Abstract
Residual dune ridges are often formed by vegetation growing along a line
some distance upwind of the lower stoss slope of migrating dunes. This
process is common in areas where vegetation germinates along the edge of
the water during the rainy period when the water level is higher and
interdune areas are flooded. The phenomenon occurs on a large scale in
North-east Brazil, because of the rise and fall in groundwater level at
the end of the rainy season. Each residual dune ridge corresponds to the
position of the dune during the wet period in each year. Therefore,
variations in the distance between these residual dune ridges could be
used potentially to monitor climatic fluctuations in rainfall and wind.
To examine the potential use of these residual dune ridges for the
reconstruction of past climatic fluctuations, a model that simulates
them under varying conditions of wind, rainfall and evaporation rates
was formulated. The model was tested for sensitivity to climatic
variability in North-east Brazil and validated against residual dune
ridge displacements as measured in the field and from high spatial
resolution satellite images. Based on the results, it is concluded that
residual dune ridges may not form in North-east Brazil in years which
are exceptionally dry, as may happen during El-Nino events. When this
type of event happens, the distance between adjacent residual dune
ridges corresponds to more than one year and, therefore, the correlation
between dune displacements and wind power becomes weak or even
disappears. Additionally, because of biotic, aeolian and hydrological
processes, these arcuate residual dune ridges may not preserve their
initial shape for long periods. The presence of residual dune ridges
testifies to the temporary flooding which may or may not be seasonal.
However, the potential for using residual dune ridges to reconstruct the
palaeo-climate of wind regime on a yearly basis or to identify past
El-Nino events seems to be limited.
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