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An OSL-dated palaeoenvironmental record from a pan-lunette complex and
the linear dunes at Witpan, southwestern Kalahari
Matt Telfer
Prof. Dave ThomasDr Mark Bateman
UK Luminescence and ESR meeting 2006, University of Liverpool
Title
Background
• Much of the Kalahari landscape is characterised by vegetated linear dunes
• These have long been interpreted as evidence of former arid periods
• Until the 1990s, the chronology for these landforms was extremely vague
Title
Aims and Objectives
• Extend the aeolian record with coring techniques.
• By using a detailed sampling strategy within cores as well as within the locale, investigate the continuity and replicability of dune records.
• Consider the implications of the record of aeolian accumulation for the regional palaeoclimatic history of the Late Quaternary.
Title
Field site
• Southwestern Kalahari of the Northern Cape, South Africa.
• Witpan contains a range of landforms which may act as palaeoenvironmental indicators – a pan-lunette complex set amongst linear dunes.
1km
Title
Field site
• Southwestern Kalahari of the Northern Cape, South Africa.
• Witpan contains a range of landforms which may act as palaeoenvironmental indicators – a pan-lunette complex set amongst linear dunes.
• Detailed sampling strategy.433
3
4
410
4
6
1 1
411
214
173
10
1km
Title
Methods
1km
• 113 samples for OSL dating
• Initially, SAR analyses on the 90-150 μm or 180-212 μm HF-etched quartz fraction.
• Generally, samples were bright, well-bleached sands showing low inter-aliquot variability.
Kal 02/7.5
0.00
0.10
0.20
0.30
0.40
0 2 4 6 8 10 12 14 16 18 20
Palaeodose (Gy)
P
Title
Methods - SGC
1km
• In view of large number of samples, experiments were conducted with the very rapid Standardized Growth Curve (SGC) methods of Duller and Roberts (2003).
• Curve created using large bank of data, pre-analysed by SAR methods.
4290 dose-response points between 0 – 50 Gy, with an uncertainty of around 5%.
0
5
10
15
20
25
30
35
0 10 20 30 40 50
Dose (Gy)
OS
LN
0
1
2
3
4
5
6
7
8
9
% U
nc
ert
ain
ty (
1 s
igm
a)
Title
Methods - SGC
1km
• 23 samples analysed by SAR and independent SGC to test validity of method.
• All samples within 1σ errors. The only sample with a SAR/SGC discrepancy of > 10% is around 0.1Gy.
• Method shows considerable potential in facilitating the analysis of very large data sets.
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35 40
Age determined by SAR (ka)
Ag
e d
eter
min
ed b
y S
GC
(ka
)
Mean ratio = 1.00 ± 0.06
Linear deposition – intra-core variation
9.0 ± 0.6
1.8 ± 0.1
10.2 ± 0.6
9.8 ± 0.6
9.1 ± 0.6
10.0 ± 0.5
9.3 ± 0.6
10.9 ± 0.7
10.8 ± 0.7
10.5 ± 0.7
3.5 ± 0.2
5.9 ± 0.4
8.9 ± 0.5
0.16 ± 0.02
Kal 02/7
Dep
th
(m)
Age (ka)
0
2
4
6
8
10
12
14
16
0 2 4 6 8 10 12 14
Linear deposition – inter-core variation
12.2 ± 0.8
56 ± 3
76 ± 8
73 ± 7
72 ± 7
12.1 ± 0.8
12.3 ± 0.7
Kal 04/6 Kal 02/1
0.15 ± 0.02
3.1 ± 0.3
4.9 ± 0.3
0.16 ± 0.02
10.1 ± 0.6
9.2 ± 0.6
10.3 ± 0.6
12.7 ± 0.8
57 ± 3
10.4 ± 0.6
9.4 ± 0.5
Age (ka)
Dep
th
(m)
0
1
2
3
4
5
6
7
8
9
10
0 20 40 60 80 100
5m
SW Kalahari linear dune accumulation
0%
2%
4%
6%
8%
10%
0 20 40 60 80 100 120
Age (ka)
Pn = 7
Stokes et al 1997
0%
2%
4%
6%
8%
10%
0 20 40 60 80 100 120
Age (ka)
P
n = 19
Stokes et al 1997, Thomas and Shaw 2002, Bateman et al 2003
SW Kalahari linear dune accumulation
0%
2%
4%
6%
8%
0 20 40 60 80 100 120
Age (ka)
Pn = 88
Stokes et al 1997, Thomas and Shaw 2002, Bateman et al 2003, This study
SW Kalahari linear dune accumulation
Title
Conclusions
• Much of the lunette is even younger than suspected previously – full-depth (9m) reworking/deposition over the past 1000 years.
• The response of the lunette dune itself is subject to the influence of differing sediment supply along its length.
• The linear and lunette dunes at Witpan respond in different ways to environmental stimuli.
• A major period of linear dune accumulation at Witpan occurred at 10-11ka, although there has been near-continuous low-intensity Holocene deposition. Previous periods of dune activity are recorded in this area.
Title
Conclusions
• Extend the aeolian record with coring techniques.… back to c. 100 ka
• By using a detailed sampling strategy within cores as well as within the locale, investigate the continuity and replicability of dune records.
… evidence of both rapid and slow accumulation discovered, as well as complex lunette development
• Consider the implications of the record of aeolian accumulation for the regional palaeoclimatic history of the Late Quaternary.
… work is ongoing, but a cyclical record does exist, apparently controlled by variations in the
strength of the continental atmospheric circulation
?
Implications for further work
• The Kalahari’s linear dunes have the potential to record long – albeit discontinuous – sequences of aeolian accumulation.
• A highly detailed and rigorous sampling strategy is required to discern details of the aeolian record.
• Palaeoclimatic interpretation as to what these depositional events mean is greatly aided by the application of other proxies.