A combined luminescence and radiocarbon dating study of the Ili loess,Central Asia

Similar to the loess in the Chinese Loess Plateau (CLP), the loess deposits in the Ili basin of Central Asia arid area play an important role in understanding the climate and environmental changes. However, in contrast to the intensively investigated loess deposits in the CLP, the Ili loess is still insufficiently known and poorly understood. The geochronology study of the Ili loess remains controversial. In order to examine the potential of optically stimulated luminescence (OSL) dating for the Ili loess, we carry out a combined luminescence and radiocarbon dating study on a 6.9 m loess section in the south margin of the Ili basin. Polymineral fine grains were investigated by post infrared (IR) OSL using a Multiple-Aliquot Regenerative-dose (MAR) protocol. Radiocarbon dating of organic carbon were carried in a 3 Megavolt (MV) multi-element Accelerator Mass Spectrometry (AMS). The results indicate that the OSL ages are in agreement with the observed stratigraphy in the field, which is well correlated with that of the CLP, but the AMS ¹⁴C ages are much younger than the OSL and assumed stratigraphical ages. Thus, the OSL dating technique may provide an absolute chronology in this loess section. Further methodological approaches and more samples analysis will lead to the improvement of this chronology for high-resolution paleoclimatic interpretation.     

An analysis of the components of the luminescence signals of selected polymineral and quartz samples from loess in western China and southern Tajikistan,and their suitability for optical dating

Previous luminescence dating studies on loess from China and Tajikistan have focused on the establishment of the regional chronology of the loess sequences. In order to improve the precision and accuracy of optical ages derived from the loess of the last glacial period in these regions, we have examined the components of luminescence signals in three loess samples from western China and southern Tajikistan. Our results show that the polymineral IRSL and post-IR OSL, and quartz OSL signals from loess of the two regions are represented by three components, which display different bleaching and growth characteristics. While the composition of the polymineral IRSL signals is similar between samples with the same age from the two regions, in the case of quartz there is significant discrepancy in the proportion of the fast and medium components of the OSL signals. Greater difference is observed in the composition of the polymineral post-IR OSL signals for the loess from the two regions. The three components of polymineral IRSL signals yield almost identical equivalent dose values as that derived from the total IRSL signal. An apparent agreement in equivalent dose is observed between the fast component of the polymineral post-IR OSL and the quartz OSL for the loess of western China but not in the loess of the same age from southern Tajikistan. The fast component of the quartz OSL yields an equivalent dose 25% higher than that based on the total signal for the sample from the base of the Late Pleistocene loess in southern Tajikistan. This demonstrates the importance of signal selection for an accurate luminescence dating of Central Asian loess.     

Implications of broad dose distributions obtained with the single-aliquot regenerative-dose method on quartz fine-grains from loess

Six samples were collected from a section of Peoria Loess in Eustis, North America, for optically stimulated luminescence (OSL) dating of quartz, and all except one (LV90) produced narrow dose distributions. A comparative study was conducted on this sample and on a ‘well-behaved’ sample (LV91), involving other dating methods and examination of the quartz OSL. These investigations revealed differences in the quartz OSL growth with dose, OSL response to thermal treatments and the range of components within the OSL signals. An ultra-fast component was found in LV90 that displayed a higher rate of sensitivity change than the fast component and this had a malign influence on the determination of the equivalent dose. The distinctive luminescence characteristics of LV90 imply either a change in wind dynamics and/or the source area for the silt.     

A high-resolution quartz OSL chronology of the Talede loess over the past ∼30 ka and its implications for dust accumulation in the Ili Basin,Central Asia

Similar to the loess deposits on the Chinese Loess Plateau (CLP), a reliable chronology plays also an important role in revisiting past climate and environment changes recorded by the loess in Central Asia. Previously, a few luminescence and ¹⁴C ages, mainly covering the last glacial, were obtained for several loess sections in Central Asia, which are often controversial. Until now, there is still a lack of reliable high-resolution chronologies covering the late Pleistocene, especially the Holocene. Here, the fine-grained (4–11 μm) quartz single-aliquot regenerative-dose (SAR) optically stimulated luminescence (OSL) dating protocol is used to establish a detailed chronology of the uppermost part (∼5 m) of Talede loess in the Ili Basin Central Asia. Conventional tests of the SAR protocol and the general luminescence characteristics indicate that this protocol is suitable for dating the Talede loess. Finally, 15 closely-spaced quartz OSL ages, ranging from 0.72 ± 0.05 to 28.9 ± 2.12 ka and with no reversals, were obtained. Consistent with previous studies in the Ili Basin, two of the three ¹⁴C ages show discrepancy when compared with quartz OSL ages for the Talede loess section. The constructed OSL chronology of the Talede loess reveals that the dust accumulation is rapid during marine isotope stage (MIS) 2, especially during the Last Glacial Maximum (LGM), and is also continuous during the Holocene, with the dust accumulation rate exhibiting a relatively low level and an increasing trend. Comparison of the dust accumulation at Talede with that at other sites in the Ili Basin indicates that the rapid dust accumulation during the LGM is not universal, and the slow dust accumulation during the Holocene is probably true for the entire Ili Basin. Comparison of different grain sized quartz OSL ages from Central Asian loess show characteristics of both consistency and inconsistency, which needs to be further investigated.     

Holocene climate changes in westerly-dominated areas of central Asia: Evidence from optical dating of two loess sections in Tianshan Mountain,China

Optical dating was applied to two loess-paleosol sections (Lujiaowan and Shuixigou) from the northern piedmont of Tianshan Mountain, Xinjiang province, China. The two sections are over 200 km apart and have a similar depositional sequence, which consists of two paleosol layers embedded by one loess layer. Two difficulties were met in optical dating. First, because the sections are located on the slope of the mountain, it was found that some cliff debris, with coarse grains (>200 μm), were mixed with the eolian sediments by rainfall, especially in the paleosol layers. Second, the optically stimulated luminescence (OSL) signals of quartz grains from the deposits were too dim to obtain a reliable equivalent dose (D_e). The 63–90 μm K-feldspar grains were separated to decrease the debris portion, and they yielded bright infrared stimulated luminescence (IRSL) signals. A multiple-elevated-temperature post-IR IRSL (MET-pIRIR) procedure was applied to determine D_e. Comparing the optical dating ages of the two sections, the Lujiaowan (LJW) and Shuixigou (SXG) sections recorded almost the same depositional process during the Holocene. The ages of the two loess layers (2.44–3.38 ka at LJW; 2.47–4.36 ka at SXG) suggested that one drought event happened widely in this westerly dominated area. The same drought event 2.5–3.5 ka ago also happened in the Chinese Loess Plateau (CLP), where the summer monsoon dominated. However, the paleosol development period (6.6–4 ka) in the study area was distinguished from the monsoon dominated area (8–4 ka), which suggests an arid early Holocene in the westerly area.     

Interpreting luminescence data from a portable OSL reader: three case studies in fluvial settings

Optically stimulated luminescence (OSL) is a technique that has been applied from the mid‐1980s, mainly for sediment dating. The OSL technique is based on sample stimulation by light to determine the luminescence signal that is stored in the crystal lattices of sediment grains after deposition, burial by later sediment and subsequent exposure to ionizing radiation from radioactive elements in the surrounding sediment. In such sediment dating, two parameters are required: the equivalent dose (i.e. the luminescence signal stored in the grains), and the dose rate (i.e. the ionizing energy from α, β and γ radiation emitted by naturally occurring radioisotopes in the deposit, plus the effects of cosmic radiation). In this research, the OSL technique is not used for dating. The study goal is to analyse only the luminescence signals (the total photon counts) in polymineral samples obtained after light stimulation in the blue (BLSL) and infrared (IRSL) wavelengths using a portable OSL reader designed and built at the Scottish Universities Environmental Research Centre (SUERC). Three fluvial case studies – from Cambodia, Australia and Mexico – are used to illustrate the geomorphological interpretations possible with the portable OSL reader data from sediments resulting from a range of different depositional processes. The case studies show that aspects of sediment's transport and depositional processes can be inferred from the portable OSL reader data, providing valuable insights into geomorphological history. Copyright © 2010 John Wiley & Sons, Ltd.     

黄土细颗粒石英光释光测年中预热温度选取的简易方法

等效剂量预热温度坪实验是光释光(OSL)测年中确定预热温度的常用手段,一般选择坪区温度作为OSL测年的预热温度。对于黄土细颗粒石英,无论采用单片再生法(SAR)还是简单多片再生法(SMAR),以10s为预热时间,其天然和再生剂量预热温度坪区间均为250～280℃,试验剂量的预热温度分布较广,如180～260℃。常规等效剂量预热温度坪实验需要耗费较长的实验室时间。通过对SMAR法测试数据的再分析发现,天然与再生剂量OSL信号校正强度的比值(N/R)随预热温度的变化与常规等效剂量坪实验中等效剂量随预热温度的变化有着相似的趋势,这样,N/R预热温度坪就可作为黄土样品细颗粒石英等效剂量预热温度坪的替代,为实验室OSL测年确定预热温度提供一个简捷途径     

Maximum age limitation in luminescence dating of Chinese loess using the multiple-aliquot MET-pIRIR signals from K-feldspar

Numerical dating of loess is important for Quaternary studies. Recent progress in post-infrared infrared-stimulated luminescence (pIRIR) signals from potassium-rich feldspar has allowed successful dating of Chinese loess beyond the conventional dating limit based on quartz optically stimulated luminescence (OSL) signals. In this study we tested the multiple-aliquot regenerative-dose (MAR) pre-dose multiple-elevated-temperature post-IR IRSL (pMET-pIRIR) procedure on samples from the palaeosol S5 (∼480 ka) and S8 (∼780 ka) layers from the Luochuan and Jingbian sections, respectively. The results show that (1) compared to sensitivity-corrected signal (L_x/T_x), a higher saturation dose is observed for the sensitivity-uncorrected MET-pIRIR signals (L_x), suggesting that MAR is advantageous for dating old samples; (2) negligible fading component can be achieved using the pMET-pIRIR procedure; (3) for the sample from the top of palaeosol S5, D_e values (1360 + 226/-167 Gy) broadly consistent with expected D_e (1550 ± 72 Gy) can be obtained using the sensitivity-uncorrected 300 °C MET-pIRIR signal. Our study suggests that a D_e value of about 1800 Gy may be the maximum dating limit of Chinese loess using the MAR pMET-pIRIR procedure.     

城市活断层探测中钻孔岩心光释光样品的采集

钻孔岩心测年样品具有较一般地质样品更为复杂的影响因素,样品特征对年龄可靠性有非常重要的影响,且主要受钻进方法和采样过程的制约。采用回转钻进取全心的方法,对光释光测年带来的不确定性影响因素主要有扰动导致软弱土层、砂层的混染;泥浆渗入对样品的污染;挤压和摩擦生热对释光信号的可能影响等。除样品特征外,含水量的变化及放射性平衡状况等对钻孔岩心样品的年龄可能存在影响。释光测年样品的基本要求、测试技术和方法是野外正确采样的依据,针对城市活断层探测工作中钻探任务的一般要求,文中提出了岩心光释光样品采集的建议,并对年代测试时需要考虑的不确定性因素进行了讨论     

Optical dating of aeolian and fluvial sediments in north Tian Shan range,China: Luminescence characteristics and methodological aspects

Loess and fluvial sand are important materials for dating river terraces and alluvial fans. This study focuses on the methodological aspects of dating loess and fluvial deposits from the northern flank of the Tian Shan range, China, using sand-sized quartz and potassium (K) feldspar. Luminescence characteristics of quartz and K-feldspar were studied for searching suitable dating procedures. Our results indicate that 1) most quartz aliquots were contaminated by feldspar, and were dated using a post-infrared optically stimulated luminescence (post-IR OSL) procedure. A Fast ratio acceptance threshold of 15 can be applied to select these aliquots with post-IR OSL signals dominated by quartz OSL; 2) the multi-elevated-temperature post-IR IR stimulated luminescence (MET-pIRIR) procedures are applicable for K-feldspar. A test dose of ∼30% of the natural dose is appropriate for dating of older (>10 ka) samples. An Age (T, t) plateau test can be used to evaluate the dating results; 3) for the loess samples, both quartz and K-feldspar were well bleached and are suitable for dating. Dating using K-feldspar is preferred for its higher efficiency; 4) for the fluvial sand samples, only the quartz grains were fully bleached. Single-aliquot dating of quartz gives reliable ages.     

Optical dating of Chinese loess using sand-sized quartz: Establishing a time frame for Late Pleistocene climate changes in the western part of the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) is of major interest to Quaternary geologists because it represents an important terrestrial archive of palaeoclimatic fluctuations. Previous multiple-aliquot luminescence dating studies of Chinese loess mainly used thermoluminescence (TL) and infrared stimulated luminescence (IRSL) signals of polymineral fine-grains; these are known to be subject to anomalous fading and thus will tend to yield age underestimations. In this paper we investigate whether the blue-light stimulated luminescence (BLSL) signals from 63 to 90 μm quartz grains extracted from three western Chinese loess sites (Zhongjiacai, Le Du and Tuxiangdao) can be used to establish a reliable chronology. The single-aliquot regenerative-dose (SAR) procedure is used for the equivalent dose (D_e) determinations and the suitability of our measurement protocol is confirmed by dose recovery tests. The influence of an IRSL signal on the quartz D_e measurements derived from BLSL has been investigated. From these results we conclude that an IRSL contamination, expressed as an IRSL/BLSL ratio, of up to 10% can be accepted before the values of D_e are significantly affected. All three sites yield stratigraphically consistent and spatially highly reproducible optical ages up to about 50–70 ka. At the Tuxiangdao site a marked hiatus in the record is identified between ∼20 and ∼30 ka; this remained undetected in previous studies and clearly highlights the importance of high-resolution optical dating in Chinese loess research. The optical ages presented in this work provide more evidence for episodic loess deposition and varying loess accumulation rates in the western part of the CLP. Our study seems to confirm the potential of optically stimulated luminescence (OSL) dating using the SAR procedure applied to the very fine sandy quartz fraction in Chinese loess back to ∼40–50 ka (∼120–150 Gy).     

High resolution dating of loess profile from Biały Kościół, south–west Poland

Results of high resolution optically stimulated luminescence (OSL) dating of one of the most complex loess/palaeosol complex in Poland located in the vicinity of the village Biały Kościół (south–west Poland) are presented. Twenty one samples were collected from the 9-m loess profile and dated using the single-aliquot regenerative method applied to coarse grained quartz fraction (90–125 μm). The obtained single-aliquot results were analysed using the central age model. OSL ages are in good agreement with the chronology inferred from the marine oxygen isotope (MOIS) stratigraphy and ages reported for similar lithostratigraphic units for other loess profiles in Europe. However, for the deepest part of the profile underestimation of OSL is observed.     

Optical dating of Romanian loess using fine-grained quartz

Romanian loess -palaeosol sequences are amongst the thickest and most complete available in Europe. These deposits represent an extended continental record of environmental and climatic change during at least five glacial/interglacial cycles. Their chronology, however, is mainly based on relative methods. In this paper, we investigate whether SAR–OSL dating of fine-grained quartz can be used to establish a reliable chronology for Romanian loess.The samples were collected from the loess–palaeosol sequence near Mircea Vod? (Dobrogea, SE Romania). The luminescence characteristics of the fine-grained quartz extracts are investigated to some extent, and indicate that the applied laboratory measurement procedure (SAR) is reliable. An internally consistent set of optical ages is obtained for the loess deposited up to ～70 ka, and evidence is presented for a varying loess accumulation rate during the Last Glacial. Comparison with independent age control (pedostratigraphy and a newly-developed palaeomagnetic time–depth model) indicates that the optical dating procedure underestimates the true burial age from the penultimate glacial period onwards (i.e. for samples below the last interglacial S1 palaeosol). These results indicate that an apparently reliable laboratory measurement procedure not necessarily yields accurate sedimentation ages. We suggest that quartz-based SAR–OSL ages obtained using the high dose linear region of the growth curve are interpreted with caution.     

Using three different approaches of OSL for the study of young fluvial sediments at the coastal plain of the Usumacinta–Grijalva River Basin,southern Mexico

We use three different approaches of optically stimulated luminescence (OSL) to study young fluvial sediments located at the main channels of one of the largest fluvial systems of North America: the Usumacinta–Grijalva. We use the pulsed photo‐stimulated luminescence (PPSL) system also known as portable OSL reader, full OSL dating and profiling OSL dating in samples extracted from vertical sediment profiles (n = 9) of riverbanks to detect changes in depositional rates of sediments and to obtain the age of the deposits. The results of the PPSL system show that the luminescence signals of vertical sediment profiles highly scattered from the top to the bottom contrast with the luminescence pattern observed on well‐reset sequences of fluvial deposits where luminescence increase from the top to the bottom of the profile. The profiling and full OSL ages yielded large uncertainty values on their ages. Based on the inconsistencies observed in both ages and luminescence patterns of profiles we suggest that these fluvial deposits were not fully reset during their transport. As an explanation, we propose that in the Usumacinta and Grijalva rivers the cyclonic storms during the wet season promote the entrainment of large volumes of sediments due to high‐erosional episodes around the basin resulting from hyper‐concentrated and turbid flows. We conclude that the PPSL, profiling and full OSL dating of sediments are useful tools to quantify and to assess the depositional patterns in fluvial settings during the Holocene. These techniques also can yield information about sites where increases in the sediment load of rivers may produce poorly resetting of grains affecting the results of OSL dating. Copyright © 2015 John Wiley & Sons, Ltd.     

Evaluating OSL-SAR protocols for dating quartz grains from the loess in Ili Basin,Central Asia

Late Pleistocene records of loess deposition are a critical archive for understanding terrestrial paleoenvironment changes in Central Asia. The age of loess is not well known for the deserts regions and surrounding high plateaus in Central Asia. Previous studies have shown that there remains a disparity between ages for loess deposition by luminescence and ¹⁴C dating. This study evaluates the potential of optically stimulated luminescence (OSL) to date a loess sequence resting on fluvial sands in the east Ili Basin, Central Asia. The single-aliquot regenerative-dose (SAR) protocol on coarse grain quartz was employed for equivalent dose determinations. The basal fluvial sand returned a secure OSL age, with low overdispersion value in equivalent doses (19 ± 2%) of ca. 36 ka and provides a close, but maximum age estimate (within 5 ka) on the initiation of loess deposition. However, the loess yielded high overdispersion values for equivalent doses and age reversals, coincident with diffuse paleosols; indicating that pedoturbation with loess deposition may be a dominant process. OSL ages between ca. 45 and 14 ka calculated using a maximum age model and OSL ages from other sites in the Basin suggests that the latest major period of loess deposition was between 70 and 10 ka ago. A future hypothesis to test based on these analyses is that there may be three periods of heightened loess deposition at ca. 45, 35 to 19 and 14 ka, when desert source areas to the west were particularly dry.     

The minimum extraction technique: A new sampling methodology for optically stimulated luminescence dating of museum ceramics

This paper introduces a novel optically stimulated luminescence (OSL) sampling protocol, referred to as the minimum extraction technique (MET), which assists in the OSL dating of museum ceramics. Here we outline how to extract a sample using this technique, as well as offering a discussion on the preparation protocols used for the method, and the calculation of the internal and external dose rates. A major benefit of MET is the minute sample size required, resulting in minimal damage to the museum object.MET permits OSL dating of material which would otherwise be usually off-limits for conventional OSL dating, for example, those housed in museum collections. In particular, MET can be used on objects which originate from regions where scientific analysis is currently not permitted on archaeological material, often owing to heritage laws.     

细颗粒石英光释光测年:简单多片再生法

文中报道了细颗粒石英光释光测年中测量等效剂量的一种可靠技术——简单多片再生法。该技术的核心是引入单片再生法中试验剂量校正感量变化的功能,即在天然和再生多测片光释光信号测量后,再对各测片辐照以试验剂量,利用试验剂量的光释光信号响应对各测片归一化,同时校正可能发生的感量变化。从方法学上,这一技术具有以下优点1)试验剂量可以校正各测片的感量变化;2)对各测片归一化,克服了实验数据分散度的问题;3)通过对比简单多片再生法与单片再生法在测年中的表现,证实简单多片再生法可以克服单片再生法中可能发生的光释光信号的积累问题。最后,文中通过测定参考年龄样品,即洛川剖面末次冰期马兰黄土(L1)的开始堆积年龄,验证了简单多片再生法的可靠性,即在提高测量准确度和精度的同时,更节约时间     

Luminescence dating of cyclone-induced washover fans at Point Lefroy (NW Australia)

Reliable age dating of coastal sedimentary landforms is crucial for inferring storm frequencies and magnitudes from geological archives. However, in highly energetic coastal settings, radiocarbon dating is often biased by reworking and/or poorly constrained marine reservoir effects. Due to this, most cyclone-driven sediment archives from the semiarid coast of NW Australia – a region frequently affected by tropical cyclones but with a historical record limited to ∼150 a, and therefore strongly in need of long-term data inferred from geological evidence – are affected by chronological inaccuracies. Optically stimulated luminescence dating (OSL) may overcome these shortcomings by dating the transport of sediment directly. In turn it may be related to other challenges when applied to cyclone deposits from semiarid environments. The cyclone-induced washover fans at Point Lefroy, NW Australia, are composed of a heterogeneous mixture of coral fragments, shell hash and siliciclastic sand. This makes them particularly prone to high dose scatter resulting from a combination of partial bleaching, sediment mixing and dose-rate heterogeneity. The washover fans are further characterised by a discontinuous nature of cyclone deposition, as indicated by erosional features and macroscopic brunification horizons. By using a combination of quartz single grain dating, autoradiography, alpha counting and gamma spectrometry, sediment mixing and dose rate heterogeneity are identified as the main sources of dose scatter. The resulting chronology allows us to discriminate at least four well constrained phases of washover fan activity at ∼180, ∼360, ∼870, and ∼1300 a ago. Older but less well constrained activity phases occurred ∼1950, ∼2300, and ∼2830 a ago. While these phases of increased cyclone activity correlate with depositional units separated by potential palaeosols, OSL ages, quasi-continuous portable OSL reader measurements and gamma spectrometry measured with increased sampling resolution point to deposition of distinct washover units within a very short period of time. However, unambiguous discrimination between deposition of individual units by single events and deposition by several cyclones within periods of only a few decades is currently not possible.     

Luminescence dating of irrigation systems: Application to a qanat in Aragón,Spain

Optically stimulated luminescence (OSL) techniques have been applied to investigate the potential for dating the deposition of upcast mounds associated with qanat ventilation shafts at the site of a medieval qanat located in Aragón, Spain. Coarse quartz grains, extracted from sediment samples taken from excavated sections of several mounds, possessed sufficiently strong OSL to enable an evaluation of equivalent dose by applying the single aliquot regenerative procedure to small aliquots, each containing an individual bright grain. The OSL dates for both palaeosol and overlying upcast indicate that a chronostratigraphic record has been preserved within the mounds investigated, and micromorphological analysis of thin sections of sediment blocks taken from the mounds is shown to provide an essential means of verifying the characteristics of the strata, in particular, the critical interface of upcast and the ancient ground surface. The earliest OSL dates for basal deposits taken from two separate sections of the same mound are in agreement, placing the mound construction during the first half of the 13th century A.D. However, in two other mounds the OSL dates for the deposition of upcast are internally consistent with the stratigraphy but significantly later, dating to the 16th and 17th centuries A.D. We interpret the differences between the dates for the upcast deposition to be the result of partial erosion of the upper shaft and later repair of the mounds, and this finding underlines the importance of both examining multiple mounds in the same qanat system and the internal structure of each sampled mound. This exploratory work demonstrates the potential for wider application of OSL for dating this important type of subterranean irrigation feature in the study of both the archaeology of human settlement and palaeoenvironmental change in arid regions.