Luminescence dating in prehistoric archaeology

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Luminescence dating in prehistoric archaeology: An overview

Daniel Richter

History of luminescence dating in archaeology The application of luminescence dating methods in archaeology started off with thermoluminescence (TL) dating of heated clay material (pottery, ceramics) in the 1950/60s1, and quickly became an established method for dating ceramics, mainly from prehistoric context. It provided a key for the verification and establishment of typology based chronologies (e.g. ancient civilizations, Central America). But with the increased precision of archaeological dating (including seriation etc.) and the possibility to provide calendar ages by calibrating radiocarbon data, the application of TL dating of ceramics and pottery became less important from the 1980s onward. Instead, the TL dating of heated flint had a large impact in Palaeolithic archaeology, because it provides direct age estimates of human occupations for periods beyond the limitations of the radiocarbon method. In the mid 1990s, and especially with the beginning of this millennium, TL methods were replaced by optical stimulated luminescence (OSL) methods for establishing depositional ages of sediments. Since then, the application of luminescence dating for providing reliable age estimates in archaeology has increased again dramatically. Luminescence dating and archaeology Especially in archaeology the definition and the correct understanding of the time scale used for age estimates for sites and the establishment of chronologies is important. Luminescence dating methods provide age estimates on a linear, calendar time scale, which is in contrast to some other methods (e.g. uncalibrated radiocarbon data). This is especially evident e.g. in the interpretation of the transitional period from Middle to Upper Palaeolithic in Europe at about 40 ka, which is beyond the accepted calibration range. The nonlinearity of the radiocarbon time scale is the probable cause why no clear chronological pattern emerges for the changes observed in the archaeological record. The most relevant questions to be answered in archaeology are related to human processes, actions, dispersals, land use etc., and can only be understood and interpreted on a linear time scale. Therefore the application of luminescence methods in archaeology is more common for earlier periods and especially useful beyond the time range provided by radiocarbon calibration. Verification of any scientific method is important, especially when it is used by fields other than the specialist ones. For dating methods this is rather difficult to achieve. An accurate and high precision comparison to radiocarbon dating is limited to the dendro-calibrated range of 14C to about 12 ka2, but possible for the entire 14C range at a lower resolution, which is quite similar to the precision obtained by luminescence dating3. However, a number of studies using independent chronometric data have shown the reliability of luminescence dating4. On the other hand, archaeological data can sometimes be used to verify luminescence data, e.g. in the selection of the appropriate model for palaeodose determination in single grain OSL dating.

3 4 1 2

E.g. Grögler et al. 1958 Stuiver et al. 2004 Fairbanks et al. 2005; Chiu et al. 2006; Hughen et al. 2006; Weninger/Jöris 2008. E.g. Dykeman et al. 2002; Lang et al. 2003; Prescott et al. 2007.

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The chronological framework provided by the archaeological data can sometimes be used for identifying potentially problematic luminescence ages, which is more difficult to achieve for geological sequences lacking any archaeological records. However, the association of the dated material and the human occupation is always a key question when applying chronometric dating methods in establishing the age of archaeological sites, or in other words the relationship between the event dated and the question asked has to be clearly shown. Of major concern in archaeology are the error estimates associated with a dating result. Many archaeological questions require such fine levels of time resolution that the dating methods available can not provide a satisfying answer. It is therefore important to understand the limitations of a dating method, and re-define the archaeological question if necessary. Interpretations and models of chronologies or time lines have to take into account the limitations of the dating methods used. At the same time the same level of the archaeological limitations have to be considered as well. Principles of luminescence dating Luminescence dating is based on the accumulation of a radiation dose (palaeodose: P) in the crystal lattice of • minerals, e.g quartz or feldspar from omnipresent ionizing radiation (dose rate: D ). This radiation originates • • • from the sample itself ( Dinternal ), the surrounding sediment ( D external ), and cosmic radiation ( Dcosmic ). The radiation dose (P) gets erased when sediment grains or rock surfaces are exposed to light, or lithic material is heated in a fire. It accumulates again as soon as the minerals are buried by sediment and are thus protected from light. The age formula, therefore, is straight forward and simple: age (a) =

P(Gy) •

D(Gy⋅a −1 )

=

P •

•

•

Dinternal + Dexternal + Dexternal

where the paleodose (P, also termed equivalent dose (DE) or dose equivalent (ED) is expressed in Gy and • the various dose rates ( D ) in µGy/a. Upon measurement the radiation energy which is stored in the crystal lattice is partially released as photons, which are called luminescence. The luminescence therefore serves as a measure of the radiation dose (P). This palaeodose (P) can be measured in the laboratory by a variety of methods5, while the dosimetry (various • dose rates, D ) are determined directly at the sites and/or by geochemical/physical methods. Especially the latter are rather variable and the number of parameters to be determined vary between materials dated and techniques used. However, the success of a dating project depends on the availability of all the necessary parameters, e.g. the lack of the context of a sample because a site has been destroyed does not allow the • determination of the external dose rate ( D external ) and thus usually does not allow the calculation of an age. Application of luminescence dating in archaeology, or, the typology of events For any scientific method used, accuracy is more important than precision. Chronometric ages are always supplied at a certain level of confidence and with a certain confidence interval, and should be treated as such6. For establishing models or scenarios of ancient developments, procedures, etc. it is more suited to provide age estimates which include the true time range at a low precision, then providing results at a high precision which does not include the true age. This paper is organized by material and question, rather than by time period, and in some examples more than one method was applied for a given site, which is a good way to check the consistency, especially of related dating methods. See overview in Bøtter-Jensen et al. 2003; or Aitken, 1985; 1998. E.g. Wagner 1998.

5 6

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The power and accuracy of luminescence dating methods has been shown in a number of cases7, while the limitations are not always evident. This paper provides examples for the most commonly used methods and materials for luminescence dating in archaeology, but is not exhaustive by any means8. Some parameters associated with the dosimetry in luminescence dating are dependant on geochemical processes in the sediments of an archaeological site. It is therefore essential to understand the influence of these parameters on the age results and establish strategies for their accurate determination. For example, sediments from sites should not be removed completely in order to allow the measurement of the external dose rates. This should be taken into consideration by the archaeologist and may result in a different strategy of excavating a site. As with any other dating method it is important to define the relationships of a sample to be dated and its context in an archaeological site. Furthermore, the formation processes of any sites have to be understood when using luminescence methods. If e.g. the nature of the deposition of a sediment is not well known, it is difficult to assign an event to the formation of the sediment (and subsequently to the bleaching event(s)) which might be of interest to the archaeologist, or the method employed might not be appropriate, e.g. the TL dating of sediment associated with the Jimnium petroglyphs at a sandstone rock shelter9. Sandstone degrades and chunks get incorporated in the sediment accumulating at its base. The incorporation of such unbleached fragments in the samples collected for luminescence dating is inevitable and the TL ages10 were therefore significantly overestimated, as it was subsequently shown by single grain OSL dating11. Tab. 1 lists the commonly used materials/samples in luminescence dating, together with the appropriate method and the event which is dated. It is especially this relationship which is sometimes not evident and needs special consideration when applying luminescence dating. In some cases it is important to take the life time of an artefact or feature into consideration, e.g. that a kiln could have been used for an extended period of time, but in general only its last use can be dated, and not the time of construction. Or in the case of dating rock surfaces, the time since the last exposure to light is dated, which can be the time of construction or destruction of a monument. Dating samples from the filling of a pit or trench provides age estimates for different events. Ceramics could be much younger than the construction or much older e.g. when they get eroded into the pit or trench from older deposits. The depositional age of sediment infill provides age information rather on the abandonment than the construction e.g. of a trench. Because most of the application of luminescence dating is concerned with several tens of thousands of years, rather than years, the time difference between e.g. a discard event and the abandonment of a site, or the time needed for the accumulation of a sedimentological deposit after a human occupation is negligible in most cases. Additionally, the time resolution (precision) of luminescence dating methods is often not sufficient for such differentiations. Therefore luminescence dating techniques for archaeological purposes are more frequently used for periods like the Palaeolithic, where a lesser precision can be tolerated. This is especially true for TL techniques, whereas OSL-SAR techniques can provide very precise sedimentation ages. However, for many archaeological sites the sediments containing the finds are often not pristine any more. This makes them more difficult to date with luminescence techniques and leads to a loss in precision. OSL methods were developed for geology and geography, where the interest lies in formation and depositional processes of sediment and when this occurred. In archaeology the complicating factor of human modification of sediments is added.

E.g. Dykeman et al. 2002; Huntley/Lian 1998. Rather comprehensive reviews of the application of luminescence dating in archaeology can be found in Roberts (1997), as well as in Feathers (2003). 9 Fullagar et al. 1996. 10 Fullagar et al. 1996. 11 Roberts et al. 1998. 7 8

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Daniel Richter

material

material details

method

dated event

common interpretation as time of:

archaeological example

brick

quartz, feldspar

TL or OSL

last firing

manufacture

Bailiff & Holland 2000

brick surface

quartz, feldspar

OSL

last exposure

construction; repair; destruction

Bailiff & Holland 2000

ceramic, pottery, tile

quartz, feldspar

TL or OSL

last firing

manufacture; authenticity

Barnett (2000); Lamothe (2004).

daub (burnt)

quartz, feldspar

TL or OSL

last heating

destruction

Quickert et al. 2003.

figurine

quartz, feldspar

TL or OSL

last heating

manufacture; authenticity

Zink & Porto 2005.

sandstone, limestone, granite (quartz, feldspar)

TL or OSL

last heating

last use

Ichikawa & Nagatomo 1978; Lamothe (2004).

quartz, feldspar

TL

last firing

last use

Hong et al. 2001.

TL

last heating

discard

Richter et al. 2007.

hearth stone kiln

lithic artefact (heated) flint, chert, quartzite, quartz, silcrete limestone (heated)

Calcite, quartz

TL

last heating

last use

Roque et al. 2001.

mortar

quartz, feldspar

TL or OSL

last light exposure

building

Goedicke 2003.

oven

quartz, feldspar

TL

last firing

last use

Roque et al. 2002.

pit/trench infill

quartz, feldspar

OSL

last light exposure

abandonment; infilling

Lang & Wagner 1996.

granite, marble, limestone (quartz, feldspar, calcite)

OSL or TL

last light exposure

construction; destruction

Greilich et al. 2005, 2006; Liritzis & Vafiadou 2005.

sediment (burnt)

quartz, feldspar

TL or OSL

last heating

last use

Godfrey-Smith & Shalev 2002.

rock surface

sediment, aeolian

quartz, feldspar

OSL

last light exposure

deposition

Jacobs et al. 2003a; 2003b.

sediment, colluvial

quartz, feldspar

OSL

last light exposure

deposition

Lang & Hönscheidt 1999.

sediment, fluvial

quartz, feldspar

OSL

last light exposure

deposition

Folz et al. 2001.

quartz

TL

last firing

last use

Haustein et al. 2003.

quartz, feldspar

OSL

last light exposure

building

Yoshida et al. 2003.

slag wasp nest

Tab.1: List of most important materials/samples from archaeological context used in luminescence dating. The appropriate method to be used for dating, and the event which is dated are given.

The archaeologist and the dating specialist Usually, samples for OSL dating are taken by a trained person, either with a tube driven into a profile or a sediment block being carved out. The light exposed portions get discarded in the laboratory. It might be required to take additional sediment samples for moisture and laboratory γ-spectrometry according to the procedures described by the individual specialist. A large range of sediment types can be dated, but at varying levels of accuracy. A detailed description of the sediment profiles is therefore helpful to establish a strategy before visiting the site. It is advisable to have more than one sedimentological layer dated by OSL in order to check the consistency of the method(s) used. However, when sediment layers are rather inhomogeneous, variable and thin, the situation is more complex. Sediments from many anthropogenic sediments (or anthropogenically altered sediments) are rather difficult to date by luminescence methods. It is therefore often advisable to select sediment layers above and below the archaeological layer in question in order to obtain termini post and ante quem age estimates. The radiation fields for heated cultural materials are often rather variable within a site. It is therefore necessary to obtain in situ measurements by γ-spectrometry or dosimeters from several different locations within a given sedimentological layer or context. This requires the presence of extended profiles still containing the layers in question and having an overburden of at least 30 cm for the in situ measurements. More favourable is the situation where the radiation field can be determined on sediment volumes still to be excavated, and thus containing the samples for dating. However, the precise radiation field can often not be established for each individual sample due to the heterogeneity of the sediments. It is therefore necessary

Luminescence dating in prehistoric archaeology

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to date several samples from the same context, which allows the calculation of mean values. Samples for additional laboratory γ-spectrometry are often needed and have to be taken during excavation. The moisture should be preserved in these samples according to instructions given by the dating specialist12. Heated sediment (pottery, bricks, fire pits, kilns, figurines, hearths, ceramics) When sediment gets heated the signal used for luminescence dating is usually zeroed very well, thus enabling the application of TL as well as of OSL. Not only fire places, kilns etc. consist of sediment, but ceramics, pottery and bricks are essentially made of sediment as well. Furthermore, light exposure allows the determination of a variety of events (see e.g. section on exposure surface dating). Later prehistoric pottery from Britain: Verification of a chronology with luminescence dating Determining the time when pottery or ceramics was fired is one of the hallmarks of luminescence dating, though it became less commonly used with the advent of accurate radiocarbon calibration and other means of establishing chronologies, e.g. seriation etc. TL-dating served as major contributor to the establishment of a variety of chronological frameworks throughout the world13. While in general typological dating provides a superior resolution, luminescence dating of pottery is nowadays still useful in regions which lack typological frameworks, stylistic distinct difference in the pottery over long periods of time, or when problems are experienced with other dating methods used to establish chronologies. Luminescence dating of sherds can also serve as a verification of established typo-chronologies. Prehistoric chronologies rely heavily on pottery typochronologies which were established by typology, seriation and fabric type, usually backed up by calibrated direct or indirect radiocarbon dating. Using these methods age ranges for certain entities for a given region are defined. Luminescence dating methods were used in an impressive study by Barnett14 to verify such typologies for the British Late Bronze and Iron Age, using sherds which are diagnostic by their shape or decoration as well as undiagnostic ones. The study used only samples from recent excavations with well documented samples from well defined contexts. With this approach all the dosimetric parameters for luminescence dating could be determined. Standard TL, OSL-SAR and TL-SAR technique were applied, which gave concordant results when the technique specific quality criteria were satisfied15. The range of luminescence ages for diagnostic pottery sherds agree at the 1σ level of confidence with the expectations, based on archaeology, surface decoration, seriation and other chronometric methods (Fig. 1). Given that only few sherds from each individual context were dated, the spread of luminescence ages is larger than the ‘known’ age ranges. However, the degree of agreement is satisfactory, which gives confidence in the ‘archaeological’ age ranges from the perspective of luminescence dating16. The correlation of luminescence ages for non-diagnostic sherds is more interesting, because many typology based chronologies do not include this find category at all, which often raises the question of the age of sites lacking diagnostic sherds. As expected, the correlation of luminescence ages for undiagnostic pottery is less strong (Fig. 2). This indicates a much lesser accuracy of chronologies based on fabric only, because the conditions for luminescence dating were not different than for the diagnostic samples. Luminescence age ranges for non-diagnostic pottery should thus yield a similar precision than luminescence dating of diagnostic sherds. As a consequence the revision of some fabric based chronologies is suggested by Barnett, based on the dating obtained by luminescence techniques.

A comprehensive review on the requirements and the evaluation of e.g. TL data of heated rock material can be found in Richter 2007. 13 E.g. Herbert 2002. 14 Barnett 2000. 15 Barnett 2000. 16 Barnett 2000. 12

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AGE (YEARS)

4000

luminescence age

associated finds

archaeological age range based on diagnostic pottery

feature

3000

Broom Quarry, Beds

2000 Easingwold 1000

Wollaston Quarry and Hardwater Road

Welland Bank Pit

Sywell

Landwade Road, Piddington Cambs

Fig. 1: Comparison of TL age estimates for decorated pottery with their archaeological age range, which is based on style, and other chronometric dating results (modified from Barnett 2000).

6000

luminescence age archaeological age based on fabric

AGE (YEARS)

5000

4000

3000

2000

1000

Broom Quarry, Beds Wollaston Quarry Hardwater Road

Willington Hill Farm

Foxhall Landwade Road

Slade Farm Fornham St Genevieve

County Farm

Fig. 2: Comparison of TL age estimates for non-diagnostic pottery with the archaeological age range based on fabric analysis (modified from Barnett 2000).

Dating the building, repair and destruction of brick walls with TL and OSL Luminescence dating of bricks from walls can provide a wealth of information on the history of a wall. Bricks are exposed to high temperatures during manufacture and to light during construction work. In both cases the luminescence signal can be expected to be zeroed well. TL or OSL dating of the core of a brick provides ages for the production and, assuming no significant delay until building work, the construction of the wall. But reuse of old bricks is a common feature. By establishing the age of the last exposure to light of the surfaces of bricks, the building and/or the age of a repair event can be dated, as well as the destruction of a wall. While for the former the mortar provides the shielding from light, for the latter it has to be the sediment accumulated after the brick or the wall fell during destruction. By comparing the results of different luminescence techniques for dating different parts of bricks, it is possible to deduce the history of an entire wall from its construction, repair to destruction. A variety of medieval bricks of known age from Newcastle upon Tyne were studied with TL and OSL techniques by Bailiff and Holland17. They were employing SAR and SARA techniques for OSL and TL, which resulted in ages very well comparable to independently known ones, with the exception of sample 225 (Tab. 2). It was suspected that the age difference was due to the re-use of an old brick. A similar result

Bailiff/Holland 2000.

17

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Luminescence dating in prehistoric archaeology

1.2

OSL IRSL

Normalized luminescence (a.u.)

1.0

0.8

IRSL

OSL 0.6

0.4

0.2

0.0

-0.2 0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

Fig. 3: Normalised OSL and IRSL intensities as a function of depth after exposure to sunlight for two and one week respectively. Bleaching depths are indicated by vertical lines (data from Vieillevigne et al. 2006).

Depth (mm)

Sample

SAR TL (A.D.)

SARA TL (A.D.)

SAR OSL (A.D.)

SARA OSL (A.D.)

Architectural date (A.D.)

225

1415 ± 70

1365 ± 100

1430 ± 85

1500 ± 95

1620–1680

227

1760 ± 30

1690 ± 35

1640 ± 40

1660 ± 50

1620–1680

228

1750 ± 30

1700 ± 35

1710 ± 30

1655 ± 45

1680–1720

257

1785 ± 25

1730 ± 35

1670–1710

Tab. 2: SAR age determinations with TL and OSL procedures for bricks of known age from Newcastle upon Tyne (data from Bailiff/ Holland 2000).

was obtained by Vieillevigne et al. by TL for bricks from the Medieval citadel of Termez in Usbekistan18. Experiments showed a difference in the sunlight bleaching depth for OSL and IRSL (Fig. 3). They attempted to date a repair of a building by using OSL on a surface sample from the re-used brick. While their feasibility study showed the applicability of dating the surface exposure of bricks (Fig. 3), the dating result was not conclusive. Establishing the age of a fire-pit by TL- and OSL-dating of heated sediment Localized reddish or blackened areas of sediment in archaeological sites are frequently interpreted as remnants of fire-places or hearths, even when no associated material like fire cracked rocks or charcoal is found. The same is true for some pits, where the macroscopic observation of alteration sometimes is more difficult to determine. This was true for a shallow pit from the Early Bronze Age context of Ashkelon Marina in Israel19. The shape of the pits was defined by a 3 cm thick rim of hardened red sediment, which was sedimentologically indistinguishable from the infill as well as from the underlying sediment. The frequent archaeological remains from copper melting from the site enforced the interpretation of the pits as being part of archaeometallurgical processes taking place at the site. But no direct association of finds linked to metal processing with any of the pits was observed. So it remained to be demonstrated that the pits were used for such activities and that they were contemporaneous to the other archaeological remains. A number of studies, including FT-IR, x-ray diffraction etc. failed to demonstrate the heating of the sediment20. Radiocarbon dating results of 5,500–5,300 cal BP on organic material show that Ashkelon Marina is one of the earliest metallurgical sites in the Levant21. 20 21 18 19

Vieillevigne et al. 2006. Godfrey-Smith/Shalev 2002. Palatnik 1999. Godfrey-Smith/Shalev 2002.

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Fig. 4: Thermoluminescence glow curves (upper row) and DE-plateau test (lower row) for the heated sediment from the rim of the feature (left column), and thermoluminescence glow curves and plateau test for the sediment infill of the pit (right column) (modified from Godfrey-Smith/Shalev 2002).

Quartz samples from the reddened rim sediment and the infill were investigated with TL and OSL22. The TL glow curves from these two samples showed marked differences (Fig. 4). While the glow curves of the reddened sediment show one dominant peak, the TL of the sediment infill exhibits several peaks. Both glow curve shapes are rather typical for heated and unheated sediment respectively, and their plateaus show the potential of the samples for dating application (Fig. 4). In general it seems possible to date the heating event with TL and OSL of the reddened sediment and the timing of the deposition of the infill of the pit, while only for the latter the completeness of the bleaching of the luminescence signal has to be questioned. The resulting palaeodoses from TL measurements have a difference by a factor of four (8 versus 36 Gy). The dosimetry is nearly identical for both samples because the sediment origin presumably is very similar. TL additive methods give ages of 5,180 ± 380 and 24,900 ± 1,580 years for the reddened rim and the infill respectively23. In fact, if the supralinearity correction would have been determined as well, the resulting ages have to be expected to be higher by a few percent. The age of the infill clearly shows the inappropriateness of the TL method for this type of deposition. In contrast, OSL SAR procedures gave identical results for two samples of about 5,000 years, which is in good agreement with the TL age of the heated sediment and the radiocarbon data24. This suggests that the pit was filled with sediment shortly after its last use as a fire pit, likely in connection to metallurgical processes at the site.

Godfrey-Smith/Shalev 2002. Godfrey-Smith/Shalev 2002. 24 Godfrey-Smith/Shalev 2002. 22 23

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Luminescence dating in prehistoric archaeology

TL-age (BC) 6,500

6,000

5,500

5,000

4,500

4,000

3,500

Sector I, middle of Dikili Tash I Oven 400 BDX 5429 BDX 5433 BDX 5434 average 5,500 ± 320 BC Sector V West, end of Dikili Tash I Oven 600 BDX 5447 BDX 5453 BDX 5454 average 4,920 ± 310 BC Sector VI, Dikili Tash II House 3

BDX 5442

BDX 5443 average 4,260 ± 280 BC Sector VI, Dikili Tash II House 4 BDX 5440: 4,510 ± 410 BC

Fig. 5: TL dating results (1σ) for burnt clay from structures at the Neolithic site of Dikili Tash in Greece (modified from Rocque et al. 2002).

TL dating of domestic ovens Ovens built from clay are numerous from the Neolithic period because of the high survival rate of fire hardened clay over time. Sometimes several generations of ovens are situated on top of each other, thus providing a sequence of occupations, or are associated with sequences of occupational layers. This is the case for the Neolithic site of Dikili Tash (Macedonia, Greece). The sequence of this Tell has been established by radiocarbon dating25, but because of the problematic association of charcoal samples with specific occupational levels, TL dating was used for a number of ovens for verification purposes26. The ovens at Dikili Tash are located either in the centre of a building or at the long sides of houses. The function of these simple structures was most likely domestic. However, a carinated vessel contained the remnants of red hematite, which was reduced in some places to iron oxide by a, presumably destructive, fire27. This find is the oldest Neolithic evidence of pigment processing. Baked clay samples from four ovens were dated with the fine grain TL-method, using a slide technique28. The external dose rates were either measured by gamma spectrometry or reconstructed using the documentation and sediment samples analysed by gamma spectrometry. The resulting TL ages of several sub-samples from three out of the four ovens showed a large dispersion of results, while the individual ages have small associated error estimates (Fig. 5). However, care was taken to ensure the selection of samples from one event (one oven) only, which allows the calculation of weighted mean ages. Such age estimates have sufficiently small associated errors providing the resolution required for interpretation and comparison in a Neolithic context. The ages are coherent and stratigraphically in the right order. TL dates of 5,500 ± 320 BC for oven 400 (middle Dikili Tash phase I) and 4,920 ± 310 BC for oven 600 (final Dikili Tash phase I) correspond well with the radiocarbon based limits of about 5,450–4,500 cal BC for Dikili Tash phase 129. The results for the two ovens belonging to Dikili Tash phase II of 4,260 ± 280 BC and 4,510 ± 410 BC fall into the range of about 500–450 cal BC given by radiocarbon dating as well. TL-dating thus gives confidence in the often assumed association of radiocarbon dating samples with occupational levels, and strengthens the Neolithic chronological framework. 27 28 29 25 26

Treuil 1992. Rocque et al. 2002. Maniatis/Tsirtsoni 2002. Rocque et al. 2002. Rocque et al. 2002.

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Authenticity testing of figurines and ceramics with TL and OSL The application of dosimetric methods for dating gets difficult as soon as material or objects are removed from their context. This applies especially to figurines and ceramics, which have received a significant, but unknown dose from the sediment in which they were once presumably buried. The use of an isochron method to at least partially overcome this problem would require samples from several positions. Such a destructive sampling strategy is usually not possible for valuable objects. In addition, the objects were exposed to different dosimetric environments, e.g. in various museums or locations. However, it is possible to determine ages based on some assumptions and differentiate genuinely old objects from recent fake ones, as has been demonstrated recently by Zink and Porto30. It has to be stressed, however, that authenticity testing, or ‘authenticity dating’, is establishing ages for objects which are based on a large number of assumptions and ‘experience’ or ‘general’ values for dosimetry and other parameters. Zink and Porto31 follow a different approach in their study of Tanagra terracotta figurines. These were numerously found in tombs from the Hellenistic period (2nd–1st century BC) in the Mediterranean area. They met the taste of 19th century European middle-classes and were thus quickly commercialized. Consequently large numbers of fake Tanagra figurines were produced in the 19th century32. Figurines created by 19th century artists are easily distinguished from ancient ones on stylistic grounds. Being proto-industrial products, moulds were used for the ancient production and for faking as well. Stylistic approaches are therefore difficult to apply for more recently faked products. About 140 of these statues from the collection of the Louvre (Paris) were investigated for their authenticity with TL and OSL by Zink and Porto. They reconstructed the dosimetric environment on basis of information provided by old excavation reports. The result of a weighted mean age for the Tanagra figurines of 2,220 ± 195 years is rather imprecise, compared to the typological dating to 4–5 different periods within the range of 330–200 BC. But moulds could have been used over even longer periods of time, and based on style, not all fake Tanagra figurines can be detected. This was achieved by Zink and Porto with good reliability by luminescence methods. A probability density plot of the ages obtained under identical assumptions for the dosimetry shows two distinct distributions (Fig. 6). One broad peak centred around 2200 years ago, which is very well differentiated from one at about 200 years ago. The broadness of the probability density peak, as well as the small peak close to the more recent one, is caused by the assumption of one single external radiation field for all objects. The precise radiation environment of each individual object is unknown, and a common single value therefore is responsible for a wide distribution of apparent ages. The point of this study is not to provide an age for each object, but to use probability statistics to differentiate fake from original ones. The differentiation between old and young Tanagra figurines appears to be clear, and it can be concluded that only a small portion of the figurines at the Louvre are fakes. Heated rocks (flint/chert, quartz, quartzite, granite, sandstone, limestone) Heating of minerals in a fire or during the production of an object is a very efficient way of zeroing the luminescence signal, and in some cases sensitizing the material. The sufficiency of the zeroing can be reasonably tested for with heating and equivalent dose plateaus test33. The obvious way to determine an age of heated material is TL, but OSL techniques can be used as well, provided the material shows an OSL sensitivity.

32 33 30 31

Zink/Porto 2005. Zink/Porto 2005. Zink/Porto 2005. Aitken 1985.

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Luminescence dating in prehistoric archaeology

0.12

0.08

0.06

0.04

0.02

0

500

1000

1500

2000

2500

3000

3500

0.00 4000

Probability distribution

0.10

Fig. 6: Luminescence age results and probability distribution for the Louvre Tanagra figurines (modified from Zink/Porto 2004).

Age (a)

TL-dating of heated flint: Did modern humans and Neanderthals meet? The transition from Middle to Upper Palaeolithic technocomplexes in Europe is regarded to represent a change in hominid species from Neanderthals to anatomically modern humans. This is much in contrast to the Levant, where modern humans were found in association with Middle Palaeolithic technology. TL dating of heated flint plays a major role in establishing the relationships in time between assemblages and hominid species. Heated flints from layers containing modern humans in Quafzeh and Skhul were dated to 90 and 120 ka respectively34. Neanderthals, on the other hand, were dated by the same techniques to about 60 ka at Kebarah35 and Amud36. Under the assumption that the stone tools used for dating are directly associated with the hominid remains, the overlap in time between those two human species is in the order of 60 ka. The earliest anatomically modern human associated with Upper Palaeolithic technology comes from Ksar Akhil, where it was found in the context of an Ahmarian assemblage. The age is estimated to about 50 ka on basis of radiocarbon data on objects from overlying layers. On the contrary, Middle Palaeolithic Mousterian assemblages occur at much later times away from the coastal area. A change in lithic technology from Middle to Upper Palaeolithic was dated to 36.0 ± 2.5 ka in the sequence of Um-el-Tlell37. Overlying layers again contain Mousterian (Middle Palaeolithic) artefacts, dated to 34.0 ± 2.5 ka by TL and 32 ± 0.58 ka with radiocarbon. An assemblage showing similar indications of transition on top of a Middle Palaeolithic sequence is reported from another Syrian site called Jerf-al-Ajla. Eight heated flint samples were dated from layer C, giving age ranges between 28.6 and 38.1 ka. However, the data belong to a normal distribution and come from an unequivocal, well defined layer, with no or little signs of bioturbation or other disturbances, which is considered to represent a short event in human occupation. It is therefore consent to calculate a weighted average age of 33.3 ± 2.3 ka for this layer38. Luminescence age estimates are often very dependent on assumed moisture contents of the sediment during burial time. Flint contains varying levels of radioactive nuclides and thus provides a stable source of ionizing radiation. In the case of Jerf-al-Ajla it is rather high, contributing between 77 and 92 % to the total dose the heated flints have received since their last heating (Tab. 3). Any change in moisture or geochemical composition of the surrounding sediment has therefore little effect on the total dose and thus the age of the samples39. The age results from Jerf-al-Ajla can thus be considered as very reliable and confirms the relatively recent occurrence of transitional or intermediary assemblages in the interior of the Levant. Therefore the 36 37 38 39 34 35

Valladas et al. 1988; Mercier et al. 1993. Valladas et al. 1987. Valladas et al. 1999. Boëda et al. 1996. Richter et al. 2002. Richter et al. 2002.

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overlap in time of the technocomplexes of Middle and Upper Palaeolithic is in the order of 20 ka, surprisingly long for such a small geographical area. If this also represents an additional overlap in time of the two hominid species remains to be shown. TL dating of heated flint artefacts has also been used to detect site disturbances. TL dating results of heated flint from an assemblage by Debenham40 provide evidence of an unnoticed severe mixing of a Palaeolithic site. The age differences of two samples, giving 38 ka and 5 ka, can only be explained by sedimentary disturbance. external dose rate (% total dose rate)

sample

internal dose rate (% total dose rate)

age (ka)

± 1σ (ka)

JA-1

92

8

28.6

3.2

JA-2

84

16

39.1

6.4

JA-3

77

22

33.8

48

JA-7

90

10

34.3

5.0

JA-8

82

18

31.5

4.2

JA-87

84

16

32.2

3.2

JA-324

80

20

37.3

4.5

JA-435

80

20

38.1

4.1

Tab. 3: TL ages on heated flint from layer C at Jerf-al-Ajla. The external and internal dose rates are expressed as percentages of the total dose rate (note that the external dose rate includes the small but stable component of the cosmic dose rate as well) (data from Richter et al. 20002). Using TL-methods to support the evidence of the earliest controlled use of fire by hominids A number of sites like Zhoukoudian, Koobi For a and Chesowanja are claimed to have the earliest evidence of fire use by early hominids. Mastering fire is regarded as one of the hallmarks in human evolution and certainly played an important role in dramatic changes in human behaviour connected with diet, defence and social interaction. However, many of these sites lack the scientific unequivocal proof of the presence of fire, not to mention evidence of intentional use of fire by hominids. For example at Zhoukoudian, the presence of fire associated with human activities could not be confirmed by scientific methods41. While fire altered lithics and bones are known form very old sites, e.g. Swartkrans42, it is difficult to show the intentional use of fire by hominids. Only distinct distributions of fire altered material (objects and/or sediment) can be used as evidence for intentional use, no matter if the fire was produced by humans or acquired from a bushfire. However, the latter can produce distinct distributions e.g. from a single long burning bush, which can be difficult to distinguish from a hearth. Clear associations with archaeological remains as well as additional analysis are therefore necessary, e.g. by phytolith analysis43 to verify if material from a single species was burning or wood from several species, as would be expected from a fire fuelled by humans. Additionally, most Lower and Middle Pleistocene archaeological sites are altered by erosional, geochemical processes, bioturbation etc. which could have destroyed the only possible unequivocal evidence of intentional fire use, like hearths or distinct spatial distributions. Controlled use of fire was recently reported from the Acheulian site of Gesher Benot Ya‘aqov in Israel44, which is dated to an age beyond the Brunhes-Matuyama boundary of 780 ka by palaeomagnetism. The evidence for Early-Middle Pleistocene fire includes burned flint artefacts, charcoal fragments, burned wood, fruits, and grains, which show distinct spatial clustering. For example the spatial distribution of very small burnt lithics is different from the unburnt material (Fig. 7). This clustering is taken as evidence against natural fire being the cause of the heating of the material, and thus as evidence for its controlled use. However, doubts remained if the flint material was altered by heat at all. TL methods were used to establish 42 43 44 40 41

Debenham 1994. Weiner et al. 1998; Goldberg et al. 2001. Brain/Sillen 1988. Rowlett/Pearsall 1993. Goren-Inbar et al. 2004.

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Fig. 7: Surface density plots of the standardized relative densities of small lithics in area C (20 m2) at Gesher Benot Ya’aqov for unburned and burnt small lithics from layers V-5 (top) and V-6 (bottom). (courtesy N. Alperson-Afil)

the unequivocal evidence of heat as agent of the macroscopically observed alterations of the flint material45. The majority of flint samples showed a heating plateau and typical glow curve shapes and peak temperatures. Because of the small size of the samples, the removal of only 0.5–1.0 mm of the outer surface was possible. Therefore sample parts which might have been bleached by sunlight during or after excavation could have been included in the analysis. But bleaching experiments showed that the high temperature TL-peak used is little, if not at all, bleachable to a degree which would result in a satisfied heating plateau test46. The samples thus can be dated by TL-methods, and the use of a new technique for dating small flint samples using a ‘short’ SAR with orange-red detection47 was suggested. Archaeometallurgical slag The invention of metal working is one of the hallmarks of humanity. Establishing the age of especially the first occurrence of metallurgy in a region is a very important. Unfortunately, many sites lack associated finds which can be dated and the slags themselves are often the only remains from places of ancient metallurgy. Temperatures in excess of 1000°C for melting and sometimes smithing guarantee the complete erasing of any luminescence and thus imply the use of especially TL for dating purposes. Using bulk samples from slags for luminescence dating provided variable results and a mineral separation thus has to be applied in order to extract quartz grains contained in slags48. The method employs the red TL-signal (RTL) and a SAR protocol. It was first tested for slags from known context with age, e.g. the Bronze Age site of ‘Versunkene Kirche’ in the Austrian Alps close to St. Lorenz. This site is typical for this northern alpine region where roasting beds were constructed into the mountain slopes with the ovens attached to these terraces. The slags then accumulated down slope and are the only remains found from early metallurgy in other places, some of which are located at elevations of up to 1,300 m asl. In the case of ‘Versunkene Kirche’ the age is known to be about 3,200 ± 200 years by radiocarbon and TL dating of associated pottery. This corresponds well with the RTL age result of 2,740 ± 240 years49. However, it has to be noted that for other sites in the same study the agreement was less convincing. The differences could be attributed to problematic associations of dating samples or problems in luminescence measurements. The study by Haustein et al.50 could show that the RTL 47 48 49 50 45 46

Alperson-Afil et al. 2007. Alperson-Afil et al. 2007. Richter/Krbetschek 2006. Haustein et al. 2003. Haustein et al. 2003. Haustein et al. 2003.

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dating of slag is possible to a sufficient accuracy. However, given the tedious and unpleasant procedure of sample preparation, it is probably advisable to use the recently developed spatially resolved high sensitivity luminescence systems (see section on surface exposure dating). Another approach, which is employing a SAR technique as the previous one has been presented for slags from the Greek island of Kythnos by Gautier51. They extracted quartz from the slags as well but used OSL for dating. The results of this study are hampered by the lack of external dosimetry data. The age of the site was established by pottery typology and confirmed by radiocarbon dating of associated material. An agreement of OSL age results with the known age could be obtained only under the assumption of extreme dosimetric values, which indicates an underestimation of the age by this technique. However, more recently developed SAR protocols are more robust and might provide better results for OSL dating of archaeometallurgical slags. Surface-exposure dating of rocks and monuments Establishing the age of an archaeological site is usually done by dating some specific artefacts or finds which are believed to be associated with the event for which an age estimate is desired. But for some of the key questions for many archaeological sites such associated material is not available, or the association is not secure. This is particularly true if the interest lies in the time of the construction or destruction of a building or monument, or the time of modification or movement of rocks. As a logical consequence from the application of luminescence methods for dating the last exposure of sediments, similar methods were developed for dating the last surface exposure of rocks and monuments. Being able to establishing the age of the last exposure of the surface of rock material (e.g. granite, sandstone, marble, etc.) provides a tool for dating a variety of events (Fig. 8). As soon as a previously light-exposed surface gets protected from light by sediment, the event leading to its protection can be dated. If rocks are turned over or moved by human (or natural) activities, the bottom side is zeroed and the luminescence signal accumulates as soon as it is protected from light. Dating of the construction of walls is possible by sampling the bottom surface of rocks or the inter-block surfaces. The timing of the sedimentation of the deposit which protects the rock surfaces from light can be established, as well as the destruction of a monument by dating the surface which was exposed up to the moment of the destruction. Obviously, sampling of such surfaces is problematic with the exception of inter-block surfaces. But in principle, surface exposure dating is possible for any material containing minerals suitable for luminescence dating. Surface luminescence (TL and OSL) dating of classical monuments The famous temple of Apollo in Delphi, Greece, has been dated by TL of the calcite from marble blocks52. The surface exposure age of 470 ± 200 BC agrees well with the established building age of 550 BC. The application of TL is hampered by a residual signal which needs to be determined and subtracted. Unfortunately calcite does not seem to be datable with OSL. Many other monuments are constructed from limestone, and it has been shown that some limestone contains a sufficient amount of quartz from a thin outer layer for OSL dating of the last light exposure53. The OSL surface-exposure age of 4,800 ± 300 BC for a pebble from the Neolithic site of Ftelia on Mykonos (Greece) agrees well with an age obtained by other methods of 5,800 ± 700 and shows the potential of the approach.

Gautier 2001. Liritzis/Vafiadou 2005. 53 Liritzis/Vafiadou 2005. 51 52

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Fig. 8: Events which can be dated by surface exposure luminescence dating of rock material. The construction can be dated on bottom or inter-block surfaces. The destruction can be dated as the last exposure of faces shielded from the light from the time they fell into sediment. Sedimentation ages of associated deposits can be obtained on side surfaces as well as the movement of stones when exposed faces are shielded after they were put down (modified from Greilich et al. 2005).

When were the Nazca lines built? Results from surface-exposure luminescence dating A novel approach was reported by Greilich et al.54, who used high resolution recording of OSL of the granite surfaces themselves in order to determine the construction age of some of the famous Nazca geoglyphs at Palpa in Peru. The geoglyphs are situated on large pediments of fanglomerates, which are covered by a dark brown desert pavement. They were constructed by removing rocks and boulders from the surfaces and thus exposing the sediment underneath, in some cases at a scale of several kilometres. This produces a striking contrast in colours between the pale silt where the rocks were removed and the brown desert pavement. The construction is assigned to a period including the Paracas to the Nazca cultures (about 500 BC–600 AD), but due to the lack of securely associated artefact material for most of these features, such assignments have to be tentative and need to be checked by chronometric methods. When the stones were moved during the construction of the geoglyphs, some of them were certainly turned upside down, which subsequently shielded the formerly exposed upper surface from light. Even those being not turned upside down are expected to be bleached sufficiently, assuming that construction activities did not take place at night. The surfaces of a number of granite rocks from one of the geoglyphs at Pampa de San Ignacio were sampled by night. SAR approaches with OSL on the quartz component and IRSL on the feldspars were employed on cores taken from these rocks by water cooled drilling. Luminescence measurements were obtained from small 100x100 µm sized areas of the rock surfaces, allowing the limitation of the signal used to only such parts of individual crystals which are showing the appropriate properties needed for OSL dating, while leaving them in context for microdosimetry. This approach overcomes difficulties of single-grain OSL dating of sediments, where the grains used are taken out of context. Some of the stones from the geoglyph were clearly moved recently, and thus gave a zero luminescence signal, others provided ages much older than expected (Fig. 9). The latter is not surprising either, because unmoved stones are hard to be distinguished from moved ones and were thus included in the analysis. The oldest age obtained with this new approach for the geoglyph of Pampa de San Ignacio is concordant with the estimates for the formation of the fanglomerate at about 50 ka ago55. The surface-exposure ages for stones which were clearly moved range from about 1000 to 2000 years, which is in accordance to a calibrated radiocarbon age obtained on associated wood of 1,350–1,410 cal BP. This confirms the attribution of this particular geoglyph to the Nazca culture. Another material for surface exposure dating is brick, for which an examples is described in the section on burnt material. 54 55

Greilich et al. 2005; 2006. Eitel et al. 2005.

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Fig. 9: Surface IRSL age (in years) estimates for rocks from a Nasca geoglyph at Palpa (Peru) (modified from Greilich/ Wagner 2006).

Sediment dating (sand, loess, colluvium) With the development of OSL dating56, the use of TL to establish depositional ages of sediments has become somewhat obsolete and is not considered here. However, recent developments for very old sediments indicate a revival of TL techniques57. Sediments containing archaeological records are often difficult to date directly with OSL because of mixing and disturbances. Turbation is a serious problem, especially in sediments related to human occupation, often termed anthropogenic sediments, where mixing can occur over a considerable depth58. While single grain dating can provide age estimates for selected population of grains, the relationship to the archaeological record and questions posed is not straightforward. Additionally, microdosimetric problems arise. It is therefore often advisable to select more suitable sedimentological units which bracket the archaeological layer of interest. OSL dating can then provide termini post quem and ante quem age estimates. Establishing the age of a deposit or using luminescence techniques to investigate it also provides information on the site formation processes, which are of increasing interest to archaeologist.

56 57 58

Huntley et al. 1985. Jain et al. 2005. E.g. Bush/Feathers 2003.

Luminescence dating in prehistoric archaeology

753

Age of the early development of modern human behaviour determined by multiple and single grain OSL dating of a sandy aeolian deposi The increasing evidence of sophisticated behaviour by ancient human populations has led to a continuous raising of the bar for the definition of ‘cultural modernity’ or ‘modern behaviour’ of anatomically modern humans during recent years. Modern behaviour is commonly associated with the emergence of the European Upper Palaeolithic at about 40 ka 14C. After blades were shown to be a common feature of the Middle Palaeolithic repertoire, other key inventions considered to be ‘modern’ had to be redefined. These are essentially bone technology and ornaments, where the latter is linked to sophisticated social behaviour, increased networks etc. However, it has become increasingly difficult to define material and behavioural differences from other earlier records, especially from Africa, which are linked to anatomically modern humans. A key site for the emergence of modern behaviour is Blombos cave in South Africa. Middle Stone Age (MSA) layers yielded not only an extensive bone technology59, but also personal ornaments in the form of shell beads60 and engraved bones and pieces of ochre61. A series of MSA layers are capped by a dune, which was followed by a sedimentological hiatus. The dune is sterile for most of its extension into the site and thus indicates that no material is intrusive from the overlying Late Stone Age (LSA) levels (Fig. 10). Bone technology and ornaments are common in the LSA context. The sterile sand layer is rather thick in most places, and therefore prevented the mixing of LSA and MSA assemblages. The association of bone tools and ornaments at Blombos with the MSA are therefore unquestionable, and a rather old age based on lithic typology and stratigraphy was postulated. TL dating of heated quartzite and silcrete from layer M1 (Fig. 10), which contains ornaments and ochre, gave a weighted average age of 74 ± 5 ka62, which is in good agreement with ESR dating results on teeth from the same level with a weighted average age of 80 ± 6 ka for the linear uptake model63. The timing of the deposition of the overlying dune sediment was dated by OSL, using quartz SAR procedures. Jacobs et al.64 obtained SAR results for multiple grain SAR samples of 69.2 ± 3.9 ka for a sample from within the cave. This age is concordant to samples from the entrance and outside the cave, giving ages of 69.6 ± 3.5 and 70.9 ± 2.8 ka respectively65. In order to investigate the potential presence of grains of different bleaching ages (mixing), single grain analyses were performed on the same samples66. The integrity of the dune layer, and thus the association of beads and bone tools with the MSA, could be confirmed by the failure to detect intrusive younger or older grains. However, only less than 5 % of grains were acceptable for analysis, which led to the construction of synthetic aliquots, giving age results of 67.3 ± 3.8 ka for the dune inside the cave, 65.6 ± 2.8 ka for the entrance and 68.8 ± 3.0 for the same dune outside67. Several models of handling single grain data were employed, all agreeing at the 1σ level, which is not surprising for an aeolian deposit where a complete bleaching of all grains is expected. However, the single grain results are about 4 % lower than the estimates obtained by multiple grains OSL (Fig. 10). This difference might be due to potential problems in single grain microdosimetry and the inclusion of results from non-quartz grains in the multiple grain analysis. When accepting bone technology and personal ornaments as evidence for modern behaviour, then this development must have evolved at least 70 ka ago, probably even as early as 100 ka68, which is indicated by recent investigations on sites from the Levant, but on sites less well dated than Blombos.

61 62 63 64 65 66 67 68 59 60

Henshilwood et al. 2001. D‘Errico et al. 2004. D‘Errico et al. 2001; Henshilwood et al. 2002. Tribolo et al. 2006. Jones 2001. Jacobs et al. 2003a. Jacobs et al. 2003a. Jacobs et al. 2003b. Jacobs et al. 2003b. Vanhaeren et al. 2006.

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Fig.10: Chronometric dating results from Blombos Cave, South Africa. OSL ages are given as multiple grain (mg) and single grain (sg) results. The TL and linear uptake ESR ages are weighted average results for heated silcretes and animal teeth respectively (modified from Henshilwood et al. 2001).

Luminescence ages of cave deposits which yielded the new species of ‘Homo floresiensis’ of the human lineage A new luminescence technique69 was developed for luminescence dating of quartz from Liang Bua on the island of Flores in Indonesia. This site provided the most spectacular hominid find of the recent decades. The skeletal remains represented by one skull, two jaws and several longbones from several individuals were found in the cave and are classified as the new species of Homo floresiensis70, dubbed ‘Hobbits’. The hominid remains are those of extremely small people of an estimated body size of around 100 cm only, above which pygmies would tower. The very small skull, nicknamed Flo, has an extremely small braincase. However, this cranium, which was used for most of the traits of the definition of this new species is viewed by others as one from a pathological individual suffering from microcephaly71. The position of this new hominid find in the human lineage tree is therefore heavily debated72. The remains were first determined as the possible descendants of homo erectus73 and later described as having more traits in common with Australopithecus74. Given the observed extreme size reduction of dwarfed stegodon on this island75, it doesn’t seem implausible that this was happening to humans as well. The lithic material, which is basically indistinguishable from stone tools produced by modern humans, is associated with a single hominid tooth76, while it originates from supposedly the same sediment layers as the hominids elsewhere in the cave. Establishing the age of the sequence which contains the hominid remains is thus crucial for placing them in the human lineage tree. The bones from Liang Buan themselves do not contain sufficient collagen for direct AMS radiocarbon dating, so only associated charcoal could be dated to 18 ka77. Given the potentially problematic association and the limitations of radiocarbon dating, various approaches in luminescence dating of quartz and feldspar from the sediments were used. The sediment from the cave yielded only feldspars of volcanic origin which are bright enough for luminescence analysis, but not sensitive to standard OSL 71 72 73 74 75 76 77 69 70

Westaway/Roberts 2006. Brown et al. 2004; Morwood et al. 2004. E.g. Hershkovitz et al. 2007 vs. Argue et al. 2007. E.g. Balter 2007. Morwood et al. 2004. Morwood et al. 2005; Tocheri et al. 2007. Morwood et al. 2004. Moore/Brumm 2007. Morwood et al. 2004.

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Luminescence dating in prehistoric archaeology

Fig.11: Schematic composite stratigraphy (not to scale) of Liang Buan. Red-TL ages of the bleachable signal were obtained with a dual-aliquot regenerative-dose protocol (DAP) for isothermal luminescence and are compared to independent chronometric data (modified from Westaway et al. 2006a).

procedure. This led to the development of the new technique of the dual-aliquot regenerative-dose protocol (DAP) dating of isothermal luminescence dating (red TL), using the red emission78. In principle, the dose since the last bleaching is determined by subtracting the dose derived from the unbleachable isothermal signal from the total isothermal dose. The results from this new technique have rather large error estimates, which stem from the low slopes of dose curves, which result in large errors on the dose estimates. However, the results are in stratigraphical order and agree with independent radiometric data for the lower part of the section (Fig. 11). The apparent tendency towards older ages of the red TL results of 95 ± 13 ka and 125 ± 55 ka in comparison with a coupled ESR/U-series of 74+14-12 ka and U/Th age of 102.4 ± 0.3 ka is explained by their lower stratigraphical positions by 3.0 and 1.6 m respectively79. But the discrepancy by a factor of two in ages for a sample of 35 ± 4 ka coming from the same stratigraphical position as a radiocarbon charcoal sample of 18,100 ± 600 cal BP remains unresolved. It could be due to the presence of populations of differently bleached grains80, but the quality and association of the charcoal sample could be equally challenged. Additional sediment ages obtained with the same approach are provided for other sections from within and outside the cave, e.g. 99 ± 33 ka as the age of the lowest layer containing lithic artefacts and which probably extends the presence of hominids on Flores back to 200 ka81. Landscape reconstruction by IRSL dating of anthropogenic sediments (colluvium) Colluvial sediments form as the result of erosion, which is predominantly caused by human activities and changes of the landscape, e.g. agriculture since the Holocene. Colluvium accumulates at lower slopes and can be used to indicate past human activities, and thus their presence, even when no archaeological material is found. In conjunction with information obtained from nearby archaeological sites the dating of the deposition of the colluvium by IRSL can provide the tools for reconstructing the use and shape of a landscape.

80 81 78 79

Westaway et al. 2007. Westaway et al. 2007. Westaway et al. 2007. Westaway et al. 2007.

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Fig. 12: Chronometric age estimates over depth for a profile of colluvium at Vaihingen-Enz. Note that the 14C data was calibrated (modified from Lang/ Hönscheidt 1999).

Late Middle Age

Late Neolithic

source

sink 1

sink 2

sink 3

source

sink 1

source

sink 1

time of first incorporation into the sedimentary pathway

sink 2

sink 3

sink 3

Modern Times

Iron Age/ Roman

Fig. 13: Cascade-model of the minimum number of sediment transportation events and colluvium formation at Vaihingen-Enz. The direction from source to sink 3 is downslope. Sink 1 represents the anthropogenic ditches and pits, sink 2 hypothetical natural depressions on the slope, and sink 3 is the colluvium on the lower slope (modified from Lang/Hönscheidt 1999).

sink 2

source

- 1 ka

- 1 ka

- 1 ka

- 1 ka

sink 1

sink 2

sink 3

Transportation process and resetting of the OSL-”clock”

The age of colluvial sediment is most commonly assessed by radiocarbon dating of organic remains, usually charcoal fragments which are preferably associated with human cultural remains. But it is often not possible to establish the association of the age or origin of the organic material and the time of deposition to a satisfying degree. Older deposits could have been reworked and mixing by bioturbation can always be a problem. However, the former is a result of the nature of the formation of colluvium by slow downslope movement of sediment particles, where stages of stagnation, accumulation and re-deposition are to be expected. Radiocarbon dating of charcoal and bone from colluvia associated with a pottery fragment from the Neolithic (Bandkeramik) site at Vaihingen-Enz (southern Germany) located uphill, gave widely scattered results82. The calibrated 14C ages range from the Neolithic to Medieval times (Fig. 12). No apparent relationship of ages to sample positions within the stratigraphy can be determined. TL dating of a potsherd gave an age consistent with the time of Bandkeramik settlement. In contrast, IRSL dating results of the deposition Lang/Hönscheidt 1999.

82

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Luminescence dating in prehistoric archaeology

of the colluvial layers increased with depth and show that the deposits at the sampling spot are not directly related to Neolithic times (Fig. 13). Instead, they are the result of activities from Iron Age/Roman to Medieval times. However, even the 14C dates can provide valuable data for the reconstruction of the redeposition of the sediments through time83. This led to the development of a model of repeated sediment erosion with deposition, erosion followed by re-deposition again and so forth (Fig. 13), and the reconstruction of the use of the landscape at Vaihingen-Enz84. Concluding remarks Luminescence dating is now in widespread use in archaeology and has proved to be a powerful tool for establishing the ages of archaeological objects, sites and chronologies. Luminescence methods are among of the few chronometric dating methods where the age of archaeological objects can be determined directly. However, age determinations of archaeological sites are not necessarily obtained directly, but indirectly by establishing the age of sediment deposition, which can be linked to the archaeological record. Even the dating of sedimentological layers above and below an archaeological layer can provide valuable age estimates, which are often more precise and accurate than typo-technological dating alone. Furthermore, OSL dating of sediment allows the reconstruction of ancient landscapes and their use by humans. The usefulness of luminescence dating in archaeology is not restricted to age ranges beyond the radiocarbon dating limit, even though this is the time range where it can show its power of dating archaeological objects directly. OSL dating of the time of the deposition of sediments provides useful chronometric information wherever applicable in the archaeological record, even for such recent times where the radiocarbon dating is not possible, as evidenced by the successful dating fluvial deposits only a few decades in age85.

References Aitken 1985: Martin Jim Aitken, Thermoluminescence dating (London 1985). Alperson et al. 2007: Nira Alperson-Afil/Daniel Richter/Naama Goren-Inbar, Phantom hearths and controlled use of fire at Gesher Benot Ya’Aqov, Israel. Paleoanthropology 2007, 1–15. Argue et al. 2006: Debbie Argue/Denise Donlon/Colin Groves/Richard Wright, Homo floresiensis: Microcephalic, pygmoid, Australopithecus, or Homo? Journal Human Evolution 51, 2006, 360–374. Balter 2007: Michael Balter, Small brains, big fight: ‘Hobbits’ called new species. Science 315, 2007, 583. Bailiff/Holland 2000: Ian K. Bailiff/N. Holland, Dating bricks of the last two millennia from Newcastle upon Tyne: a preliminary study. Radiation Measurements 32, 2000, 615–619. Barnett 2000: Sarah M. Barnett, Luminescence dating of pottery from later prehistoric Britain. Archaeometry 42, 2000, 431–457. Bird et al. 2002: Michael I. Bird/Chris S. M. Turney/L. Keith Fifield/Rhys Jones/Linda K. Ayliffe/Adrian Palmer/ Richard Cresswell/Steve Robertson, Radiocarbon analysis of the early archaeological site of Nauwalabila I, Arnhem Land, Australia: Implications for sample suitability and stratigraphic integrity. Quaternary Scien. Rev. 21, 2002, 1061–1075. Boëda et al. 1996: Eric Boëda/Jacques Connan/Daniel Dessort/Sultan Muhesen/Norbert Mercier/Hélène Valladas/ Nadine Tisnérat, Bitumen as a hafting material on Middle Palaeolithic artefacts. Nature 380, 1996, 336–338. Bowler et al. 2003: James M. Bowler/Harvey Johnston/Jon M. Olley/John R. Prescott/Richard G. Roberts/Wilfred Shawcross/Nigel A. Spooner, New ages for human occupation and climatic change at Lake Mungo, Australia. Nature 421, 2003, 837–840. Brain/Sillen 1988: C. K. Bob Brain/Andy Sillen, Evidence from the Swartkrans cave for the earliest use of fire. Nature  36, 1988, 464–466.

Lang/Hönscheidt 1999. Lang/Hönscheidt 1999. 85 Madsen et al. 2007. 83 84

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Dr. Daniel Richter Max-Planck-Institute for Evolutionary Anthropology Deutscher Platz 6 04103 Leipzig