Geoscience Reference
In-Depth Information
five former deltas of the Boteti River, indicating deposition in a series of former lakes.
The Boteti River becomes the Okavango River upstream and arises in the seasonally
wet uplands of Angola. The Okavango Delta is one of the largest inland deltas in
Africa and is separated from the Makgadikgadi Pans by a major tectonic lineament
running from north-east to south-west, called the Kunyere Fault in the south-west
and the Thamalakane Fault in the north-east. Cooke and Verstappen (
1984
) obtained
twenty radiocarbon ages from across the Makgadikgadi Basin, sixteen of which were
on calcrete (see
Chapter 15
). They ranged in age from 46 ka to 1.7 ka and cannot be
considered very reliable. The challenge of obtaining a reliable chronology was taken
up by several groups of workers (Thomas and Shaw,
2002
; Huntsman-Mapila et al.,
2006
; Burrough et al.,
2007
; Burrough et al.,
2009a
; Burrough et al.,
2009b
). Detailed
OSL ages are now available for the former high lake strandlines of Lake Ngami
(Huntsman-Mapila et al.,
2006
; Burrough et al.,
2007
) and the Mababe Depression
(Burrough and Thomas,
2008
). More than 140 OSL samples show multiple lake full
phases for the 'Palaeolake Makgadikgadi', which at its highest recorded level covered
an area of 66,000 km
2
. More recent mapping using unusually high-quality helicopter
time-domain electromagnetic data suggests an area in excess of 90,000 km
2
(Podgor-
ski et al.,
2013
). As the lake level fell, the former lake split into three component
basins (Ngami, Mababe, Makgadikgadi), all of which have well-defined shorelines
(Burrough et al.,
2009a
). High shorelines have yielded OSL ages of 8, 17, 27, 39, 64,
92 and 104 ka with relatively small error terms. Beyond that, the error terms increase:
131
25 (Burrough et al.,
2009a
). Modelling
the late Quaternary hydrology of the mega-lake suggests that once the lake attained a
threshold size, it developed the capacity to influence both local and regional climate
(Burrough et al.,
2009b
). Huntsman-Mapila et al. (
2006
) concluded that there was
an anti-phase relationship between late Quaternary rainfall in southern Africa and in
equatorial Africa, with Botswana dry when the Angolan highlands were wet, much
as occurs today. The levels in Lake Ngami were high between 19 ka and 17 ka, at the
same time that the central-southern African region showed evidence of increased arid-
ity. Put succinctly, the LGM was arid in Botswana but wet in the Angolan headwaters
of the Okavango.
In attempting to reconstruct changes in regional climate, it is important not to rely
on the record of any one lake, because local hydrologic influences may sometimes
obscure or outweigh the impact of regional climatic fluctuations. Even lakes in close
proximity to one another may show quite different response times. For example, Lake
Masoko in Tanzania shows a lag of about 1,000 years in maximum inferred effective
humidity compared to LakeMalawi, located only 30 km away (Gasse et al.,
2008
). The
late Quaternary history of Lake Masoko during the last 45 ka has been reconstructed
from sedimentary and magnetic data (Garcin,
2006
; Garcin et al.,
2006a
; Garcin
et al.,
2006b
; Garcin et al.,
2007
), pollen (Vincens et al. (
2007
) and diatom analyses
(Barker et al.,
2003
), all of which indicate driest climatic conditions from 33 to 23 ka,
±
11, 211
±
16, 267
±
27 and 288
±
