Resetting of sediments mobilised by the LGM ice-sheet in southern Norway

Former geological field investigations in the Rondane area, east-central southern Norway, have proposed that the maximum Fennoscandian ice-sheet coverage occurred during the Late Weichselian Glacial Maximum (LGM, ca. 20 ka) and that subsequent glaciofluvial sediments were first deposited in the early Holocene (after 10 ka). However, recent field investigations with ages from three internally consistent quartz optically stimulated luminescence (OSL) age series show an apparent deglaciation of northern Rondane in the period 20.0–13.8 ka. We examine here the possibility that these ages are too old because the sediment was not completely zeroed prior to deposition. Our investigations of incomplete bleaching use modern analogues, small aliquots, and single grains of quartz. First, the symmetric shape of small aliquot equivalent dose distributions suggests that the sediment was probably well bleached at deposition. This is supported by 5 modern analogue equivalent doses (D_e) of 0.6 Gy, 1.5% of the typical D_e from the deglaciation sediments. Finally, from single grain studies on three samples, we conclude that there is no evidence for poor bleaching in these samples; thus the weighted mean gives the best estimates of D_e, and these are completely consistent with both large and small aliquot estimates for these samples. These comparisons between large aliquots, modern analogues, small aliquots and single-grain analyses help to validate the OSL ages and confirm the complete resetting of these sediments prior to deposition.     

Chronology of an oyster reef on the coast of Bohai Bay, China: Constraints from optical dating using different luminescence signals from fine quartz and polymineral fine grains of coastal sediments

Fossil oyster reefs are indicators of past sea levels, and their formation is usually dated by means of radiocarbon. However, radiocarbon dating of the shells from coastal areas may be complicated by the varying sources of carbon. Here we applied optical dating methods to date the samples from above and below a fossil oyster bed in a section on the coast of Bohai Bay, China. The optical ages of the sediments were used to constrain the oyster bed. Single-aliquot regenerative-dose procedures using the OSL signal from fine grain quartz, the IRSL and post-IR OSL signals from polymineral fine grains were employed to determine equivalent dose (D_e). The behaviors of the different luminescence signals from quartz and polymineral grains during D_e measurements were examined. The results showed that the quartz OSL signal is more reliable than the polymineral IRSL and post-IR OSL signals with respect to dating for these coastal samples. The optical ages indicated that the oyster reef formed between ca. 6.2 and 5.0 ka.     

Statistical treatment of fluvial dose distributions from southern Colorado arroyo deposits

Many of the small-scale arroyo systems found across southern Colorado contain well-preserved sedimentary records of prehistoric fluvial erosion and aggradation epicycles. In the following paper, we date a set of 50 ephemeral fluvial samples from four southern Colorado arroyo systems using a combination of single-grain and single-aliquot OSL techniques. Analysis of the sample D_e distribution characteristics reveals that these arroyo sediments were subjected to a diverse array of bleaching conditions prior to deposition. The use of appropriate burial dose estimation procedures is therefore deemed vital to ensuring that accurate age estimates are produced for each of these samples. In this study we apply the formal ‘age model’ decision procedures of Bailey and Arnold [Statistical modelling of single-grain quartz D_e distributions and an assessment of procedures for estimating burial dose. Quaternary Science Reviews, in press.] and Arnold [2006. Optical dating and computer modelling of arroyo epicycles in the American Southwest. D.phil. Thesis, University of Oxford, unpublished] to our fluvial sample dataset in order to enable a more objective selection of appropriate burial dose estimates. The resultant OSL chronostratigraphies are examined and discussed. These formal decision procedures yield sample ages that are stratigraphically consistent for 94% of the 50 fluvial samples examined. The resulting OSL ages also display a greater degree of stratigraphic consistency in comparison to those ages that would have been generated by simply applying a single type of age model to all samples.     

Problems associated with luminescence dating of Late Quaternary glacial sediments in the NW Scottish Highlands

Linking the timing of glacial episodes and behaviour to climatic shifts that are documented in ice and marine sedimentary archives is key to understanding ocean-land interactions. In the NW Scottish Highlands a large number of closely spaced (‘hummocky’) moraines formed at retreating glacier margins. Independent age control on one palaeo-glacier limit is consistent with the timing of Younger Dryas (YD) glaciation in the area, but adjacent glacier lobes have remained undated due to the lack of sites and material for ¹⁴C dating. Direct dating of ice-marginal moraines using optically stimulated luminescence (OSL) techniques has never been attempted before in Scotland, but if successful, they may be the most appropriate methods for constraining the age of sediment deposition in the absence of organic material. Coarse-grained quartz and K-feldspar minerals from supraglacial sheet flow deposits and glacilacustrine sediments within ice-marginal moraines were analysed using the single-aliquot regenerative-dose (SAR) protocol. Independent age control and clear geomorphological relationships indicate that all samples should yield YD or post-Last Glacial Maximum (LGM) ages. Quartz OSL shine down curves showed low luminescence sensitivity, significant medium-to-slow components, a weak fast component, and scattered SAR data; Linearly Modulated-OSL (LM-OSL) measurements confirmed that the fast component was weak or absent. In contrast, feldspar infrared stimulated luminescence (IRSL) was highly sensitive with excellent SAR data. However, SAR data from 3 mm diameter aliquots of feldspar (200 grains) give higher than expected equivalent doses (D_e) by an order of magnitude. SAR measurements of small clusters of feldspar grains (ranging from 1–8) considerably broaden the apparent D_e distribution, but even the lowest value is about 2–3 times the expected D_e. Two possibilities arise to explain the quartz and feldspar data: (1) that glacial sequences in the NW Highlands re-work inherited (older) glacial deposits and that some of the pre-Devensian existing glacial landforms have only been modified during Devensian glaciation; or (2) that the sedimentary processes operating in these ice-marginal environments are not conducive to adequate bleaching of quartz and feldspar grains. Our study implies that ice-proximal supraglacial sediments from this region in NW Scotland reflect older ages of deposition, but dating YD sediments has not been possible.     

Developing a 150 ka luminescence chronology for the barrier dunes of the southern Cape, South Africa

The southern coastline of South Africa exhibits extensive aeolianites and spectacular barrier dunes; the ages and palaeoenvironmental significance of which have, until recently, received limited attention. We present preliminary results for samples taken from a single 6.5 m section in the seaward barrier dune near to the town of Wilderness. The reliability of these ages has been investigated using a series of dose recovery and pulsed irradiation experiments, as well as through analyses of single aliquot and single grain D_e distributions. Results indicate that the SAR protocol is providing accurate and precise D_e determinations and that adequate sensitivity correction is being made. The application of pulsed irradiations does not result in significant changes in the dose–response curves or averaged D_e's. Single grain D_e distributions are broad, with significant levels of overdispersion, which may reflect beta dose heterogeneity effects.

Peaks in aeolian activity date to 157–154 and 140–125 ka, with a slower rate of deposition recorded from 115 to 85 ka. Brief phases at 40 and 21 ka are also identified. Some ages, notably those from OIS 3 and OIS 2, are surprising from a geomorphological perspective, given the likely distance of the dune from the shoreline at such times, although they are not inconsistent with evidence from the east coast of South Africa. The results reveal multiple phases of Late Quaternary barrier dune accumulation, and suggest that aeolian activity in this region has been episodic and potentially more complex than previously thought.     

The effect of weathering on optically stimulated luminescence dating

Optically stimulated luminescence (OSL) dating was applied to quartz samples recovered from the gravel sediments of deformed marine terrace at the Suryum site, southeastern Korea, but has yielded stratigraphically inconsistent ages. Micromorphological, chemical, and mineralogical analyses of the gravels indicate the possible influence of chemical weathering on the apparent OSL ages. In the course of weathering after emerging from beach environments, originally permeable gravels underwent infilling by halloysite-rich clays in their voids, export and import of radioactive elements, and loss of mechanical strength with the creation of a porosity within pebbles. Calculation based on the evolution of the weathering profile showed a severe fluctuation of dose rate caused by the change of water content and radioactive element concentrations. Samples recovered from the weathering profile inevitably include quartz grains derived from mechanically weakened pebbles or illuviated from the upper layer, making it difficult to determine the equivalent dose (D_e) values of exclusively depositional quartz sands. Quantitative evaluation of the change of fabric and chemistry of the sediments on the basis of pedologic insight significantly aids in the derivation of OSL ages consistent with geomorphology and other independent age controls.     

OSL dating of mixed coastal sediment (Sylt,German Bight,North Sea)

As part of a study on coastal sedimentary processes this paper presents the OSL dating of mixed coastal sediment samples from the southern North Sea island of Sylt (German Bight). During coring of the swash-bar (beach) sediments, five samples were presumably contaminated by younger overwash and aeolian sediments because of the sampling method employed. To obtain reliable burial ages for these swash-bar sediments, single-grain and small aliquot measurements were used together with the Finite Mixture Model (FMM) proposed by Roberts et al. (2000) to identify the grain population containing the largest doses (from the deepest part of the core). Before the FMM was applied to dating, the parameters and performance of the FMM were first investigated by systematically comparing small aliquot (∼20 grains) and single-grain measurements of an undisturbed aeolian and swash-bar sample and a laboratory mixture of both sediments. This test case demonstrates the advantage of selecting the time interval immediately following the initial luminescence signals for background subtraction because unsuitable quartz grains were removed from the dose distribution. It is concluded that the measurement of small aliquots can be regarded as a reliable proxy for single-grain dose distribution if the sediment contains only a small proportion of quartz grains emitting a luminescence signal and that the FMM results are relatively insensitive to changes of the over-dispersion parameter between 5–40% for small aliquots and 10–40% for single-grains.We show that the burial ages of the contaminated swash-bar samples resulting from the maximum age populations from equivalent dose distributions measured using small aliquots are consistent with the stratigraphy and with ages obtained from uncontaminated samples.     

Dating of lower terrace sediments from the Middle Rhine area, Germany

Modern and known-age Pleistocene fluvial sediments were investigated by optical dating of quartz to test the suitability of the approach for dating deposits from the deeply incised Middle Rhine Valley. Samples from modern flood sediments revealed skewed distributions indicating different residual levels of equivalent dose (D_e) within the different aliquots. Nevertheless, a substantial number of aliquots from the modern deposits reflect D_e values close to zero. For the Pleistocene samples, optical ages are in general consistent with age control given by the presence of the Laacher See Tephra and radiocarbon dating. However, some samples overestimate the known age by a few thousand years when using the arithmetic mean. This is apparently explained by including aliquots in the determination of mean D_e where the optical signal was incompletely bleached at deposition. The most difficult issue in this context is identifying a suitable approach that can distinguish between the variability of D_e due to partial bleaching and microdosimetry. However, even when considering these limitations it appears that optical dating will by a quite suitable method to date Pleistocene sediments from such a complex fluvial environment, especially when focusing on a precision scale beyond a few thousand years.     

Quartz and polymineral luminescence dating of Japanese loess over the last 0.6 Ma: Comparison with an independent chronology

There are only a few luminescence dating studies of loess sediments in Japan, but interleaved with these deposits are many well-described tephras of known age based on ¹⁴C and fission track analysis; these independent age controls provide an opportunity to test the reliability of loess luminescence ages. This study provides such a comparison at two sites in central Honshu, the largest island in the Japanese archipelago. Samples were collected from sequences of interleaved volcanic tephra and loess deposited on a Middle Pleistocene river terrace in the Niigata Prefecture and on an Early to Middle Pleistocene dissected fluvial surface at in the Tochigi Prefecture, Honshu. Equivalent doses (D_e) were estimated from fine grains (4–11 μm) using both polymineral IR-OSL and (post-IR) blue-OSL, and quartz blue-OSL. The blue-stimulated luminescence signals could be represented by up to three exponentially decaying components; only the most light sensitive of these components was used in the final D_e estimation. Almost all the estimates of D_e from polymineral IR-OSL are smaller than those from polymineral (post-IR) blue-OSL and quartz blue-OSL, whereas the latter two are in good agreement. The blue-light stimulated luminescence ages using the most light sensitive component are shown to be in good agreement with the independent control, up to 500 ka. Although the IR-OSL signals underestimate the known age, a simple laboratory fading test is found empirically to correct for this underestimation. We conclude that the most rapidly blue-stimulated luminescence signals from quartz extracted from our loess samples give reliable ages, and that future loess dating studies should concentrate on these signals.     

ESR dating of sedimentary quartz: Possibilities and limitations of the single-grain approach

Usually, multiple-grain aliquots are used for electron spin resonance (ESR) dating of sediments. However, this approach excludes the ability of detecting insufficient bleaching, as would be the case when measuring several single aliquots or single grains. In this paper, we present preliminary results of single-grain ESR dating experiments on three different sedimentary deposits (fluvial, fluvio-aeolian and desert aeolian), ranging in age between 100 ka and 2 Ma. Titanium-related impurity centres (Ti–Li and Ti–H) were measured in a Q-band ESR spectrometer for estimating equivalent doses (D_e) of individual quartz grains (0.5–1 mm). Both additive and regenerative dose methods were used. The resulting D_e plots show a large scatter in D_e—from 100 Gy to more than 600 Gy—for the fluvial sample, probably reflecting different bleaching histories of the grains. On the contrary, a clear plateau can be observed in D_e for sands with a strong aeolian component. Preliminary single-grain ESR ages are in relatively good agreement with independent estimates, the best results being obtained for the desert aeolian sample (200 ka). It is concluded that ESR has a promising potential for estimating large naturally accumulated doses of well-bleached sediments. Nevertheless, the method is challenged by several experimental difficulties, such as long measurement times, excessive errors, and presumably a complex interrelationship between Ti–Li and Ti–H components.     

OSL dating of sediments from deserts in northern China

Aeolian samples from deserts in northern China were studied using the optically stimulated luminescence (OSL) of quartz and potassium (K) feldspar separates. K-feldspar has shown advantages over quartz for dating samples younger than 100 years, due to its more homogeneous equivalent dose (D_e) values, higher internal dose rate and higher luminescence sensitivity to dose. Without additional measurements, the luminescence characteristics of quartz obtained in D_e determination procedures can be used to reveal the source material and/or quartz thermal history for the deserts in northern China. The results indicated that the duration of paleosol formation in desert areas reflects regional paleoenvironments.     

Reliability of equivalent-dose determination and age-models in the OSL dating of historical and modern palaeoflood sediments

The challenge of accurately estimating the deposition age of incompletely-bleached samples in luminescence dating has motivated developments in the analysis of single grain dose distributions, and a number of statistical approaches have been proposed over the last few years. In this study, we compare the behaviour of the arithmetic average, so-called ‘robust statistics’, the Central Age Model (CAM), the Minimum Age Model (MAM) and the Internal–External Consistency Criterion (IEU) and lowest 5% method, when applied to single-grain dose distributions from a sequence of eight recent (40–1000 years) flash-flood deposits. These sediments are expected to be incompletely bleached, but all have age control from historical records. Modifications were made to allow the use of the standard CAM and MAM models with dose distributions containing near zero and negative dose values. An assessment of minimum over-dispersion (OD) is based on dose recovery tests based on gamma-irradiated samples. We then present a detailed analysis of the effect of an additional uncertainty added to the individual dose estimates on the burial dose estimates from the MAM and the IEU approach. The results of the various models are discussed in terms of the accuracy of the resulting age, and we conclude that, overall, the IEU approach generates the most accurate ages. We also demonstrate that accurate doses can be obtained for those older samples for which uncertainties of ∼40 years are unimportant by applying the IEU model to small aliquot (∼30 grains) dose distributions. From our study we conclude that these and similar young slack-water flood deposits can be accurately dated using quartz OSL, opening up the possibility of establishing time series of flood discharge in catchments for which no instrumental or historical record exists.     

Developments in optically stimulated luminescence age control for geoarchaeological sediments and hearths in western New South Wales,Australia

Research conducted by the Western New South Wales Archaeology Program (WNSWAP) provides the opportunity to assess the reliability of optically stimulated luminescence (OSL) dating of late Pleistocene and Holocene fluvial sediments and burnt stone samples from arid zone geoarchaeological contexts. A large number of radiocarbon age determinations of charcoal preserved in heat retainer hearths provides independent chronological control at these contexts. We describe a rapid OSL methodology for dating burnt hearth stones to complement previously applied radiocarbon methods, which we have tested using 37 samples from hearths with radiocarbon determinations. We propose a geoarchaeological model in which these hearths were constructed by people whose activity took place on an archaeological surface, formed by the earlier deposition of fluvial sediments. Here we demonstrate the veracity of this model by dating sediments lying stratigraphically below the hearths, and use the radiocarbon age control and chronological consistency to assess the accuracy and reliability of both small aliquot and single grain single aliquot regenerative-dose (SAR) OSL dating. While small aliquot age estimates are in most cases in agreement with independent control, the single grain determinations using a finite mixture model (FMM) appear to provide improved chronological resolution. Using single grains, we note some problems in the application of the FMM and in the dating of young samples in the range of 1–100 years. As many samples may have resided close to the surface since deposition, we have developed a mathematical function to describe gamma and cosmic dose rate contributions at burial depths down to 40 cm. These OSL age estimates allow us to reject the model of intensification of human activity as responsible for the observed pattern of archaeological radiocarbon determinations in this part of the Australian arid zone.     

Potential of quartz OSL dating on moraine deposits from eastern Tibetan Plateau using SAR protocol

Optically stimulated luminescence (OSL) dating methods have been widely applied in Quaternary glaciology. However, glacigenic deposits are considered in general as problematic for OSL dating, mainly because they are transported shorter distances prior to burial and are usually partially bleached. Thus, most researchers choose glaciofluvial and glaciolacustrine sediments (with relatively longer transportation times) for OSL dating when constraining the age of glaciation. In this study, four samples were collected from a lateral moraine series at Zhuqing Village, northern margin of Queer Shan Mountain, eastern Tibetan Plateau, in order to investigate the applicability of OSL dating for morainic deposits. Quartz grains (38–63 μm) were extracted and measured using single aliquot regenerative-dose (SAR) protocol. Internal checks and dose recovery test show that the SAR protocol is appropriate for equivalent dose (D_e) determination. The effect of thermal transfer is small for all samples and the recycling ratio for each individual sample is close to unity. The symmetry in the D_e distributions indicates that quartz grains were well-bleached prior to burial. OSL ages show good agreement with geomorphological and field investigations, and are also concordant with an independent ESR age. It is concluded that: (a) the morainic deposits in Zhuqing were well-bleached and suitable for OSL dating; (b) SAR protocol can be applied to morainic deposits for samples under study.     

An optical age chronology of late Quaternary extreme fluvial events recorded in Ugandan dambo soils

There is little geochonological data on sedimentation in dambos (seasonally saturated, channel-less valley floors) found throughout Central and Southern Africa. Radiocarbon dating is problematic for dambos due to (i) oxidation of organic materials during dry seasons; and (ii) the potential for contemporary biological contamination of near-surface sediments. However, for luminescence dating the equatorial site and semi-arid climate facilitate grain bleaching, while the gentle terrain ensures shallow water columns, low turbidity, and relatively long surface exposures for transported grains prior to deposition and burial. For this study, we focused on dating sandy strata (indicative of high-energy fluvial events) at various positions and depths within a second-order dambo in central Uganda. Blue-light quartz optically stimulated luminescences (OSL) ages were compared with infrared stimulated luminescence (IRSL) and thermoluminescence (TL) ages from finer grains in the same sample. A total of 8 samples were dated, with 6 intervals obtained at 35, 33, 16, 10.4, 8.4, and 5.9 ka. In general, luminescence ages were stratigraphically, geomorphically and ordinally consistent and most blue-light OSL ages could be correlated with well-dated climatic events registered either in Greenland ice cores or Lake Victoria sediments. Based upon OSL age correlations, we theorize that extreme fluvial dambo events occur primarily during relatively wet periods, often preceding humid-to-arid transitions. The optical ages reported in this study provide the first detailed chronology of dambo sedimentation, and we anticipate that further dambo work could provide a wealth of information on the paleohydrology of Central and Southern Africa.     

Optical dating of Holocene lake sediments: Elimination of the feldspar component in fine silt quartz samples

Optically stimulated luminescence of fine silt quartz was investigated to establish chronologies for lake sediments from the British Isles. Feldspar contamination and its potential effects on equivalent dose (D_e) determination were examined. A new feldspar contamination test based on the thermal quenching of quartz OSL and thermal assistance of feldspar OSL identified feldspar contamination in samples being treated for 20 min in 20% hydrofluoric acid. Feldspar component monitoring based on the post-IR OSL/OSL ratio was not sufficiently sensitive to monitor a feldspar component, but is a good indicator whether a standard SAR procedure can be applied successfully, despite the presence of feldspar. Based on these results we used a modified SAR protocol including a 100 s IRSL stimulation at 50 °C before OSL readout. This protocol could overcome any malign effects of feldspar even for a feldspar bearing polymineral sub-sample.     

RESIDUAL POST-IR IRSL SIGNALS OF POTASSIUM FELDSPAR FROM MODERN SAG POND DEPOSITS OF CENTRAL ALTYN TAGH FAULT: IMPLICATION FOR DATING YOUNG PALEOSEISMIC EVENTS

The Altyn Tagh Fault(ATF)is one of the most prominent active strike-slip faults in the India-Eurasia collision. Fresh features of surface ruptures, which are attributed to seismic events taking place in the last millennium, are identified at several sites along the Che'erchen River to Qingshui River section on the central part of ATF. Accurate chronology of these earthquake events would help understand the spatial-temporal relationship of the recent earthquakes. However, great difficulties are encountered. The central ATF is located in the arid area, and the vegetation cover is so limited that rare organic materials appropriate for radiocarbon dating can be found in the sediments. Luminescence dating technique may serve as an alternative to directly determine the burial ages of the earthquake related sediments. The optically stimulated luminescence(OSL)signal of quartz, which has been widely employed for luminescence dating, displays unwanted charateristics for accurate dating. Firstly, the quartz OSL signal is not sensitive to irradiation, which leads to low signal-to-noise ratio or even no measurable quartz OSL signal. Secondly, the targeted samples of the last millennium are very young, and the radiation dose received during the burial is expected to be less than 3~4Gy, which futher deteriorates the signal-to-noise ratio of the quartz OSL signal. Therefore, quartz OSL signal is not appropriate for dating the sediments relevant to the recent earthquakes on ATF.
The infrared stimulated luminescence(IRSL)signal of potassium feldspar is an alternative, and it is in usual an order of maginitude more sensitive to raidation than the quartz OSL signal. The enhanced signal-to-noise ratio makes it applicable to young samples. The post-IR IRSL signal has been successfully applied to date the sediments beyond the Holocene, however, the relatively slow bleaching of the post-IR IRSL signal poses challenges on applying it to young sediments, especially for the sediments deposited during the last millennium. In this study, we investigated the feasibility of using post-IR IRSL signal from potassium feldspar to date the earthquake events of the last millennium by employing modern sag pond deposits with different sorting and expected equivalent dose(D_e)of 0Gy. Choosing an appropriate measurement procedure and identifying the well bleached pottassium feldspar grains are essential for post-IR IRSL dating of young sediments. The non-fading characteristic of the post-IR IRSL₁₇₀ signal measured at 170℃ following a prior IR stimulation at 110℃ was verified by employing the D_e plateau test with respect to the signal integration interval and IR stimulation temperature together. Reducing the amount of potassium feldspar grains mounted on an aliquot would help reveal the among grains variation of bleaching level of post-IR IRSL₁₇₀ signal before depostion and identify the most sufficiently bleached grains. Therefore, the post-IR IRSL₁₇₀ D_e values of 2mm aliquots were measured for three samples with different sedimentary textures. The median of D_e distribution of well sorted and stratified sag pond deposits is consistent with the minimum D_e value inferred from the minimum age model(MAM-3) and finite mixture model(FMM), while for the poorly sorted deposits, the median is significantly overestimated compared with the minimum D_e values from the MAM-3 and the FMM. The minimum D_e values of 0.6~0.8Gy of all three samples are consistent with the unbleachable residual dose previously reported for post-IR IRSL signals measured at similar temperature for well bleached samples. It implies that by combined use of small aliquot and statistical age models, the well-bleached potassium feldspar grains could be identified. Such an intrinsic unbleachable component needs to be properly corrected when earthquake events of last millennium are to be dated in this area. Otherwise, the post-IR IRSL₁₇₀ age would be overestimated by 200~300a.
The post-IR IRSL₁₇₀ procedure investigated in this study is not only applicable for dating the paleoearthquake events along the Altyn Tagh Fault, but also with great potential to be applied to other tectonically active area. With consideration of the potential variability in post-IR IRSL signal characteristics of potassium feldspar grains from different origins, the signal stability needs to be routinely inspected. The modern analog sample would also be informative for justifying the measurement procedure and analytical method employed.     

Single grain optically stimulated luminescence dating of glacial sediments from the Baiyu Valley,southeastern Tibet

The Qinghai-Tibetan Plateau is an important area for the study of Quaternary glaciation. Optically stimulated luminescence (OSL) dating has the potential to contribute to the chronology of glaciation in this region, but it is important to assess the accuracy of OSL dating of these glacial sediments. In this study, single grain quartz OSL signals are examined for five glacial samples collected from the moraines outside the Baiyu Valley, southeastern Tibet. The quartz grains exhibit poor luminescence characteristics, with a small proportion of grains passing the screening criteria. Grains which pass the screening criteria have relatively low signal intensity, leading to D_e values with large uncertainties. MAM and CAM were used to determine D_e values for these samples. The OSL ages are consistent with the sequence of events derived from the geomorphological relationship of the samples, and also with previous published radiocarbon ages. However, it is more difficult to reconcile the OSL ages and the terrestrial cosmogenic nuclide (TCN) ¹⁰Be ages. Analysis of both single grain quartz OSL data and TCN ¹⁰Be data is complex in this area. Further work is required to increase confidence in the OSL ages generated for the glacial sediments from this region.     

Luminescence dating of quartz from ironstones of the Xingu River,Eastern Amazonia

This study reports on the first investigation into the potential of quartz luminescence dating to establish formation ages of ferruginous duricrust deposits (ironstones) of the Xingu River in Eastern Amazonia, Brazil. The studied ironstones comprise sand and gravel cemented by goethite (FeO(OH)), occurring as sandstones and conglomerates in the riverbed of the Xingu River, a major tributary of the Amazon River. The Xingu ironstones have a cavernous morphology and give origin to particular habitat for benthic biota in an area that hosts the largest rapids in Amazonia. So far, the Xingu ironstones have uncertain formation ages and their sedimentary origin is still poorly understood. In this way, seven samples of ironstones distributed along the lower Xingu River were collected for optically stimulated luminescence (OSL) dating of their detrital quartz sand grains. Additionally, the organic content of some samples was dated by radiocarbon (¹⁴C) for comparison with quartz OSL ages. The luminescence ages of the sand-sized quartz grains extracted from the ironstones were obtained from medium (100–300 grains) and small (10–20 grains) aliquots using the single aliquot regenerative-dose (SAR) protocol. Equivalent doses (D_e) distributions have varied overdispersion (OD) both for medium size aliquots (OD = 19–58 %) and small size aliquots (OD = 29–76 %). No significant trend was observed between D_e and aliquot size. The studied ironstones grow over the riverbed, but stay below or above water throughout the year due to the seasonal water level variation of the Xingu River. However, the effect of water saturation in dose rates is reduced due to relatively low porosity of ironstones. Water saturated dose rates (dry sample dose rates) range from 2.70 ± 0.21 (2.79 ± 0.22) Gy/ka to 12.34 ± 0.97 (13.26 ± 1.12) Gy/ka, which are relatively high when compared to values reported for Brazilian sandy sediments elsewhere (∼1 Gy/ka). Samples with high overdispersion (>40 %) are mainly attributed to mixing of grains trapped in different time periods by goethite cementation. The obtained OSL ages for water saturated (dry) samples range from 3.4 ± 0.3 (3.3 ± 0.4) ka to 59.6 ± 6.0 (58.1 ± 6.4) ka, using D_e determined from medium size aliquots and dose response curves fitted by an exponential plus linear function. Radiocarbon ages of the bulk organic matter extracted from selected ironstone varied from ca. 4 cal ka BP to ca. 23 cal ka BP. Significant differences were observed between OSL and radiocarbon ages, suggesting asynchronous trapping of organic matter and detrital quartz within the ironstone matrix. These late Pleistocene to Holocene ages indicate that ironstones of the Xingu River result from an active surface geochemical system able to precipitate goethite and cement detrital sediments under transport. The obtained ages and differences between OSL and radiocarbon ages point out that the ironstones have multiphase and spatially heterogeneous growth across the Xingu riverbed. Our results also expand the application of luminescence dating to different sedimentary deposits.     

Luminescence dating of a megaflood event on a terrace of the Jinsha River,China

The numerical dating of megaflood sediments is a worldwide challenge, a fact that has impeded a full understanding of Late Quaternary dam-outburst flood processes that occurred along the river courses of the Tibetan Plateau. Optically stimulated luminescence (OSL) dating has been widely used on such sediments. Due to their short transportation distances prior to deposition, the OSL signals of megaflood sediments are often partially bleached, resulting in age overestimations. Here, we report on a comparison of OSL ages obtained using both quartz (4–11 μm FG; 90–125, 180–250 μm CG) and K-feldspar (180–250 μm CG) extracted from sediments taken from the Binghong-Bingnong Neolithic-Bronze Age site on the second Jinshan River terrace (T2), in Yunnan Province, southwestern China. Contrary to previous experience suggesting that CG fractions are usually better bleached than FG fractions prior to deposition, our results showed that the OSL ages for the FG quartz fraction were generally younger than those for the CG fraction. This would suggest that the two fractions may have come from different sources, and may have been subject to different geomorphological processes prior to deposition. FG quartz fractions may be suitable to define the maximum age of sediments located in alpine gorge regions. CG quartz fractions can be used by applying the minimum age model (MAM) to select relatively well-bleached grains yielded ages close to the ‘true’ burial ages of the sediments. The results showed that the post-IR IRSL₂₂₅ ages of single grain K-feldspar were overestimated by > 3 ka, suggesting K-feldspar may not be suitable dating material for megaflood sediments <30 ka. The OSL dates suggest that the T2 terrace was formed ∼8.4 ka, that aeolian sediments were deposited during ∼2.6–1.5 ka, and that the megaflood event occurred after ∼1.5 ka.