Bleached Sediments Research Articles

The multiphase Eulerian-Lagrangian transport (MELT-3D) approach for modeling of multiphase mixing in fragmentation processes : Nourgaliev, R. R. et al. Progress in Nuclear Energy, 2003, 42, (2), 123–157

In this study we report on optical stimulated luminescence (OSL) ages of quartz extracted from a sedimentary record in the N-Oman mountain range. Equivalent dose (DE) distributions derived from single aliquot measurements (SAR) of small aliquots (ca. 200 grains) were investigated to test whether the skewness and broadness of the dose distribution can be used as criteria for the identification of insufficient bleaching. Furthermore, the methods proposed by Lepper and McKeever [2002 An objective methodology for dose distribution analysis. Radiation Protection Dosimetry 101 (1–4), 349-352]. Singhvi [Juyal, N., Chamyal, L.S., Bhandari, S., Bushan, R., Singhvi, A.K., in press. Continental record of the southwest monsoon during the last 130 ka: evidence from the southern margin of the Thar Desert, India. Quaternary Science Review] and Fuchs and Lang [2001 Fuchs, M., Lang, A., 2001. OSL dating of coarse-grain fluvial quartz using single-aliquot protocols on sediments from NE Peloponnese, Greece. Quaternary Science Review 20, 783–787.] to derive DE's from insufficiently bleached sediments were compared. At first, the investigations were carried out on artificially bleached, irradiated and mixed quartz material from the Oman study area to simulate insufficiently bleached sediments. Then, the various statistical methods for identifying insufficient bleaching and DE derivation were applied to the natural samples from the study area, where 18 samples were measured. For the identification of insufficient bleaching the preferential parameter is the broadness of a distribution. For DE calculation, both the Singhvi method and the Fuchs and Lang method produce similar results, which are consistent with the stratigraphic order. A drawback of both methods is their sensitivity to low outliers. The Lepper and McKeever method was not applied to the natural samples, due to limitations in its application to a small number of aliquots and due to the ambiguous identification of the rising limb of the dose distributions.

Variability of infrared stimulated luminescence properties from fractured feldspar grains

A series of OSL properties of fragments from split feldspar grains have been investigated. These are (1) the response to a radiation dose, (2) the recycling of corrected induced luminescence ( L i/ T i), (3) the corrected natural luminescence ( L N/ T N), (4) the corrected thermal transfer luminescence (termed herein L tt/ T tt) and, (5) the rate of anomalous fading (“ g”). It was found that the parameters of different parts of the same grain are the same. The relative uncertainty, at the 2 sigma level, at which we can claim that the measured parameters are correlated, is generally less than 5% but is ca 40% for the anomalous fading rate due to measurement uncertainty. For young partially bleached sediments, the result is that one could use one fragment to evaluate the equivalent dose and another to assess the contribution of thermal transfer resulting from preheating the natural luminescence.

Investigations into the potential effects of pedoturbation on luminescence dating

Much effort has been focussed on understanding the luminescence properties of natural minerals to achieve a reliable, accurate and precise dating technique. However, some field related aspects, such as the influence or effect of post-depositional disturbance on luminescence dates, are as yet underexplored. In the case of pedoturbation, depending on its intensity, the rate of sedimentation and unit thicknesses, potentially the whole sedimentary record at a site can be affected. This may lead to distorted OSL chronologies and erroneous sediment burial ages. Pedoturbation can result in sediment mixing and/or exhumation that affect luminescence both at the bulk and single grain level. Effects of these two principle processes on luminescence ages are examined using standard multigrain and single grain protocols. High resolution sampling of surface gopher mounds was used to determine the efficiency of bio-exhumation in resetting luminescence signal. Results show this is an inefficient mechanism for onsite sediment bleaching. The effects on luminescence signal of bio-mixing were explored by comparing a sample collected from within a krotovina (infilled burrow) to an adjacent undisturbed sample. Results show the difficulties in identifying pedoturbated samples at the single aliquot level and the possible inaccuracies in using the lowest palaeodose values to calculate OSL ages. Where pedoturbation of samples is suspected, use of probability plots of palaeodoses data is recommended. From these plots it is proposed that only data falling within a normal distribution centred on the peak probability be used to calculated OSL ages and to mitigate problems arising from pedoturbation.

Optical dating of quartz sediments and accelerator mass spectrometry 14C dating of bone gelatin and moa eggshell: A comparison of age estimates for non‐archaeological deposits in New Zealand

A consensus has not been reached on the validity of “old” (pre‐Polynesian settlement) 14C ages for Pacific rat bones from New Zealand. As an independent test of their validity, we have applied optical dating techniques to fossiliferous sediments at three non‐archaeological sites in the North and South Islands. In this paper, we report the optical ages obtained from quartz grains and compare them with a suite of accelerator mass spectrometry (AMS) 14C ages obtained from the bone gelatin of Pacific rats (Rattus exulans) and five species of bird (four herbivores and one omnivore). An AMS 14C age is also reported for one sample of moa (Aves: Dinornithiformes) eggshell. All dated fossil remains were collected from known stratigraphic positions. Additional chronological control is provided by two known‐age volcanic tephras at the Hukanui sites in the North Island. At the South Island site (Earthquakes #1), an infilled burrow provides independent age control, in that fossils inside the burrow should yield younger ages than the sediments and fossils outside the burrow. Bone preservation is uniformly good at all sites, as shown by surface detail, nitrogen content, and C:N ratios. In addition, amino acid profiles are consistent with those of collagen from modern Pacific rats and the laboratory rat collagen standard. Single‐aliquot optical dating protocols were employed to avoid age overestimation due to incomplete bleaching of sediments before burial and to permit the identification of any post‐depositional disturbance. At the Hukanui sites, 14C ages on bird bone and eggshell agree with the optical ages for the enclosing sediments, and both chronologies are consistent with the accepted ages for the overlying Taupo Ignimbrite (c. 1850 yr BP) and the underlying Waimihia Tephra (c. 3300 yr BP). Two 14C ages from Finsch's duck (Chenonettafinschi) bones excavated from within the Taupo Ignimbrite agree with the accepted age of the ignimbrite. In addition, a gelatin sample from a Pacific rat bone reportedly excavated from beneath the Taupo Ignimbrite has a similar amino acid profile to that of modern collagen. This measure of good bone preservation provides confidence in the 14C age for this sample (1775 ± 93 yr BP), which accords with its position beneath the ignimbrite. At Earthquakes #1, five 14C determinations were made: three on Pacific rat, one on New Zealand pigeon (Hemiphaga novaeseelandiae), and one on South Island kokako (Callaeas cinerea). All five ages are consistent with the stratigraphic locations of the fossils, and with the optical ages for the enclosing sediments. We conclude that the early arrival of rats in New Zealand is supported by the array of chronological evidence presented here: the concordance of 14C and optical ages at each of the sites; the agreement between 14C ages on different materials and for a range of species; and the consistency of the 14C and optical ages with stratigraphic and other chronological markers. The Pacific rat was present in both the North and South Islands of New Zealand at least 800 years before permanent Polynesian settlement.

Preheat-induced signal enhancement in the infrared stimulated luminescence of young and bleached sediment samples

Natural and laboratory bleached surface and young samples of potassium feldspar sand separates and polymineral silt had their infrared stimulated luminescence (IRSL) signal measured before and after preheating at 220°C for 10 min or 160°C for 16 h. For both preheats, the laboratory bleached sand samples underwent a signal enhancement which was stable with laboratory storage. The youngest samples also showed natural signal enhancement. The silt sample showed no recuperation of bleached signal on preheating, but some in the natural signal. A range of filtered bleaches was applied to one surface sand sample. Signal levels before and after preheating were reduced by filtering out the UV from the bleaching spectrum. The unfiltered bleach, however, most closely reproduced the behaviour of the natural sample.

Circumventing possible inaccuracies of the single aliquot regeneration method for the optical dating of quartz

A simple method of assessing the reliability of the single aliquot regeneration method is presented, based on the repeated measurement of identical regeneration doses. It is shown that for the majority of the samples measured the requirement of a constant repeat point in each cycle is not met. The ability to maintain a constant repeat point is shown to be dependent upon preheating conditions, the requirements for which were found to vary at both inter-sample and inter-aliquot levels. The expected effect on estimated values of D e is calculated and shown to be significant. Three possible means of circumventing these inaccuracies are presented: (i) adjustment of the measurement procedure (specifically the preheat conditions); (ii) modelling and correcting for the effect, and (iii) stabilising the response prior to construction of the regenerated growth curve. It is suggested that much of the variation in D e observed between individual aliquots and between individual grains from well-bleached sediments may possibly be attributed to differences in preheating requirements, and that such differences may also account for some of the spread in D e for partially bleached sediments.

The origin of dose distributions in fluvial sediments, and the prospect of dating single grains from fluvial deposits using optically stimulated luminescence

We examine the causes of the asymmetric distributions of dose observed from measurements of the optically stimulated luminescence emitted by small aliquots of fluvial quartz, and deduce that the asymmetry arises as a result of samples being composed of a mix of mainly well bleached grains with grains that were effectively unbleached at the time of deposition. We demonstrate that the shapes of the dose distributions can be used to assess the likelihood that aliquots consist only of grains that were well-bleached at the time of deposition. The more asymmetric the distribution, the greater the probability that the aliquots with the lowest dose most closely represent the true burial dose. Single grains with differing doses are present in each of the samples examined, and the population with the lowest dose gives an optical age consistent with the expected burial age. This result implies that the beta-dose heterogeneity in these deposits is small, and that the effects of micro-dosimetric variations on optical dating of individual grains are not significant for these samples. We demonstrate that single-grain dating of fluvial material is possible and practicable using standard Risø optical dating equipment, and we conclude that application of a new regenerative-dose protocol to single grains of quartz, using the lowest dose population to estimate the burial dose, is the best available means of obtaining reliable luminescence ages for heterogeneously bleached fluvial sediments.

Simultaneous thermoluminescence and optically stimulated luminescence dating of late Pleistocene sediments from Lake Baikal

When the thermoluminescence (TL) method is applied to dating of partially bleached sediments the need to correct for the residual TL signal left in the mineral crystals at time of deposition is a complicated problem. In dating sediments from Lake Baikal we have attempted a combination of TL and optical stimulated luminescence (OSL). The method was applied to three samples from the uppermost 50m of the sediment cored within the Baikal drilling project in 1993. The results indicate a fairly constant sedimentation rate back to 250 ka BP. This contradicts with results from radiocarbon dating of the same core but agrees reasonably well with preliminary paleomagnetic data and disequilibrium U–Th age determinations.

Comparison of 14C and luminescence chronologies at puritjarra rock shelter, central Australia

At Puritjarra rock shelter a long comparative sequence permits detailed comparisons of luminescence and radiocarbon chronologies over 35,000 years. Both techniques produce self-consistent chronologies for the Puritjarra deposit, but observed discrepancies between luminescence dates on unburnt sediments and 14C, assays on charcoal are greater than reported elsewhere. TL ages are generally older than 14C ages but dates converge at depths below 70 cm, dating first significant human occupation of the rock shelter ∼35,000 years ago. The discrepancies are not removed by calibrating 14C dates or adjusting TL ages for lower water content of sediments. The 14C chronology is broadly supported by sedimentary or palaeobotanical evidence, stone artefact typology and other archaeological data. Radiocarbon dates on intact hearths agree with those on detrital charcoal. TL ages were arrived at using both ‘total bleach’ and ‘selective bleach’ methods and the latter agree with optical ages for the same samples. Incomplete bleaching of sediments during deposition can be ruled out. Incorporation of old material into the luminescence samples via in-situ disintegration of local sandstone remains a possibility and will be an important issue to resolve as luminescence techniques are increasingly deployed to date archaeological deposits in rock shelters.

Optical dating: Insufficiently bleached sediments

Although infrared-stimulated luminescence (IRSL) signals in feldspars can be bleached rapidly by sunlight, there could still be a small amount of IRSL signal remaining in sediment grains if they have experienced only relatively short sunlight exposure before deposition. This remaining signal results in a “remnant dose” stored in the grain and is important for young samples but negligible for old samples. Several methods have been introduced to detect insufficient bleaching, and some of them can provide information on the extent of the bleaching. Others can only distinguish between samples bleached for a very long time and for a short time. Empirical methods are introduced to evaluate the possible values of equivalent dose accumulated since the grains were deposited. These methods involve the analysis of the equivalent doses and the natural IRSL signals obtained using single-disc dose determination methods.

Natural IRSL intensities and apparent luminescence ages of single feldspar grains extracted from partially bleached sediments

The single aliquot technique has been applied to single grains of K-feldspar extracted from a well-dated late-glacial marine sediment sample for which standard luminescence dating yielded ages that were in excess of the expected age. Natural infrared-stimulated luminescence (IRSL) intensities as well as single grain palaeodoses show a wide range of values. Most of the bright grains yield equivalent doses largely in excess of the expected palaeodose, considering the depositional age of the sample. The luminescence emitted by the brightest grains would dominate the signal emitted from aliquots containing many grains. This explains the overestimation of ages obtained using standard luminescence techniques for the sample investigated. Palaeodoses obtained from grains that exhibit low IRSL intensities are close to the expected dose. However, the age derived from these grains is lower than the expected age. Anomalous fading is believed to be the main cause for the age underestimation. This study is the first demonstration of the feasibility of dating a sedimentary event using the luminescence of a single mineral grain.

Aeolian stratigraphy and thermoluminescence dating of sediments of late Holocene age from Sola, southwest Norway

Thermoluminescence age determinations were performed on four aeolian sand samples of late Holocene scenceagesediments from Stavanger airport, Sola, in southwest Norway. The locality is well suited for testing thermoluminescence dating because of a good agreement between radiocarbon ages and age information based on archaeological typology and sea level changes. The conclusion from a comparison of thermoluminescence and archaeologicaltypologyradiocarbon dates is that it is possible to date well bleached sediments as young as 1600 years with thermoluminescence (TL). The TL dates for the aeolian deposits are in accord with expected ages. Six formal, diachronous aeolian/non‐aeolian phases are defined and grouped together as the Sola Episode. The first phase onousaeolian/non‐aeolianof aeolian activity was short‐lasting and took place in the middle Subboreal chronozone. A second and lianactivitywaslong‐lasting period, dominated by aeolian activity, took place from the transition between the Subboreal and Subatlantic chronozones until the present. We conclude that human activities were the releasing factor for the aeolian activity recorded in sand sheets.

Optical dating: a non-specialist review

Optical dating was developed primarily for sediments (though application to pottery has also been made) and has the particular advantage over thermoluminescence (TL) that at deposition the latent signal is orders of magnitude smaller: this is particularly important for sediments deposited in water. For routine dating of quartz the signal is stimulated by green light, from an argon ion laser, from a filtered xenon lamp, from a filtered halogen lamp or from light-emitting diodes (though at much lower intensity): the dominant emission wavelength is in the ultraviolet. With quartz, the limitation in age range is due to saturation rather than poor stability. For many types of feldspar there is a remarkable resonance in the stimulation spectrum in the near infra-red (IR). This leads to the widespread use of IR-emitting diodes for stimulation; the whole range of visible wavelengths is then available for detection and this gives the possibility of some degree of mineral selectivity by means of colour filters. The IR signal appears to have comparable stability to that of the green-stimulated signal and TL — no worse, no better; it has the advantage over both that it is more easily bleachable by daylight. In general, the signal from feldspars is much brighter than from quartz and this has allowed the development of ‘single aliquot dating’. with advantages in precision and convenience as well as in application to poorly bleached sediments.

Procedures used for luminescence dating of sediments

An account of the procedures for thermoluminescence (TL) and optically stimulated luminescence (OSL) dating of sediments used at the Nordic Laboratory for Luminescence Dating is given. The topics discussed include sampling, sample preparation, separation of quartz and feldspars using heavy liquids, dose rate estimation and methods for estimating the equivalent dose. Special attention is paid to the partial bleach TL technique of Mejdahl (1988), which appears to give reliable results for partially bleached sediments. Optically stimulated luminescence dating using stimulation by infra-red (IR) and green light has been taken up and especially added-dose methods using stimulation by IR or green light have been tried with some success. Some sources of error are discussed, including the adverse effect of shallow traps.

Perennial problems in the study of laterite: A review

There are perennial problems in the investigation of laterite. (i) The development of an acceptable terminology to describe the forms and processes associated with laterite genesis. Some workers consider that the use of the term laterite should be discontinued, because of its genetic connotations and the uncritical application of the term to a wide variety of materials. The terms ferricrete or lateritic duricrust are sometimes preferred. (ii) Identifying and classifying lateritic materials. (iii) The mode of formation of the laterite; by in situ weathering, relative or absolute accumulation or by continual weathering and erosion of a long exposed landscape. The relationships of laterite to zones of mottled and bleached bedrock and sediment are critical. (iv) Determining the age of the laterite and the weathering processes responsible for its formation and its relationship to the age of the surface on which it rests. Critical investigations include determining the age of the underlying materials, the age o...

Luminescence sensitivity change due to bleaching of sediments

Different sensitivity changes of luminescence signals were observed in colluvial and aeolian sediments. It is suggested that not only does the sensitivity change relate to the laboratory bleaching and the age of the sample, but it also relates to the exposure prior to deposition. This sensitivity change is explained by a model, which involves the competition of hard and easy-to-bleach trapped charges. The implications for optical dating of sediments are discussed.

Applications and limitations of thermoluminescence to date quaternary sediments

The thermoluminescence (TL) method dates sediment by quantifying time-dependent dosimetric properties of mineral grains. Sunlight exposure of the sediment during transportation and deposition or heating resets the TL clock to time ‘zero’. After burial of the mineral grains, ionizing radiation from the decay of radioisotopes imparts a TL signal. This accumulated TL signal is proportional to the equivalent dose (ED), which is a measure of radiation exposure during the burial period. A TL age estimate is determined from the ratio of the ED to the environmental radiation (dose rate). The calculation of an ED is influenced by the susceptibility of sediments to optical bleaching, the emission spectra of minerals, and possible long and short term fading. There are three basic methods for determining an ED: regeneration, total and partial bleach. The regeneration technique may yield under-estimates because of sensitivity changes with laboratory bleaching. The latter two techniques are most useful for well and partially light bleached sediments, respectively. A number of sediments can be dated by the TL method. Eolian and a variety of terrestrial sediments yield apparently accurate TL age estimates, though there are possible complications associated with pedogenesis. It appears that some fluvial sediment can be accurately dated by TL, but little is known about the efficiency of optical bleaching for fluvial depositional systems. Till and glacier-proximal deposits are inadequately light bleached to be dated by TL. Sediments deposited distal from the glacier margin, which are at least partially bleached, are preferred for dating. Future research is needed to understand better the relationship between sedimentary environments and the extent of light bleaching. Also, studies should concentrate on dating feldspar extracts from well-dated sequences to more fully ascertain the temporal limitations of the method. Only by understanding the interplay of sedimentology, TL dosimetry and environmental radiation can the TL method be effectively applied to the myriad of depositional environments spanning the late to middle Pleistocene.

TL dating of partially bleached sediments by the regeneration method

Thermoluminescence dating of sediments in the Warsaw laboratory is based on blue-to-green emission. The ‘partial bleach + regeneration’ method is proposed for dating sediments which have not been completely bleached at the time of burial. Correct TL ages are reported for sediments covering organic deposits of the Eemian Interglacial and also for glacial sediments of Würm and Riss age. We obtained good results for dating till using the partial bleach + regeneration method. The TL zeroing in till is explained by the ‘rubbing’ or compression of grains beneath the glacier and by the freeze-thaw activity occurring during glacial transport and deglaciation.

The plateau method for dating partially bleached sediments by thermoluminescence

The plateau method for dating sediments that have been incompletely bleached prior to deposition is described. By exposing aliquots of a sample to sunlight for different periods of time, a range of residual values is obtained and combined with the additive dose or the regeneration technique to yield a series of paleodoses. The correct paleodose is taken as that for which the longest plateau in a plot of paleodose vs. temperature is obtained.

Some experiences from an attempt to date post-glacial dunes from Finland by thermoluminescence

In an attempt to study the bleaching of sediments during deposition a few sand dunes ranging in age from 2 to 11 ka have been TL dated. Both quartz and feldspar grains were used. In calculating the ED both the R - β method and direct subtraction of the residual left after a long bleaching time were attempted. The results show that the R - β method gives somewhat lower ED values. The corresponding ages are in agreement with the assumed ages of the dunes.