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1.
Lakes over the inner Tibetan Plateau (TP) are very sensitive to the regional environmental transformations and climate changes. Well-preserved lake sediments around these lakes provide critical geomorphological and sedimentary evidence that can be used to infer the past hydroclimate changes. In this study, a lacustrine section from a sandy shoreline (∼74 m above the modern lake) situated to the northwest of modern Dawa Co in the inner TP was investigated using both luminescence and radiocarbon dating methods. Our results demonstrated: (1) the quartz optically simulated luminescence (OSL) dating yielded much younger ages (∼4 ka) than that of the post-infrared IRSL (pIRIR) dating of the K-feldspar fraction; (2) fading test showed g-values ranging between 1.34 and 4.46%/decade for quartz OSL signals, which is considered to be responsible for the underestimation of the corresponding ages; (3) the AMS 14C age of the charcoal sample from the section is in line with the K-feldspar pIRIR225 ages, confirming the reliability of the pIRIR225 dates and the underestimation of the quartz OSL ages. The anomalous fading of quartz OSL signals and the consequent age underestimation have been reported in several other lakes on the TP, we presented here for the first time firm evidence of the phenomenon with the help of a robust independent control of AMS 14C age of the charcoal. Based on the pIRIR225 and AMS 14C ages, we conclude that Dawa Co underwent a prominent highstand during the early Holocene (∼9–7 ka), which was probably controlled by the large amounts of glacial meltwater input and the increased monsoonal precipitation. 相似文献
2.
Luminescence dating has long been used for chronological constraints on marine sediments due to the ubiquitous dating materials (quartz and feldspar grains) and its applicability over a relatively long time range. However, one of the main difficulties in luminescence dating on marine sediments is partial bleaching, which causes age overestimations. Especially, partial bleaching is typically difficult to be detected in the fine grain fraction (FG) of marine sediments. The recently developed feldspar post-IR IRSL (pIRIR) protocol can detect non-fading signals and thus avoid feldspar signal instability. In the current study, fine grains were extracted from a gravity core in the northern Sea of Japan, and the aim is to test the feasibility of using different luminescence signals with various bleaching rates to explore the bleaching conditions of fine grain fraction in marine sediments. The results suggest that the quartz OSL signal and polymineral pIRIR signals at stimulation temperatures of 150 °C and 225 °C (pIRIR150 and pIRIR225) of FG were well bleached prior to deposition. The OSL ages were used to establish a chronology for this sedimentary core and the resulting age-depth relationship is self-consistent and comparable with radiocarbon dates. We conclude that different luminescence signals with various bleaching rates can be used to test the bleaching conditions of fine grain fraction in marine sediments; and the luminescence dating can be applied to marine sediments with great potential. 相似文献
3.
Sediments of river deltas provide valuable records of past coastal environments. Optically-stimulated luminescence (OSL) dating has become an alternative to radiocarbon dating for constraining the sediment chronology in large deltas that allow for sufficient sunlight bleaching of sediments during the fluvial transport. However, its applicability to smaller deltas with mountainous riverine systems has not been confirmed yet. To check this, we examine multiple signals from two Holocene sediment cores in the wave-dominated Thu Bon River delta in central Vietnam. Two cores were collected, respectively, 3.9 km and 1.2 km inland from the present shoreline and both show a >-25-m thick succession of coarsening-upward mud to sand deposits. Coarse grains (180–250 μm in diameter) of quartz and K-feldspar, and fine grains (4–11 μm in diameter) of quartz and polymineral were extracted from the upper and lower parts of the cores for multi-grain measurements of quartz OSL, and of feldspar infrared-stimulated luminescence (IRSL) at 50 °C (IR50) and post-IR IRSL at 175 °C (pIRIR175) to determine burial ages. In addition, facies analysis and radiocarbon dating were conducted. The landward core consists of transgressive to early regressive estuarine and prodelta facies, which is overlain by a sandy beach-shoreface facies. The seaward core consists of a relatively simple shallowing-upward succession from muddy prodelta facies to sandy beach-shoreface facies. All luminescence ages except for pIRIR175 of fine grains are mostly consistent with the radiocarbon ages. Instead, pIRIR175 ages of fine grains are significantly overestimated with variable offsets. OSL and IR50 of fine grains provide reasonable age estimates, as these grains were likely well bleached during the transport even along a short and steep mountainous river. Consistent age estimates of all signals from sand indicate that sand was well-bleached in the beach and shoreface owing to the frequent sediment reworking by waves and currents. These results support the hypothesis that luminescence dating is applicable to Holocene wave-dominated deltas and reiterate that comparing different luminescence signals is an effective way to check reliability of the age estimates in environments where the sunlight bleaching is not ensured. 相似文献
4.
The Late Quaternary hydroclimatic evolution of lake systems in Mongolia remains unclear. Here we present a record of lake level variations at Orog Nuur in the Valley of Gobi Lakes in southern Mongolia, since the last interglaciation, based on paleo-shoreline dating using quartz optically stimulated luminescence (OSL) and K-feldspar post-infrared infrared stimulated luminescence (pIRIR) signals. Due to feldspar contamination that could not be eliminated, the OSL signals of quartz single-aliquots (SA), except for two Holocene samples, were unsuitable for dating and a double SA regenerative-dose (SAR) protocol was used for the quartz fraction of these two samples. The pIR50IR170 and pIR200IR290 signals of K-feldspar SA were used to date Holocene samples and old samples (>100 ka), respectively, with the SAR protocol. To determine the bleaching condition of the pIR200IR290 signals, the first pIRIR dating of K-feldspar single-grains of lake shoreline sediments in Mongolia was performed. The equivalent doses of K-feldspar grains show normal distributions, suggesting that the pIR200IR290 signals are well-bleached. Overall, the results, combined with those of previous studies, show that a mega-lake developed at 56 m above the modern lake level (a.m.l.) during MIS 5e (124.2 ± 6.8–114.7 ± 8.0 ka). Holocene high-stands occurred in the last deglaciation–early Holocene (11.1 ± 1.0 ka) at 23 m a.m.l. and in the mid-Holocene (6.7 ± 0.8–3.3 ± 0.4 ka) at 20–14 m a.m.l. The dimensions of the paleo-lakes were recovered, and a hydrological index indicates that the effective moisture during MIS 5e and the mid-Holocene was 10.7 times and 3.6–5.0 times larger than today, respectively. Finally, the possible mechanisms behind the lake level history are discussed based on correlation with independent paleoclimatic records. 相似文献
5.
Testing the application of quartz and feldspar luminescence dating to MIS 5 Japanese marine deposits
The applicability of both quartz and feldspar luminescence dating was tested on twenty-five samples from a marine succession now forming a coastal cliff at Oga Peninsula, Honshu Island, Japan. The quartz optically stimulated luminescence (OSL) signal shows thermal instability and linear modulated (LM)-OSL analysis revealed the dominance of a slow component. When compared with independent age control provided by two marker tephras, the quartz OSL ages grossly underestimate the depositional age. In contrast, potassium (K)-rich feldspar is a suitable dosimeter when measured using post-IR infrared stimulated luminescence (IRSL) at 225 °C (pIRIR225). Scanning electron microscope (SEM) analyses on the feldspar extracts revealed that the grains are amorphous with small crystalline inclusions; using standard internal dose rate parameters, this would result in a too large dose rate. Dose rates were calculated using the observed grain size of 40 ± 20 μm with an assumed K concentration of 12.5 ± 0.5%. The fading corrected pIRIR225 ages agree well with independent age control, and the sediments of the Katanishi Formation were deposited between 82 ± 6 and 170 ± 16 ka. This study demonstrates that pIRIR dating of feldspar is a powerful chronological tool for the dating of sediments of volcanic origin. 相似文献
6.
Holocene flood events in the Yangtze River are associated with variations in East Asian Summer Monsoon (EASM) precipitation, and so Yangtze delta sediments may preserve information about the frequency and magnitude of EASM precipitation. These flood/drought cycles of the EASM directly affect the living standards of East Asian population. However, despite its importance, little chronological control is available for the Yangtze Delta sediments; because biogenic carbonate only occurs sporadically, it has proved the difficulty to discuss sedimentation mechanisms and rates in any detail.In 2013 two sediment cores (YD13-G3 and H1) were taken from the Yangtze subaqueous delta to investigate precipitation history. In this study, we investigate the potential of quartz OSL dating of the fine silt fraction (fine-grained quartz; 4–11 μm) from these cores to estimate the depositional age of the sediments. We test whether: (1) Yangtze subaqueous delta sediments contain quartz with suitable characteristics for dating, and (2) quartz grains are well-bleached during/before the transportation process, by examining a modern analogue of suspended particulate matter, and by cross-checking with the doses derived from infrared stimulated luminescence (IRSL) signals (both IR50 and pIRIR160) from feldspar in polymineral fine grains. We find that both the quartz and feldspar luminescence characteristics are satisfactory (quartz dose recovery ratio 1.067 ± 0.004; n = 250, pIRIR160 dose recovery ratio 1.01 ± 0.02; n = 151). Modern suspended particulate matter has measured quartz equivalent doses between 0.1 and 0.2 Gy, suggesting that this material was sufficiently bleached during/before transportation to allow dating of Holocene sediments (mean dose rates of ∼3 Gy ka−1). OSL ages of 44 samples from the 2 cores show apparently rapid accumulation at ∼6 ka between 9.65 and 5.50 m in core H1 and ∼2 ka throughout core G3 and between 5.0 and 0.0 m in core H1. The pIRIR160 signals suggest less light exposure of the core top sediments and of those from the transition layer between ∼6 ka to ∼2 ka, although there is no evidence for incomplete bleaching of quartz. The question remains as to whether significant deposition took place only at these two times, or whether the record has been disturbed by erosion/reworking. 相似文献
7.
In this study we test, for the first time, the potential of an elevated temperature post-IR IR (pIRIR290) SAR protocol for the dating of young heated artefacts. Seven heated stones and seven potshards were collected from three different archaeological sites in Denmark: one site from the early Pre-Roman Iron Age 200 BC to AD 100, and two from the Viking period between AD 800 and 1200.We first derive quartz OSL ages for these samples, to support the archaeological age control. The luminescence characteristics of the pIRIR290 signal are then investigated; in particular the dose recovery ratios are shown to be close to unity. The performance of the feldspar pIRIR290 protocol is then examined by comparing the pIRIR290 ages with those based on the quartz OSL signal; the average ratio of pIRIR290 to OSL ages is 1.14 ± 0.05 (n = 14) and there is some suggestion that the possible overestimation of the feldspar ages compared to quartz is only of significance for the heated stone samples. Nevertheless, there is no indication of incomplete heating of the stones; the ratios of De derived from the IR50 and pIRIR290 signals are independent of sample type, and consistent with complete resetting by heating. Comparison with the archaeological age control is not able to identify whether quartz or feldspar provides the most reliable dating signal. 相似文献
8.
Although the Upper Volga has been the focus of extensive research for several decades, its origin and evolution remain uncertain. According to the most popular model, the Upper Volga drainage network formed at the end of the last deglaciation (MIS 2); before that, the river basin was occupied by a large MIS2 proglacial lake. In this study we test this hypothesis by luminescence dating several fluvial terraces in the Upper Volga valley. Despite the expected late-MIS 2 age, our results show that quartz is in saturation, and the feldspar pIRIR290 signal gives much older dates than expected (∼300–∼500 ka). We argue that, in most sections, feldspar was very likely to be well bleached prior to deposition. Thus, if the ages of sedimentary structures are overestimated, this could only result from sediment transport in darkness. It is widely accepted that the MIS 6 glaciation was the last to cover the Upper Volga basin, and so such conditions would be most easily attributed to a subglacial environment. But this explanation is confounded by the absence of till on top of most sections. The alternative explanation, that the ages accurately reflect last deposition, calls into question the well-established MIS 6 border on the Russian plain, and we conclude that further studies are required to resolve this inconsistency. 相似文献
9.
Cobbles can be used as an alternative to the conventionally employed sand-sized mineral luminescence dating. In piedmont environments, cobbles are much more abundant than sand-sized material. The IRSL50 signal has been widely used in previous studies due to its greater sensitivity to exposure events. However, it is well known that the low temperature IRSL signal is more prone to fade than elevated temperature post-IR IRSL signal. In this study, to test the reliability and applicability of cobble sub-surface elevated temperature IRSL luminescence dating, six light-color granite cobbles and two sand-sized samples from silty sand lens were collected from a high terrace of Manas River on the northern piedmont of Chinese Tian Shan. A modified multi-elevated-temperature post-infrared infrared stimulated luminescence (MET-post-IR IRSL) protocol was applied. The age-temperature (A-T) plateau of MET-post-IR IRSL measurement was combined with the conventional age-depth (A-D) plateau in luminescence-depth profile to evaluate the resetting and fading of MET-post-IR IRSL signals. Uncertainties of grain-sizes of K-feldspar within solidified slices were also explored by μ-XRF mapping of potassium content. The A-T plateau was identified between MET-post-IR IRSL170 and MET-post-IR IRSL225 signals of one cobble, which suggested completeness of bleaching before burial and negligible anomalous fading during burial. This cobble yielded MET-post-IR IRSL225 ages of 15.8 ± 2.6 ka and 19.0 ± 3.2 ka for top and bottom side, respectively. These MET-post-IR IRSL225 ages were consistent with independent coarse-grained quartz MAM OSL ages (15.7 ± 3.6 ka and 14.8 ± 2.6 ka) of two sand-sized samples. The MET-post-IR IRSL225 age of 16.0 ± 1.2 ka for the bottom side of another cobble was also consistent with the independent age, even without the A-T plateau. It was inferred to be caused by anomalous fading of MET-post-IR IRSL signals other than that stimulated at 225 °C by refering to the A-D plateau observed. Our results show that MET-post-IR IRSL measurement can be employed to determine the burial ages of cobbles. The A-T plateau, complemented with the A-D plateau, could be used to assess the reliability of burial ages of cobble luminescence dating from the view of bleaching and fading. 相似文献
10.
Some of the largest catastrophic outbursts of periglacial lakes known in the geological history of the Earth have been identified in the Altai Mountains. Traces of these events are recorded in the form of large terraces, predominantly composed of gravel material with numerous horizons of large boulders and blocks. Determining the age of these large-scale events is difficult due to the lack of suitable material (e.g. organics, well-bleached sand) and the specific genesis of these sediments. The results of cosmogenic radionuclide dating suggest a post-LGM age both for the source of the flood water and for different elements of the catafluvial terraces in the Chuya and Katun river valleys. Nevertheless, the age(s) of catastrophic breakthrough remains controversial. On the basis of a few IRSL ages, and geological and other evidence, some view the event as occurring around MIS 5. In this study, we investigate loess-like loams overlying the catafluvial sediments on the surface of the highest level terrace, ∼200 m above present river level. A total of 24 samples for luminescence dating were obtained, for which the OSL, IR50, and pIRIR50,290 signals were measured to control the degree of signal zeroing and the dating reliability. The age of the loess in all three pits was from 0.5 ka at the top to 23 ka at the base of the loess strata. From a sand layer in the top of the catafluvial deposits, two ages of ∼85–90 ka were obtained from feldspar pIRIR50,290. These results provide a minimum pre-LGM age for the geomorphological surface of a major catafluvial terrace in the Altai Mountains. 相似文献
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12.
OSL and IRSL dating are applied to samples from a 152 m-long drill core to constrain the timing of three glaciolacustrine depositional periods within the infill of an overdeepened bedrock trough in the Lower Glatt valley, N Switzerland. The characterisation of the dose-response suggests that the polymineral IRSL50 and pIRIR180/225 signals are close to saturation, while quartz OSL ages are within the range of reliable dating. The demarcation of the upper quartz OSL dating limit, however, remains challenging. Dose-recovery tests performed with long storage periods were used to investigate the reliability of the high region of the dose-response curve. They suggest an upper limit for reliable dating of ∼400 Gy for these samples, which was considerably lower than the commonly used 2D0 criterion. Lifetimes were calculated for the quartz OSL and the thermal stability of the signal is not considered as problematic for the determined ages. Allowing for a contribution from inherited dose due to partial bleaching, places the infill of the overdeepened valley within the penultimate glacial cycle (MIS6). 相似文献
13.
In this study thermally transferred (TT) OSL and post-IR elevated temperature IRSL (290 °C) (pIRIR290) dating are applied to deposits covering coastal terraces on the Cap Bon peninsula, Tunisia. Both methods perform well under standard performance tests; dose recovery tests using a modern analogue show that doses relevant to our study can be recovered accurately. Residual signals in the modern analogue for both signals are very small (∼2 Gy). For the younger (<250 ka) deposits reasonable good agreement of the ages is observed between both methods, and in addition with standard quartz OSL dating. Systematic discrepancy in ages is found for the older sediments. The TT-OSL underestimate (compared to the pIRIR290 ages) is most likely due to the short lifetime of the TT-OSL trap; we estimate a lifetime of ∼0.7 Ma at 19 °C (mean ambient air temperature of the study area). This is the first time this lifetime has been derived from geological data and it is within the range of previously published laboratory estimates. This result suggests that TT-OSL is not likely to provide a significant extension of the age range beyond that available from other methods. Our preferred pIRIR290 ages suggest that the geological setting on Cap Bon is not as simple as previously suggested. 相似文献
14.
The properties of the quartz luminescence signal have been shown to be a useful tool for sediment provenance analysis. These provenance studies are based on the sensitivity of the fast optically stimulated luminescence (OSL) component, which is also used for sediment dating. Besides the widespread occurrence of quartz in terrigenous sediments, OSL sensitivity can be acquired using relatively fast and low-cost measurements compared to sediment provenance analysis methods based on accessory minerals or isotopes. Additionally, laboratories worldwide already have an extensive database of recorded quartz OSL signals primarily measured for dating studies, and these data could potentially be repurposed for provenance analysis of Quaternary sedimentary systems through OSL sensitivity calculation. Here, we investigate the use of OSL quartz signals measured in sediment dating surveys for OSL sensitivity calculation and evaluation of changes in sediment sources. The OSL sensitivity was calculated and expressed as %BOSLF, which corresponds to the percentage of the fast OSL component signal (blue stimulation) to the total OSL curve; such approach is advantageous as it does not require any normalisation of the measured signal intensity to dose or aliquot size (weight). Three sets of samples from Amazonian fluvial sediments are investigated: two sets of Holocene floodplain sediments representing different sediment sources to the Amazonian fluvial system, i.e. the Amazon craton and the Andes Mountain belt, and a set of samples from the Içá Formation, a paleo-fluvial system active during the Pleistocene whose provenance is not fully known. Results show that the quartz OSL signal derived from the first test doses (Tn) applied in dating protocols had the best performance for %BOSLF calculation when compared to results from a measurement protocol designed specifically for sediment provenance analysis. There is significant correlation (R2 = 88) between sensitivities derived from Tn and a specific OSL provenance analysis protocol. The proposed approach indicates to be appropriate for sediment provenance analysis since it is able to discriminate signal differences among samples from known sources: Brazilian cratonic quartz yield high sensitivity values (mean %BOSLF >70), in contrast to the relatively lower values from Andean quartz (mean %BOSLF <50). In general, quartz OSL sensitivities from the Içá Formation samples fall into the same range of modern sediments transported by the Içá and Japurá rivers draining the Andean Eastern Cordillera of Colombia and Ecuador. We also observe a decrease in quartz OSL sensitivity during the Holocene, notably after 4 ka, with younger deposits showing lower sensitivity. Sediment provenance variations are discussed in terms of watershed rearrangement and/or precipitation-driven changes during the Late Pleistocene and Holocene across Amazonia. 相似文献
15.
Loess deposits distributed in southeastern China play an important role for paleoclimate reconstruction of the subtropical regions. These loess-paleosol deposits are mainly spread within the middle and lower reaches of the Yangtze River as well as in the drainage area of the Huai River. The ages of loess paleosol sequences that are distributed along the Huai River are not well constrained. In this study, the standard single-aliquot regenerative dose (SAR) optically stimulated luminescence (OSL) protocol and two elevated temperature post infrared-infrared stimulated luminescence SAR protocols (pIRIR225 and pIRIR290) were applied on 4–11 μm quartz and polymineral fine grains, respectively, in order to obtain the first numerical luminescence chronology for a loess-paleosol sequence in northern Jiangsu Province. Our results show a good agreement between quartz SAR-OSL and polymineral pIRIR ages up to ~70 ka. These findings confirm that Xiashu loess accumulated during the Last Glaciation. For samples older than this, the ages increasingly deviate with depth. Fine quartz ages beyond 70 ka are interpreted as underestimates, as previous studies reported that quartz ages >70 ka from various sedimentary origins worldwide may underestimate even if they pass rejection criteria and dose recovery tests. On the other hand, the pIRIR ages are most likely overestimating the true depositional ages as indicated by the results of dose recovery tests, where a 30–60% overestimation of the recovered dose is reported for values larger than ~400 Gy. The overestimation of pIRIR protocols was also confirmed by the results obtained when large beta doses were added on top of the natural accrued dose. Moreover, our dating results suggest that L1/S1 transition (corresponding to MIS 5/4 boundary) occurred much higher in the stratigraphic sequence than may have been interpreted from the magnetic susceptibility enhancement. This inconsistency can be assigned to invalidity of magnetic susceptibility as a chronostratigraphical proxy due to ferrimagnetic minerals dissolution or transformation during paedogenesis processes in this humid subtropical region in the southeastern China. 相似文献
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17.
Although the OSL signal sourced from quartz is expected to be more stable and bleached more rapidly than the IRSL signal sourced from feldspar in a general sense, the former is much less investigated than the latter for rock surface related luminescence dating. It is mainly due to the difficulty in isolating quartz-dominated OSL signals from rock slices, and the low sensitivity and non-fast component dominated OSL signals for quartz in most of rocks. In merit of the sub-conduction band transition of trapped electrons of feldspar under both green light stimulation and IR stimulation at elevated temperature (Jain and Ankjӕrgaard, 2011), it is expected that the contribution of feldspars to the green light stimulated luminescence could be substantially reduced by a prior IR stimulation at elevated temperature. Meanwhile, more fast-component dominated quartz OSL signal (if there is any) could be obtained by green light stimulation (Bailey et al., 2011). Therefore, in this study, we investigated the feasibility of using the post-IR pulsed green stimulation, which is performed at 25 °C (PGLSL25) following two IR stimulations (IR50IR225), to isolate the quartz-dominated OSL signals from rock slices of granite cobbles for burial dating. The decay characteristics of stimulation curve, characteristic saturation dose and thermal stability of the pIR50IR225 PGLSL25 signal suggest that this signal is quartz-dominated, but still not fast-component dominated. We tentatively validated the equivalent dose (De) measurement protocol by dose recovery experiment. The luminescence-depth profiles show that the bleaching depths of pIR50IR225 PGLSL25 signals are slightly smaller than or close to that of the IR50 signals, while they are much shallower for the pIR50IR225 signals. The pIR50IR225 PGLSL25 procedure enables multiple De values determined from luminescence signals, with different bleachabilities and stabilities, from both feldspars and quartz in one measurement, which is of potential for buried age dating of cobbles. 相似文献
18.
Heavy liquid density separation is commonly utilised in isolating specific minerals from bulk sediment for luminescence dating. K-feldspars are commonly separated from other minerals by isolating the <2.58 g cm−3 fraction which yields infrared stimulated luminescence (IRSL) signals conducive to dating. However, initial measurements of the IRSL and thermoluminescence (TL) signals from the <2.58 g cm−3 fraction of alluvial fan sediments from the Hajar Mountains, northern Oman, were atypical of K-feldspar luminescence signals reported in the literature. In this study, three different density separates (<2.58 g cm−3, 2.58–2.54 g cm−3 and <2.565 g cm−3) were trialled to attempt to isolate pure K-feldspar samples for dating, to measure K-feldspar IRSL signals which are not contaminated with inputs from other feldspar, quartz and/or heavy mineral signals. The mineralogy of these separates was explored using microscopy, X-ray diffraction and mass spectrometry. We found that despite more refined heavy liquid density separation, the proportions of K-feldspar in these separates had a maximum of 40% by weight, remaining significantly contaminated by other minerals. IRSL and TL signals were atypical of K-feldspar for all density separates and it is likely that there is a contribution of signal from IR-sensitive contaminants, such as muscovite. This highlights the importance of investigating the mineralogy of samples for luminescence dating, especially in regions where little prior work has been undertaken. 相似文献
19.
Defining rates of landscape evolution in a south Tibetan graben with in situ‐produced cosmogenic 10Be 
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下载免费PDF全文 The Tangra Yum Co graben is one of the active structures that accommodate the east‐west extension of the southern Tibetan Plateau and hosts one of the largest Tibetan lakes, which experienced lake‐level changes of ~200 m during the Holocene. In this study, cosmogenic 10Be is employed to: (1) quantify catchment‐wide denudation rates in fault‐bounded mountain ranges adjacent to the Tangra Yum Co graben; (2) date palaeo‐shorelines related to the Holocene lake‐level decline; and (3) determine the age of glacial advances in this region. The fault‐bounded, non‐glaciated mountain range north of Tangra Yum Co – and presumably most other areas around the lake – erode at low rates of 10–70 mm/ka. Owing to the slow erosion of the landscape, the sediments delivered to Tangra Yum Co have high 10Be concentrations. As a consequence, accurate exposure dating of sediment‐covered terraces and beach ridges is difficult, because the pre‐depositional 10Be concentration may exceed the post‐depositional 10Be concentration from which exposure ages are calculated. This difficulty is illustrated by a rather inaccurate 10Be exposure age of 2.3 ± 1.4 ka (i.e. an error of 60%) for a terrace that is located 67 m above the lake. Nevertheless, the age is consistent with luminescence ages for a series of beach ridges and provides further evidence for the decline of the lake level in the late Holocene. At Tangra Yum Co exposure dating of beach ridges via 10Be depth profiles is not feasible, because the pre‐depositional 10Be component in these landforms varies with depth, which violates a basic assumption of this approach. 10Be ages for boulders from two moraines are much older than the early Holocene lake‐level highstand, indicating that melting of glaciers in the mountain ranges adjacent to Tangra Yum Co has not contributed significantly to the lake‐level highstand in the early Holocene. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
20.
XiangJun Liu ZhongPing Lai QiShun Fan Hao Long YongJuan Sun 《Quaternary Geochronology》2010,5(2-3):218-222
Qinghai Lake is situated in the northeast of the Qinghai-Tibetan Plateau (QTP). Its size and proximity to the junction of three major climate systems make it sensitive to climate changes. Some investigations on shorelines of Qinghai Lake suggested highstands during MIS 3, but to what extent the lake level was higher than today is yet undetermined. Others proposed that the maximum highstands probably dated to MIS 5. It has also been shown that the lake level 120 m higher than today occurred at around 12 ka. Most of these previous ages were obtained using 14C dating or multiple-aliquot IRSL/OSL dating. For 14C dating, because of the dating limit (<40 ka) and the lack of suitable dating materials in this arid area, it is difficult to establish reliable chronological control. In the present study, seven samples collected from lacustrine deposits (five samples) and sand wedges (two samples) were dated using quartz optically stimulated luminescence (OSL) with the single aliquot regenerative-dose (SAR) protocol. OSL dating results showed that (1) the lake had experienced two high lake levels, one was in MIS 5 and another in early to middle MIS 3; (2) no evidence of high lake levels in MIS 4 has been found; (3) the alluvial gravels, whose surface is at an elevation of ～3246 m, were formed at least 28.8 ± 2.3 ka ago, and the widespread sand wedges within the alluvial gravels were formed during the period of 15.1–28.8 ka, which implied that the lake level had not reached an elevation of ～3240 m after 28.8 ± 2.3 ka. 相似文献