Michel Lamothe

Measurement of anomalous fading for feldspar IRSL using SAR

Abstract Age underestimates and wide equivalent paleodose distributions are commonly observed in IRSL dating of feldspar minerals using both multiple and single aliquot methods. For well-bleached sediments, this is thought to be caused primarily by anomalous fading. This article explains how to assess a fading rate ( g value), which can be used to correct the measured optical ages. It also shows how different fading rates are obtained on single-aliquots using different protocols. It is found that the g value deduced from delayed L i / T i measurements is strongly dependent on the timing of the preheat treatment. For the single-aliquot regenerative-dose protocol (SAR), best results are obtained using delayed L i / T i ratios if the samples are preheated before storage.

Towards a prediction of long-term anomalous fading of feldspar IRSL

Anomalous fading of the infrared stimulated luminescence in feldspar is described by a new equation in which the unwanted loss of luminescence is given as a function of three variables, the measured fading rate, the laboratory radiation dose rate, and the environmental radiation dose rate. This equation is labelled dose rate correction (DRC) and it is tested on (a) young sediment samples for which the radiation growth curve is in the linear part of the dose–response curve, and (b) geologically old sediments in luminescence field saturation. Most of the experimental results are consistent with predictions that can be deduced from this equation. Given the common range of dose rates in luminescence laboratories and in natural soil environments, the time necessary for the unstable luminescence generated by artificial irradiation to fade away is predicted to be ca. 104–105 years.

Initiation and development of the Laurentide and Cordilleran Ice Sheets following the last interglaciation

Abstract Fossil records from sites overridden by or adjacent to the Laurentide Ice Sheet indicate that the climate of the last interglaciation (Oxygen-Isotope Substage 5e, ca. 130-116 ka) was warmer than today. Following the last interglaciation, the Laurentide Ice Sheet first developed during Stage 5 over Keewatin, Quebec and Baffin Island. Along its northern margin, the ice sheet reached its maximum extent of the last glaciation during Stage 5. The ice sheet advanced across Baffin Island onto the continental shelf early during Stage 5 (5d?), whereas the advance into the western Canadian Arctic occurred late during Stage 5 (5b?). The ice sheet also may have advanced into the St Lawrence Lowland during Substage 5b, although this event may be younger (Stage 4). The Hudson Bay lowland became ice-free during Substage 5a. Retreat of the ice sheet on Baffin Island occurred during late Stage 5, probably Substage 5a. The exact timing of retreat from the western Canadian Arctic is unknown, but it occurred before 48 ka. The southern sector, including the St Lawrence Lowland, was ice-free during late Stage 5. The Hudson Bay lowland may have remained ice free through Stage 4 and much of Stage 3. Because of conflicting chronologies, however, it is more likely that this area was glaciated throughout Stage 3 and perhaps Stage 4. Nevertheless, the data demonstrate that the lowland was ice-free during part of the last glaciation. An independent ice cap developed over the Appalachian Uplands and advanced across Nova Scotia during Stage 4, perhaps as far as the edge of the continental shelf. The ice cap remained active over Nova Scotia as a setellite to the Laurentide Ice Sheet throughout the remainder of the last glaciation. The ice sheet advanced into the St Lawrence Lowland during Stage 4 and subsequently overwhelmed the local ice cap in the Appalachian Uplands, advancing perhaps into northern New England, but not farther south. The Lowland remained covered by the ice sheet until late Stage 2. The ice sheet may also have advanced into the Lake Ontario basin during Stage 4. The position of the northern margin of the Laurentide Ice Sheet during Stage 4 is not known, but it remained an unknown distance behind its maximum position reached during Stage 5. Cores from Baffin Bay indicate a substantial decrease in high-latitude glaciation during Stage 4. Following retreat, the Keewatin sector of the ice sheet may have remained over much of northwestern Canada as a quasi-stable ice mass until it readvanced during Stage 2. Similarly, the Baffin Island sector of the ice sheet may have remained largely intact. The southern margin of the ice sheet may have advanced into the Lake Ontario basin and upper Mississippi Valley during the middle of Stage 3 (ca. 50 ka), reaching its maximum extent of the last glaciation during Stage 2 (ca. 18–21 ka). Advance of the northern margin was younger (ca. 8–13 ka) than that of the southern margin; this advance was less extensive than the penultimate advance (Stage 5). Paleoenvironmental records indicate that the last interglaciation in areas covered by and near the Cordilleran Ice Sheet was as warm as, or warmer than, present. The Cordilleran Ice Sheet appears to have developed during Stage 5 or 4. At that time, it advanced over southern British Columbia and into the northern Puget Lowland. There is no record of this event in northern areas that were later covered by the ice sheet. The ice sheet disappeared before 59 ka, at the beginning of a lengthy nonglacial interval. Paleoenvironmental records indicate that climate was similar to the present during part of this interval. The ice sheet was absent, and glaciers probably were confined to mountain areas, throughout Stage 3. Climatic deterioration marking the end of this nonglacial interval may have begun as early as 29 ka. By 14–15 ka, the ice sheet had achieved its maximum extent of the last glaciation. Because there are few suitable dating methods capable of resolving events beyond the radiocarbon limit and because sites that preserve a record of events from the last glaciation are spatially restricted, we consider this synthesis as tentative and subject to significant revision as dating methods improve. Nevertheless, this perspective of the North American ice sheets through the last glaciation demonstrates their complex and dynamic behavior and attendant rapid fluctuations in ice volume.

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

Abstract 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.

The blue emission of K-feldspar coarse grains and its potential for overcoming TL age underestimation

Abstract This paper presents preliminary TL ages on K-feldspar coarse grains from raised shallow-marine sediments of NW Europe and Southern Italy. Comparisons between TL ages using different optical filters suggest the use of the blue-transmitting Corning 7–59 filter allows measurement of an apparently stable TL component in K-feldspar which could overcome the problem of age underestimation in the 75–300 ka range.

Comparison of TL and IRSL age estimates of feldspar coarse grains from waterlain sediments

Abstract The infrared stimulated luminescence (IRSL) and thermoluminescence (TL) dating methods are applied to alkali feldspar coarse grains from shallow marine, estuarine, shallow lacustrine and sublittoral sediments in the range 10–200 ka. No significant difference was observed between the equivalent doses obtained using IRSL and TL. Most luminescence dates are consistent with the expected geological age estimates obtained from radiocarbon, amino acid and ESR dates. This study provides evidence of the reliability of IRSL age estimates on alkali feldspars from well-bleached waterlain sediments older than 100 ka, provided that a correction for long-term thermal stability is applied in each case.

Variability of infrared stimulated luminescence properties from fractured feldspar grains

Abstract 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.

Luminescence chronology of Pleistocene loess deposits from Romania: testing methods of age correction for anomalous fading in alkali feldspars

The IRSL dating method is applied to silt-sized alkali feldspar grains (40–60mm) from Upper and Middle Pleistocene loess deposits of southeastern Romania (sites of Tuzla and Giurgiu Malu-Rosu), using both the multiple aliquot g dose technique and the SAR method. All samples show evidence of anomalous fading; the measured IRSL ages are in correct stratigraphic order but systematically underestimate the expected geological age (up to 40%). Three protocols of age correction for the observed fading have been tested: the correction of Huntley and Lamothe (Can. J. Earth Sci. 38 (2001) 1093), the correction of Mejdahl (Quat. Sci. Rev. 7 (1988) 357) and the correction model suggested by Wintle (Quat. Sci. Rev. 9 (1990) 385). The three protocols yield corrected IRSL ages that are in better agreement with the expected ages. This study demonstrates the potential of the IRSL dating method to provide chronological information on Upper and Middle Pleistocene loess deposits of Romania. r 2003 Elsevier Science Ltd. All rights reserved.

Assessing the datability of young sediments by IRSL using an intrinsic laboratory protocol

Abstract A series of young sediment samples, for which most of the IRSL dates are in complete disagreement with available stratigraphic information, are used to develop a laboratory protocol based only on luminescence measurements to assess their datability. A parameter R 1 is introduced, defined as the increase of luminescence upon added dose for individual grains. The statistical distribution of this parameter for a sediment is shown to be an indicator of homogeneity of bleaching at time of deposition. A second set of measurements is performed on the same group of grains in which the luminescence is monitored over a limited period of time. This is in order to assess luminescence stability and, hence, fading. This protocol should be used for acceptance or rejection of samples in the course of a luminescence dating program.

Concepts and approaches to in situ luminescence dating of martian sediments

In this paper we present the concept of a robotic instrument for in situ luminescence dating of near-surface sediments on Mars. The scientific objectives and advantages to be gained from the development of such an instrument are described, and the challenges presented by the Mars surface environment to the design and operation of the instrument are outlined.