Geoscience Reference
In-Depth Information
the sapropel record in the eastern Mediterranean is incomplete, with some evidence
of complete removal of sapropels by post-depositional oxidation (Higgs et al.,
1994
),
it is a longer and more complete record than that presently available on land, and so
can serve as a useful surrogate record for Nile floods and phases of enhanced summer
monsoon precipitation.
10.10 Nile Delta records of Holocene fluctuations in Nile flow
Marriner et al. (
2012
) carried out a rigorous statistical comparison between mean rates
of Nile deltaic sedimentation during the past 8,000 years (8 ka) and well-dated proxy
records of climatic change in sites as far away as the Cariaco Basin off Venezuela,
speleothems from Oman and China, a detailed lake diatom record from Ethiopia
and Neolithic pastoral sites from the eastern Sahara. Two main conclusions emerged
from their analysis. One, evident in high rates of deltaic sedimentation, confirmed
that the early to mid-Holocene moister interval was associated with a northward
displacement of the Intertropical Convergence Zone (ITCZ). Desiccation after 5.5 ka
reflected a southward displacement of the ITCZ and a progressive decline in deltaic
deposition. Nile Delta accretion during the Holocene depended on suspended sediment
inputs from upstream, and these in turn were related to changes in monsoon strength
controlled ultimately by the earth's orbital geometry.
The second significant conclusion concerns the link between El Ni no-Southern
Oscillation (ENSO) events, Nile flow, Nile sediment discharge and regional hydro-
climatic changes at submillennial time scales. This link has long been very well-
established for the time of instrumental records (see
Chapter 23
) but had not been
shown explicitly for the Holocene. One tantalizing suggestion is that a weak statistical
correlation between Nile deltaic sedimentation and proxy climate records during the
interval 4.6-2.5 ka may reflect increasing human impacts in the Nile Valley, triggering
changes in deltaic deposition.
10.11 River channel incision and deposition
It can be argued that the Blue Nile depositional model illustrated in
Figures 10.12
and
10.13
is based on an unproven assumption, namely, glacial aridity. After all, other
workers had used the evidence afforded by late Pleistocene Nile sands and gravels
flanking the Nile in northern Sudan and southern Egypt to argue for greater fluvial
competence and consequently higher discharge and more pluvial glacial conditions
(Butzer and Hansen,
1968
). The contentious issue of glacial aridity versus glacial
pluvial is discussed in
Chapter 12
. Suffice to say here that the inference by Adamson
et al. (
1980
) that the late Pleistocene was a time of greater aridity in the Nile head-
waters was based on the fact that during the Last Glacial Maximum, lake levels in
Ethiopia were low (Gasse,
1975
; Gasse,
2000a
; Gasse,
2000b
), as they were in Kenya
