The development and application of luminescence dating to loess ...

The development and application of luminescence dating to loess deposits: a perspective on the past, present and future HELEN M. ROBERTS

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Roberts, H. M. 2008 (November): The development and application of luminescence dating to loess deposits: a perspective on the past, present and future. Boreas, Vol. 37, pp. 483–507. 10.1111/j.1502-3885.2008.00057.x. ISSN 0300-9483. Loess deposits preserve important records of Quaternary climate change and atmospheric dust flux; however, their full significance can only be revealed once a reliable chronology is established. Our understanding of loesspalaeosol sequences and the development of luminescence dating techniques have progressed hand-in-hand over the past 25 years, with each subject informing the advancement of the other. This article considers the development and application of luminescence dating techniques to loess deposits from the early days of thermoluminescence (TL) to the optically stimulated luminescence (OSL) methods utilized today. Recent technological and methodological advances have led to a step-change in the accuracy and precision of quartz OSL ages; this has led to an expansion of high-resolution luminescence studies, which in turn are informing loess studies and challenging some of the basic ideas regarding the nature of loess records, their formation and their significance. Future luminescence research efforts are likely to focus on extending the age range of luminescence techniques, possibly by utilizing new luminescence signals; this, again, will allow investigation of the long-term variability of loess records in comparison with other long records of climate change to which they are frequently compared. Helen M. Roberts (e-mail: [email protected]), Institute of Geography and Earth Sciences, Aberystwyth University, Ceredigion, Wales, SY23 3DB, UK; received 21st March 2008, accepted 20th July 2008.

Loess deposits were the first terrestrial sediments to which luminescence dating was systematically applied, and on which it was tested and developed. Since the initial ground-breaking work of Wintle (1981, 1982) and Wintle & Brunnacker (1982), a great complementary link has developed between loess and luminescence studies, with both fields of study supporting the development and understanding of the other. Loess deposits provided an ideal testing-ground for early luminescence techniques, because the stratigraphy was so clearly defined and the broad framework of the chronology was reasonably well established, and, due to the long timespan and essentially continuous nature of many loess deposits, records of both glacial and interglacial periods were preserved. It was also easier to obtain loess samples than samples from marine cores, where the chronology from oxygen isotopes was more direct. Furthermore, the characteristics of loess make it an ideal material for testing luminescence techniques; luminescence dating techniques have been principally developed for application to grains of feldspar and quartz, which are both typically abundant in loess, and, because loess is an aeolian deposit and its fine-grained nature implies medium-to-long transport distances, any previous luminescence signal should be removed (or ‘bleached’) prior to deposition. Similarly, luminescence dating is an important tool for developing numerical chronologies for loess deposits, thereby allowing comparison of different records and the investigation of correlations between records. The age range of luminescence techniques is also well

suited to loess studies, comfortably spanning a wide time period from 10’s of years to approximately 55–70 kyr for quartz from some loess deposits, but perhaps up to 125 kyr or even much further in some cases (this is discussed in further detail later). In addition, the event being dated is the last exposure of the sediments to daylight, meaning that the luminescence age relates directly to the time of deposition of the sediment. Given that loess deposits provide a direct record of atmospheric circulation, and also that, because of their temporally extensive nature, they provide one of the most detailed and significant terrestrial records of climate change found on Earth, the ability to date directly the deposition event over a glacial–interglacial time scale is one of the great strengths of determining a numerical chronology using luminescence techniques. Thus, the study of loess records and the development of luminescence dating complement each other strongly. Consequently, over the years the development and testing of luminescence techniques and their application to loess deposits has gone hand-in-hand. One inevitable consequence of so much pioneering luminescence research being conducted on loess deposits is that some luminescence ages produced during developmental phases are now questionable. Unfortunately for the non-specialist, the rapid pace of change has resulted in a plethora of potentially confusing techniques and terminology associated with luminescence dating; as a result, it can be difficult for the non-specialist to compare and assess the reliability of published luminescence ages. This article seeks to provide an overview

DOI 10.1111/j.1502-3885.2008.00057.x r 2008 The Author, Journal compilation r 2008 The Boreas Collegium

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of the development of luminescence dating techniques and of the application to loess deposits, from the early days of luminescence dating of sediments to recent advances; hopefully, this will help non-specialists assess published luminescence ages. The article also looks to the future, discussing some of the challenges posed in loess research and the potential solutions offered by luminescence dating.

Physical characteristics and definition of loess Many workers have attempted to define ‘loess’ (e.g. see discussions by Smalley & Vita-Finzi 1968; Liu et al. 1985; Pye 1987, 1995; Pe´csi 1990). A simple definition offered by Pye (1995) is that loess is a ‘terrestrial clastic sediment, composed predominantly of silt-size particles, which is formed essentially by the accumulation of wind-blown dust’. Most loess deposits are dominated by silt-sized (
4–63 mm) quartz and feldspars, and ‘typical’ loess has a modal grain size of 30 mm (Pye 1995). However, the sand (63 mm to 2 mm) and clay (o
4 mm) content, and thus the median and modal grain size, can vary greatly between deposits. Indeed, variation in loess particle size is often exploited to infer changing palaeowind characteristics (see review in Muhs & Bettis 2003). The grain size used for luminescence dating of loess can vary; most luminescence ages published for loess, particularly early luminescence ages, have been determined using fine-silt sized (4–11 mm diameter) grains of mixed-mineralogy (i.e. quartz and feldspar) (e.g. Forman 1991; Zhou et al. 1992; Musson et al. 1994; Musson & Wintle 1994; Richardson et al. 1997; Frechen 1999a, b; Frechen et al. 2005), although more recently fine-silt sized quartz (e.g. Watanuki et al. 2005; Lu et al. 2006; Zhang & Zhou 2007) and coarsesilt sized quartz (within the range 35–63 mm diameter, e.g. Roberts et al. 2003; Lai & Wintle 2006; Stevens et al. 2006; Lai et al. 2007a) and sand-sized quartz grains (463 mm diameter, e.g. Zander et al. 2000; Hilgers et al. 2001; Miao et al. 2005; Zhao et al. 2007; Buylaert et al. 2008) have also been used. Ideally, the grain size selected for luminescence dating should reflect the modal grain size of the deposit in order for the grains to be representative. However, where possible it is prudent to avoid grain size ranges that invoke complex dosimetry calculations, and also to use a narrow grain size range in order to minimize grain size-dependent effects related to the use of a beta source in the laboratory (e.g. see Wintle & Aitken 1977 and Armitage & Bailey 2005, for further discussion). The minerals typically making a significant contribution to the composition of loess deposits are quartz (50–70%), feldspars (5–30%), mica (5–10%), carbonates (0–30%) and clay minerals (10–15%) (Pye 1987). However, the geology of the loess source areas influences the mineral composition of the deposits and

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can cause them to vary significantly world-wide and even regionally (e.g. Muhs & Bettis 2003). Both feldspars and quartz have been used for luminescence dating of loess; the advantages and disadvantages of both of these minerals for luminescence dating are considered later in this article. Loess and palaeosol units are often clearly discernible in the field. The classic Quaternary loess sequence preserved at Luochuan in the Chinese Loess Plateau is shown in Fig. 1, to illustrate the intercalated yellow-buff coloured loess units (prefix ‘L’, with each unit being numbered) and the red-brown coloured palaeosol sequences (prefix ‘S’), formed during alternating cold and warm climate conditions, respectively. The triple ‘S5’ palaeosol complex can clearly be identified in Fig. 1 as the darkest and most developed palaeosol unit in this sequence, corresponding to Oxygen Isotope Stages (OIS) 13, 14 and 15. It is this linking of these soils to the marine oxygen isotopes stages, and their chronology based on the orbital time scale, that is used to provide an independent time scale for the glacial/ interglacial deposits of loess and palaeosols.

Principles of luminescence dating techniques Luminescence dating techniques are typically applied to mineral grains of quartz and feldspar. In the laboratory, the mineral grains are stimulated with heat or with light of a certain wavelength, and this gives rise to the emission of a light signal which is measured. If the sample is stimulated in the laboratory using heat, the

Fig. 1. The uppermost part of the classic Quaternary loess–palaeosol sequence at Luochuan, central Chinese Loess Plateau. Some of the prominent yellow-buff coloured loess (‘L’) and red-brown coloured palaeosol (‘S’) units are labelled to demonstrate the alternating loess–palaeosol sequence. Note the three persons for scale, located towards the centre of the image, just above palaeosol S4.

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Development and application of luminescence dating to loess deposits

resultant signal is termed ‘thermoluminescence’ (TL), and if the sample is stimulated using light, then the term ‘optically stimulated luminescence’ (OSL) is used. The luminescence signals observed arise from exposure of the grains to ionizing radiation (principally from the decay of uranium, thorium and potassium in the sediment, and from cosmic radiation) in the natural environment, and the signal builds up steadily over time following deposition of the sediment grains. For sedimentary grains, the mechanism of resetting the luminescence signal is typically exposure to sunlight during transport, termed ‘bleaching’. Following deposition, the bleached grains are covered by other grains that form new sediments, allowing the luminescence signal to build up again over time. Measurements of the luminescence signal observed in the laboratory are used to calculate the total amount of radiation that the sediment has been exposed to since the time of deposition, expressed in terms of equivalent laboratory dose in Grays (Gy), the SI unit of absorbed dose. Dividing this ‘equivalent dose’ or ‘De’ value by the rate of dose delivered by the exposure to radiation (termed the ‘dose rate’, Gy/kyr), gives the luminescence age expressed in kyr, i.e. thousands of years ago since the time of measurement (Eqn. 1). AgeðkyrÞ ¼ Equivalent doseðGyÞ=Dose rateðGy=kyrÞ

ð1Þ

Further details of the specifics of luminescence dating techniques not covered in this article are provided by various reviews, including Wintle (1993, 1997), Duller (1996, 2000, 2004), Prescott & Robertson (1997), Stokes (1999) and Lian & Roberts (2006). Luminescence dating of desert loess and coversands is discussed by Singhvi et al. (2001) and Singhvi & Porat (2008), while reviews specifically provided for luminescence studies of loess include Wintle (1987a, 1990), Berger (1988) and Zoller & Wagner (1990). ¨

Early developments in luminescence dating of loess deposits The first luminescence methods applied to loessic sediments involved measurements of the TL signal from polymineral fine grains (i.e. 4–11 mm diameter grains of mixed mineralogy) (e.g. Wintle 1981; Li 1982; Wintle et al. 1984). In such measurements, the TL signal is dominated by the signal from feldspars, although the quartz grains present also contribute to the TL signal. The relative contribution from these two components varies from sample to sample, depending on the proportion of each mineral by volume, and their relative luminescence intensities, which will vary according to the optical filter through which the TL passes to reach the detector. While the mineral composition may be measured relatively commonly, the relative intensities

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have rarely been assessed. Thus, the major problem associated with measurements of such TL signals is that it is not clear which mineral is being analysed, and the relative contribution of the signals from feldspars and quartz is likely to vary between samples. A second concern in TL dating is the long light-exposure times (several hours) required to remove the light-sensitive TL signal, and the fact that a residual, unbleachable, TL signal remains after this time. Assessing the size of this unbleachable signal, left after exposure to sunlight prior to deposition and re-burial, may be difficult, but it is particularly important to determine this residual signal for young sediments, e.g. those of Holocene age, otherwise the calculated TL ages will be overestimated. No standard measurement protocol was developed for TL dating, and a variety of light sources, laboratory bleaching times and preheat temperatures and durations have been employed (cf. Aitken 1985; Wintle 1990, 1997; Zoller & Wagner 1990); this means that re¨ porting of the exact measurement procedure and conditions is essential if the quality of published ages is to be assessed and age determinations between different laboratories compared, and, in practice, even then such comparisons and assessments are extremely difficult to make. TL remained the only luminescence signal used for sediment dating until use of the optically stimulated luminescence (OSL) signal from quartz was proposed by Huntley et al. in 1985. This development was followed swiftly by the introduction of a new OSL signal for dating by Hutt ¨ et al. (1988), namely the infrared stimulated luminescence (IRSL) signal from feldspars. OSL dating, or ‘optical dating’, is a term that can be used to cover the use of both OSL signals from quartz and IRSL signals from feldspars. For sediment dating, optical dating offers several advantages over TL dating; of key significance is the fact that the OSL signal is more rapidly and easily bleached (a few minutes of light exposure only) than the TL signal (Godfrey-Smith et al. 1988), both in nature and in the laboratory, and the residual OSL signal is bleached to a lower level than for TL, allowing young sediments of only a few hundred years, or less, to be dated successfully. In addition, in optical dating of sediments, only optically stimulable traps are examined, thereby mimicking more closely the light exposure that occurs in nature, and samples are not heated to as high temperatures as those used for measurement of TL, thus avoiding concerns over potential phase changes, thermal quenching and stimulation of deep traps. Perhaps the most significant advantage offered by optical dating, however, is that multiple measurements can be made on the same aliquot using short light exposures, allowing the proposal (Huntley et al. 1985) and development of single-aliquot measurement techniques that allow the determination of an equivalent dose for each individual aliquot examined and hence

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yield age determinations of higher precision (e.g. 5–10%) than those from multiple-aliquot techniques (e.g. 10–15%; where measurements are made on many aliquots and combined to calculate one equivalent dose (De) value). Both additive-dose (e.g. Duller 1991, 1994, 1995 for feldspars) and regenerative-dose (e.g. Murray & Wintle 2000 for quartz, but also applied to feldspars) methods have been developed for single aliquots. However, prior to 2000, the majority of published luminescence dating studies of loess were conducted using multiple-aliquot techniques for either TL or IRSL signals from polymineral fine-grained material.

Early applications of luminescence dating to loess deposits During the 1980s, the signal used predominantly for luminescence dating studies of loess deposits was TL, although a variety of measurement methods were employed, each offering various advantages and disadvantages (cf. reviews by Wintle 1987a; Berger 1988; Wintle 1990; Zoller & Wagner 1990). Throughout the ¨ 1990s, IRSL was increasingly utilized for dating loess sediments, taking advantage of the greater bleachability of the IRSL signal compared to the TL signal. By the late 1990s, a bewildering array of measurement procedures were in existence, consisting of differing measurement conditions and utilizing different luminescence signals (chiefly IRSL and TL signals); for loess studies, these techniques were still applied to grains of mixed-mineralogy (i.e. quartz and feldspar) typically of 4–11 mm diameter. Although various checks were proposed, including the analysis of scatter in multiple aliquots (Huntley & Berger 1995), the plateau test for TL measurements (see Aitken 1985, and discussions in Berger & Anderson 1994), plots of De versus stimulation time (Huntley et al. 1985; Aitken & Xie 1992), the reliability of many of these IRSL and TL ages generated is often difficult to assess in the absence of any independent age control. Several studies therefore used a combined IRSL and TL approach in the dating of loess and loess-like deposits (e.g. Musson & Wintle 1994; Frechen et al. 1997; Frechen 1999a, b; Frechen & Yamskikh 1999; Frechen et al. 1999, 2001; Novothny et al. 2002) with IRSL and then TL measurements being made on the same aliquots using multiple-aliquot regenerative dose and additive dose methods, thus enabling comparison of the age determinations. However, such studies demonstrated inconsistencies between the IRSL and TL ages, and between different methods utilizing the same luminescence signal. For example, IRSL ages of loess are frequently reported to underestimate TL results, for both regenerative dose and additive dose methods (e.g. Musson et al. 1994; Musson & Wintle 1994; Richardson et al. 1997; Frechen et al. 2001), although some studies do show an agreement between the

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results obtained for these two procedures and the expected chronology up to around 70–80 kyr (e.g. Frechen & Dodonov 1998; Frechen 1999a). TL studies of loess older than
50–100 kyr, however, frequently report age underestimates in comparison with existing knowledge, attributed to the lack of sensitivity corrections or anomalous fading of the feldspar contribution to the TL signal (e.g. Debenham 1985; Zoller et al. ¨ 1994; Zhou et al. 1995). ‘Anomalous fading’ is a phenomenon that affects the IRSL and TL signals from feldspars (Spooner 1992, 1994), and uncertainty regarding the presence and degree of anomalous fading presents the largest single obstacle to the widespread adoption and development of luminescence studies using feldspars. The phenomenon was first noted by Wintle (1973), and refers to the decay of an unstable luminescence signal on laboratory measurement time scales which, if uncorrected for, results in underestimation of the true age of the sample using either IRSL or TL signals. There are three major problems associated with anomalous fading that currently contribute to the concern over using feldspars for dating. First, there is the question of whether anomalous fading affects all samples taken for luminescence dating; allied to this is the question of whether all feldspars behave in a similar fashion. The laboratory preparation of samples, even of sand-sized grains, achieves only a rudimentary separation of plagioclase and alkali feldspars; consequently, the composition of prepared feldspar samples, and hence also their luminescence properties, would be expected to vary widely. Anomalous fading is frequently reported in luminescence studies using feldspar, and indeed some workers suggest that the phenomenon may in fact be ubiquitous (Huntley & Lamothe 2001; Little et al. 2002); in spite of this, not all published luminescence work using feldspars considers anomalous fading. Other workers do not detect significant anomalous fading (e.g. Frechen & Dodonov 1998; Frechen et al. 2001); in such cases, multiple-aliquot procedures were used and aliquots were stored at ambient temperatures for several days prior to measurement of either the TL or IRSL signal (this is not practicable for single-aliquot methods, where many irradiations are performed on each individual aliquot). Storing the aliquots before measurement reduces the effect of the unstable luminescence component, giving a De value closer to the dose received in nature. However, when fading tests are subsequently made on this material (e.g. comparing IR decay curves and glow curves at 4 weeks and 12 weeks after irradiation, Frechen & Dodonov 1998), it is then extremely difficult to detect fading in the laboratory due to the short time scale over which the measurements are compared; a small degree of fading observed on a short time scale of laboratory measurement will have a much more significant impact on the luminescence signal over geologic time. This raises the second

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Development and application of luminescence dating to loess deposits

issue of concern, which relates to the detection of anomalous fading. At present, there is no agreed method of testing for and assessing anomalous fading, and various methods have been proposed (e.g. Aitken 1985; Lamothe & Auclair 1999; Visocekas 2000; Huntley & Lamothe 2001; Auclair et al. 2003; Lamothe et al. 2003; Visocekas & Gue´rin 2006) although not all are suitable for application to loess due to its fine-grained nature and hence the homogenous nature of prepared aliquots (e.g. Lamothe & Auclair 1999). Allied to this is the third and final problem, namely that when anomalous fading is detected, there is then the issue of if and how the loss of signal might be corrected for; again there is no universally agreed method of doing this (e.g. Huntley & Lamothe 2001; Auclair et al. 2003; Lamothe et al. 2003). The situation is further complicated due to the potential variability of fading rates for different types of feldspar, and the different composition of individual aliquots of the same sample, meaning that any correction is particularly complicated where dating measurements have been made using multiple-aliquot methods, and correction should ideally be made for each individual aliquot measured. Where corrections have been made for anomalous fading, the success of the results is found to be variable; agreement with independent age control tends to occur when corrections are applied to young samples (e.g. using the correction method of Huntley & Lamothe (2001) success is achieved for corrections made within the linear part of the dose-response (‘growth’) curve) rather than for older samples (e.g. Little et al. 2002; Auclair et al. 2007), although a new method by Kars et al. (2008) may hold promise for older samples. Despite the various concerns outlined above that became apparent during the early studies applying TL and/or IRSL dating to loess deposits, the application of luminescence methods did help to refine loess chronostratigraphy. Previously, a broad chronostratigraphic framework was devised for loess-palaeosol sequences based on radiocarbon dating for the youngest loesses (e.g. Ruhe 1983), magnetic reversals for older loesses (e.g. Kukla 1975; Heller & Liu 1982), the location of distinct stratigraphic markers, and later by correlation of records of magnetic susceptibility with orbitally tuned marine oxygen isotope records (e.g. see review by Liu & Ding 1998). By the mid-late 1980s, it was thought by some that loess records provided a terrestrial archive equivalent in resolution and detail to that of the marine isotope record (e.g. Liu et al. 1985); with the introduction of luminescence dating, it became possible to develop a direct numerical chronology for the time of deposition of the loess–palaeosol sequences, thereby providing an independent test of both the validity of intra- and inter-regional terrestrial stratigraphic correlations, and the degree of synchronicity between these terrestrial records and the marine record. For example, Musson et al. (1994) obtained an age of 55.13.4 kyr

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for a sample taken just below the interstadial (Sm) palaeosol from Liujiapo, near Xian, on the Chinese Loess Plateau, thereby confirming the proposed timing of an intensified summer monsoon between 55 and 30 kyr due to an interstadial climate and based on magnetic susceptibility correlation across three sites (An et al. 1991). However, the same study by Musson et al. (1994) also highlighted the potential pitfalls of chronostratigraphy based on magnetic susceptibility, which relies on counting down from the top of the stratigraphic units in a profile, by obtaining an age of 15.80.5 kyr for a sample taken from 0.5 m depth, hence demonstrating that significant erosion and human interference is possible even in apparently geomorphologically stable loess sites. Notwithstanding the difficulties associated with luminescence dating at the time, other studies also resulted in the proposal of a revised chronostratigraphy at various loess sites (e.g. Singhvi et al. 1987; Juvigne´ & Wintle 1988; Wintle & Packman 1988; Singhvi et al. 1989; Zoller et al. 1994). While some luminescence stu¨ dies demonstrated good agreement with radiocarbon dates obtained for palaeosols within loess sequences (e.g. Wintle 1987b; Zoller & Wagner 1989a, b; Buch & ¨ Zoller 1990; Maat & Johnson 1996), other studies offered ¨ confirmation of the initial suspicion that much of the early radiocarbon chronology developed for palaeosols was unreliable due to the paucity of suitable organic material and/or due to contamination by modern carbon (e.g. from rootlets) giving rise to age underestimates (e.g. Juvigne´ & Wintle 1988 and Singhvi et al. 1987, respectively). The lack of independent numerical dating control beyond the upper limit of radiocarbon in loess–palaeosol sequences became a real hindrance to the further development and application of luminescence dating because, although the broad pattern of glacial–interglacial cycles could clearly be identified in the sequences, counting the number of palaeosols present gave rise to some ambiguity as to which palaeosol was which. The palaeosol(s) associated with Oxygen Isotope (OI) Stage 5 is often noted to be a particularly well-developed interglacial soil which can be identified by eye in the field (Fig. 1). However, it is not always possible to identify palaeosols originating from all three substages of Stage 5, i.e. 5e, 5c and 5a, and where only one well-developed palaeosol exists it can be unclear whether this represents the whole of Stage 5 or only the warmest event, Substage 5e. This uncertainty highlights the limitations of the rather broad time control offered by loess– palaeosol sequences; for example, the timing of the end of formation of a well-developed palaeosol observed in the field could reasonably be expected to be either
115 kyr (OI Substage 5e/5d transition) or
70 kyr (OI Stage 5/4 transition) if the application of the SPECMAP chronology (Martinson et al. 1987) is valid. In either scenario, the presence of this palaeosol does provide a threshold for the minimum age of the loess underlying it, which must be
130 kyr and older

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(Martinson et al. 1987). However, the value of this stratigraphic marker for providing a check on luminescence ages was soon diminished by the fact that TL studies, which represented the majority of published luminescence studies, appeared to show that the upper limit of TL dating was
100 kyr owing to saturation of the luminescence signal (e.g. Lu et al. 1987; Singhvi et al. 1989; Zhou & Wintle 1989; Wintle 1990; Zoller & ¨ Wagner 1990; Forman 1991; Prescott & Robertson 1997; Frechen & Dodonov 1998; Singhvi et al. 2001). However, other studies argued that reliable loesspalaeosol sequence TL ages could be obtained for the past
100 kyr (e.g. Singhvi et al. 1989; Zoller & Wagner ¨ 1989b) and even beyond (e.g. Zoller & Wagner 1989a; ¨ Berger et al. 1992). The ambiguity concerning the onset and cessation of palaeosol formation, and also the potential for misidentification of palaeosols, contributed to the uncertainty concerning the significance and testing of luminescence dating results in those early days. While the application of early luminescence studies to loess did lead to a greater understanding and appreciation of the shortcomings and pitfalls of the methods and, hence, encouraged further development of luminescence dating methods, by the 1990s it had become apparent that the chronostratigraphy of many loess sections was not as clearly defined as was initially thought. Beyond the upper limit of radiocarbon dating, little independent chronology exists for loess–palaeosol sequences, aside from where tephra may be present, but even then unambiguous identification of the eruption is not always made (e.g. Westgate et al. 2008). Given the uncertainties associated with both the IRSL and TL signals used for dating, the diverse range of measurement protocols used, and the inconsistencies in results, as discussed above, the lack of well-dated loess deposits with accurate independent age control that fell within the time range of luminescence dating (up to
100 kyr in most parts of the world) presented a major impediment to the further advance of luminescence studies in loess. It seemed that, aside from the advantage offered by the aeolian nature of the deposits (i.e. offering wellbleached sediments), loess deposits were perhaps not the ideal testing-ground for luminescence studies that they were initially envisaged to be. Clearly, another strategy was required if luminescence studies were to contribute to the further understanding of the significance of loess deposits.

Recent developments in luminescence dating of loess Methodological developments Given the concerns discussed in the previous section, regarding use of either IRSL or TL signals for dating, and hence over assessment and comparison of pub-

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lished luminescence ages, some authors (e.g. Wintle & Huntley 1982; Berger 1988) advocated that each study required support from independent age control, thus limiting the use of luminescence techniques as standalone dating methods. Although a broad chronostratigraphic framework can often be proposed for loess sequences based on the intercalated loess/palaeosol units, it is not always possible to obtain rigorous independent dating evidence at each site, and in any case the need for this to be established clearly makes redundant the value of any luminescence dating. When coupled with questions regarding the ubiquity and impact of anomalous fading of signals from feldspars, and the difficulty of assessing and correcting for this, plus the large ‘unbleachable’ TL signal often observed, this means that IRSL and TL methods of determining luminescence ages still do not yet provide a routine and reliable dating method. The other mineral that had occasionally been examined during the early days of luminescence dating was quartz. The great advantage of using quartz for luminescence dating is that unlike feldspar it has not been found to suffer from anomalous fading. Indeed, in an early study of loess by Wintle (1982), pure quartz of 4–11 mm diameter was prepared and its TL signal was used to check that the polymineral TL signal for the same sample did not suffer from anomalous fading. However, for loess studies, the fine-grained nature of the deposits and the concomitant labour- and time-intensive sample preparation procedures required to separate out pure quartz gave rise to the dominance of studies upon mixed-mineralogy (quartz and feldspar) fine grains (4–11 mm). This trend was also encouraged by the availability of inexpensive IR diodes for IRSL that could easily be retro-fitted onto existing luminescence equipment, in contrast to the expensive and cumbersome argon-ion lasers required for quartz studies of OSL until the early-mid 1990s. In the mid-1990s, advances in the equipment available made the routine measurement of OSL signals from quartz a possibility for the first time, with the introduction of first halogen lamps filtered to provide blue-green light (Btter-Jensen & Duller 1992), followed by green (Galloway 1994) and then blue (Btter-Jensen et al. 1999) light emitting diodes (LEDs), all of which could be easily and inexpensively fitted to existing automated luminescence equipment. The use of quartz for luminescence dating, however, was not without its limitations; the principal concerns were the occurrence of sensitivity changes within the measurement sequence, and again the lack of any checks on the reliability of the ages generated, aside from those offered by any available independent age control. The dose–response curve for OSL signals from quartz was also known to saturate below that of feldspar (Fig. 2), indicating that the maximum luminescence age that could be achieved using quartz was lower

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Development and application of luminescence dating to loess deposits

Fig. 2. Dose-response curves for the IRSL signal from polymineral fine grains (closed triangles) and the OSL signal from 63–90 mm diameter sand-sized quartz grains (filled circles) obtained for a sample from Luochuan, China, demonstrating the difference in dose response (modified from Buylaert et al. 2007); in each case a single aliquot measurement protocol has been followed and the sensitivity corrected luminescence signal is plotted. Recycled points are shown as open symbols. Inset shows the same data set, but with the dose displayed on a logarithmic scale.

that that using feldspar. Nevertheless, the ability to circumvent completely the problem of anomalous fading that had plagued TL and IRSL dating studies made the use of quartz an extremely attractive option. The introduction of quartz OSL as an additional widely available luminescence signal for use in dating loess made the need for independent dating evidence all the more pressing as the range of materials, signals and measurement techniques expanded. Watanuki & Tsukamoto (2001) compared IRSL ages of polymineral fine grains, and OSL ages of fine grains of pure quartz using stimulation with light of blue/green wavelength, and also TL ages of fine-grained quartz, prepared for two loess samples from Northern Kyushu, southwestern Japan, containing tephra of known age (Fig. 3); a young sample of unknown age from China was also examined. All three samples (Nt-6 and Nt-11 from Japan, Zh-1 from China) were examined using multiple–aliquot methods; the additive-dose method was used for IRSL and OSL measurements, while the total bleach and regenerativedose methods were employed for the measurement of TL. Sensitivity change following optical bleaching was noted using the regenerative-dose method; thus, the TL measurements discussed were made using the total bleach method only. All three samples gave age overestimates of at least 40% for TL of quartz compared to both polymineral IRSL and quartz OSL ages (Fig. 3); this was particularly pronounced for the young sample from China, where the TL age overestimated the OSL and IRSL ages by more than factors of 3.5 and 5, respectively. This discrepancy was explained by the suggestion that the TL signal of the two Holocene age

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Fig. 3. Comparison of luminescence ages obtained using different methods for three loess samples from Japan and China (redrawn from Watanuki & Tsukamoto 2001). Independent dating evidence from tephrochronology was available for the Japanese samples, shown as shaded regions on the plot. TL measurements (diamond symbol) were conducted on fine-grained quartz using the total bleach method. OSL measurements of fine-grained quartz were made using stimulation with light of blue/green wavelength (square symbol), while IR stimulation was applied to polymineral fine grains (triangle symbol); the multiple-aliquot additive dose method was used in both cases. OSL of fine-grained quartz using blue/green light for stimulation was also measured following stimulation with IR for sample Zh-1 from China (circle symbol).

samples, one from Japan and one from China, may have been incompletely bleached on deposition, leaving a large residual TL component which was underestimated, hence leading to overestimation of the TL age (Fig. 3). There was good agreement (within 1s error) between ages determined using IRSL of feldspars and OSL of quartz. The two loess samples from Japan (of Holocene and last glacial age) showed particularly good agreement between the tephrochronology and the OSL ages from fine-grained quartz (Watanuki & Tsukamoto 2001). Using material of known age, this study demonstrated the great potential of dating using the OSL signal from fine-grained quartz, and it proved to be the most appropriate signal for dating the loess samples from Japan. It should be noted, however, that no sensitivity correction was applied to the quartz OSL data set, and a multiple-aliquot additive-dose method was used. Only recently have single-aliquot techniques been successfully applied to loess; in these techniques, each aliquot examined gives rise to a De value, and these many De values are then combined in the final age determination. The primary advantage offered by singlealiquot techniques is that the ages generated are of much greater precision than those using multiple aliquots (Duller 1991). Aliquot-to-aliquot variability is less of an issue for fine-grained material (where each aliquot typically contains in excess of a million 4–11 mm diameter grains) than for coarse grains (where the total number of grains can be 2–5 orders of magnitude less; Duller 2008). However, the use of single-aliquot techniques, where all of the measurements are made on one aliquot, is still more desirable even for loess. The major hindrance to the development of single-aliquot

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techniques for quartz was the need to monitor and, if necessary, correct for, sensitivity change occurring within the measurement sequence itself which, if not addressed, could yield erroneous OSL age determinations. It was not until the single-aliquot regenerative-dose (SAR) protocol was published by Murray & Wintle (2000) that a reliable and routine sensitivity correction for quartz was available. The need to correct for sensitivity change was examined by Zhou & Shackleton (2001) in a study applied to fine-grained quartz prepared from loess. They used light of blue/green wavelength to stimulate a sample of fine-grained quartz prepared using hydrofluorosilicic acid, and compared multiple- and single-aliquot regenerative-dose methods, both with and without a sensitivity correction. Zhou & Shackleton (2001) noted that significant sensitivity changes occurred for fine-grained quartz during the measurement procedures, primarily as a result of preheating; these changes affected both single- and multiple-aliquot techniques. Sensitivity corrections were made using the 1101C TL peak, and also using the responses to a test dose; these two sensitivity corrections yielded different De values. As for so many loess studies, there was no independent chronology for the fine-grained quartz sample prepared from loess in this study; however, Zhou & Shackleton (2001) concluded that the sensitivity correction using the OSL response to a test dose in the SAR procedure of Murray & Wintle (2000) was favoured, giving De values that were consistently of high precision, with uncertainties of o5%. The significance of the SAR protocol introduced for the study of quartz (Murray & Wintle 2000) should not be underestimated. The introduction of this quartz measurement protocol and the further refinements proposed thereafter have revolutionized luminescence dating, causing a step-change in accuracy and precision, and hence also in confidence of both luminescence researchers and the ‘user community’. One of the great strengths offered by the technique is that it has a suite of internal checks on the reliability of the method (Murray & Wintle 2000), and several additional checks can be conducted (Fig. 4; see Wintle & Murray (2006) for further technical detail regarding these checks) which, if satisfactory, increase the confidence in the final OSL age generated, thereby negating the need for independent dating control at every site studied. Of course, cross-checks against independent age control are still desirable where possible, but an initial study by Murray & Olley (2002) compiling OSL ages obtained from a wide range of depositional environments suggests that applying the SAR measurement protocol to pure quartz offers extremely good agreement with independent age control (Fig. 5); a more recent compilation has been provided by Rittenour (2008). A study by Watanuki et al. (2005) for loess of known age from Japan also demonstrated excellent agreement between the OSL ages from fine-grained quartz dated using the

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SAR protocol and the independent age control provided by tephrochronology (Fig. 6A, B). This study also showed that the IRSL ages obtained for polymineral fine grains consistently underestimated the quartz OSL ages by an average of
20% across a wide range of De values from
40 to 250 Gy (Fig. 6A, B); the cause of this IRSL age underestimate was attributed to anomalous fading. This study encapsulates the arguments in support of the use of pure quartz SAR measurements for dating, because the SAR method appears to be robust when applied to quartz below saturation doses, and the signal from quartz does not suffer from anomalous fading; furthermore, SAR offers a series of checks on the behaviour of the luminescence signals generated and the suitability of the method for application to each sample studied (see Fig. 4). One of these criteria is that for a quartz OSL signal to be acceptable for dating, it must be dominated by what is called a ‘fast component’ (Bailey et al. 1997). Watanuki et al. (2005) noted that not all of the quartz OSL signals in their study were dominated by a fast component (e.g. compare Fig. 6C, in which the OSL signal is dominated by a fast component, to Fig. 6F, in which it is not); using component fitting to extract the fast component mathematically, Watanuki et al. (2005) calculated OSL ages based on this fast component alone (Fig. 6A, B) and these gave excellent agreement with the independent dating control from tephrochronology. If a fast component is not dominant, and if none can be extracted mathematically, then the OSL age would be rejected as being unreliable because the slower components (seen in Fig. 6F) are less light-sensitive and thus less likely to have been zeroed by sunlight prior to deposition, and some slower components have been shown to be thermally unstable (e.g. Jain et al. 2003). Figure 6(D, E) shows the quartz OSL ages obtained using the standard, bulk OSL signal, with no regard for assessment of the dominance of a fast component, demonstrating poor agreement with independent age control where the OSL signal is not dominated by the fast component. Thus, the great strength of quartz SAR measurements is that problematic samples, and hence erroneous OSL ages, can be identified without the need for independent dating control; the strength of these tests has been confirmed in studies of known age materials (e.g. Watanuki et al. 2005).

Isolating an OSL signal from quartz Preparation of pure quartz is vital for quartz OSL studies, because OSL signals can also be obtained from feldspar grains. Coarse-grained quartz for luminescence dating (i.e. sand-sized grains, typically of 490 mm diameter) is relatively simple to prepare, with separation of quartz being achieved on the basis of density (e.g. using heavy liquid separation techniques) and then

Development and application of luminescence dating to loess deposits

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1) Examining OSL signal characteristics

OSL signal is derived from quartz? (!)

NO

Further sample preparation required

NO

Tests of the suitability of other components for dating required

YES

OSL signal is dominated by the fast component? YES 2) Determining appropriate measurement conditions The sensitivity change is monitored and corrected for appropriately (i.e. recycling ratio of 1 ± 0.1)?

The decay and growth curve shapes are appropriate, and with sufficient points to define the curves and constrain the Natural dose?

No recuperation of signal? (i.e. recuperation of signal in response to a ‘zero dose’ is < 5% of the Natural signal)

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Further investigations required

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Further investigations required

YES 3) Confirmation of appropriate laboratory measurement conditions A laboratory-given radiation dose of known value can be accurately measured or ‘recovered’ for unheated materials? YES

Sample confirmed assuitable for OSL dating, using the SAR measurement conditions defined by laboratory tests Fig. 4. Checks that may be applied in OSL dating of quartz to assess the suitability of the signal and the laboratory measurement conditions used for dating, and hence to infer the reliability of the age determination.

removal of any remaining feldspar grains by a short treatment (
40 min duration) in concentrated hydrofluoric acid (HF) followed by re-sieving to remove any partially dissolved feldspar grains. This treatment also removes the outer alpha-irradiated layer (
10 mm) of the quartz grains, thereby making the dosimetry calculations simpler. However, loess deposits, by their very nature, tend to be dominated by finer-grained material and it is difficult to separate such fine-grained (i.e. siltsized grains, typically 4–11 mm diameter) quartz using density separation, and aggressive HF etches that remove the outer 10 mm of material are clearly inappropriate; instead, a more lengthy chemical treatment is necessary, using hydrofluorosilicic acid (H2SiF6) (e.g. Jackson

et al. 1976; Berger et al. 1980; Roberts 2007) preferably followed by re-sieving or re-settling the grains after the chemical treatment (Roberts 2007); weak hydrofluoric acid (e.g. Prasad 2000; Mauz & Lang 2004) has also been proposed but unlike H2SiF6, HF will eventually etch the quartz grains, so care must be taken when using even weak HF. One drawback of working with fine-grained quartz is that the preparation is labourintensive and time-consuming, and the hydrofluorosilicic acid treatments are lengthy (e.g. 1–2 weeks; Roberts 2007). For this reason, a modified version of the SingleAliquot Regenerative-dose (SAR) procedure of Murray & Wintle (2000) developed for quartz was proposed by Banerjee et al. (2001) and developed for Holocene

Helen M. Roberts

OSL/Independent age

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Aeolian sand Loess Fluvial Marine Glacial

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Chinese loess deposits by Roberts & Wintle (2001, 2003); the procedure uses a combination of stimulation sources to examine mineral components within a fine grain polymineralic sample (i.e. containing a mixture of quartz and feldspar grains). This combined or ‘doubleSAR’ measurement procedure exploits the fact that quartz does not respond to stimulation with IR, while feldspars can respond to stimulation using both IR and blue LEDs; the polymineralic sample is stimulated with IR to reduce the contribution from feldspars, prior to stimulation with blue LEDs which should then yield a quartz-dominated signal (termed the ‘[post-IR] OSL’ signal). Thus, the ‘double SAR’ measurement protocol gives rise to two estimates of equivalent dose from each aliquot examined (i.e. one De value from the IRSL signal and one from the [post-IR] OSL signal), and these De values may be compared. However, because the measurements are made in a system with no interchangeable filter system, the filters selected (Hoya U340, suitable for collection of signals stimulated using both IR and blue LEDs) are suboptimal compared with those that would be selected if the IRSL signal alone was to be used for dating. However, as the signal used

100

1000

Fig. 5. Summary of age comparisons made by Murray & Olley (2002) for different sedimentary environments, comparing quartz OSL ages obtained using the single aliquot regenerative dose (SAR) measurement protocol with independent age control (redrawn from Murray & Olley 2002). Note the logarithmic axes in the main diagram. Inset shows the same data plotted on a linear scale, and including error bars for each age. The uppermost plot shows the ratio of the OSL age to the independent age (linear scale), plotted against the independent age (log scale); one data point for a fluvial sample has been omitted from this upper plot because the independent age is poorly known.

for dating is the ‘post-IR’ OSL signal, then this choice of filters is the only one available. A modification to the ‘double SAR’ protocol was proposed by Wang et al. (2006b) in a study comparing SAR fine-grain quartz OSL De values with those obtained using IRSL and [post-IR] OSL signals from polymineral fine grains (4–11 mm). Wang et al. (2006b) found good agreement between the [post-IR] OSL De values and those from the quartz OSL signal, once a sufficiently lengthy IR pretreatment had been applied (
300–500 s; Fig. 7A); the duration of this IR exposure was established by identifying the plateau region of a plot showing [post-IR] OSL De values as a function of IR stimulation time (Fig. 7B). The IRSL De values underestimated the OSL De values, presumably due to anomalous fading (Fig. 7A); this was particularly notable for older samples (De
450 Gy), and the ratio of the IRSL/[post-IR] OSL signal was proposed as a measure of the degree of fading of the IRSL signal (Wang et al. 2006b). This ratio was also proposed as a proxy for weathering intensity in Chinese loess by Wang & Miao (2006), who state that the ratio of the IRSL/[post-IR] OSL signal reflects the relative

Development and application of luminescence dating to loess deposits

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Fig. 6. Fine grain polymineral IRSL, [post-IR] OSL, and quartz OSL age determinations obtained using a SAR protocol for two sample localities in Japan; (A) and (D) show data for Niigata, and (B) and (E) data from Tochigi. Independent age control for these sites is also shown in (A), (B), (D) and (E), provided by tephra dated using fission track and radiocarbon. The quartz OSL ages shown in (A) and (B) are calculated using the fast component of the OSL signal; (D) and (E) show the quartz OSL age determined using the standard OSL signal, with no consideration of the contribution of the fast component to the total signal. Where an OSL signal is dominated by the fast component (C), there is no significant difference between the OSL ages shown in (A) and (D), or (B) and (E), respectively (e.g. sample Tm-3). Where an OSL signal is not dominated by the fast component (F), the OSL ages shown in (A) and (B) are different to those shown in (D) and (E), respectively (e.g. sample Tm-10). The shaded areas in (C) and (F) indicate the portion of the OSL signal used for dating. The OSL ages determined using the fast component are in good agreement with the independent chronology. Redrawn from Watanuki et al. (2005).

abundance of feldspar and quartz which varies with depth down-section (and hence, over time), due to their different resistance to weathering. The importance of determining an appropriate IR stimulation duration was also noted by Zhang & Zhou (2007) in their investigations of the ‘double SAR’ protocol. They also examined the effect of IR stimulation at different temperatures, concluding that IR stimulation, preferably at room temperature, but up to a maximum of 1251C, was optimal and had no detrimental effect on the [post-IR] OSL De value obtained from quartz.

Using the ‘double SAR’ measurement technique is not straightforward, however. Relying on the use of stimulation sources to isolate the luminescence signals from mixed mineralogy samples can sometimes bring about problems, and false security can be gained from obtaining the same equivalent dose determinations from IRSL and [post-IR] OSL signals for the same aliquot (Roberts 2007). For example, Roberts (2007) demonstrated that it is possible for a dominant feldspar component to mask the signal from quartz. In this study of loess from Nebraska, North America, the effect of

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Fig. 7. (A) shows a comparison of the De values obtained using the IRSL and [post-IR] OSL signals plotted as a function of the De values obtained for fine-grained quartz, while (B) plots the [post-IR] OSL De value obtained as a function of IR stimulation time, used to identify the optimum IR stimulation duration required prior to stimulation with blue diodes and subsequent measurement of the resultant [post-IR] OSL signal for use in dating. Redrawn from Wang et al. (2006b).

180 IRSL De [post-IR] OSL De OSL De

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Fig. 8. The value of De determined for loess of 35–50 mm diameter treated with hydrofluorosilicic acid (H2SiF6) for different durations (redrawn from Roberts 2007). Measurements were made following a SAR protocol, using a preheat temperature of 1601C/10 s and a cut heat of 1601C. The IRSL and [post-IR] OSL De values shown for each etch time are paired values obtained for the same aliquot; the OSL De values were obtained for a second suite of aliquots using only stimulation with blue diodes (i.e. no IR treatment). The dashed and dotted lines shown indicate general trends only.

the progressive removal of feldspar (using H2SiF6) on the De values obtained for a mixed-mineralogy loess sample of 4–11 mm and also 35–50 mm diameter using the ‘double-SAR’ procedure was investigated (Fig. 8). Prior to any etching, the De values of the IRSL and [post-IR] OSL signals were essentially the same. However, as the feldspar component was progressively removed with etching time, the De value obtained from the [post-IR] OSL signal increased, while the IRSL signal remained at the same value. Further tests suggested that the feldspar component showed signs of fading (Roberts 2007), and that the feldspar dominated the [post-IR] OSL signal both for unetched material and also during the early stages of etching. Etching with H2SiF6 reduces the feldspathic component, thereby enhancing the contribution from quartz to the [post-IR] OSL signal and the OSL signal obtained without pre-

vious IR stimulation. The ages derived from the [postIR] OSL signal of etched material were in agreement with the independent dating control at the site (Roberts et al. 2003; Roberts 2007), and the IRSL ages calculated as part of this ‘double-SAR’ procedure were clearly incorrect (and therefore the [post-IR] OSL ages of the unetched material were also incorrect, being the same as the IRSL ages) (Roberts 2007). This study sounds a note of caution regarding use of the ‘double-SAR’ measurement protocol, and emphasizes the importance of careful investigation of the origin of the signals used for dating; in this example, getting the same De value twice using the two different optical signals of the ‘double SAR’ technique did not serve as any confirmation of accuracy. A test of whether the [post-IR] OSL signal was dominated by quartz or feldspar was proposed by Roberts (2007) and also demonstrated for Chinese loess (Roberts & Duller 2004). By making measurements using aliquots measured across a range of preheat temperatures, the form of the dose-response curves at different preheat temperatures can be examined (Fig. 9). Signals dominated by feldspars produce a distinctive spread of sensitivity-corrected dose-response curves when a range of preheat temperatures (but a fixed cutheat temperature) is used, with the curves decreasing with increasing preheat temperature from 160 to 3001C (as shown for the IRSL signal from unetched and H2SiF6-etched ‘polymineral’ fine grains in Fig. 9A, C, respectively). In contrast, quartz-dominated sensitivitycorrected dose-response curves tend to cluster together at preheat temperatures between
160 and 2601C, and then drop off systematically with increasing preheat temperatures of 2801C and above (as shown for the [post-IR] OSL signal from H2SiF6-etched ‘polymineral’ fine grains in Fig. 9D). The [post-IR] OSL signal for unetched polymineral fine-grained material shown in Fig. 9B displays the same pattern of dose–response curves across the range of preheat temperatures as that of the IRSL signal from unetched polymineral fine grains (Fig. 9A), suggesting that the [post-IR] OSL

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Development and application of luminescence dating to loess deposits

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C

D

495

Fig. 9. Standardized growth curves for luminescence signals measured following application of different preheat temperatures (160–3001C at a rate of 51C/s, held for 10 s) and a fixed cut-heat temperature (1601C) for, (A) unetched polymineral IRSL and (B) unetched polymineral [postIR] OSL signals, and C H2SiF6-etched ‘polymineral’ IRSL and D H2SiF6-etched ‘polymineral’ [post-IR] OSL signals. The key for these data is shown in (B). The inset to each figure uses the data for a given regenerative dose of
30 Gy (also shown in the main plots), replotting these data to demonstrate the change in sensitivity (‘standardized luminescence signal’, where ‘Lx’ is the luminescence signal arising from the natural or regenerative dose, ‘Tx’ is the luminescence signal arising from the test dose used to monitor sensitivity change, and ‘TD’ is the test dose in Gy) across a range of preheat temperatures. Redrawn from Roberts (2007).

signal is dominated by a signal from feldspars (see Roberts 2007, for further discussion). Identifying the source of the [post-IR] OSL signal is crucial, because, for the ‘double SAR’ technique to generate accurate results, the [post-IR] OSL signal used for dating has to meet all of the criteria for quartz SAR measurements (Fig. 4), and most critically it relies upon the [post-IR] OSL signal being derived from quartz (as shown in Fig. 9D) and having a dominant fast component (Wintle & Murray 2006). To avoid any of the complications of the use of two stimulation sources to isolate a signal from quartz, the measurement of chemically purified quartz is preferable for most studies. This conclusion has been reached by numerous loess researchers, including Watanuki & Tsukamoto (2001), Watanuki et al. (2003, 2005), Wang et al. (2006b), Roberts (2007), Zhang & Zhou (2007), in spite of the lengthy pretreatment that is required to isolate fine-grained quartz. A reminder of the problems posed by use of the IRSL signal from feldspars for dat-

ing is provided in an article by Little et al. (2002) reporting a recent multiple-aliquot IRSL study of loess from the east European Plain, Russia. Little et al. (2002) noted anomalous fading of the IRSL signal from the polymineral fine grain samples examined, and tried to correct for this fading, but this was only successful for the younger known-age samples (i.e. o50 kyr). The great advances made in the ability to determine accurate OSL ages for quartz is attested to in the conclusion of Little et al. (2002), where the examination of quartz from these loess deposits is advocated as a means of completely circumventing the problems of anomalous fading posed by working with feldspars.

Streamlining measurements to facilitate high-resolution studies Quartz OSL ages obtained using the SAR protocol have been demonstrated to be accurate, and are also of

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high precision (typically 5–10%). The high precision offered by SAR techniques makes high-resolution OSL dating studies feasible (e.g. Roberts et al. 2001, 2003; Lai & Wintle 2006). However, the relatively high dose rate of most loess deposits means that the measurement times for loess samples can be significantly longer than for coarse-grained quartz from samples of the same age; this is because more measurements and/or measurements to higher radiation doses are needed to constrain accurately the dose–response curve and hence to determine the De value. The ‘standardized growth curve’ method proposed by Roberts & Duller (2004) offers the potential to increase the number of samples studied within a fixed measurement duration by streamlining the measurements required to determine De values at given sites without compromising the precision. This approach permits the resolution of study to be increased due to the increased throughput of samples, and has consequently been adopted by loess researchers interested in studying the temporal fluctuations in accumulation rates (e.g. Lai 2006; Stevens et al. 2006). The standardized growth curve (SGC) has been shown to generate OSL ages that are in excellent agreement with the ages generated using full SAR measurements both for [post-IR] OSL signals from polymineral samples prepared from loess from China, and also for fine- and coarse-grained quartz from various continents (Roberts & Duller 2004; Lai 2006; Lai et al. 2007b). Although one standardized growth curve can be constructed to represent the dose–response from quartz from around the world, Roberts & Duller (2004) advocated that, where many samples are to be dated from one section or region, ideally a site-specific (or possibly a regional) standardized growth curve should be developed and applied to the samples in that specific study. One consideration that is of great importance in the construction of the SGC is that care must be taken to ensure that sufficient dose–response points are measured to characterize the growth curve required in order to optimize the mathematical fit applied to the data. The standardized growth curve should also only be used up to the dose–response values of interest; this is of particular importance when the dose–response curve has been characterized to high doses approaching saturation, yet the curve is to be used for De values below these high values. The incorporation of an additional measurement step, in the form of measuring the response to a given radiation dose in order to be able to check that this lies on the SGC (Burbidge et al. 2006), offers reassurance of the integrity of the De value determined using the standardized growth curve; this still allows a significant reduction of measurement time, thereby offering the opportunity to increase the number of aliquots examined and hence also to increase the sampling resolution possible in a dating study.

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Recent applications of luminescence dating to loess deposits The new-found confidence that has arisen from the development of optical dating of quartz using SAR measurement protocols (Murray & Wintle 2000), and the associated development of a suite of checks on the luminescence behaviour of the OSL signal used for dating (discussed in Wintle & Murray 2006), has led to a dramatic rise in the number of studies applying quartz SAR techniques to loess–palaeosol sequences. Indeed, the method of choice for dating loess-palaeosol sequences is now considered to be quartz OSL utilizing a SAR measurement protocol (e.g. Watanuki & Tsukamoto 2001; Watanuki et al. 2003, 2005; Wang et al. 2006b; Roberts 2007; Zhang & Zhou 2007), for the reasons discussed earlier in this article; however, given the high dose rate typically found for loess deposits, the maximum age range that can be achieved for these deposits using quartz is typically only
55 kyr (e.g. Zhou & Shackleton 2001; Buylaert et al. 2007, 2008). Some studies have developed fine-grained quartz OSL chronologies for loess beyond this limit, e.g. Lu et al. (2007) obtained ages ranging from
1 to 125 kyr (De of
3 to 400 Gy) using a sensitivity-corrected multiple-aliquot method applied to loess from China, and Kemp et al. (2003, 2006) obtained ages of up to
100–200 kyr (De of up to
410–750 Gy) for loess-palaeosol sequences from Argentina. However, the more typical maximum quartz OSL ages of
55 kyr are consistent with De values of
125 Gy, beyond which Roberts & Duller (2004) noted that an additional linear component was required in order to be able to adequately fit the dose–response curve data for coarse-grained quartz. Recent studies discussed below have demonstrated, however, that there is still much to be learned from loess–palaeosol sequences even over this relatively short time-span. The advances made in the luminescence dating of quartz using the SAR techniques discussed above have allowed some of the fundamental ideas regarding the development of loess–palaeosol sequences to be investigated; this has only been possible due to the proven reliability of SAR methods developed for optical dating of quartz and the resulting transformation in confidence in the ages generated without the need for support from independent dating control.

Investigating sediment accumulation rates A high-resolution study of 19 samples of Holocene loess at Duowa, China, on the western fringes of the Loess Plateau, used the ‘double SAR’ measurement protocol (Roberts & Wintle 2001) to obtain a [post-IR] OSL signal for dating, generating high-precision ages (Roberts et al. 2001). This study demonstrated that, contrary to previously held notions that sediment

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Development and application of luminescence dating to loess deposits

accumulation rates were constant, accumulation rates had actually varied at this site during the Holocene ranging from 0.2 mm/yr from the early Holocene until 2500 years ago, to a fourfold increase to
0.8 mm/yr from 2500 to 680 years ago, and reaching
3.4 mm/yr between 680 and 410 years ago when anthropogenic enrichment of the soil is believed to be responsible for artificially enhancing this latter accumulation rate. The distinct increase in accumulation rate after
2500 years ago is believed to be linked to the development of agriculture (Roberts et al. 2001) which expanded into marginal areas during the West Han Period due to large-scale population movement (Sun 2000); similar increases in Holocene accumulation rate have been observed at several other sites on the central Chinese Loess Plateau (Beiguoyuan, Xifeng, Shiguanzhi) and also attributed to expansion of agriculture (Stevens et al. 2006). The study by Roberts et al. (2001) demonstrated the power of high-resolution luminescence dating studies applied to loess, and highlighted the potential magnitude of anthropogenically induced modifications in dust accumulation rates during the Holocene, which is of interest to future dust production studies. Critically, however, the study by Roberts et al. (2001) contributed to the debate concerning the validity of climofunctions derived from magnetic susceptibility measurements. Magnetic susceptibility has been reported as being ‘simple to measure, but not always simple to interpret’ (Evans & Heller 2001: p. 136), and this statement would seem to be particularly appropriate where such measurements are used to develop climofunctions. Following the development of a climofunction based on modern-day observations (Maher et al. 1994), Maher & Thompson (1995) used the magnetic susceptibility measurements of palaeosols to derive estimates of palaeoprecipitation for the Chinese Loess Plateau. However, such work is not without its critics, not just because quantitative estimates of palaeoprecipitation vary greatly (e.g. Heller et al. 1993; Liu et al. 1995; Maher & Thompson 1995; Han et al. 1996), but also because it inherently relies upon the rapid development and stabilization of the magnetic susceptibility signal following changes in climate, and principally precipitation. If the magnetic susceptibility signal does not respond quickly to climate and rapidly reach equilibrium, then the effect of time (i.e. soil forming duration) cannot be ruled out as a factor controlling the magnetic susceptibility values measured (Maher 1998). Indeed, soil chronosequence studies in California have shown that magnetic susceptibility in soils continues to increase over hundreds of thousands of years (Singer et al. 1992). Contrary to the established ideas at the time, Roberts et al. (2001) demonstrated that no variation in accumulation rates was observed between the Holocene loess and palaeosol units at Duowa, and hence there was no reduction in dust ac-

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cumulation rate during soil forming periods, suggesting in turn that phases of accretionary soil development occurred while dust continued to accumulate. Roberts et al. (2001) concluded that because the source of material, the topography and the accumulation rate (and hence time) had remained constant during the development of the Holocene loess–palaeosol sequence, climate must be the primary control on soil development at this site. This idea was investigated further by Maher et al. (2003), who noted that the degree of soil development (indicated by magnetic and geochemical properties) was actually weakest at Duowa when accumulation rates (Roberts et al. 2001) were lowest, and hence when soil forming durations were greatest, clearly demonstrating that the degree of soil formation did not vary as a function of time. By process of elimination, therefore, Maher et al. (2003) concluded that the factor responsible for the changes in magnetic and geochemical properties must be climate (and associated changes in organic activity), and specifically palaeoprecipitation in this semi-arid region. From the constant accumulation rates observed through the loess and intercalated palaeosols, Maher et al. (2003) also concluded that ‘dust dilution’ effects (Kukla et al. 1988; Porter et al. 2001) were not a significant influence at this site and did not control the magnetic susceptibility variations between loess and palaeosols. Maher & Hu (2006) went on to use the [post-IR] OSL ages of Roberts et al. (2001) to demonstrate an antiphase relationship between the southeast Asian monsoon and the north African/ Indian monsoon records. Another model which relies upon the presence of continuous accumulation is that using grain-size variations in loess to derive a numerical time scale, based on the Oxygen Isotope Stage (OIS) boundaries of Martinson et al. (1987) which are tied to the major climatic changes in the loess as indicated by grain-size variations (Vandenberghe et al. 1997). Thus, an accumulation rate can be determined for each loess-palaeosol unit, using the time difference between the ages of stratigraphic tie-points and dividing this by the thickness of the unit; hence a numerical age can be determined for any given depth in the profile, assuming a constant accumulation rate over time for that specific unit, and assuming that loess accumulation is continuous and no erosion takes place. Further adjustments and refinements to the grain-size time scale have been proposed by Nugteren et al. (2004), to account for changing compaction over time and to detrend the grain-size data which coarsen upwards. The broad correspondence between the grain-size data from Luochuan and the marine oxygen isotope curve (Vandenberghe et al. 1997) is compelling over Quaternary time scales; on a more recent time scale, OSL dating offers the opportunity to examine in more detail the validity of the assumptions inherent in numerical age determinations based on grain-size models. A study by Lai et al.

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(2007a) examined samples taken at
1 m intervals through the Malan Loess (loess unit ‘L1’, above the first interglacial palaeosol) at Yuanbao, near Linxia in the western Chinese Loess Plateau, to investigate the assumption of continuous sediment accumulation during the last glaciation. Based on coarse-silt sized (45–63 mm) quartz OSL ages derived using the SAR protocol, sediment accumulation was noted to be essentially continuous at Yuanbao over the
50 kyr time scale studied, with no major hiatuses being identified (Lai et al. 2007a). A dramatic increase in sediment accumulation rate was noted after
26 kyr, with a threefold increase from 0.33 m/kyr to 1.1 m/kyr being calculated; however, an earlier study of the site by Chen et al. (1997) did not note an abrupt change in grain-size (i.e. % grains 440 mm; Porter & An 1995) accompanying the increase in accumulation rate noted by Lai et al. (2007a). Lai et al. (2007a) use this observation to challenge the ability of grain-size models to provide a reliable chronology. Lu et al. (2006) also challenged the fundamental assumption of the grain-size models, namely that there is a direct relationship between grain size and dust flux; they used a sensitivity corrected finegrained quartz OSL dating method to identify sediment accumulation rate changes at Luochuan, on the Chinese Loess Plateau, during a period when grain-size models suggest that dust accumulation is constant.

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quality of the age determination. As recommended by Roberts & Duller (2004), Stevens et al. (2006) constructed site-specific SGCs based on the full SAR dose–response curves for aliquots from the Beiguoyuan Main site and the Xifeng site (the latter was also used to derive SGC ages for Beiguoyuan section). Although the agreement between ages determined using SAR and SGC methods was typically shown to be acceptable (Stevens et al. 2006), it is interesting to note that when the data are plotted down section, the SGC ages seem to be systematically offset when compared to the ages determined using SAR, with a slight underestimate in age being noted for the SGC compared to SAR ages (Fig. 10). This discrepancy is minor, and the two suites of ages are in agreement within errors, but this does highlight the care required in constructing and utilizing SGCs. The SGC constructed by Stevens et al. (2006, 2007a) was not shown, so it is difficult to comment on the extent to which SGC curves vary between sites, and to speculate on the potential reasons. Contrary to the assertion of Stevens et al. (2006, 2007a) that site-specific SGC curves were required for their study due to differences in the dose–response characteristics, various other workers have found very good inter- and intrasite agreement between the SGCs for quartz and loess,

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High-resolution studies of sediment accumulation rates 2

100 Depth below surface (cm)

Inter- and intra-site variability in sediment accumulation rates was also demonstrated in a high-resolution study (sampling interval 10–20 cm) by Stevens et al. (2006) spanning the last
30 kyr, based on coarse-silt sized (40–63 mm) quartz OSL ages determined using the SAR protocol for three sites (Shiguanzhi, Xifeng and Beiguoyuan) along a southeast–northwest transect across the Chinese Loess Plateau; unlike the study by Lai et al. (2007a), hiatuses in the loess records were also reported (Stevens et al. 2006). Consequently, not only do Stevens et al. (2006, 2007b) challenge the validity of models based on variations in proxies (e.g. grain-size or magnetic susceptibility) that inherently assume a constant accumulation rate within a stratigraphic unit, they also conclude that at these loess sites sediment accumulation is not continuous. Furthermore, Stevens et al. (2006, 2007b) contend that their sites were prone to erosion and to postdepositional mixing and diagenesis that would otherwise not be detected using proxy measures, and these effects are only identified using OSL dating. The high-resolution study by Stevens et al. (2006) was only really made feasible due to the adoption of the SGC (Roberts & Duller 2004) which, if implemented appropriately, reduces the measurement time required to determine an OSL age without compromising on the

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Fig. 10. OSL ages determined for ‘Beiguoyuan Main’ loess section using the single-aliquot regenerative-dose (SAR) measurement protocol and using a standardized growth curve (SGC). Redrawn from Stevens et al. (2006).

Development and application of luminescence dating to loess deposits

respectively, from various sites around the world (Roberts & Duller 2004; Lai 2006; Lai et al. 2007b). The study by Stevens et al. (2006) demonstrates the great opportunity afforded by the ability to undertake such high-resolution studies. It should be remembered, however, that sampling a section in greater resolution than can be discerned by the dating technique applied is a waste of resources; if the SGC ages generated by Stevens et al. (2006) were removed from Fig. 10, there would be essentially no loss of information. This is because the very high sampling resolution (typically 10–20 cm) causes adjacent age determinations to overlap within errors, even where the standard error is used and hence the uncertainty is reduced to o5%. Where such overlap occurs, all that can be concluded is that the sediment accumulation rate is too rapid to be resolved and the ages are regarded as coeval. However, it is an almost inevitable outcome of dating studies that must be conducted in one phase of analysis that one will generate great detail in areas of a section that do not necessarily warrant such study (e.g. Fig. 10 and discussions above), and yet fail to capture adequately all of the crucial time periods within the samples that are examined (e.g. the need for closer sampling is discussed by Zhao et al. 2007 and Lai et al. 2007a).

Mass accumulation rates A common oversight in the various studies of the use of OSL to determine sediment accumulation rates in loess is that no allowance is usually made for compaction of the sediments at depth or changing grain size. Simple thickness accumulation rates (e.g. cm/kyr) are typically quoted (e.g. Roberts et al. 2001; Miao et al. 2005; Lai & Wintle 2006; Stevens et al. 2006; Lu et al. 2007; Zhao et al. 2007; Buylaert et al. 2008); instead, the determination of mass accumulation rates (MARs; e.g. g/ cm2/kyr), which incorporates the bulk density of the sediment, is much more useful and permits the simple calculation of dust flux, hence enabling a more direct comparison to be made with dust from marine and ice core records (e.g. Roberts et al. 2003 and Lai et al. 2007a use assumed bulk density values; Muhs et al. 2003 present measured bulk densities). Bulk density is easy to measure, but has been largely overlooked in many studies; this is unfortunate, given the great care, time and attention given to the determination of luminescence ages. Figure 11A uses values from the Chinese Loess Plateau (cited within Fig. 6 of Kohfeld & Harrison 2001) to show the dramatic variation that may be obtained for MARs calculated using the average bulk density of loess (1.65 g/cm3; Pye 1987) compared to the bulk density values of last glacial loess from the Chinese Loess Plateau (typically 1.281–1.632 g/cm3; Liu 1966). Muhs et al. (2003) also demonstrate that bulk density values in Alaskan loess can vary both within the loess

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(1.19–1.56 g/cm3) and across and between soil horizons (0.2–0.5 g/cm3 in soil O horizons; 1.07–1.55 in soil B horizons). If bulk density values have not been determined for a site, Sun et al. (2000) propose that an average bulk density of 1.48 g/cm3 is the most appropriate value for samples of loess from the Chinese Loess Plateau. Data from Liu et al. (1985) and Nugteren et al. (2004) demonstrate the effect of depth on the bulk density determinations for loess deposited during four different climatic conditions (Fig. 11B), highlighting the great variability of the bulk density values and hence the need to consider mass accumulation rates rather than simple loess thicknesses over time. Mass accumulation rates were determined for three thick last glacial (Peoria) loess sections within the A Difference in mass accumulation rate (MAR using BD 1.65 g /cm MAR using another BD value)

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Fig. 11. The potential variability in bulk density (BD) values of loess and, hence, in the MARs calculated (MAR, g/m2/yr = accumulation rate, m/yrBD, g/m3). A. The difference (in g/m2/yr) between MARs calculated using a fixed, average bulk density value of 1.65 g/cm3 (Pye 1987) and MARs calculated using different bulk density values appropriate for various types of loess (e.g. sandy loess = 1.38 g/cm3, silty loess = 1.46 g/cm3 and clay loess = 1.5 g/cm3; Kohfeld & Harrison (2001)), plotted against the MAR calculated using the average bulk density value of 1.65 g/cm3 (Pye 1987). The accumulation rate calculated using bulk density values of 1.38 g/cm3, 1.46 g/cm3 and 1,50 g/cm3 differs by 16.4%, 11.5% and 9%, respectively, compared to the average bulk density value of 1.65 g/cm3. B. The change in bulk density values for loess deposited during four different climatic conditions. Redrawn from Nugteren et al. (2004).

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mid-continent of North America by Roberts et al. (2003), based on OSL ages determined using a SAR protocol applied to coarse-silt sized (35–50 mm) quartz grains, and using a bulk density value of 1.45 g/cm3 which is typical for Peoria loess (Bettis et al. 2003). This study revealed that MARs were not constant over time, and had varied enormously during the last glacial period. The MARs reached exceptionally high values between
18 and 14 kyr ago, ranging from 11 500 g/m2/yr for Bignell Hill (the site with the thickest last glacial loess) to 3500 g/m2/yr for Eustis, a Nebraskan site further from the source area (Roberts et al. 2003). The timing of this unparalleled dust flux (higher than reported for any other pre-Holocene location worldwide) coincides with a mismatch between palaeoecological evidence for the presence of extralimital northern or Cordilleran (cool climate) flora and fauna persisting in central North America for several thousands of years after summer insolation values exceeded those of the present day; this was proposed as evidence of regional scale dust-induced radiative forcing of climate, highlighting the need for atmospheric general circulation models (AGCMs) to incorporate dust loading into regional scale models (Roberts et al. 2003). The recent dust modelling work of Mahowald et al. (2006) is an advance towards this goal. Rousseau et al. (2007) also dated the Eustis site using the same OSL methodology as Roberts et al. (2003) to increase the number of age determinations at this site; based on the fact that the grain size does not increase during the time of the Heinrich 1 event, Rousseau et al. (2007) concluded, contrary to expectations, that the climatic record at Eustis demonstrated greater similarity to records from the Santa Barbara Basin, rather than the North Atlantic Ocean. Future climate modelling work may be able to address the questions thrown up by such studies. Another key consideration for loess studies is the balance between the conditions required for the generation and transportation of silt, and those conditions required to trap silt; both conditions must be satisfied in order to generate and preserve a record of climate change in loess deposits. Additionally, the position of a section within the landscape is important in determining the likelihood of preserving a full climate record within the loess at any given site (e.g. Lai et al. 2007a). For example, continuous sediment accumulation is most likely for sites located in geomorphologically stable settings (e.g. large sedimentary basins), although exceptions to this are observed (e.g. Stevens et al. 2006); hiatuses are most likely for unstable sites such as hillslopes and exposed hilltops (Sun et al. 2000), although, again, they have been identified at sites traditionally regarded as ‘stable’ (e.g. Stevens et al. 2006; Buylaert et al. 2008). Proximity to source areas will also influence the (mass) accumulation rates determined since the thickest loess deposits will usually be located closest to the source (e.g. see summaries of MAR data within

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the DIRTMAP special issue of Quaternary Science Reviews, Derbyshire 2003). However, it should be noted that even modest accumulations of source-distal loess represent atmospheric dust loadings that are potentially capable of modifying climate through radiative forcing, because they contain fine material; it is this far-travelled, fine (o2 mm) material that is the most radiatively active component (Tegen et al. 1996) of loess deposits.

Investigating the synchronicity of records The new found confidence in dating loess deposits using high-precision (typically o5% uncertainty) quartz OSL SAR techniques permits investigations into the synchronicity, or otherwise, of stratigraphic boundaries occurring within loess–palaeosol sequences, without invoking any of the previous concerns regarding the circularity of arguments stemming from reliance on proxies (e.g. magnetic susceptibility measurements) rather than on numerical dating controls. For example, one can compare the quartz OSL age determinations obtained for the last glacial ‘L1’ (‘Malan’) loess unit in China; this unit is capped by the ‘S0’ palaeosol termed ‘Black Loam’. Comparing the luminescence ages determined around the L1/S0 boundary in one study (Stevens et al. 2006), the quartz OSL ages at the top of the L1 (Malan Loess) unit are between 20 and 21 kyr at all three sites in the southeast to northwest transect across the central Chinese loess Plateau; however, the basal age of the overlying unit S0 (Black Loam) varies greatly between 20 kyr at the most southeasterly sites Shiguanzhi and Xifeng, and 10 kyr for the most northwesterly site, Beiguoyuan. Given the strong colour changes noted between the units, Stevens et al. (2006) define the position of the L1/S0 boundary on the basis of the visual appearance in the field. The difficulty of defining the boundary between loess-palaeosol units was considered by Lai & Wintle (2006), who determined the L1/S0 boundary at the Yuanbao section on the basis of linear regression of the fine-grained quartz OSL ages (obtained using SAR) with depth for the two distinctive zones of accumulation. Using this method, Lai & Wintle (2006) placed the L1/S0 boundary at 13.5 kyr for Yuanbao, located on the western edge of the Chinese Loess Plateau (2006). Lai & Wintle (2006) pointed out that, if the boundary were to be defined using the colour change observed in the field, it would be placed at 15.1 kyr, while if the mid-point of the magnetic susceptibility measurements was used, the L1/S0 boundary would be placed at 9.8 kyr. These two studies (Lai & Wintle 2006; Stevens et al. 2006) used the same OSL methodology applied in the same laboratory, and as such they demonstrate the siteto-site variability that it is possible to preserve within the loess deposits. These studies also emphasize the

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Development and application of luminescence dating to loess deposits

difficulty of defining a clear boundary between loesspalaeosol units, highlighting the concerns regarding the use of magnetic susceptibility to identify a change in climatic regime and/or to develop an indirect chronology (Lai & Wintle 2006). Considering the different timing of the boundary between the upper L1 and lower S0 units for the various sites discussed for the Chinese Loess Plateau, it would seem unlikely that these variations in magnetic susceptibility or colour represent regional differences in the onset and cessation of climatic changes; instead, the variations in timing may be associated with local conditions at the site, reflecting local sediment accumulation rates. For example, where sediment accumulation rates are low during soil forming periods, the palaeosol will develop by weathering down into the loess unit. However, where accumulation rates are maintained at a high-enough level during soil forming periods, the palaeosol will still develop by weathering downwards through the profile, but it will develop in an accretionary profile and hence the zone of weathering will effectively move up through the profile as it accumulates further material. The resultant age profile for these two examples would not be the same; in the first example, a large hiatus could potentially develop (e.g. as observed for the L1/S0 boundary at Shiguanzhi; Stevens et al. 2006) and the colour and magnetic susceptibility changes could be quite pronounced, while in the second example no such hiatus would necessarily be observed and the transitions in colour and magnetic susceptibility parameters are likely to be smeared through the profile with no sharp boundary being defined. Of course, if a palaeosol weathers down into an established loess unit (rather than developing syndepositionally as sediment continues to accumulate), then any OSL age from the resultant palaeosol would be prone to all of the issues of postdepositional reworking and bioturbation that are often discussed for other sedimentary deposits (e.g. Bateman et al. 2003, 2007a, b; Duller 2008). However, given the typically fine-grained nature of loess deposits, the difficulty for loess would be that such mixing would be almost impossible to identify given the large number (41 million) of grains that make up even a modest finegrained aliquot. For this reason, where postdepositional mixing or bioturbation is suspected, it would be prudent to work with sand-sized grains if at all possible to investigate the likelihood of such mixing having taken place, even if these coarser grains are, by definition, not as far travelled as fine silt and not the size typically associated with such deposits of wind-blown ‘silt’. Coarse (sand-sized) grains do, however, exist in many loess deposits and have been used successfully to determine quartz OSL ages using the SAR protocol (e.g. Mason et al. 2003; Miao et al. 2005; Buylaert et al. 2008). One advantage of working with sand-sized materials is that the dose rate of the surrounding sediment matrix

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tends to be lower for sandy (particularly for quartzrich) sediments compared to that of fine silts. This means that the maximum limit of the OSL technique applied to quartz may be slightly higher for sandy loess than for very fine loess. Nevertheless, the limitations imposed by the relatively low upper limit of the quartz OSL signal are still frustrating, particularly now that the benefits of deriving a reliable, independent numerical chronology for loess using the SAR protocol have been explored, and several basic notions concerning loess-palaeosol sequences have been challenged as a result. The availability of data that contest the ideas of constant and continuous sediment accumulation rates has led to a lack of certainty over the exact age of loess–palaeosol boundaries, and it seems that once again the loess–palaeosol sequences have become a lessthan-favourable testing ground for luminescence studies due to the confusion generated by the observed intra- and inter-site variability. As the quartz OSL technique has typically been confined to the last
55 kyr of deposition in loess, attention has had to focus upon this relatively recent time period; however, looking at this time period with the high-resolution studies permitted by SAR quartz OSL, things seem to have reached the point where almost more differences are identified rather than similarities.

Future developments in luminescence dating of loess deposits If luminescence studies of loess are to make yet another major advance, it will probably be by adopting a broader time scale of scrutiny to extend beyond the last interglacial–glacial cycle. Typically, the maximum age range of loess using quartz OSL is
55 kyr (Zhou & Shackleton 2001; Buylaert et al. 2007, 2008), although ages have been obtained that are in agreement with stratigraphic evidence back to 126 kyr (Lu et al. 2007) and even beyond (Watanuki et al. 2005). The study by Watanuki et al. (2005) is rather unusual for quartz OSL studies of loess because the ages achieved are very old (reaching a maximum value of 4300 kyr). The mixture of minerals present and the fine grain size of loess means that the annual dose rate is typically much higher for loess (typically
3.5–6 Gy/kyr) than for coarser sediments (typically
1–2 Gy/kyr), meaning that the maximum age that can be achieved before the materials reach saturation is lower for loess than for coarser-grained materials. However, the very high water content (64–140% calculated as mass of water over mass of dry sediment) recorded in the study by Watanuki et al. (2005), resulting from the location of the samples on a river terrace, gives rise to a very low annual dose rate of
1 Gy/kyr, making these older ages (4300 kyr) possible. However, given the rather restricted upper age limit typically faced for quartz OSL

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dating of loess deposits, which is particularly frustrating when one considers that some loess–palaeosol sequences can span the entire Quaternary Period, efforts in the future will concentrate on extending the upper age range of luminescence dating techniques. One way of doing this may be to revisit feldspars, because feldspars saturate at much higher doses than quartz (Fig. 2) and are therefore capable of extending the age range further back in time than quartz; however, efforts would need to be redoubled to develop methods to circumvent the problems that have hitherto hindered the routine use of feldspars for dating (principally anomalous fading). Therefore, the future may also lie in applying new OSL methods to quartz, and also working with new luminescence signals. One approach that may hold promise for extending the upper age limit of luminescence dating is to exploit other emission wavelengths. All of the examples considered in this article thus far have examined that part of the luminescence signal that is emitted in the blue or near-UV (ultraviolet) part of the spectrum. However, luminescence may also be emitted at other wavelengths; for example, some quartz samples emit a strong TL signal in the red part of the spectrum (e.g. detection between 575 and 675 nm; Lai & Murray 2006). Such ‘Red TL’ was originally developed to date heated materials (e.g. Miallier et al. 1991; 1994a, b; Fattahi & Stokes 2000); however, it has recently been tested for use in dating loess deposits (Lai et al. 2006; Lai & Murray 2006) because this signal grows to a much higher dose than the signal of the conventional blue/ UV OSL emission in quartz. Lai & Murray (2006) note that the saturation dose for the red TL signal from a sample of loess from China is
550 Gy, compared to a value of only
90 Gy for the UV OSL from the same sample. However, high residual doses of
100 Gy have also been observed from both modern and bleached samples, meaning that the technique is likely to be restricted to loess samples older than
30 kyr (based on a typical dose rate for loess of
3.5 Gy/kyr; Lai et al. 2006). The technique is still under development, with the focus being on defining the ‘bleachability’ of each sample and determining the residual signal level, plus monitoring and correcting for the dose-dependent sensitivity changes observed when laboratory doses in excess of 1500 Gy are applied (Lai & Murray 2006). Nevertheless, the technique holds promise if these problems can be surmounted. Isothermal thermoluminescence (ITL) has also recently been proposed as a potential signal for extending the upper limit of luminescence dating. The technique utilizes the UV emission from ITL of quartz (i.e. measurements made at a constant temperature) and offers the benefit over traditional TL (i.e. measurements made during ramped temperature) that the signal is potentially more fully reset, and measurements can be made using a single-aliquot protocol (bringing with it all the

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associated advantages, such as multiple determinations of De and hence De distributions, improved precision and excellent reproducibility). Choi et al. (2006) found that there was good agreement between quartz SARITL and SAR-OSL De values for samples originating from a variety of sediments. The loess samples in this study were beyond the limits of conventional OSL dating; however, Choi et al. (2006) concluded that having measured a De value of 550 Gy for the oldest loess sample, ITL offers a potential means of extending the age range of luminescence dating. Measurements of quartz ITL and OSL were carried out by Buylaert et al. (2006) for Chinese loess using both SAR and single aliquot regeneration added dose (SARA) techniques. Unlike SAR (Murray & Wintle 2000), SARA (Mejdahl & Btter-Jensen 1994) is not strictly a ‘single-aliquot’ technique, because it requires at least two aliquots to yield a De value; however, the benefit of SARA is that it can be used to obtain reliable De values irrespective of the sensitivity changes which might occur during measurement of an aliquot. Buylaert et al. (2006) concluded that measuring ITL using a SARA technique potentially offered a large increase in the dose range compared to measurements made using OSL, and showed consistency between the SARA-ITL and SAR-OSL De values in regions where the OSL signal is not saturated. However, in contrast to Choi et al. (2006), the use of a SAR-ITL protocol gave significantly higher De values than using SAR-OSL, due to large sensitivity changes occurring during the first heating applied to measure the natural signal (Buylaert et al. 2006). Interestingly, use of the SARA-ITL in the study by Huot et al. (2006) did not give results which were in agreement with known age samples; this study is worth mentioning in this review because, although it was not conducted on loess, it noted the important point that a SARA approach is inherently less useful in the strongly nonlinear region of the growth curve, which is of course the region of interest for extending the dating range. For this reason, a key problem that remains to be addressed when applying ITL concerns the use of SAR and therefore the question of sensitivity change occurring during the first measurement; this must be addressed before more widespread adoption of the technique is likely. Another newly proposed technique that may play a role in the future of luminescence dating is that of Wang et al. (2006c), using thermally transferred OSL (TT-OSL) to potentially extend significantly the age range for dating using quartz (Wang et al. 2006a). This is a multiple-aliquot regenerative-dose protocol in two parts which measures a signal that has a much higher saturation dose (
2500 Gy) than the conventional quartz fast component OSL signal (typically
125 Gy, and in some cases up to
400 Gy). The TT-OSL technique is in the relatively early stages of development and testing; however, initial work applying the

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Development and application of luminescence dating to loess deposits

technique to loess deposits in China suggests that it may potentially be able to extend the age range back by approximately an order of magnitude to the Brunhes/ Matuyama magnetic reversal
776 000 years ago (Wang et al. 2006a). Future efforts are likely to focus on further testing and validation of the technique, and the development and testing of a single-aliquot procedure (e.g. Wang et al. 2007; Tsukamoto et al. 2008).

Future applications of luminescence dating to loess deposits If the aim to extend the age range of luminescence techniques proves successful, there will be many applications of these new techniques, but loess will remain particularly attractive as a testing-ground for new techniques due to the long, quasi-continuous records that can be preserved. If reliable luminescence techniques that span numerous glacial–interglacial cycles can be developed, then loess records could be examined from a new perspective and efforts would be likely to focus on investigating the long-term synchronicity of climate records. Establishing a long record of change using a numerical chronology would reduce concerns regarding circularity of arguments that can be a factor when using palaeoclimate proxies, and would eliminate the requirement to assume relationships between records or proxies. In this way, records could be linked unambiguously and thus correlations could be really examined. Questions regarding the long-term continuity of the terrestrial record of climate change preserved in loess could also be addressed, and the variability of loess records could be examined in concert with those of other long records with which they are compared, such as the marine record. As methods used for luminescence dating are developed, tested and proved, and quality checks on the reliability of the signals and the methods are introduced, the need for luminescence studies to be reassured by other independent chronologic evidence will be reduced thereby further increasing the value of luminescence dating; this has already happened in the case of pure quartz of less than
100 kyr age measured using SAR. As confidence in luminescence chronologies increases, the future will be likely to give rise to an increased number of interdisciplinary studies involving luminescence dating, with luminescence chronologies playing an increasingly significant role in these studies. Recent examples of such a successful interdisciplinary study are those of Antoine et al. (2001) and Rousseau et al. (2002) based on the loess/palaeosol sequences of Nussloch, near Heidelberg, Germany. Luminescence and radiocarbon dating provided the chronological framework, taking advantage of recent advances in AMS radiocarbon dating that require only very small quan-

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tities (o0.1% by weight) of organic carbon (Hatte´ et al. 2001). When combined with stratigraphy, palaeopedology, sedimentology, palynology, malacology and isotope geochemistry, the study by Antoine et al. (2001) and Rousseau et al. (2002) provided a detailed reconstruction of wind dynamics, palaeoclimate and dust flux through the last interglacial–glacial cycle. Loess deposits preserve records of atmospheric circulation and dust flux, and increasingly the importance of these records, not only as a record of climate change but also as a potential agent of climate change, is being recognized. Traditionally, high rates of dust (loess) deposition were believed to be the result of climate change, arising from the shift to a dry, cold, windy glacial period with abundant sediment supply and little vegetation cover. However, recent climate models challenge this view, suggesting instead that increased levels of atmospheric dust, regardless of how they are brought about, can actually cause climate change (Overpeck et al. 1996; Tegen et al. 1996; Harrison et al. 2001; Bar-Or et al. 2008). At present, we have a limited understanding of the role of dust in climate change (Harrison et al. 2001), but interest in dust flux and the role of dust in climate change is growing, with a number of programmes and interdisciplinary research projects, such as the ‘Dust Indicators and Records from Terrestrial and MArine Palaeoenvironments (DIRTMAP)’ initiative (Kohfeld & Harrison 2001), recognized as important formal activities of the INQUA loess commission. In spite of these efforts to collect and collate data, the record of the spatial and temporal variability in dust flux is currently inadequate for satisfactory testing of atmospheric global circulation models (AGCMs). However, in the future, luminescence dating is likely to play an increasingly important role in furthering our knowledge and understanding of records of past dust flux because the increased precision of luminescence ages obtained by using single-aliquot regenerative-dose (SAR) techniques has made highresolution dating studies feasible (e.g. Roberts et al. 2001, 2003; Lai & Wintle 2006; Lai et al. 2007a), and streamlining the measurements required to determine OSL ages at some sites (e.g. using the SGC approach of Roberts & Duller (2004)) offers the potential to increase the sheer number of samples studied while maintaining the precision and hence increasing the resolution still further (e.g. Stevens et al. 2006). The detailed investigation of spatial and temporal variations in dust-flux records is anticipated to be the new, exciting direction of loess research, and the recent developments and improvements made in luminescence dating offer an unprecedented opportunity to access these significant terrestrial records of climate change. Acknowledgements. – Thanks to Dan Muhs and an anonymous reviewer for valuable comments on an earlier version of this

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manuscript. Thanks also to Ian Gulley and Antony Smith for help in preparing some of the diagrams.

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