Richard M. Bailey

Partial bleaching and the decay form characteristics of quartz OSL

Abstract Three exponential components have been isolated from observed high temperature OSL decay data of quartz. These components have been found to display differential bleaching and growth characteristics. It is postulated that changes in the ratio of the first two components (the “fast” and the “medium”) are responsible for the changes in decay form observed in partially bleached samples. Measurements have shown that a comparison of the observed ratios to the expected ratios (had a sample been fully bleached) may be able to differentiate sediments for which the OSL has only been partially reset prior to deposition.

Biodiversity baselines, thresholds and resilience: testing predictions and assumptions using palaeoecological data

Fossil records are replete with examples of long-term biotic responses to past climate change. One particularly useful set of records are those preserved in lake and marine sediments, recording both climate changes and corresponding biotic responses. Recently there has been increasing focus on the need for conservation of ecological and evolutionary processes in the face of climate change. We review key areas where palaeoecological archives contribute to this conservation goal, namely: (i) determination of rates and nature of biodiversity response to climate change; (ii) climate processes responsible for ecological thresholds; (iii) identification of ecological resilience to climate change; and (iv) management of novel ecosystems. We stress the importance of long-term palaeoecological data in fully understanding contemporary and future biotic responses.

Towards a general kinetic model for optically and thermally stimulated luminescence of quartz

An attempt to produce a self-consistent general model of quartz luminescence behaviour (thermally and optically stimulated) is presented. Results from a numerical simulation of the quartz electronic system are described. The processes of choosing and testing the numerical parameters are described in detail and the results of the modelling are compared to a wide range of laboratory measurements of several different natural quartz samples. The model comprises five electron trapping centres and four hole trapping (recombination) centres (empirical evidence for multiple hole trapping centres is described). The model is shown to reproduce, to a high level of accuracy, the empirical data for measurements of dose response, phototransfer efficiency, dose quenching, thermal activation, optical de-sensitisation, optical sensitisation, OSL temperature dependence and power dependence, among others, of the natural sedimentary quartz samples measured. The relationship of TL and OSL sensitivity is also examined and the effects of dose quenching on OSL described. A development of the Zimmerman model for dose quenching and pre-dose sensitisation is suggested and the variation in sensitisation characteristics between samples is examined. It is shown that by varying the concentration of hole trapping centres in the model over a relatively small range, it is possible to produce a similarly wide variation in both dose quenching and subsequent thermal activation characteristics to that observed empirically.

The Early to Middle Stone Age transition and the emergence of modern human behaviour at site 8-B-11, Sai Island, Sudan

Site 8-B-11 at Sai Island in northern Sudan is a stratified site containing late Middle and early Upper Pleistocene occupation levels in excellent conditions of preservation. In Middle Pleistocene times, the banks of a small gully were repeatedly occupied by human groups leaving Acheulean and Sangoan material cultures in an interstratified pattern. Optical age determinations on aeolian intercalations within the gully sediments range between 220 and 150 ka. This sequence is truncated by Nile floodplain silts in which three occupation levels with Lupemban-related Nubian Complex assemblages (Van Peer, 1998) are stratified. The long archaeological sequence at 8-B-11 is a rare African case to document the Early to Middle Stone Age transition by means of primary context situations in direct stratigraphic super-position (Clark, 2001; Tryon & McBrearty, 2002). In contrast to the Acheulean, the early MSA Sangoan levels show sophisticated behaviours involving considerable technological and symbolic investment. Quartzite cobbles were used in the grinding of vegetal materials. Yellow and red ochre were exploited and ground to pigments using shaped mortars and selected chert nodules. We conclude that 8-B-11 is a key site with regard to the initial emergence of modern human behaviour outside subsaharan Africa (McBrearty Brooks, 2000).

Component-resolved bleaching spectra of quartz optically stimulated luminescence: preliminary results and implications for dating

Abstract Bleaching spectra of the ‘fast’ and ‘medium’ optically stimulated luminescence (OSL) components of quartz are reported. A dependence of photoionization cross-section, σ , on wavelength was observed for the fast and medium components and a significant difference in their responses to stimulation wavelength was found. The ratio of the fast and medium photoionization cross-sections, σ fast / σ medium , varied from 30.6 when stimulated with 590 nm light to 1.4 at 375 nm . At 830 nm the fast and medium photoionization cross-sections were found to be sufficiently different that infrared bleaching at raised temperatures allowed the selective removal of the fast component with negligible depletion of the medium. A method for optically separating the OSL components of quartz is suggested, based on the wavelength dependence of photoionization cross-sections.

Style and timing of glaciation in the Lahul Himalaya, northern India: a framework for reconstructing late Quaternary palaeoclimatic change in the western Himalayas

This paper presents a revised glacial chronology for the Lahul Himalaya and provides the most detailed reconstruction of former glacier extents in the western Himalayas published to date. On the basis of detailed geomorphological mapping, morphostratigraphy, and absolute and relative dating, three glaciations and two glacial advances are constrained. The oldest glaciation (Chandra glacial stage) is represented by glacially eroded benches and drumlins (the first to be described from the Himalaya) at altitudes of .4300 m and indicates glaciation on a landscape of broad valleys that had minimal fluvial incision. The second glaciation (Batal glacial stage) is represented by highly weathered and disssected lateral moraines and drumlins representing two phases of glaciation within the Batal glacial stage (Batal I and Batal II). The Batal stage was an extensive valley glaciation interrupted by a readvance that produced superimposed bedforms. Optically stimulated luminescence (OSL) dating, indicates that glaciers probably started to retreat between 43 400 ± 10 300 and 36900 ± 8400 yr ago during the Batal stage. The Batal stage may be equivalent to marine Oxygen Isotope Stage 4 and early Oxygen Isotope Stage 3. The third glaciation (Kulti glacial stage), is represented by well-preserved moraines in the main tributary valleys that formed due to a less-extensive valley glaciation when ice advanced no more than 12 km from present ice margins. On the basis of an OSL age for deltaic sands and gravels that underlie tills of Kulti age, the Kulti glaciation is younger than 36900 ± 8400 yr ago. The development of peat bogs, having a basal age of 9160 ± 70 14C yr BP possibly represents a phase of climatic amelioration coincident with post-Kulti deglaciation. The Kulti glaciation, therefore, is probably equivalent to all or parts of late Oxygen Isotope Stage 3, Stage 2 and early Stage 1. Two minor advances (Sonapani I and II) are represented by small sharp-crested moraines within a few kilometres of glacier termini. On the basis of relative weathering, the Sonapani advance is possibly of early mid-Holocene age, whereas the Sonapani II advance is historical. The change in style and extent of glaciation is attributed to topographic controls produced by fluvial incision and by increasing aridity during the Quaternary.  1997 by John Wiley & Sons, Ltd.

Identification of partial resetting using De as a function of illumination time

Abstract Modern age samples from various depositional environments were examined for signal resetting. For 19 modern aeolian/beach samples all D e values obtained were Gy , with ∼70% having D e Gy . For 21 fluvial/colluvial samples, all D e values were Gy with ∼80% being Gy . D e as a function of illumination (OSL measurement) time ( D e ( t )) plots were examined for all samples. Based on previous laboratory experiments, increases in D e ( t ) were expected for partially reset samples, and constant D e ( t ) for fully reset samples. All aeolian samples, both modern age and additional ‘young’ samples ( D e ( t ) while all modern, non-zero D e , fluvial/colluvial samples showed increasing D e ( t ). ‘Replacement plots’, where a regenerated signal is substituted for the natural, yielded constant (flat) D e ( t ). These findings support strongly the use of D e ( t ) as a method of identifying incomplete resetting in fluvial samples. Potential complicating factors, such as illumination (bleaching) spectrum, thermal instability and component composition are discussed and a series of internal checks on the applicability of the D e ( t ) for each individual aliquot/grain level are outlined.

Fire and climate change impacts on lowland forest composition in northern Congo during the last 2580 years from palaeoecological analyses of a seasonally flooded swamp

The mixed semi-evergreen forests in lowland central equatorial Africa can contain many elements of secondary vegetation. This raises the question of what factors have determined the current forest composition in this region. Is this forest in the process of succession after natural climatic variation and/or anthropogenic disturbances in the past, or is it a stable forest type? This paper presents a multiproxy palaeoecological analysis of a sedimentary sequence taken from a small sedimentary basin located in logged semi-evergreen lowland forest in northern Congo-Brazzaville which addresses these questions. Analyses undertaken included fossil pollen, geochemical and microscopic charcoal. Geochemical results were interpreted as a proxy for rainfall, and showed that northern Congo has experienced changes in rainfall during the past 2580 years, while microscopic charcoal concentrations indicated increased burning from approximately 1240 cal. yr BP to the present. Analysis of the fossil pollen assemblages showed that although light-demanding taxa were a major constituent of this forest throughout the sequence, shade-tolerant taxa were more abundant in the forest prior to 1345 cal. yr BP. Increases in aridity and/or droughts after 1345 cal. yr BP, and an increase in fire after 1240 cal. yr BP resulted in a semi-evergreen forest with abundant pioneers that persisted with minor changes in composition during the last 900 years, even during a period of increased rainfall from 400 to 100 cal. yr BP. These data may guide conservation policy by predicting potential consequences of future climate change and the impact of logging.

OSL properties of NaCl relative to dating and dosimetry

Abstract The luminescence properties of NaCl are discussed. Attention is focussed predominantly upon the optically stimulated luminescence (OSL) signal (UV emission) of NaCl (420–560 nm stimulation). The results from experiments relevant to geological dating applications, such as signal resetting, dose response and thermal stability, are described. The behaviour of the OSL signal observed was found to be favourable for dating, with both the thermal stability and dose response suggesting a range of at least 10 ka. Signal resetting via optical exposure was found to be extremely rapid and no evidence of either thermoluminescence or OSL signals was found following re-crystallization. The potential for dating natural salt deposits, as found in various arid environments, appears to be good.

Paper I: The use of measurement-time dependent single-aliquot equivalent-dose estimates from quartz in the identification of incomplete signal resetting

Abstract The identification of incomplete signal re-setting of optically stimulated luminescence signals in sedimentary quartz is a vitally important step in the continued improvement of optical dating. It is shown that narrow spectrum (blue-green) laboratory partial bleaching of aliquots of natural sedimentary quartz has a significant effect on equivalent dose (measured using a single aliquot procedure) calculated as a function of measurement time (De(t)). The blue-green stimulation spectrum mimics that found underwater and the results suggest that incomplete re-setting of waterlain sediments may be possible using the De(t) method.