THERMOLUMINESCENCE (TL) DATING OF INACTIVE DUNES FROM THE RIO
GRANDE DO NORTE COAST, BRAZIL
MárcioYee*, Sonia Hatsue Tatumi**, Kenitiro Suguio***, Alcina M. F. Barreto****, Erica
F.Momose*****, Rosemeire P.Paiva*****, Casimiro S.Munita*****
Abstract
Thermoluminescence dating of inactive
dunes from the State of Rio Grande do Norte coast
(Brazil) were performed in 19 samples. The age
should represent time elapsed after the last sunlight
exposition of the grain, for the time when the dunes
became fixed. The age showed important phases of
eolic activity in the region, during Upper
Pleistocene, between 390-190 k years BP and 64-15
k years BP, also after Middle Holocene up to the
present.
Introduction
The ages of samples were determined by
Thermoluminescence
(TL)
method,
whose
applications to age determination of sedimentary
deposits was initiated in the 80’s decade (Wintle &
Huntley, 1980), especially in eolian deposits ( Less
et al., 1990). In Brazil, after the pioneer wiks of
Poupeau et al. (1984 and 1988), this is one if the few
systematic application of this method, except for the
study of Barreto (1996; apud Tatumi et al., 1998).
Dose dependence of the TL intensity of
gamma irradiated samples and effect of sunlight
exposure were studied. Theoretical fitting of the TL
glow cure emission was perfoemed by using second
order kinetics. The sediment equivalent dose (AD)
was estimated by the additive dose method ( Singhvi
et al., 1982) and the total bleach regeneration
method ( Debenham and Walton, 1983; Proszynska,
1983).
TL dating allows to obtain ages of few tens
up to 106 years BP, depend on sample properties and
the sensitivity of the TL reader. In this method it can
be used the quartz grains, that were obtained, for
example, from eolian, fluvial and marine sediments.
Inactive dunes from the State of Rio
Grande do Norte coast (Brazil), with regional
dimensions, could be considered as geomorphlogical
and sedimentological indicators of paleoclimates
drier than, the present, during the Quaternary.
Probably, these coastal dunes were generated due to
intensified eolian activity, attributable to past more
arid conditions in the area.
On the other hand, their most pronounced
expansion periods could be correlated with sea-level
drop episodes. Otherwise, sand nourishment to the
coastal plains occurred dominantly during the
regression following a sea-level rise, being intensely
reworked by the winds.
Consequently, TL dating of these natural
environmental changes are crucial to better
understand geological evolutionary history of the
area in the Quaternary.
Study Area
The State of Rio Grande do Norte coast
stretch-out for about 400km in the Northeastern
Brazil (Figure 1). According to Silveira's (1964)
classification it belongs to the “Barreiras” littoral,
which is characterized by a ubiquitous presence of
the “Barreiras” Formation outcrops. These
continental deposits are Tertiary in age, and form a
gentle seaward dipping tableland, between the
coastal plain and the sublittoral zone. This tableland
is moderately dissected by steep-slope valleys, and
finishes bordering the beaches as active sea-cliffs.
The coastal plains made-up of beach- ridges are
unimportant or even absent in this state, and other
very common coastal features include lakes, lagoons
and beach rocks. Inactive and active eolian dunes are
conspicuously observed along the State of Rio
Grande do Norte coast. The inactive dunes are more
frequent between Tibau do Sul and Touros, with an
estimated area of 120km2. The active dunes, on the
other hand, are situated dominantly from Touros
town to the State of Ceará boundary.
* Tecnólogo em Processos de Produção pela FATEC-SP.
** Professor Pleno do Departamento de Ensino Geral da FATEC-SP/CEETPS, Mestre em Física do Estado Sólido pelo IFUSP, Doutor
em Ciências pelo IFUSP.
*** Instituto de Geociências/USP, Rua do Lago, 562, cep. 05508-900.
**** Universidade Federal de Pernambuco, Departamento de Geologia, Avenida Acad. Hélio Ramos, s/n, Cid. Universitária, cep.
50740-530, Pernambuco.
***** Instituto de Pesquisas Energéticas Nucleares, Supervisão de Radioquímica, Rua Travessa R, 400, cep. 05508-900, São Paulo.
Á
Touros
A
R
5º
RR
E
Natal
C
AP
RN
AL
SE
TO
RO
A
PE
AC
A
R
PB
PI
B
A
P
CE
MA
I
PA
AM
BA
MT
GO
45º W
MG
ES
MTS
SP
RJ
0º
PR
SC
RS
0
50
75º
100 km
30º
35º
Figure 1 – Location map of the Study in State of Rio Grande do Norte, Brazil.
The inactive dunes are overspreaded on the
“Barreiras” Formation tableland. In general, they are
vegetated by fragmental forests with Atlantic coast
psamophytic plants. These dunes, sometimes with
gigantic dimensions, present a parabolic compound
shape, whose original morphology is partially
modified by erosional and/or pedogenetic processes.
The parabolic dunes are variable in height from 15
to 70m and in length from less than 2km to l9km.
Sometimes, the dune morphology was completely
destroyed, and only a sandy and gently undulated
surface was observed by Barreto et al. (1998).
Gomes et al (1981) distinguished the
inactive and active eolian dunes, in this coastal
plain, based on the presence or absence of
vegetation, respectively. On the other hand,
Nogueira et al (1975) and Perrin & Costa (1982)
established two generations of dunes based on the
color of the sands. According to these authors, the
darkish color (red to orange) of fixed dunes would
be older, and the lightish color (yellow and white) of
mobile dunes would be younger.
Finally, the ages of inactive coastal dunes
and the most important sea-level and climate
oscillations episodes could be correlated, when
adequately supported by the following assumptions.
The first is that, there is a close relationship
between these environmental changes and the origin
of several generations of coastal dunes (Short,
1988), and the second is that, these environmental
changes occurred at least in a semi-regional scale
(Giannini,1993).
Materials And Methods
Nineteen sediment samples were collected
from inactive eolian dunes, Figure 2 shows the all
the ages found in the present work.
The collected samples were submitted to
chemical treatments composed by HCl (20%) to 20
minutes and HF (20%) during 2 hours, to selected
only grains of quartz and feldspar. The grains of
quartz were obtained to the heavy liquid “Sodium
Polytungstate”, followed by sieving to separate the
grain sizes between 88 and 180 µm. All γ irradiations were performed with a 6OCo source of
the IPEN-CNEN (Instituto de Pesquisas Energéticas
e Nucleares - Comissão Nacional de Energia
Nuclear) laboratories. For the TL measurements, a
Daybreak
Nuclear
and
Medical
Systems
Incorporated, Model 1100-Series Automated
TL/OSL System, with a Schott BG-39 optical filter,
was used.
The accumulated doses were obtained using
Additional Doses Method following the Multiple
Aliquot Protocol with Natural Normalization (NN),
and the Total Bleach Method from Aitken (1985).
The annual doses were calculated using 235U, 238U,
232
Th and 40K concentrations and the Bell's
equations.
Radioactive
concentrations
were
measured by Neutron Activation Analysis, see Table
I.
SAMPLE
U
(ppm)
0.45±0.01
0.85±0.02
0.41±0.01
1.08±0.02
0.25±0.01
0.30±0.02
0.19±0.01
0.31±0.01
0.86±0.04
1.20±0.09
0.89±0.06
0.31±0.05
0.84±0.02
0.45±0.02
0.59±0.02
0.63±0.02
1.12±0.03
2.01±0.05
0.94±0.07
1.14±0.07
0.53±0.06
0.33±0.04
1.29±0.05
RN-TL1
RN-TL2
RN-TL3
RN-TL4
RN-TL5
RN-TL6
RN-TL7
RN-15/98
RN-TL22/98
RN-TL32/98
RN-TL38-98
RNTL38/98-A
RN 02
RN03
RN07
RN27
RN40
RN41
RN42
RN43
RN44
RN45
RN46
Th
(ppm)
1.61±0.02
2.08±0.02
2.33±0.02
6.70±0.05
2.25±0.02
0.97±0.01
0.40±0.01
1.09±0.01
4.03±0.04
6.76±0.06
4.52±0.04
0.70±0.01
1.02±0.01
2.24±0.02
3.93±0.03
2.25±0.02
4.29±0.03
12.29±0.06
2.28±0.02
2.69±0.02
1.57±0.02
1.07±0.01
4.56±0.04
Results And Interpretations
TL glow curves of the studied samples
supplied peaks at 325 and 375ºC. The intensities of
the 375ºC peak were used, to obtain the paleodoses
and equivalent doses values. TL growth curves have
been linear, sublinear and exponential behaviors in
the Total Bleach Method. Figure 2 shows an
K-40
(10-5%)
1.59 ± 0.03
1.246 ± 0.001
0.591 ± 0.005
0.063 ± 0.005
0.226 ± 0.003
2.94 ± 0.02
0.340 ± 0.007
7.5 ± 0.1
1.91± 0.03
5.81± 0.08
6.02 ± 0.07
5.35 ± 0.07
1.2±0.1
3.09±0.2
1.9±0.1
0.71±0.2
0
18.5±0.9
18±1
20±1
7.7±0.6
8.6±0.5
10.5±0.5
Annual Dose
(µGy/ano)
419±4
557±6
463±4
963±9
415±4
332±6
261±3
344±3
706±13
999±28
751±19
315±14
475±6
466±7
629±7
513±7
793±10
1.623±17
596±20
678±20
437±17
348±11
857±16
P
( Gy )
14.5±2
0.12±0.01
1.8±0.3
0.4±0.1
89±2
70±9
0.9±0.3
71±3
45±3
117±7
141±5
84±3
185±5
112±3
150±7
18±1
4.5±0.4
40.1±0.4
1.5±0.1
6.2±0.2
4.1±0.2
22.0±0.4
128±3
example of the natural and 20 Gy of γ - dose
irradiated and previously submitted to 16 hours of
sunlight TL glow curves of the sample RN38-98. TL
growth curve the above cited sample is shown in
Figure 3, in this case it was observed single
saturating exponential growth.
1,2x106
Thermoluminescence (arb. units)
8,0x10
Thermoluminescence (arb. units)
38-98-RN
Natural
16h sunlight + 20Gy
5
6,0x105
4,0x105
2,0x105
0,0
1,0x106
RN38-98
8,0x105
6,0x105
P=(141+-5)Gy
AD=(751+-19)µGy/yr
I= (1,9+-0,1)105years BP
4,0x105
2,0x105
0,0
0
50
100
150
200
250
300
o
Temperature C
350
400
450
-50
0
50
100
150
200
250
Dose (Gy)
Figure 2 - TL glow curve of the natural quartz grains and 16 Figure 3 – TL growth curve of quartz grains fitted single
saturating exponential equation.
hours of sunlight and γ- rays irradiated quartz grains.
Figure 4 shows TL glow curves of the same
sample with additive dose and Figure 5 the
respectively growth curve, in this case the
experimental points were fitted with exponential
growth curve. It was noted the same exponential
growth in both methods. The age in the six samples
using the two already cited methods showed a
difference from 3% to 17%.
1,60E+008
TL ( arbitrary units)
2,0x107
RN38-98
nat + 25Gy
nat + 75Gy
nat + 100Gy
nat + 150Gy
nat + 200Gy
residual
natural
1,40E+008
RN 38-98
TLpeak 375 0C
1,20E+008
TL Intensity (arb.units)
2,5x107
1,5x107
1,0x107
6
5,0x10
1,00E+008
8,00E+007
Q=(142+-9) Gy
AD=(751+-19)µ Gy/yr
I=(1.89+- 0.1)105yearsBP
6,00E+007
4,00E+007
residual TL(16hourssun-light)
2,00E+007
0,00E+000
0,0
100
-250
200
300
400
-200
-150
500
-100
-50
0
50
100
150
200
Dose (Gy)
0
Temperature ( C)
Figure 4 – TL glow curve of the natural quartz grains and Figure 5 - TL growth curve of the quartz grains fitted
with single saturating exponential equation.
samples submitted to crescent doses of γ- rays.
Conclusion
The TL glow curves of the studied samples
supplied peaks at 325 and 375ºC. The intensities of
the 375ºC peak were used, to obtain the paleodose
and the equivalent dose values.
They have linear and sublinear TL growth
curves in the majority of samples isung the Total
Bleaching method.
In the Additive Dose Method has been used
the exponential curve to determine the equivalent
dose.
The age in the six samples using the two
already cited methods showed a difference from 3%
to 17%.
The obtained ages (Table I) would
represent the interval elapsed after their last
subaerial exposition, that is, the time when the
inactive dunes have been stabilized. According to
the obtained data, eolian activities have been very
intensified between 190-390 ky (5 samples) and 6414.9 ky (6 samples); both intervals belong to the
Upper Pleistocene. The obtained ages suggest that
probably the older interval is attributable to the
Illinoian and the younger could be ascribed to the
Wisconsinian glacial stades, when the sea-levels
dropped, and the sands accumulated during the
previous high sea-level episodes were intensely
reworked by winds.
After the Middle Holocene eolian activities,
in the studied area, became again important, possibly
due to renewed sea-level drop, that occurred after
the Holocene sea-level culmination stage about, 5ky
ago (Suguio et al.,1985).
Finally, several generations of dune sands,
in the study area, cannot be distinguished only by
difference in colors. This is, probably, because the
colors of dune sands are not related only to the
burial time, but are also related to the nature of
source area, climate during the sedimentation, type
and content of clay minerals, etc.
Acknowledgements
Field works partially supported by FAPESP
(Fundação de Amparo à Pesquisa do Estado de São
Paulo).
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