Quaternary Geochronology 10 (2012) 430e435
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Quaternary Geochronology
journal homepage: www.elsevier.com/locate/quageo
Research paper
Challenges in constraining pluvial events and hominin activity: Examples of ESR
dating molluscs from the Western Desert, Egypt
B.A.B. Blackwell a, b, *, A.R. Skinner a, F. Mashriqi b, A.E. Deely b, R.A. Long b, J.J.J. Gong b, M.R. Kleindienst c,
J.R. Smith d
a
Dept. of Chemistry, Williams College, Williamstown, MA 01267-2692, USA
RFK Science Research Institute, Glenwood Landing, NY 11547-0866, USA
Dept. of Anthropology, University of Toronto at Mississauga, Mississauga, ON L5L 1C6, Canada
d
Dept. of Earth & Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130-4862, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 October 2011
Received in revised form
10 January 2012
Accepted 11 January 2012
Available online 20 January 2012
Receiving <0.1 mm/y of precipitation, Egypt’s hyperarid Western Desert, today lacks naturally occurring
surface water. Artesian spring deposits, tufa deposited by springs and carbonate-rich silty lacustrine
sediment attest that oases in the Western Desert had surface water during the Pleistocene. Paleolithic
artefacts, fossil ungulate teeth, and snails occurring within the Pleistocene deposits and dotting the
surface record times when higher rainfall and/or groundwater tables during pluvial events allowed
surface water to exist in wetlands, small ponds and lakes, enabling hominin habitation. Archaeological
finds ranging from Early to Later Stone Age (ESAeLSA) occur in gravel lags, within sedimentary deposits,
and on the older geomorphic surfaces. Near Kharga, large tufa deposits ranging from a few hectares to
more than 10 km2 in area, such as Matana and Medauwara, dot the edge of the Libyan Plateau. Molluscs
were dated using standard ESR protocols. To test for reworked fossils, multiple samples from a single
sample were dated independently. In some units at Medauwara, multiple gastropod populations from
different times have been preserved, while others appear to only preserve a single population. To see the
effects of the cosmic dose rate on ESR ages, ages were calculated using zero cosmic dose rate, the full
modern cosmic dose rate, and time-averaged cosmic and sedimentary dose rates. For gastropods from
Matana, no significant difference in ESR ages resulted from different cosmic dose rate assumptions.
Therefore, at Matana 2, the shells dated at 27.7 1.9 assuming time-averaged external dose rates, while
at Matana 3, they averaged 65.1 4.1 ka, suggesting that water was present for hominin use at times
during OIS 2 and 4.
Ó 2012 Published by Elsevier B.V.
Keywords:
ESR (electron spin resonance) dating
Matana
Kharga oasis
Egypt
Medauwara
Kharga oasis
Egypt
Cosmic dose rate modeling
External dose rate modeling
Reworking
1. Introduction
Today, the hyperarid Western Desert, Egypt, receives
w0.01e0.07 mm/y precipitation, but suffers >2 m/y of evaporation,
ensuring that area lacks naturally occurring standing water (Smith
et al., 2007). In past pluvial events, however, this area may have
received as much as 30 cm/y (Hawkins et al., 2001). Hominins
inhabited the Western Desert at sites around Kharga Oasis (Fig. 1),
and left behind their Earlier (ESA), Middle (MSA), and Later Stone
Age (LSA) artefacts. At Kharga, fossil freshwater molluscs that sit in
lacustrine silt and mud beds record times when surface water did
* Corresponding author. Dept of Chemistry, Williams College, Williamstown,
MA 01267-2692, USA. Tel./fax: þ1 516 759 6092.
E-mail address: bonnie.a.b.blackwell@williams.edu (B.A.B. Blackwell).
1871-1014/$ e see front matter Ó 2012 Published by Elsevier B.V.
doi:10.1016/j.quageo.2012.01.005
exist for hominins and their game to drink, probably thanks to
higher rainfall during pluvial events (Adelsberger and Smith, 2010).
Dating these events, however, has proved to be more challenging
than finding the artefacts that document the hominin presence.
Rarely do the artefacts occur within any sedimentary context that
can be correlated to any datable materials, while intermittent
fluvial and the pervasive aeolian erosion have removed most
paleoenvironmental indicators. Thus, dating proxy materials
becomes imperative.
Pleistocene lake sediment, artesian spring and tufa deposits, all
show that Kharga Oasis had surface water repeatedly during the
Pleistocene. Tufa can only form when spring water brings dissolved
salts to the surface and evaporates to deposit the carbonate. Where
tufa dams block water flow, small basins form within the tufa
deposits. In order for basins to trap, and hence, preserve silt or mud
deposits, those basins must contain standing water. Freshwater
431
B.A.B. Blackwell et al. / Quaternary Geochronology 10 (2012) 430e435
Table 1
Samples in the study.
Fig. 1. The Kharga Oasis Depression. Near Kharga, large tufa deposits dot the escarpment edge between the Libyan Plateau and the Kharga Oasis Depression. Lacustrine
silts containing freshwater snails were sampled at sites within Matana (also spelled as
Metana in some publications) and Medauwara.
snails, like Melanoides tuberculata, Gyraulus sp., Lymnaea stagnalis,
and Plantries planorbis, can only survive and breed when abundant
surface water fills lakes. Hominids, and many ungulate species,
require relatively fresh surface water to drink daily. At times during
the Pleistocene, water was sufficiently abundant to maintain
a large, relatively diverse ecosystem attractive to hominins, who
left numerous Paleolithic artefacts scattered across the landscape
ranging from Oldowan choppers to LSA settlements, and even
Neolithic hearths and beads to Roman pottery.
For dating the archaeological artefacts, datable materials are
rare. Although 230Th/234U can sometimes date tufas younger than
w400 ka, detrital 230Th contaminates the tufa complicating or
preventing successful 230Th/234U dating (Blackwell and Schwarcz,
1995). Moreover, the porous travertines and lake carbonates can
contain multiple generations of calcite deposition that preclude
accurate dates with any method. Some tufa units from Kharga have
been dated successfully, mainly from Medauwara (Kleindienst
et al., 2008; Smith et al., 2004a,b). ESR (electron spin resonance)
can directly date freshwater molluscs from 5 ka to 2 Ma in age
(Blackwell, 2006). No tufa units at Kharga have yielded large
molluscs, but Matana and Medauwara (Fig. 1) have produced
freshwater gastropods for ESR dating. Freshwater gastropods from
Matana (also spelled Metana) and Medauwara were analyzed to
assess their suitability for ESR dating (Table 1). Potential effects
from variable cosmic dose rates, sedimentary dose rates, and
evidence for reworked fossils within deposits were examined.
2. Geology and archaeology at Kharga Oasis
Below the Libyan Plateau in Egypt’s Western Desert, roughly
650 km southwest of Cairo at 25 N 30 E and 100 m asl, the Kharga
Oasis depression extends w200 km north-south, and w75 km westeast (Fig.1). The oasis formed as a depression ablated by the wind into
Cretaceous-Tertiary marine limestone, shale and sandstone units.
Eocene Thebes Group limestone now rims the depression (Smith
et al., 2004a,b; Nicoll et al., 1999). In 1930e1933, Caton-Thompson
and Gardner studied the archaeology of Kharga Oasis (Caton-
Number
Location
ESR
Outcrop
GPS coordinates
Species
Catalogue
Name
North
East
RM47
KAR17
Big Snail Gully
Medauwara
N2760628
24 56.9290
E0307807
31 05.9050
Melanoides
tuberculata
RM51-RM54
KAR17a-d
Big Snail Gully
Medauwara
N2760628
E0307807
Melanoides
tuberculata
FM77-FM80
KAR102a-f
Parking Lot
Medauwara
24 570 49.600
31 050 15.800
Melanoides
tuberculata
FM81-FM84
KAR105a-d
Railway 3
Medauwara
24 570 16.500
31 050 02.100
Melanoides
tuberculata
FM85
KAR105e
Railway 3
Medauwara
24 570 16.500
31 050 02.100
Gyraulus
sp.
AM08a,b
KAR106a,d
Matana 2
25 010 59.100
30 570 17.800
Melanoides
tuberculata
AM07a-b
KAR107a-b
Matana 3
25 010 59.000
30 570 18.300
Melanoides
tuberculata
Thompson, 1952). Kleindienst et al. (2008) and Smith et al. (2007)
relocated some of their localities on the escarpment at Naqb el
Refûf, Bulaq, and Matana. The Dakhleh Oasis and Kharga Oasis
Prehistory Projects have investigated Medauwara area since 1996.
Along the Libyan Plateau that rims the depression, springs have
deposited several large tufa accumulations, such as Medauwara
and Matana (Figs. 1 and 2), which mark nickpoints for the local
groundwaters at slope breaks. Locally, tufa dammed ponds up to
a few square metres and even small lakes up to 1e5 km2. These
ponds and lakes accumulated lacustrine silts, which often comprise
large amounts of carbonates mixed with small inputs of clastic
sediment. Both the lacustrine silts, and the tufa preserves freshwater gastropods, including M. tuberculata and Gyraulus sp. (Smith
et al., 2004a). Since these gastropods can only live in fresh or
brackish water, dating their shells indicates when hominins could
inhabit the area (Adelsberger and Smith, 2010). Since tufa resists
aeolian erosion better than does the lacustrine and playa sediment,
the tufa itself and the lake deposits rimmed or overlain by tufa
often preserve the only evidence for human occupation in the
Western Desert (Hawkins et al., 2001). Because the springs that
result from aquifer storage deposit tufa from water at ambient
temperature, tufa also records when an ample water supply existed
(Nicoll et al., 1999). Freshwater gastropods also record wetter
climatic conditions, because they can only exist when ample water
is present in ponds or lakes.
A region hosting substantial tufa deposits >12 km2 in area near
an ancient caravan route (Nicoll et al., 1999), Naqb (Wadi) el
Medauwara contains Earlier Stone Age, Upper Acheulean, Older and
Younger MSA, Aterian, Khargan, Epipaleolithic, and Neolithic
occurrences. Chert nodules which may have served as the raw
material for lithic production occur in the gravel lags within some
basins. Although Smith et al. (2004a,b) recognized three tufa units,
the Plateau Tufa, and Wadi Tufas 1 and 2 (Fig. 2), more recent
analyses suggest a much more complex stratigraphy. Complicating
the problem is the limited extent for many small lacustrine basins,
coupled with the extensive erosion that has removed intervening
mapable units. Deposited by direct precipitation near spring
orifices, these tufas occur at different altitudes on the escarpment.
Tufa dammed small ponds with areas of 1e5 m2 that trapped silts.
Small lakes with areas up to 1e5 km2 also formed in scattered
locations, including at Railway Outcrop and Big Snail Gully.
Unfortunately, subsequent erosion and some anthropogenic
surface modification has left isolated outcrops containing molluscs
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B.A.B. Blackwell et al. / Quaternary Geochronology 10 (2012) 430e435
Fig. 2. The spring tufa deposits at Medauwara. Medauwara contains several generations of spring tufas. The tufas dammed ponds and small lakes which collected carbonate
lacustrine silt and freshwater gastropods, such as Melanoides tuberculata. Gastropods were collected from three localities.
that are difficult to relate to one another (Kieniewicz and Smith,
2007). At Kharga, however, few sections containing more than
a few stratigraphic units occur anywhere. Since mapped tufa units
do not occur as one outcrop, but are discontinuously distributed
laterally across the area, interbasinal correlation has been difficult,
and correlation between scattered tufa deposits even more challenging. Even where two outcrops occur only few tens of metres
apart, correlating the units is often impossible. Hence, chronostratigraphic dating will help to map their stratigraphy and
archaeological associations. Thus, ages for the samples dated here
will be used to build a more detailed chronostratigraphic map of
the area.
During the Oxygen Isotope Stage (OIS) 6/5e pluvial event, the
same precipitation source fed all the regions and the aquifer
sources, likely deriving from air masses originating over the
Atlantic Ocean. This rainfall produced low d18O ratios in the tufa, as
well as deep and shallow groundwater in Nubian and Libyan
aquifers, and maintained a small recurring paleolake, which
provided a stable water source at Medauwara. In the early Holocene
pluvial period before 5 ka, relatively abundant C4 species, such as
grasses, carpeted the area near the springs (Smith et al., 2004a;
Kieniewicz, 2007).
A smaller tufa accumulation than Medauwara, Matana clings to
the Libyan Plateau in a region not accessed by roads, which requires
a lengthy climb over dunes and small cliffs up the escarpment edge.
Few studies have occurred at Matana due to the difficulty in
accessing the site without camels. Nonetheless, Gardner reported
several MSA localities there (Caton-Thompson, 1952).
Although Kharga tufas are well preserved, colluvial and eolian
silt and sand can contaminate tufas, thereby complicating
230
Th/234U dating efforts. Tufas overlying lake silts were dated at
126 4 ka by 230Th/234U at Medauwara (Smith et al., 2004a),
103 14 ka at KOPP Matana 02 (Smith et al., 2007), 240 5 ka and
125 þ 1.6 ka at El Refûf (Kleindienst et al., 2008).
3. ESR Dating
At Medauwara, molluscs from three areas were tested to
determine the prevalence of reworked fossils (Table 1, Figs. 1 and 2).
All samples were collected from within the snail-bearing horizons
in areas where snails were found eroding from the underlying
sandy silts. While the snail-bearing horizons at all three localities
may or may not represent penecontemporaneous lake(s) deposits
given their altitudes and locations within the tufa deposit, the
presence of visibly different taphonomic characteristics in at least
two of them makes it necessary to examine the collections for
possible reworked samples. Detailed stratigraphic relationships
between the outcrops are impossible to verify except through
chronological analyses.
At Matana, two mollusc samples were collected for ESR dating.
Both came from silty sand units that were capped by dirty
discontinuously eroded tufa units, that overlie the two sampling
locations (Table 1, Fig. 1). Approximately 5 m apart vertically, the
two sampling locations were separated by some 30 m laterally,
from which a significant portion of the capping travertine had been
eroded or had never existed. Intervening fine sediment had also
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B.A.B. Blackwell et al. / Quaternary Geochronology 10 (2012) 430e435
been winnowed away, making it impossible to relate these two
units stratigraphically.
ESR can provide absolute dates for aragonitic molluscs over
5 kae2 Ma using the stable signal at g ¼ 2.007 (Skinner, 1988;
Molod’kov, 1993; Blackwell, 2001). Ages are found using Eq. (1):
A
S
¼
Z
t
0
ðDint ðtÞ þ Dsed ðtÞ þ Dcos ðtÞÞdt
(1)
where A S ¼ the total accumulated dose in the sample,
Dint(t) ¼ the dose rate from internal sources: U, its daughters,
and any other radioisotopes,
Dsed(t) ¼ the dose rate from external sources: sedimentary U, Th,
and K,
Dcos(t) ¼ the dose rate from external sources: cosmic radiation,
t ¼ the sample’s age (Deely et al., 2011).
Due to the climatic variability in the Sahara during the Pleistocene,
successful ESR dating must consider how reworking and deflation
affect the calculation for the sedimentary and cosmic dose rates.
Molluscs were prepared using standard ESR dating procedures
(Skinner, 1988; Skinner and Shawl, 1994). Using a 60CO g source, all
but one aliquot were irradiated at a dose rate of 56e132 Gy/s with
added doses ranged from 8 to 1300 Gy. Molluscs were annealed for
1.0 h at 90 C to remove unstable interference. At 25 C, aliquots
were scanned in a JEOL RE1X ESR spectrometer at 2 mW power
under a 100 kHz field modulation of 5 mT, using a 0.3 s time
constant. The spectra were scanned over 50 mT, centered at 330 mT
for molluscs with an 8.0 min sweep time. Gains were set to maximize signal intensity.
To measure Dint(t), U concentrations within the fossils were
analyzed geochemically with NAA (see Blackwell, 1989 for
analytical details) and were modelled using the standard early
uptake (EU, p ¼ 1), linear uptake (LU; p ¼ 0), and recent uptake
(RU, p ¼ 10) models using Rosy (Brennan et al., 1997). To determine
Dext(t), volumetrically averaged sedimentary geochemical analysis
was used. For sedimentary geochemistry, sediment samples from
within 30 cm of the sample were geochemically analyzed by NAA,
for radioactive elements and then volumetrically averaged for each
layer around the fossil(s). Due to low sedimentary radioactivity,
Dsed(t) values for the molluscs constitute w20e50% of DS(t), the
total dose rate. Thus, as the source for the remaining 20e35% of
DS(t), Dcos(t) may present a significant uncertainty in the calculated
ages. To examine such uncertainties for each dating sample, several
time-averaged cosmic dose rates, Dcos ðtÞ, were modelled as noted
above (Deely et al., 2011).
To test for reworked molluscs, where sample sizes permitted,
several random grab samples from each mollusc-bearing bed were
tested for age coherence. Mean ages were calculated by inversely
weighting the ages by their associated standard deviations.
4. Results
Dating multiple samples from in situ stratigraphic deposits
associated with lacustrine or fluvial units will often demonstrate if
a deposit contains fossils of different ages that were mixed during
reworking. If their ages, U concentrations, and accumulated doses
agree statistically, then the possibility for reworked fossils becomes
less likely (e.g., Table 2; Blackwell, 1994). To test for reworking at
three outcrops, several samples were analyzed. For the reworking
tests, only the LU ages will be discussed. The trends in the EU or RU
ages give similar results.
For KAR17 from Big Snail Gully, the shells were chosen randomly
from within the whole sample. U concentrations in these
subsamples show little variation, all falling in the range
0.66e0.81 0.02 ppm U (Table 2a). Although the accumulated
Table 2
Tests for reworked snails from Medauwara, Kharga Oasis.
Accumulated
dose, A S
(Gray)
Agea,b
LU
(ka)
0.66 0.02
67.8 5.3
104.1 9.2
grab sample
0.76 0.02
61.0 2.2
94.1 5.2
grab sample
0.75 0.02
58.6 2.9
90.1 5.9
grab sample
0.67 0.02
62.3 3.3
95.9 6.5
grab sample
0.81 0.02
68.7 4.0
105.5 7.6
3
unweathered
7.46 0.02
29.5 1.6
24.0 3.0
25.97 0.02
56.5 3.1
31.8 2.8
15.34 0.02
42.8 4.2
34.4 5.1
26.09 0.02
75.2 3.4
40.1 5.3
11.52 0.02
57.3 3.3
50.2 5.8
1.16 0.02
40.5 4.0
63.6 8.2
0.93 0.02
54.7 4.7
87.2 10.5
1.06 0.02
42.4 2.1
67.1 6.5
1.26 0.02
54.8 4.6
84.3 10.0
Sample
Description
a. Big Snail
RM47
KAR17
RM51
KAR17c
RM54
KAR17b
RM53
KAR17a
RM52
KAR17d
Gully
grab sample
b. Railway
FM85
KAR105e
FM84
KAR105d
FM82
KAR105b
FM83
KAR105c
FM81
KAR105a
light, rugose
unweathered
dark, rugose
unweathered
light, smooth
weathered
dark, smooth
weathered
c. Parking Lot Basin
FM77
small, light, rugose
KAR102a unweathered
FM80
medium, red-stained,
KAR102f rugose, unweathered
FM79
medium size, gray,
KAR102e smooth; weathered
FM78
medium, light beige,
KAR102d smooth; weathered
[Umol]a
(ppm)
a
Abbreviations: LU ¼ assuming linear (continuous) U uptake, p ¼ 0,
[Umol] ¼ U concentration in the molluscs.
b
Ages calculated with parameters listed in Table 4 and cosmic dose rate,
Dcos ðtÞ ¼ 0.293 0.010 mGy/y.
doses show somewhat more variation, the ages do not differ
significantly given their 2 s errors (95% confidence limit). Hence, it
appears that KAR17 does not contain multiple populations of shells.
For KAR105, however, the samples had visually identifiable
subpopulations within the sample. For each, these were first separated into the subpopulations and then analyzed as separate splits
(Table 2b). Among the M. tuberculata splits, the U concentrations
ranged from 12 to 26 ppm, while the Gyraulus had only 7.5 ppm.
Their accumulated doses showed similar variation. For KAR105,
unweathered shells that retained their rugose outer surface gave
younger ages, ranged from 24.0 3.0 to 34.4 5.1 ka. In the strongly
polished shells, the ages ranged from 40.1 5.3 to 50.2 5.8 ka.
Although the weathered shells do not differ statistically from the
oldest of the unweathered shells, they are statistically different from
the other two unweathered shell populations. Therefore, at Railway
3, at least two populations appear to be present, an older weathered
population, and a younger unweathered population.
At Parking Lot Basin, KAR102 again visually identifiable
subpopulations were separated before analysis (Table 2c). With U
concentrations ranging from 0.9 to 1.3 ppm, these shells appear
similar. Their accumulated doses and ages, however, hint that two
populations may be mixed here, one with accumulated doses
averaging 42.0 1.9 Gy and ages at 65.8 5.1 ka and another with
accumulated doses at 54.8 3.3 Gy and ages of 85.7 7.3 ka. Based
on their accumulated doses, these two groups are statistically
different at the 95% confidence limit. In this instance, therefore,
neither the gastropod size nor the presence of weathering uniquely
distinguished the two subpopulations.
434
B.A.B. Blackwell et al. / Quaternary Geochronology 10 (2012) 430e435
ages. Nonetheless, modelling both Dsed ðtÞ, and Dcos ðtÞ allows us to
constrain the fossils’ ages. To model Dcos ðtÞ, species environmental
preferences, geological and geomorphological indicators are used
to estimate past water or sediment depths and, thus, the minimum
and maximum cover thicknesses, as well as the amount of erosion
and its relative timing. With ramped box models with these
constraints, minimum and maximum Dsed ðtÞ and Dcos ðtÞ; minimum
and maximum ages can then be derived (for a detailed discussion,
see Deely et al., 2011).
Fig. 3 illustrates such an analysis for snails from Matana. At both
sites, DBG
ðtÞ ranged from 0.40 0.04 to 0.48 0.05 mGy/y, and
sed:b
DBG
ðtÞ from 0.461 to 0.496 0.03 mGy/y (Table 3). Despite the
sedg
Fig. 3. AM07b’s age vs. external dose rate, Dext(t). As Dext(t) rises, the ages drop and the
differences between the different model ages decrease. Considering the 2 s errors (95%
confidence limit), the ages calculated with a cosmic dose rate, Dcos(t) ¼ 0 mGy/y, do not
differ significantly from those calculated using the time-averaged external dose rate,
Dext ðtÞ, which was modeled for changes in both Dcos(t) and Dsed(t) with time. The ages
calculated using the modern Dcos(t), 292.9 mGy/y, do not significantly differ from those
ages calculated with Dext ðtÞ.
Possibly, reworking or deflation produced these two sets of
mixed shell populations. In a blowout, as the wind winnows away
the fine sediment and the blowout deepens, the heavier grains,
such as hominin artefacts, gastropod shells, mammalian teeth and
bones, drop onto the sediment surface. All traces for some units
may disappear. Thus, deflation can mix coarser grains removed
from any higher eroded units, making it more challenging to date
the artefacts or fossils. Consequently, before final ages can be
calculated, the region must be mapped more extensively to attempt
to determine the original units from which these shells derived,
and to estimate the time-averaged cosmic dose rate, Dcos ðtÞ, and
time-averaged sedimentary dose rate, Dsed ðtÞ, for the shells in
Railway 3 and Parking Lot Basin.
In open air desert sites, determining Dcos ðtÞ, is essential. Since
deflation reworks fossils, changing both their Dsed(t) and their
Dcos(t), this requires modelling in order to calculate reliable ESR
age difference between the two sites, the similarity in the sedimentary geochemical data suggests that both Matana 2 and 3
sediment derived from the same source, whether that source was
aeolian sediment, authigenic carbonate deposited from lacustrine/
pond/spring water at the time the snails lived or later. At Matana 3,
potential sediment cover thicknesses fell in the range of w1e2 m,
while the water depths, although likely of limited duration,
varied from possibly 10 cm to not more than 1 m. The presence
of a significant tufa unit above the snail-bearing silts does suggest
that the water ponded existed here for more than a few years.
Using Dsed ðtÞ and Dcos ðtÞ for AM07b led to a mean
Dext ðtÞ ¼ 1036 80 mGy/y, yielding an LU age of 69.2 6.9 ka
(Fig. 3). This age does not differ significantly from the age,
62.8 5.9 ka calculated assuming continuous exposure to the
modern Dcos(t) (Table 4). AM07b’s age with time-averaged dose
rates also does not differ significantly, 82.5 7.7 ka, calculated
assuming Dcos(t) ¼ 0 mGy/y. While this is clearly an impossibly low
Dcos ðtÞ; given that the snails now sit under w1.0e1.5 m of sediment,
it does set the absolute minimum age. At Matana 2, the mean timeaveraged Dext(t) at 1106 80 mGy/y gave a mean LU age of
27.7 1.9 ka. Again, these ages did not differ significantly from
those using the modern Dcos(t) ¼ 292.9 mGy/y or Dcos(t) ¼ 0 mGy/y
(Table 4). The reality lies somewhere between these extrema, but
does not yield any significant change in the calculated ages for
these snails.
Since two samples at both Matana 2 and 3 constituted the same
species, these samples can be averaged to give a mean for each site.
At Matana 2, the U in the molluscs, [Umol], averaged
0.73 0.09 ppm, but [Umol] ¼ 0.55 0.06 ppm at Matana 3
(Table 4). At Matana 2, A S agree well with a mean at 31.6 1.5 Gy,
while at Matana 3, they agree well, averaging 69.0 2.0 Gy. At
Matana 2, using the time-averaged Dext(t) of 1106 80 mGy/y, gave
a mean LU age of 27.7 1.9 ka. At Matana 3, the time-averaged
Dext(t) ¼ 1036 80 mGy/y, yielding a mean LU age of
65.1 4.1 ka. Therefore, Matana 2 and 3 were deposited during the
Late Pleistocene, in OIS 2 and 4 respectively, indicating that freshwater was available then for hominin use.
Table 3
Sedimentary radioactivity at Matana.
Dose ratesa
Sample
Concentrations
NAA/type
Field
U
(ppm)
Th
(ppm)
K
(wt%)
DBG
ðtÞb
sed;b
(mGy/y)
DBG
ðtÞc
sed;g
(mGy/y)
AM08a-b sed
KAR106b
AM07a-b sed
KAR107d
1.60 0.02
4.68 0.28
0.54 0.02
0.476 0.050
0.496 0.033
1.76 0.02
4.12 0.27
0.44 0.01
0.396 0.043
0.461 0.032
a
Abbreviations: DBG
ðtÞ ¼ sedimentary dose rate from b sourcesDBG
ðtÞ ¼ sedimentary dose rate from g sourcesCalculated with aragonite density, rmol ¼ 2.95 0.01 g/cm3,
sed;g
sed;b
carbonate sediment density, rcal ¼ 2.96 0.01 g/cm3, quartz sediment density, rqtz ¼ 2.66 0.01 g/cm3, sedimentary water concentration, Wsed ¼ 10.0 5.0 wt%, All errors are 1 s.
b
Uses thicknesses from the nearest shell.
c
Uses cosmic dose rate, Dcos ðtÞ ¼ 0.0 0.0 mGy/y.
435
B.A.B. Blackwell et al. / Quaternary Geochronology 10 (2012) 430e435
Table 4
ESR ages for Matana molluscs.
Sample
AM07a
KAR107a
AM07b
KAR107b
Matana 3
Mean (n ¼ 2)
AM08a
KAR106a
AM08b
KAR106d
Matana 2
Mean (n ¼ 2)
[Umol]
(ppm)
Accumulated
dose, AS
(Gray)
ESR Agesa,b
ESR Agesa,c
Dext(t)
(mGy/y)
EU
(ka)
LU
(ka)
RU
(ka)
Dext(t)
(mGy/y)
EU
(ka)
LU
(ka)
RU
(ka)
0.65 0.02
67.30 2.22
1149.9 80.0
54.1 1.9
56.5 1.9
58.1 1.9
1036.2 80.0
59.3 4.7
62.4 5.1
64.4 5.4
0.82 0.02
75.63 5.00
1149.9 80.0
59.3 5.5
62.8 5.9
65.2 6.2
1036.2 80.0
65.9 6.3
69.2 6.9
72.2 7.3
0.74 0.09
68.96 2.04
1149.9 80.0
54.8 1.8
57.2 1.8
58.9 1.8
1036.2 80.0
62.0 3.8
65.1 4.1
67.6 4.4
0.61 0.02
31.90 1.95
1264.9 80.0
24.0 2.0
24.6 2.2
25.1 2.3
1106.1 80.0
27.2 2.5
28.0 2.6
28.7 2.7
0.49 0.02
31.05 2.26
1264.9 80.0
25.7 2.6
26.3 2.6
26.9 2.7
1106.1 80.0
26.7 2.7
27.4 2.8
27.9 2.9
0.55 0.06
31.55 1.48
1264.9 80.0
24.7 1.6
26.4 1.7
25.9 1.8
1106.1 80.0
27.0 1.8
27.7 1.9
28.3 2.0
a
Abbreviations: EU ¼ assuming early U uptake, p ¼ 1, LU ¼ assuming linear U uptake, p ¼ 0, RU ¼ assuming recent U uptake, p ¼ 10, Ages calculated using a/g factor,
ka ¼ 0.10 0.01, initial U activity ratio, (234U/238U)0 ¼ 1.2 0.20, aragonite density, rmol ¼ 2.95 0.01 g/cm3, radon loss from the shells, Rnmol ¼ 0.0 0.0 vol%, carbonate
sediment density, rcal ¼ 2.96 0.01 g/cm3, quartz sediment density, rqtz ¼ 2.66 0.01 g/cm3, sedimentary water concentration, Wsed ¼ 10.0 5.0 wt%, All errors are 1 s.
b
Uses cosmic dose rate, Dcos ðtÞ ¼ 0.293 0.020 mGy/y.
c
Uses Dsed ðtÞ and Dcos ðtÞ.
5. Conclusions
Testing for reworked gastropod assemblages at Medauwara
revealed that two of three did likely contain more than one population. For these assemblages, further geological sampling is
needed to reveal the sources for the two populations before accurate ages can be calculated.
At Matana, two different units were dated by building box
models to calculate the time-averaged cosmic and sedimentary
dose rates. The gastropods from Matana 2 averaged 27.7 1.9 ka,
while those from Matana 3, had a mean LU age of 65.1 4.1 ka.
Thus, during OIS 2 and 4, water was available at Matana for hominin
uses. Therefore, the Khargan Complex occupations that occur atop
the Libyan Plateau and on the escarpment face above Kharga Oasis
depression would have had sufficient water.
Acknowledgements
A.C. Montoya and S.M. Baboumian assisted with some sample
preparation. Jean Johnson and Alice Pedruczny, McMaster University Nuclear Reactor, did the NAA. We thank the National Science
Foundation (EAR-0447357 to JRS, and ILI 9151111 to ARS), Williams
College, RFK Science Research Institute, McMaster University
Nuclear Reactor for logistical and financial support.
Editorial handling by: R. Grun
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