Drone photogrammetry and KMeans point cloud filtering to create high resolution topographic and inundation models of coastal sediment archives

Coastal areas are vulnerable to the impacts of tropical cyclones (TC), tsunamis and other water super‐elevation events, but the frequency of these events is often poorly represented by conventional records. Coastal overwash deposits (including washover fans) can provide a longer‐term archive of event frequency. Because of their low‐gradient geomorphic form, washover fans require high accuracy (centimetre‐resolution) topographic models to understand patterns of connectivity and dynamics that control archive formation. Using images collected by a remotely piloted aircraft system (RPAS, or ‘drone’) and Structure‐from‐Motion (SfM) photogrammetry techniques, we apply a novel point‐cloud filtering technique based on KMeans classification of the R‐G‐B colour of each X‐Y‐Z point to remove vegetation and create a centimetre‐resolution and accuracy bare‐earth digital terrain model (DTM) of a washover fan in Exmouth Gulf (Western Australia). Using the RPAS‐SfM orphophoto and DEM data, supported by ground‐penetrating radar (GPR) and field stratigraphic analysis, we show how this approach can be applied to understand dynamics controlling low‐gradient geomorphic landforms, using an example of a washover fan sedimentary archive in northwestern Australia created by extreme overwash events. Our approach reveals the likely role of backflooding and terrestrial runoff in creating backwater environment for sub‐aqueous deposition and good sediment preservation and identifies key areas to target for detailed dating and stratigraphic analysis of a potentially decadal to sub‐millennial resolution sediment archive of TC activity. Copyright © 2018 John Wiley & Sons, Ltd.     

The complexity of luminescence dating of tidal sand body revealed by the discrepancy of paired quartz OSL ages

Optically stimulated luminescence (OSL) dating is becoming a useful technique to yield absolute age of organic-poor sandy deposits. The buried tidal sand body (BTSB) in the coastal zone of northern Jiangsu Province, China, has been suggested to have the same origin as the offshore radial sand ridge in the Yellow Sea. However, chronological constrain of the BSTB is still quite limited. In this study, OSL measurements were conducted using silt-sized multi-grain and coarse-grained single-grain quartz to constrain the depositional history of a 25.6 m core from the BTSB. A low luminescence sensitivity of quartz was observed, and only ∼1.04% of the grains passed the standard rejection criterion for single-grain measurement. Analysis of paired OSL ages from two grain-size fractions using different protocols showed that silt-sized quartz ages were underestimated of 0.14–1.35 ka in comparison to coarse-grained quartz in the depth interval of 5.8–22.4 m. We interpret such an age discrepancy as the effects of lateral infiltration of fine-grained sediment into the sand body due to dynamic feature of channel-ridge system on the shelf. As far as we know, it is the first time that such infiltration is demonstrated through OSL dating. Our OSL data indicated that there is a significant hiatus between the Late Pleistocene stiff clay layer (50–18 ka) and the Holocene sequence. Holocene deposits only occurred in the last 2 ka, with rapid accumulation of ∼17 m-thick sediments at ∼2–1 ka, a slower accumulation between ∼1 and 0.1 ka and rapid land emergence through an accretion of ∼4 m-thick sediment over the past ∼0.1 ka. This study highlights the complexity of OSL dating in highly dynamic sedimentary environments. Therefore, examining different grain size fractions and comparing different measurement protocols are highly deserved in carrying out OSL dating in such environments.     

Luminescence dating and archaeology of Holocene fluvio-lacustrine sediments of Abu Tartur, Eastern Sahara

Endorheic basins, filled by a rhythmic stratification of fluvio-lacustrine sediments, are useful archives for studying the interaction of palaeoenvironment and prehistoric development. Archaeological evidence indicates an occupational maximum during the Early to Mid-Holocene humid phase that can be linked to geochronological dates. A wadi has cut these sediments and a wadi profile of about 6 m was sampled for luminescence dating.

Palaeodose estimates are based on single aliquot measurements of the sand-sized quartz fraction using the regenerative dose technique. The comparison of equivalent dose results from different aliquot sizes (<100 and >1000 grains) provides interesting insights into heterogeneous degrees of bleaching. Problems arising from substantial variations in environmental dose rates of such heterogeneous sediments are discussed.

The luminescence ages indicate two major phases of deposition, both of Holocene age. The lower 2 m of sediments were deposited about 9400 years ago. The sedimentation of the upper 2 m started about 7200 and terminated around 6300 years ago. Radiocarbon ages of charcoals contained in the upper part of the section disagree with that OSL dating. This discrepancy is explained by re-deposition of the charcoal fragments.     

A method for estimating sediment budgets of washover deposits using digital terrain models

Washover fans are located on small barriers in fetch-limited micro-tidal coastal environments in Denmark. These washover fans are formed during high-energy storm events and we present a method to quantify their volumes and to estimate sediment exchanges between washover fans and their adjacent morphologies. We use high resolution digital terrain models (DTMs) based on light detection and ranging (LiDAR) data. We have delineated landforms using known methods of scale analysis and geomorphometric classification. We quantified volumes of the delineated landforms and estimated the related sediment budgets. These computed volumes were compared using different pre-depositional surfaces. Finally, we assessed the sediment exchange and associated sources of sediments of the washover fans. We applied a scale analysis to determine suitable DTM resolution and focal statistics window size as input to a geomorphometric classification analysis. Landform areas and landforms were delineated using morphometric threshold values, and volumes and sediment budgets of the delineated landforms were computed using different assumptions to define the pre-depositional surface. Resulting washover fan volumes were validated against digital elevation model (DEM) of difference (DoD) derived volumes. Sediment budgets were derived from representative volumes of the washover fans and adjacent berms. We show that quantification of washover features derived from DTMs, using geomorphometric analysis is feasible and that the presented approach provides estimates of washover deposit volumes with an accuracy between 1% and 28% compared to control volumes. © 2021 John Wiley & Sons, Ltd.     

High-resolution OSL chronology of a well-preserved inland dune in the Lys valley (Sint-Martens-Latem,NW Belgium)

The inland dune “Molenberg” (Lys valley, NW Belgium) is an exceptionally well-preserved dune of more than 10 m high. To determine the phases of dune formation and identify possible levels for human occupation, this dune was sampled at closely spaced vertical intervals for Optically Stimulated Luminescence (OSL) dating (every 50 cm) and grain-size analysis (intervals of 10–25 cm) over its entire depth, including the top of the underlying sediments. The resulting ages show dune formation during the late glacial period (GI-1 and GS-1), with a probable extensive deposition during the Allerød (GI-1a-c) and Younger Dryas (GS-1). Interestingly, the OSL dataset for the main body of the dune shows significant cyclical age inversions that may be caused by reworking of sediment causing incomplete bleaching of sand grains. This may further indicate the potential of high-sampling resolution OSL to contribute to the reconstruction of complex dune dynamics.     

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.     

Evaluating isothermal thermoluminescence and thermally transferred optically stimulated luminescence for dating of Pleistocene sediments in Amazonia

The paleogeography of Amazonia lowlands during the Pleistocene remains hampered by the lack of reliable absolute ages to constrain sediment deposition in the hundred thousand to few million years timescales. Optically stimulated luminescence (OSL) dating applied to quartz has provided important chronological control for late Quaternary sediments, but the method is limited to the last ∼150 ka. In order to extend the age range of luminescence dating, new signals from quartz have been investigated. This study tested the application of isothermal thermoluminescence (ITL) and thermally transferred optically stimulated luminescence (TT-OSL) signals of quartz for dating of fluvial terraces from eastern Amazonia. ITL and TT-OSL signals measured in a modern fluvial sediment sample have shown small residual doses (4 and 16 Gy), suggesting adequate bleached sediments for the target dose range (>150 Gy). This sample responded well to dose recovery test, which showed that the ITL and TT-OSL signals grow to higher doses compared to the doses estimated by the conventional OSL signal. The ITL signal saturated for doses significantly lower than doses reported in the literature. Most dating samples were beyond the ITL saturation doses and only TT-OSL signals were suitable to estimate equivalent doses. Burial ages ranging from 107 to 340 ka were estimated for the fluvial terraces in the lower Xingu River. The main ages uncertainties are related to dose rate changes through time. Despite the uncertainties, these ages should indicate a higher channel base level during the Middle Pleistocene followed by channel incision, possibly due to episodes of increased precipitation in the Xingu watershed.     

Towards dating Quaternary sediments using the quartz Violet Stimulated Luminescence (VSL) signal

Quartz optically stimulated luminescence (OSL) dating is widely used to determine the time of deposition and burial of Late Quaternary sediments. Application of the method is usually limited to the past 150,000 years due to early saturation of the OSL signal. Here we explore the potential to date Quaternary sediments using the violet (402 nm) stimulated luminescence (VSL) signal of quartz. We develop and test a new post-blue VSL single aliquot regenerative dose dating protocol, and demonstrate that the VSL signal originates from a deep trap at about 1.9 eV with a thermal lifetime of 10¹¹ years at 10 °C, and that this trap is bleachable by sunlight. The VSL signal grows with dose to ∼6400 Gy, a factor ∼20 higher than the conventional quartz OSL signal, and with the proposed protocol we recover a known dose of 1000 Gy in three out of four samples. The potential of the VSL protocol for dating Quaternary sediments is highlighted by its successful application to a suite of geological samples ranging in age between 13 and 330 ka. Based on our investigations, we propose that the VSL protocol has the potential to extend the quartz dating range to cover the full Quaternary.     

Luminescence dating of sediments from a Palaeolithic site associated with a solution feature on the North Downs of Kent,UK

Optically stimulated luminescence (OSL) dating has been applied to determine the burial age of sediment horizons within a sequence containing Lower Palaeolithic artefacts on an upland site associated with a solution feature (doline) at West Cliffe, located on the North Downs in Kent, UK. The study makes use of a novel extension of the single aliquot OSL measurement procedure to investigate the nature of significant overdispersion in equivalent dose values with very small aliquots of quartz inclusions, enabling single grain resolution to be approached with ∼90 μm diameter quartz extracted from the relatively fine-grained brickearth. A detailed examination of the uniformity of the distribution of radionuclide sources in the sampled volumes was also performed and this included the application of a spatially-resolved technique for beta dose rate measurement. The OSL ages, obtained for the burial of brickearth positioned stratigraphically below and above a clay and flint clast layer containing the artefacts and debitage, place the deposition of the artefacts to between ca 140 and 80 ka ago. This is significantly later than indicated by the artefact typology (>300 ka) and contrary to the expectation of in situ burial indicated by earlier research in this region. If displacement occurred on other upland sites this finding has important implications for establishing the timing of hominin use of the upland areas which, beyond broad attribution to Lower or Middle Palaeolithic origin, is uncertain and similar doubts apply to the interpretation of the environments that prevailed.     

OSL chronology of a sedimentary sequence from the inner-shelf of the East China Sea and its implication on post-glacial deposition history

Sedimentary records from the inner-shelf of the East China Sea (ECS) are unique for the reconstruction of post-glacial palaeoclimate and sea-level changes. So far, the chronology of sediment succession from this region has mainly been based on radiocarbon dating, which might be problematic due to reworked deposition or old carbon contamination. In this study we tested the applicability of optically stimulated luminescence (OSL) dating to a drilling core (ECS-DZ1) taken from the northern ESC. A total of 20 OSL samples and two radiocarbon samples were collected from the upper 58 m of this core. The results indicate the likely sufficient reset of OSL signal of fine-grained (4–11 μm) quartz before burial, and thus reliable chronology for the studied core sediments. For one sample, however, the extracted coarse-grained (100–200 μm) quartz overestimated the deposition age significantly, presumably resulting from partial bleaching prior to deposition. The fine-grained quartz ages are generally consistent with the stratigraphical order, and the reliability of these OSL ages are further validated by two selective robust ¹⁴C dates. The chronological framework of core ECS-DZ1 reveals striking sedimentation-rate changes. By comparison with other chronostratigraphical records, we infer that post-glacial deposition history (since ∼15 ka) of the study site is likely related to regional sea-level rise and delta-estuary environment evolution, as well as strengthened human activities and/or coastal currents.     

Dating desert pavements – First results from a challenging environmental archive

Desert pavements are widespread landforms of arid environments. They consist of a monolayer of clasts at the surface, associated with an underlying unit of eolian fines. In some situations, buried desert pavements can be observed, which is interpreted as a change in the environmental conditions. Therefore, it is believed that desert pavements represent important paleoenvironmental sediment archives, especially for arid environments, where natural archives of past environments are rare. To better understand the formation process of desert pavements and to enable the paleoenvironmental interpretation of these valuable sediment archives, reliable chronologies are of crucial importance. Thus, OSL dating was applied to samples from well-developed desert pavements in two different study areas, the Cima Volcanic Field, eastern Mojave Desert, USA, and the desert of northeastern Badia, Jordan. To test the suitability of the sediments for OSL dating, the luminescence characteristics of the fine- and coarse-grain quartz fraction are described and compared. Finally, first OSL ages are presented.     

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.     

Quartz OSL and K-feldspar pIRIR dating of a loess/paleosol sequence from arid central Asia,Tianshan Mountains,NW China

Loess deposits surrounding the high mountainous regions of arid central Asia (ACA) play an important role in understanding environmental changes in Eurasia on orbital and sub-orbital time scales. However, problems with dating loess in ACA have limited the interpretation of climatic and environmental data, especially Holocene data. We selected a typical loess/paleosol sequence (LJW10) on the northern slope of the Tianshan Mountains in ACA consisting of 280 cm of loess with multiple paleosols formed in the upper 170 cm of the section. We applied quartz OSL dating to coarse-grained (63–90 μm) fractions, and newly developed K-feldspar pIRIR dating protocols to both coarse-grained and medium-grained (38–63 μm) fractions of the samples from LJW10 section. Internal checks of the quartz OSL dating indicate that the single-aliquot regenerative-dose protocol on large aliquots (5 mm) is appropriate for equivalent dose (D_e) determinations and that the quartz ages of the loess samples are likely to be reliable. Luminescence characteristics and internal checks of the pIRIR dating indicate the pIRIR signal at a 170 °C stimulation temperature with a 200 °C preheat can be used for both coarse-grained and medium-grained D_e determinations. Anomalous fading tests for the pIRIR 170 °C signal indicate the pIRIR signals are stable and the anomalous fading of the pIRIR 170 °C signal can be ignored. Sunlight bleaching tests of the loess indicate the residual dose for the pIRIR 170 °C signal can also be ignored as it corresponds to only ∼9 years for the medium-grained K-feldspar and ∼85 years for the coarse-grained K-feldspar. The pIRIR ages of five medium-grained and coarse-grained K-feldspar samples are consistent with coarse-grained quartz OSL ages, and both the medium-grained and coarse-grained ages increase uniformly with depth, indicating these pIRIR ages are reliable. Based on the coarse-grained quartz OSL ages, and on coarse-grained and medium-grained K-feldspar pIRIR ages, an age-depth model for the paleosol-loess sequence was established by using a Bacon age-depth model. This model suggests eolian loess deposition began by at least ∼16 ka ago and that paleosol development on these eolian loess deposits began ∼5.5 ka, continuing to the present, with periods of high effective moisture at 5.5–4.9, 4.6–4.1, and 3.4–3.1 ka. This sequence suggests overall relative aridity during the early Holocene and an increase in effective moisture beginning ∼5.5 ka during the mid-late Holocene in ACA.     

Refining late Quaternary plunge pool chronologies in Australia's monsoonal 'Top End'

Plunge pool deposits from Australia's 'Top End' are considered as important archives of past monsoonal activity in the region. The available chronology of these deposits was so far based on thermoluminescence (TL) dating and indicated maximum flood magnitudes during the Last Glacial Maximum in contrast with more arid conditions as deduced from other archives of the region. This study revisits plunge pool deposits at Wangi Falls by applying multiple and single-grain Optically Stimulated Luminescence (OSL) dating of quartz and high-resolution gamma spectrometry, supported by radiocarbon dating of organic material. The aim is to reappraise the existing chronology and investigate if the deposits are affected by partial bleaching, post-depositional mixing and/or problems related to annual dose determination. The latter seems to have a minor impact on the ages at most. Equivalent Dose (D_e) distributions are broad, in particular for single grains, but apparently not result from partial bleaching or post-depositional mixing. Rather, microdosimetry caused by radiation hotspots in the sediment and zircon inclusions in the quartz grains is considered problematic for these sediments. The results presented here imply that the previous TL chronology overestimated the real deposition age of the sediments.     

Luminescence dating without sand lenses: An application of OSL to coarse-grained alluvial fan deposits of the Lost River Range,Idaho, USA

Optically stimulated luminescence (OSL) dating is increasingly used to estimate the age of fluvial deposits. A significant limitation, however, has been that conventional techniques of sampling and dose rate estimation are suitable only for thick (>60 cm) layers consisting of sand size or finer grains. Application of OSL dating to deposits lacking such layers remains a significant challenge. Alluvial fans along the western front of the Lost River Range in east-central Idaho, USA are one example. Deposits are typically pebble to cobble sheetflood gravels with a sandy matrix but thin to absent sand lenses. As a result, the majority of samples for this project were collected by excavating matrix material from gravelly deposits under light-safe tarps or at night. To examine the contributions of different grain-size fractions to calculated dose-rates, multiple grain-size fractions were analyzed using ICP–MS, high resolution gamma spectrometry and XRF. Dose rates from bulk sediment samples were 0.4–40% (mean of 18%) lower than dose-rate estimates from the sand-size fractions alone, illustrating the importance of representative sampling for dose rate determination. We attribute the difference to the low dose-rate contribution from radio-nuclide poor carbonate pebbles and cobbles that occur disproportionately in clast sizes larger than sand. Where possible, dose rates were based on bulk sediment samples since they integrate the dose-rate contribution from all grain sizes. Equivalent dose distributions showed little evidence for partial bleaching. However, many samples had significant kurtosis and/or overdispersion, possibly due to grain-size related microdosimetry effects, accumulation of pedogenic carbonate or post-depositional sediment mixing. Our OSL age estimates range from 4 to 120 ka, preserve stratigraphic and geomorphic order, and show good agreement with independent ages from tephra correlation and U-series dating of pedogenic carbonate. Furthermore, multiple samples from the same deposit produced ages in good agreement. This study demonstrates that with modified sampling methods and careful consideration of the dose rate, OSL dating can be successfully applied to coarse-grained deposits of climatic and tectonic significance that may be difficult to date by other methods.     

Colluvial and alluvial sediment archives temporally resolved by OSL dating: Implications for reconstructing soil erosion

Colluvial and alluvial sediments represent important geoarchives to reconstruct long-term soil erosion and to gain insight into the complex system of sediment cascades and sediment fluxes within a catchment. In this respect, the temporal information of sediment archive formation is essential and achievable through optical stimulated luminescence (OSL) dating. In this study, colluvial and alluvial sediments from a mesoscale catchment in northern Bavaria were investigated and dated by OSL. Insufficient sediment bleaching was detected for some of the samples. In these cases, the method proposed by Fuchs and Lang [Fuchs, M., Lang, A., 2001. OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peloponnese, Greece. Quaternary Science Reviews 20, 783–787.] was applied for equivalent dose (D_e) determination. The calculated OSL ages are in stratigraphic order and their accuracy is confirmed by ¹⁴C age control. Based on the chronostratigraphies for alluvial and colluvial archives, there is a dominant synchronous sedimentation history for the Medieval and Modern period, but initiation of Holocene sedimentation occurred at different times in the upper and lower parts of the catchment. The latter phenomenon might be explained by the decoupled sediment fluxes between both the slope–channel system and the upper and lower catchment.     

Investigating the amount of zeroing in modern sediments of River Danube, Austria

Sediments attributed to flooding events of River Danube concerning the bleaching of the optical stimulated luminescence (OSL) signal were investigated. It is demonstrated that the OSL signal in both quartz and feldspar is not completely but differentially bleached in the sediment grains. Partial bleaching of the samples is clearly indicated by the scatter of equivalent dose determined for several individual single aliquots. It is also shown that residual ages in feldspars are significantly higher than those calculated for quartz. It is furthermore demonstrated that analysing measurement-time dependent equivalent dose estimates is not a suitable method to identify partial bleaching in the investigated sediment grains. However, the transport and deposition process of the investigated samples was probably disturbed by an artificial input of sediment, and this case study may thus not be representative of undisturbed high flood events in the past.     

Optical dating of tidal sediments: Potentials and limits inferred from the North Sea coast

The accuracy of optical ages derived from tidal sediments depends largely upon the transport processes. These processes constrain the degree of bleaching by the time of deposition and the choice of grain size for dating. This study looks at flow regime, sediment bedding, particle size and suspended sediment concentration (SSC) over tidal flats in order to identify the tidal sub-environment from which reliable multigrain optical ages are most likely to be achieved. The resulting conceptual model is then compared with empirical OSL data obtained from Holocene sediments of the southern North Sea tidal coastal plain of continental Europe. Optical dating of the tidal sediments included single-aliquot-regenerative dose protocol applied to multigrain aliquots of fine sand and fine silt, statistical analysis using weighted skewness, standardised kurtosis and over-dispersion. It is inferred from the model that smaller grains should be better bleached than larger grains. However, because transport and deposition processes are extremely variable in both space and time, unequivocal “bleaching rules” could not be assigned to a particular tidal sub-environment. In this context more than 85% of our samples return accurate ages and around 13% of our optical ages are overestimated when compared with ages from established well-constrained stratigraphic frameworks. The empirical study confirms the concept of “variable bleaching rules”: both accurate and inaccurate ages are obtained from silty and sandy OSL samples regardless of the sub-environment and well-bleached samples may be obtained from all tidal sub-environments. Although our study is based on multiple-grain aliquots it also shows that an independent statistical treatment of equivalent dose data is an indispensable procedure to detect and correct for insufficient bleaching.     

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.     

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.