Geoscience Reference
In-Depth Information
12.5-11.5 ka), which was marked by aridity in Lake Victoria and in the Sahara; flow in
the Nile was probably curtailed and more seasonal during the YD
Sudden decline in Saharan desert dust export to Atlantic at 15.5 ka, with low levels of dust
flux until 5 ka
(
IV. 5-0 ka
Lakes in Blue and White Nile source regions continue to overflow
Weaker summer monsoon and ITCZ retreats
500 km to the south during the NH summer
More seasonal flow in Blue and White Nile rivers and main Nile; Blue Nile carries a mixed
load of sand, silt and clay
Lakes dry out in the Sahara, which becomes abandoned by Neolithic pastoralists who
move south into West Africa or east into the Nile Valley
Deposition of sapropel S1 in the Eastern Mediterranean Sea may have persisted until
5 kyr, when the Nile deep-sea turbidite system also became inactive
Increase in Saharan dust flux to Atlantic
re-established, allowing Upper Palaeolithic hunter-gatherers and Neolithic herders to
penetrate far into the Sahara. Along the northern margins, in contrast, the climate
was mostly dry with minor wet intervals, rivers were less active and the plant cover
indicated warmer-than-present conditions. After about 5 ka, the climate became drier
along both margins and conditions became cooler. Long-term desertification set in,
with human activities aggravating the ecological damage caused by more frequent
droughts and sparser vegetation (Chapters
17
and
24
).
In peninsular Arabia, the evidence from lakes is sparse (Rosenberg et al.,
2011
)
and many of the so-called Holocene lakes may have actually been wetlands fed by
groundwater with limited inputs from local run-off (Enzel,
2013
). However, in con-
trast to the Sahara, there is a rich paleoclimatic archive in the form of speleothems
from limestone caves in Oman and Yemen. As a general rule, speleothems will not
form in desert caves unless there is a significant and reasonably prolonged increase in
regional precipitation. Analysis of the oxygen isotopic composition and fluid inclu-
sions preserved within speleothems dated by U-series has provided a 330,000-year
record of wetter phases in northern Oman and southern Yemen (Fleitmann et al.,
2003
;
Fleitmann et al.,
2009
; Fleitmann et al.,
2011
). Speleothem deposition was rapid at
330-300, 200-180, 130-120, 82-78 and 10.5-6.3 ka at Hoti Cave in northern Oman
(Fleitmann et al.,
2003
; Fleitmann et al.,
2009
). Analysis of the speleothemD/H ratios
(
18
O values revealed that speleothem deposition coincided with interglacial
or interstadial conditions during which groundwater was primarily recharged from
moisture derived from the Indian Ocean when the monsoon rainfall belt extended
further north and reached northern Oman. At Mukalla Cave in southern Yemen, spe-
leothems likewise only formed during interglacial periods, such MIS 9, 7e, 7a, 5e, 5c,
5a and 1 (early to mid-Holocene) (Fleitmann et al.,
2011
). Maximum precipitation
occurred during the last interglacial (MIS 5e) and the lowest precipitation occurred
D
)and
