Over the last decades, luminescence dating of sediments has been established as an indispensable tool in Quaternary research (e.g. Rittenour, 2018). This is in particular due to several methodological developments that have improved over the years the reliability, expanded the dating range and introduced new materials and concepts. Most of these developments such as the single-aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000), post-IR infrared stimulated luminescence (IRSL) (Buylaert
First and of ubiquitous importance, it has to be highlighted that the determination of dose rate-relevant elements (K, Th, U) is not without problems. In a comparative study, it was the dose rate that showed large differences between different luminescence laboratories (Murray
Presented here is a dating application that addresses all four aforementioned issues related to dose rate determination. The material investigated is from two scientific drill cores in the village of Niederweningen in northern Switzerland that is known for the Pleistocene fauna that has been discovered there, in particular the remains of several individuals of mammoth, and related environmental reconstructions (Furrer
Niederweningen is located in the overdeepened trough of Wehntal, a 5-km-long east-west orientated valley in the northern Alpine foreland of Switzerland (
Previous research has focussed on sites located a few 100 m apart from the sites analysed here (
In this study, we illustrate the challenges of dating sediments deposited in such a complex setting with luminescence methods by analysing two drill cores with a diameter of 220 mm, which were recovered during a drilling campaign utilising a rotary drill rig in late 2018. The cores NW2018/2 (16.0 m length) and NW2018/3 (16.7 m length) are located more than 500 m NW of previous coring locations (
Logs of the cores NW2018/2 and NW2018/3 illustrating the LTs, lithology and OSL sample locations as well as the layer model used for the individual OSL samples. LTs, lithotypes; OSL, optically stimulated luminescence.
Within core NW2018/2, the organic-rich sequence spans from 7.62 m to 4.17 m depth and is associated with thin interbedded silts and clays. Beneath 7.10 m, these silts are light grey to green and contain mollusc shells (LT: 3c), while above, the clastics interbedding with the peats are predominantly dark grey, organic-rich silty clays (LT: 2b) or dark brown organic-rich sandy silts (LT: 3d). The organic-rich deposits can be differentiated into dark brown silty gyttia with few plant remains (LT: 1a), compacted dark brown to black muddy-silty peat with few wood fragments (LT: 1b) and dark brown to silty peat many wooden fragments (LT: 1c). Above the organic-rich interval, sandy silts of light grey to dark grey colour and with small plant remains occur (LT: 3f).
Within core NW2018/3, the organic-rich sequence spans from 5.05 m to 2.47 m depth. In contrast to NW2018/02, the interbedded clastic layers are generally of coarser nature, highlighted by the occurrence of a dark grey pebbly silty sand in which plant remains are observed (LT: 4b), and the presence of pebbles within one of the peat layers. Above the peat interval, graded sands with large pebbles occur (LT: 4a).
After drilling, the two cores were stored indoors in wooden boxes and were split in half just prior to sampling for luminescence dating. The cut surfaces were carefully prepared and photo-documented prior to further analysis. Following a macroscopical sedimentological description, samples were taken every 10 cm for grain size and compositional analysis. For laser-optical grain size analysis, a first batch of all samples were pre-treated with 20% H2O2 at 70°C for 24 h to degrade organic matter. Thereafter, Calgon (a solution of 33 g sodium hexametaphosphate and 7 g sodium carbonate) was added to the samples, and after a 24-h period, analysis was performed with a Malvern Mastersizer 3000 (Abdulkarim
Since the cored sediments were directly transferred to wooden boxes (no use of opaque liners), samples for luminescence dating had to be taken as large pieces of compact material (typically 10–20 cm length in all dimensions; a minimum of 1 kg of material). A first batch of samples was taken few weeks after the drilling operation, when cores were kept in a cool place (sample numbers up to 11). A second batch of samples (sample numbers above 20) was taken more than 1 year after drilling, when sediment in part had substantially dried out. An overview of the location and sedimentary context of the different samples is given in
Overview of samples and their sedimentary context.
NW18/2-11 | 148 | Located within a thick sequence of light grey to dark grey sandy, muddy silts |
NW18/2-10 | 224 | Same as aNW18/2-11, with the sand fraction more dominant |
NW18/2-09 | 375 | Located about 40 cm above the peat-rich interval in a dark grey silt, with 15–cm-thick organic-rich silty clay separating the sampled silt from the uppermost peat |
NW18/2-21 | 430 | Within the topmost muddy-silty peat, which has a thickness of ~30 cm. Over and underlain by organic-rich silty clays |
NW18/2-08 | 442 | Located at the top of 20-cm-thick silty clay, which sits in between two at least 30-cm-thick compacted silty peat layers |
NW18/2-22 | 455 | Same as NW18/2-08 but at the bottom of the silty clay layer |
NW18/2-23 | 530 | Within a thin (~15 cm) silty gyttia layer, which is over and underlain by black muddy peats with few wood fragments |
NW18/2-07 | 545 | Within a thin (~10 cm) organic-rich sandy silt with plant remains. Overlain by black muddy peats and underlain by a dark brown peat with many large wood fragments |
NW18-2-24 | 570 | Within a dark brown silty peat with many large wooden fragments, overlain by a thin (~10 cm) organic-rich sandy silt with plant remains and overlying a thick dark grey silty clay with organic-rich layers |
NW18/2-06 | 610 | Located at the top of dark grey silty clay with organic-rich layers, overlain by a dark grey organic-rich silt |
NW18/2-25 | 749 | Located within a thin (~15 cm) light grey to white silt with small plant remains and mollusc shells which is interbedded between a black muddy peat (top) and silty gyttia (bottom) |
NW18/2-05 | 788 | Within a thick light grey silt with mollusc shells |
NW18/3-11 | 232 | Located within a grey sandy, pebbly silt with plant remains. Overlain by a graded sand and overlying a compacted thin black silty peat |
NW18/3-21 | 323 | A thin (8 cm) grey silt with small plant remains, interbedded within compacted silted peats |
NW18/3-10 | 335 | Dark grey silty organic-rich clay, over and underlain by very thin (<5 cm) silt and sandy layers which are packaged within black peats |
NW18/3-09 | 406 | Within a dark grey sandy silt with plant remains and peat fragments. Overlain by a dark grey pebbly sand and underlain by a black compacted peat |
NW18/3-22 | 438 | Within a thick (~45 cm) black peat layer, which is over and underlain by organic-rich grey silts |
NW18/3-23 | 496 | Within a thin (10 cm) light grey silt with mollusc shells. Overlain by a dark brown organic-rich sand silt and overlying a thin layer of black peat |
NW18/3-08 | 518 | At the top of a thick (>1 m) light grey sandy silt, overlain by an organic-rich sand silt with small plant remains |
Under subdued red-light conditions in the laboratory, the outer light-exposed parts of the lumps were first generously removed (>1 cm), gaining some 100 g of sediment that was not exposed to light. The removed sediment was used for dose rate and other analyses. Part of the remaining material (some 10 g) was subsequently treated with 10% HCl, 30% H2O2 and Na-oxalate to remove carbonates and organic material as well as to dissolve clay particles. The fraction 4–11 μm was enriched by settling (Stokes’ law) using a combination of Atterberg cylinders and centrifuging (cf. Frechen
The measurement set-up was chosen to be similar (close to identical) to previous studies at this site (Preusser and Degering, 2007; Anselmetti
Characterisation of luminescence properties exemplified for sample NW19-3-8. Shown are
The characteristics of the luminescence behaviour are summarised in
High-resolution gamma spectrometry at the University of Freiburg was carried out using a high-purity germanium (HPGe) detector (Ortec GMX30P4-PLB-S, n-type coaxial, 30% relative efficiency, 1.9 keV Full Width at Half Maximum at 1.33 MeV). After drying the sample material at 50°C, containers with a diameter of 75 mm and a height of 30 mm were completely filled with homogenised sediment (crushed to <2 mm), sealed with adhesive tape, and stored for at least 4 weeks to build up equilibrium between radon (Rn-222) and its daughters. The sediment mass varied between 75 g and 161 g per sample depending on its density. After storage, the sample containers were placed on the carbon fibre endcap of the detector and were measured for 3 days or 4 days, respectively, to determine the activities of primordial radionuclides K-40, Th-232 and U-238. The detector is installed in a massive lead shielding to minimise the influence of the environmental radioactivity. Additionally, a blank sample (empty container) was measured to account for background radiation.
The gamma spectrometer was calibrated using the certified Weichselian loess from Nußloch quarry near Heidelberg, Germany (Koeln Loess, Potts
Results of repeated gamma spectrometry measurements of reference material ‘Koeln Loess’ in comparison with NAA and reference values.
U-238 | Th-234 | 63.3 | 28.0 ± 1.2 | 35.6 ± 1.4 | 33.9 ± 1.4 | 34.3 ± 1.4 | ||
Pb-214 | 295.2 | 32.1 ± 0.7 | 34.1 ± 0.7 | 33.8 ± 0.7 | 34.2 ± 0.7 | |||
Pb-214 | 351.9 | 32.0 ± 0.7 | 33.2 ± 0.7 | 33.6 ± 0.7 | 33.3 ± 0.7 | |||
Bi-214 | 609.3 | 31.9 ± 0.7 | 33.3 ± 0.7 | 33.5 ± 0.7 | 33.0 ± 0.7 | |||
Bi-214 | 1120.3 | 29.5 ± 0.6 | 36.5 ± 0.8 | 33.5 ± 0.7 | 35.1 ± 0.7 | |||
Bi-214 | 1764.5 | 32.8 ± 0.7 | 34.1 ± 0.7 | 33.6 ± 0.7 | 32.7 ± 0.7 | |||
Concentration | (ppm) | 2.56 ± 0.10 | 2.77 ± 0.10 | 2.72 ± 0.01 | 2.72 ± 0.08 | 2.49 ± 0.19 | 2.70 ± 0.19 (2.75 ± 0.09) | |
Th-232 | Ac-228 | 338.3 | 32.4 ± 0.7 | 32.5 ± 0.7 | 34.1 ± 0.8 | 32.0 ± 0.7 | ||
Ac-228 | 911.1 | 34.0 ± 0.8 | 32.9 ± 0.7 | 32.4 ± 0.7 | 32.7 ± 0.7 | |||
Ac-228 | 969.1 | 34.2 ± 0.8 | 33.3 ± 0.7 | 31.7 ± 0.7 | 31.7 ± 0.7 | |||
Pb-212 | 238.6 | 32.1 ± 0.6 | 33.3 ± 0.7 | 33.1 ± 0.7 | 32.8 ± 0.6 | |||
Tl-208 | 583.2 | 33.3 ± 0.8 | 33.7 ± 0.8 | 33.0 ± 0.7 | 33.6 ± 0.8 | |||
Concentration | (ppm) | 8.16 ± 0.24 | 8.16 ± 0.12 | 8.10 ± 0.45 | 8.02 ± 0.17 | 7.68 ± 0.39 | 8.11 ± 0.47 (8.27 ± 0.21) | |
K-40 | - | 1460.8 | ||||||
Concentration | (%) | 1.10 ± 0.02 | 1.09 ± 0.02 | 1.08 ± 0.02 | 1.09 ± 0.02 | 1.04 ± 0.06 | 1.08 ± 0.02 (1.11 ± 0.01) |
NAA, neutron activation analyses.
The spectrometer used was calibrated against values given in Potts
Conversion of activity into concentrations follows Guérin
Bold values represent mean activity values.
Gamma ray spectrum of the reference loess material (Koeln Loess). Sample weight: 160.5 g, measurement time: 584,786 s. Selected peaks are labelled with the corresponding nuclide names, red: U-238 daughter nuclides, blue: Th-232 daughter nuclides. The inset focusses on the lower energy range of the spectrum from 0 keV to 400 keV.
Additional long time measurements on selected samples were performed by VKTA – Radiation Protection, Analytics & Disposal, Rossendorf in the underground laboratory ‘Felsenkeller’ (Niese
Multi-element NAA analyses (cf. Laul, 1979; Greenberg
The measured concentration of dose rate-relevant elements and radionuclides is given in
Concentration of dose rate-relevant elements (K, Th, U) determined by both NAA and HR-GS.
Loess Ref | - | 1.11 ± 0.01 | 1.11 ± 0.01 | 8.27 ± 0.21 | 8.27 ± 0.21 | 2.75 ± 0.09 | 2.75 ± 0.09 | - | - |
Loess Test | - | 1.04 ± 0.06 | 1.08 ± 0.02 | 7.68 ± 0.39 | 8.27 ± 0.45 | 2.49 ± 0.19 | 2.33 ± 0.23 | 2.62 ± 0.13 | - |
NW18/2-11 | 148 | 1.85 ± 0.09 | 1.68 ± 0.03 | 8.2 ± 0.4 | 7.86 ± 0.17 | 2.01 ± 0.10 | 2.02 ± 0.13 | 2.99 ± 0.08 | 0.67 |
NW18/2-10 | 224 | 1.72 ± 0.09 | 1.85.0.04 | 8.7 ± 0.4 | 8.26 ± 0.16 | 8.38 ± 0.42 | 7.36 ± 0.20 | 3.19 ± 0.06 | 2.31 |
NW18/2-09 | 375 | 1.65 ± 0.08 | 1.75 ± 0.04 | 5.8 ± 0.3 | 5.34 ± 0.18 | 3.37 ± 0.22 | 2.94 ± 0.14 | 2.14 ± 0.06 | 1.37 |
NW18/2-21 | 430 | 0.61 ± 0.03 | 0.70 ± 0.02 | 7.0 ± 0.4 | 7.54 ± 0.30 | 5.03 ± 0.30 | 7.30 ± 0.24 | 4.46 ± 0.13 | 1.64 |
NW18/2-08 | 442 | 1.07 ± 0.06 | 1.31 ± 0.03 | 9.2 ± 0.5 | 10.51 ± 0.20 | 5.20 ± 0.31 | 6.57 ± 0.27 | 5.66 ± 0.35 | 1.16 |
NW18/2-22 | 455 | 0.90 ± 0.05 | 0.96 ± 0.02 | 7.9 ± 0.4 | 8.28 ± 0.26 | 4.69 ± 0.27 | 5.98 ± 0.27 | 4.49 ± 0.15 | 1.33 |
NW18/2-23 | 530 | 0.79 ± 0.04 | 0.86 ± 0.02 | 11.6 ± 0.6 | 12.59 ± 0.36 | 22.20 ± 1.11 | 29.03 ± 0.64 | 15.24 ± 0.27 | 1.90 |
NW18/2-07 | 545 | 1.31 ± 0.07 | 1.44 ± 0.03 | 9.1 ± 0.5 | 9.18 ± 0.31 | 13.79 ± 0.68 | 12.29 ± 0.35 | 7.56 ± 0.21 | 1.63 |
NW18-2-24 | 570 | 0.81 ± 0.04 | 0.84 ± 0.02 | 8.8 ± 0.4 | 8.70 ± 0.20 | 21.47 ± 1.09 | 24.88 ± 0.58 | 9.42 ± 0.09 | 2.64 |
NW18/2-06 | 610 | 1.22 ± 0.06 | 1.46 ± 0.03 | 16.4 ± 0.8 | 17.15 ± 0.35 | 9.80 ± 0.47 | 9.79 ± 0.32 | 7.80 ± 0.22 | 1.26 |
NW18/2-25 | 749 | 1.16 ± 0.06 | 1.25 ± 0.03 | 8.5 ± 0.4 | 8.70 ± 0.20 | 5.62 ± 0.36 | 5.14 ± 0.20 | 3.88 ± 0.13 | 1.32 |
NW18/2-05 | 788 | 1.13 ± 0.06 | 1.21 ± 0.03 | 6.7 ± 0.3 | 6.55 ± 0.14 | 2.05 ± 0.1 | 1.73 ± 0.12 | 2.05 ± 0.13 | 0.85 |
NW18/3-11 | 232 | 1.52 ± 0.08 | 1.69 ± 0.04 | 9.5 ± 0.5 | 9.92 ± 0.09 | 12.57 ± 0.68 | 12.10 ± 0.33 | 4.34 ± 0.18 | 2.79 |
NW18/3-21 | 323 | 1.44 ± 0.07 | 1.47 ± 0.03 | 11.5 ± 0.6 | 11.96 ± 0.39 | 5.62 ± 0.24 | 6.71 ± 0.24 | 4.67 ± 0.07 | 1.44 |
NW18/3-10 | 335 | 1.84 ± 0.09 | 1.98 ± 0.04 | 14.4 ± 0.7 | 14.13 ± 0.22 | 4.17 ± 0.21 | 4.00 ± 0.20 | 4.45 ± 0.07 | 0.90 |
NW18/3-09 | 406 | 1.86 ± 0.10 | 1.92 ± 0.04 | 10.9 ± 0.6 | 10.21 ± 0.22 | 2.57 ± 0.17 | 2.14 ± 0.13 | 2.93 ± 0.03 | 0.73 |
NW18/3-22 | 438 | 1.20 ± 0.06 | 1.28 ± 0.03 | 8.8 ± 0.4 | 8.32 ± 0.19 | 8.03 ± 0.40 | 8.48 ± 0.26 | 5.73 ± 0.12 | 1.48 |
NW18/3-23 | 496 | 1.47 ± 0.08 | 1.53 ± 0.03 | 8.5 ± 0.4 | 8.38 ± 0.13 | 3.03 ± 0.20 | 3.05 ± 0.16 | 2.95 ± 0.06 | 1.03 |
NW18/3-08 | 518 | 1.40 ± 0.07 | 1.53 ± 0.03 | 8.6 ± 0.4 | 8.22 ± 0.17 | 2.50 ± 0.17 | 2.17 ± 0.14 | 2.39 ± 0.08 | 0.91 |
HR-GS, high-resolution gamma spectrometry; NAA, neutron activation analyses.
For the latter method, the concentration of U-238 and Ra-226 (average of all post-radon isotopes) is given.
The ratio of the two values indicates surplus (if >1.00) or deficit (if <1.00) of nuclides from the early part of the decay chain.
The implications of such radioactive disequilibria are discussed in Section 4.4.
The concentration of Ra-226 is given as uranium equivalent concentrations (UE), that is, as uranium concentration of an equilibrated decay system with the Ra-226 activity as analysed.
The real Ra-226 concentration is by a factor of 2.8 106 lower than the U-238 concentration.
Comparison of the results of NAA and HR-GS for potassium
For a realistic assessment of the water content, the sediment moisture at the time of sampling as well as the water absorption capacity of each sample were determined (
Sediment moisture at the time of sampling plotted versus water absorption capacity. Note that the water absorption capacity of all samples is higher than the sediment moisture. The organic content also influences the water content, with samples with a high organic content featuring higher water contents. Dashed = 1:1 line, dotted line: y = x + 25.
Samples with a high organic content generally have a higher water content than samples with a low organic content; however, even peat samples only show a water absorption capacity of 54%–160% (
Listing the three different water contents used to test the influence of water content on dose rate calculations.
NW18/2-11 | 3f | 3 | 15 | 38 | - | 25 ± 5 |
NW18/2-10 | 3f | 5 | 20 | 41 | - | 30 ± 5 |
NW18/2-09 | 3f | 3 | 12 | 34 | - | 25 ± 5 |
NW18/2-21 | 1b | 51 | - | 160 | 200 | 180 ± 10 |
NW18/2-08 | 2b | 47 | 96 | 100 | 125 | 110 ± 10 |
NW18/2-22 | 2b | 36 | 73 | 95 | 119 | 100 ± 10 |
NW18/2-23 | 1a | 24 | 40 | 96 | 120 | 100 ± 10 |
NW18/2-07 | 3d | 18 | 57 | 74 | 92 | 75 ± 5 |
NW18-2-24 | 1c | 51 | 21 | 68 | 85 | 75 ± 5 |
NW18/2-06 | 2b | 20 | 43 | 77 | 96 | 75 ± 5 |
NW18/2-25 | 3c | 7 | - | 62 | - | 60 ± 5 |
NW18/2-05 | 3c | 2 | 25 | 50 | - | 40 ± 5 |
NW18/3-11 | 3f | 17 | 24 | 48 | - | 40 ± 5 |
NW18/3-21 | 3f | 24 | - | 64 | - | 55 ± 5 |
NW18/3-10 | 2b | 6 | 27 | 59 | - | 45 ± 5 |
NW18/3-09 | 3f | 3 | 17 | 40 | - | 30 ± 5 |
NW18/3-22 | 1b | 20 | 35 | 54 | 67 | 60 ± 5 |
NW18/3-23 | 3c | 15 | 29 | 54 | 67 | 60 ± 5 |
NW18/3-08 | 3b | 2 | 19 | 42 | - | 35 ± 5 |
Samples with numbers larger than 20 had been sampled more than a year after the core had been taken.
As these samples had almost completely dried out, sediment moisture was not determined.
For the organic-poor silts, the long-term water content is more likely to have between the sediment moisture at the time of sampling and the experimentally determined maximum water absorption capacity. To test the impact of the long-term water content on dose rate and age determination, three different water contents are used: (1) the sediment moisture during sampling; (2) the water adsorption capacity, including a correction for peat-rich samples (adding 25% on top of the observed value); and (3) an estimated long-term water content based on the sediment properties and the water contents calculated by approaches (1) and (2) (
Illustrating the impact of different water contents on the IRSL ages of core NW18/2
The effect of applying different sediment moisture scenarios is displayed in
The studied sediment sequence captures a change in the depositional environment from a lacustrine setting to alluvial- and fluvial-influenced wetlands under changing climatic conditions. As a result, the sedimentary sequence comprises interlayered silts, clays and peats, with individual bed thicknesses often less than 30 cm (
We use a three-layer model to account for the different dose rates that is implemented in ADELE-2017: the sampled layer L, the overlying layer A and the underlying layer B (
Illustration of the layer model used to analyse the impact of inhomogeneous radiation fields. The parameters t, x1 and x2 describe the location of the sample within the stratigraphic setting.
Boxplots of the radionuclide content of each LT.
The layer models for the samples from the two studied cores are illustrated in
For core NW2018/2, the layer model correction for the top three samples (NW18/2-09 to NW18/2-11) is not more than 200 years (max. effect on age 0.3%), and it is more relevant for some of the samples from the middle and lower parts of the core (
Illustrating the impact of layer correction on the IRSL ages of core NW18/2
Radioactive disequilibria were detected solely in the U-238 decay series since the low half-lives of all Th-232 successors results in a recovery of any disturbed equilibrium within some decades. Samples with indication of significant disequilibrium show a surplus of U-238 over Ra-226. Additional analyses of Th-230 were used to decide whether a model for radium loss or for uranium uptake must be chosen. The long-term measurements at VKTA Rossendorf show a clear trend of equilibrium between Th-230 and Ra-226, indicating an enrichment of U-238.
Radioactive disequilibria indicate the occurrence of an open system, either at deposition and/or during certain periods after deposition. The mass exchange of open systems is generally linked to migration processes in the aqueous phase. Both uranium and radium are soluble under oxidising conditions, whereas thorium is virtually insoluble (cf. Ivanovich and Harmon, 1992). Because of their half-lives, U-238/Th-230 disequilibria exist for some 100 ka, whereas Th-230/Ra-226 imbalances disappear after about 10 ka.
Only uranium uptake in open systems thus explains the observed disequilibria; any Ra loss in the past cannot be excluded but is at present no longer detectable. In the applied models, we assume the presence of two phases: a mineral detritus showing a radioactive equilibrium and an additional phase (e.g. organics) with the ability of uranium accumulation. The detrital uranium content was derived from the thorium content of the sample, using the ratios found in the equilibrated material. Furthermore, it was assumed that open system behaviour was in the past limited to periods without permafrost or long-lasting seasonal ground frost. While permafrost was most likely present in central Europe during the Last Glaciation Maximum (LGM) (e.g. Murton, 2021), data presented by Andrieux
With these boundary conditions, two scenarios explaining the current activity ratios were investigated:
Scenario 1: At the time of deposition (t = 0), a certain amount of detrital U-238 was present in the sample that was in equilibrium with the rest of the decay chain. In addition, uranium (U-238, U-234) was already absorbed at this time, representing a very early uptake situation. An open system with gain or loss of uranium existed only during the youngest period (15–0 ka) and resulted in the present concentration of radionuclides. A higher initial concentration of uranium would require open system behaviour during additional periods and possibly an exchange of radium. Hence, this model described the highest possible uranium content at the time of deposition.
Scenario 2: At the time of deposition, the U-238 decay chain is in radioactive equilibrium at the level equal to the detrital value. Exchange of uranium was possible during three periods (P5 > 70 ka, P3 = 60–45 ka, P1 = 15–0 ka), whereas the system remained closed during times of permafrost. Due to the presence of peat and based on the results by Dehnert
The effect of dose rate correction using the two different scenarios is illustrated in
Illustrating the impact of different scenarios of correction for radioactive disequilibrium on the IRSL ages of core NW18/2
The assignment of OSL and IRSL ages to the sediment sequences after ultimate dose rate correction is displayed in
Calculated dose rate data with sample code and composite depth, and estimated long-term water content.
NW18/2-11 | 148 | 25 ± 5 | 2.89 ± 0.14 | 2.89 ± 0.14 | 2.84...2.94 ± 0.16 | 3.18 ± 0.22 | 3.18 ± 0.22 | 3.12...3.24 ± 0.25 |
NW18/2-10 | 224 | 30 ± 5 | 2.96 ± 0.12 | 2.95 ± 0.12 | 2.91...2.93 ± 0.13 | 3.25 ± 0.21 | 3.24 ± 0.21 | 3.18...3.21 ± 0.23 |
NW18/2-09 | 375 | 25 ± 5 | 2.48 ± 0.10 | 2.48 ± 0.10 | 2.44...2.47 ± 0.12 | 2.69 ± 0.15 | 2.70 ± 0.15 | 2.66...2.69 ± 0.15 |
NW18/2-21 | 430 | 180 ± 10 | - | - | - | 1.21 ± 0.09 | 1.23 ± 0.09 | 1.05...1.17 ± 0.09 |
NW18/2-08 | 442 | 110 ± 10 | - | - | - | 2.59 ± 0.22 | 2.46 ± 0.22 | 1.99...2.10 ± 0.19 |
NW18/2-22 | 455 | 100 ± 10 | 1.66 ± 0.13 | 1.60 ± 0.12 | 1.44...1.53 ± 0.15 | 1.88 ± 0.15 | 1.82 ± 0.14 | 1.61...1.73 ± 0.14 |
NW18/2-23 | 530 | 100 ± 10 | - | - | - | 3.97 ± 0.40 | 3.73 ± 0.39 | 4.59 ± 0.51 |
NW18/2-07 | 545 | 75 ± 5 | - | - | - | 3.07 ± 0.24 | 2.98 ± 0.23 | 2.85...2.98 ± 0.23 |
NW18-2-24 | 570 | 75 ± 5 | - | - | - | 3.12 ± 0.27 | 3.10 ± 0.27 | 2.51...2.94 ± 0.27 |
NW18/2-06 | 610 | 75 ± 5 | - | - | - | 3.61 ± 0.29 | 3.60 ± 0.29 | 3.50...3.58 ± 0.29 |
NW18/2-25 | 749 | 60 ± 5 | 1.71 ± 0.09 | 1.82 ± 0.09 | 1.84...2.09 ± 0.13 | 1.89 ± 0.12 | 2.00 ± 0.12 | 2.06...2.34 ± 0.18 |
NW18/2-05 | 788 | 40 ± 5 | - | - | - | 2.02 ± 0.19 | 2.02 ± 0.19 | no dis. |
NW18/3-11 | 232 | 40 ± 5 | 2.98 ± 0.15 | 2.93 ± 0.15 | 2.88...2.90 ± 0.17 | 3.32 ± 0.23 | 3.27 ± 0.15 | 3.24...3.25 ± 0.24 |
NW18/3-21 | 323 | 55 ± 5 | 2.72 ± 0.19 | 2.49 ± 0.18 | 2.47...2.49 ± 0.23 | 3.05 ± 0.22 | 2.82 ± 0.22 | 2.75...2.81 ± 0.23 |
NW18/3-10 | 335 | 45 ± 5 | 3.35 ± 0.15 | 3.24 ± 0.15 | no dis. | 3.73 ± 0.26 | 3.62 ± 0.26 | no dis. |
NW18/3-09 | 406 | 30 ± 5 | - | - | - | 3.36 ± 0.27 | 3.32 ± 0.21 | 3.34...3.51 ± 0.24 |
NW18/3-22 | 438 | 60 ± 5 | - | - | - | 2.82 ± 0.21 | 2.83 ± 0.21 | 2.54...2.74 ± 0.21 |
NW18/3-23 | 496 | 60 ± 5 | 2.01 ± 0.11 | 2.08 ± 0.10 | no dis. | 2.31 ± 0.15 | 2.30 ± 0.15 | no dis. |
NW18/3-08 | 518 | 35 ± 5 | 2.36 ± 0.10 | 2.37 ± 0.10 | no dis. | 2.60 ± 0.16 | 2.61 ± 0.16 | no dis. |
Dose rates are given for both quartz (Q) and polymineral fine-grains (F) for three different scenarios: (1) using ‘standard’ scenario with estimated water content, (2) using estimated water content and layer correction and (3) estimated water content, layer correction and considering radioactive disequilibrium with the uncertainty related to the two different scenarios (given are the lower and upper dose rate estimates, which share the same uncertainty).
For the latter column, a mean value is given as the dose rate in the presence of radioactive disequilibrium changes with time.
Mean De values (CAM) and resulting ages with sample code, composite depth and number of replicate measurement (quartz/feldspar) for OSL and IRSL.
NW18/2-11 | 148 | 5/7 | 303.9 ± 8.7 | 341.6 ± 11.6 | 105 ± 5 | 105 ± 5 | 103...107 ± 5 | 107 ± 7 | 107 ± 7 | 105...109 ± 7 |
NW18/2-10 | 224 | 5/7 | 235.4 ± 4.7 | 302.8 ± 9.3 | 79.6 ± 3.3 | 79.8 ± 3.3 | 80...81 ± 3 | 93.1 ± 6.1 | 93.3 ± 6.1 | 94...95 ± 6 |
NW18/2-09 | 375 | 5/7 | 212.3 ± 4.8 | 190.8 ± 3.3 | 85.6 ± 3.6 | 85.5 ± 3.6 | 86...87 ± 4 | 70.9 ± 3.9 | 70.8 ± 3.9 | 71...72 ± 4 |
NW18/2-21 | 430 | -/6 | - | 206.3 ± 5.1 | - | - | - | 171 ± 13 | 167 ± 13 | 177...197 ± 13 |
NW18/2-08 | 442 | -/7 | - | 191.8 ± 7.0 | - | - | - | 74.1 ± 6.4 | 77.8 ± 7.0 | 91...96 ± 7 |
NW18/2-22 | 455 | 5/6 | 165.9 ± 10.0 | 223.1 ± 5.2.4 | 100.1 ± 7.6 | 103.9 ± 7.9 | 109...115 ± 8 | 119 ± 9 | 123 ± 10 | 129...138 ± 10 |
NW18/2-23 | 530 | -/6 | - | 148.9 ± 5.5 | - | - | - | 37.5 ± 3.7 | 39.9 ± 4.2 | 28...32 ± 3 |
NW18/2-07 | 545 | -/7 | - | 197.8 ± 3.6 | - | - | - | 64.5 ± 5.0 | 66.3 ± 5.2 | 66...69 ± 5 |
NW18-2-24 | 570 | -/6 | - | 254.7 ± 7.5 | - | - | - | 125 ± 7 | 115 ± 10 | 87...102 ± 8 |
NW18/2-06 | 610 | -/7 | - | 248.0 ± 4.6 | - | - | - | 68.8 ± 5.5 | 68.8 ± 5.5 | 69...71 ± 6 |
NW18/2-25 | 749 | 5/6 | 246.50 ± 9.0 | 310.80 ± 8.7 | 144 ± 8 | 135 ± 7 | 118...134 ± 7 | 164 ± 11 | 155 ± 10 | 133...151 ± 11 |
NW18/2-05 | 788 | -/7 | - | 170.8 ± 4.6 | - | - | - | 84.4 ± 5.5 | 84.4 ± 5.5 | no dis. |
NW18/3-11 | 232 | 5/7 | 202.6 ± 6.5 | 198.4 ± 5.2 | 67.9 ± 3.5 | 69.0 ± 3.5 | 70 ± 4 | 59.7 ± 4.2 | 60.7 ± 2.7 | 61 ± 4 |
NW18/3-21 | 323 | 5/6 | 181.8 ± 10.4 | 243.0 ± 5.7 | 67.0 ± 4.7 | 72.9 ± 5.3 | 73...74 ± 5 | 79.7 ± 5.8 | 86.0 ± 6.7 | 86...88 ± 7 |
NW18/3-10 | 335 | 5/6 | 276.0 ± 6.3 | 288.0 ± 7.6 | 82.5 ± 3.7 | 85.2 ± 3.9 | no dis. | 77.3 ± 5.4 | 79.6 ± 5.7 | no dis. |
NW18/3-09 | 406 | -/7 | - | 310.8 ± 6.6 | - | - | - | 92.6 ± 7.5 | 93.5 ± 5.9 | 89...93 ± 6 |
NW18/3-22 | 438 | -/6 | - | 242.9 ± 5.8 | - | - | - | 86.2 ± 6.6 | 85.9 ± 6.5 | 89...96 ± 7 |
NW18/3-23 | 496 | 5/5 | 202.2 ± 6.0 | 236.2 ± 7.3 | 97.0 ± 4.5 | 97.1 ± 4.5 | no dis. | 102 ± 7 | 103 ± 7 | no dis. |
NW18/3-08 | 518 | 5/7 | 239.8 ± 5.7 | 259.7 ± 4.9 | 102 ± 5 | 101 ± 4 | no dis. | 99.9 ± 6.2 | 99.5 ± 6.1 | no dis. |
IRSL, infrared stimulated luminescence; OSL, optically stimulated luminescence.
Ages were calculated using three different scenarios: (1) using ‘standard’ scenario with estimated water content, (2) using estimated water content and layer correction and (3) estimated water content, layer correction and considering radioactive disequilibrium, with the uncertainty related to the two different scenarios (given are the lower and upper age estimates, which share the same uncertainty).
Final results after correction indicated by bold letters.
For core NW18/3, the quartz OSL ages show a steady increase with depth from 70.5 ± 3.5 ka to 101 ± 4 ka. For polymineral IRSL, the increase in age with depth is a little less uniform, but overall, there is a very good agreement between the two approaches; the ratio of IRSL/OSL ages is 1.01 ± 0.12. This is despite the lack of fading correction for the present data set, but not applying such a correction is in agreement with previous reports from Niederweningen (Anselmetti
The different dose rate corrections carried out within the frame of this study have, for core NW18/3, only a minor to moderate effect, not exceeding 10% of the final age estimate. According to the dating results, core NW18/3 covers the period ca. 100–70 ka; hence, most of the Early Würmian including its two well-developed interstadials (cf. Preusser, 2004). These apparently reflect equivalents of the Brørup and Odderade of northern Central Europe. Neglecting the dose rate corrections would not have led to a different interpretation, and overall the chronological assignment appears reliable.
For core NW18/2, the situation is very different. The age data set is internally highly contradictory, both with regard to stratigraphic inconsistencies and some moderate conflicts between OSL and IRSL ages, the IRSL/OSL age ratio is 1.07 ± 0.15. Interestingly, age discrepancies within the sequence become often even more pronounced after applying dose rate corrections, that is, opposite of what was expected and what represented the initial motivation for this entire study. This requires a detailed discussion of individual data before conclusions considering the applied procedures and implications regarding the age assignment can be drawn.
For the basal part of the core, two samples have been investigated, one at a depth of 788 cm (NW18/2-05) at the top of rather homogenous silty (lacustrine) deposits, and one at 749 cm depth (NW18/2-25) within a sequence of silty and peaty layers. The first sample is neither significantly affected by layer correction nor is there any indication for radioactive disequilibrium. Hence, the IRSL age of 84.4 ± 5.5 ka appears at a first glance reliable. For sample NW18/2-25, there is quite some effect from layer correction and radioactive disequilibrium, which leads to a decrease in the final corrected OSL and IRSL ages of just below 10%, compared to the uncorrected age estimates. The ages after correction of 134 ± 7 ka (OSL) and 151 ± 11 ka (IRSL) just overlap within uncertainties. The tendency towards slight discrepancy of ages could be explained by the effect of partial resetting of the IRSL signal, as suggested by Dehnert
The lower central part of the sediment sequence, between 610 cm and 530 cm depth, is covered by four samples, for which only IRSL ages could be determined. All samples are apparently only to a minor, or at worst to a moderate, degree affected by layer heterogeneity and/or radioactive disequilibrium. Two of these samples have consistent IRSL ages of 70.8 ± 5.6 ka (NW18/2-06) and 69.4 ± 5.2 ka (NW18/2-07), whereas the sample located between these two has a significantly higher IRSL age of 102 ± 9 ka (NW18/2-24). While the first two ages point towards a correlation of this part of the sequence with late MIS 5a (85–71 ka), the older sample would attribute this part to MIS 5c (102–91 ka). Entirely outside of the rest of all other samples is the IRSL age of 32.4 ± 3.4 ka for sample NW18/2-23. This sample shows both the lowest De value of the entire data set (148.9 ± 5.5 Gy) and the highest dose rate (3.73 ± 0.39 Gy ka−1). Striking for this sample is the high U-238 concentration (29.03 ± 0.64 ppm) and the clear evidence of radioactive disequilibrium (daughter isotopes at 15.24 ± 0.27 ppm). A mechanism to explain the low age could be a very late but extremely high U-238 uptake, but calculating any scenarios for this would be arbitrary in the absence of supporting information. Nevertheless, for other samples with evidence for disequilibrium, a late U-238 uptake would also have a significant effect on age calculation that is not covered by the presented data, which is based on the scenarios most reasonably to be expected.
The upper central part of the sequence, between 500 cm and 390 cm depth, is characterised by organic-rich and very fine-grained (fine silt and clay) clastic deposits (palustrine). A clastic layer sampled within the peaty layer at 455 cm (NW18/2-22) and 442 cm (NW18/2-08) returned ages of 115 ± 9 ka (OSL), 139 ± 10 ka (IRSL) and 96.3 ± 7.6 ka (IRSL). These samples are, only to a minor extent, affected by layer inhomogeneity and moderately by radioactive disequilibrium and indicate an early MIS 5, possibly Last Interglacial (MIS 5e) age. However, the sample taken slightly higher, at 430 cm (NW18/2-21), has a corrected age of 197 ± 13 ka. It should be noted this is mainly due to the enormously high water content of 180 ± 10% assumed for this organic-rich sample (51% organic matter). Present moisture was not measured for this sample as it had dried out almost entirely during storage. Using a much lower but likely still possibly reasonable water content for this particular setting would dramatically lower the age but not younger than early MIS 5.
The upper part of the sequence (from 390 cm to 120 cm depth) is composed of quite homogenous clastic fine-grained deposits with organic matter contents not exceeding 5%. While there is some evidence for radioactive disequilibrium, dose rate correction has only a minor effect on the ages. While the ages show some scatter, the mean (CAM) of all six OSL and IRSL ages of 90 ± 6 ka for this upper part of the sequence corresponds to MIS 5b (91–85 ka), the cool period separating the two Early Würmian interstadials.
This study has focussed on dose rate determination in what Brennan