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

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

Lunette dune development

Telfer, M.W. and Thomas, D.S.G. (2006) Geology 34, 10, p. 853-856

Lunette dune development

Lunette/linear dune interactions

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.