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fragile, secondary recrystallisation processes are
probable and could make the ages too young. As we
cannot be completely sure that acid leaching removes
all the possible recrystallisation, we prefer to express
the results as minimum ages, except of course for charcoal
samples.
4.2 - OSL DATES (tab. 2 and fig. 4)
The optical dating method (Aitken, 1998) was applied
to 6 sediment samples from Unit 2. Their precise
stratigraphic positions are indicated on figure 4, and
they span the full sedimentary sequence, from near the
base (layer 6) to the top layer 35. All samples are fine
silts, except those of layer 31 which are sands.
For each sample, middle-sized quartz grains (40-
50 μm) were extracted and isolated, according to the
procedure described in Mercier et al., (2003). To determine
the mean equivalent dose (ED), a single-aliquot
regenerative dose (SAR) protocol (Murray & Wintle,
2000) was applied to several sub-samples (at least 24
aliquots per sample). All luminescence analyses were
done with a Risoe TL/OSL DA-15 reader equipped
with blue light emitting diodes for optical stimulation,
and artificial doses were given by a calibrated Sr/Y-90
beta radioactive source fitted to the reader.
Radioisotopic contents of bulk sediment samples
were measured with a high-purity Ge detector; no disequilibrium
in the U-series and Th-series was detected.
Alpha and beta dose-rates were deduced from these
values using the conversion factors given by Adamiec
and Aitken (1998) and from attenuation factors relevant
to the 40-50 μm granulometric fraction (Mejdahl,
1979; Brennan et al., 1991). Quartz grains were assumed
free of radioelements. α-sensitivity of 5 μGy/a/103 σ was
used for all samples.
Gamma and cosmic dose-rates were deduced from
field measurements done with a portable gamma
spectrometer. Annual dose-rates were computed by assuming
a mean past water content of 10% (±3%) by
weight.
For each sample, at least 24 aliquots, each composed
of hundred of grains, were analysed in order to evaluate
the distribution of the individual equivalent doses. Depending
on the sample, the distributions indicated a
more or less important scatter of the individual values
around the arithmetic mean, probably due to incomplete
bleaching of the grains or because of the presence
53
Sample Layer U Th K Alpha Beta Gamma Cosmic Annual Equi.
Dose
Age
n ° (ppm) (ppm) (%) Doses (m Gy/a) + – (Gy) + – (ka) + –
OSL 19 35 2,96 8,32 2,32 130 2132 1162 200 3624 43 112 1,5 30,9 2,5
OSL 1 31 2,68 6,43 2,42 110 2767 1238 144 4259 130 119 1,2 27,9 2,4
OSL 2 31 2,76 6,03 2,28 75 2608 1183 148 4014 127 124 2 30,9 2,7
OSL 18 28 2,95 7,35 2,02 123 1904 1054 200 3281 42 128 2 39 3,2
OSL 3 14 3,83 7,44 1,77 144 2314 1031 124 3613 113 133 3 36,8 3,2
OSL 17 6 2,45 7,56 2,28 113 2025 1069 110 3317 42 163 5 49,1 4,3
Tab. 2: OSL dates of Fort Oued Noun section.
Travertines
Layers
238U ppm 234/238U mes 230/234U Age ka 234/238U 232Th ppm 230/232Th Age ka,
corrected*
+/–
35 0,854 +/– 0,020 4,931 +/– 0,096 0,250 +/– 0,003 30 +2,3 -2,2 5,275 +/– 0,104 0,924 +/–0,019 3,47 21,6 +4,3
33 3,07 +/– 0,053 3,73 +/– 0,042 0,313 +/– 0,003 38,9+1,6 –1,6 4,041 +/– 0,05 4,284 +/– 0,06 2,55 24,4 +1,6
–1,5
3 1,81 +/– 0,088 3,79 +/– 0,14 0,528 +/– 0,009 73,6 +7,9 –7,3 4,421 +/– 0,172 3,178 +/0,04 3,46 55,9 +9,3
–8,6
3, leaching 1,48 +/– 0,098 4,64 +/– 0, 245 0,445 +/– 0,011 59,0 +10 –9,1 5,291 +/– 0, 289 1,52 +/– 0,06 6,1 50,9 +10,8
–9,8
3-5 2,11 +/– 0,034 3,39 +/– 0,061 0,677 +/– 0,009 105 +8,7 –8,1 4,20 +/– 0,98 3,12 +/– 0,07 4,73 87,6 +9,1
–8,5
5 1,39 +/– 0,03 3,90 +/– 0,08 0,643 +/– 0,01 96,3 +11 -10 4,80 +/– 0,11 1,79 +/– 0,06 5,92 Isochron age
Sample 5
5 1,81 +/– 0,041 4,25 +/– 0,088 0,618 +/– 0,007 90,7 +8,3 –7,7 5,19 +/– 0,11 1,053 +/– 0,04 13,8 92,2
5 1,72 +/– 0,063 3,64 +/– 0,133 0,723 +/– 0,025 115 + 33 - 25 4,64 +/– 0,18 2,691 +/– 0,22 5,13
Waters
Hotel
Oued Noun
238U ppb
2,83 +/– 0,10
5,82 +/– 0,20
5,12 +/– 0,12
5,07 +/– 0,10
Tab. 3: U/Th dates of Fort Oued Noun section.
Errors quoted are one sigma counting statistics.
: corrected ages = corrected for detrital contamination with 230Th/232Th =1, measured in the sedimentary detrital fraction.