feathers et al luzia
TRANSCRIPT
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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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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
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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).
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GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4400
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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HOW OLD IS LUZIA? LUMINESCENCE DATING
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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GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4402
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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HOW OLD IS LUZIA? LUMINESCENCE DATING
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.
GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4404
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
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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.
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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
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GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4408
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.
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Figure8.Microphotographofth
insectionN6showingcalciumoxalateashpseudo-crystalsincellvoids
ofcharredplanttissuein(a)plainpolarized
light(PPL),topleft,and(b)cross-polarizedlight(XPL),topright.(c)
Microphotographinplainpolarized
lightofthinsectionN6showingmic
roscopic
bonefragmentformingpartofsiltymicromassembeddingcolluvialc
layeyaggregates.Notemicroscopic
charcoalfragments.
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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
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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.
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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 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
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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
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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
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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 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.)
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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.
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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 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
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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
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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.
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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.
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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 0.91.1 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
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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 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
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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
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GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4420
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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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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GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4422
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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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
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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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GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4426
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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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
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GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, VOL. 25, NO. 4428
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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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