How old is Luzia? Luminescence dating and ... - Wiley Online Library

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How Old Is Luzia? Luminescence Dating and Stratigraphic Integrity at Lapa Vermelha, Lagoa Santa, Brazil James Feathers,1,* Renato Kipnis,2 Luis Piló,2 Manuel Arroyo-Kalin,3 and David Coblentz4 1

Department of Anthropology, Box 353100, University of Washington, Seattle, WA. 98195-3100 2 Laboratório de Estudos Evolutivos Humanos, Sala 244, Departamento de Genética and Biologia Evolutiva, IB/USP, Rua do Matão, 277. Cidade Universitária, São Paulo, SP, Brazil 05508-900 3 Department of Archaeology, Durham University, South Road, Durham DH1 3LE, UK 4 Comparative Religion Program of the Henry M. Jackson School of International Studies, University of Washington, Seattle, WA 98195-3650

During an excavation in the 1970s, a disarticulated female human skeleton, later nicknamed Luzia, was discovered at 12m depth at Lapa Vermelha rockshelter in central Brazil. Radiocarbon dating of associated charcoal suggested an age of 11.4-16.4 ka for the skeleton. The scattering of the skeletal parts, some uncertainty about the exact provenience of the skeleton, and evidence of pervasive insect turbation in the archaeological layers have raised doubts about the accuracy of the age. Luminescence dates for the depositional ages of the sediments at Lapa Vermelha are reported here. Single-grain optically stimulated luminescence (OSL) of quartz along with grain-size, chemical and micro-morphological analyses of the sediments were employed to assess stratigraphic integrity, particularly the degree of sediment mixing. These various lines of evidence point to high stratigraphic integrity with little mixing at Lapa Vermelha. Sediments closest to where Luzia was recovered give OSL ages ranging from 12.7 to 16.0 ka, thus not refuting the original dates. © 2010 Wiley Periodicals, Inc.

INTRODUCTION Understanding the initial migration of humans to the New World requires sound dating at relevant archaeological sites. Minimum dating criteria for evaluating early sites usually include the application of a chronometric method combined with stratigraphic integrity (e.g., Haynes, 1969). This is because most chronometric methods do not date relevant events directly but depend on stratigraphic association or correlation. Luminescence dating, which provides an age for sediment deposition, has

*Corresponding author; E-mail: [email protected]. Geoarchaeology: An International Journal, Vol. 25, No. 4, 395–436 (2010) © 2010 Wiley Periodicals, Inc. Published online in Wiley Interscience (www.interscience.wiley.com). DOI:10.1002/gea.20316

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recently been employed in a number of Paleoindian studies (e.g., Mayer, 2003; Feathers et al., 2006a, 2006b). Single-grain optically stimulated luminescence (OSL) in particular is important because it addresses the deposition of individual grains and thus combines chronometric dating with an evaluation of stratigraphic integrity. This paper reports on the application of single-grain OSL to an important Paleoindian site in South America where mixing may be an issue. Sedimentary and micromorphological analyses were also carried out to better understand site formation processes. ARCHAEOLOGICAL BACKGROUND North America has played a central role in the debate over the initial colonization of the New World for at least two major reasons: the presumed route of entry along the northern margin of the Pacific Ocean and the confinement to North America of the earliest widely accepted lithic tradition (Clovis). North American research has not only included many disputed (as well as accepted) early sites but also detailed consideration of the environments, possible migration routes, adaptations of the earliest settlers, genetic affinities of early peoples, and evolution of technology (e.g., Jablonski, 2002). However, a full understanding of the peopling of the New World must also consider the South American evidence. This includes not only claims for a few early sites, from controversial ones like Pedra Furada in Brazil (Guidon and Delibrias, 1986; Meltzer et al., 1994) to the widely accepted Monte Verde in Chile (Dillehay, 1989, 1997; Meltzer et al., 1997), whose pre-Clovis date has caused rethinking of the North American evidence, but also the broader context in which early colonization of the southern continent occurred. Any explanation of the colonization process must account for the evidence that early South Americans are different from early North Americans in technology (Clovis seemed to reach no further than Panama [Pearson, 2004, and although some fluted points are found [Borrero et al., 1998; Jackson, 2007], much of the continent contains distinct lithic technology and in many areas is dominated by unifacial industry [Dillehay, 2000]); in adaptation (broad-scale foraging in very different environments and across varied landscapes [Kipnis, 1998; Prous and Fogaça, 1999; Roosevelt, 2002; Meggers and Miller, 2003]); and in physical appearance (different skeletal morphology [Neves et al., 2007]). The University of São Paulo (USP), under the direction of biological anthropologist Walter Neves, has in the last decade carried out a multidisciplinary research project in one portion of South America—the Lagoa Santa region of central Brazil— to better understand this broader context in one locality. The dating and geoarchaeological evidence presented here is aimed at understanding the depositional context at the site of Lapa Vermelha IV in Lagoa Santa and the age of some human skeletal remains. Lagoa Santa is located just north of Belo Horizonte, Brazil’s fourth largest city, in the state of Minas Gerais (Figure 1). It is a karstic region with abundant limestone outcrops, semipermanent lakes, and rock shelters that contain a rich archaeological and palaeontological record. Study of the region dates to the 1830s, when a

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Figure 1. Location of Lapa Vermelha and other sites mentioned in the text within the Lagoa Santa region, Brazil.

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Danish naturalist, Peter Lund, began investigating a number of rock shelters. He discovered not only a rich faunal record but also abundant human skeletal remains. On the basis particularly of work at Sumidouro Cave, where he found 29 human skeletons in apparent stratigraphic association with extinct forms of large mammals, he argued not only that the association with extinct mammals was real but also that human settlement in the New World must be much older than previously thought (Lund, 1845). Lund’s claims were not accepted (the association of humans with extinct mammals had not yet been established even in Europe), but study of the skeletons was continued by others throughout the late nineteenth and early twentieth centuries. Most of these scholars noted that the cranial morphology was distinct from that of other native Americans (see review by Neves et al., 2007). An American physical anthropologist, Ales Hrdlicˇ ka, disputed that the Sumidouro cranial morphology was outside the range of modern native Americans and doubted that the skeletons were old, arguing that the apparent association with extinct mammals was a consequence of post-depositional mixing (Hrdlicˇ ka, 1912). A recent evaluation at Sumidouro has favored Lund’s original interpretation for the antiquity of the human remains (Piló et al., 2005; Neves et al., 2007), but is equivocal for the association with extinct fauna. After Hrdlicka’s criticisms, little professional work took place in Lagoa Santa until the 1950s, when Hurt and Blasi excavated at Cerca Grande and Boleiras, two other large rock shelters (Hurt, 1960, 1964; Hurt and Blasi, 1969). Radiocarbon ages obtained from Cerca Grande (Hurt, 1964) were the first evidence of a Paleoindian age for the skeletons, but establishing the contemporaneity between humans and megafauna proved elusive. Then in 1971, Annette Laming-Emperaire began excavating at Lapa Vermelha IV (Laming-Emperaire, 1979). The excavation, carried out over several seasons, progressed through 14 m of sediments in the back of the shelter, most of it archaeologically sterile. The remains of an extinct ground sloth (Glossoterium gigas) were encountered at 11 m, and another meter down the disarticulated remains of a human female were uncovered (Neves et al., 1999). The skeleton has since been nicknamed Luzia (Portuguese for Lucy). Conventional radiocarbon dates on charcoal produced bracketing uncalibrated ages of 10,220 and 12,960 14C yr BP (11.4–16.4 ka calibrated [all calibrations by OxCal 4.1]), raising the possibility that Luzia might be pre-Clovis (Laming-Emperaire, 1979). A later attempt to date the skeleton itself (Neves et al., 1999) was not successful due to lack of collagen, but the radiocarbon lab reported a minimum AMS date derived from organic residue (either degraded collagen or exogenous organics [D. Hood, Beta Analytic, personal communication, 2010]) obtained from the bone of 9,330 ⫾ 60 14C yr BP (10.4–10.6 ka calibrated). Charred material associated with the sloth produced a conventional date of 9,580 ⫾ 200 14C yr BP (10.6–11.2 ka calibrated) (Neves et al., 1999). Unfortunately, the untimely death of Laming-Emperaire in 1977 prevented her from publishing her results, and public information on the site is largely restricted to secondary sources (e.g., Neves et al., 1999; Prous and Fogaça, 1999). In order to clarify the ages, nature, and archaeological context of the Lagoa Santa skeletons and increase their sample number, Neves and colleagues initiated the 398


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University of São Paulo’s research project in the late 1990s. The project has reevaluated older excavations at Lapa Vermelha IV (Neves et al., 1999), Boleiras (Araujo et al., 2002, 2008), and Sumidouro Cave (Neves et al., 2007) and also initiated research in previously unstudied rock shelters such as Lapa do Santo as well as some openair sites near Sumidouro (e.g., Araujo and Feathers, 2008). The more significant finding has not been the age of the skeletons; with the possible exception of Luzia, most skeletons date around 10,000 years old, on the basis of both luminescence and radiocarbon (e.g., Neves et al., 2007; Araujo et al., 2008). But the cranial morphology of them raises questions. Cranial measurements by Neves on nearly 100 skeletons, including Luzia, show them to be morphologically distinct from modern native Americans and northeast Asians, as well as from Archaic-aged American specimens (Neves and Pucciarelli, 1991; Neves et al., 1999, 2003, 2004, 2007). Instead, the crania appear more similar to South Asians, aboriginal Australians, and even Africans. Neves has hypothesized an earlier migration to the Americas originating from South Asia prior to the migration that was ancestral to modern native Americans (Neves et al., 1996, 2003; Neves and Hubbe, 2005). Lagoa Santa provides the largest sample, by an order of magnitude, of Paleoindian skeletons in the New World. Because of their abundance and more importantly because of their distinctive morphology, they require an accounting in any scenario for the colonization of the Americas. Unfortunately the great majority of the human remains uncovered at Lagoa Santo do not present collagen for 14C dating. It is therefore important to clarify their age, and the USP project has initiated a program of luminescence dating of sediments to complement radiocarbon dating at several sites in Lagoa Santa (e.g., Araujo et al., 2008). Here, we report on dating at Lapa Vermelha IV to evaluate the radiocarbon claims for the age of Luzia.

LAPA VERMELHA IV Lapa Vermelha IV is one of a series of caves overlooking a small lake in the southern part of the karstic region (Figure 1). Its geometry, which effectively shelters an area 50 m long and 7.5 m wide (Figure 2), is most likely the result of collapse of an older configuration. Fallen rocks, cobbles, and boulders along the drip line have created a closed basin in the interior of the shelter, facilitating the accumulation of deposits behind the rocks. At the base of the shelter is a nowdormant sinkhole. Laming-Emperaire’s excavations reached 14 m and removed most of the rock shelter’s deposits (Laming-Emperaire et al., 1975, Laming-Emperaire, 1979). The only remaining sediments are at the north and south ends (Figures 3 and 4) and a small irregular baulk at the rear of the shelter, between the two end profiles (Figure 5). The baulk is referred to as the central profile in this paper. While much of the depositional record is lost, these surviving deposits permit the identification of two main strata (here designated A and C) and a series of additional lenses of variable relationship to each other and to the main strata and which we have lumped together, for present purposes, as stratum B.


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c Drip line

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Figure 2. Plan view of Lapa Vermelha IV published by Laming-Emperaire (1979) with the current locations of the profiles added. The central profile is the same as the central baulk mentioned in the text. The lettered squares represent excavation units begun in the first season, 1971.

Luzia was recovered somewhere near the interface of Strata A and C, in the vicinity of the surviving central baulk. The precise location is not known because some of Laming-Emperaire’s field notes appear to have been lost after her death. Some surviving notes, reports, and an inscription on the wall put there during the 1970s excavation, indicate the skull was found at 12.9 m below the current surface. A right upper incisor, pelvis, and femur were found at approximately 10.0 m and 5 m north from the location of the skull (Mello e Alvim, 1977; Cunha and Guimarães, 1978). Cunha and Guimarães (1978:291) argued that the human skeleton had originally been deposited near a depth of 9.7 m and that it had become slowly disarticulated and gravitationally displaced as a result of a pond forming seasonally in the shelter. On the basis of this presumed original location and the presence of red clay analogous to that of stratum A inside the bones, they suggested a Holocene age for the skeleton. Alternatively, the topography of the basin’s surface at the time may have formed a slope from north to south, and some of the skeleton’s elements rolled down with time, the skull—the roundest piece—moving farthest. If this surface (now the interface) is terminal Pleistocene, this might suggest a somewhat older age. These uncertainties of provenience, coupled with bioturbation and the disarticulated state of the skeleton, raise doubts about the association between Luzia and the charcoal used to bracket the age. Laming-Emperaire’s excavations in the 1970s produced 29 14C assays (Delibrias et al., 1986), all processed by the Gif-sur-Yvette laboratory in France. Table I lists the ages in chronological order. Some dates are labeled with level numbers rather than depth, with A the highest in the stratigraphy. The exact depths of the levels are not known to us at present, but the dates arranged by either level or depth are in rough stratigraphic order, although some inversions may suggest mixing. We also do not know where, in plan view, the samples were obtained from within the shelter. The bracketing of the age of Luzia by radiocarbon samples Gif-3727 and Gif-3906 is based on comparative depths of charcoal and Luzia’s cranium, which was found at about the same depth as sample Gif-3906. From these Laming-Emperaire (1979) surmised that the skeleton must be older than 12 ka (uncalibrated radiocarbon years). 400


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Figure 5. Photograph of central profile showing OSL sample locations. Stratum A is the darker layer near the top; the lighter layer is stratum C. The boundary here is rather diffuse, but at the time of collection it appears UW1385 was taken from the bottom of stratum A. The plastic pipe in the sample holes contain at their ends dosimeters, although these particular ones were never retrieved. The two samples are 55 cm apart.

To address the issue of Luzia and better understand site formation, we first present geoarchaeological data to characterize the surviving stratigraphy at Lapa Vermelha. We then apply OSL single-grain dating to obtain a measure of the depositional age of the sediments and assess the extent of mixing in the deposit. GEOARCHAEOLOGICAL STUDIES Regionally, the limestone comprising the rockshelters and other karstic features of Lagoa Santa is known as the Sete Lagoas Formation. It is overlain by a yellow to red soil mantle resulting from weathering of pelites of the Serra de Santa Helena Formation. The soil mantle also includes nodules weathered from quartz veins. The deposits in the rock shelters, Lapa Vermelha included, are composed of limestone eroded from the walls combined with colluvium derived from the weathered pelites, with the eroded quartz providing the material for OSL dating. The main goals of the geoarchaeological study were to (1) characterize the stratigraphy of the site, (2) develop some inferences about the main depositional processes, (3) assess the degree of bioturbation affecting the deposit, and (4) provide information on the abundance and source of quartz particles on which the OSL dates are based. Evidence was provided by macroscopic field observations; bulk sample particle size analysis on six samples from the south profile and one from the Latosol (Oxisol) soil mantle above the rock shelter (pipette method as adapted by the Laboratório do Instituto Mineiro de Agropecuária); bulk sample X-ray fluorescence analysis on the same samples (lithium tetraborate fusion, using a Philips PW 1988 GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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FEATHERS ET AL. Table I. Published radiocarbon dates (Delibrias et al., 1986). Sample #

Depth (m) from Surface or Excavation Level

Gif-2732 Gif-2735 Gif-3222 Gif-3220 Gif-3221 Gif-3211 Gif-3218 Gif-3219 Gif-3210 Gif-2734 Gif-2545 Gif-2733 Gif-3209 Gif-2543 Gif-3215 Gif-2544 Gif-3213 Gif-3214 Gif-3907* Gif-3207 Gif-3217 Gif-3216 Gif-3208 Gif-3727 Gif-3726* Gif-3906 Gif-3905 Gif-3725* Gif-3908*

1.15 0.2 Base level B Surface Level D Not given Base level D Base level C Level E 2.1 1.9 1.5 Level E 4.35 Level G 5.0 Level F Level G 12.95–13.15 9.65 Level I Level H 10.3–10.8 11.7–11.9 11.7 12.6–12.8 13.55–14.5 11.7–11.8 12.6–13.55

Uncalibrated Ages (14C years BP) 300 ⫾ 110 320 ⫾ 80 1620 ⫾ 100 1880 ⫾ 140 3070 ⫾ 110 3260 ⫾ 110 3370 ⫾ 110 3430 ⫾ 130 3580 ⫾ 130 3660 ⫾ 110 3720 ⫾ 120 3740 ⫾ 110 3750 ⫾ 110 4170 ⫾ 120 4350 ⫾ 120 4400 ⫾ 120 4550 ⫾ 130 5120 ⫾ 130 5400 ⫾ 500 6830 ⫾ 150 6950 ⫾ 140 8490 ⫾ 160 9580 ⫾ 200 10200 ⫾ 220 11680 ⫾ 500 12960 ⫾ 300 15300 ⫾ 400 ⱖ25000 22410 ⫾ 400

Calibrated Range** (years BC or AD) 1450–1950 AD 1480–1650 AD 260–550 AD 40 BC–320 AD 1450–1130 BC 1670–1430 BC 1860–1520 BC 1900–1541 BC 2130–1750 BC 2200–1890 BC 2290–1950 BC 2330–1980 BC 2340–1980 BC 2890–2580 BC 3330–2880 BC 3330–2910 BC 3500–3030 BC 4050–3710 BC 4800–3660 BC 5890–5620 BC 5990–5720 BC 7730–7330 BC 9250–8700 BC 10400–9450 BC 12250–10950 BC 14400–13150 BC 16900–16100 BC 25750–24400 BC

* Reported undersized or mixed sample. ** Calibration to 1 sigma using version 4.1 of OxCal.

spectrometer); and analysis of 13 sediment thin sections from both north and south profiles using soil micromorphological methods (Courty et al., 1989; Stoops, 2003). Results for the grain size and chemical analyses are given in Table II. Figures 3–5 show the profiles and the location of collected samples. Stratum A This unit makes up the bulk of the deposits, extending to the modern surface and having generally sharp boundaries with other units. Macroscopically, the stratum can be characterized as a red (5YR 5/8) sandy clay with inclusions of charcoal fragments, limestone cobbles, and rare speleothem fragments. The sediments are riddled 404


18.9 18.2 26.2 26.0 23.7 26.1 12.7

Silt (%) 45.9 39.6 33.1 30.4 35.0 15.9 60.8

Clay (%) 35.40 36.50 34.10 34.60 34.10 27.90 39.00

SiO2 30.60 31.70 27.60 25.80 28.80 16.00 28.50

Al2O3 1.60 1.70 1.50 1.40 1.50 0.83 1.50

TiO2 15.70 13.30 13.00 12.90 13.00 8.00 11.80

Fe2O3 0.31 0.16 0.24 0.22 0.21 0.20 0.39

MnO 0.65 0.55 0.94 1.20 1.00 1.30 0.54

MgO

1.10 0.84 5.30 5.40 3.40 21.70 0.30

CaO

K2O 0.76 0.69 0.98 0.90 0.85 0.72 0.70

Na2O ⬍ 0.1 ⬍ 0.1 ⬍ 0.1 0.11 ⬍ 0.1 ⬍ 0.1 ⬍0.1

0.67 0.61 0.92 0.86 0.79 0.64 0.56

P2O5

*Definitions of grain sizes are clay, ⬍ 0.002 mm; silt, 0.002–0.06 mm; and sand, 0.06–2 mm. Chemical proportions do not sum to 100% because the chemical analysis was done after loss on ignition removed volatiles and hydroxides. The differences between the sum percentages and 100 equal the percent LOI.

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with holes from burrowing ants. From particle size analysis (Table II), stratum A is higher in clay and lower in silt than sediments from other strata, while X-ray fluorescence data (Table II) show closer similarities to a modern soil sample from atop the karst formation than to other samples, in particular the much lower calcium oxide contents. The lower CaO reflects the likely origin of stratum A sediment from the soil mantle atop the shelter. This soil is characterized by a high degree of weathering from which CaO and other bases are highly leached. Stratum A did not receive significant inputs of dissolved CaO from the shelter walls or from ashy deposits from human activity, partly because of its younger age than the older strata. Drier climatic conditions (so less carbonate dissolution) during the Holocene may also be factors. Behind the drip line, stratum A sediments are characterized by clear stratification in the form of primarily horizontal laminations of coarse (fine gravel, quartz sand, and iron nodules) and fine (fine to coarse sand-sized granules of colluvial origin) sediments, as well as localized cross laminations and muddy lenses as thick as 4 cm. Outside the drip line, laminations disappear as a result of mixing associated with roots and soil fauna. Micromorphological analysis of south profile samples 1–3 (and top of 4) show in plane and cross-polarized light that the sediments are made of aggregates or crumbs of reddish hematite-rich undifferentiated clayey material embedding intrapedally 1–5% subangular quartz grains. Laminations appear as alternating microscopic beds composed of well-sorted granular, coarse to fine sand-sized clayey peds with 20%–40% porosity (Figure 6). As many as 16 alternating laminations were found within a 12-cm section in the lower part of the deposit. Mud lenses appear as stacks of well-sorted and bedded microscopic layers that alternate between 250- and 125-mm granules and fine sand to silt-sized crumbs, suggesting the settling of fine debris in an aqueous medium with minimal faunal reworking. About 20% of granules show edge morphology, cappings, and contrasts in optical properties which indicate in-mixing of reworked colluvial material. Packing voids in south profile sample 4 are in-filled by silt-sized clayey crumbs, calcium carbonate–replaced plant matter, and very rare charcoal fragments, perhaps associated with microscopic debris from occupations. Beyond the drip line, north profile samples 1 to 4 show a composite crumb to channel microstructure (Fitzpatrick, 1993), channels in-filled with silt-sized crumbs and large irregular peds with rounded morphologies that suggest soil faunal activity. Sample 3 is exceptional in including rare silt- to sand-sized charcoal fragments, very rare calcium oxalate pseudomorphs (ash “crystals”), and sand-sized bone fragments, the isolated presence of which suggests that fauna reworking has not obliterated all stratification. Stratum B Stratum B is a heterogeneous unit that consists of a number of structurally massive lenses that either extend at 25° angles into stratum A, usually with abrupt boundaries, or extend subhorizontally on the irregular surface of stratum C, with varying sharp to diffuse boundaries. Field observations suggest a mixed reddish-gray (5YR 4/2) and reddish-brown (5YR 4/2 and 5YR 6/6) sandy mud composed of small clayey 406


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Figure 6. Thin section S2 showing alternating microscopic beds composed of well-sorted granular peds of a similar size range. The fine mineral fraction is a hematite-rich reddish-brown (5YR 4/4) clayey material (PPL) with an undifferentiated b-fabric (XPL). Stratification is expressed by contrasts (often fine–coarse–fine) in the modal size of granules which dominate each bed. Note stack of well-sorted and bedded microscopic layers made of fine sand-sized or smaller granules and crumbs which point to settling fine debris in an aqueous medium.

blocks in which frequent charcoal fragments and small cobbles are observable. The lenses are also laminated but have higher-silt and lower-clay content than stratum A (Table II). Strong reaction to HCl and higher Ca content indicated by XRF suggest the grayer colors may result from calcium carbonates. Micromorphological analysis allows distinctions to be made among the lenses. A lower lens visible on the south profile and sampled by S5 and S6 is similar to a lens on the north profile sampled by N6 and N7. Both are made of dense clayey aggregates of soil crumbs intermixed with silt-sized debris. Porosity is reduced and neither a welldeveloped soil structure nor stratification is evident. The fine-mineral fraction of most aggregates (95%) is a hematite-rich reddish-brown (5YR 4/4) clayey material and an undifferentiated b-fabric, bearing a resemblance to stratum A sediments. The other 5% are made of 10YR 7/6 goethite-rich yellow clay with a speckled to circular GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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Figure 7. Microphotograph of thin section N6 showing dense clayey aggregates embedded in reducedporosity micromass composed of soil crumbs and silt to very fine sand-sized porous clayey crumbs, fragments of amorphous organic matter, relatively common microscopic charcoal fragments, common individual ash crystals (see Figures 8a and 8b), rare, microscopic bone fragments (see Figure 8c) and rare plant tissue replaced by calcium carbonate, in plain polarized light.

striated b-fabric. In all cases, the aggregates embed small quantities (1%–5%) of fine sand or smaller quartz grains and rare silt-sized charcoal. The silty micromass includes abundant silt to very fine sand-sized porous clayey crumbs, fragments of amorphous organic matter, common microscopic charcoal fragments, rare plant tissue replaced by calcium carbonate, ash crystals, and rare bone fragments (Figures 7–9). The proportion of these varies from sample to sample. A higher lens on the south profile, represented by samples S3 and S4, differs in having a higher proportion of coarse sand-sized aggregates, suggesting more faunal reworking, less ash crystals and bone fragments, and some indication of stratification as an upward decreasing size of embedded granules. These differences highlight variability within stratum B. Stratum C The lowest deposits make up stratum C, a reddish-yellow (7.5YR 6/4 and 7.5YR 6.6) laminated sandy mud with gravel, the latter mainly limestone boulders and cobbles originating from roof fall. Smaller clasts are partially weathered. In the south profile many clasts are inclined 37° toward the back of the shelter, in disagreement with the orientation of the unit as a whole. In other places, clasts are largely absent. Stratum C has much higher sand content and much less clay than the other deposits (Table II). It is also distinct chemically with slightly lower values of silicon, aluminum, and iron and higher values of calcium. While no micromorphological samples from stratum C were collected, goethite-rich clayey granules identified in south profile samples S5 and S6 are presumed to be more common in stratum C, because of its 408


Figure 8. Microphotograph of thin section N6 showing calcium oxalate ash pseudo-crystals in cell voids of charred plant tissue in (a) plain polarized light (PPL), top left, and (b) cross-polarized light (XPL), top right. (c) Microphotograph in plain polarized light of thin section N6 showing microscopic bone fragment forming part of silty micromass embedding colluvial clayey aggregates. Note microscopic charcoal fragments.

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Figure 9. Microphotograph of thin section N6 showing calcite fragment (“CaCO3”), clay aggregate embedding silt-sized quartz grain (“q”), and calcium oxalate ash pseudo-crystals (“ash”) in (a) plain polarized light (PPL), top, and (b) cross-polarized light (XPL), bottom.

yellowish color. As such, they can be related tentatively to the yellow soil horizon described by regional pedological studies (Boulet et al., 1992; Piló, 1998). Interpretation The collapse of the old cave resulted in accumulation of rock fall, cobbles, and boulders along the drip line, where there has also been formation of stalagmites. These processes formed a closed basin that acted as a sediment trap. Stratum C appears to be related to this rock fall, as the grain size and chemical data do not indicate that it was derived primarily from the lateral colluvial dejection or debris 410


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cones that have formed on either side of the rock shelter. Nevertheless, some of the sediments in stratum C must have origins outside the shelter. In contrast, chemical, particle size, and micromorphological data for stratum A suggest an origin from the still-active dejection cones. As mentioned, the chemical similarity of stratum A with the red Latosol soil from above is evidence of the ultimate origin of this deposit. Stratum B represents a number of different depositions but the lenses appear to be admixtures of stratum A–like colluvium, debris associated with human occupation, and some calcium carbonate precipitation (most likely from ash). We hypothesize that a depositional hiatus exists between strata A and C and infer that stratum A is deposited as a result of gravitational transport, the transportation of larger particles in a viscous sludge, and the rhythmic settling of fine sediments in an aqueous environment, perhaps a small, shallow seasonal pond inside the rock shelter. Stratum A forms distinct boundaries with other units and seems to fill irregularities produced by adjacent units. The heterogeneous stratum B sediments were most likely displaced gravitationally, reworked by human trampling, and/or sheet-washed from occupation at the front of the shelter. The inclined lenses suggest more erosion from the external part of the shelter than from the dejection cones. The micromorphological and the chemical evidence indicates that stratum B is bulked up by anthropological sedimentation, perhaps the result of ash production in fires built close to the shelter’s opening and near the drip line or, alternatively, at spots that have been removed by earlier excavations. The depositional processes of stratum B seem to have occurred about the same time as the beginnings of stratum A deposition. Our observations indicate that bioturbation affecting the deposit has not obliterated stratigraphic integrity, especially behind the drip line and in the deeper part of the deposit. Despite being riddled with small holes left by burrowing ants, these deposits preserve sedimentary structures, even more so in the deepest parts of the deposit, where Luzia was recovered and limited sun exposure appears to have restricted ant activity. Finally, as regards the abundance and source of quartz particles in these sediments, micromorphological observations show that most quartz grains are embedded in red clayey aggregates of colluvial origin. A minority of quartz in south profile samples 5 and 6 are embedded in yellow clayey aggregates which can be associated with much older colluvial inputs (Piló, 1998). LUMINESCENCE PROCEDURES Because luminescence dating addresses a depositional event—the last time the sediment was exposed to sunlight—it provides a more direct measure of the sediments than the radiocarbon of charcoal, which relies on an associational argument between the charcoal and the sediment. Employing single-grain dating, moreover, allows for an evaluation of the extent of mixing, because grains with different doses, which presumably represent different exposure ages, can be identified. Nine samples were collected, three from the north, two from the central, and four from the south profiles (Table III, Figures 3–5; ages given will be discussed later). Notice that there is some ambiguity about UW1385. At the time of collection, it was GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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FEATHERS ET AL. Table III. Luminescence age of samples arranged in stratigraphic order from youngest to oldest. Component specifies which component from the finite mixture model is being used for the age and its percentage of all grains. Sample

Stratum

Burial Depth (m)

Component (%)

Age (ka)

South Profile UW853

Upper A

9.6

Lower A B C

11.5 10.3 11

2nd (57.5) 3rd (23.2) 2nd (76.5) 2nd (75.1) 3rd (63.4) 2nd (24.2)

4.5 ⫾ 0.4 6.3 ⫾ 0.6 5.2 ⫾ 0.3 6.2 ⫾ 0.4 22.8 ⫾ 1.7 13.7 ⫾ 1.5

8.8 9.8 6.6 7.2 7.6 11.1

Upper A Lower A B

7.5 11.5 10.3

2nd (95.6) 1st (100) 1st (100)

6.5 ⫾ 0.4 7.9 ⫾ 0.5 9.2 ⫾ 0.6

6.6 6.3 6.4

14 14

1st (100) 1st (100)

12.7 ⫾ 0.8 16.0 ⫾ 1.0

6.7 6.2

UW850 UW852 UW851 North Profile UW1387 UW1420 UW1386 Center Profile UW1385 UW1384

C or A C

% error

intended that UW1384 be obtained from stratum C and UW1385 from stratum A, both samples from near the bottom of the rock shelter. There was also an intention to keep UW1385 some distance from the rock shelter wall to simplify dose rate calculations. From Figure 5 it can be seen that stratum A is very narrow at this depth, and the boundary with stratum C does not appear as clear in Figure 5 as elsewhere. While the sample possibly straddles the boundary, the conclusion from field observations is that it lies entirely within stratum A. Samples were collected by driving lighttight metal tubes into the profiles and capping both ends. The light-exposed ends were removed under red light in the laboratory. Grain-Size Effect The samples were sorted into size fractions by screening. The 125- to150-mm grain fraction was employed for dating on the samples from the south profile (UW850–853), which were collected in 2003. This size fraction may compromise single-grain resolution to some extent because two or three grains may fit into the 300-mm holes on the single-grain disks used for measurement. A recent modeling work (Arnold and Roberts, 2009) has cautioned against using grains smaller than 180 mm in the Risø single-grain disks because averaging effects from multigrains may produce misleading results including “phantom” equivalent dose components. Measurements were acquired prior to knowledge of this work, and the smaller grain size was chosen to increase chances that any one position would produce a usable signal. The 150- to 180-mm-size fraction was used on the samples from the north and central profiles (UW1384–1387,1420), collected in 2005. At this size, it is more likely each

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position will contain one grain, although some may contain two. (Two grains per hole cannot be easily avoided even for 180- to 212-mm grains. The lead author has observed two 180- to 212-mm grains stacked on top of each other in one hole—a situation that may not be detected by visual analysis.) Because the south profile samples have higher dispersion with more components, the effect of using smaller grain sizes was assessed by measuring 180- to 212-mm grains on one sample, UW1851. The percentage of positions (holes) yielding a measurable signal did not differ (both 22%) between the 125- to 150-mm and 150- to 180mm samples (Table IV), but it was significantly less for the 180- to 212-mm grains from UW1851, only 3% (the corresponding proportion for 125- to 150-mm grains for this particular sample was 15%). Including grains that had a signal but were rejected for other reasons, these percentages would increase to about 30% for the smaller grain sizes and 5% for 180 to 212 mm. If the percentage of grains with a measurable signal is 5%, then with three grains in each hole, the probability of any hole producing a signal is 15% and the probability of two or more grains within each hole producing a signal is less than 1% (or about 6% of acceptable grains). Even if the percentage of grains with a measurable signal is 10%, the probability of any hole producing a signal is 30%, and the probability of two or more grains within each hole producing a signal is still only 3% (10% of acceptable grains). This is the maximum effect, because many holes will contain less than three grains, so significant deviation from singlegrain resolution is not likely. This probability of more than one grain in a hole producing a measurable signal is much smaller than the 50% considered by Arnold and Roberts (2009:224) in their model.1 Chemical Treatment The screened material was treated with HCl and H2O2, etched for 40 min in 48% HF, and density separated using a sodium polytungstate solution of 2.67 specific gravity. The HCl removed from 10% to 50% by weight of the screened material. Such variation in carbonate content was verified for the whole sample by treating unscreened material. The carbonate content varied by weight from 40% for UW851 to 4.2% for UW1420. The HF etch removed more than 90% by weight from the 125- to 180-mm fractions. Much of this loss is thought due to the breakup of conglomerated pelite. Quartz was thus not abundant, probably not enough for large multigrain aliquot analysis, but was sufficient for single grains. Such low abundance of quartz was borne out by the micromorphological observations discussed earlier.

1. An additional possibility is that two grains that individually would not produce a signal above background might do so together. This might account for the somewhat higher acceptance ratio for the smaller grain size of UW851 than would be expected from the acceptance ratio for the 180- to 212-mm grains and the number of grains that physically fit into a hole. However, for the small number of grains in each hole and the generally low sensitivity of the samples, we do not think this will be significant, although it could account for some of the low-proportion, small-value components discussed later. (Later discussion also suggests any “phantom” components are not significant.)


413

414

1097 1096 896 600 1095 998 500 699 695 7676

UW850 UW851 UW852 UW853 UW1384 UW1385 UW1386 UW1387 UW1420 Total % of total

775 764 641 382 793 712 295 443 473 5278 68.8

26 35 15 22 37 20 8 27 15 205 2.7

Recycling Test 17 128 26 31 70 80 21 23 21 417 5.4

Natural OSL Exceeding Highest Regenerated OSL 8 4 2 0 3 4 3 2 0 26 0.3

De Not Significantly Different from Zero 0 0 0 0 1 23* 1 11 0 36 0.4

Recuperation

2 0 1 0 2 2 6 15 1 29 0.4

Feldspar Contamination

269 (24.5) 165 (15.1) 211 (23.5) 165 (27.5) 189 (17.3) 157 (15.7) 166 (33.2) 178 (25.5) 185 (26.6) 1685 22.0

Accepted (% of total)

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* Twenty-two of these were from one disk.

Measured

Sample

Poor Signal

Table IV. Number of grains sampled, number of grains rejected and rejection criteria.

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Dose Rate Radioactivity was measured for all samples by thick source alpha counting for U and Th using the pairs technique, flame photometry for K, and thick source beta counting. Conversion to dose rates followed Adamiec and Aitken (1998). Alpha and beta dose rates were adjusted for attenuation because of grain size and etch. A derived 0.03 to 0.6 Gy/ka for the alpha dose rate was assumed to subsume any internal alpha contribution. Gamma dose rates were estimated from laboratory measurements (alpha counting, assuming equilibrium, and flame photometry) of the samples and of selected additional material from within 30 cm of the sample if such material (such as rocks) likely differed in radioactivity from the samples. Where layering of strata or sediment vis-à-vis rocks was apparent, gradients in the gamma dose rate were employed following Aitken (1985:appendix H). Copper (99.999% pure) dosimeter capsules containing CaSO4:Dy (from Teledyne Isotopes) were also left at sample locations for 1.09 years, although dosimeters from the north profile were never retrieved. The copper was of sufficient thickness to exclude beta doses, so only gamma and cosmic irradiation was absorbed. Thermoluminescence from the CaSO4:Dy was calibrated against a laboratory beta source (with a low-dose rate achieved by keeping the shutter closed) to determine the gamma and cosmic dose rates. Cosmic radiation dose rates were independently calculated after Prescott and Hutton (1988). The resulting values were then divided by 3 to approximate attenuation due to the configuration of the rock shelter, taking into consideration the height and width of the shelter opening, the thickness of overburden on top of the shelter, the burial depth, and the distance of the sample from the drip line. On the basis of current assessments, the moisture contents, as ratio of water to dry sediment weight, were estimated at 0.10 ⫾ 0.04 for the two deepest samples (UW1384 and UW1385) and 0.06 ⫾ 0.03 for all others.

Equivalent Dose Luminescence was measured on a Risø TL-DA-15 reader with single-grain attachment. Equivalent dose (De), which is a measure of the total absorbed dose through time, was determined using the single-aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000; Wintle and Murray, 2006). Parameters are given in Table V. An age is the quotient of De and the dose rate. It is only the De that is measured at single-grain resolution. Dose rates are measured on the bulk sample. A principal reason for using single-grain analysis is to evaluate the integrity of the stratigraphy by identifying the mixture of different-aged grains. To do this, other sources of variation in De among grains must be controlled. Some of this variation is simply statistical due to the differential precision in obtaining De from grains with different luminescence sensitivity. The common age model and central age model of Galbraith (Galbraith et al., 1999, 2005) are often used statistical tools in evaluation of De distributions. These models are used in reference to De and not “age” per se, although dividing the De values by the bulk dose rate provides an “age” for each grain (not accounting for differential dose rates for individual grains). De distribution is GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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FEATHERS ET AL. Table V. Single-grain OSL measurement parameters. System: Risø TL-DA-15, Single-Grain Attachment Excitation: 532 nm laser (90% of 50 W/cm2) Detection filters: 7.5 mm U340 (ultraviolet) Preheat: 240°C 10 s Cut heat: 160°C or 200°C Test dose: 3 Gy Exposure: 0.8 s at 125°C Analysis: 0.06 s, background 0.65–0.8 s Irradiation source: 90Sr delivering ⬃ 0.1 Gy/s to quartz SAR sequence Dose (Di, where i ⫽ 0 for natural signal) Preheat OSL (Li) Test dose Cut heat OSL (Ti) Repeat steps for i ⫽ different regeneration doses: commonly 20, 10, 30, 40, 50, 0, 20 Gy. For each sample, steps with regeneration doses of 15 and 25 Gy were added but with a 40 s IR (880 nm) exposure at 125°C prior to the OSL (Li) step.

implied in usage of these terms in this paper. The common age model controls for differential precision by computing a weighted average using log De values. The central age model is similar except rather than assuming a single true value, it assumes a natural distribution of De values, even for single-aged samples, because of nonstatistical sources of variation. It computes an overdispersion parameter (sb) interpreted as the relative standard deviation (or coefficient of variance) of the true De values or the deviation beyond what can be accounted for by measurement error. Empirical evidence suggests that sb of between 10% to 20% are typical for singleaged samples (Olley et al. 2004; Jacobs et al., 2006). Another source of variation in De is instrumentation error. We have included measured 2% systematic error in all luminescence measurements to account for error in reproducibility. Other instrumentation error arises from the variation in the calibration of the laboratory beta source for different grains. A number of laboratories have found that the calibration of the laboratory beta source varies across the disks that contain the single-grains (these disks have a 10 ⫻ 10 grid of small holes in which the grains are placed) (Ballarini et al., 2006). In our machine the calibration varies by a factor of 2 (Figure 10), and in converting the De values from seconds of beta irradiation to Gy, the average calibration for the horizontal row of holes in which a particular hole is located was used (coefficient of variance along each row was less than 3.2%, and averaging smoothed some of the noise). Taking into account differential calibration does not affect the central tendency of the distributions (because the differences average out) but does affect the amount of overdispersion, particularly for samples with lower relative overdispersion. In a subset of grains from one 416


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Figure 10. Density plot of beta source calibration. Numbers on axes represent holes along the horizontal and vertical grid of the single-grain disks for Risø TL-DA-15 reader. Values are in Gy/s.

sample (UW1384), assuming a uniform calibration produced sb of 20.2%, while applying differential calibration reduced sb to 11.6%. In another sample (UW851), overdispersion was not significantly reduced—only from 45.1% to 41.7%—probably because other causes of overdispersion predominate. A third source of variation in De is the presence of grains that do not meet the assumptions of the SAR protocol. Grains may be unsuitable for dating for a variety of reasons. A large number simply do not have a measurable signal. Others may be contaminated with feldspar inclusions, which may have reduced De values because of anomalous fading. Still others might produce inaccurate De values because the signal is dominated by slowly bleaching components. An advantage of single-grain dating is the opportunity to remove from analysis grains with unsuitable characteristics by establishing a set of criteria grains must meet. In this study, grains were eliminated from analysis if they 1.

2.

3. 4. 5.

had poor signals (as judged from errors on the test dose greater than 30% or from net natural signals not at least three times above the background standard deviation), did not produce, within 20%, the same signal ratio (often called recycle ratio) from identical regeneration doses given at the beginning and end of the SAR sequence, suggesting inaccurate sensitivity correction, yielded natural signals that did not intersect saturating growth curves, had a signal larger than 10% of the natural signal after a zero dose, produced a zero De (within 1 sigma of 0), or GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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6.

contained feldspar contaminates ( judged visually on growth curves by a reduced signal from infrared stimulation before the OSL measurement [Duller, 2003]); done on two doses to lend confidence the reduction in signal is due to feldspar contamination).

The recycling ratio threshold of 20% is higher than normally used, but allowed a larger sample size without significantly affecting results. For example, for a subset of UW1384, using a 0.9–1.1 window resulted in a central age value of 37.3 ⫾ 0.9 and an overdispersion of 9.9 ⫾ 3.2% (n of 96), while a 0.8–1.2 window resulted in a central age value of 37.7 ⫾ 0.8 and an overdispersion of 11.6 ⫾ 2.8% (n increased to 113). Of more than 7600 grains measured for all samples, only 22% were acceptable (Table IV). The largest number of rejections, other than those due to poor signal, were those where the natural signal did not intersect a saturating growth curve. This phenomenon is thought to be related to large laboratory dose rates and is mainly a problem for grains close to saturation (Bailey et al., 2005). Two-thirds of these in this study came from the three oldest samples, which because of them, had an acceptance rate (Table IV) less than the others. Beyond these various factors and removal of unsuitable grains, there is still another source of variation in De values among single-aged single grains. This relates to the fact that the analysis of De is at single-grain resolution, but evaluation of dose rate is only at bulk sample resolution. Grains may be the same age but have different De values because they experienced different dose rates, primarily because of heterogeneity in the distribution of relatively short-ranged beta radiation. Most of the radioactivity in the sample probably stems from the fine-grained pelite. Limestone contains few radioactivity impurities, and to the extent limestone rocks are distributed unevenly in the sampling area, grains close to limestone rocks will receive less dose than those grains further away (Nathan et al., 2003). If all these sources of variation can be controlled, any further overdispersion can be attributed to grains of different ages, either because of post-depositional mixing or partial bleaching at the time of deposition. Galbraith et al. (1999) recommended a minimum-age model for partially bleached deposits, but this is not used here because partial bleaching is not considered a major problem, as discussed below. For analysis of post-depositionally mixed sediments, Galbraith (1988; Roberts et al., 2000; Jacobs et al., 2006) has proposed a finite mixture model, a statistical method that uses maximum likelihood to separate the grains into single-aged components on the basis of the input of a given sb value and the assumption of a log normal distribution of each component. The model estimates the number of components, the weighted average of each component, and the proportion of grains assigned to each component. The model provides two statistics for estimating the most likely number of components, maximum log likelihood (llik) and Bayes Information Criterion (BIC). The latter was used in this analysis, although the conclusions would not have differed had llik been used (see Jacobs et al., 2008a). Roberts et al. (2000) (see also Jacobs et al., 2006) found that the model successfully isolated the correct components of a synthetic mixture of known dosed grains, provided the overdispersion for any particular component is not different from others due to 418


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intrinsic luminescence characteristics, the example given of recuperation (signal after zero dose, caused by the preheat) in low-dose components. We tested for recuperation by first giving a laser bleach (100 s at 45 W/cm2 at 125°C) on 100 grains each from four samples. The SAR protocol was then applied with the expectation of 0 De if no recuperation was present. Ninety-one grains passed the above criteria (with the exception of the zero-dose criterion), and the weighted average De for any one sample was not significantly different from 0. Seventy-three percent of all grains for all samples yielded De values within 1 sigma of 0, and 95% within 2 sigma. This information plus the low rejection rate for grains other than those with little sensitivity suggest that the finite mixture model will not produce significantly biased results. We also tested the sufficiency of the 240°C preheat employed. Single-grain analysis of UW1384 was performed using four different preheats: 170°C, 220°C, 240°C, and 260°C, all with a 10-s hold at the maximum temperature. At least 40 grains were acceptable from each preheat. Resulting central age De values (Gy) were 36.1 ⫾ 2.3, 33.2 ⫾ 1.8, 37.7 ⫾ 0.8, and 37.2 ⫾ 2.1 Gy for the respective temperatures. Corresponding overdispersion values were 31.0 ⫾ 5.4, 27.6 ⫾ 4.7, 11.6 ⫾ 2.8, and 22.3 ⫾ 5.8 and average recycle ratios were 1.01 ⫾ 0.04, 0.97 ⫾ 0.03, 1.00 ⫾ 0.02, and 0.99 ⫾ 0.03. Except for somewhat lower De values at the 220°C preheat, and lower overdispersion for 240°C, the differences are not significant, nor is any trend detected of increasing De with increasing preheat, as might be expected if any preheat-caused transfer of charge into the main OSL trap was occurring. A final test of procedures is an attempt to recover a known dose. Grains from several samples are initially bleached and then given a laboratory dose. The SAR protocol is applied next to see if the known dose can be derived. Because the applied dose is the same for all grains in this situation, any overdispersion must be attributed to other causes than dose rate heterogeneity or grains of different ages. Some 200 grains from each of six samples were bleached (with the green laser for 100 s at 45 W/cm2 and at 125°C to avoid phototransfer into shallow peaks) and then given a 200-s beta irradiation with a 90Sr beta source delivering 0.1 Gy/s. Table VI shows that the adopted protocol seems to be working in terms of both central tendency and the number of grains with De values consistent with 200 s at 1 or 2 sigma. Overdispersion is zero overall or small for individual samples, suggesting that most overdispersion in the natural samples is due to causes related to depositional or post-depositional conditions (e.g., dose rate heterogeneity, mixing of grains of different ages, or insufficient bleaching prior to burial). The lack of overdispersion in the dose recovery of these samples is unusual. Tests using the same parameters and the same machine on samples from a nearby rock shelter, Boleiras, produced relatively high overdispersion (Araujo et al., 2008). The reasons for this discrepancy are not clear to us. LUMINESCENCE RESULTS Dose Rate Table VII gives information relevant to dose rate from laboratory measurements for each sample as well as for limestone rocks and other strata that contribute to the gamma dose rate of some samples. Total dose rates are also given. There is substantial GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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FEATHERS ET AL. Table VI. Dose recovery results 200-s beta irradiation.

Sample

# of Accepted Grains

UW850 UW851 UW852 UW1384 UW1385 UW1386

55 42 28 26 28 24 203

Total

Beta Irradiation Time Based on Central Age Model (s)*

Over-dispersion, sb

# within 1 Sigma (%)

210 ⫾ 7 203 ⫾ 7 206 ⫾ 10 206 ⫾ 9 198 ⫾ 7 185 ⫾ 7

0.06 ⫾ 0.07 0.08 ⫾ 0.06 0 0 0 0

46 (84) 33 (79) 24 (86) 24 (92) 26 (93) 14 (58)

202 ⫾ 3

0

167 (82)

# within 2 Sigma (%) 55 (100) 39 (93) 27 (96) 26 (100) 28 (100) 18 (75) 193 (95)

* Doses are given in terms of seconds of beta irradiation. The source delivers about 0.1 Gy/s.

variation from sample to sample, reflecting in part differential proximity to rocks and shelter wall and differential intrinsic limestone content, although in terms of the latter, dose rates were only weakly dependent on carbonate content. A regression yielded an R 2 of only 0.3 for the total dose rate and .46 for the beta dose rate, which is more relevant because of the short ionization range of beta irradiation and the limitation of the carbonate determination to the size of the sample collected for dating.2 Results from in situ dosimetry from the south profile have low precision because of some uncertainty due to the travel control dosimeter being zeroed some time after the other dosimeters were retrieved. They are nevertheless consistent within 1 sigma of the laboratory measurements except for the one associated with UW852. This dosimeter gave a slightly higher external dose rate than the laboratory measurement, although within 2 sigma, suggesting some inhomogeneity in the gamma ionization sphere of this sample. Because of low precision, the dosimeter results were not used in age calculations. Equivalent Dose Figure 11 gives examples of decay curves and corresponding growth curves on four grains, two from UW1384 and two from UW851. With nearly 1700 grains with acceptable signals, it is difficult to claim these curves are representative, but there were many curves like these. Two of them (a and c) have very sharp decays typical of a grain dominated by a fast bleaching component. The other two (b and d) have somewhat more gradual decays, indicating the presence of a slower component, although the fast component still dominates. Table VIII gives the equivalent dose as determined by the central age model as well as the overdispersion value. The latter varies from sample to sample, being lowest 2. Major differences in Th content apparent in Table VII are probably not too meaningful. The U and Th contents were determined using the pairs technique in alpha counting, a technique that can lead to large errors in the relative proportions of the two, but not in the total contribution to the dose rate. The differences in total dose rate are more meaningful.

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HOW OLD IS LUZIA? LUMINESCENCE DATING Table VII. Dose rate data. %K

Total Dose Rate* (Gy/ka)

18.23 ⫾ 2.02 7.90 ⫾ 1.23 24.62 ⫾ 2.19 26.67 ⫾ 2.30 9.74 ⫾ 1.19 15.09 ⫾ 1.84 10.80 ⫾ 1.38 10.26 ⫾ 1.51 20.20 ⫾ 2.14

0.51 ⫾ 0.02 0.49 ⫾ 0.01 0.62 ⫾ 0.03 0.56 ⫾ 0.01 0.66 ⫾ 0.03 0.53 ⫾ 0.01 0.52 ⫾ 0.02 0.51 ⫾ 0.06 0.60 ⫾ 0.01

3.07 ⫾ 0.14 1.93 ⫾ 0.09 3.42 ⫾ 0.15 3.68 ⫾ 0.16 2.38 ⫾ 0.10 2.38 ⫾ 0.18 2.33 ⫾ 0.10 2.61 ⫾ 0.12 3.34 ⫾ 0.14

3.94 ⫾ 0.25

6.56 ⫾ 1.10

0.82 ⫾ 0.02

1.43 ⫾ 0.09 0.54 ⫾ 0.07 4.71 ⫾ 0.33

0.15 ⫾ 0.19 2.52 ⫾ 0.05 15.02 ⫾ 1.59

0.00 ⫾ 0.01 0.69 ⫾ 0.02 0.51 ⫾ 0.04

Sample (stratum)

238

UW850 (A) UW851 (C) UW852 (B) UW853 (A) UW1384 (C) UW1385 (C or A) UW1386 (B) UW1387 (A) UW1420 (A) Additional Measurements

6.68 ⫾ 0.44 4.27 ⫾ 0.27 5.40 ⫾ 0.42 6.35 ⫾ 0.47 5.76 ⫾ 0.34 6.78 ⫾ 0.43 5.26 ⫾ 0.33 6.76 ⫾ 0.40 7.13 ⫾ 0.47

Slightly different colored sediment below UW1384 Limestone wall near UW1385 Limestone rock near UW1420 Grayish level near UW1420

U (ppm)

232

Th (ppm)

*Total dose rates were based on the given concentrations, derived from alpha counting and flame photometry, assuming secular equilibrium, plus cosmic contribution (see text). The bulk of the dose rate is contributed by beta and gamma radiation. Small alpha contribution has been adjusted using a b-value of 1.0 ⫾ 0.5 (Gy mm2). Gamma dose rates for the relevant samples were adjusted to take into account the additional measurements listed, using gradients for strata of different radioactivity, using Aitken (1985:appendix H). Beta dose rates were also determined by beta counting, but these did not differ significantly for any sample from beta dose rates derived, assuming equilibrium, from alpha counting and flame photometry. This and the agreement of the dosimeters with laboratory measurements for gamma dose rates are taken as evidence for secular equilibrium in the samples. The beta-counting results (not shown) were therefore not used in the calculation of the total dose rate.

for the two deepest samples, UW1384 and UW1385, farthest under the overhang. These samples are beyond extensive ant turbation, although other reasons may contribute to lower overdispersion as well. They both had relatively low limestone content (10% and 15% carbonate content respectively), for example, although overall abundance may not affect overdispersion as much as the size and spatial distribution of the limestone particles (Nathan et al., 2003). Table VIII also gives the number of components derived from the finite mixture model when overdispersion is assumed to be zero. Three of the samples, UW1384, UW1385, and UW1420, have only two components when overdispersion is zero. UW1385 has the lowest measured overdispersion at 13.3 ⫾ 2.6%. The two components are near in proportions (59 ⫾ 19% to 41 ⫾ 19%) and close in average value (31.1 ⫾ 1.6 to 39.6 ⫾ 2.0). Moreover, if the assumed overdispersion for a single-aged sample is 3% or higher, UW1385 is statistically consistent with a single component by the finite mixture model. Because natural overdispersion values for single-aged samples is commonly greater than 3% (e.g., Galbraith et al., 2005), we make the assumption that the 13% overdispersion of UW1385 is consistent with a single-age distribution. While allowable overdispersion for a single age might vary from sample to sample, depending in part on the content and distribution of limestone, the 13% is used here as a benchmark to judge the likelihood a GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

421

Figure 11. Decay and growth curves for four grains, two from UW1384 and two from UW851. The decay curves (luminescence versus time) are for the natural signal. The point of initial rise marks when the stimulating light from the laser was turned on. Rapid decay is shown in (a) and (c), somewhat slower decay in (b) and (c). The growth curves plot luminescence against regeneration dose. The natural signal appears on the y-axis. A horizontal line from it intersects the growth curve at the equivalent dose value.

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sb (%) 32 ⫾ 2 42 ⫾ 3 23 ⫾ 2 26 ⫾ 2 15 ⫾ 2 13 ⫾ 3 18 ⫾ 3 31 ⫾ 3 18 ⫾ 3

De (Gy) Central Age Model

16.3 ⫾ 0.4 35.8 ⫾ 1.3 21.9 ⫾ 0.5 18.7 ⫾ 0.5 38.1 ⫾ 0.7 34.3 ⫾ 0.7 21.5 ⫾ 0.5 17.3 ⫾ 0.6 26.3 ⫾ 0.6

Sample

UW850 UW851 UW852 UW853 UW1384 UW1385 UW1386 UW1387 UW1420

4 5 3 4 2 2 3 3 2

# Components when sb ⫽ 0 3 4 3 3 1 1 1 3 1

# Components when sb ⫽ 13

Table VIII. Equivalent dose, central age, and finite mixture models.

2nd 3rd 2nd 2nd 1st 1st 1st 2nd 1st

Most Common Component 76.5 63.4 75.1 57.5 100 100 100 95.6 100

Percentage of Most Common Component (%)

16.0 ⫾ 0.4 44.1 ⫾ 2.0 21.1 ⫾ 0.9 16.7 ⫾ 1.1 38.1 ⫾ 0.7 34.3 ⫾ 0.7 21.5 ⫾ 0.5 17.0 ⫾ 0.4 26.3 ⫾ 0.6

De (Gy) of Most Common Component

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sample represents a single age. This is similar to the value of 12% obtained by Jacobs et al. (2006) for some presumably single-aged samples from South Africa. To judge potential error from an inaccurate overdispersion value, Table IX gives the number of components and the De of the most common component when the finite mixture model is applied using sb values for single-age components of 8%, 13%, and 18%. Significant differences in De of the most common component are only present for UW853 and UW1420. Using the 8% value, UW1386 and UW1420 gain a component, but in the case of UW1386 it does not significantly change the value of the dominant component. Using the 18% value, UW851, UW853, and UW1387 lose a component. In the case of UW851 and UW1387, the component lost accounts for 5% or less of the grains. For UW853, two components are combined, changing the value of the dominant component significantly. We conclude the choice of sb allows considerable latitude. Over the 10% range considered here, significant effects are present only for UW853 and UW1420. The number of components detected might in part be a function of sample size. One might expect the number of components to increase, analogous to increases in sample richness, as sample size increases. We modified the finite mixture model program to include a bootstrapping routine, which involved repeated sampling while increasing sample size from some small amount to the full available sample. Table X shows the detected number of components as the sample size increases for all samples. Most samples, with perhaps the exception of UW851, seem to be holding steady in terms of number of components after about half the available sample is achieved, although we cannot exclude the possibility that additional components might be resolved with larger samples. The results of this test can be interpreted as a matter of resolution; that is, smaller sample size will produce fewer components with lower precisions (R. Roberts, personal communication, 2008). Table VIII gives the number of components from the finite mixture model and the equivalent dose of the most common component for all samples, using an overdispersion value of 13% as representative of a single-age component. Four of the samples, all from the north or central profile, appear as single component, with the fifth from those profiles having 95.6% of the grains assignable to one component. More heterogeneity is present in the south profile samples, with two samples having only about 60% of the grains assignable to the most common component. Figure 12 shows radial graphs (Galbraith et al., 1999) of three samples. The construction of the graphs is explained in the caption. Figure 12a shows UW1385, where all grains are compatible with a single component. Figure 12b shows UW852, where 75% of the grains are assignable to one component, and Figure 12c shows UW851 where 63% of the grains are assignable to the most common component (44 Gy). For UW851, a second reference showing the second most common component (27 Gy) is also shown. Causes of Overdispersion For the north and central profile samples, where the De distributions are consistent with a single age (or nearly so in the case of UW1387), the De from the central 424


3 4 3 3 1 1 2 3 2


# of Components 3 4 3 3 1 1 1 3 1

De of Most Common Component 15.9 ⫾ 3.4 44.8 ⫾ 1.9 20.7 ⫾ 0.6 16.4 ⫾ 0.8 38.1 ⫾ 0.7 34.3 ⫾ 0.7 19.4 ⫾ 3.5 16.8 ⫾ 0.4 22.5 ⫾ 1.2

16.0 ⫾ 0.4 44.1 ⫾ 2.0 21.1 ⫾ 0.9 16.7 ⫾ 1.1 38.1 ⫾ 0.7 34.3 ⫾ 0.7 21.5 ⫾ 0.5 17.0 ⫾ 0.4 26.3 ⫾ 0.6

De of Most Common Component

sb ⫽ 0.13

3 3 3 2 1 1 1 2 1

# of Components

16.2 ⫾ 0.5 47.2 ⫾ 2.8 22.6 ⫾ 3.7 19.2 ⫾ 0.4 38.1 ⫾ 0.7 34.3 ⫾ 0.7 21.5 ⫾ 0.5 17.6 ⫾ 0.4 26.3 ⫾ 0.6

De of Most Common Component

sb ⫽ 0.18

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*For those samples where only one component is present, the De is calculated using the central age model.

# of Components

Sample

sb ⫽ 0.08

Table IX. Number of components and equivalent dose (Gy) as a function of overdispersion.*

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FEATHERS ET AL. Table X. Bootstrapping results. # of Components* Sample

N

N/9

2N/9

N/3

4N/9

5N/9

2N/3

7N/9

8N/9

N


269 165 211 165 189 157 166 178 185

2 2 2 2 1 1 1 2 1

2 2 2 2 1 1 1 2 1

3 3 2 2 1 1 1 2 1

3 3 2 2 1 1 1 2 1

3 3 3 3 1 1 1 2 1

3 3 3 3 1 1 1 3 1

3 4 3 3 1 1 1 3 1

3 4 3 3 1 1 1 3 1

3 4 3 3 1 1 1 3 1

*Each column represents the number of components for different sample sizes: 1/9 to 9/9 of N.

age model (or in the case of UW1387 the average value of the dominant component) is appropriate for determining the age. For the south profile samples, where the distributions are not consistent with a single age, it is more difficult to select an appropriate De. The multiple components reflect either mixing of differently aged grains or an underestimation of the overdispersion relevant to a single-grain distribution. This latter would be the case if heterogeneity in the beta dose rate were greater for the south profile samples. As mentioned, the most likely cause of heterogeneity is the limestone content, although the range of carbonate concentrations in the south profile (10.9% to 40.2%) does not differ greatly from the range in the north-central profile (4.2% to 34.0%). Overdispersion of all samples is only weakly dependent on carbonate content (R2 ⫽ 0.37), although again it may be the size and spatial distributions that are more important than overall abundance. Nevertheless, we attempted to model beta dose heterogeneity by assuming that grains next to limestone pieces would experience only half the beta dose rate as those grains some distance away (Nathen et al., 2003; Jacobs et al., 2008b). Calculating the age of the lowest component of these samples using such a reduced dose rate, however, still significantly underestimates the age compared to the age of the most abundant component assuming the full dose rate for all four south profile samples (Table XI). This is even the case when assuming a beta dose rate of zero for the low component. The presence of this low component then cannot be accounted for by variation in dose rate. The middle component is the most abundant for UW850, UW852, and UW853. Assuming only half the beta dose rate when calculating the age of this component, however, does bring it into agreement with the age of the third, higher but less abundant component calculated using the full dose. However, it does not seem likely that the majority of the grains would be close enough to limestone pieces to have significantly reduced dose rates (compared to the bulk average) and only a minority experiencing the full dose rate, particularly given that many quartz grains are found embedded in pelite granules (see micromorphology discussion in Geoarchaeological Studies.) Another possibility is that the high component in these samples consists of grains that experienced higher than average dose rates by being 426


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Figure 12. Radial graphs for three samples: (a) UW1385, (b) UW852, and (c) UW851. Radial graphs plot precision as a function of equivalent dose, normalized by the number of standard deviations from a reference point, in this case the equivalent dose of the most common component from the finite mixture model, or for UW851 the two most common components, the second at 27 Gy and the third at 44 Gy (see text). The shaded area encompasses all points within two standard deviations of the reference. A line drawn from the origin through any point intersects the vertical scale to the right at the calculated equivalent dose for that point.

located near radioactive hotspots, the most likely candidates being K-feldspars, 40K being a major beta contributor (Mayya et al., 2006). However, K-feldspars are scarce in these sediments, and the bulk of the beta dose rate probably stems from the clay particles in the pelite, which would provide a much more homogeneous dose rate. In sum, while dose rate variation cannot be ruled out completely, the most likely cause of the multiple components in the south profile samples is mixture of differently aged grains (either from partial bleaching at the time of deposition or from post-depositional processes). For UW850 and UW852, the De from the main component (consisting of more than 75% of the grains) probably is the best estimate for GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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FEATHERS ET AL. Table XI. Luminescence ages recalculated assuming different dose rates. Age (ka) Sample

Dominant Component Full Beta Dose Rate

UW1850 UW1851 UW1852 UW1853

5.2 ⫾ 0.3 22.8 ⫾ 1.7 6.2 ⫾ 0.4 4.5 ⫾ 0.4 Middle Component Half Beta Dose Rate

UW1850 UW1851 UW1852 UW1853

6.8 ⫾ 0.4 21.0 ⫾ 2.0 7.9 ⫾ 0.5 5.8 ⫾ 0.5

Lowest Component Half Beta Dose Rate 2.9 ⫾ 0.3 7.7 ⫾ 0.9 1.7 ⫾ 0.3 0.8 ⫾ 0.2

Lowest Component Zero Beta Dose Rate 4.2 ⫾ 0.5 11.6 ⫾ 1.5 2.3 ⫾ 0.4 1.1 ⫾ 0.3

High Component Full Beta Dose Rate 8.5 ⫾ 0.7 22.8 ⫾ 1.7 8.3 ⫾ 1.1 6.3 ⫾ 0.6

determining the age. For UW853, using the De of the main component is less secure because it represents only 57% of the grains, and a higher component represents another 41%. UW851 is the only sample with four components. It does have the highest carbonate content of all samples, 40.2%, and assuming a reduced dose rate for the second component (24.2% of all grains) does bring the age of that component into agreement with that of the third component (63.4% of all grains) using the full-dose rate. The sample also has a much younger and a much older component, neither of which can be accounted for by beta heterogeneity. The ages of the second and third components are discussed later. Partial Bleaching One possibility accounting for multiple components is partial bleaching. Many quartz grains are coated with fine-grained material, perhaps sufficient to prevent full bleaching. One way to address this problem is to determine De for different parts of the OSL signal (Singarayer and Bailey, 2005). The overall OSL signal is a composite of signals that are differentially affected by exposure to sunlight. The SAR protocol assumes the signal is dominated by a fast bleaching component, but mediumand slow-bleaching components are known as well (e.g., Jain et al., 2003), and different grains may contain different proportions of these signals. While OSL curves can be resolved into individual components by sophisticated curve fitting, a simpler method for separating components, at least roughly, is with linear modulated OSL (LM-OSL) (Buhur et al., 2002; Singarayer et al., 2004). Conventional OSL (called continuous wave OSL [CW-OSL]) is measured using a constant stimulating wavelength at a constant power. LM-OSL varies the wavelength or, more commonly, the power (Bulur, 1996). LM-OSL was measured here on 100–200 grains each from three samples 428


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Figure 13. LM-OSL curves for three different grains from UW851. The luminescence is plotted as a function of laser power in terms of percentage of maximum power (50 W/cm2). The power was linearly increased over 30 s. The solid line represents a luminescence signal dominated by the fast component. The dotted line represents a signal with a fast component but dominance by a slower component. The dashed line represents a signal dominated by the fast component, but containing a significant slower component.

Table XII. Linear modulated OSL results. Sample # measured grains using LM-OSL # grains with fast component # grains with slow component # grains with both components Central age De for fast component (Gy) Central age De given in Table VI (Gy)* Central age De for slow component (Gy)

UW850

UW851

UW1384

200 70 4 3 17.3 ⫾ 1.2 16.3 ⫾ 0.4 50.1 ⫾ 8.6

200 77 26 17 34.7 ⫾ 2.3 35.8 ⫾ 1.3 12.4 ⫾ 3.0

100 21 1 1 29.7 ⫾ 2.4 38.1 ⫾ 1.0 76.3 ⫾ 131.5

*Central age for conventional OSL is calculated for a different set of grains.

by increasing the laser power from 0% to 90% (of maximum 50 W/cm2) at a linear rate for 30 s. The early part of this signal will be dominated by the fast-bleaching component, while the latter part of the signal will be dominated by slower components. We calculated De, using SAR, for the first and last 5 s of the LM-OSL signal, using a second 30-s LM-OSL exposure for background (David et al., 2007). Some curves are shown in Figure 13 for UW851. Two observations can be made about the results given in Table XII. First, the luminescence from most grains is dominated by the fast component. The slow component was detected for only a few grains, and most of these with rather poor precision because of a small signal. Both a fast and slow component could be measured on only 21 grains (17 of them from UW851), and on 18 of these, the slow component produced a De that was statistically equivalent (within 1 s) GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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to or less than that of the fast component. The large number of small De values obtained for the slow component (particularly UW851) could be an artifact of low signal to noise ratios, where actual noise is mistaken for a small signal. Second, the central age De for LM-OSL fast component was for two of the samples equivalent to that from conventional OSL (which was measured on a different set of grains, those given in Table IV). It was somewhat smaller for UW1384. The influence of a poorly bleached slow component on the signal of grains without the components separated could increase the central age value of conventional OSL when compared with the fast component of LM-OSL. This is not observed for UW850 or UW851, but it is the case for UW1384. However, only one grain from those measured for this sample even had a measurable slow component (and with very poor precision). The low overdispersion for this sample using CW-OSL would also not be expected if some grains were poorly bleached. The reason for discrepancy here is not clear, but the smaller sample size for LM-OSL (n ⫽ 21) as compared to CW-OSL (n ⫽ 189) is worth noting. While comparison of fast with slower components provides an argument against partial bleaching, it is not sufficient, for even the fast component can be partially bleached. With poor or no bleaching, both the fast and slower components could still yield the same De. However, the south profile sediments were deposited in much the same way as the north profile ones, and in both cases the quartz was embedded in clay particles. The single-component distributions of the north profile samples suggest no partial bleaching. It is unlikely that could be the explanation for the south profile ones. Age Table III gives the ages based on the selected finite mixture component. Two ages are given for both UW851 and UW853, the samples displaying the greatest mixing. The ages of the most common component for each sample are in correct stratigraphic order, although there is a discrepancy between the north and south profiles. Samples at the same depth are older in the north profile than in the south profile. This might be a function of the slope of the sediments from north to south. The rock shelter floor does follow such a slope. It might also suggest a greater amount of mixing in the south profile samples. For example, UW851, from stratum C, has a less abundant component that yields an age of 13.8 ka, more in line with the samples from stratum C (or A-C interface) on the central profile which dates from 12.7 to 16 ka. UW851 even has some older grains, but an older component is absent in the central profile samples. Samples S5 and S6 from stratum B just above where UW851 was sampled revealed in micromorphological analysis the presence of yellowish granules that embed what may be older quartz grains. Stratum C where UW1384 was collected was not sampled for micromorphology, but here stratum C was far under the overhang, close to the bottom of the deposits and to the sink. Older deposits may not be as well preserved here as they are farther out from the back wall, such as where UW851 was collected. UW853 also appears highly mixed and has an older component yielding an age of 6.3 ka, which is similar to an analogous sample on the northern side that dates to 430


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6.5 ka. This older age, however, puts it out of stratigraphic order with the less mixed UW850 and UW852. While earlier discussion concluded that mixing was a better explanation for the overdispersion in the south profile samples than beta heterogeneity, the latter may still have some influence. The central age model, which assumes a single-aged sample and would average out differences in dose rate, does not, however, change the picture much for the upper three samples, although it does produce an age of 18.5 ⫾ 1.3 for the sediment from stratum C, again more in line with the samples from the central profile. The bulk sample dose rates are also more variable on the south profile than on the north profile; this reflects a more complicated environment. Part of the problem could therefore be an overestimation of the dose rate for the top three samples (and the in situ dosimetry information was not precise enough to resolve this) and an underestimation for UW851. Finally, interpreting the mixing in these samples may be complicated by the smaller grain size used, although the reanalysis of UW851 using 180- to 212-mm grains does not suggest this. Some 1500 180- to 212-mm grains were analyzed for UW851. There were only 43 acceptable grains, compared to 165 for the smaller grains. The overdispersion for the larger grains was the same (45%) as that for the smaller. The larger grains did divide into three components instead of four, but the De (36.2 ⫾ 2.2 Gy) and percentage of the largest component (52%) were not much different from those of the smaller grains (44.1 ⫾ 2.0 and 63%). The one less component in the larger grain size could well be the result of smaller sample size, since Table X shows that for a sample size around 43, only three components were resolved for the smaller grain size. To look at the sample size issue in more detail, the 165 acceptable grains for the 125- to 150-mm fraction of UW851 were resampled independently nine times, each sample totaling 43 to match the 180- to 212-mm sample size. The finite mixture model was run on all nine. The number of components resolved ranged from 1 (n ⫽ 1) to 3 (n ⫽ 5). The average De of the component with the highest proportion of grains ranged from 34.8 to 48.0 Gy (compare with 36.2 Gy for the 180- to 212-mm fraction). Most had a second component in the range 19.4 to 30.2 Gy (compare with 17.6 Gy for the 180- to 212-mm fraction). The other components were either high (53.1 to 79.5 Gy), or low (4.8 to 12.4 Gy) (compare with 8.0 Gy for the 180- to 212 mm fraction). The four components of the full 165-grain sample of the 125- to 150 mm fraction are 11.2, 27.1, 44.8, and 72.2 Gy. All of the subsamples are sampling a portion of the larger sample, and the 180- to 212-mm fraction is no different in this regard from the 125- to 150-mm subsamples. There is thus no clear evidence of any “phantom” components and no differences between the two grain-sized fractions that cannot be accounted for by sample size. This exercise does show how much variation in number of components and component values can be obtained with small sample sizes. Nevertheless, for all these reasons, the north and central profile samples are less problematic. They do not indicate any broad-scale mixing, in line with the micromorphological observations. If Luzia, whose skull was recovered near the central profile, was located somewhere near the interface of strata A and C, the best date for that interface is between 16.0 (stratum C) and 12.7 (stratum A or A-C interface) ka, GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4

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fully consistent with the radiocarbon bracketing of 11.4–16.4 ka. The date of 9.2 ⫾ 0.6 ka (UW1386) for the anthropogenic stratum B on the north profile, which is stratigraphically above both UW1384 and UW1385, also suggests that Luzia is at least that old and it is possible she is related to that occupation. Some parts of the skeleton were recovered from shallower depths, more in line with the depth of UW1386. This would put the age of Luzia more in line with other early skeletons in the Lagoa Santa region. Nevertheless, we cannot discount an older age. The age of the sloth is not resolved by our data. The radiocarbon date associated with the sloth is older than stratum A deposits of the same depth from either profile (5.2 ka on the south and 7.9 ka on the north). This and the wider dispersion of grain ages from at least two of the samples from the south profile suggest that some localized mixing is present among the deposits and that the sloth remains are likely intrusive. Nevertheless, the overall results suggest that the stratigraphy, at least in the central and northern profiles, does have broad integrity. Pinning down a more precise age of Luzia does not seem likely unless the bones themselves can be better dated or the provenience better defined, but the results here do confirm a late Pleistocene or very earliest Holocene age for the skeleton.

CONCLUSION Luminescence dating provides a critical complement to radiocarbon dating in Paleoindian studies. Not only does it provide an independent age estimation, but when applied to single grains, it can provide an evaluation of stratigraphic integrity—an important criterion for all early sites—particularly when combined with micromorphological information. The female skeleton, nicknamed Luzia, from Lapa Vermelha IV lays claim as the oldest reported human remains in the New World, but the circumstances of her discovery, the degraded nature of the bones, and the evidence of bioturbation at the site have raised doubts about the reported age based on radiocarbon of associated charcoal (e.g., Schmitz, 2004). While luminescence cannot improve on the resolution provided by radiocarbon (given uncertainties about provenience), it can assess the probability that the association of the skeleton and charcoal is fortuitous. The results presented here demonstrate that the mixing of sediments is not so severe that the stratigraphy in the northern and central portions of the shelter, where Luzia was discovered, is greatly compromised. The best estimates of luminescence dates (12.7–16 ka) support the radiocarbon determinations that have been used to bracket the age of Luzia (11.4–16.4 ka). However, the results also indicate that the rock shelter’s surface sloped from north to south, thus explaining skeletal-elements dispersion as a product of natural skeletal dismembering and sloping along the floor’s inclination. Depth considerations of the various skeletal parts could support a slightly younger age for Luzia of about 9 ka, based on a date from a layer with presumed occupation debris. While questions about Luzia’s provenience prevent sufficient resolution by either radiocarbon or luminescence to demonstrate that Luzia is

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pre-Clovis or even the oldest human remains in the Americas, the luminescence results do confirm that the skeleton is old (late Pleistocene–early Holocene) and, along with the rest of Lagoa Santa’s skeletons, must be taken into account in any understanding of human migration to the New World. This research was funded by the National Science Foundation (Grant # 0407002) and the State of São Paulo Research Foundation (Grant # 99/00670-7). Renato Kipnis also thanks the State of São Paulo Research Foundation for a postdoctoral scholarship (Fellowship # 01/06881-1), and the National Council for Scientific and Technological Development for a research scholarship from CNPq (Fellowship # 300892/2005-5). Luis B. Piló thanks the State of São Paulo Research Foundation for a postdoctoral scholarship (Fellowship # 00/14917-3). We also thank Rex Galbraith for sending us his program for the finite mixture model. This program was rewritten in a format friendlier to nonstatistical researchers by one of us (Coblentz). For Galbraith’s central age model we used a program written by Hiro Yoshida. The single-grain discussion benefited from valuable comments by Richard G. (Bert) Roberts, even if the discussion does not reflect all his good advice. Michel Fontugne of Gif-sur-Yvette laboratory helped track down the radiocarbon dates. Micromorphological analysis was conducted by Manuel Arroyo-Kalin at the McBurney Geoarchaeology Laboratory, University of Cambridge, with useful comments from Charles French. Additional thanks to Walter Neves and Astolfo G. M. Araujo, and to two thoughtful and thorough reviewers, including Daniel Richter.

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Received 11 October 2009 Accepted for publication 14 March 2010 Scientific editing by Andreas Lang

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