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| Titel |
Quaternary palaeoenvironments in Namibia: new records from optically stimulated dating of Kalahari linear dune accumulation and northern Namib Sand Sea interdune deposits. |
| VerfasserIn |
Abigail Stone, David Thomas, Heather Viles, Richard Bailey |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044111
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| Zusammenfassung |
| In this study we consider new linear dune accumulation records from the west of
the southern Kalahari linear dunefield (~24o S 18oE) and a new chronology for
three sites along the former course of the Tsondab River in the northern Namib
Sand Sea (23o49’15”S, 14o57’30”E, 23o48’39”S, 15o02’21”E and 23o56’11”S,
15o17’89”E) from Namibia, southern Africa. Optically stimulated luminescence dating
(OSL) has been applied to both sites in order to: (i) provide a chronology for linear
dune accumulation in a previously unstudied region of the southern Kalahari linear
dunefield and (ii) to provide a chronological reassessment for water-lain units from the
Namib Sand Sea, previously dated using radiocarbon applied to inorganic carbonate
sediments.
The 48 OSL ages from the linear dunes include the oldest linear dune sediment ages
recorded in the southern Kalahari and also reveal that the dunefield has been partially active
throughout much of the past 120 ka. These ages are considered alongside the net
accumulation record for the entire southern Kalahari linear dunefield (141 optical ages) and
we make some important observations about the influence of sampling strategy. We attempt
to isolate a rigorous signal that does not depend on sampling strategy by removing cores and
comparing the net accumulation record from different parts of the dataset. We consider
the young bias resulting from the predominance of shallow sampling in the total
dataset.
The bracketing OSL ages (from sand units that inter-bed water-lain calcareous rich
sediments) in the northern Namib Sand Sea suggest that the series of seven mud units at
Narabeb (23o49’15”S, 14o57’30”E) are substantially older than the existing radiocarbon
chronology from the mud units themselves. Twelve of the thirteen OSL ages fall within
MIS 5. By contrast, at Ancient Tracks (23o56’11”S, 15o17’89”E) the OSL-based
chronological reassessment suggests the water-lain units were 1 to 2 ka younger than
the existing radiocarbon ages, with deposition between 12.8 ± 0.8 ka and 12.0
± 0.8 ka and between 11.5 ± 0.6 ka and 10.5 ± 0.5 ka. Together the three sites
provide a new estimate for the timing of the progressive desiccation of the Tsondab
River.
We conclude that larger datasets of linear dune accumulation (1000s not 100s of OSL
ages) are needed, and also to demonstrate that altering sampling strategy within this dataset
does not bias the combined-core results, before we have a reliable dataset of dune
accumulation in this region. This is before tackling the additional issue of interpreting the
palaeoenvironmental significance of the dune accumulation record. For the Namib Sand
Sea we conclude that the revised timing for increased moisture availability to the
catchment of the Tsondab River still reinforces the idea that changes in moisture
availability are driven by precipitation changes over the catchments in the Great
Escarpment and further inland. Finally, we make some brief observations regarding how
these two new records compare with the current regional palaeoclimatic syntheses. |
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