to provide new pulsed IR50 ages. However, we only conducted comparative bleaching measurements of different IRSL signals on one coarse-grained sample (sample NG2) which is probably more prone to insufficient bleaching than the fine-grained sample of NG5. A definite exclusion of insufficient bleaching for all of the coarse-grained samples NG1 to NG4 can therefore not be made. However, as the last depositional ages of the coarse-grained samples are consistent within their age errors, we conclude that insufficient bleaching does not seem to be of great significance for these samples. The comparison of ages obtained from different IRSL signals for sample NG5 also demonstrated that although the fading corrected pIRIR225 and pIRIR290 ages slightly overestimated the pulsed IR50 age, the three ages agreed within their 2-sigma uncertain-

Fig. 6. Results of bleaching tests for the CW IR50 (white circle), pIRIR225 (black circle) and pulsed IR50 signals (white square) of sample NG2 and NG5, clearly showing that the pulsed IR50 signal bleaches a lot faster than the pIRIR225 signal, which is considered to be even harder bleachable than the CW IR50 signal.

m a.s.l.

Lithology Interpretation

* i • NG4

NG3 • fluvial

\ NG2 .

\NG1 •

Final ages

100±5 ka 73±4 ka

68±5 ka 77±6 ka 73±3 ka

0 limnic a. £ peat E 0 -i peat D n O) u peat B z peat A2



177±8 ka 206±6 ka

201+15/-13 ka 227+9/-8 ka

I • | fluvial deposits with locations of luminescence samples and a major erosional bounding surface (Rohde et al., _ 2012; this study)

^^ limnic deposits with fen peat layers and locations of 230U/Th samples (Kleinmann etal., 2011; Waas et al., 2011)

Fig. 7. Schematic log of the investigated deposits of the Nachtigall pit, showing lithology, interpretation and final pulsed IR50 and 230U/Th ages. The major erosional bounding surface is indicated by a (dashed) line (see Fig. 2B). The altitudes of the fluvial sediments are derived from Rohde et al. (2012) and from GPS measurements of the recent Nachtigall pit (this study). The altitudes and lithology boundaries of the interglacial Nachtigall-Complex deposits were based on Kleinmann et al. (2011) and Waas et al. (2011) who derived their data from a core drilled about 175 m northwest of sample NG1 (see Fig. 2C). Here, altitude of the surface during drilling was 108.55 m a.s.l.

ties (Table 4B and 4C). Again, this suggests that the bleaching condition prior to deposition does not seem to have been a major problem for sample NG5.

Stratigraphic significance of the luminescence results

Our ages are stratigraphically agree with each other and are also consistent with the 230U/Th ages of the underlying interglacial deposits, which were correlated with MIS 7c to early MIS 6 (Waas et al., 2011; Fig. 7). The obtained luminescence ages are of great value when evaluating the previously established Middle to Late Pleistocene fluvial depositional model (e.g. Rohde et al., 2012). On the one hand, the occurrence of fluvial sediments which were assumed to be younger than the underlying interglacial sediments could be proven. However, fluvial deposition did not occur during the (Middle Pleistocene) Saalian, as had previously been assumed (e.g. Rohde et al., 2012), but during the (Late Pleistocene) Early Weich-

selian to Early Pleniglacial (MIS 5d, late MIS 5b to MIS 4; Table 4). This is comparable to the study of Cor-dier et al. (2014) who also found by using luminescence dating techniques that deposits of a presumably Saalian age were actually deposited during the Weichselian.

On the other hand, the occurrence of Late Pleistocene (Weichselian) fluvial deposits was expected to occur only in the floodplain area of River Weser (cf. Rohde et al., 2012). The previous depositional model has to be revised due to the obtained luminescence ages of samples NG1 to NG4, pointing to an Early Weichselian to Early Plenigla-cial deposition for this part of the pit. It is, however, likely that the adjacent valley area is characterised by gravelly and sandy fluvial sediments (referred to as Lower Terrace deposits, cf. Rohde et al., 2012), which had been deposited afterwards and which may be underlain by Saalian fluvial deposits, as has been described by Rohde et al. (2012). So far, these fluvial sediments have not been dated. Therefore, the Late Pleistocene sedimentary complex seems to have been subdivided into two fluvial sediment bodies.

The first phase of fluvial aggradation occurred around 100 ± 5 ka, correlating with MIS 5d, whereas the second phase of fluvial aggradation was found to have occurred from 77 ± 6 to 68 ± 5 ka, mainly correlating with late MIS 5b to MIS 5a (Table 4). It may, however, have continued until early MIS 4. The timing of vertical erosion (incision) of about 9 m (Figs. 2A and 2B) is difficult to determine but is likely to have occurred somewhere during MIS 5d to MIS 5c, which would be in accordance with data from France (Moselle and Meurthe: e.g. Cordi-er et al., 2010; Somme, Seine, Yonne: Antoine, 1994; Antoine et al., 2007) and Germany (Leine valley: Winsemann et al., 2015)

The luminescence dating results have shown that comparison with independent age control is of great importance. Given the important value of the obtained luminescence ages, additional numerical dating approaches need to be performed and thus complement the chronos-tratigraphic framework of the deposits of the Nachtigall pit.


We present new luminescence ages from five fluvial samples of the Nachtigall pit in the southern Weser valley in northwestern Germany. Luminescence measurements on monomineralic feldspar coarse-grains and poly-mineralic fine-grains were performed using a pulsed IRSL SAR protocol. Luminescence ages are consistent with 230U/Th ages of underlying interglacial deposits (Waas et al., 2011).

- Luminescence samples passed required performance tests, and results of dose recovery and recycling ratio tests as well as recuperation values were satisfyingly acceptable.

- Additional bleaching tests for samples NG2 and NG5, and fading testes and measurements using the pI-RIR225 and pIRIR290 signals were performed on sample NG5. The effect of insufficient bleaching of the coarse-grained sample cannot be entirely ruled out because only one sample was measured. However, as the ages for all coarse-grained samples agree within their age errors, insufficient bleaching is considered to be a random feature and assumed to only play a negligible role. Comparison with the obtained g-values of the pulsed IR50 signal showed that the pulsed IR50 signal does bleach faster (sample NG5) or bleaches in a smiliar way (sample NG2). For sample NG5, De values were in the order of pIRIR225 < pulsed IR50 < pIRIR290 but all fading corrected ages agreed within their 2-sigma uncertainties. Based on these observations, insufficient bleaching is not considered a major issue for the studied samples.

- Numerical dating results point to two phases of fluvial aggradation, which occurred during the Late Pleistocene. One polymineralic fine-grained sample (NG5) was derived from fine-grained overbank deposits from the western part of the succession and gave an uncorrected feldspar age of 100 ± 5 ka (MIS 5d; Early Weichselian). Four coarse-grained samples (NG1, NG2, NG3, NG4) were derived from medium- to fine-grained sandy deposits, interbedded into gravel sheet deposits, lateral and downstream accretion macroforms in the eastern part of the succession, giving corrected feldspar ages ranging from 77 ± 6 ka to 68 ± 5 ka (late MIS 5b to MIS 4; Early Weichselian to Early Pleniglacial).

- The fluvial deposits overlying the Nachtigall-Complex are indeed younger as has previously been assumed (Rohde et al., 2012). However, the obtained luminescence ages contradict an expected Middle Pleistocene Saalian deposition. The depositional model for this part of the Nachtigall pit has to be revised, indicating the possible occurrence of a solely Late Pleistocene laterally attached terrace complex. This complex is characterised by a major erosional bounding surface which separates the western and older from the eastern and younger fluvial deposits.


We gratefully acknowledge financial support by the LU Hannover. Constructive comments by two anonymous reviewers are highly appreciated and helped to improve the manuscript. Many thanks are due to J.-U. Müller of Bauunternehmen Jens Müller GmbH for permitting us to work on his property at the Nachtigall pit. Sincere thanks are given to P. Rohde for drawing our attention to the Nachtigall pit, for assistance during field work, for fruitful discussions and personal comments on the draft version of the manuscript. J. Lang, J. Lepper, A. Osman, L. Pollok, A. Weitkamp and J. Winsemann are

thanked for field work and helpful discussion, and S. Riemenschneider is thanked for technical support in the luminescence laboratory.


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