Luminescence Dating Of Feldspar Research Articles

Analyzing complex single grain feldspar equivalent dose distributions for luminescence dating of glacially derived sediments

Overdeepened valleys are structures scoured into the underlying substrate by glaciers that have been subsequently filled with sediment, which recorded the environmental history of the adjacent landscape. Investigated here is an overdeepened structure that was formed beneath the Salzach Paleoglacier west of Freilassing, Bavaria (southern Germany). In a previous study, infinite infrared stimulated luminescence (IRSL) ages have been determined for the lower part of the core using a multi-grain approach. Applying single-grain feldspar luminescence dating allows to identify the unsaturated signals, previously masked by multi-grain signal averaging. Identification of the normally distributed leading edge in equivalent dose (De) distributions allows for the dismissal of low value De (LOVED) grains that appear to reflect underestimating outliers. Measurement of eleven samples distributed along the entire length of the core yielded two distinct age clusters, for which fading corrected IRSL50 and uncorrected pIRIR225 ages agree within uncertainties. Whereas the younger age cluster at the top of the sequence is attributed to Marine Isotope Stage (MIS) 3, the older age cluster is assigned to MIS 6. It is shown that single grain pIRIR measurements will allow to resolve the infill chronology of overdeepened structures in the Alps, at least for the last ca. 200 ka. Overall, the present study represents a further step towards understanding and handling complex De distributions often encountered in single grain feldspar luminescence dating. This contributes to better understand the frequently observed age offset between single grain and single-aliquot measurements.

Testing the potential of using coarse-grain feldspars for post-IR IRSL dating of calcium sulphate-wedge growth in the Atacama Desert

The growth of vertically laminated calcium-sulphate wedges in the Atacama Desert is assumed to be driven by the interaction of moisture supply and salt dynamics in the subsurface. Geochronological data of these wedge laminations is yet sparse but indispensable to resolve wedge-growth phases and episodes of moisture supply and to use these deposits as a palaeoclimate archive in the hyperarid environment. Our pilot study presents a first approach of dating a calcium-sulphate wedge from the Atacama Desert using coarse-grain feldspar luminescence dating. Our results show a widespread and clustered equivalent-dose distribution of two wedge samples from ∼20 Gy up to saturation. Optically stimulated luminescence (OSL) of quartz revealed unsuitable properties for dating wedge deposits. Consequently, we applied post-infrared infrared stimulated luminescence (post-IR IRSL) to coarse-grained feldspars. Since feldspar single-grain measurements yielded a low number of luminescent grains, we used 1 mm aliquots as reliable single-grain proxies for genuine single-grain measurements. Data from energy-dispersive x-ray spectroscopy (EDX) showed that the feldspar single grains have large differences in their internal K content, resulting in an averaged internal K content of 3.9 ± 1.0 % for all luminescent grains. This result was subsequently used for dose rate and age calculations. Our results of equivalent-dose distributions and palaeodoses derived from the minimum age model reveal most recent wedge-growth activities at 10.6 ± 2.2 ka and 7.9 ± 1.8 ka for the two wedge samples.

Late Holocene rupture history of the Ash Hill fault, Eastern California Shear Zone, and the potential for seismogenic strain transfer between nearby faults

AbstractUnderstanding how fault systems interact and transfer strain over seismogenic timescales (seconds to ka) requires temporal records of past ruptures along adjacent and intersecting fault networks. Here we document the record of Late Holocene ruptures as recorded in the geomorphology of alluvial deposits along the Ash Hill fault, in the Eastern California Shear Zone (ECSZ). We leverage a multi‐faceted approach to evaluate the relative timing of Ash Hill fault ruptures to those of nearby faults in the ECSZ. We determine the number and timing of Late Holocene earthquakes on the Ash Hill fault using high‐resolution tectono‐geomorphic mapping, a locally calibrated alluvial fan stratigraphy, feldspar luminescence dating, and fault offset analysis from field observations, LiDAR, and drone‐based digital surface elevation models. We find evidence for three surface‐rupturing earthquakes that have occurred since ~4 ka, each with ~1.0 ± 0.2 m of oblique slip per event (Mw ~ 6.9–7.0), and we constrain the timing of these earthquakes by dating deposits that bracket each event. The timing of these three ruptures is similar to the paleoseismic record along the adjacent range‐bounding fault in southern Panamint Valley. Specifically, the two adjacent faults exhibit similar numbers of earthquakes during the Late Holocene, with similar recurrence intervals and rupture timing. These data suggest that it is possible that these two faults have ruptured in the same or closely temporally related events throughout the Late Holocene. Similar spatio‐temporal clustered earthquakes have been recognized in both historic and paleoseismic records in the region, and such behaviour may be common in complexly interlinked fault networks, like those that exist in the ECSZ.

Circumvention of anomalous fading in feldspar luminescence dating using Post-Isothermal IRSL

Abstract In luminescence dating of feldspar, laboratory induced luminescence necessary to assess the naturally acquired dose in the environment suffers from a poorly understood instability over time, known as anomalous fading (AF). AF is the source of commonly observed age underestimation in optical dating of feldspar. Approaches to circumvent AF include correction methods as well as attempts to measure directly an unfading component of feldspar luminescence, e.g. post-IR IRSL protocols. The former has the advantage of measuring the easily bleachable traps but requires extrapolations of fading rates. Post-IR IRSL methods can be limited by the use of more difficult-to-bleach traps and hence by potential age overestimations. In this paper, we show how post-isothermal laboratory induced luminescence (pIt-IR) allows measuring an equivalent dose (De) that is not dependent on time elapsed since irradiation. In this procedure, one measures two IRSL signals, first from an IR stimulation at low temperature (i.e. IR50) followed by a second one, at a higher temperature (i.e. IR225). Since both signals fade at different rates, De from IR50 is lower than that of IR225. In pIt-IR, a succession of thermal treatments is carried out before the measurement of laboratory-induced IR50 and IR225 luminescence. The dependence of De on thermal annealing is different for each IR signal so one can find which thermal treatment will yield the same De for both signals. This De is assumed to be the true total radiation dose received by the feldspar minerals in nature, more properly known as the paleodose (P). This new methodology is herein applied to three samples of different ages and different geological contexts. Post-isothermal luminescence is thus proposed as a way to circumvent AF, even though extended isothermal annealing treatment is not shown to eradicate AF.

Chronology and formation processes of the Middle to Upper Palaeolithic deposits of Ifri n'Ammar using multi-method luminescence dating and micromorphology

The existence of an early Upper Palaeolithic culture at the transition from the Middle Palaeolithic to the Upper Palaeolithic in North African cave sites is currently under debate. We studied Ifri n‘Ammar in North-East Morocco, which is one of the oldest settlement sites of anatomically modern humans (AMH) in the Maghreb and contains several sediment layers which are attributed to Middle and Upper Palaeolithic occupations. In order to investigate processes of sediment accumulation and postdepositional alteration, we studied thin sections from these levels. According to micromorphological analysis, aeolian input considerably contributed to sediment accumulation and postdepositional mixing by bioturbation occurred. We compared multiple and single-grain quartz and multiple-grain feldspar luminescence dating of three samples from corresponding sediment layers to achieve a comprehensive chronology. The single-grain dose distributions scatter strongly and the source of the scatter is unclear. We used an arithmetic mean to calculate the equivalent doses. Archaeological evidence and age control from radiocarbon dating was essential to interpret the data. Quartz and feldspar multiple-grain luminescence ages are between 15 and 80 ka. The central part of the profile shows an intermediate accumulation, which lacks specified lithic artefacts. This supports the idea of an occupational gap between Middle and Upper Palaeolithic layers.

Testing feldspar luminescence dating of young archaeological heated materials using potshards from Pella (Tell Tabqat Fahl) in the Jordan valley

Recent developments in the use of more stable feldspar signals in the luminescence dating of sediments offer the possibility of obtaining accurate feldspar luminescence ages for ceramic artefacts; this is especially interesting in locations which do not provide suitable quartz extracts. Here we examine the application of the stable infrared stimulated luminescence signal measured at elevated temperature (in this case 290°C; pIRIR<sub>290</sub>) after stimulation at about room temperature to Levantine pottery samples. A total of 52 potsherds were collected from three superimposed iron-age units at Pella (Jordan); based on <sup>14</sup>C dating, typology and seriation these units were deposited between 700 and 900 BCE. Sand-sized quartz extracts were unsuitable, and there was insufficient sand-sized feldspar, and so polymineral fine grains were chosen for dating. Various tests for reliability were undertaken (dose recovery, dependence of D<sub>e</sub> on first stimulation temperature <i>etc</i>.). The pIRIR signals are weak, and 14 potsherds were rejected on this basis. Of the remainder, 3 were confidently identified as outliers. Based on those sherds for which IR signals were sufficiently intense, we use the ratio of the IR<sub>50</sub> to pIRIR<sub>290</sub> signals to argue that these outliers do not arise from incomplete resetting during manufacture. The ages from each layer are considerably over dispersed (typically by ∼25%) but average ages for each unit are consistent with each other and with the expected age range. The average OSL age for the site is 2840 ± 220 years (n = 35), with the overall uncertainty dominated by systematic uncertainties; this average is consistent with the range of <sup>14</sup>C ages from 970–1270 BCE reported from across the destruction horizon. We conclude that the pIRIR<sub>290</sub> signal is delivering accurate ages, but that the variability in age from shard to shard is much greater than would be expected from known sources of uncertainty. This demonstrates the need for site ages to be based on multiple samples; individual shard ages are unlikely to be sufficiently accurate.

A lake-level chronology based on feldspar luminescence dating of beach ridges at Tangra Yum Co (southern Tibet)

Many lakes on the Tibetan Plateau exhibit strandplains with a series of beach ridges extending high above the current lake levels. These beach ridges mark former lake highstands and therefore dating their formation allows the reconstruction of lake-level histories and environmental changes. In this study, we establish a lake-level chronology of Tangra Yum Co (fifth largest lake on the Tibetan Plateau) based on luminescence dating of feldspar from 17 beach-ridge samples. The samples were collected from two strandplains southeast and north of the lake and range in elevation from the current shore to 140m above the present lake. Using a modified post-infrared IRSL protocol at 170°C we successfully minimised the anomalous fading in the feldspar IRSL signal, and obtained reliable dating results. The luminescence ages indicate three different stages of lake-level decline during the Holocene: (1) a phase of rapid decline (~50m) from ~6.4 to ~4.5ka, (2) a period of slow decline between ~4.5 and ~2.0ka (~20m), and (3) a fast decline by 70m between ~2ka and today. Our findings suggest a link between a decrease in monsoonal activity and lake-level decline since the early Holocene.

Testing the potential of a transferred IRSL (T-IRSL) feldspar signal for luminescence dating

One of the major dilemmas in feldspar luminescence dating is that the infra-red stimulated luminescence (IRSL) signals are either stable and difficult to bleach when measured at elevated temperatures, or unstable and easy to bleach when measured at low temperatures. To identify a signal for sediment dating that is both stable and easy to bleach, we investigate the potential of an optically and thermally transferred IRSL (T-IRSL) signal. Based on the mechanisms described in Wang et al. (2014), we develop a T-IRSL single-aliquot regenerative-dose (SAR) measurement protocol. We investigate the a-thermal stability of six different T-IRSL signals from a sample of infinite age using fading experiments, and by comparing field and laboratory saturation levels. The T-IRSL signal measured at 125 °C (T-IRSL125) following a preheat of 280 °C, is found to be as stable as the post-IR IRSL 290 °C signal (pIRIR290). Furthermore, laboratory bleaching experiments show that the T-IRSL125 signal bleaches faster than the pIRIR290 signal, and that the corresponding residual doses are more than 40% lower. This indicates that T-IRSL signals may be superior to pIRIR methods for dating young and/or insufficiently bleached deposits. However, a SAR protocol performance test of the T-IRSL125 signal yielded a systematic underestimation of 8 ± 2%. This is possibly caused by a sensitivity change during the first preheat and requires further investigation.

Bleaching of the post‐IR IRSL signal: new insights for feldspar luminescence dating

Post‐infrared (pIR) stimulated luminescence dating of sedimentary feldspar largely avoids the effects of anomalous fading that affect conventional infrared stimulated luminescence (IRSL) dating. However, optical resetting of pIR signals is more difficult than resetting the conventional IRSL signal, which may undermine the crucial assumption that pIR signals were effectively bleached upon deposition and burial of sediment grains. In this study, we quantify the bleaching properties of several pIR signals on various samples using laboratory‐simulated bleaching in full sunlight and water‐attenuated sunlight. Our data show that bleaching is most efficient under full spectrum conditions for all pIR signals and that pIR signals measured at elevated temperature are increasingly harder to bleach than IR and pIR signals measured at low temperature (e.g. IR at 50°C). All bleaching curves exhibit a very slow and steady decrease, indicating that a fixed un‐bleachable residual level cannot be reached within the 11 days of solar simulator exposure undertaken here. We show that the magnitude of a laboratory‐determined residual dose depends on the adopted bleaching protocol and cannot be used as a proxy for the dose that remains in the sample at the time of burial (remnant dose). Our data emphasize the importance of finding a balance between sufficient signal stability and a minimized contribution of a remnant dose when using pIR procedures for feldspar luminescence dating.

Validating post IR-IRSL dating on K-feldspars through comparison with quartz OSL ages

Recent developments have opened up the possibilities of using potassium feldspar for dating Pleistocene sediments; a stable (less-fading) part of the infrared stimulated luminescence (IRSL) signal can be selected by largely depleting the unstable part of the IR signal, using a combination of thermal and IR stimulation: post IR-IRSL dating (pIR-IRSL).This study aims to test the validity of pIR-IRSL dating on feldspars. We obtained pIR-IRSL ages on a large suite of samples from several locations in the Netherlands area, covering a wide range of depositional environments and ages. Age control was provided by quartz optically stimulated luminescence (OSL) ages on the same samples; these ages were shown to accord with geological age constraints. Comparison with IRSL ages enabled us to evaluate the improvement that pIR-IRSL dating provides over conventional IRSL methods.The majority of feldspar ages obtained with pIR-IRSL showed good agreement with both the quartz OSL ages and the geological age constraints. Our study demonstrates that pIR-IRSL dating is more robust than conventional IRSL and should be the method of choice in feldspar luminescence dating of Pleistocene sediments.

Study on feldspar thermoluminescence spectra and its luminescence dating implication

Thermoluminescence spectra of feldspar in pegmatite from South China were experimentally investigated using a newly-constructed three-dimensional thermoluminescence spectrometer, under variant experimental conditions: the storage time period between sample irradiation and spectral measurements, the dose rate used for sample irradiation. The experimental results show that (1) the feldspar TL spectra display in a pattern of wavelength bands, centered on eight different distinguishable wavelengths; (2) the thermoluminescence components of feldspar from pegmatite fade away heterogeneously depending on wavelength and thermal stimulated temperature; (3) the two different dose rates used for artificial irradiation result in distinguishable diverse thermoluminescence spectra. All these luminescence spectral characterization features suggest that more complicated ingredients influencing feldspar luminescence dating need to be ruled out than expected. Especially it is necessary to correct the normal luminescence dating results in regard to these influential factors when feldspars extracted from sediments but originating from rocks are used for luminescence dating. On the other hand the experimental results also suggest the feasibility of optimization of feldspar luminescence dating.

Radioluminescence dating: A new tool for quaternary geology and archaeology

Natural minerals, such as widespread quartz and feldspars, have physical properties which enable them to be used as radiation dosimeters. The underlying luminescence phenomena have made it possible in recent decades to determine the age of a variety of materials important for quaternary geochronometry and archaeochronometry. We present a new luminescence dating method based on radioluminescence measurements of potassium feldspar. For the first time we have been able to investigate the light-emitting transition of electrons from the conduction band to an optically sensitive electron trap. Many advantages can be derived from such direct measurements of the metastable electron density in this particular trap, which produces the age-dependent signal. The method can be used to date the last light exposure of feldspar grains within a range of a few hundred to more than 200,000 years. Examples are presented of age determination of various waterlaid quaternary sands. The results of a basic study of feldspar radioluminescence also shed light on effects not sufficiently understood in conventional dating by luminescence techniques, especially in infrared optically stimulated luminescence dating of feldspar. http://link.springer. de/link/service/journals/00114/bibs/9086009/90860441.htm</HEA