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Optical dating of fluvio-deltaic clastic lake-fill sediments – A feasibility study in the Holocene Rhine delta (western Netherlands)
Quaternary Geochronology 5 (2010) 602–610
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Research Paper
Optical dating of fluvio-deltaic clastic lake-fill sediments – A feasibility study in the Holocene Rhine delta (western Netherlands) Jakob Wallinga a, *, Ingwer J. Bos b, c a
Netherlands Centre for Luminescence dating, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands Department of Physical Geography, Utrecht University, Heidelberglaan 2, NL-3508 TC Utrecht, The Netherlands c TNO Built Environment and Geosciences/Geological Survey of The Netherlands, Princetonlaan 6, NL-3584 CB Utrecht, The Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2009 Received in revised form 29 October 2009 Accepted 10 November 2009 Available online 17 November 2009
We test the applicability of quartz optically stimulated luminescence (OSL) dating on clastic lake sediments to investigate whether this dating method can be applied to study the timing and rate of deposition in Holocene fluvio-deltaic lakes. Our study concerns the filling of a lake by the Angstel-Vecht system, part of the Rhine delta in the western Netherlands. Age constraints are provided by radiocarbon dates on the development and abandonment of the fluvial channels debouching into the lake. Results indicate that light exposure prior to deposition and burial was sufficient to reset the OSL signal of the vast majority of the quartz grains. Special attention was given to accurate estimation of the dose rate in the laminated and bedded deposits. The OSL ages obtained are in good agreement with the age constraints, especially for the relatively coarse sediments. OSL results indicate that the filling of the lake took about 700 years, with a sedimentation rate of w3 mm per year. This study is a demonstration of the use of OSL dating of sand-sized quartz to determine the timing and rate of sedimentation in a Holocene fluviodeltaic environment. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: OSL dating Holocene Lacustrine Fluvial Quartz Delta Rhine Heterogeneous deposits
1. Introduction River deltas are among the most densely populated areas. Understanding of deltaic development and sediment composition is important to predict delta responses to climate and sea level changes, and because delta sediments can develop into water and hydrocarbon reservoirs. Lakes are common features in distal – or low-gradient – delta plains (see examples from the Columbia River (Makaske et al., 2002) and the Cumberland Marshes (Smith et al., 1989)). Similarly, sediments that fill these lakes (organic-clastic lake fills, including lacustrine deltas (e.g., Tye and Coleman, 1989a)), are important architectural elements within distal delta-plain successions (Bos, in press). Understanding the formation and development of these environments has a twofold relevance. First, lake basins function as sediment traps for channels that debouch into them. Thereby, the lake sediments reflect the sediment volume and composition transported by the fluvial system. The sediment yield (and upstream erosion rate) of the fluvial distributary can be quantified from the volume of the deposits and the duration of
* Corresponding author. Tel.: þ31 15 2781056. E-mail address: [email protected] (J. Wallinga). 1871-1014/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2009.11.001
lacustrine deposition. Second, compared to most other architectural elements in distal delta plains, organic-clastic lake fills may be rich in sand. The incorporation of lake fills in reservoir-modelling studies will result in more accurate estimations of, for example, the hydraulic conductivity of deltaic deposits. Understanding of the composition and development of distal delta plains, including organic-clastic lake fills, is limited because investigations have been biased to proximal delta plains. Among the studies that focused on Holocene distal delta plains are those in the Atchafalaya Basin, USA (Tye and Coleman, 1989a, 1989b), the Cumberland Marshes, Canada (Pe´rez-Arlucea and Smith, 1999; Smith et al., 1989; Smith and Pe´rez-Arlucea, 1994) and the RhineMeuse delta, The Netherlands (Bos et al., 2009; Van der Woude, 1984; Weerts and Bierkens, 1993). These studies demonstrate that the composition and geometry of overbank deposits in distal delta plains are highly variable and probably more complex than those in the proximal zone. A clastic lake fill, which is defined as ‘a sharply bounded sediment body that was deposited in a delta-plain lake, which comprises gyttja and overlying clastic deposits – lacustrine deltas – that exhibit a coarsening-upward succession’ (following Bos, in press) are commonly found in and restricted to the distal zone of deltaic plains (e.g., Smith and Pe´rez-Arlucea, 1994; Tye and Coleman, 1989a).
J. Wallinga, I.J. Bos / Quaternary Geochronology 5 (2010) 602–610
Age constraints are required to define the period of sedimentation within a lake basin. For modern and recent clastic lake fills, these can be provided by series of aerial photographs (Smith and Pe´rez-Arlucea, 1994). For older Holocene systems, age constraints can be obtained from radiocarbon analyses of underlying and overlying organic material (To¨rnqvist and Van Dijk, 1993). However, radiocarbon dating provides no direct measurement of the timing and duration of clastic lacustrine deposition. The same holds for evidence from archaeological artefacts, which can be found on top of natural levees along the channels debouching into the lake, and along channels that transverse the former lake after filling (Louwe Kooijmans, 1974). In some cases, pollen analyses on clastic lake deposits may provide rough age estimates through correlation with dated sequences (e.g. Bos et al., 2009). The application of optically stimulated luminescence (OSL) dating on (sub)recent clastic lake deposits is a promising alternative dating method as it directly determines the age of the clastic lake deposits. However, OSL dating of clastic lacustrine sediments in a fluvio-deltaic environment is not straightforward. Firstly, light exposure of the mineral grains during fluvial transport may be too limited to completely reset the OSL signal prior to deposition and burial in the fluvio-lacustrine environment (Fiebig & Preusser, 2007; Singarayer et al., 2005; Wallinga, 2002a). Secondly, estimation of the natural dose rate in lacustrine deposits may be complicated due to heterogeneity of the deposits, varying water contents, and radioactive disequilibria (e.g. Bubenzer et al., 2007; Olley et al., 1996). To our knowledge, OSL dating of clastic lake fills (including lacustrine deltas) of Holocene age has not been attempted before. Promising results have, however, been obtained for similar environments like oxbow lakes in the Mississippi Delta (Rowland et al., 2005). In this paper, we test the applicability of quartz OSL dating of Holocene fluvio-deltaic clastic lake-fill sediments. OSL results are compared to age constraints provided by the radiocarbon dated
603
period of activity of feeding channels. Our goal is to determine the timing of lake-fill sedimentation using OSL dating methods and to use this information to calculate sedimentation rates. 2. Site and samples The Aetsveldse organic-clastic lake fill is genetically associated with the channel network of two Holocene Rhine-distributaries, the Angstel and Vecht Rivers (Figs. 1–3). It is an excellent study area for two reasons. First, the time constraints for the formation of the organic-clastic lake fills could be accurately determined. Fluvial sedimentation occurred during a single episode as identified by uninterrupted formation of levee and floodbasin deposits that are associated with the feeding channel, the River Angstel (i.e. no soil horizons within the overbank deposits; Bos et al., 2009). Second, the position of the organic-clastic lake fills near the top of the Holocene sequence enabled accurate mapping of the clastic lake-fill facies (Fig. 3), which subsequently offered the possibility to select the most suitable sample locations. The palaeogeography of the late Holocene Angstel and Vecht Rivers has been outlined in detail by Bos et al. (2009) and is briefly discussed below with emphasis on the chronological constraints; radiocarbon ages, sample codes and references are provided in Table 1. Throughout this contribution, radiocarbon ages are presented with two sigma confidence intervals. Calibrated ages (cal. BP) are obtained using Reimer et al. (2004) calibration curve, and are given relative to 1950 AD. When comparing radiocarbon and OSL chronologies we add 56 years to the calibrated age to express all dates relative to 2006 AD (the year of OSL sampling). The beginning of clastic sedimentation in the study area is determined by the onset of the Angstel and Vecht Rivers, which has been dated at 2970 100 cal. yr BP (Bos et al., 2009). This date has been obtained by combining three radiocarbon samples (UtC14574, 82, 84) that indicate the beginning the Angstel-Vecht
Fig. 1. Map showing the location of the study area within the Holocene Rhine delta (The Netherlands).
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two cross sections (Fig. 3). Four OSL samples were taken in a vertical sequence from a site located 350 m away from the Gd1 lacustrine distributary channel (Fig. 3, cross-section A). This channel is part of the channel network that developed in the organic-clastic lake fill. The sequence comprised two samples that were taken from the bottom clayey facies and two samples from the overlying sandy facies interpreted as mouth-bar deposits of lacustrine distributary channel Gd1. We took two additional samples from sandy distributary-channel deposits (Gd1 and Gd2) that were overlain by organic residual-channel deposits. Ages for abandonment of these channels are provided by radiocarbon dates on terrestrial macrofossils from the residual-channel fills (Gd1: 2530 180 cal. yr BP; Gd2: 2510 190 cal. yr BP, see Table 1). 3. Optical dating 3.1. Sample preparation
Fig. 2. Map of the study area. The extent of the organic-clastic lake-fill deposits, the courses of the feeding channels (the Angstel and Vecht Rivers), and the locations of the radiocarbon dating samples are shown. General flow direction is from south to north.
system upstream of the lakes (Fig. 2, Table 1). This analysis was performed using OxCal v4.0.5 software (Bronk Ramsey, 1995, 2001). The more downstream positioned sample that indicates the onset of sedimentation by the Angstel (UtC-14578) is of comparable age, which implies that the small lakes upstream of the Aetsveldse Lake were rapidly filled (probably within decades). The clastic succession in the former Aetsveldse Lake is approximately 4-m thick and displays a coarsening-upward sequence that overlies gyttja deposits (Fig. 3). Sandy mouth-bar deposits overly a clayey bed near the lacustrine distributary channels, whereas further away from these channels the mouth-bar deposits are finer grained. The lake-fill sediments are mostly laminated, with thicknesses of sediment laminae varying from millimetre to decimetre scale. Bedding is especially pronounced for the mouth-bar deposits. Further details about the sedimentology of the lake deposits are provided by Bos (in press). Between 2650 and 2200 cal. yr BP sediment supply to the Aetsveldse Lake strongly decreased as is evidenced by the abandonment of the lacustrine distributary channels in the clastic lake fill (UtC14577, 73; Fig. 2, 3 and Table 1). However, the westernmost part of the Aetsveldse Lake received clastic sediment at least until 1580 130 cal yr BP and probably until the Angstel was abandoned around 1460 90 cal yr BP (UtC-14580, 75; Fig. 2 and Table 1). For this study, we collected six OSL samples from three mechanical cores that were positioned along or in the vicinity of
Using a mechanical bailer drilling unit (Oele et al., 1983), we obtained 1-m long sediment core segments contained in opaque PVC tubes of 10-cm diameter. To obtain samples for optical dating, the sediment cores were split in a dark room equipped with subdued orange/red lights. For each core, one half was brought into the light to describe the sediments and select suitable depth intervals for OSL sampling. The OSL samples were taken from the other half of the core. We sampled relatively coarse intervals and tried to avoid sampling close to lithological boundaries to facilitate estimation of the gamma dose rate. For many samples this was not possible, and we took additional samples from adjacent material to allow estimation of the gamma dose-rate contribution from those sediments. Information on sample locations and depths is provided in Table 2 and Fig. 3. Where sediments were relatively homogeneous, large vertical intervals (30–40 cm) were sampled, and the mixed material was used both for equivalent-dose and dose-rate analysis. In bedded layers, we sampled a single homogeneous bed (5–15 cm) for equivalent-dose estimation and we took additional samples above and/or below these intervals for dose-rate estimation. For one sample from a bedded layer (NCL-3206025), we were able to perform dose-rate measurements on the centre sample as well. Upon arrival in the luminescence dating laboratory, the relatively large samples (NCL-3206025-27) were split in two; w200 g of material was used for dose-rate analysis, from the remainder we extracted a sand-sized quartz fraction for equivalent-dose analysis. The other samples were used for equivalent-dose estimation (NCL numbers in Table 2) or dose-rate estimation (those without NCL number in Table 2). Quartz is the mineral of choice as residual signals for fluvial deposits are usually lower than those for feldspar (Fiebig and Preusser, 2007), and because the quartz OSL signal is stable over geological timescales (Wintle and Murray, 2006). We prefer sand-sized grains for equivalent-dose determination as these allow OSL measurements on aliquots containing only a few grains, which facilitates interpretation of the completeness of OSL resetting prior to deposition and burial (e.g. Duller, 2008). Samples for equivalent-dose estimation were treated with HCl and H2O2 to remove carbonates and organic material and were wet-sieved to obtain the grain size range of 90–180 mm (180–212 mm for sample NCL-3206027). Subsequently, the samples were etched with concentrated HF (40%) to dissolve feldspars and remove the outer layer of the quartz grains which was exposed to alpha radiation in the natural environment. The samples were then rinsed with HCl and water, and finally re-sieved to discard the grains that were heavily damaged by the HF treatment. Samples for dose-rate estimation were heated to 105 C for 24 h to determine water contents, ashed for 24 h at 500 C to determine
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Fig. 3. Detailed map of the area of the former Aetsveldse Lake. The location of the cross-sections A and B, the courses of distributary channels (Gd1, 2 and 3), and the positions of the cores used for this study are indicated (see Fig. 2 for the legend of the map). Cross-sections A and B (after Bos et al., 2009) show the different facies of the organic-clastic lake-fill sequence; note the general coarsening-upward trend. Locations of radiocarbon and OSL samples are indicated in the cross sections, as well as the ages obtained on the samples.
the organic content, and then ground to homogenize the samples. The ground sediment was mixed with wax (weight proportion 7:3) and cast into a 2-cm thick puck with 10-cm diameter for measurement with a gamma spectrometer. 3.2. Equivalent-dose estimation For equivalent-dose estimation, we employed a Risø TL/OSL DA 15 reader (Bøtter-Jensen et al., 2000) equipped with a Sr/Y beta source (dose rate w0.030 Gy/s). Quartz OSL signals were obtained through stimulating with blue LEDs (470 nm, w30 mW/cm2), infrared (IR) stimulation was provided by IR diodes (875 nm,
w116 mW/cm2). Signal collection used an EMI9235QA photomultiplier tube shielded by a 7.5 mm Hoya U-340 filter. Tests with infrared stimulation indicated that some feldspar contamination remained after HF treatment. To minimize the feldspar contribution to the OSL signal, we adopted a post-IR blue single-aliquot regenerative dose (SAR) procedure for equivalent-dose estimation (Wallinga et al., 2002). Only the centre 2 mm of aliquots were covered with material (w200 grains per disc); because only a small portion of quartz grains usually contributes to the OSL signal (Duller et al., 2000) we expect the OSL signal of each aliquot to be dominated by a small number of grains. Therefore, incomplete resetting of the OSL signal in part of the grains is expected to result
Table 1 Radiocarbon age control on Aetsveldse Lake deposits (modified from Bos et al., 2009). Laboratory nr
14
Calendar ageb (cal yr BP)
Coordinatesc (x/y) (m)
Surface elevationd (m O.D.)
Depth below surface (m)
Sample name
Relevance
UtC-14574 UtC-14582 UtC-14584 UtC-14578 UtC-14577 UtC-14573 UtC-14580 UtC-14575
2920 70 2810 50 2870 46 2853 46 2420 50 2367 44 1694 45 1577 43
3100 220 2930 140 3010 150 3000 150 2530 180 2510 190 1580 130 1460 90
123751/464139 129342/466569 125711/465662 128508/473158 128141/477478 130221/478555 124678/474147 129542/470254
1.67 1.15 1.63 1.55 1.50 1.44 2.05 0.72
1.77–1.81 2.17–2.18 2.02–2.03 1.46–1.47 1.33–1.34 1.21–1.22 2.13–2.14 1.94–1.95
Spengen 2 Weeresteijn 2 Portengen 3 Angstel 1 Gein 1 Weesp 1 Winkel 5 Loenen 1
Beginning Angstel/Vecht Beginning Angstel/Vecht Beginning Angstel/Vecht Beginning Angstel End Gd1 in Aetsveldse Lake End Gd2 in Aetsveldse Lake End sedimentation Aetsv. Lake west End Angstel
a b c d
C agea
All dated material consisted of selected terrestrial macrofossils. Calibrated using Reimer et al. (2004). In Dutch coordinate grid (Rijksdriehoekstelsel). O.D.: Dutch Ordnance datum.
1.3 0.3 – 1.3 0.3 24 5 – 24 5 358 6 – 358 6 a
b
location according to Dutch coordinate grid, surface elevation relative to NAP (Dutch Ordnance datum). samples from above and below the optical dating samples were combined for measurement of activity concentrations and water and organic contents.
14.7 3.8 – 14.7 3.8 21.5 0.7 – 21.5 0.7 18.4 0.9 – 18.4 0.9 Sandy clay Sand with clay laminae Sandy clay
21.6 0.3 – 21.6 0.3
22.5 0.6 – 22.5 0.6
20.2 1.9 – 20.2 1.9
20.4 1.3 – 20.4 1.3
3.7 0.7 – 3.7 0.7 68 14 – 68 14 463 3 – 463 3 32.8 4.3 – 32.8 4.3 34.3 0.6 – 34.3 0.6 28.8 1.2 – 28.8 1.2 Sandy clay Sandy clay/ sand Sandy clay
35.4 0.3 – 35.4 0.3
35.9 0.4 – 35.9 0.4
33.6 1.2 – 33.6 1.2
32.5 4.6 – 32.5 4.6
5.6 1.1 1.7 0.3 64 13 73 15 477 4 374 4 28.7 4.6 23.8 4.3 36.1 0.6 29.5 0.7 29.3 1.1 27.1 0.8
NCL-3206023 M4 3.24–3.57 NCL-3206022 M3 4.06–4.46 Core 25G1054 (x: 128144; y: 477486; z: 1.10)a NCL-3206026 M1b 2.20–2.28 M2 2.28–2.40 b 2.40–2.50 M3 Core 25H0740 (x: 130220; y: 478551; z: 1.35)a b NCL-3206027 M2a 1.52–1.57 M1 1.57–1.70 1.70–1.75 M2bb
Clay with sand laminae Silty clay, Sand admixture
36.5 0.3 30.4 0.2
37.5 0.5 31.1 0.4
34.0 29.3 33.2 1.3
34.5 3.3 29.4 3.2
– 26 5 – 406 6 – 18.9 4.0 – 19.8 0.7 NCL-3206024
M5 M6bb
2.34–2.39 2.39–2.49
Fine sand Sandy clay
– 17.2 0.9
– 20.5 0.3
– 20.8 0.5
– 18.9 17.2
– 16.9 2.7
1.6 0.3 0.4 0.1 1.6 0.3 1.8 0.4 34 7 20 3 34 7 26 5 418 4 422 3 418 4 406 6 15.6 3.7 11.8 2.1 15.6 3.7 18.9 4.0 18.9 0.5 14.3 0.3 18.9 0.5 19.8 0.7 17.6 17.5 13.1 13.3 17.6 17.5 18.9 17.2 19.8 0.5 14.5 0.3 19.8 0.5 20.8 0.5 19.4 0.2 14.2 0.1 19.4 0.2 20.5 0.3 17.5 1.6 13.3 0.5 17.5 1.6 17.2 0.9 Sandy clay Fine sand Sandy clay Sandy clay Core 25G1057 (x: 128526; y: 477358; z: 1.30)a NCL-3206025 M8ab 1.55–1.63 M7 1.63–1.76 b 1.76–1.83 M8b 2.27–2.34 M6ab
Th-series
18.4 1.5 14.1 1.2 18.4 1.5 16.9 2.7
K-40 Ac-228 Th-234
U-series
Pb-214
B-214
Pb-210
Pb-212
Bi-212
K
Water content (% by weight) Activity concentrations (Bq/kg) Lithology Depth below surface (m) Sample
Table 2 Samples used for OSL dating, with information on radionuclide concentrations, and water and organic contents.
– 1.8 0.4
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Organic content (% by weight)
606
in a scattered equivalent-dose distribution (Wallinga, 2002b; Duller, 2008). Based on preheat-plateau tests on three samples, we selected a preheat of 220 C for 10 s; heating prior to the test dose measurements was to 200 C. Data was accepted if the recycling ratio was within 20% of unity. The adopted protocol allowed us to accurately determine a laboratory given dose of 5 Gy (dose recovery ratio 1.04 0.02, n ¼ 16). A typical shine-down curve and doseresponse curve is shown in Fig. 4. For each sample, equivalent-dose estimates were made until at least 22 aliquots passed the acceptance criteria. This value was selected based on the distributions obtained; we do not expect large improvements in precision by measuring additional aliquots. Equivalent-dose distributions (see Fig. 5) indicated internal agreement of the vast majority of equivalent-dose estimates (i.e. spread in results can be explained by uncertainties on individual estimates), but that some aliquots returned extraordinary high or in some cases low values. High outliers are likely caused by incorporation of quartz grains for which the OSL signal was not completely reset prior to deposition and burial. Low outliers may be due to accidental light exposure or contamination during sample handling. To avoid bias of the results due to these outliers, we iteratively rejected single-aliquot equivalent-dose estimates yielding values more than two standard deviations away from the sample mean. The average of the remaining estimates was then used for age calculation. 3.3. Dose-rate estimation Radionuclide concentrations were measured using a Canberra broad energy HPGe gamma spectrometer following a procedure similar to the one described by Murray et al. (1987). To allow Rn buildup in the wax pucks, samples were stored for at least three weeks after casting before they were measured. U-series activity was determined by measurement of Th-234, Pb-214, Bi-214 and Pb210 activity concentrations. Th-series activity was determined from Ac-228, Pb-212 and Bi-212 activity concentrations. The K-40 activity concentration was measured directly. These activity concentrations (see Table 2) were converted to dose rates using standard conversion tables (Olley, 1996), with an uncertainty of 3%. Dose-rate attenuation due to grain size (Mejdahl, 1979), water content (Zimmerman, 1971) and organic content (Madsen et al.,
Fig. 4. Typical shine-down curve (A) and dose-response curve (B). Examples shown are for sample NCL-3206024; the dose-response curve is an average of three aliquots, and is fitted with an exponential plus linear function.
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607
laminated lacustrine deposits. The samples were taken from depth intervals that contained layers of sand, varying in thickness from a millimetre up to 12 cm (see Table 3). As the sediments within each bed or lamina are well sorted, we assume that the sand-sized grains used for equivalent-dose estimation originate from these sand layers. The range of beta particles in sediment is w2 mm (e.g. Aitken, 1985). Hence for sand laminae thicker than w1 cm it is safe to assume that the beta dose originates entirely from within the sand layer. For thinner sand laminae, the beta dose originates partly from surrounding material. Depending on the thickness of the sand layer we calculated how to mix the contributions (based on Aitken, 1985, p. 295 eq. H.13; Aitken et al., 1985); the relative weighting of the beta dose rate from outside the sample layer (Wo,b) is given by:
n Wo;b ¼
o 1 exp mb t mb t
(1)
Where mb (mm1) is the linear attenuation coefficient in sediment for b particles and t (mm) is the layer thickness. As linear attenuation coefficients are similar for U, Th and K, we assumed mb to be 1.14, the value suggested by Aitken (1985) for the U spectrum. The weighting of the beta dose rate from the sample layer itself is 1 Wo,b. Only for sample NCL-3206025 we had enough material to determine the radionuclide concentrations for the sandy sediments. We assumed the results for this sample to be representative for all sand deposits, and we base the beta dose from all sand layers on the radionuclide concentrations measured on this sample. The beta dose originating from surrounding material was determined from the radionuclide activity concentrations in that material. The relative importance of the contribution from outside the sand layer is up to 60% (for sand laminae of 1 mm, see Table 3). For estimating gamma dose rates we used a similar approach as for the beta dose rates. However, gamma radiation originates from a larger sphere around the sample because its range is much wider (w30 cm). Infinite matrix gamma dose rates for the sands were based on measurements on sample NCL-3206025. Infinite matrix gamma dose rates from material bordering the sand layers were based on gamma spectrometry results on that material. For samples taken from very thin sand layers (<0.5 cm), it is safe to assume that the gamma dose rate of a bulk sample taken around this depth is valid. For samples taken from thicker sand layers, we used a combination of gamma dose-rate values from the sand layers, and those obtained on adjacent material; relative contributions (see Table 3) were calculated based on equation H.10 of Aitken (1985, p. 294), where the weighting of the gamma dose rate from outside the sample layer is given by:
Wo;g ¼ Fig. 5. Equivalent-dose distributions of all investigated samples shown as radial plots (Galbraith, 1990). Note that more than 75% of the single-aliquot equivalent-dose estimates agrees with the value adopted for age calculation (grey bar), indicating that the OSL signal of the vast majority of quartz grains was reset at the time of deposition and burial.
2005) was taken into account. An internal alpha contribution of 0.06 0.03 Gy/ka was assumed. In addition to the environmental dose rate, a contribution from cosmic rays was calculated based on Prescott and Hutton (1994), assuming immediate burial to the present depth. Special attention was needed to quantify the dose rate as the surrounding sediment was not homogeneous in the bedded or
o 65 n 0:94 exp mg t ðt 4Þ
(2)
Where mg (mm1) is the linear attenuation coefficient in sediment for gamma rays (taken to be 0.015 mm1 based on Aitken, 1985), and t (mm) is the layer thickness. The weighting of the gamma dose rate from the sample layer itself is 1 Wo;g . Both water and organic material shield the mineral grains from radiation; this effect is referred to as attenuation of the external dose rate. As organics and water have similar stopping power, attenuation was calculated from the combined water and organic content (following Madsen et al., 2005). For the sandy layers, the water content may be affected by disturbance during sampling. Therefore, we based the water content for the sandy intervals on the porosity of fluvial sands (w34% by volume; Weerts, 1996, p. 101) yielding a water content of 20% by weight. Organic content of
1.69 0.06 0.17 0.01 0.93 0.05 –
25G1057 M7
27%
25H0740 M2ab
73%
0.06 0.03
0.53 0.03
0.16 0.01 0.58 0.04 0.92 0.07 0.06 0.03
4. Results Equivalent doses, dose rates and resulting OSL ages for all samples are given in Table 4; ages are given relative to the year of sampling (2006) with a 1-sigma confidence interval (including all random and systematic uncertainties). Ages range from 3.49 0.21 ka to 2.45 0.12 ka. Results for the four samples in a vertical sequence (core 25G1057; samples NCL-3206022 to 25) are in correct stratigraphical order (within uncertainties); a linear fit of the lower three data points provides an accumulation rate of 2.8 1.0 mm per year for the fine-grained facies (Fig. 6). OSL ages for the upper two samples are identical (both w2.8 ka), which agrees with anticipated rapid deposition of a coarse facies in a mouth bar. The OSL age of 2.87 0.16 obtained on associated deposits of distributary channel Gd1 (core 25G1054; sample NCL3206026) is identical to those obtained on the mouth-bar deposits (2.83 0.12 ka, combined uncertainty). The OSL age obtained on channel deposits of distributary channel Gd-2 (core 25H0740; sample NCL-3206027, 2.45 0.12 ka) is some 400 years younger than those for Gd-1 (w2.8 ka). This time lag is in accordance with gradual filling of the lake from the south, although more samples should be dated to allow determination of the progradation rate based on OSL data.
– 100% 25G1057 M7 35
100% 25G1054 M1,M3 – – 25G1054 M1,M3 39% 61% 25G1057 M7 2 2.34
the sandy layers was low (determined on sample NCL-3206025: 0.4% by weight). For all other materials, we used water and organic contents as measured on the samples (see Table 2) to calculate attenuation factors (following Zimmerman, 1971). Changes in water content due to compaction of the deposits are not taken into account as compaction likely occurred quickly after formation of the deposits. Radionuclide concentrations for the Th-232 decay series are all in agreement, indicating that there is no disequilibrium in this series. For the U-238 decay series, we found that Pb-214 and Bi-214 radionuclide concentrations were on average 19 2% higher than those of Th-234 (Table 2). This likely indicates the presence of excess Th-230 at the time of deposition. Because the age of the deposits is small compared to the half life of Th-230 (75 ka), the measured activity concentrations can be used to calculate dose rates.
5. Discussion 5.1. Resetting of the OSL signal Our samples are from three different depositional environments within the fluvio-deltaic lake setting. Samples NCL-3206022 and 23 were taken from clayey deposits formed in a low-energy lacustrine setting, relatively far away from the river mouth. Samples NCL3206024 and 25 are taken from sandy mouth-bar deposits, formed in a higher energy environment when the river had prograded close Table 4 Quartz optical dating results.
Core 25G1054 NCL-3206026 M2
1.64
NCL sample
Core 25H0740 NCL-3206027 M1
25G1057 25G1057 25G1057 25G1057 25G1057 M7 57% 25G1057 M7 38% – – – – – – 28% 60% – – 25G1057 M4 25G1057 M3 M7 100% M7 100% M7 72% M7 40% 25G1057 25G1057 25G1057 25G1057 120 60 3 1 M7 M5 M4 M3 Core 25G1057 NCL-3206025 NCL-3206024 NCL-3206023 NCL-3206022
1.70 2.37 3.41 4.26
Weight
Sample
Weight Sample
M8ab M6ab M4 M3
Weight
43% 62% 100% 100%
Total Cosmic Gamma Beta
Dose rates (Gy/ka)
Alpha Surrounding Sand layer
Samples used for gamma dose estimation
Depth Thickness Samples used for beta dose estimation (m) of sand Sand layer Surrounding laminae (mm) Sample Weight Sample De on sample NCL sample
Table 3 Relative dose-rate contributions from the OSL-sampled sand layer and surrounding deposits and resulting dose rates for the OSL samples.
1.71 0.08
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0.06 0.03 0.96 0.05 0.49 0.02 0.17 0.01 1.68 0.06 0.06 0.03 0.96 0.05 0.52 0.02 0.16 0.01 1.69 0.06 0.06 0.03 0.94 0.06 0.61 0.04 0.14 0.01 1.74 0.08 0.06 0.03 0.81 0.07 0.50 0.03 0.13 0.01 1.50 0.08
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Depth (m)
Equivalent dose (Gy)
Dose rate (Gy/ka)
OSL age (ka)
Core 25G1057 NCL-3206025 NCL-3206024 NCL-3206023 NCL-3206022
1.70 2.37 3.41 4.26
4.76 0.15 4.74 0.12 5.51 0.16 5.22 0.14
1.68 0.06 1.69 0.06 1.74 0.08 1.50 0.08
2.83 0.14 2.80 0.13 3.16 0.17 3.49 0.21
Core 25G1054 NCL-3206026
2.34
4.91 0.15
1.71 0.08
2.87 0.16
Core 25H0740 NCL-3206027
1.64
4.13 0.15
1.69 0.06
2.45 0.12
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Fig. 6. Plot of OSL ages with depth for core 25G1057 penetrating mouth bar and distal fine-grained deposits of distributary channel Gd1 (see Fig. 3 for location). Error bars indicate one sigma uncertainty interval. A linear fit of the lower three points indicates a sedimentation rate of w3 mm per year for the fine-grained facies. Results for the top two samples are identical, in line with expected rapid aggradation for coarse-grained mouth-bar deposits.
to the sampling site. Finally, samples NCL-3206026 and 27 are taken from channel deposits formed when the fluvial channels traversed the (then filled in) lake area. For all samples, more than 75% of the single-aliquot equivalent-dose estimates agree (within 2s) with the sample mean determined after iterative removal of outliers (see Fig. 5). These results indicate that the vast majority of quartz grains had their OSL signal reset prior to deposition and burial. We see no clear dependency of the spread in single-aliquot equivalent-dose estimates on the depositional environment. This may indicate that resetting of the OSL signal occurred during fluvial transport of the grains (which is similar for all samples), and does not depend on the exact depositional environment within the fluvio-deltaic system (i.e. the facies in which they are preserved). We note that, due to low sedimentation rates in combination with a large sample size, samples NCL-3206022 and 23 are expected to encompass over a century of deposition. This will add to the scatter in equivalent-dose results (w4% difference in De between grains at the top and bottom of the sample), but this source of spread is small compared to the measurement uncertainties. Singarayer et al. (2005) have tested residual doses in modern fluvial deposits from a range of environments; unfortunately, no fluvio-deltaic deposits were included in their study. They found mean equivalent doses ranging from 0.17 to 3.27 Gy, with an average of 0.98 Gy (standard deviation 0.76 Gy). For most of our samples, the mean equivalent dose is greater than the equivalent dose obtained through the iteration procedure that was adopted for age calculation. The average difference between the two estimates is 0.4 Gy (standard deviation 0.4 Gy), which is similar in magnitude to the average remnant mean equivalent dose reported by Singarayer et al. (2005). We suggest that it is important in studies of young fluvial deposits to exclude outliers from the equivalent-dose distribution for calculating burial ages. Our results confirm previous findings (reviewed in Wallinga, 2002a; Rittenour, 2008) that quartz OSL signals in downstream stretches of large river systems are well-enough reset to allow accurate age determination through quartz OSL dating for deposits that are more than a thousand years old. 5.2. Dose-rate assumptions The dose rates calculated for samples from heterogeneous deposits depend on the assumptions made with respect to dose-
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rate contributions from different layers. For our samples, however, the dependence is limited because of two counterbalancing factors: (1) the radionuclide concentrations and (2) the attenuation due to water and organic contents. The sand layers have relatively low radionuclide concentrations and low water and organic contents, resulting in an infinite matrix dose rate of the sands of w1.45 Gy/ka (b: w0.96 Gy/ka þ g: w0.49 Gy/ka). The silt and clay deposits have higher radionuclide concentrations and also higher water and organic contents, resulting in an average infinite matrix dose rate of w1.41 Gy/ka (b: w0.85 Gy/ka þ g: w0.55 Gy/ka). Owing to the small difference in infinite matrix dose rates for the different facies, the dependency of the dose rate on the weighting factors used (Eqs. (1) and (2); Table 3) is limited. Dose rates also depend on the assumptions with regard to water content and organic content; a 1% increase in either will induce a w1% decrease in dose rate and thus a w1% increase in OSL age. Water contents may have changed over geological time due to compaction. In addition, pushing the core into the sediments may have induced compaction (lowering water contents) or disturbed packing (enlarging water contents) and samples may have dried during storage (although care was taken to avoid this). We used a large relative uncertainty of 20% on the measured water contents to encompass the full range of possible values during geological time. As water contents are largest for the more clayey deposits, results for samples from those deposits are most sensitive to the assumptions made. 5.3. Comparison with independent age constraints Sedimentation in the lake must post-date the start of activity of the fluvial channels feeding into the lake. Yet, the OSL determined start of lacustrine sedimentation (NCL-3206022, 3.49 0.21 ka) is slightly older than the onset of clastic sedimentation based on radiocarbon dating (2970 100 cal yr BP). Although the age intervals for both methods slightly overlap, the OSL age seems to be a bit too old (based on two sigma confidence intervals for both methods and adding 56 years to the calibrated 14C BP age to allow direct comparison – 14C age: 3.12 2.94 ka; OSL age: 3.91 3.07 ka). As there is strong evidence that sedimentation in the lake did not start earlier than suggested by the radiocarbon ages (discussed in Bos et al., 2009), and there is no evidence for incomplete resetting of the OSL signal, we attribute this possible age overestimation to an underestimation of the dose rate. The most likely cause is overestimation of the water content of the sample. OSL results for the final stages of clastic sedimentation at the two sites (Gd1: samples NCL-3602024-26; 2.83 0.12 ka (combined uncertainty); Gd2: NCL-3602027; 2.45 0.12 ka) agree well with the decreased sediment input between 2650 and 2200 cal. yr BP. 6. Conclusions Results of our investigation of clastic lake-fill deposits in the former Aetsveldse Lake (Rhine delta, western Netherlands) indicate that: Quartz OSL signals of the vast majority of grains are well reset prior to deposition and burial. Radioactive disequilibria yield no problems for dose-rate estimation. Dose rates in bedded/laminated sediments can be obtained from radionuclide concentrations and water contents of the different sediment layers. High water contents may contribute to the uncertainty and accuracy of the OSL dates obtained, especially for clayey facies.
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Quartz OSL dates for all but one sample agree with radiocarbon age constraints. Average sedimentation rates of the clayey facies at our main sampling site was w3 mm per year. This study demonstrates that OSL dating may be a powerful tool to determine the timing and duration of clastic lake sedimentation, especially of sandy facies. Thereby, OSL methods may complement other chronometric approaches in such environments. Acknowledgements We thank Candice Johns for sample preparation and measurements. We would like to thank Torbjo¨rn To¨rnqvist and Tammy Rittenour for their constructive reviews of this manuscript, and Bert Roberts for editorial handling. We also acknowledge two anonymous reviewers for their comments on an earlier version of this paper. JW is supported by Technology Foundation STW (DSF.7553). Editorial handling by: R. Roberts References Aitken, M.J., 1985. Thermoluminescene Dating. Academic Press, London. Aitken, M.J., Clark, P.A., Gaffney, C.F., Løvborg, L., 1985. Beta and gamma gradients. Nuclear Tracks and Radiation Measurements 10, 647–653. Bos, I.J., Feiken, H., Bunnik, F.P.M., Schokker, J, 2009. Influence of organics and clastic lake fills on distributary channel processes in the distal Rhine-Meuse delta (The Netherlands). Palaeogeography, Palaeoclimatology. Palaeoecology 284, 355–374. Bos, I. J. Architecture and facies distribution of organic-clastic lake fills in the fluviodeltaic Rhine-Meuse system, The Netherlands. Journal of Sedimentary Research, in press. Bronk Ramsey, C., 1995. Radiocarbon calibration and analysis of stratigraphy: the OxCal program. Radiocarbon 37 (2), 425–430. Bronk Ramsey, C., 2001. Development of the radiocarbon calibration program OxCal. Radiocarbon 43 (2A), 355–363. Bubenzer, O., Hilgers, A., Riemer, H., 2007. Luminescence dating and archaeology of holocene fluvio-lacustrine sediments of Abu Ttartur, Eastern Sahara. Quaternary Geochronology 2, 314–321. Bøtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 57–73. Duller, G.A.T., 2008. Single-grain optical dating of Quaternary sediments: why aliquot size matters in luminescence dating. Boreas 37, 589–612. Duller, G.A.T., Bøtter-Jensen, L., Murray, A.S., 2000. Optical dating of single sand-sized grains of quartz: sources of variability. Radiation Measurements 32, 453–457. Fiebig, M., Preusser, F., 2007. Investigating the amount of zeroing in modern sediments of River Danube, Austria. Quaternary Geochronology 2, 143–149. Galbraith, R.F., 1990. The radial plot – graphical assessment of spread in ages. Nuclear Tracks and Radiation Measurements 17, 207–214. Louwe Kooijmans, L. P. (1974). The Rhine/Meuse delta. Four studies on its prehistoric occupation and Holocene geology. Published PhD thesis. Analecta Praehistorica Leidensia, 7. Madsen, A.T., Murray, A.S., Andersen, T.J., Pejrup, M., Breuning-Madsen, H., 2005. Optically stimulated luminescence dating of young estuarine sediments: a comparison with 210Pb and 137Cs dating. Marine Geology 214, 251–268. Makaske, B., Smith, D.G., Berendsen, H.J.A., 2002. Avulsion, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada. Sedimentology 49, 1049–1071. Mejdahl, V., 1979. Thermoluminescence dating: beta dose attenuation in quartz grains. Archaeometry 21, 61–72.
Murray, A.S., Marten, R., Johnston, A., Marten, P., 1987. Analysis for naturally occurring radionuclides at environmental concentrations by gamma spectrometry. Journal of Radioanalytical and Nuclear Chemistry 115, 263–288. Oele, E., Apon, W., Fischer, M.M., Hoogendoorn, R., Mesdag, C.S., De Mulder, E.F.J., Overzee, B., Seso¨ren, A., Westerhoff, W.E., 1983. Surveying The Netherlands: sampling techniques, maps and their applications. In: Van den Berg, M.W., Felix, R. (Eds.), Special Issue in the Hounour of J.D. de Jong. Geologie en Mijnbouw 62, 255–372. Olley, J.M. (1996). The use of U-238 and Th-232 decay series radionuclides in sediment tracing. Unpublished Ph.D. Thesis, University of New South Wales. Olley, J.M., Murray, A., Roberts, R.G., 1996. The effects of disequilibria in the uranium and thorium decay chains on burial dose rates in fluvial sediments. Quaternary Science Reviews 15, 751–760. Pe´rez-Arlucea, M., Smith, N.D., 1999. Depositional patterns following the 1870’s avulsion of the Saskatchewan River (Cumberland Marshes, Saskatchewan). Journal of Sedimentary Research 69, 62–73. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and log-term time variations. Radiation Measurements 23, 497–500. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Bertrand, C.J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg, A.G., Hughen, K.A., Kromer, B., McCormac, G., Manning, S., Bronk Ramsey, C., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., Van der Plicht, J., Weyhenmeyer, C.E., 2004. IntCal04 terrestrial radiocarbon age calibration, 0–26 cal kyr BP. Radiocarbon 46, 1029–1058. Rittenour, T.M., 2008. Luminescence dating of fluvial deposits: applications to geomorphic, palaeoseismic and archaeological research. Boreas 37, 613–635. Rowland, J.C., Lepper, K., Dietrich, W.E., Wilson, C.J., Sheldon, R., 2005. Tie channel sedimentation rates, oxbow formation age and channel migration rate from optically stimulated luminescence (OSL) analysis of floodplain deposits. Earth Surface Processes and Landforms 30, 1161–1179. Singarayer, J.S., Bailey, R.M., Ward, S., Stokes, S., 2005. Assessing the completeness of optical resetting of quartz OSL in the natural environment. Radiation Measurements 40, 13–25. Smith, N.D., Pe´rez-Arlucea, M., 1994. Fine-grained splay deposition in the avulsion belt of the lower Saskatchewan River, Canada. Journal of Sedimentary Research 64, 159–168. Smith, N.D., Cross, T.A., Dufficy, J.P., Clough, S.R., 1989. Anatomy of an avulsion. Sedimentology 36, 1–23. To¨rnqvist, T.E., Van Dijk, G.J.,1993. Optimizing sampling strategy for radiocarbon dating of Holocene fluvial systems in a vertically aggrading setting. Boreas 22, 129–145. Tye, R.S., Coleman, J.M., 1989a. Depositional processes and stratigraphy of fluvially dominated lacustrine deltas: Mississippi delta plain. Journal of Sedimentary Petrology 59, 973–996. Tye, R.S., Coleman, J.M., 1989b. Evolution of Atchafalaya lacustrine deltas, southcentral Louisiana. Sedimentary Geology 65 (1-2), 95–112. Van der Woude, J.D., 1984. The fluviolagoonal palaeoenvironment in the Rhine/ Meuse deltaic plain. Sedimentology 31, 395–400. Wallinga, J., 2002a. Optically stimulated luminescence dating of fluvial deposits: a review. Boreas 31, 303–322. Wallinga, J., 2002b. Detection of OSL age overestimation using single-aliquot techniques. Geochronometria 21, 17–26. Wallinga, J., Murray, A.S., Bøtter-Jensen, L., 2002. Measurement of the dose in quartz in the presence of feldspar contamination. Radiation Protection Dosimetry 101, 367–370. Weerts, H.J.T., 1996. Complex confining layers. Architecture and hydraulic properties of Holocene and Late Weichselian deposits in the fluvial Rhine Meuse delta, The Netherlands. Nederlandse Geografische Studies 213, 1–189. Weerts, H.J.T., Bierkens, M.F.P., 1993. Geostatistical analysis of overbank deposits of anastomosing and meandering fluvial systems; Rhine-Meuse delta, The Netherlands. Sedimentary Geology 85 (1–4), 221–232. Wintle, A.G., Murray, A.S., 2006. A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols. Radiation Measurements 41, 369–391. Zimmerman, D.W., 1971. Thermoluminescent dating using fine grains from pottery. Archaeometry 13, 29–52.
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Research Paper
Optical dating of fluvio-deltaic clastic lake-fill sediments – A feasibility study in the Holocene Rhine delta (western Netherlands) Jakob Wallinga a, *, Ingwer J. Bos b, c a
Netherlands Centre for Luminescence dating, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands Department of Physical Geography, Utrecht University, Heidelberglaan 2, NL-3508 TC Utrecht, The Netherlands c TNO Built Environment and Geosciences/Geological Survey of The Netherlands, Princetonlaan 6, NL-3584 CB Utrecht, The Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2009 Received in revised form 29 October 2009 Accepted 10 November 2009 Available online 17 November 2009
We test the applicability of quartz optically stimulated luminescence (OSL) dating on clastic lake sediments to investigate whether this dating method can be applied to study the timing and rate of deposition in Holocene fluvio-deltaic lakes. Our study concerns the filling of a lake by the Angstel-Vecht system, part of the Rhine delta in the western Netherlands. Age constraints are provided by radiocarbon dates on the development and abandonment of the fluvial channels debouching into the lake. Results indicate that light exposure prior to deposition and burial was sufficient to reset the OSL signal of the vast majority of the quartz grains. Special attention was given to accurate estimation of the dose rate in the laminated and bedded deposits. The OSL ages obtained are in good agreement with the age constraints, especially for the relatively coarse sediments. OSL results indicate that the filling of the lake took about 700 years, with a sedimentation rate of w3 mm per year. This study is a demonstration of the use of OSL dating of sand-sized quartz to determine the timing and rate of sedimentation in a Holocene fluviodeltaic environment. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: OSL dating Holocene Lacustrine Fluvial Quartz Delta Rhine Heterogeneous deposits
1. Introduction River deltas are among the most densely populated areas. Understanding of deltaic development and sediment composition is important to predict delta responses to climate and sea level changes, and because delta sediments can develop into water and hydrocarbon reservoirs. Lakes are common features in distal – or low-gradient – delta plains (see examples from the Columbia River (Makaske et al., 2002) and the Cumberland Marshes (Smith et al., 1989)). Similarly, sediments that fill these lakes (organic-clastic lake fills, including lacustrine deltas (e.g., Tye and Coleman, 1989a)), are important architectural elements within distal delta-plain successions (Bos, in press). Understanding the formation and development of these environments has a twofold relevance. First, lake basins function as sediment traps for channels that debouch into them. Thereby, the lake sediments reflect the sediment volume and composition transported by the fluvial system. The sediment yield (and upstream erosion rate) of the fluvial distributary can be quantified from the volume of the deposits and the duration of
* Corresponding author. Tel.: þ31 15 2781056. E-mail address: [email protected] (J. Wallinga). 1871-1014/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2009.11.001
lacustrine deposition. Second, compared to most other architectural elements in distal delta plains, organic-clastic lake fills may be rich in sand. The incorporation of lake fills in reservoir-modelling studies will result in more accurate estimations of, for example, the hydraulic conductivity of deltaic deposits. Understanding of the composition and development of distal delta plains, including organic-clastic lake fills, is limited because investigations have been biased to proximal delta plains. Among the studies that focused on Holocene distal delta plains are those in the Atchafalaya Basin, USA (Tye and Coleman, 1989a, 1989b), the Cumberland Marshes, Canada (Pe´rez-Arlucea and Smith, 1999; Smith et al., 1989; Smith and Pe´rez-Arlucea, 1994) and the RhineMeuse delta, The Netherlands (Bos et al., 2009; Van der Woude, 1984; Weerts and Bierkens, 1993). These studies demonstrate that the composition and geometry of overbank deposits in distal delta plains are highly variable and probably more complex than those in the proximal zone. A clastic lake fill, which is defined as ‘a sharply bounded sediment body that was deposited in a delta-plain lake, which comprises gyttja and overlying clastic deposits – lacustrine deltas – that exhibit a coarsening-upward succession’ (following Bos, in press) are commonly found in and restricted to the distal zone of deltaic plains (e.g., Smith and Pe´rez-Arlucea, 1994; Tye and Coleman, 1989a).
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Age constraints are required to define the period of sedimentation within a lake basin. For modern and recent clastic lake fills, these can be provided by series of aerial photographs (Smith and Pe´rez-Arlucea, 1994). For older Holocene systems, age constraints can be obtained from radiocarbon analyses of underlying and overlying organic material (To¨rnqvist and Van Dijk, 1993). However, radiocarbon dating provides no direct measurement of the timing and duration of clastic lacustrine deposition. The same holds for evidence from archaeological artefacts, which can be found on top of natural levees along the channels debouching into the lake, and along channels that transverse the former lake after filling (Louwe Kooijmans, 1974). In some cases, pollen analyses on clastic lake deposits may provide rough age estimates through correlation with dated sequences (e.g. Bos et al., 2009). The application of optically stimulated luminescence (OSL) dating on (sub)recent clastic lake deposits is a promising alternative dating method as it directly determines the age of the clastic lake deposits. However, OSL dating of clastic lacustrine sediments in a fluvio-deltaic environment is not straightforward. Firstly, light exposure of the mineral grains during fluvial transport may be too limited to completely reset the OSL signal prior to deposition and burial in the fluvio-lacustrine environment (Fiebig & Preusser, 2007; Singarayer et al., 2005; Wallinga, 2002a). Secondly, estimation of the natural dose rate in lacustrine deposits may be complicated due to heterogeneity of the deposits, varying water contents, and radioactive disequilibria (e.g. Bubenzer et al., 2007; Olley et al., 1996). To our knowledge, OSL dating of clastic lake fills (including lacustrine deltas) of Holocene age has not been attempted before. Promising results have, however, been obtained for similar environments like oxbow lakes in the Mississippi Delta (Rowland et al., 2005). In this paper, we test the applicability of quartz OSL dating of Holocene fluvio-deltaic clastic lake-fill sediments. OSL results are compared to age constraints provided by the radiocarbon dated
603
period of activity of feeding channels. Our goal is to determine the timing of lake-fill sedimentation using OSL dating methods and to use this information to calculate sedimentation rates. 2. Site and samples The Aetsveldse organic-clastic lake fill is genetically associated with the channel network of two Holocene Rhine-distributaries, the Angstel and Vecht Rivers (Figs. 1–3). It is an excellent study area for two reasons. First, the time constraints for the formation of the organic-clastic lake fills could be accurately determined. Fluvial sedimentation occurred during a single episode as identified by uninterrupted formation of levee and floodbasin deposits that are associated with the feeding channel, the River Angstel (i.e. no soil horizons within the overbank deposits; Bos et al., 2009). Second, the position of the organic-clastic lake fills near the top of the Holocene sequence enabled accurate mapping of the clastic lake-fill facies (Fig. 3), which subsequently offered the possibility to select the most suitable sample locations. The palaeogeography of the late Holocene Angstel and Vecht Rivers has been outlined in detail by Bos et al. (2009) and is briefly discussed below with emphasis on the chronological constraints; radiocarbon ages, sample codes and references are provided in Table 1. Throughout this contribution, radiocarbon ages are presented with two sigma confidence intervals. Calibrated ages (cal. BP) are obtained using Reimer et al. (2004) calibration curve, and are given relative to 1950 AD. When comparing radiocarbon and OSL chronologies we add 56 years to the calibrated age to express all dates relative to 2006 AD (the year of OSL sampling). The beginning of clastic sedimentation in the study area is determined by the onset of the Angstel and Vecht Rivers, which has been dated at 2970 100 cal. yr BP (Bos et al., 2009). This date has been obtained by combining three radiocarbon samples (UtC14574, 82, 84) that indicate the beginning the Angstel-Vecht
Fig. 1. Map showing the location of the study area within the Holocene Rhine delta (The Netherlands).
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two cross sections (Fig. 3). Four OSL samples were taken in a vertical sequence from a site located 350 m away from the Gd1 lacustrine distributary channel (Fig. 3, cross-section A). This channel is part of the channel network that developed in the organic-clastic lake fill. The sequence comprised two samples that were taken from the bottom clayey facies and two samples from the overlying sandy facies interpreted as mouth-bar deposits of lacustrine distributary channel Gd1. We took two additional samples from sandy distributary-channel deposits (Gd1 and Gd2) that were overlain by organic residual-channel deposits. Ages for abandonment of these channels are provided by radiocarbon dates on terrestrial macrofossils from the residual-channel fills (Gd1: 2530 180 cal. yr BP; Gd2: 2510 190 cal. yr BP, see Table 1). 3. Optical dating 3.1. Sample preparation
Fig. 2. Map of the study area. The extent of the organic-clastic lake-fill deposits, the courses of the feeding channels (the Angstel and Vecht Rivers), and the locations of the radiocarbon dating samples are shown. General flow direction is from south to north.
system upstream of the lakes (Fig. 2, Table 1). This analysis was performed using OxCal v4.0.5 software (Bronk Ramsey, 1995, 2001). The more downstream positioned sample that indicates the onset of sedimentation by the Angstel (UtC-14578) is of comparable age, which implies that the small lakes upstream of the Aetsveldse Lake were rapidly filled (probably within decades). The clastic succession in the former Aetsveldse Lake is approximately 4-m thick and displays a coarsening-upward sequence that overlies gyttja deposits (Fig. 3). Sandy mouth-bar deposits overly a clayey bed near the lacustrine distributary channels, whereas further away from these channels the mouth-bar deposits are finer grained. The lake-fill sediments are mostly laminated, with thicknesses of sediment laminae varying from millimetre to decimetre scale. Bedding is especially pronounced for the mouth-bar deposits. Further details about the sedimentology of the lake deposits are provided by Bos (in press). Between 2650 and 2200 cal. yr BP sediment supply to the Aetsveldse Lake strongly decreased as is evidenced by the abandonment of the lacustrine distributary channels in the clastic lake fill (UtC14577, 73; Fig. 2, 3 and Table 1). However, the westernmost part of the Aetsveldse Lake received clastic sediment at least until 1580 130 cal yr BP and probably until the Angstel was abandoned around 1460 90 cal yr BP (UtC-14580, 75; Fig. 2 and Table 1). For this study, we collected six OSL samples from three mechanical cores that were positioned along or in the vicinity of
Using a mechanical bailer drilling unit (Oele et al., 1983), we obtained 1-m long sediment core segments contained in opaque PVC tubes of 10-cm diameter. To obtain samples for optical dating, the sediment cores were split in a dark room equipped with subdued orange/red lights. For each core, one half was brought into the light to describe the sediments and select suitable depth intervals for OSL sampling. The OSL samples were taken from the other half of the core. We sampled relatively coarse intervals and tried to avoid sampling close to lithological boundaries to facilitate estimation of the gamma dose rate. For many samples this was not possible, and we took additional samples from adjacent material to allow estimation of the gamma dose-rate contribution from those sediments. Information on sample locations and depths is provided in Table 2 and Fig. 3. Where sediments were relatively homogeneous, large vertical intervals (30–40 cm) were sampled, and the mixed material was used both for equivalent-dose and dose-rate analysis. In bedded layers, we sampled a single homogeneous bed (5–15 cm) for equivalent-dose estimation and we took additional samples above and/or below these intervals for dose-rate estimation. For one sample from a bedded layer (NCL-3206025), we were able to perform dose-rate measurements on the centre sample as well. Upon arrival in the luminescence dating laboratory, the relatively large samples (NCL-3206025-27) were split in two; w200 g of material was used for dose-rate analysis, from the remainder we extracted a sand-sized quartz fraction for equivalent-dose analysis. The other samples were used for equivalent-dose estimation (NCL numbers in Table 2) or dose-rate estimation (those without NCL number in Table 2). Quartz is the mineral of choice as residual signals for fluvial deposits are usually lower than those for feldspar (Fiebig and Preusser, 2007), and because the quartz OSL signal is stable over geological timescales (Wintle and Murray, 2006). We prefer sand-sized grains for equivalent-dose determination as these allow OSL measurements on aliquots containing only a few grains, which facilitates interpretation of the completeness of OSL resetting prior to deposition and burial (e.g. Duller, 2008). Samples for equivalent-dose estimation were treated with HCl and H2O2 to remove carbonates and organic material and were wet-sieved to obtain the grain size range of 90–180 mm (180–212 mm for sample NCL-3206027). Subsequently, the samples were etched with concentrated HF (40%) to dissolve feldspars and remove the outer layer of the quartz grains which was exposed to alpha radiation in the natural environment. The samples were then rinsed with HCl and water, and finally re-sieved to discard the grains that were heavily damaged by the HF treatment. Samples for dose-rate estimation were heated to 105 C for 24 h to determine water contents, ashed for 24 h at 500 C to determine
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Fig. 3. Detailed map of the area of the former Aetsveldse Lake. The location of the cross-sections A and B, the courses of distributary channels (Gd1, 2 and 3), and the positions of the cores used for this study are indicated (see Fig. 2 for the legend of the map). Cross-sections A and B (after Bos et al., 2009) show the different facies of the organic-clastic lake-fill sequence; note the general coarsening-upward trend. Locations of radiocarbon and OSL samples are indicated in the cross sections, as well as the ages obtained on the samples.
the organic content, and then ground to homogenize the samples. The ground sediment was mixed with wax (weight proportion 7:3) and cast into a 2-cm thick puck with 10-cm diameter for measurement with a gamma spectrometer. 3.2. Equivalent-dose estimation For equivalent-dose estimation, we employed a Risø TL/OSL DA 15 reader (Bøtter-Jensen et al., 2000) equipped with a Sr/Y beta source (dose rate w0.030 Gy/s). Quartz OSL signals were obtained through stimulating with blue LEDs (470 nm, w30 mW/cm2), infrared (IR) stimulation was provided by IR diodes (875 nm,
w116 mW/cm2). Signal collection used an EMI9235QA photomultiplier tube shielded by a 7.5 mm Hoya U-340 filter. Tests with infrared stimulation indicated that some feldspar contamination remained after HF treatment. To minimize the feldspar contribution to the OSL signal, we adopted a post-IR blue single-aliquot regenerative dose (SAR) procedure for equivalent-dose estimation (Wallinga et al., 2002). Only the centre 2 mm of aliquots were covered with material (w200 grains per disc); because only a small portion of quartz grains usually contributes to the OSL signal (Duller et al., 2000) we expect the OSL signal of each aliquot to be dominated by a small number of grains. Therefore, incomplete resetting of the OSL signal in part of the grains is expected to result
Table 1 Radiocarbon age control on Aetsveldse Lake deposits (modified from Bos et al., 2009). Laboratory nr
14
Calendar ageb (cal yr BP)
Coordinatesc (x/y) (m)
Surface elevationd (m O.D.)
Depth below surface (m)
Sample name
Relevance
UtC-14574 UtC-14582 UtC-14584 UtC-14578 UtC-14577 UtC-14573 UtC-14580 UtC-14575
2920 70 2810 50 2870 46 2853 46 2420 50 2367 44 1694 45 1577 43
3100 220 2930 140 3010 150 3000 150 2530 180 2510 190 1580 130 1460 90
123751/464139 129342/466569 125711/465662 128508/473158 128141/477478 130221/478555 124678/474147 129542/470254
1.67 1.15 1.63 1.55 1.50 1.44 2.05 0.72
1.77–1.81 2.17–2.18 2.02–2.03 1.46–1.47 1.33–1.34 1.21–1.22 2.13–2.14 1.94–1.95
Spengen 2 Weeresteijn 2 Portengen 3 Angstel 1 Gein 1 Weesp 1 Winkel 5 Loenen 1
Beginning Angstel/Vecht Beginning Angstel/Vecht Beginning Angstel/Vecht Beginning Angstel End Gd1 in Aetsveldse Lake End Gd2 in Aetsveldse Lake End sedimentation Aetsv. Lake west End Angstel
a b c d
C agea
All dated material consisted of selected terrestrial macrofossils. Calibrated using Reimer et al. (2004). In Dutch coordinate grid (Rijksdriehoekstelsel). O.D.: Dutch Ordnance datum.
1.3 0.3 – 1.3 0.3 24 5 – 24 5 358 6 – 358 6 a
b
location according to Dutch coordinate grid, surface elevation relative to NAP (Dutch Ordnance datum). samples from above and below the optical dating samples were combined for measurement of activity concentrations and water and organic contents.
14.7 3.8 – 14.7 3.8 21.5 0.7 – 21.5 0.7 18.4 0.9 – 18.4 0.9 Sandy clay Sand with clay laminae Sandy clay
21.6 0.3 – 21.6 0.3
22.5 0.6 – 22.5 0.6
20.2 1.9 – 20.2 1.9
20.4 1.3 – 20.4 1.3
3.7 0.7 – 3.7 0.7 68 14 – 68 14 463 3 – 463 3 32.8 4.3 – 32.8 4.3 34.3 0.6 – 34.3 0.6 28.8 1.2 – 28.8 1.2 Sandy clay Sandy clay/ sand Sandy clay
35.4 0.3 – 35.4 0.3
35.9 0.4 – 35.9 0.4
33.6 1.2 – 33.6 1.2
32.5 4.6 – 32.5 4.6
5.6 1.1 1.7 0.3 64 13 73 15 477 4 374 4 28.7 4.6 23.8 4.3 36.1 0.6 29.5 0.7 29.3 1.1 27.1 0.8
NCL-3206023 M4 3.24–3.57 NCL-3206022 M3 4.06–4.46 Core 25G1054 (x: 128144; y: 477486; z: 1.10)a NCL-3206026 M1b 2.20–2.28 M2 2.28–2.40 b 2.40–2.50 M3 Core 25H0740 (x: 130220; y: 478551; z: 1.35)a b NCL-3206027 M2a 1.52–1.57 M1 1.57–1.70 1.70–1.75 M2bb
Clay with sand laminae Silty clay, Sand admixture
36.5 0.3 30.4 0.2
37.5 0.5 31.1 0.4
34.0 29.3 33.2 1.3
34.5 3.3 29.4 3.2
– 26 5 – 406 6 – 18.9 4.0 – 19.8 0.7 NCL-3206024
M5 M6bb
2.34–2.39 2.39–2.49
Fine sand Sandy clay
– 17.2 0.9
– 20.5 0.3
– 20.8 0.5
– 18.9 17.2
– 16.9 2.7
1.6 0.3 0.4 0.1 1.6 0.3 1.8 0.4 34 7 20 3 34 7 26 5 418 4 422 3 418 4 406 6 15.6 3.7 11.8 2.1 15.6 3.7 18.9 4.0 18.9 0.5 14.3 0.3 18.9 0.5 19.8 0.7 17.6 17.5 13.1 13.3 17.6 17.5 18.9 17.2 19.8 0.5 14.5 0.3 19.8 0.5 20.8 0.5 19.4 0.2 14.2 0.1 19.4 0.2 20.5 0.3 17.5 1.6 13.3 0.5 17.5 1.6 17.2 0.9 Sandy clay Fine sand Sandy clay Sandy clay Core 25G1057 (x: 128526; y: 477358; z: 1.30)a NCL-3206025 M8ab 1.55–1.63 M7 1.63–1.76 b 1.76–1.83 M8b 2.27–2.34 M6ab
Th-series
18.4 1.5 14.1 1.2 18.4 1.5 16.9 2.7
K-40 Ac-228 Th-234
U-series
Pb-214
B-214
Pb-210
Pb-212
Bi-212
K
Water content (% by weight) Activity concentrations (Bq/kg) Lithology Depth below surface (m) Sample
Table 2 Samples used for OSL dating, with information on radionuclide concentrations, and water and organic contents.
– 1.8 0.4
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Organic content (% by weight)
606
in a scattered equivalent-dose distribution (Wallinga, 2002b; Duller, 2008). Based on preheat-plateau tests on three samples, we selected a preheat of 220 C for 10 s; heating prior to the test dose measurements was to 200 C. Data was accepted if the recycling ratio was within 20% of unity. The adopted protocol allowed us to accurately determine a laboratory given dose of 5 Gy (dose recovery ratio 1.04 0.02, n ¼ 16). A typical shine-down curve and doseresponse curve is shown in Fig. 4. For each sample, equivalent-dose estimates were made until at least 22 aliquots passed the acceptance criteria. This value was selected based on the distributions obtained; we do not expect large improvements in precision by measuring additional aliquots. Equivalent-dose distributions (see Fig. 5) indicated internal agreement of the vast majority of equivalent-dose estimates (i.e. spread in results can be explained by uncertainties on individual estimates), but that some aliquots returned extraordinary high or in some cases low values. High outliers are likely caused by incorporation of quartz grains for which the OSL signal was not completely reset prior to deposition and burial. Low outliers may be due to accidental light exposure or contamination during sample handling. To avoid bias of the results due to these outliers, we iteratively rejected single-aliquot equivalent-dose estimates yielding values more than two standard deviations away from the sample mean. The average of the remaining estimates was then used for age calculation. 3.3. Dose-rate estimation Radionuclide concentrations were measured using a Canberra broad energy HPGe gamma spectrometer following a procedure similar to the one described by Murray et al. (1987). To allow Rn buildup in the wax pucks, samples were stored for at least three weeks after casting before they were measured. U-series activity was determined by measurement of Th-234, Pb-214, Bi-214 and Pb210 activity concentrations. Th-series activity was determined from Ac-228, Pb-212 and Bi-212 activity concentrations. The K-40 activity concentration was measured directly. These activity concentrations (see Table 2) were converted to dose rates using standard conversion tables (Olley, 1996), with an uncertainty of 3%. Dose-rate attenuation due to grain size (Mejdahl, 1979), water content (Zimmerman, 1971) and organic content (Madsen et al.,
Fig. 4. Typical shine-down curve (A) and dose-response curve (B). Examples shown are for sample NCL-3206024; the dose-response curve is an average of three aliquots, and is fitted with an exponential plus linear function.
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607
laminated lacustrine deposits. The samples were taken from depth intervals that contained layers of sand, varying in thickness from a millimetre up to 12 cm (see Table 3). As the sediments within each bed or lamina are well sorted, we assume that the sand-sized grains used for equivalent-dose estimation originate from these sand layers. The range of beta particles in sediment is w2 mm (e.g. Aitken, 1985). Hence for sand laminae thicker than w1 cm it is safe to assume that the beta dose originates entirely from within the sand layer. For thinner sand laminae, the beta dose originates partly from surrounding material. Depending on the thickness of the sand layer we calculated how to mix the contributions (based on Aitken, 1985, p. 295 eq. H.13; Aitken et al., 1985); the relative weighting of the beta dose rate from outside the sample layer (Wo,b) is given by:
n Wo;b ¼
o 1 exp mb t mb t
(1)
Where mb (mm1) is the linear attenuation coefficient in sediment for b particles and t (mm) is the layer thickness. As linear attenuation coefficients are similar for U, Th and K, we assumed mb to be 1.14, the value suggested by Aitken (1985) for the U spectrum. The weighting of the beta dose rate from the sample layer itself is 1 Wo,b. Only for sample NCL-3206025 we had enough material to determine the radionuclide concentrations for the sandy sediments. We assumed the results for this sample to be representative for all sand deposits, and we base the beta dose from all sand layers on the radionuclide concentrations measured on this sample. The beta dose originating from surrounding material was determined from the radionuclide activity concentrations in that material. The relative importance of the contribution from outside the sand layer is up to 60% (for sand laminae of 1 mm, see Table 3). For estimating gamma dose rates we used a similar approach as for the beta dose rates. However, gamma radiation originates from a larger sphere around the sample because its range is much wider (w30 cm). Infinite matrix gamma dose rates for the sands were based on measurements on sample NCL-3206025. Infinite matrix gamma dose rates from material bordering the sand layers were based on gamma spectrometry results on that material. For samples taken from very thin sand layers (<0.5 cm), it is safe to assume that the gamma dose rate of a bulk sample taken around this depth is valid. For samples taken from thicker sand layers, we used a combination of gamma dose-rate values from the sand layers, and those obtained on adjacent material; relative contributions (see Table 3) were calculated based on equation H.10 of Aitken (1985, p. 294), where the weighting of the gamma dose rate from outside the sample layer is given by:
Wo;g ¼ Fig. 5. Equivalent-dose distributions of all investigated samples shown as radial plots (Galbraith, 1990). Note that more than 75% of the single-aliquot equivalent-dose estimates agrees with the value adopted for age calculation (grey bar), indicating that the OSL signal of the vast majority of quartz grains was reset at the time of deposition and burial.
2005) was taken into account. An internal alpha contribution of 0.06 0.03 Gy/ka was assumed. In addition to the environmental dose rate, a contribution from cosmic rays was calculated based on Prescott and Hutton (1994), assuming immediate burial to the present depth. Special attention was needed to quantify the dose rate as the surrounding sediment was not homogeneous in the bedded or
o 65 n 0:94 exp mg t ðt 4Þ
(2)
Where mg (mm1) is the linear attenuation coefficient in sediment for gamma rays (taken to be 0.015 mm1 based on Aitken, 1985), and t (mm) is the layer thickness. The weighting of the gamma dose rate from the sample layer itself is 1 Wo;g . Both water and organic material shield the mineral grains from radiation; this effect is referred to as attenuation of the external dose rate. As organics and water have similar stopping power, attenuation was calculated from the combined water and organic content (following Madsen et al., 2005). For the sandy layers, the water content may be affected by disturbance during sampling. Therefore, we based the water content for the sandy intervals on the porosity of fluvial sands (w34% by volume; Weerts, 1996, p. 101) yielding a water content of 20% by weight. Organic content of
1.69 0.06 0.17 0.01 0.93 0.05 –
25G1057 M7
27%
25H0740 M2ab
73%
0.06 0.03
0.53 0.03
0.16 0.01 0.58 0.04 0.92 0.07 0.06 0.03
4. Results Equivalent doses, dose rates and resulting OSL ages for all samples are given in Table 4; ages are given relative to the year of sampling (2006) with a 1-sigma confidence interval (including all random and systematic uncertainties). Ages range from 3.49 0.21 ka to 2.45 0.12 ka. Results for the four samples in a vertical sequence (core 25G1057; samples NCL-3206022 to 25) are in correct stratigraphical order (within uncertainties); a linear fit of the lower three data points provides an accumulation rate of 2.8 1.0 mm per year for the fine-grained facies (Fig. 6). OSL ages for the upper two samples are identical (both w2.8 ka), which agrees with anticipated rapid deposition of a coarse facies in a mouth bar. The OSL age of 2.87 0.16 obtained on associated deposits of distributary channel Gd1 (core 25G1054; sample NCL3206026) is identical to those obtained on the mouth-bar deposits (2.83 0.12 ka, combined uncertainty). The OSL age obtained on channel deposits of distributary channel Gd-2 (core 25H0740; sample NCL-3206027, 2.45 0.12 ka) is some 400 years younger than those for Gd-1 (w2.8 ka). This time lag is in accordance with gradual filling of the lake from the south, although more samples should be dated to allow determination of the progradation rate based on OSL data.
– 100% 25G1057 M7 35
100% 25G1054 M1,M3 – – 25G1054 M1,M3 39% 61% 25G1057 M7 2 2.34
the sandy layers was low (determined on sample NCL-3206025: 0.4% by weight). For all other materials, we used water and organic contents as measured on the samples (see Table 2) to calculate attenuation factors (following Zimmerman, 1971). Changes in water content due to compaction of the deposits are not taken into account as compaction likely occurred quickly after formation of the deposits. Radionuclide concentrations for the Th-232 decay series are all in agreement, indicating that there is no disequilibrium in this series. For the U-238 decay series, we found that Pb-214 and Bi-214 radionuclide concentrations were on average 19 2% higher than those of Th-234 (Table 2). This likely indicates the presence of excess Th-230 at the time of deposition. Because the age of the deposits is small compared to the half life of Th-230 (75 ka), the measured activity concentrations can be used to calculate dose rates.
5. Discussion 5.1. Resetting of the OSL signal Our samples are from three different depositional environments within the fluvio-deltaic lake setting. Samples NCL-3206022 and 23 were taken from clayey deposits formed in a low-energy lacustrine setting, relatively far away from the river mouth. Samples NCL3206024 and 25 are taken from sandy mouth-bar deposits, formed in a higher energy environment when the river had prograded close Table 4 Quartz optical dating results.
Core 25G1054 NCL-3206026 M2
1.64
NCL sample
Core 25H0740 NCL-3206027 M1
25G1057 25G1057 25G1057 25G1057 25G1057 M7 57% 25G1057 M7 38% – – – – – – 28% 60% – – 25G1057 M4 25G1057 M3 M7 100% M7 100% M7 72% M7 40% 25G1057 25G1057 25G1057 25G1057 120 60 3 1 M7 M5 M4 M3 Core 25G1057 NCL-3206025 NCL-3206024 NCL-3206023 NCL-3206022
1.70 2.37 3.41 4.26
Weight
Sample
Weight Sample
M8ab M6ab M4 M3
Weight
43% 62% 100% 100%
Total Cosmic Gamma Beta
Dose rates (Gy/ka)
Alpha Surrounding Sand layer
Samples used for gamma dose estimation
Depth Thickness Samples used for beta dose estimation (m) of sand Sand layer Surrounding laminae (mm) Sample Weight Sample De on sample NCL sample
Table 3 Relative dose-rate contributions from the OSL-sampled sand layer and surrounding deposits and resulting dose rates for the OSL samples.
1.71 0.08
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0.06 0.03 0.96 0.05 0.49 0.02 0.17 0.01 1.68 0.06 0.06 0.03 0.96 0.05 0.52 0.02 0.16 0.01 1.69 0.06 0.06 0.03 0.94 0.06 0.61 0.04 0.14 0.01 1.74 0.08 0.06 0.03 0.81 0.07 0.50 0.03 0.13 0.01 1.50 0.08
608
Depth (m)
Equivalent dose (Gy)
Dose rate (Gy/ka)
OSL age (ka)
Core 25G1057 NCL-3206025 NCL-3206024 NCL-3206023 NCL-3206022
1.70 2.37 3.41 4.26
4.76 0.15 4.74 0.12 5.51 0.16 5.22 0.14
1.68 0.06 1.69 0.06 1.74 0.08 1.50 0.08
2.83 0.14 2.80 0.13 3.16 0.17 3.49 0.21
Core 25G1054 NCL-3206026
2.34
4.91 0.15
1.71 0.08
2.87 0.16
Core 25H0740 NCL-3206027
1.64
4.13 0.15
1.69 0.06
2.45 0.12
J. Wallinga, I.J. Bos / Quaternary Geochronology 5 (2010) 602–610
Fig. 6. Plot of OSL ages with depth for core 25G1057 penetrating mouth bar and distal fine-grained deposits of distributary channel Gd1 (see Fig. 3 for location). Error bars indicate one sigma uncertainty interval. A linear fit of the lower three points indicates a sedimentation rate of w3 mm per year for the fine-grained facies. Results for the top two samples are identical, in line with expected rapid aggradation for coarse-grained mouth-bar deposits.
to the sampling site. Finally, samples NCL-3206026 and 27 are taken from channel deposits formed when the fluvial channels traversed the (then filled in) lake area. For all samples, more than 75% of the single-aliquot equivalent-dose estimates agree (within 2s) with the sample mean determined after iterative removal of outliers (see Fig. 5). These results indicate that the vast majority of quartz grains had their OSL signal reset prior to deposition and burial. We see no clear dependency of the spread in single-aliquot equivalent-dose estimates on the depositional environment. This may indicate that resetting of the OSL signal occurred during fluvial transport of the grains (which is similar for all samples), and does not depend on the exact depositional environment within the fluvio-deltaic system (i.e. the facies in which they are preserved). We note that, due to low sedimentation rates in combination with a large sample size, samples NCL-3206022 and 23 are expected to encompass over a century of deposition. This will add to the scatter in equivalent-dose results (w4% difference in De between grains at the top and bottom of the sample), but this source of spread is small compared to the measurement uncertainties. Singarayer et al. (2005) have tested residual doses in modern fluvial deposits from a range of environments; unfortunately, no fluvio-deltaic deposits were included in their study. They found mean equivalent doses ranging from 0.17 to 3.27 Gy, with an average of 0.98 Gy (standard deviation 0.76 Gy). For most of our samples, the mean equivalent dose is greater than the equivalent dose obtained through the iteration procedure that was adopted for age calculation. The average difference between the two estimates is 0.4 Gy (standard deviation 0.4 Gy), which is similar in magnitude to the average remnant mean equivalent dose reported by Singarayer et al. (2005). We suggest that it is important in studies of young fluvial deposits to exclude outliers from the equivalent-dose distribution for calculating burial ages. Our results confirm previous findings (reviewed in Wallinga, 2002a; Rittenour, 2008) that quartz OSL signals in downstream stretches of large river systems are well-enough reset to allow accurate age determination through quartz OSL dating for deposits that are more than a thousand years old. 5.2. Dose-rate assumptions The dose rates calculated for samples from heterogeneous deposits depend on the assumptions made with respect to dose-
609
rate contributions from different layers. For our samples, however, the dependence is limited because of two counterbalancing factors: (1) the radionuclide concentrations and (2) the attenuation due to water and organic contents. The sand layers have relatively low radionuclide concentrations and low water and organic contents, resulting in an infinite matrix dose rate of the sands of w1.45 Gy/ka (b: w0.96 Gy/ka þ g: w0.49 Gy/ka). The silt and clay deposits have higher radionuclide concentrations and also higher water and organic contents, resulting in an average infinite matrix dose rate of w1.41 Gy/ka (b: w0.85 Gy/ka þ g: w0.55 Gy/ka). Owing to the small difference in infinite matrix dose rates for the different facies, the dependency of the dose rate on the weighting factors used (Eqs. (1) and (2); Table 3) is limited. Dose rates also depend on the assumptions with regard to water content and organic content; a 1% increase in either will induce a w1% decrease in dose rate and thus a w1% increase in OSL age. Water contents may have changed over geological time due to compaction. In addition, pushing the core into the sediments may have induced compaction (lowering water contents) or disturbed packing (enlarging water contents) and samples may have dried during storage (although care was taken to avoid this). We used a large relative uncertainty of 20% on the measured water contents to encompass the full range of possible values during geological time. As water contents are largest for the more clayey deposits, results for samples from those deposits are most sensitive to the assumptions made. 5.3. Comparison with independent age constraints Sedimentation in the lake must post-date the start of activity of the fluvial channels feeding into the lake. Yet, the OSL determined start of lacustrine sedimentation (NCL-3206022, 3.49 0.21 ka) is slightly older than the onset of clastic sedimentation based on radiocarbon dating (2970 100 cal yr BP). Although the age intervals for both methods slightly overlap, the OSL age seems to be a bit too old (based on two sigma confidence intervals for both methods and adding 56 years to the calibrated 14C BP age to allow direct comparison – 14C age: 3.12 2.94 ka; OSL age: 3.91 3.07 ka). As there is strong evidence that sedimentation in the lake did not start earlier than suggested by the radiocarbon ages (discussed in Bos et al., 2009), and there is no evidence for incomplete resetting of the OSL signal, we attribute this possible age overestimation to an underestimation of the dose rate. The most likely cause is overestimation of the water content of the sample. OSL results for the final stages of clastic sedimentation at the two sites (Gd1: samples NCL-3602024-26; 2.83 0.12 ka (combined uncertainty); Gd2: NCL-3602027; 2.45 0.12 ka) agree well with the decreased sediment input between 2650 and 2200 cal. yr BP. 6. Conclusions Results of our investigation of clastic lake-fill deposits in the former Aetsveldse Lake (Rhine delta, western Netherlands) indicate that: Quartz OSL signals of the vast majority of grains are well reset prior to deposition and burial. Radioactive disequilibria yield no problems for dose-rate estimation. Dose rates in bedded/laminated sediments can be obtained from radionuclide concentrations and water contents of the different sediment layers. High water contents may contribute to the uncertainty and accuracy of the OSL dates obtained, especially for clayey facies.
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Quartz OSL dates for all but one sample agree with radiocarbon age constraints. Average sedimentation rates of the clayey facies at our main sampling site was w3 mm per year. This study demonstrates that OSL dating may be a powerful tool to determine the timing and duration of clastic lake sedimentation, especially of sandy facies. Thereby, OSL methods may complement other chronometric approaches in such environments. Acknowledgements We thank Candice Johns for sample preparation and measurements. We would like to thank Torbjo¨rn To¨rnqvist and Tammy Rittenour for their constructive reviews of this manuscript, and Bert Roberts for editorial handling. We also acknowledge two anonymous reviewers for their comments on an earlier version of this paper. JW is supported by Technology Foundation STW (DSF.7553). Editorial handling by: R. Roberts References Aitken, M.J., 1985. Thermoluminescene Dating. Academic Press, London. Aitken, M.J., Clark, P.A., Gaffney, C.F., Løvborg, L., 1985. Beta and gamma gradients. Nuclear Tracks and Radiation Measurements 10, 647–653. Bos, I.J., Feiken, H., Bunnik, F.P.M., Schokker, J, 2009. Influence of organics and clastic lake fills on distributary channel processes in the distal Rhine-Meuse delta (The Netherlands). Palaeogeography, Palaeoclimatology. Palaeoecology 284, 355–374. Bos, I. J. Architecture and facies distribution of organic-clastic lake fills in the fluviodeltaic Rhine-Meuse system, The Netherlands. Journal of Sedimentary Research, in press. Bronk Ramsey, C., 1995. Radiocarbon calibration and analysis of stratigraphy: the OxCal program. Radiocarbon 37 (2), 425–430. Bronk Ramsey, C., 2001. Development of the radiocarbon calibration program OxCal. Radiocarbon 43 (2A), 355–363. Bubenzer, O., Hilgers, A., Riemer, H., 2007. Luminescence dating and archaeology of holocene fluvio-lacustrine sediments of Abu Ttartur, Eastern Sahara. Quaternary Geochronology 2, 314–321. Bøtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 57–73. Duller, G.A.T., 2008. Single-grain optical dating of Quaternary sediments: why aliquot size matters in luminescence dating. Boreas 37, 589–612. Duller, G.A.T., Bøtter-Jensen, L., Murray, A.S., 2000. Optical dating of single sand-sized grains of quartz: sources of variability. Radiation Measurements 32, 453–457. Fiebig, M., Preusser, F., 2007. Investigating the amount of zeroing in modern sediments of River Danube, Austria. Quaternary Geochronology 2, 143–149. Galbraith, R.F., 1990. The radial plot – graphical assessment of spread in ages. Nuclear Tracks and Radiation Measurements 17, 207–214. Louwe Kooijmans, L. P. (1974). The Rhine/Meuse delta. Four studies on its prehistoric occupation and Holocene geology. Published PhD thesis. Analecta Praehistorica Leidensia, 7. Madsen, A.T., Murray, A.S., Andersen, T.J., Pejrup, M., Breuning-Madsen, H., 2005. Optically stimulated luminescence dating of young estuarine sediments: a comparison with 210Pb and 137Cs dating. Marine Geology 214, 251–268. Makaske, B., Smith, D.G., Berendsen, H.J.A., 2002. Avulsion, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada. Sedimentology 49, 1049–1071. Mejdahl, V., 1979. Thermoluminescence dating: beta dose attenuation in quartz grains. Archaeometry 21, 61–72.
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