BOREAS. Luminescence dating of fluvial deposits: applications to geomorphic, palaeoseismic and archaeological research

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1 Luminescence dating of fluvial deposits: applications to geomorphic, palaeoseismic and archaeological research TAMMY M. RITTENOUR BOREAS Rittenour, T. M (November): Luminescence dating of fluvial deposits: applications to geomorphic, palaeoseismic and archaeological research. Boreas, Vol. 37, pp /j x. ISSN Fluvial deposits and landforms are important archives of river response to climate, tectonics and base level change and are commonly associated with archaeological sites. Unlike radiocarbon dating, the target material for optically stimulated luminescence (OSL) dating (sands and silts) is nearly ubiquitous in fluvial deposits and the age range for OSL spans the last glacial interglacial cycle, a time period of interest to many Quaternary scientists. Recent advances in OSL techniques and the development of single-grain dating capabilities have now allowed fluvial deposits, and other deposits commonly afflicted with incomplete zeroing of the luminescence signal, to be dated. The application of OSL dating to fluvial deposits is discussed with respect to its potential to provide important contributions to research in the fields of geomorphology, palaeoseismology and archaeology. Examples are given from each research field. Tammy M. Rittenour ( tammy.rittenour@usu.edu), Department of Geology, Utah State University, 4505 Old Main Hill, Logan UT, 84322, USA; received 25th February 2008, accepted 18th July River systems are important geomorphic agents in sculpting the Earth s surface and are excellent monitors of environmental change because they integrate signals related to the geology, geomorphology, climate, hydrology, vegetation and tectonics from within their catchments (e.g. Schumm 1977). As such, fluvial deposits and landforms provide important archives of river response to changes in climate, tectonics and base level. Additionally, fluvial deposits are commonly associated with archaeological sites. Obtaining age control from fluvial deposits, however, has been difficult due to limited organic material for radiocarbon dating and problems with reworking of old carbon in many fluvial sediments (e.g. Blong & Gillespie 1978; Gillespie et al. 1992; Stanley & Hait 2000). Other techniques, such as cosmogenic nuclide dating of terrace surfaces and U-series dating of pedogenic carbonate, provide minimum ages on sediment deposition and landform abandonment (e.g. Gosse & Phillips 2001; Sharp et al. 2003). Optically stimulated luminescence (OSL) dating has the benefit of directly dating the time of sediment deposition and is a rapidly growing technique in the fields of sedimentology, geomorphology and archaeology (see reviews by Stokes 1999; Feathers 2003; Lian & Roberts 2006). This paper provides a review of new applications of OSL dating to fluvial deposits. Technical descriptions of OSL techniques are given elsewhere (e.g. Aitken 1998; Btter-Jensen et al. 2003). Wintle (2008a) provides a general introduction to the minerals used for dating (quartz and feldspars) and dating methodologies. Wallinga (2002) and Jain et al. (2004a) have provided excellent reviews of the applications and problems of OSL dating in fluvial settings. The goal of this article is to provide an updated review of innovative applications of OSL dating to fluvial deposits to solve questions related to fluvial response to climate and base level change, palaeoseismic studies and archaeological applications. Luminescence dating of fluvial sediments OSL dating provides an age estimate for the last time sediments were exposed to sunlight, which resets the luminescence signal (Huntley et al. 1985). After burial, this signal grows with time due to exposure to ambient radiation in the surrounding sediments and from incoming cosmic rays. The longer the sample is buried, the longer it is exposed to this low-level radiation and the greater the intensity of the luminescence signal subsequently measured. In the laboratory, the age of a sample is calculated by dividing the amount of ionizing radiation the sample absorbed during burial (called the equivalent dose, D e ) by the dose rate derived from the environment surrounding the sample. A number of techniques for D e determination have been developed (e.g. Wintle 1997; Aitken 1998; Lian & Roberts 2006). This article focuses on applications using the most recent single-aliquot regenerative dose (SAR) technique for quartz sand (blue and green light stimulated OSL) (Murray & Wintle 2000), feldspar (infrared stimulated luminescence, IRSL) (Wallinga et al. 2000a) and finegrained (silt) IRSL dating (Banerjee et al. 2001). In this paper, quartz SAR OSL ages are referred to as quartz OSL ages, while luminescence ages obtained from other methods and minerals are identified differently. All methods of OSL dating rely on the luminescence signal acquired during the preceding burial history to have been removed by light exposure prior to deposition. Incomplete solar resetting of the luminescence DOI /j x r 2008 The Author, Journal compilation r 2008 The Boreas Collegium

2 614 Tammy M. Rittenour BOREAS signal at deposition (partial bleaching) results in age overestimation and can be a problem in some fluvial settings (e.g. Murray et al. 1995; Gemmell 1997; Olley et al. 1999, 2004b; Stokes et al. 2001). Partial bleaching (zeroing) of the luminescence signal prior to deposition is likely in fluvial environments for a number of reasons. Solar resetting of water-transported sediment is limited by the attenuation of light through the water column (e.g. Berger 1990). This effect is enhanced by increased suspended sediment concentrations (e.g. Berger & Luternauer 1987). Water depth, the mode of sediment transport (suspension, saltation or bedload) and transport distance are also important controls on the bleaching of fluvial sediments. The direct input of non-bleached sediment from the erosion of older deposits and river banks is common in fluvial systems and also contributes to scatter in results. Additionally, floods, storms and other high-discharge events cause rapid erosion and transport of sediments, limiting solar exposure. A number of methods have been proposed to combat the influence of partial bleaching on D e values and resulting age calculations. One group of methods utilizes the multiple components of the quartz OSL signal to isolate and date the most light-sensitive OSL traps (e.g. Tsukamoto et al. 2003; Jain et al. 2005a; Li & Li 2006). A second group of methods takes advantage of recent advances in the OSL technique that have led to the measurement of smaller and smaller aliquot sizes (Olley et al. 1998), culminating in the development of single-grain dating techniques (Duller et al. 1999; Duller 2000, 2008) and instrumentation (Btter-Jensen et al. 2000). In partially bleached samples, analysis of large aliquots of sand may produce age overestimates from the contribution of non-bleached grains to the total signal measured (e.g. Jacobs et al. 2003; Thomas et al. 2005; Porat et al. 2008). Small-aliquot (o100 grains) and single-grain dating may allow the true burial age of a sample to be isolated by allowing the population of grains not bleached at deposition to be identified. Singlegrain results commonly show positively skewed D e distributions, with the youngest population representing the grains fully bleached at deposition (Fig. 1). Owing to the large distribution of D e results in singlegrain analysis, a number of statistical methods have been produced to isolate grains representative of the true burial dose. These include methods that calculate the mean of the lowest 5% of the D e values (Olley et al. 1998), methods that fit a Gaussian distribution to the leading edge (youngest values) of a D e distribution histogram (Lepper et al. 2000) and the central age model (CAM), minimum age model (MAM) and finite mixture model (FMM) (Galbraith et al. 1999; Galbraith 2005). The optimum choice of statistical methods may be different for each sample and is dependent on the dominant mechanisms affecting D e scatter (e.g. partial bleaching, post-depositional mixing or dose-rate heterogeneity) (Bailey & Arnold 2006). Considerable success has been achieved using singlegrain dating from some of the most challenging depositional environments, including fluvial deposits (Olley et al. 2004b; Thomsen et al. 2007; see also review by Duller 2008). Single-grain dating is most important for, and applicable to, younger samples (less than a few thousand years old) (Jain et al. 2004a). Collection of modern samples from a number of depositional environments has indicated residual luminescence signal in some settings. This level of non-bleached signal will contribute a large proportion of the measured D e in young samples and can produce large age overestimates (e.g. Murray et al. 1995). This effect is expected to be reduced in older samples that have higher burial doses. As an example, DeLong & Arnold (2007) report large overestimates (up to 50% and greater) of single-aliquot measurements as compared to singlegrain measurements from samples under 1000 years old. Negligible differences were found in late Pleistocene samples. Similar overestimates in age have been seen in other analyses of large aliquots from partially bleached samples (e.g. Thomas et al. 2005; Porat et al. 2008). Figure 1 gives examples of single-grain and large aliquot results from young fluvial samples collected from an ephemeral stream in western Nebraska (Hanson 2006). It is clear from the dose distributions of the single-grain results that these samples have high D e scatter that is skewed toward higher doses, indicative of partial bleaching. However, results from the large aliquot (5 mm) analyses do not show this same skewness and have much higher mean and minimum D e values. Averaging of thousands of grains has masked signs of partial bleaching and the true burial age of the sample. For these reasons, single-grain and small-aliquot analysis is preferred for identification and potential mitigation of problems associated with partial bleaching in young fluvial samples. Despite potential pitfalls, many studies have had considerable success in dating fluvial deposits using single-aliquot (multi-grain) techniques (e.g. Törnqvist et al. 2000; Cheong et al. 2003; Rittenour et al. 2005; Rodnight et al. 2005, 2006; Tooth et al. 2007). As with single-grain dating, samples with evidence of partial bleaching may require statistical methods to isolate the true burial dose (e.g. Olley et al. 1998; Galbraith et al. 1999, 2005; Lepper et al. 2000; Roberts et al. 2000; Fuchs & Lang 2001), but, overall, most sediments have proved datable by OSL with only a few exceptions (Wallinga 2002; Jain et al. 2004a). Research by a number of groups over the last couple of decades has identified interesting results regarding the bleaching of sediments in fluvial environments. The first is related to the observation that modern and recently deposited sediment in fluvial systems is

3 BOREAS Luminescence dating of fluvial deposits 615 Frequency Frequency Large aliquot (multi-grain, MG) Rocky Hollow 7 1 n = De (Gy) Rocky Hollow 8 3 n = De (Gy) Frequency Frequency De (Gy) Single grain (SG) Rocky Hollow 7 1 n = 207 Rocky Hollow 8 3 n = De (Gy) Age Results SG = 150±20 yr MG = 1400±180 yr Lowest MG De = 430±530 yr SG = 550±40 yr MG = 1400±140 yr Lowest MG De = 630±50 yr Frequency Rocky Hollow 14 1 n = De (Gy) Frequency Rocky Hollow 14 1 n = De (Gy) SG = 500±50 yr MG = 1270±140 yr Lowest MG De = 340±30 yr Fig. 1. Comparison of large-aliquot and single-grain results from three young fluvial samples from an ephemeral stream in western Nebraska (Hanson 2006). Results from the large-aliquot analysis (5 mm, thousands of grains) show some signs of equivalent dose (D e ) scatter, such as bimodal distributions (sample 7-1) and skew toward younger values (sample 14-1). Multi-grain results from sample 8-3 are normally distributed, which may be interpreted to indicate a well-bleached sample. However, single-grain results show skewed distributions toward higher D e values and provide a clearer image of the partial bleaching within these samples. Averaging of thousands of grains within the large aliquots has masked these signs of partial bleaching and the true burial age of the sample. Under the age results column: SG = single-grain results using a minimum age model; MG = multi-grain (large-aliquot) results. Data kindly provided by P. R. Hanson, University of Nebraska. commonly less well bleached than samples collected from older deposits from the same river system (see Jain et al. 2004a). The large influence of small residual luminescence signals on modern age deposits compounds this difference but does not completely explain it. Collection of modern analogue samples has been proposed to identify the bleaching efficiency of different depositional environments (Murray et al. 1995). However, not all modern samples are good analogues for bleaching of older deposits. It is likely that sediments in terrace deposits and older floodplain and channel deposits have undergone considerably longer and more numerous transport and deposition cycles prior to final deposition (allowing for greater bleaching) than the transient sediment found in modern channel and bar deposits (Jain et al. 2004a). A second unexpected result in the bleaching of fluvial sediment is related to the difference between coarser and finer grain sizes. It might be expected that finegrained sediment (silt to very fine sand) would be better bleached due to the greater potential for these sediments to be transported in suspension in the water

4 616 Tammy M. Rittenour BOREAS column. Coarser sediment (medium-coarse sand) is more likely to have been transported as bedload by saltation or tractive forces at the base of the water column. However, many studies have found that the coarser grain sizes have lower D e values and are better bleached at deposition (Olley et al. 1998; Colls et al. 2001; Truelsen & Wallinga 2003; Alexanderson 2007; Vandenberghe et al. 2007). The reasons for the difference in bleaching between finer and coarser grain sizes are not fully understood but are probably related to the mode of transport. Coarser grained sediments are transported at a slower rate through a fluvial system than silts and very fine sand in suspension, allowing more time for sunlight exposure between initial erosion from an older deposit and final deposition. Coarser sediment is also more likely to be deposited on channel bars and exposed to light numerous times during transport. In addition to mud coatings on fine grains, the cohesive properties of silts and very fine sand may cause these grains to be transported as aggregates and hinder solar bleaching. Regardless of the cause of the difference in bleaching between grain sizes, it is recommended that coarser grain sizes are dated in fluvial deposits where partial bleaching is a problem. Comparison between dating techniques A number of studies have compared OSL ages with other dating techniques. Murray & Olley (2002) compared quartz OSL ages from known age deposits from a number of depositional settings and found a good correlation, with accurate OSL ages obtained to at least 350 kyr. Figure 2 is an updated version of the figure produced by Murray & Olley (2002) with additional samples included. Note that despite potential problems with partial bleaching in fluvial settings, there is no systematic offset for fluvial samples as compared to other depositional environments. Most studies have compared OSL with radiocarbon due to its wide application and acceptance in the Quaternary research community and have found good correspondence in many cases (e.g. Wallinga et al. 2001; Jain et al. 2004a; Olley et al. 2004a; Rittenour et al. 2005), but not all (e.g. Folz et al. 2001; Kolstrup et al. 2007; Owen et al. 2007). However, while inconsistencies between these chronometers may indicate errors in the OSL chronology (e.g. due to partial bleaching or dose rate uncertainty), it is also likely that they reflect problems with radiocarbon ages due to contamination or reworking of organic material (e.g. Goble et al. 2004; Cupper 2006; DeLong & Arnold 2007). Additionally, there is added uncertainty when comparing OSL and radiocarbon due to problems with calibrating radiocarbon ages to calendar years beyond 15 kyr (Reimer et al. 2004). Despite potential problems with both techniques, there is good agreement in samples collected from fluvial deposits from throughout the radiocarbon age range (0 40 kyr) (Fig. 2). Fewer comparisons are available for older samples, but there is no evidence of systematic departure between OSL and independent ages over the last OSL/Independent Age Ratio Fluvial (n = 61) OSL Age (kyr) Aeolian (n = 43) Marine (n = 33) Glacial (n = 6) 1:1 line OSL age over estimate (partial bleaching) 400 Independent Age (kyr) Independent Age (kyr) OSL Age (kyr) Fig. 2. Comparison of quartz SAR OSL ages from fluvial (filled squares) and other deposition settings to independent age control. Modified and updated from Murray & Olley (2002) with more recently published results. Age data and references provided in Appendix Table 1.

5 BOREAS Luminescence dating of fluvial deposits 617 several hundred thousand years (e.g. Murray & Olley 2002; Murray & Funder 2003; Watanuki et al. 2005; Murray et al. 2007, 2008). Applications and examples Fluvial deposits and environments are important archives of past changes in climate, base level and tectonics, and commonly contain archaeological horizons. Therefore, accurate age control from fluvial deposits is important for a number of research fields. OSL dating of fluvial deposits has become increasingly widely used and accepted over the past decade. Examples of the application of OSL dating to a number of fluvial settings and research questions are discussed below. Fluvial response to glaciation and sea-level change River channel morphology, longitudinal valley profile, sedimentary architecture and dynamic state (aggrading/incising/stable) are controlled by complex interactions between external controls such as climate, tectonics and base level (e.g. Blum & Tornqvist 2000). Although fluvial deposits and landforms do not provide a direct record of past base level and climate change, they can provide information about the response of the river system to these external variables. Large river systems such as the lower Mississippi River in the United States, the Rhine Meuse River system in The Netherlands, and the River Thames and former Solent River in southern England have been extensively studied with respect to fluvial response to glaciation and sea-level change (e.g. Bridgland 1994, 2000; Maddy et al. 1998; Tornqvist 1998; Blum et al. 2000; Törnqvist et al. 2000, 2004; Rittenour et al. 2007). These rivers have also been extensively dated with radiocarbon, although most of these ages are limited to Holocene deposits due to the lack of organic material in the Pleistocene strata and the upper age limit for radiocarbon dating. OSL dating has been applied to these river systems to extend the fluvial chronologies over the past several glacial interglacial cycles. Mississippi River, USA. An OSL chronology based on over 80 quartz OSL ages has been developed for Pleistocene and Holocene fluvial deposits in the lower Mississippi valley (Rittenour et al. 2003, 2005, 2007; Holbrook et al. 2006). OSL ages were collected from outcrops and shallow surface cores from late Pleistocene braid belts and last interglacial and Holocene meander belts and range from 0 to 85 kyr (Fig. 3) (Rittenour et al. 2005, 2007). These ages agree with the available radiocarbon chronology and relative age constraints from loess stratigraphy and cross-cutting relationships. Additionally, OSL results show only minimal influence of partial bleaching despite sediment transport under high suspended sediment load conditions and late Pleistocene glacial meltwater discharge. Fig. 3. Generalized cross-section of fluvial landforms and deposits in the northern lower Mississippi valley with the OSL chronology plotted graphically above each terrace surface. Modified from Rittenour et al. (2005) with the addition of Holocene meander belt ages.

6 618 Tammy M. Rittenour BOREAS Adequate bleaching prior to deposition was likely due to long transport distances. Insights gained from this new OSL chronology and field relationships suggest that braid belt formation and abandonment was controlled by fluctuations in glacial sediment and meltwater discharge, while sea level controlled the elevation to which the river was graded (Rittenour et al. 2007). Rhine Meuse River, The Netherlands. OSL dating has also been conducted on the Rhine Meuse River system in The Netherlands (Törnqvist 1998; Tornqvist et al. 2000, 2003; Wallinga 2001; Wallinga et al. 2001, 2004; Busschers et al. 2005, 2007). Over 90 luminescence samples have been collected from a number of deep cores collected within the Rhine Meuse valley (Busschers et al. 2005, 2007). A majority of the samples were processed to extract quartz for luminescence measurements, while some were processed for both quartz and feldspar dating. Where comparisons were made, feldspar IRSL ages consistently underestimated both quartz OSL ages and constraints from known-age deposits (Wallinga et al. 2000b, 2001). This may be due to anomalous fading of the luminescence signal in feldspars and problems with its correction (e.g. Wintle 1973; Wallinga et al. 2007). Quartz OSL ages range from Holocene to over 200 kyr, and are stratigraphically consistent with the available radiocarbon and pollen biostratigraphic age control (Busschers et al. 2005, 2007). OSL results and core descriptions have been used to reconstruct river response to sea level and have helped to refine the subsurface fluvial stratigraphy of the Rhine Meuse River system (Törnqvist et al. 2000, 2003; Busschers et al. 2005, 2007). Additionally, hundreds of cores and stratigraphic descriptions from the Rhine Meuse Delta have been used to reconstruct channel patterns and fluvial response to ice advance, sea level and climate change during the last glacial cycle (Busschers et al. 2005, 2007, 2008). Thames and Solent River systems, southern England. The large river systems south of the Anglian (marine oxygen isotope stage, MIS 12) and Late Devensian (MIS 2) ice limits in southern England contain extensive fluvial records that extend back to the Middle Pleistocene and roughly correlate to glacial interglacial climate, sea-level and isostacy changes in the region (e.g. Sandford 1924; Bridgland 1994, 2000; Maddy et al. 1998; Lewis et al. 2001; Briant et al. 2006). Fluvial deposits and terraces of the River Thames, and to a lesser extent those in the Solent basin, have been dated by pollen and mammalian biostratigraphy, aminoacid geochronology and Palaeolithic artifact assemblages (e.g. Briggs et al. 1985; Bridgland 1994). New quartz OSL chronologies have been developed to refine the chronostratigraphies in these river systems (e.g. Maddy et al. 1998; Lewis et al. 2001; Briant et al. 2006). These OSL chronologies for the most part match previous age models; however, there are some inconsistencies and scatter in age results for deposits older than 300 kyr in the Solent basin, possibly due to samples nearing the maximum limit for OSL dating and saturation of the luminescence signal (Briant et al. 2006). Fluvial response to climate change River systems and climate are intimately linked through the hydrological cycle and climate-mediated changes in sediment yield within catchments (e.g. Schumm 1977; Bull 1991). Despite potential problems with partial bleaching, considerable success has been obtained applying OSL dating to a number of different fluvial settings for the reconstruction of fluvial response to climate change (e.g. Leigh et al. 2004; Schokker et al. 2005; Briant et al. 2006; Brook et al. 2006; Williams et al. 2006; Sohn et al. 2007; Thomas et al. 2007b). Studies from a number of regions and climate settings are discussed below. Hillslope and fluvial response to climate, Australia. Luminescence dating has played a key role in Quaternary studies and climate reconstruction from fluvial sequences in Australia (e.g. Page et al. 1991, 1996; Page & Nanson 1996; English et al. 2001; Ogden et al. 2001; Banerjee et al. 2002; Kemp & Spooner 2007). Thomas et al. (2007a) provide an excellent example of the application of OSL dating to hillslope and fluvial deposits along the northeastern coast of Queensland, Australia to decipher fluvial response to climate change. Ages were obtained using radiocarbon (n = 6), thermoluminescence (TL) (n = 16) and quartz OSL methods (n = 36). Owing to scarcity of organic material for radiocarbon dating, age control was only possible through luminescence dating techniques at most locations. Results indicate marked differences in alluvial/colluvial systems between coarse-grained fanglomerate deposition in MIS 3 (64 28 kyr), fine-grained alluviation and vertical fan accretion in MIS 2 (28 24 kyr) and incision at kyr. Correlation to the regional pollen stratigraphy (Kershaw et al. 2007) suggests that these changes in hillslope and fluvial processes were associated with climate and vegetation shifts. Tributary and trunk channel response to climate, Grand Canyon and southwestern USA. Studies of arid fluvial systems have suggested that they are sensitive indicators of climate-related changes in sediment supply and discharge (e.g. Bull 1991). The Colorado River and its tributaries in the Grand Canyon and Grand Wash Trough, just downstream of the Grand Canyon, have been the focus of research efforts to understand the response of different scale fluvial systems to local and regional climate (Anders et al. 2005; DeJong et al. 2006; Pederson et al. 2006; Rittenour et al. 2006; DeJong 2007). Fluvial deposits and terrace landforms have been dated by a number of methods. The time of fluvial

7 BOREAS Luminescence dating of fluvial deposits 619 aggradation and sediment deposition has been determined by quartz OSL dating of sediment and U-series dating of interbedded travertine that was deposited contemporaneously with fluvial deposition. Terrestrial cosmogenic nuclide (TCN) surface dating of desert pavements and sediment profiles provides minimum age estimates for river incision and terrace surface abandonment (Gosse & Phillips 2001). Geochronologic and stratigraphic results from four study areas within and adjoining the Grand Canyon are presented in Fig. 4. Age results are consistent between the different dating methods and are coherent with stratigraphic relationships. OSL results are roughly equivalent to U-series ages of fluvial aggradation in Travertine Grotto in western Grand Canyon (Fig. 4C). In eastern Grand Canyon, OSL and TCN samples were collected from a number of terraces from the Colorado River and its tributaries. As expected, TCN ages of terrace surface abandonment and landform stabilization are younger than OSL ages of fluvial aggradation of the underlying terrace fill (Fig. 4A, B). Equivalent dose distributions from samples collected from quartzrich, long-transported deposits of the mainstem Colorado River showed very little evidence for partial bleaching, while samples of short-transported and locally sourced sediments from tributary catchments contained much greater scatter and evidence for partial bleaching (DeJong et al. 2006; Rittenour et al. 2006; DeJong 2007). OSL ages from deposits of the Colorado River and its tributaries in eastern and western Grand Canyon and Grand Wash Trough have provided important information about incision rates (Pederson et al. 2006) and the timing of the response of these arid fluvial systems to regional and extra-regional climate (Anders et al. 2005; DeJong et al. 2006; Rittenour et al. 2006; DeJong 2007). OSL and other chronostratigraphic results from terrace deposits of the main stem Colorado River indicate that aggradation and incision were predominantly controlled by MIS 4 glaciation in its headwaters. In contrast, tributary chronologies indicate a local response to climate-related sediment supply and have very few similarities with contemporaneous dynamics in the main stem river (Anders et al. 2005; DeJong et al. 2006; DeJong 2007) (Fig. 4B, C). Terrace stratigraphies from Grand Wash indicate lower late Pleistocene incision rates and record a different sequence of fluvial terraces in this much larger and more arid drainage basin (Rittenour et al. 2006) (Fig. 4D). A 180 B 150 Colorado River - Eastern Grand Canyon Eastern Grand Canyon Tributaries m above river m above creek C Travertine Grotto - Western Grand Canyon D Grand Wash - Grand Wash Trough m above creek m above wash Fig. 4. Schematic cross-sections and age constraints from terrace sequences from (A) the Colorado River in eastern Grand Canyon (modified from Pederson et al. 2006), (B) tributary valleys in eastern Grand Canyon (modified from Anders et al. 2005), (C) Travertine Grotto, a small tributary in western Grand Canyon, and (D) Grand Wash in the Grand Wash Trough, just downstream of Grand Canyon. Age control has been provided by OSL ages on fluvial sediment, U-series ages on travertine interbedded within fluvial and colluvial sediments and terrestrial cosmogenic nuclide (TCN) dating of terrace surfaces. As expected, OSL and U-series age from the same terrace fills provide similar ages while TCN ages of terrace surface abandonment and landform stabilization are younger than OSL ages of fluvial aggradation of the underlying terrace fill.

8 620 Tammy M. Rittenour BOREAS Climate and tectonic record from fluvial systems on the Gangetic Plains, India. The broad Gangetic Plains in the Himalayan Foreland Basin of northern India contain a rich archive of fluvial response to climate change and regional tectonic activity. Radiocarbon, quartz OSL and feldspar IRSL ages have been obtained from fluvial deposits of the Ganga River and its tributaries (Srivastava et al. 2003a; Gibling et al. 2005; Williams et al. 2006; Sinha et al. 2007). Despite potential partial bleaching problems in these fluvial settings, luminescence ages are in close agreement with radiocarbon age control where present. Gibling et al. (2005) identified disconformity-bounded floodplain sequences on now dissected interfluves on the southern Gangetic Plain. Interpretations of these and other stratigraphies from the Gangetic Plains suggest that the Ganga River system has switched between modes of aggradation and incision several times during the last glacial cycle, most likely due to changes in the monsoonal precipitation regime (Gibling et al. 2005; Williams et al. 2006; Sinha et al. 2007). Superimposed on these climate-driven changes in fluvial dynamics, geomorphic evidence suggests that some incision events were driven by tectonic activity (Srivastava et al. 2003a, b). Indian monsoon variability, Thar Desert, India. A detailed palaeoclimate record of Indian monsoon variability is emerging from luminescence-dated fluvial deposits from the Thar Desert in western India. Juyal et al. (2006) combined quartz OSL ages and stratigraphic descriptions from the Mahi and Orsang Rivers to reveal sub-milankovitch-scale changes in fluvial regime related to variations in monsoonal precipitation over the last 130 kyr (see also Tandon et al.1997)(fig.5). This OSL-dated fluvial record shows similar changes in palaeomonsoon intensity as a marine productivity record from the Arabian Sea (Leuschner & Sirocko 2003) (Fig. 5). Luminescence-dated chronologies from the Luni River and other fluvial systems in the Thar Desert also suggest a regional fluvial response to climate change (Jain & Tandon 2003; Jain et al. 2005b). Equivalent dose distributions from fluvial samples displayed asymmetrical distributions skewed toward higher values. In order to mitigate partial bleaching problems and remove the contribution of aliquots containing non-bleached grains from the final age calculation, Juyal et al. (2006) used a subset of the youngest D e values (defined as the minimum D e value to the minimum D e value1(2error)) to calculate their OSL ages. This method is similar to other methods proposed to isolate the youngest population of D e values, assumed to be representative of the true burial age. Slackwater flood sequences from the Thar Desert have also been analysed to reconstruct monsoonrelated large flood frequency over the last millennium. Kale et al. (2000) described several slackwater flood deposits along the Luni River and used OSL dating to provide age control. OSL was chosen over radiocarbon dating due to the ability to systematically sample desired flood units. Because of the possibility of partial bleaching in sediment transported during floods with high suspended sediment loads, multiple OSL dating techniques and different mineral fractions were analysed. The primary age control for the sections was obtained by OSL dating quartz using a multiple aliquot (MA) additive-dose technique. Duplicate analyses of OSL ages (kyr) Stratigraphy 11±1 28±3 30±3 49±4 60±6 69±6 98±8 Southern Thar Desert Fluvial Record (Juyal et al. 2006) cl si s g grain size Inferred environmental processes incision, river adjustment aeolian deposition flood plain aggradation, pedogenesis braided channel, soil development flood plain aggradation, bedded calcrete formation braided channel flood plain fine deposition Inferred monsoonal activity weak monsoon enhanced monsoon arid/episodic monsoon activity enhanced monsoon with seasonality weak monsoon enhanced monsoon increased monsoon acitivity Age (kyr) Arabian Sea (Leuschner & Sirocko 2003) Ba/Al Fig. 5. Synthesis diagram from the research of Juyal et al. (2006) on fluvial deposits in the southern Thar Desert, India. OSL ages were collected from fluvial channel and floodplain deposits, depositional environments described and monsoonal activity interpreted. This fluvial record of palaeomonsoon activity correlates nicely with marine Ba/Al productivity records of monsoon strength from the Arabian Sea (rightmost panel, Leuschner & Sirocko 2003).

9 BOREAS Luminescence dating of fluvial deposits 621 each sample followed SAR (quartz) and MA differential partial bleach (feldspar) (Singhvi & Lang 1998) techniques. SAR results indicate that the samples were fairly well bleached at deposition. Results from the study indicate variability in flood frequency over the last 1000 years that match records of changes in regional monsoon precipitation (Bryson & Swain 1981) and other palaeoflood reconstructions from central India (Ely et al. 1996; Kale 1999). Summary. Fluvial deposits contain valuable records of river response to changes in sea level, climate and glaciation. Systematic collection of OSL samples from silty and sandy material within fluvial sequences can provide valuable age control on deposits that are otherwise difficult to date due to limited organic material for radiocarbon dating. Additionally, the age range for OSL dating is sufficiently long, up to 300 kyr in some cases (e.g. Murray & Olley 2002), to allow for the reconstruction of fluvial response to climate and sealevel change over the last glacial cycle. Moreover, recent statistical and technical advances in OSL dating have reduced or mitigated partial bleaching problems found in some fluvial settings (e.g. Duller et al. 1999; Galbraith et al. 1999; Murray & Wintle 2000; Galbraith 2005). These advances and developments allow fluvial deposits to be accurately dated with OSL and records of fluvial response to climate and sea-level change to be constructed. Palaeoseismic reconstruction Fluvial deposits, terraces and alluvial fans can be important archives of tectonic activity (e.g. Schumm 1986; Holbrook & Schumm 1999; Pearce et al. 2004). A number of studies have used OSL and other dating methods on offset fluvial deposits or landforms to determine the timing of fault displacement, slip rates and seismic recurrence intervals (e.g. Chen et al. 2003; Cheong et al. 2003; Caputo et al. 2004; Zuchiewicz et al. 2004; Mahan et al. 2006; Mason et al. 2006; Mukul et al. 2007). OSL dating has also been directly applied to fault-scarp colluvium to more directly date seismic events (e.g. Lu et al. 2002; Fattahi & Walker 2007; Porat et al. 2008). Some examples of the application of OSL dating of fluvial deposits for palaeoseismic reconstructions are given below. Offset fluvial deposits and landforms. One of the most common uses of OSL dating for palaeoseismic studies is dating offset or deformed fluvial landforms or deposits (e.g. Thompson et al. 2002; Cheong et al. 2003; Cox et al. 2006; Amos et al. 2007). Fluvial landforms can commonly be correlated over long distances and have fairly consistent slopes, allowing crossing faults and structures to be identified and the displacement of features to be calculated. Age control on displaced landforms and deposits is necessary for the calculation of slip rates and recurrence intervals on fault movement and earthquake activity. Age control on past seismic events is especially important for the development of accurate hazard assessments of tectonically active regions near population centres. Fattahi et al. (2006, 2007) have studied displaced and deformed alluvial fans and terraces from the highly seismically active Alborz-Kopeh Dagh ranges in northeastern Iran. For example, Fattahi et al. (2006) examined folded and faulted alluvial fan deposits associated with the Sabzevar thrust fault. Quartz OSL ages from deformed alluvial sediments indicate fan deposition from 32 to 13 kyr, faulting and graben formation by 9 kyr and additional faulting on the Sabzevar thrust fault in the last 3 kyr (Fig. 6A). Slip rates as high as 1 mm/yr and a 3000-year recurrence interval for large earthquakes were calculated for this active fault segment. Reassessment of a young colluvial sample by Fattahi & Walker (2007) has produced a SAR IRSL age of kyr on pre-faulting colluvium (Fig. 6A), suggesting that the most recent faulting and displacement on the Sabzevar thrust likely represents the earthquake that destroyed the town of Sabzevar in AD In southern California, DeLong et al. (2007) studied a fluvial terrace that was cross-cut and offset by the eastern Big Pine oblique-reverse fault, one of a number of faults associated with the broad San Andreas transform fault system. They used quartz OSL ages to confirm geomorphic evidence that the deposits on either side of the fault scarp were originally part of the same terrace surface (Fig 6B). Although there is some scatter in the data, OSL ages from the hanging wall ( to kyr) and the foot wall ( to kyr) are consistent. From these OSL ages and the amount of vertical displacement on the fault (10 m), a dip-slip rate estimate of 0.9 m/kyr was calculated. Geomorphic response to tectonic uplift, folding and faulting. In addition to dating offset fluvial landforms, OSL dating has been used by a number of researchers to assess tectonic activity by studying the fluvial geomorphic response of rivers to deformation (e.g. Holbrook et al. 2006; Mason et al. 2006; Mathew et al. 2006; Mukul et al. 2007; Srivastava & Misra 2008). Localized river incision and/or channel pattern changes near known structures are commonly used criteria to identify fault activity or fold deformation (e.g. Schumm, 1986; Holbrook & Schumm, 1999). Mathew et al. (2006) used OSL to date incised channel deposits from a number of rivers along an active fault-related fold associated with the Kachchh Mainland Fault in northwestern India. Incision ages on fluvial deposits progressively decreased in age eastward.

10 622 Tammy M. Rittenour BOREAS A 9±1 kyr hinge graben infilled with aeolian sand not to scale 13±0.7 kyr 24±5 kyr ~9.5 m 32±3 kyr B Elevation (m) horizontal strata alluvial fan surface tilted beds low angle thrusts colluvial wedge 24.3± ±1.9 kyr (n=3) 22.0± ±3.0 kyr (n=4) deformed tread estimated dip-slip rate: 0.9 m/kyr Big fine fault 10 m Distance (m) scarp 3±0.3 kyr (quartz) 1.71±0.29 kyr (feldspar) Fig. 6. Examples of the applications of OSL dating to palaeoseismic research and reconstruction of fault slip rates. A. Schematic drawing of faulting and folding of an alluvial fan along the Sabzevar thrust fault in northeastern Iran (Fattahi et al. 2006; Fattahi & Walker 2007). OSL ages indicate fan syndepositional folding and faulting from 32 to 13 kyr, graben formation by 9 kyr and a SAR IRSL age from faulted colluvium suggests that the most recent faulting on the Sabzevar thrust occurred within the last 1.7 kyr. Modified from Fattahi et al. (2006). B. OSL age ranges collected from a fluvial terrace that has been offset by the eastern Big Pine fault in southern California, USA. OSL ages were used to confirm that the now displaced fluvial landforms were once the same terrace surface and to calculate dipslip rates on the fault. Modified from De- Long et al. (2007). These data, along with geomorphic evidence, were used to suggest eastward lateral fold migration during the Holocene due to fault length extension and vertical uplift rates of 101 mm/yr. Mukul et al. (2007) and Srivastava & Misra (2008) have dated fluvial deposits related to incision from active uplift and faulting along the Himalayan front in northeastern India. Mukul et al. (2007) used an innovative approach of applying both TL ages from fault-gouge material (see discussion below) and quartz OSL ages from unpaired strath terraces to provided evidence for the timing and response of the Tista River to uplift and seismic activity surrounding the South Kalijhora Thrust and several out-of-sequence, surfacebreaking faults. Srivastava & Misra (2008) combined geomorphic analysis of strath terraces and quartz OSL dating to examine the uplift history of the Himalayan front along the Kameng River. Their results indicate that the greatest river incision occurred at about 7 kyr (11.9 mm/yr), with average incision rates during the 14 kyr of record of 7.5 mm/yr. Geomorphic analysis of the ratio between strath height and alluvial cover thickness (Starkel 2003) suggest that the strath terraces were formed predominantly in response to tectonic uplift with lesser climate influence. Holbrook et al. (2006) used quartz OSL and radiocarbon dating to determine the timing of channel straightening events in Mississippi River meander belts in the New Madrid Seismic Zone, USA. They interpreted the switch from a highly sinuous channel to a straightened channel course as a response to periodic clustering of seismic events and decreased gradient due to fault displacement. Two and possibly three straightening events were identified through mapping crosscutting relationships and extensive coring (Holbrook et al. 2006). Age control was provided by radiocarbon ages on carefully selected macrofossils from channel fill deposits (n = 12) and OSL samples from point bar deposits (n = 14) (Fig. 7). OSL and radiocarbon ages were consistent, and where paired samples were collected, point bar samples were older than their associated abandoned channel-fill deposits, as expected based on geomorphic relationships. Fault-scarp colluvium. Direct dating of discrete fault rupture events can be achieved by OSL dating faultscarp colluvium. Trenching of faults is a routine technique for many palaeoseismic studies (e.g. McCalpin 1998). Traditionally, colluvial wedges and fault planes are identified from these trenches, and organic material from buried soils or other organics within the faulted sediments are radiocarbon dated to obtain a fault-rupture chronology. However, organic material is commonly scarce in many arid settings and therefore radiocarbon sample collection is mostly opportunistic in nature. Luminescence dating, on the other hand, allows more selective sampling and has been used by a number of researchers (e.g. Forman et al. 1989, 1991; Porat et al. 1997; Zilberman et al. 2000; Amit et al. 2002; Lu et al. 2002). Early analyses using TL dating have been largely replaced by OSL and single-grain techniques to allow measurement of the most light-sensitive traps and most well-bleached grains. Porat et al. (2008) examined colluvial wedge deposits from the base of a normal fault that has displaced a middle Holocene alluvial fan in the Dead Sea Transform, southern Israel. They compared large aliquot

11 BOREAS Luminescence dating of fluvial deposits 623 Late Holocene Cycle Meandering Phase Straight Phase Middle Holocene Cycle Meandering Phase Straight Phase 1250±70 OSL RC RC 2490±180 OSL Early Holocene Cycle? Meandering Phase Straight Phase Channel Fill Stratigraphic data from core OSL and calibrated radiocarbon (RC) age control points (yr) 1030±70 OSL Reelfoot Fault Scarp 7250±360 OSL RC 7484±367 OSL 4590±230 OSL RC 2300±220 OSL RC RC 4244±269 OSL 3304±306 OSL 3620±220 OSL 0 5 km 3680±270 OSL 4720±300 OSL Mississippi River Fig. 7. OSL and radiocarbon ages collected from point bar and channel fill deposits of the Mississippi River in the New Madrid Seismic Zone, USA. Meanderbelt abandonment and straightening of channel courses, as dated by OSL and radiocarbon, have been interpreted to have occurred in response to clustered seismic activity on the Reelfoot fault. Note that OSL ages from point bar deposits are older than their associated channel fill deposits, as expected from geomorphic relationships. Modified from Holbrook et al. (2006). (1000 s grains), small-aliquot (100 s grains) and singlegrain results from samples collected from proximal (less than 1 m from the fault) and distal (2 m from the fault) parts of a stack of colluvial wedges (Fig. 8). Results indicate partial bleaching in all samples, with greater partial bleaching in proximal colluvial wedge samples (Table 1). Age reversals in SAR multi-grain analyses were attributed to progressive exposure and erosion of older sediments with continual fault displacement. Large aliquot ages substantially overestimated the time of faulting, providing additional support for the use of small-aliquot or single-grain dating for environments where partial bleaching may be a problem. Single-grain results analysed using the minimum age model of Galbraith et al. (1999) produced the most reasonable ages (Table 1). OSL dating of fault gouge and liquefaction features. Innovative research is being developed to directly date faulting by luminescence dating fault gouge and liquefaction features. During seismic activity and fault slip it may be possible that sediment within the fault gouge receives enough energy from friction, vibration, crushing and heat to zero the luminescence signal. Initial studies tested the application of thermoluminescence (Fukuchi 1992) and electron-spin resonance (ESR) (Singhvi et al. 1994) to date fault-gouge materials and found results consistent with radiocarbon ages. More recently, Mukul et al. (2007) applied TL dating of faultgouge deposits in northeastern India and found TL ages much younger than the source rock (42 45 kyr compared to Z2 Myr), suggesting the samples were zeroed during fault movement. Owing to uncertainties with partial resetting of the TL signal, the ages were considered maximum ages for faulting. Research involving OSL analysis of fault-gouge deposits may provide more sensitive indicators of fault movement and luminescence resetting (Rink et al. 1999). Liquefaction features such as injection dikes and sand blows (deposition from the surface rupture of an injection dike) are commonly produced during earthquakes in sandy fluvial deposits. New research is investigating the potential applications of OSL to these liquefaction features in an attempt to directly date seismic events.

12 624 Tammy M. Rittenour BOREAS XI distal colluvial wedge proximal colluvial wedge XI VII X IX VIII medial colluvial wedge 7 IV VI II III 3 V m m 1 1 Fig. 8. Locations of OSL samples collected from fault-scarp colluvial wedges adjacent to a fault that cuts across an alluvial fan in the Dead Sea Transform, Israel (Porat et al. 2008). OSL samples were collected from proximal, medial and distal colluvial wedges to determine relationships between bleaching and distance from the fault scarp. Samples were analysed for quartz and feldspar using large aliquots, small aliquots and single-grain analysis. Results are presented in Table 1. Arabic numerals refer to colluvial wedges and Roman numerals to deposits in the faulted alluvial fan (numbered from oldest to youngest). Modified from Porat et al. (2008). Table 1. OSL ages from colluvial wedges in Trench-18, Dead Sea Transform, from Porat et al. (2008). Sample no. and location Feldspar IRSL ages (kyr) Quartz OSL ages (kyr) Large aliquot SAAD Small aliquot SAR Large aliquot SAR Single-grain SAR Mean Youngest Mean Youngest MAM Mean Youngest Mean MAM Distant colluvial wedge 5 Upper wedge (X) (34) Lower wedge (VII) (72) Medial colluvial wedge 7 Lowest colluvium (30) Proximal colluvial wedge 11 Upper wedge (IX) (35) Middle wedge (VIII) (26) Lowest wedge (VII) (23) Mean is the age calculated from the average of all aliquots measured. Youngest is the age calculated from the lowest measured D e. MAM = minimum age model of Galbraith et al. (1999). Numbers of grains included in single-grain age estimates are given in parentheses. See Fig. 8 for sample locations. Porat et al. (2007) investigated the use of OSL to determine the difference between clastic dikes (filled from above) and injection dikes (formed by liquefaction during earthquakes) in the southern Dead Sea basin of Israel. It was proposed that injection dikes would have the same OSL age as the late Pleistocene source material, OSL dated to kyr. However, quartz OSL ages from both the depositional and injection dikes were dated to kyr. These results imply that the OSL signals in the injection dikes may have been reset during liquefaction and that OSL may be used to date earthquake-induced liquefaction features directly. However, a similar study by Thomas et al. (2007c) in NE India has produced indistinguishable quartz OSL

13 BOREAS Luminescence dating of fluvial deposits 625 ages between injection dikes and their source sediment, suggesting that further research into the applicability of OSL for dating liquefaction is needed. Additional age constraint on the timing of liquefaction and seismic activity may be obtained by OSL dating sand blows formed by the surface rupture of liquefaction features and injection dikes. Thomas et al. (2007c) used thin sampling tubes to collect the upper 1.5 cm of sediment from a sand blow deposit, hoping to collect the sediment exposed to sunlight during the deposition of the sand blow, but prior to subsequent floodplain deposition. Results were scattered suggesting that non-bleached sediments were collected and the possible influence of bioturbation. Future sampling strategies have been proposed including careful mmscale excavation of the upper sediment from sand blows (Thomas et al. 2007c). Cox et al. (2007) applied a different approach to dating sand blows from the lower Mississippi valley, USA. Instead of sampling the sand blow directly, they collected fine-grained samples from buried soils and sediment layers underlying the target sand blows. For the most part, their multiple aliquot IRSL results fit with the radiocarbon ages obtained from the underlying deposits and within the sand blows, with some evidence of partial bleaching resulting from the presence of reworked older sedimentary grains. Summary. OSL dating has great potential for palaeoseismic reconstruction owing to its versatility to directly sample targeted features and deposits. OSL samples from offset or deformed fluvial terraces and alluvial fans provide critical age control for slip-rate calculations. Dating proximal settings such as faultscarp colluvium and sand blows is more challenging due to partial bleaching, but may provide more detailed information of recurrence intervals. New advances in OSL dating techniques (Murray & Wintle 2000), single-grain dating (Duller et al. 1999; Btter-Jensen et al. 2000; Duller 2000) and statistical analyses of D e results (e.g. Galbraith et al. 1999; Galbraith 2005) have improved the accuracy of OSL dating and have allowed new applications of OSL dating to palaeoseismic studies. Archaeological applications Rivers, lakes and other water sources have always been important human resources and the focal points of past cultures and occupation sites. As such, archaeological sites are commonly associated with, and interleaved within, fluvial deposits. Traditionally, archaeological sites and occupation horizons have been dated with radiocarbon, commonly from charcoal or wood. While radiocarbon ages have been instrumental in placing archaeological features and artefacts in a chronological framework, there are several caveats that limit their use. For example, radiocarbon ages may not provide the desired age of occupation due to reuse of materials or reworking of detrital charcoal and wood, which is common in fluvial settings. Radiocarbon dating is further limited by uncertainties in calibration to calendar years and is constrained to the last 40 kyr. For these reasons, luminescence dating may be preferable in some cases and has been increasingly applied to archaeological settings. There has been a long-standing connection between luminescence dating and archaeological research. Thermoluminescence dating has been the keystone technique for dating fired pottery (e.g. Zimmerman 1971; Aitken, 1985), with more recent applications of OSL dating of pottery (see reviews by Roberts 1997; Feathers 2003; Wintle 2008b). Additionally, OSL dating has been applied to sediments associated with archaeological sites for geoarchaeological and environmental studies (e.g. Folz et al. 2001; Sommerville et al. 2001; Feathers 2003; Gibling et al. 2008). Luminescence techniques have been particularly useful for dating sites older than 40 kyr (e.g. Bowler et al. 2003; Grine et al. 2007; Mercier et al. 2007), and sites associated with initial colonization of Australia (e.g. Cupper & Duncan 2006; Olley et al. 2006; David et al. 2007; Prescott et al. 2007) and the Americas (e.g. Feathers et al. 2006; Gonz alez et al. 2006). The focus of this article is on the application of OSL dating to fluvial deposits, but it should be noted that there are many applications of OSL dating that are unique to archaeological studies. These include dating fired sediments and rocks associated with hearths (Tribolo et al. 2003; Lamothe 2004; Lian & Brooks 2004; Fanning et al. 2008), single-grain dating mud wasp nests associated with rock art (Roberts et al. 1997; Yoshida et al. 2003), single-grain dating grave infills to determine age of human burials (Olley et al. 2006), dating sediment cemented within late Pleistocene human remains (Grine et al. 2007), and dating brick and mortar to determine the timing of building construction (Bailiff & Holland 2000; Jain et al. 2004b; Vieillevigne et al. 2006). Dating Palaeolithic sites. Palaeolithic archaeological sites provide important information about the evolution and dispersal routes of Neanderthal and modern humans. While late Upper Palaeolithic sites (40 10 kyr) may be dated with radiocarbon, Middle Palaeolithic and older Upper Palaeolithic sites are near or beyond the applicable limit for radiocarbon dating and are associated with large calibration uncertainties and multiple calibration curves available for this interval (e.g. Kitagawa & van der Plicht 1998; Hughen et al. 2004; Fairbanks et al. 2005). For these reasons, OSL dating has been increasingly used to provide age control at a number of Palaeolithic sites. Many important Palaeolithic sites in rock shelters and non-fluvial settings have been dated with OSL (e.g. Bowler et al. 2003; Cupper & Duncan 2006; Olley et al. 2006; Rhodes et al.

14 626 Tammy M. Rittenour BOREAS 2006; David et al. 2007; Mercier et al. 2007; Prescott et al. 2007). Applications of OSL dating to Palaeolithic sites found along river terraces in Russia and India are discussed here. The Kostenki Palaeolithic sites, found on the low terraces of the Don River in westernmost Russia, have been the focus of archaeological research since the initial discovery of stone artifacts and mammoth bones in the late 1800s (see Hoffecker et al. 2002, 2004 and references therein). Initial age control was obtained from stratigraphic relationships and radiocarbon ages from bone. More recently, Holliday et al. (2007; see also Sinitsyn & Hoffecker 2006; Anikovich et al. 2007) have developed a more detailed chronostratigraphy of the locality from fine-grained MA IRSL ages and radiocarbon dating of charcoal (Fig. 9). Additional age control was obtained from identification of the Campanian Ignimbrite Y5 tephra (40 kyr) and palaeomagnetic correlation to the Laschamp Excursion (39 45 kyr). Luminescence ages are consistent with the other age control and provide the most reliable ages for the oldest cultural horizons and lowest part of the section. In a similar study, Gibling et al. (2008) used quartz OSL and radiocarbon dating to provide age control for archaeological sites from river terraces along the Belan River in India. Terrace deposits containing Middle Palaeolithic artifacts were dated to between 8511 kyr and 728 kyr and stratigraphically younger aeolian and fluvial channel fills were OSL dated to 13 8 kyr. These younger ages are consistent with radiocarbon results and correlate with strata containing Neolithic artifacts. Irrigation canals. In addition to providing age constraints beyond the limit of radiocarbon, OSL dating may, in some cases, provide improved age control in young sediments, where there are greater radiocarbon calibration uncertainties and reworking of older organic material is common. Examples of the application of OSL dating to young canal sediments are given below. Canal networks on the Mekong Delta in southern Cambodia have been investigated to determine the utility of OSL dating to date channel use and infilling (Sanderson et al. 2003, 2007; Bishop et al. 2004).Although underwater bleaching experiments and subaqueous spectral measurements suggest reduced bleaching efficiency in canal settings with high suspended sediment loads (Sanderson et al. 2007), many quartz OSL ages are consistent with radiocarbon ages (Sanderson et al.2003; Bishop et al. 2004). Age overestimates from some samples are interpreted to be due to incorporation of nonbleached grains in multi-grain aliquots. Smaller aliquot sizes or single-grain dating may reduce these problems. In the American southwest, Berger et al. (2004) examined irrigation canals near Phoenix, Arizona. Samples were collected and analysed for fine-grained MA IRSL, SAR IRSL and post-ir SAR (analysis of quartz signal). Results indicate that some samples were incompletely zeroed at deposition. However, as with the samples from canals on the Mekong Delta, these outliers were easily identified within the stratigraphy and the age range of canal use could be determined. Summary. OSL dating can provide important age control for fluvial sediments associated with archaeological sites. While charcoal and other material for radiocarbon dating may be common within cultural features, OSL samples can be collected from horizons that lack suitable material for radiocarbon dating and can provide invaluable environmental information about the timing and rate of deposition between cultural horizons. Additionally, OSL ages do not need to be calibrated, reducing age uncertainties. The larger age range of OSL dating makes this technique particularly useful for Palaeolithic sites that are older than or near the limit for radiocarbon dating. In addition, OSL dating is important for geoarchaeological investigations that relate people to their environment by dating Fig. 9. IRSL, radiocarbon and other age control obtained for the Kostenki Upper Palaeolithic site 12, east wall, along the Don River in western Russia. Luminescence dating is especially useful in dating Palaeolithic sites such as this due to their antiquity (near the upper limits for radiocarbon) and the abundant and wellsuited target material for luminescence dating in the fluvial sediments associated with the cultural horizons. Modified from Anikovich et al. (2007) and Holliday et al. (2007).