Precise and accurate dating of fluvial deposits is essential to understand floodplain evolution during the Holocene. Although radiocarbon dating has been commonly used to reconstruct floodplain evolution (Aslan and Autin, 1999; Berendsen and Stouthamer, 2000; Funabiki
Quartz sand grains are generally used for the OSL dating of fluvial deposits because (1) incomplete bleaching can be detected from the dose distribution of small aliquots or single grains (Wallinga, 2002), and (2) coarser grains are better bleached in many cases, possibly because of longer residence time on the riverbed and sunlight exposure on channel bars (Olley
The Mekong River is one of the largest river systems in the world, with a large sediment discharge comparable to other larger Asian river systems such as Yellow and Ganges–Brahmaputra rivers. In Cambodia, the river is actively migrating and characterised by a series of abandoned, mud-dominated, inner bank levees upstream of Phnom Penh (
(A) Map showing the drainage basin (shaded) of the Mekong River. (B) Relief map of the study area. Sampling locations for the modern flood sediments are shown by red circles, which are labelled with the residual doses of fine-grained quartz samples. The white circle represents the location of MEK-2 and MEK-2B. (C) Digital Surface Model (DSM) around the MEK-2 and MEK-2B sites. The DSM used is ALOS World 3D – 30 m (AW3D30), released from the Japan Aerospace Exploration Agency (JAXA). (D) Photograph of the section at site MEK-2B.
The Mekong River originates from the Tibetan Plateau and flows approximately 4000 km before entering the alluvial lowland downstream of Kratie, Cambodia (
The drainage basin of the Mekong River is mainly located in the Indian monsoon region (Wang
In southern Cambodia, rapid floodplain aggradation occurred in the Mekong River delta during the sea-level rise from 8.4–10 ka and mangrove forests were established between 6.3–8.4 ka (Tamura
Between Kampong Cham to Phnom Penh, the flood-plain is 10–20 km wide and abandoned natural levees due to channel migration occur frequently (
During February 2018, a
Samples of modern flood deposits were collected at six sites in September 2018, just after the monsoon flooding, to examine the residual doses of fluvial deposits (
A laser diffraction particle size analyser (SALD-3000J, Shimadzu Corporation, Japan) was used to obtain the grain-size distribution of sediment samples. A sub-sample of 0.1–0.2 g was treated with 10% H2O2 to remove organic matter. The samples were placed in an ultrasonic bath for 10 minutes prior to analysis to disperse the aggregates. The median grain size was determined from the grain-size distribution.
OSL samples were processed under subdued red light to avoid the loss of luminescence signals. Sediments within 20–25 mm from each end of the sample tubes were used for measurements of water content and dosimetry. The rest of the samples were treated with 10% HCl and 10% H2O2 to remove carbonate and organic matter, and after that, sand-sized samples were sieved to obtain different grain size fractions. Sand-sized quartz and feldspar grains were separated using sodium polytungstate liquids to extract quartz-rich fraction (2.58–2.70 g/cm3) and K-feldspar fraction (2.53–2.58 g/cm3). The quartz-rich fraction was etched with 40% HF for 1 h, to eliminate feldspar contamination and remove the outer portion of the quartz grains, and then treated with 10% HCl. The etched grains were re-sieved to remove any remaining small feldspar grains. The fine-grained (4–11 μm) fractions were separated using Stokes’ Law. Polymineral fine grains were treated with H2SiF6 for four days to dissolve feldspars, followed by 10% HCl treatment to obtain quartz grains. Three aliquots per sample were checked for feldspar contamination with IRSL. All quartz samples showed <10% OSL depletion after IRSL and thus are considered to be successfully purified.
Luminescence measurements were made with an automated Risø TL/OSL DA-20 reader. The blue-light (∼470 nm) stimulated signal from quartz samples were detected through a 7.5 mm Hoya U-340 filter. The infrared-light (∼875 nm) stimulated signal from feldspar, or fine-grained polymineral samples were detected through a combination of Schott BG3 (3 mm thick), BG39 (2 mm), and GG400 (3 mm) filters. Small (2 mm) aliquots were used for sand-sized quartz samples to examine the
The single-aliquot regenerative-dose (SAR) protocol (Murray and Wintle, 2000) was used for quartz OSL measurements, and modified SAR protocols based on Buylaert
Examples of decay curves and dose-response curves (insets) for (A) fine-grained quartz, (B) IR50 of polymineral fine grains, (C) pIRIR150 of polymineral fine grains, and (D) pIRIR150 of K-feldspars. White points in the insets denote recycling points.
Summaries of OSL, IRSL and post-IR IRSL SAR protocols used in this study.
1 | Preheat at 200 or 220°C for 10 s | Preheat at 180°C for 60 s |
2 | Blue stimulation at 125°C for 20 s | IR stimulation at 50°C for 100 s |
3 | Test dose | IR stimulation at 150°C for 100 s |
4 | Cut-heat at 160°C | Test dose |
5 | Blue stimulation at 125°C for 20 s | Preheat at 180°C for 60 s |
6 | Dose and return to step 1 | IR stimulation at 50°C for 100 s |
7 | IR stimulation at 150°C for 100 s | |
8 | Dose and return to step 1 |
Preheat plateau and dose recovery tests on quartz sand (gsj18032) and fine-grained samples (gsj18031) were performed (
(A) Preheat plateau tests and (B) preheat dose recovery tests for samples gsj18031 (4–11 μm) and gsj18032 (62–90 μm).
To examine the bleaching rates for OSL and IR50 signals, 14 batches comprising three aliquots of quartz and feldspar sands (gsj18034), and quartz and polymineral fine grains (gsj18033) were bleached under artificial sunlight for different durations. The sensitivity-corrected light intensity of bleached aliquot was normalised using the average of the three natural aliquots.
The environmental dose rate was calculated with DRAC of Durcan
Summary of dated samples and dosimetry data. The number of measured and accepted aliquots is shown. We added 5% to the water content values obtained from samples collected during dry season to account the seasonal changes. The “Q”, “F”, and “P” denotes quartz, K-feldspar, and polymineral fine grains, respectively.
gsj18025 | 0.85–1.00 | 4–11 | 2.2 | 3.0 | 14.9 | 138.0 | 27.5 | 3.79 ± 0.19 | 4.39 ± 0.22 |
gsj18026 | 1.90–2.05 | 4–11 | 1.4 | 1.9 | 11.2 | 77.7 | 14.0 | 2.93 ± 0.15 | 3.43 ± 0.17 |
gsj18027 | 2.90–3.05 | 4–11 | 1.6 | 2.3 | 12.6 | 100.0 | 24.8 | 2.99 ± 0.15 | 3.50 ± 0.17 |
gsj18028 | 3.85–4.00 | 4–11 | 1.5 | 2.1 | 11.8 | 86.6 | 31.4 | 2.61 ± 0.13 | 3.04 ± 0.15 |
gsj18029 | 4.90–5.05 | 4–11 | 1.7 | 2.5 | 13.7 | 109.0 | 38.9 | 2.86 ± 0.14 | 3.34 ± 0.16 |
gsj18035 | 4.75 | 120–150 | 1.2 | 2.0 | 14.5 | 62.0 | 8.9 | 2.51 ± 0.15 | - |
211–250 | 2.45 ± 0.14 | - | |||||||
gsj18034 | 6.6 | 62–90 | 1.2 | 1.6 | 9.0 | 60.3 | 9.5 | 2.04 ± 0.10 | 2.78 ± 0.19 |
211–250 | 1.96 ± 0.12 | 2.92 ± 0.15 | |||||||
gsj18033 | 7.4 | 4–11 | 1.7 | 3.0 | 13.8 | 104.0 | 36.6 | 3.06 ± 0.17 | 3.65 ± 0.17 |
gsj18032 | 8.2 | 62–90 | 1.3 | 1.3 | 7.0 | 64.2 | 30.4 | 1.62 ± 0.10 | 2.19 ± 0.14 |
211–250 | 1.55 ± 0.09 | 2.46 ± 0.12 | |||||||
gsj18031 | 9.0 | 4–11 | 1.8 | 3.0 | 13.9 | 108.0 | 43.0 | 2.86 ± 0.14 | 3.38 ± 0.16 |
gsj18030 | 10.0 | 211–250 | 1.0 | 0.8 | 3.8 | 50.3 | 10.7 | 1.34 ± 0.09 | - |
The OSL sample at 7.4 m depth (gsj18033) was taken from the midpoint of a mud layer 30 cm thick, so we calculated the gamma contributions from the over- and underlying layers using the principle of superposition (Aitken, 1985: Appendix H). The external infinite-matrix gamma dose rate of over- and underlying layers were determined using the values from upper and lower OSL samples (gsj18034 and gsj18032). Water contents of the three layers were averaged and used for calculation of water-content attenuation.
To investigate the difference in water content between wet and dry seasons, we collected samples near the site MEK-2B in September 2018. The water content in the dry season was
Depth profiles of water content in the dry and wet seasons at site MEK-2.
Laboratory fading rates (g2days-values) of IR50 were measured on aliquots that were used for
The section MEK-2B consists of basal
Sedimentary columns of sites MEK-2 and MEK-2B. Vertical grain size profile and age-depth plots are also shown. The OSL ages are shown with 1 σ errors. The age-depth model was constructed by Bacon software (Blaauw and Christen, 2011). The dashed line shows ‘best’ model from the weighted average age and the shaded area indicates the 95% confidence interval. The single-grain ages calculated using the minimum age model (10% overdispersion value) are shown.
The auger core MEK-2, just 40 m from the MEK-2B outcrop, comprises
A 1.5 cm thick, coarsening-upward silt layer was observed at site MO-1. Samples gsj18320 and gsj18321 were collected from the upper and lower halves of the layer, respectively. At site MO-2, a >30 cm thick, massive, fine-grained sand layer occurred on the bar and was apparently overlain by a
The quartz OSL ages obtained from 62–90 μm fractions of sand at 6.6 m and 8.2 m depth (gsj18034, 18032) are 610 ± 50 and 590 ± 40 a, respectively (
Quartz equivalent doses obtained from different grain size fractions of sand sample (gsj18034).
Frequency distribution of equivalent doses obtained from multi-grain measurements of (A) 62–90 μm fractions and (B) 211–250 μm fractions of gsj18034, and (C) single-grain measurements of 211–250 μm fractions of gsj18034.
Luminescence ages obtained from point-bar and natural levee deposits. “OD” is overdispersion value of quartz sample calculated using central age model. OSL ages with errors exceeding 10 a are rounded to the nearest decade.
gsj18025 | 0.85–1.00 | 4–11 | 6 (6) | 6 (6) | 0.7 ± 0.02 | 1.7 ± 0.66 | - | 190 ± 10 | 390 ± 150 | 480 ± 180 | - | 5.2 |
gsj18026 | 1.90–2.05 | 4–11 | 6 (6) | 6 (6) | 1.67 ± 0.04 | 2.32 ± 0.14 | - | 570 ± 30 | 680 ± 50 | 840 ± 60 | - | 3.6 |
gsj18027 | 2.90–3.05 | 4–11 | 6 (6) | 6 (6) | 1.19 ± 0.04 | 1.91 ± 0.06 | - | 400 ± 20 | 550 ± 30 | 680 ± 40 | - | 5.9 |
gsj18028 | 3.85–4.00 | 4–11 | 5 (6) | 6 (6) | 1.22 ± 0.02 | 1.89 ± 0.05 | - | 470 ± 30 | 620 ± 30 | 770 ± 40 | - | 0 |
gsj18029 | 4.90–5.05 | 4–11 | 6 (6) | 6 (6) | 1.15 ± 0.04 | 1.97 ± 0.09 | - | 400 ± 20 | 590 ± 40 | 730 ± 60 | - | 6.7 |
gsj18035 | 4.75 | 120–150 | 17 (20) | - | 1.36 ± 0.10 | - | - | 540 ± 50 | - | - | - | 29.9 |
211–250 | 17 (20) | - | 1.86 ± 0.24 | - | - | 760 ± 110 | - | - | - | 52.9 | ||
gsj18034 | 6.6 | 62–90 | 20 (20) | 6 (6) | 1.24 ± 0.06 | 1.92 ± 0.02 | 11.87 ± 0.27 | 610 ± 50 | 690 ± 50 | 910 ± 80 | 4270 ± 300 | 22.3 |
211–250 | 13 (20) | 6 (6) | 1.75 ± 0.24 | 2.46 ± 0.06 | 9.48 ± 0.50 | 890 ± 130 | 840 ± 50 | 1120 ± 70 | 3250 ± 240 | 48.6 | ||
gsj18033 | 7.4 | 4–11 | 6 (6) | 5 (6) | 1.47 ± 0.02 | 1.96 ± 0.15 | - | 480 ± 30 | 530 ± 50 | 650 ± 60 | - | 0.9 |
gsj18032 | 8.2 | 62–90 | 20 (20) | 6 (6) | 0.96 ± 0.03 | 1.81 ± 0.03 | 11.42 ± 0.30 | 590 ± 40 | 830 ± 50 | 1100 ± 70 | 5220 ± 350 | 11.8 |
211–250 | 17 (20) | 6 (6) | 1.55 ± 0.15 | 2.33 ± 0.21 | 9.02 ± 0.67 | 1000 ± 110 | 950 ± 100 | 1270 ± 120 | 3680 ± 330 | 38.2 | ||
gsj18031 | 9.0 | 4–11 | 6 (6) | 5 (6) | 1.57 ± 0.03 | 1.84 ± 0.18 | - | 550 ± 30 | 550 ± 60 | 680 ± 80 | - | 2.3 |
gsj18030 | 10.0 | 211–250 | 17 (20) | - | 1.53 ± 0.25 | - | - | 1150 ± 200 | - | - | - | 66.6 |
In the single-grain measurements, we measured 2400 grains for samples gsj18034 and 18032, respectively. Only
Results of single-grain measurements. The number of measured and accepted grains is shown. The ages were calculated using central age model (CAM) and minimum age model (MAM) assuming the overdispersion values of 10% (OD = 10) and.20% (OD = 20). OD is calculated as part of central age model.
gsj18034 | 6.6 | 211–250 | 30 (2400) | 1.49 ± 0.24 | 800 ± 160 | 1.00 ± 0.06 | 510 ± 40 | 1.15 ± 0.07 | 590 ± 50 | 81.2 |
gsj18032 | 8.2 | 211–250 | 26 (2400) | 1.57 ± 0.29 | 960 ± 160 | 0.98 ± 0.05 | 630 ± 50 | 1.01 ± 0.07 | 650 ± 60 | 96.3 |
Fine-grained quartz yields younger age estimates than coarse quartz grains (
IR50 ages of fine-grained samples are generally younger than those of sand samples (
Fine-grained quartz from modern flood deposits on levees yielded small
We found that five out of seven modern fine-grained quartz samples are almost completely bleached (
Residual doses of modern flood deposits. The “Q”, “F”, and “P” denotes quartz, K-feldspar, and polymineral fine grains, respectively. “OD” is overdispersion value of quartz sample calculated using central age model.
gsj18320 | 4–11 | levee | 6 (6) | 6 (6) | 0.02 ± 0.00 | 0.79 ± 0.03 | - | 32.2 |
gsj18321 | 4–11 | levee | 6 (6) | 6 (6) | 0.07 ± 0.00 | 0.82 ± 0.03 | - | 3.2 |
gsj18322 | 90–120 | bar | 20 (20) | 6 (6) | 0.68 ± 0.20 | 3.89 ± 0.24 | 39.5 ± 2.4 | 131.6 |
211–250 | bar | 18 (20) | 6 (6) | 0.32 ± 0.20 | 3.91 ± 1.44 | 28.4 ± 6.0 | 263.7 | |
gsj18323 | 4–11 | bar | 6 (6) | 6 (6) | 0.04 ± 0.00 | 1.38 ± 0.05 | - | 16.8 |
gsj18324 | 90–120 | bar | 20 (20) | 6 (6) | 0.62 ± 0.27 | 4.04 ± 0.62 | 42.5 ± 2.8 | 194.1 |
211–250 | bar | 20 (20) | 6 (6) | 0.32 ± 0.15 | 6.11 ± 1.19 | 33.6 ± 2.9 | 202.0 | |
gsj18325 | 4–11 | levee | 6 (6) | 6 (6) | 0.04 ± 0.02 | 1.22 ± 0.05 | - | 101.9 |
gsj18326 | 4–11 | levee | 6 (6) | 6 (6) | 0.06 ± 0.00 | 0.93 ± 0.04 | - | 17.8 |
gsj18327 | 211–250 | levee | 20 (20) | 6 (6) | 0.43 ± 0.13 | 1.41 ± 0.13 | 15.3 ± 1.0 | 130.0 |
gsj18328 | 62–90 | bar | 20 (20) | 6 (6) | 0.49 ± 0.12 | 4.02 ± 0.13 | 39.4 ± 1.0 | 110.0 |
gsj18329 | 4–11 | bar | 6 (6) | 6 (6) | 1.55 ± 0.05 | 3.36 ± 0.20 | - | 7.6 |
gsj18330 | 4–11 | levee | 6 (6) | 6 (6) | 2.43 ± 0.04 | 1.87 ± 0.03 | - | 2.3 |
At sites MEK-2 and MEK-2B, fine-grained quartz ages are younger than sand quartz ages (
The finer fractions of quartz sand have larger residual doses than the coarser fractions for two modern samples (gsj18322 and gsj18324), and the residual dose of 62–90 μm fractions from gsj18328 are almost equivalent to that of 211–250 μm fractions from gsj18327 (
The bleaching test using artificial sunlight shows that the quartz OSL signal decreases more rapidly than the IR50 signal (
Bleaching rates for OSL and IR50 of fine-grained and sand-sized samples (gsj18033 and gsj18034). The error bars represent one standard error.
The residual doses of incompletely bleached fine-grained quartz grains from modern flood deposits (2.43 ± 0.04 Gy; gsj18330) are comparable to those from suspended sediments collected in the middle reach of the Yellow River (0.6–1.6 Gy; Hu
Based on radiocarbon dates of bulk sediments or plant fragments, the point bar at site MEK-2B is estimated to have formed between 1700 and 2500 cal yr BP (Oketani and Haruyama, 2011), but this estimate is significantly older than the quartz OSL ages. The details of dated materials were not described in detail by Oketani and Haruyama (2011), and the older radiocarbon dates may reflect contamination with older organic carbon (Grimm
The time scales for establishing natural levees on point bars are essential for understanding floodplain evolution but have rarely been investigated. The quartz OSL age obtained from the lowest part of natural levee deposits (400 ± 20 a) is relatively close to those obtained from mud beds in the point-bar deposits (480 ± 30 and 550 ± 30 a). Furthermore, the natural levee deposits seem to be deposited within a few centuries (
The age-depth modelling shows that the median sedimentation rates of natural levee deposits at sites MEK-2 are 42-50 mm/yr. This rate is comparable to Mississippi Delta (10–40 mm/yr; Shen
This study investigated the degree of bleaching of fluvial deposits in the Mekong River floodplain by comparing the ages obtained from quartz OSL, feldspar and polymineral IR50, and pIRIR150 signals. The fading-corrected IR50 ages were older than the quartz OSL ages for all samples, and it is difficult to ensure sufficient bleaching of the quartz OSL signal. However, we infer that fine-grained quartz grains are generally well bleached based on the low residual doses of modern flood deposits. A comparison of fine-grained quartz OSL ages with single-grain ages obtained from the point-bar deposits also suggests that the fine-grained deposits are well bleached. Therefore, a plausible age-depth model can be made using fine-grained quartz OSL ages. Fine-grained quartz may be sufficiently bleached in other large rivers because of the long transport distances.