OSL dating is now increasingly being used to determine the timing of the deposition of fluvial sediments (
A number of approaches have been proposed to test the completeness of bleaching prior to deposition in the past. First, one could determine the ages and assess the residual signals present in modern (recently-deposited) samples from a range of contexts, and then use these modern analogues to assess the degree of bleaching in older materials which have experienced the same depositional conditions (
The Yellow River is famous for its high sedimentary load. It is the second largest river in the world in terms of sedimentary load (Milliman and Meade, 1983). The high turbidity of the Yellow River’s water limits light penetration, potentially causing the partial bleaching of OSL signals in suspended and bedload sediments (Ditlefsen, 1992). The Inner Mongolian reaches of the Yellow River are characterised by desert environments which extend along either side of the river channel, as well as by high sedimentary loads, by severe sedimentary siltation, and by extensive river channel migration (Ta
Numerous studies have tested the residual signals found in modern fluvial sediments (Jain
Our study area was located along the Inner Mongolian reaches of the Yellow River, starting at Shizuishan (the Ningxia Autonomous Region), and ending at Hekou Town (the Inner Mongolia Autonomous Region). The total river-channel length was 673 km (Fig. 1). Due to the shallow gradient, a meandering channel, loose riverbed materials, a high sedimentary load and surrounding vast alluvial plains (the Hetao Plain) (Hou, 1996), these meandering reaches of the Yellow River experience high lateral erosion, which in turn leads to frequent changes in the course of the river channel (Li
Previous studies conducted in this area have mainly focused on the evolution of river channel deposition during the past 50 years. These studies have been principally based on the interpretation of sectional and hydrological data (Ta
All OSL samples were opened under a subdued red light in the luminescence-dating laboratory. Minerals were extracted from the unexposed middle parts of the cores and tubes for De determination. All raw samples were treated with 10% HCl and 20% H2O2 to remove carbonates and organic matter. For modern samples, Grain size fractions of medium fraction (38–63 μm) and/or coarse fractions (63–90, 63–150, 90–150, 150–200 μm) were extracted by wet sieving. The core samples were wet sieved to obtain the medium (MG, 38–63 μm) and coarse (CG, 90–125 μm) fraction. The 38–63 µm fraction was etched with 35% fluorosilicic acid for about two weeks to dissolve feldspars (Lai and Wintle, 2006; Lai
OSL signal measurements were made using an automated Risø TL/OSL-DA-15 reader (Markey
The preheat plateau test was used to isolate suitable preheat temperatures. The dose recovery test was employed to evaluate the suitability of the SAR protocol. A modern sample (LFQ-07) and a core sample (HDZ03-06) were selected for the preheat plateau test. The test temperatures of the modern samples were set at 180, 200, 220, 240 and 260°C, with four aliquots’ being used at each preheat temperature. No significant difference was found between 180 and 240°C. Thus, a combination of a 200°C preheat (10 s) temperature and a 180°C cut heat was chosen for the SAR measurement of modern samples (Fig. 2a). For core sample HDZ03-06, the test temperatures were set at 220, 240, 260, 280 and 300°C, whereas the preheat plateau was clearly identified from 220 to 260°C. Therefore, a preheat temperature of 260°C for 10 s was chosen for both the natural and regenerative doses, and a cut heat of 220°C was selected for test doses (Fig. 2c).
For the dose recovery test, a new set of six aliquots from each of the two samples (LFQ-04, HDZ03-06) was first bleached with blue LED for 100 s at room temperature, following which a known dose, which was nearly equal to their natural dose (De), was administered, and the same preheat/cut-heat conditions, as employed in the standard SAR measurement protocol, were used. The dose recovery ratios for these two samples are shown in Fig. 2b and Fig. 2d, respectively. The measured -to-given dose ratios were 1.04 and 0.90 (their mean values), falling within the 0.9–1.1 acceptance range. The given dose was well reproduced, indicating that, in this study, the SAR protocol is suitable for De determination.
In order to determine an independent chronological control, we compared OSL dates with AMS14C dates. Although it was difficult to obtain reliable AMS14C dates from fluvial sediments, due to the limited number of 14C samples, the plant residues and a clay layer rich in organic matters found within each core were suitable for 14C dating. 11 14C samples from seven cores were dated using the AMS14C dating method at the Radiocarbon Dating Laboratory of Peking University and Beta Analytic, Inc. (Miami, FL, USA). 14C dates were calibrated to the calendar year before the present (cal BP) using the IntCal13 curve (Reimer
Fig. 3a shows a typical natural OSL decay curve for modern sample LFQ-06. The OSL natural signal exhibits a very low value close to the measurement background, and this signal was rapidly bleached by blue light. Almost all the samples showed similar characteristics. The growth curve of all the modern samples was averaged to construct an SGC (Fig. 3b). The recycling ratios of all the samples were within the 0.9–1.1 acceptance range.
The De values and OSL apparent ages of different grain sizes for the modern samples are given in Table 1, and the De values of different grain sizes versus downstream distance obtained are shown in the Fig. 4. The mean De values of silt grains (38–63 μm) for most samples ranged from 0.16 to 0.49 Gy, except for samples LFQ-5 and LFQ-13, which showed De values of 0.74 Gy and 1 Gy, respectively, and it appears that no dependence of De with distance downstream was observed. However, for the coarser grains (63–90, 90–150 μm), the De values showed a larger correlation with distance downstream. It also can be seen from Figs. 4 and 5 that the modern samples show a general increase in the equivalent dose with an increasing grain size.
OSL dating results for 14 modern fluvial samples.Sample ID Depth (m) K (%) Th (ppm) U (ppm) Water content (%) Grain size (μm) Dose rate (Gy/ka) De (Gy) OSL age (a) LFQ-1 0.2 ± 0.05 1.80 ± 0.06 11.70 ± 0.33 2.72 ± 0.11 15 ± 5 38–63 2.99 ± 0.20 0.32 ± 0.03 110 ± 10 63–90 2.97 ± 0.20 0.40± 0.05 130 ± 20 LFQ-2 0.2 ± 0.05 1.69 ± 0.06 9.52 ± 0.29 2.70 ± 0.11 15 ± 5 38–63 2.77 ± 0.19 0.49 ± 0.04 180 ± 20 63–90 2.74 ± 0.19 0.55 ± 0.06 200 ± 30 90-150 2.71 ± 0.19 1.18 ± 0.20 440 ± 80 LFQ-3 0.2 ± 0.05 1.64 ± 0.05 10.80 ± 0.30 2.54 ± 0.10 15 ± 5 38-63 2.77 ± 0.19 0.16 ± 0.01 60 ± 10 63-150 2.72 ± 0.18 0.35 ± 0.03 130 ± 10 LFQ-4 0.2 ± 0.05 1.81 ± 0.06 11.60 ± 0.32 3.06 ± 0.11 15 ± 5 38–63 3.06 ± 0.21 0.21 ± 0.02 70 ± 10 63–90 3.00 ± 0.20 0.35 ± 0.03 120 ± 10 LFQ-5 0.2 ± 0.05 1.87 ± 0.06 10.90 ± 0.31 2.84 ± 0.11 15 ± 5 38–63 3.03 ± 0.20 0.74 ± 0.09 240 ± 30 63–90 2.97 ± 0.20 0.74 ± 0.05 250± 20 90-150 2.76 ± 0.19 0.98 ± 0.06 330 ± 30 LFQ-6 0.2 ± 0.05 1.60 ± 0.05 10.9 ± 0.31 2.70 ± 0.11 15 ± 5 38–63 2.77 ± 0.19 0.37 ± 0.05 130 ± 20 63–90 2.75 ± 0.19 0.57 ± 0.12 210 ± 50 90-150 2.72 ± 0.19 1.36 ± 0.14 500 ± 60 150–200 2.68 ± 0.18 2.33± 0.31 870 ±130 LFQ-7 0.2 ± 0.05 1.67 ± 0.06 10.30 ± 0.30 2.80 ± 0.11 15 ± 5 38–63 2.82 ± 0.19 0.36 ± 0.03 130 ± 10 90±150 2.76 ± 0.19 1.55 ± 0.11 560 ± 60 LFQ-8 0.2 ± 0.05 1.78 ± 0.06 10.1 ± 0.29 2.43 ± 0.10 15 ± 5 38–63 2.82 ± 0.19 0.24 ± 0.02 90 ± 10 63–150 2.81 ± 0.19 1.18 ± 0.21 430 ± 80 LFQ-9 0.2 ± 0.05 1.51 ± 0.05 13.5 ± 0.36 3.38 ± 0.12 15 ± 5 38–63 2.99 ± 0.20 0.24 ± 0.01 80 ± 10 63–90 2.97 ± 0.20 1.17 ± 0.14 390 ± 50 90–150 2.94 ± 0.19 2.07 ± 0.19 710 ± 80 150–200 2.89 ± 0.19 3.72 ± 0.50 1290 ±190 LFQ-10 0.2 ± 0.05 1.66 ± 0.05 12.00 ± 0.34 3.36 ± 0.12 15 ± 5 38–63 3.02 ± 0.20 0.29 ± 0.05 100 ± 20 63–90 3.00 ± 0.20 0.53 ± 0.05 180 ± 20 LFQ-11 0.2 ± 0.05 1.65 ± 0.05 10.6 ± 0.31 2.60 ± 0.10 15 ± 5 38–63 2.78 ± 0.19 0.43 ± 0.04 150 ± 20 63–90 2.75 ± 0.19 0.67 ± 0.06 240 ± 30 90–150 2.72 ± 0.18 1.62 ± 0.13 590 ± 60 LFQ-12 0.2 ± 0.05 1.75 ± 0.06 9.95 ± 0.29 2.60 ± 0.10 15 ± 5 38–63 2.82 ± 0.19 0.44 ± 0.05 160 ± 20 63–90 2.80 ± 0.19 1.42 ± 0.16 510 ± 70 90–150 2.77 ± 0.19 2.35 ± 0.24 850 ±100 LFQ-13 0.2 ± 0.05 1.76 ± 0.06 11.1 ± 0.31 2.38 ± 0.10 15 ± 5 38–63 2.85 ± 0.19 0.33 ± 0.02 120 ± 10 63–150 2.81 ± 0.19 0.86 ± 0.08 310 ± 40 LFQ-14 0.2 ± 0.05 1.63 ± 0.05 7.90 ± 0.24 1.89 ± 0.08 15 ± 5 38–63 2.45 ± 0.17 1.00 ± 0.09 410 ± 40 63–90 2.43 ± 0.17 1.11 ± 0.08 460 ± 50 90–150 2.41 ± 0.17 1.41 ± 0.13 590 ± 70 150–200 2.37 ± 0.19 3.61 ± 0.31 1520 ±170
The OSL ages of each sample, with their De values and dose rate information, are given in Table 2. A total of 26 OSL dating results are displayed in Fig. 6. It is apparent that the OSL ages from all drilling cores increase with an increasing depth. The OSL ages of fluvial sediments fall within the 114 to 0.8 ka range. The 38–63 μm grain size fraction was measured; when the samples lack this fraction, we used the coarser 90–125 μm fraction.
OSL dating results for 26 samples from seven fluvial cores.Sample ID Grain Size (μm) Depth (m) K (%) Th (ppm) U (ppm) Water content (%) Dose rate (Gy/ka) De (Gy) OSL age (ka) HDZ03-06 38–63 3.9 1.54 ± 0.06 9.32 ± 0.29 2.09 ± 0.15 24 ± 5 2.18 ± 0.16 2.39 ± 0.12 1.1 ± 0.1 HDZ03-14 38–63 14.3 1.54 ± 0.06 4.15 ± 0.18 1.13 ± 0.14 20 ± 5 1.79 ± 0.14 2.53 ± 0.17 1.4 ± 0.1 HDZ03-17 38–63 18.7 1.65 ± 0.06 4.95 ± 0.19 1.03 ± 0.14 23 ± 5 1.66 ± 0.13 58.77 ± 1.51 35.3 ± 2.9 HDZ08-11 38–63 11.2 1.52 ± 0.06 5.03 ± 0.20 1.24 ± 0.13 21 ± 5 1.72 ± 0.13 8.19 ± 0.38 4.8 ± 0.4 HDZ12-03 38–63 1.5 1.89 ± 0.09 11.35 ± 0.33 2.45 ± 0.20 23 ± 5 2.81 ± 0.20 2.31 ± 0.15 0.8 ± 0.1 HDZ12-07 38–63 7.0 1.70 ± 0.08 5.69 ± 0.20 1.19 ± 0.13 21 ± 5 2.00 ± 0.16 2.58 ± 0.23 1.3 ± 0.2 HDZ12-09 38–63 11.2 1.62 ± 0.09 4.96 ± 0.19 1.06 ± 0.14 22 ± 5 1.81 ± 0.15 2.45 ± 0.11 1.4 ± 0.1 DKZ06-07 38–63 5.1 1.81 ± 0.07 6.09 ± 0.23 1.30 ± 0.14 8 ± 5 2.59 ± 0.20 19.74 ± 0.84 7.6 ± 0.7 DKZ06-10 90–125 9.0 1.59 ± 0.06 3.33 ± 0.18 0.92 ± 0.13 15 ± 5 1.75 ± 0.14 16.28 ± 1.20 9.3 ± 1.0 DKZ06-12 90–125 12.0 1.58 ± 0.06 3.65 ± 0.17 0.79 ± 0.13 13 ± 5 1.75 ± 0.14 19.74 ± 1.05 11.3 ± 1.1 DKZ06-15 38–63 15.7 1.51 ± 0.06 5.00 ± 0.20 1.09 ± 0.13 19 ± 5 1.83 ± 0.14 41.91 ± 1.30 23.0 ± 1.9 DKZ06-16 90–125 18.0 1.62 ± 0.06 3.72 ± 0.19 0.92 ± 0.14 18 ± 5 1.68 ± 0.13 48.71 ± 2.46 29.0 ± 2.7 DKZ09-08 90–125 2.6 1.65 ± 0.06 2.59 ± 0.16 0.68 ± 0.12 9 ± 5 1.94 ± 0.15 15.52 ± 0.81 8.0 ± 0.8 DKZ09-11 90–125 9.0 1.52 ± 0.06 2.60 ± 0.15 0.63 ± 0.11 14 ± 5 1.61 ± 0.13 49.61 ± 2.09 30.7 ± 2.7 DKZ09-16 38–63 12.0 1.36 ± 0.05 2.95 ± 0.17 0.63 ± 0.11 15 ± 5 1.53 ± 0.12 55.30 ± 1.36 36.0 ± 3.1 DKZ09-18 90–125 17.0 1.44 ± 0.06 3.10 ± 0.17 0.74 ± 0.11 18 ± 5 1.48 ± 0.12 77.01 ± 1.92 52.2 ± 4.3 DKZ09-21 90–125 20.0 1.46 ± 0.06 2.53 ± 0.14 0.70 ± 0.10 15 ± 5 1.51 ± 0.12 86.44 ± 3.89 57.4 ± 5.3 DKZ10-08 90–125 4.1 1.71 ± 0.06 3.29 ± 0.19 1.06 ± 0.16 12 ± 5 2.00 ± 0.15 23.68 ± 1.18 11.8 ± 1.1 DKZ10-16 90–125 9.7 1.52 ± 0.06 3.22 ± 0.19 0.76 ± 0.12 16 ± 5 1.62 ± 0.13 25.76 ± 1.03 15.9 ± 1.4 DKZ10-20 90–125 13.2 1.75 ± 0.06 4.49 ± 0.21 1.20 ± 0.15 16 ± 5 1.95 ± 0.15 156.25 ± 5.79 80.2 ± 6.8 DKZ10-23 38–63 19.5 1.76 ± 0.07 4.58 ± 0.21 0.97 ± 0.12 16 ± 5 1.93 ± 0.15 197.41 ± 8.21 102.1 ± 9.0 DKZ13-06 38–63 3.2 1.77 ± 0.08 5.64 ± 0.19 1.27 ± 0.13 22 ± 5 2.02 ± 0.15 3.10 ± 0.60 1.5 ± 0.3 DKZ13-08 38–63 5.2 1.44 ± 0.07 4.24 ± 0.17 0.86 ± 0.12 20 ± 5 1.62 ± 0.12 4.47 ± 0.35 2.8 ± 0.3 DKZ13-16 38–63 11.9 1.76 ± 0.08 10.88 ± 0.32 3.18 ± 0.22 17 ± 5 2.74 ± 0.20 276.16 ± 12.15 100.8 ± 8.5 DKZ13-19 38–63 15.1 1.83 ± 0.08 7.64 ± 0.24 2.14 ± 0.16 20 ± 5 2.29 ± 0.17 261.27 ± 11.52 114.1 ± 9.9
A total of 11 14C dates were collected from seven drilling cores. Their 14C dates are listed in Table 3 and displayed in Fig. 6. All 14C dates fall within the 48130 to 2160 cal BP range. The 14C ages were either in agreement with the OSL ages, or fell into the same stratigraphic order as the OSL ages (Fig. 7). However, the 14C ages of the three samples HDZ03-09, HDZ12-09 and DKZ06-08 were 2340–2650, 2210–2360 and 13460–13720 cal BP, i.e. much older than the OSL ages in the strata above and below. Samples HDZ03-09 and HDZ12-09 both included fragments of plant stems collected from the sand strata, at a depth of 9.1 m in Core HDZ03, and 3.2 m in Core HDZ12. If calibrated 14C results were presented on the same time scale as OSL dates(ka), these samples yielded ages of 2.56 ± 0.15 and 2.35 ± 0.08 ka, respectively. Sample DKZ06-08, composed of organic matter found in the clayey sediment, gave an age of 13.65 ± 0.13 ka.
Sample data and 14C age results for 11 samples from seven fluvial cores.Sample ID Lab Code Depth (m) Material δ 13C (‰) 14C age (BP) Calibrated age 95.4% confidence interval (cal BP) HDZ03-09 LZU1424 9.1 plant residue –36.4 2375 ± 30 2340–2650 HDZ08-09 Beta-385805 6.5 clay −23.1 3260 ± 30 3400–3570 HDZ08-14 Beta-385806 17.6 clay −25.7 31400 ± 170 34850–35700 HDZ12-09 LZU1316 3.2 plant residue −35.6 2315 ± 25 2210–2360 DKZ06-02 Beta-385798 1.5 clay −22.5 2310 ± 40 2160–2430 DKZ06-08 Beta-385799 5.8 clay −23.3 11740 ± 50 13460–13720 DKZ06-17 Beta-385800 19.5 clay −23.6 31410 ± 170 34860–35710 DKZ09-18 LZU1422 16.7 plant residue −33.0 43680 ± 520 45780–48130 DKZ10-20 LZU1426 13 plant residue −37.2 38730 ± 320 42230–43140 DKZ13-11 Beta-385802 8.1 clay −23.4 24770 ± 30 28600–28960 DKZ13-12 LZU1425 9.7 clay −26.3 33210 ± 30 36910–37900
A typical natural OSL decay curve for modern fluvial sediment shows that the OSL natural signal is very low (Fig. 3a). The comparison of the distribution pattern in Fig. 5 shows that the De values of the different quartz fractions have small residuals. We extracted modern samples from a depth of 15 to 20 cm below the surface, which might be the reason for the small residuals. If an average deposition rate of 0.12 cm/a for the river channel in the Inner Mongolian reaches of the Yellow River (Pan
Assuming that the modern (recently deposited) samples and core samples experienced the same depositional conditions, we could use these modern analogues to assess the degree of bleaching in older core samples (
The rate and completeness of bleaching is dependent on a number of variables in fluvial environments (Jain
This study shows that medium-grained quartz seems to be better bleached than coarse-grained quartz. Coarser grains (150–200 μm) have significantly larger doses than medium and coarse grains. For most of the modern samples, there is also a slight increase in De with an increasing grain size (Fig. 4). This is contrary to the results from previous investigations of different grain-size fractions of fluvial sediments (e.g. Alexanderson, 2007; Rittenour, 2008; Hu
The Inner Mongolian reaches of the Yellow River represent a typical alluvial channel in terms of the highly complex alternation between runoff and sedimentation, as well as the high-frequency evolution of the river channel (Ta
As shown in Fig. 4, the De values for the coarser-grained fractions (150–200 μm) from LFQ-6, LFQ-9 and LFQ-14 are 2.33, 3.72 and 3.61 Gy, respectively, and the individual De value reaches up to 7.5 Gy (Fig. 5h), which is greater than, or equal to, those of medium and coarse quartz grains (<150 μm). The scattered De distributions for the coarser-grained fractions (150–200 μm) may indicate that the coarser grains were both poorly and heterogeneously bleached prior to deposition, and might be better explained by a mixture of a small number of poorly-bleached grains and a larger number of well-bleached grains (Olley
In our study, the De values of medium-grained (38–63 μm) quartz fraction from all the modern fluvial samples exhibited no decrease with an increasing transport distance (Fig. 4). Sedimentary inputs from riverbank erosion or tributaries may as well change this pattern. Shields (1936) used critical shear stress and critical mean flow velocity curves to predict the critical conditions for initial sedimentary movement. This defines the critical bed shear stress necessary to set particles of a given size in motion. For clay (0–3.9 μm) and silt (3.9–63 μm) particles to erode and then remain in suspension, they require higher shear stresses and flow velocity. Once set in motion within the fluvial environment, they can be transported over considerable distances. When the velocity of flow falls below the settling velocity, they are then deposited, and can only be re-eroded with difficulty (Miller
By contrast to the medium-grained (38–63 μm) quartz fraction, the De values of coarse-grained fractions (63–90, 90–150 μm) show a stronger correlation with the distance downstream, which is readily explained by the more complex interactions with the adjacent dune sediments, tributary confluences and sandy bank failures, occurring along the length of the channel network. As discussed above, most coarse sediment (>80 μm) inputs from the local desert regions are wind-blown, or result from bank erosion, tributary confluences and sandy bank failures through bedloads. It is most likely that the bedload sediments from the desert reaches to the tributary sections were not efficiently transported further downstream, and the coarse particles in the tributary reaches come mainly from the ten tributaries (Pan
Accurate and precise dating is essential for correlating fluvial sedimentation with external forcing (Cunningham and Wallinga, 2012). Laboratory tests of luminescence and OSL age characteristics were used to assess the precision of particular dating methods, but could not test the accuracy of the ages found (Zhang
The OSL ages were compared with the 14C ages on the stratigraphic columns for the seven cores (Figs. 6 and 7). The 14C ages were either in agreement with the OSL ages, or fell into the same stratigraphic order as the OSL ages, except for the 14C ages of the three samples HDZ03-09, HDZ12-09 and DKZ06-08, much older than the OSL ages in the strata above and below. If incomplete bleaching was a common problem, for the OSL-dated samples, incomplete zeroing would result in an overestimation of age, thus resulting in older OSL ages than 14C ages. Sediments are known to be repeatedly exchanged through frequent river channel migration and riverbank erosion in floodplains (Dunne
The residual De values of modern fluvial samples from the Inner Mongolian sections on the upper reaches of the Yellow River range principally between 0.16 and 0.49 Gy for silt grains (38–63 μm), and between 0.35 and 3.72 Gy for sand grains of the modern fluvial sediment samples. The small residual dose for modern samples, and their luminescence signal characteristics, indicate that fluvial sediments have been relatively well bleached during transport, when compared to the residual doses of modern fluvial sediments from the middle reaches of the Yellow River and from other rivers around the world. Medium-grained quartz fractions are better bleached and more suitable than coarse-grained quartz when dating fluvial sand samples, and the De values of coarse-grained (63–90, 90–150 μm) fractions show a stronger correlation with distance downstream, suggesting that the tributaries change the pattern of De values of coarse-grained fractions with distance. In addition, a total of 11 14C ages were compared with 26 OSL results for drilling cores, to give confidence that the initial bleaching of these sediments was sufficient. We would therefore propose that these fluvial samples were well bleached prior to deposition, and that the OSL ages we obtained can reasonably be taken as the best estimates for the dates of initial sedimentary deposition.