Feathers Et Al Luzia

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Geoarchaeology: An International Journal, Vol. 25, No. 4, 395436 (2010) 2010 Wiley Periodicals, Inc.Published online in Wiley Interscience (www.interscience.wiley.com). DOI:10.1002/gea.20316

*Corresponding author; E-mail: [email protected].

How Old Is Luzia? Luminescence Dating

and Stratigraphic Integrity at LapaVermelha, Lagoa Santa, Brazil

James Feathers,1,* Renato Kipnis,2 Luis Pil,2

Manuel Arroyo-Kalin,3 and David Coblentz4

1Department of Anthropology, Box 353100, University of Washington, Seattle,WA. 98195-31002Laboratrio de Estudos Evolutivos Humanos, Sala 244, Departamento deGentica and Biologia Evolutiva, IB/USP, Rua do Mato, 277. Cidade

Universitria, So Paulo, SP, Brazil 05508-9003Department of Archaeology, Durham University, South Road, Durham DH13LE, UK4Comparative Religion Program of the Henry M. Jackson School ofInternational 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 evi-

dence 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 earlysites usually include the application of a chronometric method combined with strati-

graphic integrity (e.g., Haynes, 1969). This is because most chronometric methods

do not date relevant events directly but depend on stratigraphic association or cor-

relation. Luminescence dating, which provides an age for sediment deposition, has


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FEATHERS ET AL.

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4396

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 partic-

ular is important because it addresses the deposition of individual grains and thus com-

bines chronometric dating with an evaluation of stratigraphic integrity. This paperreports 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 researchhas 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 con-

sider 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 processmust 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 Fogaa, 1999; Roosevelt, 2002; Meggers and Miller, 2003]); and in physi-

cal appearance (different skeletal morphology [Neves et al., 2007]).

The University of So Paulo (USP), under the direction of biological anthropolo-

gist Walter Neves, has in the last decade carried out a multidisciplinary researchproject in one portion of South Americathe Lagoa Santa region of central Brazil

to better understand this broader context in one locality. The dating and geoar-

chaeological 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, Brazils 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 archaeologi-

cal and palaeontological record. Study of the region dates to the 1830s, when a


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Ri

b

b

.Jequ

iti

Rio

Jabuticatubas

Rib

od

eir

aMata

Lagoa doSumidouro

LagoaSanta

RIO DA

L

SVE

HAS

MG-424

MG-010

SeteLagoas

SantanadoRiacho

Belo Horizonte

600590580 km E 610 620 630

7.850

7.840

7.830

7.820 km N

7.8604400'

1930'

1945'

Vespasiano

Santa Luzia

So Josda Lapa

Lagoa Santa

Confins

Pedro Leopoldo

Esmeraldas

Ribeiro das Neves

MatozinhosMocambeiro

Fidalgo

Lapinha

Doutor Lund

Capim Branco

Sete Lagoas Baldim

Jabuticatubas

Prudentede Morais

Funilndia

LAPA DO SANTO

CERCA GRANDE VI

LAPA DAS BOLEIRAS

LAPA VERMELHA

5km 0 5 10kmSource:IBGE, Escala 1:50.000,folhas Pedro Leopoldoe Lagoa

Santa;Escala1:100.000, folhasSete Lagoas e Baldim.

MINAS GERAIS

RioS

o

Franc

isco

Rio

dasVe

lh

as

o

o

o

oo 40

15

20

4550 W

S

S

WW

BELOHORIZONTE

Matozinhos

200km

BRAZIL

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 mam-mals, 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).

Lunds claims were not accepted (the association of humans with extinct mam-

mals 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 Hrdlicka, 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 con-sequence of post-depositional mixing (Hrdlicka, 1912). A recent evaluation at

Sumidouro has favored Lunds original interpretation for the antiquity of the human

remains (Pil et al., 2005; Neves et al., 2007), but is equivocal for the association

with extinct fauna.

After Hrdlickas 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 LapaVermelha 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 char-

coal produced bracketing uncalibrated ages of 10,220 and 12,960 14C yr BP

(11.416.4 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 (10.410.6 ka

calibrated). Charred material associated with the sloth produced a conventional

date of 9,580 200 14C yr BP (10.611.2 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 sec-

ondary sources (e.g., Neves et al., 1999; Prous and Fogaa, 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

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HOW OLD IS LUZIA? LUMINESCENCE DATING

399

University of So Paulos research project in the late 1990s. The project has reeval-

uated 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 open-air sites near Sumidouro (e.g., Araujo and Feathers, 2008). The more significant find-

ing 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 radio-

carbon (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 cra-

nia 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 (Neveset 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 there-

fore important to clarify their age, and the USP project has initiated a program of lumi-

nescence 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 shel-

ters an area 50 m long and 7.5 m wide (Figure 2), is most likely the result of col-

lapse 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 accu-

mulation of deposits behind the rocks. At the base of the shelter is a now-

dormant sinkhole.Laming-Emperaires excavations reached 14 m and removed most of the rock shel-

ters 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 pres-

ent purposes, as stratum B.


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Luzia was recovered somewhere near the interface of Strata A and C, in the vicin-

ity of the surviving central baulk. The precise location is not known because some

of Laming-Emperaires 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 exca-

vation, 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 Guimares, 1978).

Cunha and Guimares (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 analo-

gous to that of stratum A inside the bones, they suggested a Holocene age for the

skeleton. Alternatively, the topography of the basins surface at the time may have

formed a slope from north to south, and some of the skeletons elements rolled down

with time, the skullthe roundest piecemoving 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 disarticu-

lated state of the skeleton, raise doubts about the association between Luzia and

the charcoal used to bracket the age.

Laming-Emperaires excavations in the 1970s produced 29 14C assays (Delibrias

et al., 1986), all processed by the Gif-sur-Yvette laboratory in France. Table I liststhe 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 Luzias 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 12ka (uncalibrated radiocarbon years).

FEATHERS ET AL.


B

c

A

3.0m

5.3m

6.2m

5.0m

5.0m

7.5m

7.0m

6.5m

5.5m

Drip lineSouth profile

North profile

Central

profile

Figure 2. Plan view of Lapa Vermelha IV published by Laming-Emperaire (1979) with the current loca-

tions 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.


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401

UW

1420

7.9

00500

UW

1386

9.200600

UW

1387

6.5

00400

3

24

1

6

5

7

0

1

2m

SCALE

LAPAVERMELHA

NORTHPROFILE

2

OS

Lsamples

Micromorphologysamples

Stra

tumA

Stra

tumB

Stra

tumC

Limestonerock

101

102

103

104

105

106

107

108

91

92

93

94

95

96

97

98

99

100

101

102

103

Figure3.NorthprofileshowingOSLandmicromorphologysamples.Numbersontheverticalandhorizontalaxesrepresentdistancesinm

eters

fromanarbitrarydatumduring

theoriginalexcavation.Luziawasfoundatdepth96.5m,butclosetothecentralprofile.


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FEATHERS ET AL.


UW

850

5.200300

UW

851

13.7001.500

22.8001.700

UW

853

4.500400

6.300600

UW

852

6.200400

6

1

2

3

4

56

Surfacein1971

LAPA

VERMELHA

SOUTHPROFILE

2

OSLsamples

Micromorphologysamples

Sedimentsamples

StratumA

StratumB

StratumC

Limestonerock

40

1

2m

SCALE

1

2

4

3

5

6

97

98

99

100

101

102

103

104

105

106

105

10

4

103

102

101

100

99

98

97

96

95

Figure4.Southprofileshowing

locationofOSL,sediment,andmicr

omorphologysamples.Numberson

theverticalandhorizontalaxesrepresent

distancesinmetersfromanarbitrarydatumduringtheoriginalexcav

ation.


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403

To address the issue of Luzia and better understand site formation, we first pres-

ent geoarchaeological data to characterize the surviving stratigraphy at Lapa

Vermelha. We then apply OSL single-grain dating to obtain a measure of the deposi-tional 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 stratig-

raphy of the site, (2) develop some inferences about the main depositional processes,

(3) assess the degree of bioturbation affecting the deposit, and (4) provide infor-

mation on the abundance and source of quartz particles on which the OSL dates are

based. Evidence was provided by macroscopic field observations; bulk sample par-

ticle 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

Laboratrio do Instituto Mineiro de Agropecuria); bulk sample X-ray fluorescence

analysis on the same samples (lithium tetraborate fusion, using a Philips PW 1988

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.


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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 35 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 frag-

ments, limestone cobbles, and rare speleothem fragments. The sediments are riddled

FEATHERS ET AL.


Table I. Published radiocarbon dates (Delibrias et al., 1986).

Depth (m) from Surface Uncalibrated Ages Calibrated Range**

Sample # or Excavation Level (14C years BP) (years BC or AD)

Gif-2732 1.15 300 110 14501950 AD

Gif-2735 0.2 320 80 14801650 AD

Gif-3222 Base level B 1620 100 260550 AD

Gif-3220 Surface 1880 140 40 BC320 AD

Gif-3221 Level D 3070 110 14501130 BC

Gif-3211 Not given 3260 110 16701430 BC

Gif-3218 Base level D 3370 110 18601520 BC

Gif-3219 Base level C 3430 130 19001541 BC

Gif-3210 Level E 3580 130 21301750 BC

Gif-2734 2.1 3660 110 22001890 BC

Gif-2545 1.9 3720 120 22901950 BC

Gif-2733 1.5 3740

110 23301980 BCGif-3209 Level E 3750 110 23401980 BC

Gif-2543 4.35 4170 120 28902580 BC

Gif-3215 Level G 4350 120 33302880 BC

Gif-2544 5.0 4400 120 33302910 BC

Gif-3213 Level F 4550 130 35003030 BC

Gif-3214 Level G 5120 130 40503710 BC

Gif-3907* 12.9513.15 5400 500 48003660 BC

Gif-3207 9.65 6830 150 58905620 BC

Gif-3217 Level I 6950 140 59905720 BC

Gif-3216 Level H 8490 160 77307330 BC

Gif-3208 10.310.8 9580 200 92508700 BC

Gif-3727 11.711.9 10200 220 104009450 BC

Gif-3726* 11.7 11680 500 1225010950 BC

Gif-3906 12.612.8 12960 300 1440013150 BC

Gif-3905 13.5514.5 15300 400 1690016100 BC

Gif-3725* 11.711.8 25000

Gif-3908* 12.613.55 22410 400 2575024400 BC

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


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TableII.Grainsizeanalysisandche

micalanalysisbyX-rayfluorescence

spectroscopy(in%).*

S

tratum

Sample

Sand(%)

Silt(%)

Clay(%)

SiO2

Al2O3

TiO2

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

A

02

35.1

18.9

45.9

35.40

30.60

1.60

15.70

0.31

0.65

1.10

0.1

0.76

0.67

06

42.1

18.2

39.6

36.50

31.70

1.70

13.30

0.16

0.55

0.84

0.1

0.69

0.61

B

03

40.6

26.2

33.1

34.10

27.60

1.50

13.00

0.24

0.94

5.30

0.1

0.98

0.92

04

43.5

26.0

30.4

34.60

25.80

1.40

12.90

0.22

1.20

5.40

0.11

0.90

0.86

05

41.2

23.7

35.0

34.10

28.80

1.50

13.00

0.21

1.00

3.40

0.1

0.85

0.79

C

01

57.9

26.1

15.9

27.90

16.00

0.83

8.00

0.20

1.30

21.70

0.1

0.72

0.64

M

odernSoil

10

26.4

12.7

60.8

39.00

28.50

1.50

11.80

0.39

0.54

0.30

0.1

0.70

0.56

*

Definitionsofgrainsizesareclay,0.002mm;silt,0.0020.06mm;andsand,0.062mm.Chemicalproportionsdonotsumto100%becausethechemical

analysiswas

d

oneafterlossonignitionremovedvolatilesandhydroxides.Thedifferences

betweenthesumpercentagesand100

equalthepercentLOI.


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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 fluo-

rescence 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 calciumoxide 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 weath-

ering 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) sedi-

ments, 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 withroots and soil fauna.

Micromorphological analysis of south profile samples 13 (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

15% 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 aque-ous medium with minimal faunal reworking. About 20% of granules show edge mor-

phology, 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 carbonatereplaced 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 chan-

nel 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 allstratification.

Stratum B

Stratum B is a heterogeneous unit that consists of a number of structurally mas-

sive lenses that either extend at 25 angles into stratum A, usually with abrupt bound-

aries, 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

FEATHERS ET AL.



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407

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 suggestthe 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 well-

developed 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

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 mate-

rial (PPL) with an undifferentiated b-fabric (XPL). Stratification is expressed by contrasts (often

finecoarsefine) 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 set-

tling fine debris in an aqueous medium.


14/42

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 amorphousorganic matter, common microscopic charcoal fragments, rare plant tissue replaced

by calcium carbonate, ash crystals, and rare bone fragments (Figures 79). The pro-

portion 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

FEATHERS ET AL.


Figure 7. Microphotograph of thin section N6 showing dense clayey aggregates embedded in reduced-

porosity 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 indi-

vidual 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.


15/42



409

Figure8.Microphotographofth

insectionN6showingcalciumoxalateashpseudo-crystalsincellvoids

ofcharredplanttissuein(a)plainpolarized

light(PPL),topleft,and(b)cross-polarizedlight(XPL),topright.(c)

Microphotographinplainpolarized

lightofthinsectionN6showingmic

roscopic

bonefragmentformingpartofsiltymicromassembeddingcolluvialc

layeyaggregates.Notemicroscopic

charcoalfragments.


16/42

yellowish color. As such, they can be related tentatively to the yellow soil horizondescribed by regional pedological studies (Boulet et al., 1992; Pil, 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

FEATHERS ET AL.


Figure 9. Microphotograph of thin section N6 showing calcite fragment (CaCO3), clay aggregate embed-

ding 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.


17/42



411

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 sim-ilarity 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 Alike 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 pro-

duced by adjacent units. The heterogeneous stratum B sediments were most likely

displaced gravitationally, reworked by human trampling, and/or sheet-washed fromoccupation 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 micromor-

phological and the chemical evidence indicates that stratum B is bulked up by anthro-

pological sedimentation, perhaps the result of ash production in fires built close to

the shelters 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 obliter-

ated 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, thesedeposits 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 sedi-

ments, micromorphological observations show that most quartz grains are embed-

ded 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 (Pil, 1998).

LUMINESCENCE PROCEDURESBecause luminescence dating addresses a depositional eventthe last time the

sediment was exposed to sunlightit provides a more direct measure of the sedi-

ments 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 35; ages given will be discussed later).

Notice that there is some ambiguity about UW1385. At the time of collection, it was


18/42

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 cal-

culations. 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 obser-

vations is that it lies entirely within stratum A. Samples were collected by driving light-

tight 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 (UW850853),

which were collected in 2003. This size fraction may compromise single-grain reso-

lution to some extent because two or three grains may fit into the 300-mm holes onthe single-grain disks used for measurement. A recent modeling work (Arnold and

Roberts, 2009) has cautioned against using grains smaller than 180mm in the Ris

single-grain disks because averaging effects from multigrains may produce mis-

leading results including phantom equivalent dose components. Measurements

were acquired prior to knowledge of this work, and the smaller grain size was cho-

sen 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 (UW13841387,1420), collected in 2005. At this size, it is more likely each

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) % error

South Profile

UW853 Upper A 9.6 2nd (57.5) 4.5 0.4 8.8

3rd (23.2) 6.3 0.6 9.8

UW850 Lower A 11.5 2nd (76.5) 5.2 0.3 6.6

UW852 B 10.3 2nd (75.1) 6.2 0.4 7.2

UW851 C 11 3rd (63.4) 22.8 1.7 7.6

2nd (24.2) 13.7 1.5 11.1

North Profile

UW1387 Upper A 7.5 2nd (95.6) 6.5 0.4 6.6

UW1420 Lower A 11.5 1st (100) 7.9 0.5 6.3

UW1386 B 10.3 1st (100) 9.2

0.6 6.4Center Profile

UW1385 C or A 14 1st (100) 12.7 0.8 6.7

UW1384 C 14 1st (100) 16.0 1.0 6.2


19/42



413

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 holea sit-

uation 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 meas-

urable signal did not differ (both 22%) between the 125- to 150-mm and 150- to 180-

mm 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 212mm. 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 sig-

nal is 15% and the probability of two or more grains within each hole producing asignal 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 sig-

nal 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 single-

grain resolution is not likely. This probability of more than one grain in a hole pro-

ducing 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 varia-

tion 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 frac-

tions. 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 backgroundmight do so together. This might account for the somewhat higher acceptance ratio for the smaller grainsize of UW851 than would be expected from the acceptance ratio for the 180- to 212-mm grains and thenumber of grains that physically fit into a hole. However, for the small number of grains in each hole andthe generally low sensitivity of the samples, we do not think this will be significant, although it couldaccount for some of the low-proportion, small-value components discussed later. (Later discussion alsosuggests any phantom components are not significant.)


20/42

FEATHERS ET AL.


TableIV.Numberofgrainssampled,numberofgrainsrejectedandrejectioncriteria.

NaturalOSL

Poor

Recycling

ExceedingHighest

DeNotSignificantly

Feldspar

Accepted

Sample

Measured

Signa

l

Test

RegeneratedO

SL

DifferentfromZero

Recuperation

Contamination

(%oftotal)

UW850

1097

775

26

17

8

0

2

269(24.5)

UW851

1096

764

35

128

4

0

0

165(15.1)

UW852

896

641

15

26

2

0

1

211(23.5)

UW853

600

382

22

31

0

0

0

165(27.5)

UW1384

1095

793

37

70

3

1

2

189(17.3)

UW1385

998

712

20

80

4

23*

2

157(15.7)

UW1386

500

295

8

21

3

1

6

166(33.2)

UW1387

699

443

27

23

2

11

15

178(25.5)

UW1420

695

473

15

21

0

0

1

185(26.6)

Total

7676

5278

205

417

26

36

29

1685

%oftotal

68

.8

2.7

5.4

0.3

0.4

0.4

22.0

*Twenty-twoofthesewerefromon

edisk.


21/42



415

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 andbeta 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 inter-

nal alpha contribution. Gamma dose rates were estimated from laboratory meas-

urements (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--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 werenever 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 attenu-

ation 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 Ris TL-DA-15 reader with single-grain attach-

ment. 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 theDe values by the bulk dose rate provides an age for each grain

(not accounting for differential dose rates for individual grains). De distribution is


22/42

implied in usage of these terms in this paper. The common age model controls for

differential precision by computing a weighted average using logDe values. The cen-

tral age model is similar except rather than assuming a single true value, it assumesa natural distribution of De values, even for single-aged samples, because of non-

statistical sources of variation. It computes an overdispersion parameter (sb) inter-

preted as the relative standard deviation (or coefficient of variance) of the true Devalues 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 single-

aged samples (Olley et al. 2004; Jacobs et al., 2006).

Another source of variation inDe is instrumentation error. We have included meas-

ured 2% systematic error in all luminescence measurements to account for error in

reproducibility. Other instrumentation error arises from the variation in the cali-

bration 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 disksthat 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 theDe values from seconds of beta irra-

diation to Gy, the average calibration for the horizontal row of holes in which a par-

ticular 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 differ-

ential calibration does not affect the central tendency of the distributions (because

the differences average out) but does affect the amount of overdispersion, particu-

larly for samples with lower relative overdispersion. In a subset of grains from one

FEATHERS ET AL.


Table V. Single-grain OSL measurement parameters.

System: Ris TL-DA-15, Single-Grain Attachment

Excitation: 532 nm laser (90% of 50 W/cm2)

Detection filters: 7.5 mm U340 (ultraviolet)

Preheat: 240C 10 s

Cut heat: 160C or 200C

Test dose: 3 Gy

Exposure: 0.8 s at 125C

Analysis: 0.06 s, background 0.650.8 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 doseCut 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 25Gy were added but with a 40s IR (880 nm)

exposure at 125C prior to the OSL (Li) step.


23/42



417

sample (UW1384), assuming a uniform calibration produced sb of 20.2%, while apply-

ing differential calibration reduced sb to 11.6%. In another sample (UW851), overdis-

persion was not significantly reducedonly from 45.1% to 41.7%probably becauseother 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 inaccurateDe values because the sig-

nal is dominated by slowly bleaching components. An advantage of single-grain

dating is the opportunity to remove from analysis grains with unsuitable character-

istics by establishing a set of criteria grains must meet. In this study, grains were

eliminated from analysis if they1. 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 stan-

dard deviation),

2. did not produce, within 20%, the same signal ratio (often calledrecycle ratio)

from identical regeneration doses given at the beginning and end of the SAR

sequence, suggesting inaccurate sensitivity correction,

3. yielded natural signals that did not intersect saturating growth curves,

4. had a signal larger than 10% of the natural signal after a zero dose,

5. produced a zero De (within 1 sigma of 0), or

Figure 10. Density plot of beta source calibration. Numbers on axes represent holes along the hori-

zontal and vertical grid of the single-grain disks for Ris TL-DA-15 reader. Values are in Gy/s.


24/42

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 0.91.1 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 0.81.2 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 inthis study came from the three oldest samples, which because of them, had an accept-

ance 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 Devalues because they experienced different dose rates, primarily because of hetero-

geneity 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 distrib-uted 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 givensb value and the assump-tion of a log normal distribution of each component. The model estimates the num-

ber 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 con-

clusions 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

FEATHERS ET AL.



25/42



419

intrinsic luminescence characteristics, the example given of recuperation (signal after

zero dose, caused by the preheat) in low-dose components. We tested for recupera-

tion by first giving a laser bleach (100 s at 45 W/cm2 at 125C) on 100 grains each from

four samples. The SAR protocol was then applied with the expectation of 0 De if norecuperation was present. Ninety-one grains passed the above criteria (with the excep-

tion 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 sam-

ples 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 240C preheat employed. Single-grain analy-

sis of UW1384 was performed using four different preheats: 170C, 220C, 240C,

and 260C, 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 2.1Gy for the respective temperatures. Correspondingoverdispersion 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 220C preheat, and lower overdisper-

sion for 240C, the differences are not significant, nor is any trend detected of increas-

ing 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 sev-

eral samples are initially bleached and then given a laboratory dose. The SAR proto-

col 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 toother 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 125C 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 condi-

tions (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 sam-ples 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


26/42

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 lat-

ter, dose rates were only weakly dependent on carbonate content. A regression yielded

an R2 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 accept-

able 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 agrain dominated by a fast bleaching component. The other two (b and d) have some-

what 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

FEATHERS ET AL.


Table VI. Dose recovery results 200-s beta irradiation.

Beta Irradiation

Time Based on

# of Accepted Central Age Over-dispersion, # within 1 # within 2

Sample Grains Model (s)* sb Sigma (%) Sigma (%)

UW850 55 210 7 0.06 0.07 46 (84) 55 (100)

UW851 42 203 7 0.08 0.06 33 (79) 39 (93)

UW852 28 206 10 0 24 (86) 27 (96)

UW1384 26 206 9 0 24 (92) 26 (100)

UW1385 28 198 7 0 26 (93) 28 (100)

UW1386 24 185 7 0 14 (58) 18 (75)

Total 203 202 3 0 167 (82) 193 (95)

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

2. Major differences in Th content apparent in Table VII are probably not too meaningful. The U and Thcontents were determined using the pairs technique in alpha counting, a technique that can lead to largeerrors in the relative proportions of the two, but not in the total contribution to the dose rate. The dif-ferences in total dose rate are more meaningful.


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421

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 over-

all abundance may not affect overdispersion as much as the size and spatial distri-

bution of the limestone particles (Nathan et al., 2003). Table VIII also gives the num-

ber 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 at13.3 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 con-

sistent with a single component by the finite mixture model. Because natural overdis-

persion 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 con-

sistent 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 distri-

bution of limestone, the 13% is used here as a benchmark to judge the likelihood a

Table VII. Dose rate data.

Total Dose Rate*

Sample (stratum) 238U (ppm) 232Th (ppm) % K (Gy/ka)

UW850 (A) 6.68 0.44 18.23 2.02 0.51 0.02 3.07 0.14

UW851 (C) 4.27 0.27 7.90 1.23 0.49 0.01 1.93 0.09

UW852 (B) 5.40 0.42 24.62 2.19 0.62 0.03 3.42 0.15

UW853 (A) 6.35 0.47 26.67 2.30 0.56 0.01 3.68 0.16

UW1384 (C) 5.76 0.34 9.74 1.19 0.66 0.03 2.38 0.10

UW1385 (C or A) 6.78 0.43 15.09 1.84 0.53 0.01 2.38 0.18

UW1386 (B) 5.26 0.33 10.80 1.38 0.52 0.02 2.33 0.10

UW1387 (A) 6.76 0.40 10.26 1.51 0.51 0.06 2.61 0.12

UW1420 (A) 7.13 0.47 20.20 2.14 0.60 0.01 3.34 0.14

Additional Measurements

Slightly different colored 3.94 0.25 6.56 1.10 0.82 0.02

sediment below UW1384Limestone wall near UW1385 1.43 0.09 0.15 0.19 0.00 0.01

Limestone rock near UW1420 0.54 0.07 2.52 0.05 0.69 0.02

Grayish level near UW1420 4.71 0.33 15.02 1.59 0.51 0.04

*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 bybeta 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 alsodetermined 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 withlaboratory 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.


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FEATHERS ET AL.


Figure11.Decayandgrowth

curvesforfourgrains,twofromUW

1384andtwofromUW851.Thedecaycurves(luminescenceversustime

)areforthe

naturalsignal.Thepointofinitialrisemarkswhenthestimulating

lightfromthelaserwasturnedon.

Rapiddecayisshownin(a)and(c)

,somewhat

slowerdecayin(b)and(c).Thegrowthcurvesplotluminescenceagainstregenerationdose.Thenaturalsignalappearsonthey-axis.Ahorizontalline

fromitintersectsthegrowthc

urveattheequivalentdosevalue.


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423

TableVIII.Equivalentdose,centralage,andfinitemixturemodels.

De(Gy)Central

#Components

#Com

ponents

MostCommon

PercentageofMost

De(Gy)ofMost

Sample

AgeModel

sb(%)

whensb

0

when

sb

13

Component

Com

monComponent(%)

Common

Component

UW850

16.3

0.4

32

2

4

3

2nd

76.5

16.0

0.4

UW851

35.8

1.3

42

3

5

4

3rd

63.4

44.1

2.0

UW852

21.9

0.5

23

2

3

3

2nd

75.1

21.1

0.9

UW853

18.7

0.5

26

2

4

3

2nd

57.5

16.7

1.1

UW1384

38.1

0.7

15

2

2

1

1st

100

38.1

0.7

UW1385

34.3

0.7

13

3

2

1

1st

100

34.3

0.7

UW1386

21.5

0.5

18

3

3

1

1st

100

21.5

0.5

UW1387

17.3

0.6

31

3

3

3

2nd

95.6

17.0

0.4

UW1420

26.3

0.6

18

3

2

1

1st

100

26.3

0.6


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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 thefinite mixture model is applied using sb values for single-age components of 8%, 13%,

and 18%. Significant differences inDe of the most common component are only pres-

ent for UW853 and UW1420. Using the 8% value, UW1386 and UW1420 gain a com-

ponent, but in the case of UW1386 it does not significantly change the value of the dom-

inant 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 ofsb 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 sam-

ples. 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 lowerprecisions (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 overdis-

persion value of 13% as representative of a single-age component. Four of the sam-

ples, 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 com-

ponent (27 Gy) is also shown.

Causes of Overdispersion

For the north and central profile samples, where the De distributions are consis-

tent with a single age (or nearly so in the case of UW1387), the De from the central

FEATHERS ET AL.



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425

TableIX.Numberofcomponentsandequivalentdose(Gy)asafunctionofoverdispersion.*

sb

0.08

sb

0.13

sb

0.18

DeofMostCommon

DeofMostCommon

#ofComponents

DeofMostCommon

Sample

#ofComponents

Component

#ofCom

ponents

Component

Component

UW850

3

15.9

3.4

3

16.0

0.4

3

16.2

0.5

UW851

4

44.8

1.9

4

44.1

2.0

3

47.2

2.8

UW852

3

20.7

0.6

3

21.1

0.9

3

22.6

3.7

UW853

3

16.4

0.8

3

16.7

1.1

2

19.2

0.4

UW1384

1

38.1

0.7

1

38.1

0.7

1

38.1

0.7

UW1385

1

34.3

0.7

1

34.3

0.7

1

34.3

0.7

UW1386

2

19.4

3.5

1

21.5

0.5

1

21.5

0.5

UW1387

3

16.8

0.4

3

17.0

0.4

2

17.6

0.4

UW1420

2

22.5

1.2

1

26.3

0.6

1

26.3

0.6

*Forthosesampleswhereonlyone

componentispresent,theDeiscalculatedusingthecentralagemodel.


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FEATHERS ET AL.


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 dis-

tributions are not consistent with a single age, it is more difficult to select an appro-

priate 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 oncarbonate content (R2 0.37), although again it may be the size and spatial distri-

butions 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

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

UW850 269 2 2 3 3 3 3 3 3 3

UW851 165 2 2 3 3 3 3 4 4 4

UW852 211 2 2 2 2 3 3 3 3 3

UW853 165 2 2 2 2 3 3 3 3 3

UW1384 189 1 1 1 1 1 1 1 1 1

UW1385 157 1 1 1 1 1 1 1 1 1

UW1386 166 1 1 1 1 1 1 1 1 1

UW1387 178 2 2 2 2 2 3 3 3 3

UW1420 185 1 1 1 1 1 1 1 1 1

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


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427

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 differ-

ently 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 compo-

nent (consisting of more than 75% of the grains) probably is the best estimate for

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 ref-

erence 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.


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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 car-

bonate 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-doserate. 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 determineDe for different parts

of the OSL signal (Singarayer and Bailey, 2005). The overall OSL signal is a com-

posite of signals that are differentially affected by exposure to sunlight. The SARprotocol assumes the signal is dominated by a fast bleaching component, but medium-

and slow-bleaching components are known as well (e.g., Jain et al., 2003), and dif-

ferent 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 con-

tinuous 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 100200 grains each from three samples

FEATHERS ET AL.


Table XI. Luminescence ages recalculated assuming different dose rates.

Age (ka)

Dominant Component Lowest Component Lowest Component

Sample Full Beta Dose Rate Half Beta Dose Rate Zero Beta Dose Rate

UW1850 5.2 0.3 2.9 0.3 4.2 0.5

UW1851 22.8 1.7 7.7 0.9 11.6 1.5

UW1852 6.2 0.4 1.7 0.3 2.3 0.4

UW1853 4.5 0.4 0.8 0.2 1.1 0.3

Middle Component High Component

Half Beta Dose Rate Full Beta Dose Rate

UW1850 6.8 0.4 8.5 0.7

UW1851 21.0 2.0 22.8 1.7

UW1852 7.9 0.5 8.3 1.1

UW1853 5.8 0.5 6.3 0.6


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429

by increasing the laser power from 0% to 90% (of maximum 50 W/cm2) at a linear

rate for 30s. The early part of this signal will be dominated by the fast-bleaching com-

ponent, 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 com-

ponent. 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 compo-

nent could be measured on only 21 grains (17 of them from UW851), and on 18 of

these, the slow component produced aDe that was statistically equivalent (within 1s)

Figure 13. LM-OSL curves for three different grains from UW851. The luminescence is plotted as a func-

tion 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 UW850 UW851 UW1384

# measured grains using LM-OSL 200 200 100

# grains with fast component 70 77 21

# grains with slow component 4 26 1

# grains with both components 3 17 1

Central age De for fast component (Gy) 17.3 1.2 34.7 2.3 29.7 2.4

Central age De given in Table VI (Gy)* 16.3 0.4 35.8 1.3 38.1 1.0

Central age De for slow component (Gy) 50.1 8.6 12.4 3.0 76.3 131.5

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


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to or less than that of the fast component. The large number of smallDevalues

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