Geoscience Reference
In-Depth Information
(
Chapter 6
defines PDB.) Ten new AMS
14
C ages obtained on shells from two of the
clay pans have confirmed that the interval between 9.9 ka and 7.6 ka, and especially
the 600-year interval from 9.0 ka to 8.4 ka, was perhaps three times wetter than
today (Williams and Jacobsen,
2011
). These ages are comparable to the ages of
the Mesolithic barbed bone harpoon sites of Tagra and Shabona east of the lower
White Nile (Adamson et al.,
1974
; Adamson et al.,
1987a
), as well as the age of a
recently mapped 450 km
2
lake that was fed by an overflow channel from the main
Nile in presently arid northern Sudan between 9.5 ka and 7.5 ka (Williams et al.,
2010b
). Both Blue and White Nile floods were also high during this time (Williams,
2009b
), which suggests that regionally wetter conditions in the eastern Sahara away
from the Nile coincided with times of high Nile flow in the early Holocene (see
Chapter 10
).
Lezine et al. (
1990
) investigated the stable carbon and oxygen isotope stratigraphy
and associated pollen grains in a sediment core fromChemchane sebkha inMauritania
that had originally been studied by Chamard (
1973
). The present salt pan, or sebkha,
was a relatively deep freshwater lake between approximately 8.3 and 6.5 ka, at which
time there was a 400-500 km northward shift of savanna vegetation. This shift of
the early Holocene savanna vegetation belt is consistent with that inferred by Ritchie
et al. (
1985
) and Ritchie and Haynes (
1987
) in northern Sudan some 3,000 km further
east, suggesting that the vegetation may have migrated northwards along the entire
southern Sahara.
7.5 The oxygen, carbon and hydrogen isotope record in desert
speleothems and tufas
18
O,
13
Cand
The results of
D (D/H) analyses of speleothems and, in some
instances, tufa deposits from arid, semi-arid and seasonally wet localities in the
Americas, China, peninsular Arabia, southern Africa and Israel are covered in detail
in
Chapter 14
. As a result, it is only necessary to consider very briefly some of the
assumptions underlying the interpretations of past environments and climatic changes
presented in that chapter. Because the
18
O values in cave speleothems do not afford
an unequivocal temperature signal and can vary with the source of rain water (Vaks
et al.,
2006
) and with rain shadow effects (Vaks et al.,
2003
), it is always useful to
provide an independent quantitative assessment of mean annual cave temperature by
analysing, for example, the
D fluctuations recorded in fluid inclusions within the
speleothems being studied (Matthews et al.,
2000
; McGarry et al.,
2004
). Another
promising (but very time-consuming) approach may be to use 'clumped isotope'
thermometry (Affek et al.,
2008
). In many cases, none of these methods has been used
for independent validation of temperature estimates and until they have, temperatures
inferred from the
18
O fluctuations will remain qualitative inferences.
