Climate change in arid and tropical humid regions
679
have an age of 10,000 years, and Lancaster (1989a) estimated in the age of the large dunes
of the Namib Desert to be some 42,000 years. Dunes of 70 m height exist on the coast of
the Huelva province, in E1 Asperillo of southern Spain, showing an age, by means of
archaeological and historical data, from the 18th century (Borja et al., 1999; Zazo et al.,
1999).
Today it is well known that there are extensive areas of stabilized dunes on all the
continents, in tropical zones as well as at high latitudes, in relation to periglacial climates
(Sarnthein, 1978) (Figure 25.35). Dunes formed, during the last glacial maximum,
2.5 million klTl 2 in Africa, which means the largest desert in the world (Thomas and
Shaw, 1991). Comparing the extension of these dune fields with the current area with
active dunes, it can be deduced that great changes in have occurred precipitation and in
the percentage of land cover. At present, active dune deserts, located between 30~
and 30~ occupy only 10%, while 18,000 years ago their extension was 50% (Goudie,
1992).
In the boundaries of most zonal deserts there are stabilized longitudinal dunes. The
most extensive ones are located in the Sahara, over 5~ (Figure 25.18) and stretch from the
Atlantic coast of Senegal to Sudan. They extend toward the south over 500 kin, in areas
that nowadays have a precipitation of 500 mm. Three generations of dunes can be
recognized (Grove and Warren, 1968; Talbot, 1980). The oldest group is older than 20,000
years, the second between 20,000 and 13,000 years and the most recent has an age of about
5000 years. Most of them were stabilized between 11,000 and 7000 years ago. The last
droughts and deforestation have reactivated some dunes and have generated new ones
(Nickling and Gillies, 1993). Other examples can be found in the Kalahari, northern
Arabia, Thar Desert, Australia, Venezuela, and elsewhere. All these dunes are completely
colonized by vegetation and have soils developed on them.
1.3.3. Paleoclimatic information provided by dune systems
Active dunes that developed on continents are located around and below the isohyet of
150 mm (Mainguet et al., 1980), with the southern Sahara as an excellent example of that
spatial distribution (Figure 25.36). The existence of these relict dunes represents an
irrefutable evidence of previous dryer stages. The situation of these dunes in areas of high
precipitation, as for example the selva of the Congo, reveals the drastic change that
occurred. Moreover, it is necessary to take into account that the reactivation or formation
of dunes can be the result of changes in the velocity of the wind and their interactions with
humidity (Thomas, 1992).
Comparing wind directions that mobilize sand with dune alignments, valuable
paleoclimate information can be obtained. There are wind systems that at present blow in
the same direction as during the late Pleistocene, as in the southern Sahara (trade winds).
Figure 25.35. Active dune fields and long-term resultant effective wind regimes for the present,
18,000 yr BP (glacial maximum), and 6000 yr BP Climatic Optimum. Continental outlines at 18,000 BP
represent sea-level lowering of 120 m. Hatched areas indicate dune fields and major dune contours.
Arrows are long-term resultant surface-wind directions. H signs stand for fossil wet conditions excluding
dune formation (after Sarnthein (1978)).
679
have an age of 10,000 years, and Lancaster (1989a) estimated in the age of the large dunes
of the Namib Desert to be some 42,000 years. Dunes of 70 m height exist on the coast of
the Huelva province, in E1 Asperillo of southern Spain, showing an age, by means of
archaeological and historical data, from the 18th century (Borja et al., 1999; Zazo et al.,
1999).
Today it is well known that there are extensive areas of stabilized dunes on all the
continents, in tropical zones as well as at high latitudes, in relation to periglacial climates
(Sarnthein, 1978) (Figure 25.35). Dunes formed, during the last glacial maximum,
2.5 million klTl 2 in Africa, which means the largest desert in the world (Thomas and
Shaw, 1991). Comparing the extension of these dune fields with the current area with
active dunes, it can be deduced that great changes in have occurred precipitation and in
the percentage of land cover. At present, active dune deserts, located between 30~
and 30~ occupy only 10%, while 18,000 years ago their extension was 50% (Goudie,
1992).
In the boundaries of most zonal deserts there are stabilized longitudinal dunes. The
most extensive ones are located in the Sahara, over 5~ (Figure 25.18) and stretch from the
Atlantic coast of Senegal to Sudan. They extend toward the south over 500 kin, in areas
that nowadays have a precipitation of 500 mm. Three generations of dunes can be
recognized (Grove and Warren, 1968; Talbot, 1980). The oldest group is older than 20,000
years, the second between 20,000 and 13,000 years and the most recent has an age of about
5000 years. Most of them were stabilized between 11,000 and 7000 years ago. The last
droughts and deforestation have reactivated some dunes and have generated new ones
(Nickling and Gillies, 1993). Other examples can be found in the Kalahari, northern
Arabia, Thar Desert, Australia, Venezuela, and elsewhere. All these dunes are completely
colonized by vegetation and have soils developed on them.
1.3.3. Paleoclimatic information provided by dune systems
Active dunes that developed on continents are located around and below the isohyet of
150 mm (Mainguet et al., 1980), with the southern Sahara as an excellent example of that
spatial distribution (Figure 25.36). The existence of these relict dunes represents an
irrefutable evidence of previous dryer stages. The situation of these dunes in areas of high
precipitation, as for example the selva of the Congo, reveals the drastic change that
occurred. Moreover, it is necessary to take into account that the reactivation or formation
of dunes can be the result of changes in the velocity of the wind and their interactions with
humidity (Thomas, 1992).
Comparing wind directions that mobilize sand with dune alignments, valuable
paleoclimate information can be obtained. There are wind systems that at present blow in
the same direction as during the late Pleistocene, as in the southern Sahara (trade winds).
Figure 25.35. Active dune fields and long-term resultant effective wind regimes for the present,
18,000 yr BP (glacial maximum), and 6000 yr BP Climatic Optimum. Continental outlines at 18,000 BP
represent sea-level lowering of 120 m. Hatched areas indicate dune fields and major dune contours.
Arrows are long-term resultant surface-wind directions. H signs stand for fossil wet conditions excluding
dune formation (after Sarnthein (1978)).