Knowledge of the age of fluvial deposits is an important aspect in the understanding of river dynamics, which is pre-requisite for sustainable river management and restoration back to more natural conditions and processes. Presented here is a case study on using feldspar Infrared Stimulated Luminescence (IRSL) to date low-energy fluvial sediments that formed after correction of the Upper Rhine River in the first half of the 19th century. A rigorous testing programme is carried out to characterise the IRSL properties of the samples, including thermal transfer, dose recovery and fading. All samples reveal complex distributions of equivalent dose, implying the presence of differential bleach-ing in the samples. It is shown that multi-grain aliquots overestimate the known-age by up-to 200 years, i.e. apparent IRSL ages are twice as old as the true age of the sediment. The use of single grains results in ages that are in excellent agreement with the expected age, therefore the age overestimation in multi-grain aliquot measurements is likely explained by signal averaging effects. While the application of single grains appears mandatory for dating young low-energy fluvial deposits, the small absolute offset associated with the multi-grain approach might be acceptable when dating sediments of such type that are older than a few 1000 years.
Keywords
- luminescence (IRSL) dating
- fluvial sediments
- historic
- feldspar
- Rhine
Understanding centennial to millennial scale morpho-dynamics of fluvial systems is considered important for sustainable management and restoration of rivers (Brierley and Fryirs, 2005; David
The most common method in this context is radiocar-bon dating, but it suffers from two major problems. First, most organic material in fluvial environments is reworked and radiocarbon dates may hence overestimate the true age of fluvial deposition (e.g., Howard
Optically Stimulated Luminescence (OSL) dating utilises a light-sensitive latent signal in quartz and feldspar grains induced by ionising radiation, mainly originating from radioactive decay of K, Th and U and their daughter isotopes (cf. Preusser
For young sediments, little signal will have accumulated as grains have been exposed to ionising radiation for only a short periods of time, which can be problematic with regard to the signal-to-noise ratio. In particular, this applies to quartz and appears to be related to a lack of dosing and exposure cycles that would cause a sensitisation of the electron traps in quartz (Pietsch
Thermal transfer and recuperation, both representing a transfer of electrons from thermally shallow, light-insensitive traps to deeper, light-sensitive traps, occur when grains are preheated during the measurement procedure to remove unstable components of the OSL signal that are induced by laboratory irradiation (Aitken and Smith, 1988;Rhodes and Bailey, 1997). Following Wintle and Murray (2006), ‘thermal transfer’ is used here only for the transfer of charge during the first preheating of the sample, i.e. it effects only the natural signal. As thermally transferred OSL is not present in the build-up of the signal in nature, i.e. it is a laboratory artefact, this phenomenon will lead to overestimation of the real OSL deposition age. The effect of thermal transfer can be monitored by removing the light sensitive signal by light exposure and measuring the signal induced by preheat (thermal transfer test). The term ‘recuperation’ describes charge transfer during preheat following laboratory irra-diation. It is monitored by including a zero dose step in the measurement procedure (Murray and Wintle, 2000).
Overestimation of ages will also occur when the OSL signal was incompletely reset at the time of deposition, as the signal acquired since the event to be dated will add on top of the residual signal. However, as each grain has its own daylight exposure history, the residual level in grains will differ in the presence of incomplete resetting; grains will be differentially bleached (Duller, 1991). Hence, measuring several dozen to a few hundred individual grains and investigating the spread will provide information on the degree of bleaching at the time of deposition (Li, 1994; Murray
deally, luminescence dating would be carried out using single grain measurements as this will provide the full information about individual grain history. However, as this approach is quite laborious, De measurements are often made on aliquots consisting of several dozen grains. Here, it has to be considered that usually only few percent of the grains (in particular for quartz) exhibit OSL signals (e.g. Duller, 2004). Hence, using aliquots with not more than 100 grains will almost mimic single grain measurements. However, in particular for feldspar separates a much larger number of grains may exhibit luminescence signals. In such cases multi-grain approaches can cause averaging of the signal originating from individual grains and mask incomplete bleaching at least to some extent (Wallinga, 2002).
Several previous studies have shown that young aeolian and coastal sediments are suitable for the dating of young deposits using quartz, as partial bleaching has little effect. There are quite a few studies that have used quartz to date young fluvial deposits (e.g. Murray
Presented here is a case study testing the potential of feldspar IRSL to date very young fluvial deposits from a moderate/low-energy braiding/anastomosing river system from a temperate environmental setting. We took five samples from a sediment sequence that deposited in a former main channel of the River Rhine near Strasbourg, and which has been disconnected upstream by engineering works in the early part of the 19th century, as documented by a large set of historical maps. A series of performance tests to characterise the signal, and a comparison of multi-grain (small aliquot) and single grain IRSL dating was carried out. Besides the importance for future projects on river systems in Western Europe, the results presented are considered relevant for similar studies in others parts of the planet, where quartz shows nonsuitable OSL signals.
The Upper Rhine is one of the most altered fluvial systems in Europe ( a.) Overview of the River Rhine catchment with the location of Strasbourg. b.) Comparison of the Upper Rhine system before and after river engineering works in the 19th and 20th century. c.) Historic development of the study area from AD 1778 until 2010.
The study of old maps reveals that the upstream reach of the main watercourse of the island (the ‘Bauerngrundwasser’) was the thalweg of the River Rhine in AD 1828 (
Samples have been taken by hammering opaque tubes into the freshly cleaned surface of the exposure in a small trench ( Schematic sketch of the sampled exposure.
Multi-grain measurements were performed on a
Storage tests to monitor IRSL signal decay due to fading have been carried out at room temperature (
Measurement of De was done using the Single Aliquot Regenerative Dose (SAR) protocol (Murray and Wintle, 2000) modified for feldspar following Preusser (2003) and Blair
Measurement results have been analysed using Analyst 4.31.9 (Duller, 2016) and were further handled in home-made Excel sheets. All multi-grain discs passed the usual rejection criteria (recycling ratios within 10% of unity, 10% test dose error, test dose signal min. three times above background; cf. Wintle and Murray, 2006). Grains with a signal >3 sigma above background, a test dose error <15% and a recycling ratio between 0.90 and 1.10 (considering uncertainties) were selected for De determination. Further grains were rejected, if the recuperation expressed in percentage of the regenerative signal was >10%, or if the natural signal was above dose response saturation level. Of all measured single grains between 7.3 and 11.2% passed all applied rejection criteria and were used for De calculation. For all samples, mean De was calculated using the Central Age Model (CAM) and the MAM-3 (Galbraith
The concentration of dose rate relevant elements (K, Th, U) was measured using high-resolution gamma spectrometry, similar to the procedures described by Preusser and Kasper (2001). Using the approach of Preusser and Degering (2007), we observed no evidence for radioactive disequilibrium in the Uranium decay chain. Age calculations were carried out using ADELE 2015 software using present day geographic position and burial depth, assuming an a-value of 0.07 ± 0.02, and estimating sediment moisture based on sediment composition and hydrological context (allowing for large uncertainties). An internal K-content of 12.5 ± 1.0% was used based on values reported by Gaar
Summary data of luminescence dating for both multiple grain (MG) and single grain (SG) approaches with sampling depth below surface, the concentration of dose rate relevant elements (K, Th, U), assumed average water content (W) during burial, dose rate (D), mean De values calculated for the Central Age Model (CAM) and Minimum Age Model (MAM), using σb values of 0.10 (MG) and 0.20 (SG), the fading corrected MAM De, the resulting IRSL ages for MAM (before AD 2015), and the age transferred into AD time scale.
Sample | Depth (cm) | K (%) | Th (ppm) | U (ppm) | W (%) | D (Gy ka−1) | n | od. | De CAM (Gy) | De MAM (Gy) | Age MAM (a) | Age MAM corr. (a) | Age MAM (AD) | Age MAM corr. (AD) |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
RO1-6MG | 15 | 1.51 ∔ 0.02 | 5.65 ∔ 0.23 | 1.19 ∔ 0.02 | 10 ∔ 5 | 2.62 ∔ 0.10 | 48 | 0.79 | 0.99 ∔ 0.11 | 0.54 ∔ 0.04 | 206 ∔ 17 | 244 ∔ 21 | 1792-1826 | 1750-1792 |
RO1-6SG | 15 | 1.51 ∔ 0.02 | 5.65 ∔ 0.23 | 1.19 ∔ 0.02 | 10 ∔ 5 | 2.62 ∔ 0.10 | 48 | 0.62 | 0.85 ∔ 0.08 | 0.37 ∔ 0.05 | 141 ∔ 20 | 166 ∔ 24 | 1854-1894 | 1825-1873 |
RO1-5MG | 39 | 1.07 ∔ 0.02 | 4.34 ∔ 0.19 | 1.24 ∔ 0.02 | 20 ∔ 5 | 2.07 ∔ 0.09 | 47 | 0.62 | 0.91 ∔ 0.08 | 0.62 ∔ 0.03 | 300 ∔ 20 | 356 ∔ 33 | 1695-1735 | 1626-1692 |
RO1-5SG | 39 | 1.07 ∔ 0.02 | 4.34 ∔ 0.19 | 1.24 ∔ 0.02 | 20 ∔ 5 | 2.07 ∔ 0.09 | 29 | 0.78 | 0.83 ∔ 0.12 | 0.32 ∔ 0.09 | 154 ∔ 44 | 182 ∔ 50 | 1817-1900 | 1783-1883 |
RO1-4MG | 48 | 1.10 ∔ 0.02 | 3.90 ∔ 0.10 | 1.02 ∔ 0.03 | 20 ∔ 5 | 2.02 ∔ 0.09 | 24 | 0.31 | 0.87 ∔ 0.06 | 0.67 ∔ 0.05 | 332 ∔ 29 | 395 ∔ 35 | 1654-1712 | 1585-1655 |
RO1-4SG | 48 | 1.10 ∔ 0.02 | 3.90 ∔ 0.10 | 1.02 ∔ 0.03 | 20 ∔ 5 | 2.02 ∔ 0.09 | 52 | 0.55 | 0.83 ∔ 0.07 | 0.41 ∔ 0.05 | 203 ∔ 27 | 239 ∔ 30 | 1785-1839 | 1746-1806 |
RO1-3 | 55 | 1.04 ∔ 0.02 | 5.80 ∔ 0.24 | 1.66 ∔ 0.05 | 20 ∔ 5 | - | - | - | - | - | - | - | - | - |
RO1-2MG | 72 | 1.08 ∔ 0.01 | 3.96 ∔ 0.13 | 1.19 ∔ 0.03 | 20 ∔ 5 | 2.06 ∔ 0.09 | 50 | 0.20 | 0.78 ∔ 0.02 | 0.69 ∔ 0.04 | 334 ∔ 31 | 399 ∔ 30 | 1650-1712 | 1586-1646 |
RO1-2SG | 72 | 1.08 ∔ 0.01 | 3.96 ∔ 0.13 | 1.19 ∔ 0.03 | 20 ∔ 5 | 2.06 ∔ 0.09 | 67 | 0.70 | 0.98 ∔ 0.09 | 0.40 ∔ 0.06 | 194 ∔ 30 | 230 ∔ 36 | 1791-1851 | 1749-1821 |
The feldspar shows a moderate response to IR stimulation in the multi-grain approach with close to linear growth of dose response curves (
Example of IRSL decay for a multi-grain disc and a dose response curve of sample RO1-4 (inset). Representative single grain IRSL decay of sample RO1-4. Results of thermal transfer tests experiments reveal a slight increase of De with increasing preheat temperature. Same applies for recuperation as monitored in the dose recovery experiments.
The multi-grain dose recovery experiment reveals an excellent reproducibility of the applied dose with a recovery ratio of 1.03 and a relative standard deviation of 2.3%, which corresponds to the average uncertainties of individual measurements ( Results of the dose recovery tests for both multi-grain aliquots and single grains.
The light-sum plots ( Light sum curve of single grain data of four samples.
The storage tests carried out on multi-grain samples to estimate the fading rate (
This example of a fading test displays a g-value of 1.6 ± 0.4 %/decade.

Result of the single grain fading test.

For all four samples we found complex De distributions for both single grain and multi-grain aliquots that show a positive skew together with large overdispersion reaching up to 0.79 ( Dose distributions plots of multi-grain aliquot and single measurements of all samples investigated. Three De values of 19.1 ± 1.6 Gy (RO1-5 MG), 11.9 ± 0.4 Gy and 17.2 ± 0.8 Gy (RO1-6 MG) are not shown in the plot to improve presentation.
The above mentioned observations call for the appli-cation of the MAM to extract mean De estimate from the grains/aliquots presenting the well-bleached fraction. As there is little experience with samples from the region, a sensitivity test was carried out to investigate the effect of the input value σ Change of De when assuming different overdispersion values in the Minimum Age Model of Galbraith et al. (1999).
IRSL ages have been calculated both without and including fading correction; the difference is about 18% ( Plot of IRSL ages for different approaches versus the timeline reconstructed from historic records.
This study highlights that feldspar IRSL can be used successfully to date very young fluvial sediments, despite the several challenges related to this issue. As averaging effects in multi-grain aliquots mask part of the differential bleaching in the samples, this required application of the single grain approach. The resultant ages are in excellent agreement with the expected deposition history as inferred from the study of old maps. As the offset of the multi-grain aliquot IRSL ages is only 100–200 years, this method may well be suitable for samples being older than a few millennia. Hence, the approach is considered as an alternative in settings where quartz exhibits poor OSL properties. Nevertheless, it is necessary to investigate the effect of thermal transfer, partial bleaching and fading in the samples in more detail, as these have the potential to substantially interfere with accurate calculation of sediment deposition ages.












Summary data of luminescence dating for both multiple grain (MG) and single grain (SG) approaches with sampling depth below surface, the concentration of dose rate relevant elements (K, Th, U), assumed average water content (W) during burial, dose rate (D), mean De values calculated for the Central Age Model (CAM) and Minimum Age Model (MAM), using σb values of 0.10 (MG) and 0.20 (SG), the fading corrected MAM De, the resulting IRSL ages for MAM (before AD 2015), and the age transferred into AD time scale.
Sample | Depth (cm) | K (%) | Th (ppm) | U (ppm) | W (%) | D (Gy ka−1) | n | od. | De CAM (Gy) | De MAM (Gy) | Age MAM (a) | Age MAM corr. (a) | Age MAM (AD) | Age MAM corr. (AD) |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
RO1-6MG | 15 | 1.51 ∔ 0.02 | 5.65 ∔ 0.23 | 1.19 ∔ 0.02 | 10 ∔ 5 | 2.62 ∔ 0.10 | 48 | 0.79 | 0.99 ∔ 0.11 | 0.54 ∔ 0.04 | 206 ∔ 17 | 244 ∔ 21 | 1792-1826 | 1750-1792 |
RO1-6SG | 15 | 1.51 ∔ 0.02 | 5.65 ∔ 0.23 | 1.19 ∔ 0.02 | 10 ∔ 5 | 2.62 ∔ 0.10 | 48 | 0.62 | 0.85 ∔ 0.08 | 0.37 ∔ 0.05 | 141 ∔ 20 | 166 ∔ 24 | 1854-1894 | 1825-1873 |
RO1-5MG | 39 | 1.07 ∔ 0.02 | 4.34 ∔ 0.19 | 1.24 ∔ 0.02 | 20 ∔ 5 | 2.07 ∔ 0.09 | 47 | 0.62 | 0.91 ∔ 0.08 | 0.62 ∔ 0.03 | 300 ∔ 20 | 356 ∔ 33 | 1695-1735 | 1626-1692 |
RO1-5SG | 39 | 1.07 ∔ 0.02 | 4.34 ∔ 0.19 | 1.24 ∔ 0.02 | 20 ∔ 5 | 2.07 ∔ 0.09 | 29 | 0.78 | 0.83 ∔ 0.12 | 0.32 ∔ 0.09 | 154 ∔ 44 | 182 ∔ 50 | 1817-1900 | 1783-1883 |
RO1-4MG | 48 | 1.10 ∔ 0.02 | 3.90 ∔ 0.10 | 1.02 ∔ 0.03 | 20 ∔ 5 | 2.02 ∔ 0.09 | 24 | 0.31 | 0.87 ∔ 0.06 | 0.67 ∔ 0.05 | 332 ∔ 29 | 395 ∔ 35 | 1654-1712 | 1585-1655 |
RO1-4SG | 48 | 1.10 ∔ 0.02 | 3.90 ∔ 0.10 | 1.02 ∔ 0.03 | 20 ∔ 5 | 2.02 ∔ 0.09 | 52 | 0.55 | 0.83 ∔ 0.07 | 0.41 ∔ 0.05 | 203 ∔ 27 | 239 ∔ 30 | 1785-1839 | 1746-1806 |
RO1-3 | 55 | 1.04 ∔ 0.02 | 5.80 ∔ 0.24 | 1.66 ∔ 0.05 | 20 ∔ 5 | - | - | - | - | - | - | - | - | - |
RO1-2MG | 72 | 1.08 ∔ 0.01 | 3.96 ∔ 0.13 | 1.19 ∔ 0.03 | 20 ∔ 5 | 2.06 ∔ 0.09 | 50 | 0.20 | 0.78 ∔ 0.02 | 0.69 ∔ 0.04 | 334 ∔ 31 | 399 ∔ 30 | 1650-1712 | 1586-1646 |
RO1-2SG | 72 | 1.08 ∔ 0.01 | 3.96 ∔ 0.13 | 1.19 ∔ 0.03 | 20 ∔ 5 | 2.06 ∔ 0.09 | 67 | 0.70 | 0.98 ∔ 0.09 | 0.40 ∔ 0.06 | 194 ∔ 30 | 230 ∔ 36 | 1791-1851 | 1749-1821 |