Geoscience Reference
In-Depth Information
forming nebkhas (Fig.
7.6
) There are many reports in the
agricultural literature on the use of different plants to sta-
bilize dunes found among or close to croplands or orchards,
as well as to decrease the loss of topsoil to the wind. At
present, the Earth is the only place within our solar system
where vegetation plays an important role in the modification
of aeolian transportation of sand.
the shallow seabeds of the continental shelves would have
been dry, making the sands there available for transport
inland (e.g., much of the Arabian Gulf was dry, with the
exposed
carbonate-rich
sand
blowing
into
the
Arabian
deserts).
Although both dryer and wetter periods can prevail
(notably, a few thousand years ago the Sahara was wetter
and thus much more habitable than today) the climate of the
last few centuries has been wetter and therefore less con-
ducive to dune migration than, say, 20,000 years ago. Thus
what were previously active dunes are now covered in soils
and vegetation; there are many such dune systems in Eur-
ope, and in America the Nebraska sand hills serve as an
example. However, the more recent warming of the climate
may lead to dryer soils and thus reactivation of dune sys-
tems, notably in the Kalahari. Although dunes are likely
never to rival lake sediment cores or ice cap profiles for
paleoclimate reconstructions, some important information
can be recovered from examination of dune sands with
particular relevance to human survival, notably, measure-
ment of the amount of organic matter, and age via carbon-
Wind patterns can also change as the climate changes.
For example, the annual monsoonal flow coming up from
the Indian ocean may today not penetrate as far into the
Arabian peninsula as it once did. This means that some
areas (e.g., Fig.
7.11
) which previously had seen a bidi-
rectional wind regime now see a more unidirectional one,
and the dune type (linear vs barchan) may be changing as a
result. We discuss this morphodynamic age, and its appli-
11.6
Links Between Sand Dunes and Climate
The formation of dunes requires mobile (i.e., dry) sand, and
wind. Clearly, changing climate can affect these quantities
directly, but the influence of moisture on Earth is magnified
by the role of vegetation. If sand is damp, not only is its
cohesion higher (and thus the threshold wind stress higher),
but it may allow plants to grow. Thus an increase in fre-
quency of damp conditions can lead to further binding of
the sand by plant roots themselves, and ultimately by
organic matter from dead plants and animals that may feed
upon it. To this is added the effect of plants on the aero-
dynamic roughness.
Changes in climate, particularly those over tens of
thousands and hundreds of thousands of years, associated
with the Croll-Milankovich cycles (periodic variations in
orbital and rotational parameters of the Earth which influ-
ence the distribution of sunlight at high latitudes) are
therefore expected to affect sand mobility. Notably, during
the ice ages (when sea level was *100 m lower than today
because the water was locked up in continental ice sheets)
the atmosphere overall may have been dryer. Furthermore,
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