Manas Lake is a closed lake basin located in the western part of the Junggar Basin in northern Xinjiang Province, northwestern China. It is adjacent to Gurbantunggut Desert to the east (Fig. 1). The lake is elongated in the NE-SW direction (Fig. 2), and its current lake bed is at an elevation of 244 m a.s.l. As hydrological systems in such arid region are sensitive to climate change, valuable information about the paleoenvironmental changes can be obtained through studies of sedimentary records in Manas Lake. Understanding past changes of the lake is important in predicting future hydrological changes, as well as managing water resources in the region.
Map of northern Xinjiang showing the major water systems, including the Manas Lake west of the Gurbantunggut Desert. Inset map shows the northern Xinjiang area (in dotted rectangle) and the present wind systems in Asia (Herzschuh, 2006).Fig. 1
Google Earth image showing the Manas Lake area and two NE-trending faults. Sampling locations for this study (red dots) and for previous studies by Fan et al. (2012) and Wang (2014) (blue dots) are also shown, together with paleoshorelines (white lines). The elevation shown is in meters above sea level.Fig. 2
In previous studies on Manas Lake, radiocarbon dating (e.g. Rhodes
Using OSL dating, Fan
Later, Wang (2014) focused on dating young, late Holocene sediments of Manas Lake. He identified two paleoshoreline terraces at elevation 255 and 262 m a.s.l. (11 and 18 m above lake bed respectively) on both northwestern and southeastern sides of the lake (Fig. 2). He obtained an OSL age of ~300 a for the upper paleoshoreline (sites MS-2A and MS-2B), while the lower paleoshoreline (sites MS-1A and MS-1B) was too young to be OSL-dated. This suggests a cold and wet Little Ice Age, when the lake level once reached ~262 m a.s.l., forming the upper paleoshoreline. The lake level then dropped steadily towards the present, with the lower paleoshoreline (255 m) formed more recently (within the last century) due to a short period of stable lake level (Wang, 2014). Manas Lake became completely dried in 1962, only being recovered occasionally in summer after 1999 (Zhang and Li, 2004).
Wang (2014) also found a sedimentary break in his sampling section of the upper paleoshoreline in the southeastern side (MS-2B of Fig. 2) of Manas Lake, in which the sample from the layer beneath the Little Ice Age sediments gives a much stronger natural OSL signal. Similar to the results of Fan
Adjacent to the Gurbantunggut Desert (Fig. 1), the regional climate is arid, and is dominantly controlled by the Westerlies (Wang
Previous geomorphological studies show that Manas Lake is a remnant of a much larger lake, called the Paleo-Manas Lake. The large lake was a wandering lake, originally formed due to tectonic subsidence in the western part of Junggar Basin in the early Pleistocene (Wang, 1991). Based on the distribution of lacustrine sediments and satellite data, the lake level once reached ~93 m above the present Manas Lake bed (Fan
It is noted that the evolution of Manas Lake was controlled not only by climate change, but also neotectonic activities (Wang, 1991; Mu
Samples were collected from two sites in Manas Lake (sites 1 and 3 on Fig. 2). OSL samples were taken either by hammering stainless steel tubes (30-cm long with 5-cm diameter) into an exposed sedimentary section (site 1), or by taking block samples (site 3). Both ends of the tubes were then sealed immediately with tissue papers and tapes to avoid any exposure to light or loss of moisture content. For block samples, they were sealed in plastic bags and were wrapped with tapes to avoid breakage during transport.
Site 1 (45°44′53.1″N, 85°55′16.6″E) is located on the upper paleoshoreline in the southeastern side of Manas Lake at an elevation of 262 m a.s.l. This site is close to Wang’s (2014) section (MS-2B) along the paleoshoreline terrace. A trench was dug to expose a sedimentary section of ~1 m deep (Fig. 3). The 15 cm-thick top layer of the section consists of dark greyish rounded granules and pebbles interbedded with light yellowish coarse sand containing occasional granules. They were likely deposited in recent flooding events. The next ~30 cm is made of horizontally-bedded light yellowish coarse sand with small amount of sub-rounded gravels. It is the same layer as Wang’s (2014) paleoshoreline sample, and is expected to have an age of a few centuries based on the OSL date by Wang (2014). This is underlain by ~40 cm thick of cross-bedded yellowish brown medium to coarse sand, which is finer and wetter than the light yellowish sand above. The relatively coarse grain size of sand indicates a near-shore environment, with a paleo-current dominantly flowing to the center of the lake as indicated by cross-bedding. This layer is further underlain by relatively wet, yellowish brown coarse sand with some sub-rounded gravels at the bottom of the section. The coarse-grain nature again indicates an environment near a paleoshoreline. OSL samples were collected from each of the three sandy layers (Fig. 3). The purpose of dating these samples is to establish the size of the sedimentary age gap described in section 1, in the southeastern side of Manas Lake. It also helps to determine the time when relatively wet episodes occurred in the lake area.
Field photo showing the excavated trench at paleoshoreline site 1, and the schematic diagram of the section.Fig. 3
Site 3 (45°51′51.6″N, 85°52′38.6″E) is located on the northwestern side of Manas Lake at an elevation of 270 m a.s.l. An exposed vertical sedimentary section was found (Fig. 4). The top of the section is a thin layer of yellowish grey gravels, indicating a fluvial environment. The next ~55 cm of the section consists of very light yellowish fine to medium sand. The relatively fine-grain nature of the sand suggests an offshore shallow lacustrine environment. It is underlain by a ~20 cm thick layer of brown mud. The brownish color indicates an oxidizing environment. Thus, the brown mud was likely deposited in a very shallow and quiet water environment, occasionally exposed to the air. Below the muddy layer, there is another ~25 cm thick layer of very light yellowish fine to medium sand, which indicates a shallow lacustrine environment again. The bottom of the section is made of yellowish grey, poorly sorted fluvial gravels (dominantly pebbles and granules), which is more than 2 m thick. OSL samples were collected from the middle of each of the two sandy layers (Fig. 4). As it was too hard for hammering stainless steel tube into the section, block samples were taken directly instead.
Field photo and stratigraphic log of the sedimentary section at site 3.Fig. 4
In the laboratory, pretreatment of OSL samples was conducted under subdued red light conditions, following a standard technique (Aitken, 1998; Li
All luminescence measurements were performed using an automated Risø TL/OSL-DA-20 reader in the Luminescence Dating Laboratory, The University of Hong Kong. The reader was equipped with blue LEDs (470 ± 20 nm) and IR LEDs (870 ± 40 nm) for optical stimulation. The maximum power that can be delivered by the blue LEDs and the IR LEDs to the sample position are ~80 mW/cm2 and ~135 mW/cm2, respectively. 90% of the maximum power was used for all luminescence stimulations. Luminescence was detected by a bialkali EMI 9235Q photomultiplier tube (PMT). Three 2.5-mm thick Hoya U-340 filters were equipped in front of the PMT, allowing transmission of UV light with wavelengths of 290–370 nm. Irradiation was performed by a calibrated 90Sr/90Y beta source.
A single aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000; Wintle and Murray, 2006) was used to determine the equivalent dose (De). Forty-eight aliquots were measured for each sample, using the protocol detailed in Table 1. Most aliquots showed some weak, measurable IRSL signals, but they are all less than 10% of the intensity of initial quartz OSL. A 100 s IR stimulation at 125°C was used to remove the contribution from feldspar contaminations, before measuring the wanted OSL signals (Lx and Tx) by blue light stimulation at 125°C for 40 s (Banerjee
The single aliquot regenerative dose (SAR) protocol for quartz post-IR OSL measurements in this study. For the ‘natural’ sample, i = 0 and D0 = 0. The whole sequence is repeated for several regenerative doses including a zero dose and a repeat dose. For the young sample MN15-1-1, the preheat was at 220°C and the cut-heat was at 180°C. For other samples, the preheat was at 260°C and the cut-heat was at 220°C. For the young sample MN15-1-1, the preheat was at 220°C and the cut-heat was at 180°C. For other samples, the preheat was at 260°C and the cut-heat was at 220°C.Step Treatment Observed 1 Give regenerative dose, Di 2 Preheat at 260°C / 220°C 3 IR stimulation at 125°C for 100 s 4 Blue light stimulation at 125°C for 40 s Lx 5 Give test dose, DT 6 Cut-heat at 220°C / 180°C 7 IR stimulation at 125°C for 100 s 8 Blue light stimulation at 125°C for 40 s Tx 9 Blue light bleaching at 280°C for 100 s 10 Return to step 1
A preheat plateau test was conducted on the sample MN15–1-2, using preheat temperatures from 200°C to 280°C with increments of 20°C. A De plateau was obtained for preheat temperatures from 220°C to 280°C (Fig. 5). Based on the De plateau, a preheat temperature of 260°C for 10 s and a cut-heat temperature of 220°C were selected, except the sample MN15-1-1 in which the preheat temperature was 220°C and the cut-heat was 180°C to minimize thermal transfer effects in such young samples (Wintle and Murray, 2006). To further test the results, a dose recovery test was conducted on sample MN15-3-1. Twenty-four aliquots of the sample were first bleached by an Oriel solar simulator for 2 h, and were then given laboratory dose of 188 Gy, close to the mean natural De value. Such known dose was then treated as unknown, and the aliquots were measured with the SAR protocol in Table 1. Nineteen aliquots have passed recycling ratio and recuperation test. Their results are shown using histograms and radial plots (Fig. 6). The mean and central recovered De values are 203.8 ± 6.7 and 199.0 ± 7.3 Gy respectively, with a small overdispersion of 12.6%. The mean and central dose recovery ratios are thus 1.08 ± 0.04 and 1.06 ± 0.04 respectively, lying within the range of 1.0 ± 0.1. This confirms the reliability of the De measurement results.
Preheat plateau test for sample MN15-1-2, with a De plateau obtained for preheat temperatures from 220°C to 280°C.Fig. 5
The distribution of recovered dose values in a dose recovery test of the sample MN15-3-1 is shown using histogram and radial plot. The given laboratory dose was 188 Gy.Fig. 6
For environmental dose rates, the contribution from U and Th decay chains was determined using a Littlemore 7286 thick source alpha counting system, while the contribution from 40K was evaluated by measuring the K content using X-ray fluorescence (XRF). The measured alpha count rate and K content can be converted to dose rates using conversion factors from Adamiec and Aitken (1998). Attenuation of beta dose was calculated using the beta dose absorption fractions reported by Fain
Typical OSL decay curves and dose response curves from samples MN15-1-1 and MN15-3-2 are shown in Fig. 7. The OSL decay curves show rapid decay in the OSL signals, suggesting that the signals are dominated by the fast component. The fast component has been considered to be the most reliable signal for OSL dating (Wintle and Murray, 2006). The influence of medium and slow components was tested by altering the stimulation time integrals used for calculating Lx and Tx. It was found that there was no obvious De dependence on stimulation time for all samples. When an “early background subtraction” method is used instead, more than 80% of aliquots give consistent individual De values with those calculated by “late background subtraction” as described in the procedures above. Also, no systematic differences in the mean and central De are found between both methods, but the “early background subtraction” leads to a larger error due to subtraction of a stronger signal. Thus, subtraction of late background signals from the last 5 s of OSL decay curves were used to calculate the De in the following.
Typical OSL decay curves and dose response curves (insets) from samples MN15-1-1 and MN15-3-2.Fig. 7
De distributions of all the five samples were analyzed using histograms and radial plots (Fig. 8). The arithmetic mean De values, the De derived from central age model (CAM) of Galbraith
De distributions of the five samples shown in histograms (left) and radial plots (right).Fig. 8
The equivalent doses derived from the mean age model and the central age model, with the overdispersion values for the five samples. 48 aliquots were measured for each sample. Aliquots were rejected by various reasons, including failure in recycling ratio test (4, 4, 4, 2, 2) or recuperation test (2, 1, 1, 1, 0), failure in fitting the dose response curve (0, 0, 0, 0, 2) and when the natural Lx/Tx cannot be interpolated onto the dose response curve (0, 3, 5, 2, 2). The five numbers in brackets refer to the number of aliquots rejected by that specific criteria for the five samples (MN15-1-1, 1-2, 1-3, 3-1, 3-2) respectively. Due to insufficient bleaching problem, the minimum age model was used for the sample MN15-1-1, which gives a De of 2.67 ± 0.14 Gy.Sample ID Number of aliquots Mean age model De (Gy) Central age model De (Gy) Over-dispersion(%) MN15-1-1 42 31.1 ± 5.9 16.3 ± 2.5 118 MN15-1-2 40 219.8 ± 11.7 196.3 ± 8.3 22.5 MN15-1-3 38 199.7 ± 8.6 186.5 ± 7.5 22.2 MN15-3-1 43 183.9 ± 11.7 169.8 ± 10.0 38.0 MN15-3-2 42 229.5 ± 11.2 216.9 ± 10.8 31.4
It is noticed that these four samples have relatively high De values close to 200 Gy. Thus, the “2D0” values were also considered. D0 is a constant related to the shape of a dose response curve which can be described by a single saturated exponential of the form Lx/Tx = (Lx/Tx)0[1-exp(−D/D0)]. If De exceeds 2D0, then the result may not be reliable. For our four old samples, the average D0 values for individual samples vary between 112 and 159 Gy. None of the mean nor CAM De values reported in Table 2 exceeded the average 2D0 values, although a small portion of individual aliquots have De > 2D0. The possible dependence of De on D0 for individual aliquots was also investigated (Fig. 9). It was observed that there are no or little dependence of De on D0. Thus, the results can still be regarded as reliable despite larger uncertainties. For these four samples, their arithmetic mean De values are about 10 to 20 Gy larger than the CAM De values (Table 2), but they are consistent with each other within 2σ. Since the mean age model does not consider the errors of individual aliquots, it will give an upward bias if the few aliquots with large De (which have larger errors due to near saturation) are treated equally with the other aliquots. Thus, the CAM De will be used in the following discussion, except the sample MN15-1-1.
Plots of De vs D0 for individual aliquots of samples MN15-1-2, MN15-1-3, MN15-3-1 and MN15-3-2.Fig. 9
For sample MN15-1-1, the extremely large scatter of De values (Fig. 8a) and an overdispersion of 118% indicates that the sample was insufficiently bleached before deposition. This is in contrast to Wang’s (2014) paleoshoreline sample MS-2B, which was well bleached with a mean natural De of 0.9 Gy and all aliquots give individual De values between 0.8 and 1.1 Gy. This shows that bleaching by sunlight was heterogeneous, even though the samples are from the similar paleoshoreline layer (Figs. 2 and 10). Due to insufficient bleaching problem, the minimum age model (Galbraith
A summary of the dose rate measurements for different samples are shown in Table 3. The major difficulties in estimating the dose rates come from uncertainties in the water content variation in the past. For site 1, the as found sample water content was assumed in the dose rate calculations, with a relative error of 20%. However, for site 3, the sedimentary section has been exposed for a long time before sample collection, losing most of the moisture content. As the samples from this site were deposited in a lacustrine environment, they must have been under water for a considerable amount of time. Thus, the water content for samples at site 3 was assumed to be 25 ± 5%, which is close to the saturation water content of ~30% estimated in the laboratory. The final calculated dose rates, together with the OSL ages of the samples, are shown in Table 3.
OSL dating results for the five samples in the study area. The α-count rate is measured through a 42-mm-diameter ZnS screen. K content was measured using XRF. The error is assumed to be ±10% (relative). The error in water content is estimated to be ±20% (relative). See text for discussion. The error in cosmic ray dose rate is assumed to be ±0.02 Gy/ka Minimum age model is used for sample MN15-1-1 due to insufficient bleaching problem. Central age model (CAM) is used for other samples. Minimum age model is used for sample MN15-1-1 due to insufficient bleaching problem. Central age model (CAM) is used for other samples.Sampling site Sample ID Grain size (µm) Depth (cm) α-count rate (cts/ks) K content (%) Water content (%) Cosmic ray (Gy/ka) Dose rate (Gy/ka) Equivalent dose (Gy) OSL age (ka) Site 1 MN15-1-1 180–212 22 4.97 ± 0.12 1.48 0.12 0.21 2.36 ± 0.11 2.67 ± 0.14 1.13 ± 0.08 MN15-1-2 150–180 50 3.12 ± 0.10 1.64 1.9 0.20 2.22 ± 0.13 196.3 ± 8.3 88.5 ± 6.3 MN15-1-3 150–180 70 2.82 ± 0.09 2.03 10.6 0.20 2.32 ± 0.15 186.5 ± 7.5 80.3 ± 6.0 Site 3 MN15-3-1 150–180 30 4.09 ± 0.12 2.20 25 0.21 2.32 ± 0.15 169.8 ± 10.0 73.3 ± 6.4 MN15-3-2 125–150 90 7.71 ± 0.17 2.17 25 0.19 2.71 ± 0.16 216.9 ± 10.8 80.2 ± 6.1
The upper layer of medium to coarse sand with gravels yielded an OSL age of ~1.1 ka (sample MN15-1-1). This age is older than that dated by Wang (2014) from a section along the same paleoshoreline, in which his sample MS-2B (see Figs. 2 and 10) was dated to be ~0.3 ka, corresponding to Little Ice Age. It is noted that this sample is poorly bleached, in contrast to the well-bleached sample of Wang (2014). This means that even aliquots with small De would contain a mixture with a small amount of poorly bleached grains. Thus, the minimum age model would still give an overestimation of ages for MN15-1-1. It may be treated as the maximum age. There may also be uncertainties in dose rate due to heterogeneity of this layer. The exact age of this layer is beyond the scope of the following discussion.
The lower two samples MN15-1-2 and MN15-1-3 give OSL ages of 88.5 ± 6.3 and 80.3 ± 6.0 ka respectively. Although a slight reversal in ages was found, these two ages are consistent with each other within errors. It is noted that the sample MN15-1-3 is mixed with some pebbles which may have different radioactive contents from their surrounding sandy matrix. This potentially gives a large error in the estimation of dose rate. However, based on the ages of these two samples, it is suggested that during ~90–80 ka ago, site 1 was in a paleoshoreline or a near-shore environment, as indicated by the relatively coarse grain sizes of sand. At that time, the paleo-lake level fluctuated at around this site (~262 m a.s.l.). At a certain time after ~80 ka, the lake level retreated. Between ~80 ka and the last ~1 ka, there were no sedimentary records at this site. It is interpreted that a large scale erosion may have occurred in the area. Even if there were lacustrine episodes during this period, the deposited lacustrine sediments would have been eroded away when they were exposed during low lake stands. The eroded materials were likely transported to the adjacent Gurbantunggut Desert by the Westerlies.
At site 3, the two lacustrine sand samples yielded consistent OSL ages of 73.3 ± 6.4 and 80.2 ± 6.1 ka respectively. Based on the sedimentary properties and the OSL ages, we deduced that there was a fluvial episode before ~80 ka ago at this site, depositing > 2 m thick layer of greyish gravels. Around 80 ka ago, the lake level of Manas Lake rose and reached above site 3. As a result, fine sand layers were deposited during 80–70 ka ago. But it was interrupted by a brownish mud layer in between. The mud layer could indicate a deeper lake environment. Alternatively, it could be a very shallow lake and calm environment with limited water flow, because the brownish color probably indicates can oxidizing environment. Regardless of how the brownish mud layer formed, it can still be interpreted that the lake level was generally high during 80–70 ka ago. After that, the lake level retreated, with another fluvial episode recorded at the site.
It is not known how long the lacustrine episode has lasted for at this site around 80–70 ka ago, due to the errors of the OSL ages and possible phases of erosion. However, the ages are consistent with that of Fan
The results from this study, as well as those of Fan
Sites 1 and A currently have similar elevations but are on the opposite side of each other. When the sedimentary sections of the two sites are compared with each other, it can be noted that the sizes of the age gap are different (Fig. 10). A few lacustrine episodes were recorded at site A in the northwestern side of the lake from before 66 ka ago to ~27 ka ago (Fan
Summary of results from this studies (sites 1 and 3) and previous studies by Fan et al. (2012) and Wang (2014). Note that the top of each sedimentary section shown corresponds to the ground surface.Fig. 10
Lacustrine sediments from the northwestern and southeastern sides of Manas Lake were used to study the paleoenvironmental and neotectonic changes in the area, with an OSL chronology established. On the northwestern side of the lake, site 3 (270 m a.s.l.) recorded lacustrine episodes at around 80–70 ka ago. On the other side of the lake, site 1 (262 m a.s.l.) recorded a paleoshoreline to near-shore environment at ~80–90 ka ago, followed by a sedimentary age gap between ~80 ka ago and the last ~1 ka. Combining with the results with those from previous studies (see Fig. 10), it is concluded that breaks in the sedimentary records are common in the area at an elevation of > 260 m a.s.l. Large scale aeolian erosion likely occurred in the area during periods of low lake stands. Furthermore, by comparing sedimentary environments at different times from opposite sides of Manas Lake, it is suggested that a small amount of uplift of the northwestern side relative to the southeastern side of the lake have occurred in the last ~80 ka. Uncertainties still exist in the amount, timing and the spatial extent of the uplift. Future studies may aim to look for lacustrine sediments aged at ~73–27 ka in the southeastern side of the lake, in order to further constrain the paleoenvironment on that side and the extent of differential uplift.