670
Climatic Geomorphology
of the lake Bonneville during the period 25,000 to 11,000 yr BP and the one on the right
indicates the isostatic rebound produced by lake desiccation, which is 20 % of the column
of water (Crittenden, 1963).
The Aral-Caspian Sea system is one of the most spectacular cases of lake expansion
during the Pleistocene era, which is formed by wide shallow deformations. Huge amounts
of glacial melt water went into this system from Syr Darya and Amu Darya rivers, to the
Aral Sea, and from Volga and Ural to the Caspian Sea. When the level of the lake reached
76 m above the Caspian Sea, it met the Aral Sea, generating the largest "pluvial" lake in
the world, flooding an area of 1,100,000 klTl 2 and penetrating 1300 km into the river Volga
(Goudie, 1992). The Caspian Sea also reached the Black Sea through the Mantych
depression.
Despite the difficulties of interpreting climate on the basis of geomorphologic studies of
paleolakes, these are crucial for paleoclimatic reconstructions, especially if they are based
on multidisciplinary paleoenvironmental studies.
1.3. Dune systems
1.3.1. Introduction
Large aeolian accumulations or ergs, together with the erosive landforms existing in
intertropical deserts, occupy 20 to 25% of the Earth's surface (Livingstone and Warren,
1996). The extension of these dune fields is well known in most of the deserts in the world
(Figure 18.2) (Thomas, 1997b), from aerialphotos and satellite images. Dunes have been
generated in periods of 1000 to 100,000 years (Lancaster, 1995). Expansions and
contractions of the principal ice sheets, which considerably affected lower latitudes,
changing climate and sea level, occurred during dune-field formation. The most significant
variations become evident through changes of the boundaries of great sand deserts, in
sand-particle supply and in the mobility of the dunes. These sand deserts are not
exclusively Pleistocene, but many of them also became larger during the global climate
cooling (Figure 25.26) (Shackleton and Kennet, 1975; Goudie, 1992); this is what
Williams (1994) call "upper Cenozoic desiccation."
Many periods of dryness can be identified in the course of the Quaternary. The study of
the ice cores dated in the upper part of the Greenland ice sheet (GISP2: Greenland Ice
Sheet Project 2), show fluctuations in the electrical conductivity of the ice, at scales of < 5
to 20 years. These reveal rapid oscillations in the atmospheric dust content (Figure 25.27)
in periods in which the atmosphere was very dusty, separated by others without dust. The
analyses of continental sediments within deep oceanic cores can also be used to interpret
the extension, intensity and duration of the arid zones during the Quaternary. Continents in
intertropical regions provide fluvial and aeolian sediments to the marine bottom.
Furthermore, there is a relatively constant supply of organic carbonates, and thus, those
cores with a high content of carbonates indicate a scarce supply from continents. Aeolian
supplies are represented by high percentage of silt-sized quartz grains, which can be
differentiated from fluvial ones because of their tendency for reddish staining (DiesterHaas, 1996). Increments of aeolian dust took place during Quaternary cold periods, in
which the quartz content at the bottom of the Atlantic Ocean moved toward the south,
Climatic Geomorphology
of the lake Bonneville during the period 25,000 to 11,000 yr BP and the one on the right
indicates the isostatic rebound produced by lake desiccation, which is 20 % of the column
of water (Crittenden, 1963).
The Aral-Caspian Sea system is one of the most spectacular cases of lake expansion
during the Pleistocene era, which is formed by wide shallow deformations. Huge amounts
of glacial melt water went into this system from Syr Darya and Amu Darya rivers, to the
Aral Sea, and from Volga and Ural to the Caspian Sea. When the level of the lake reached
76 m above the Caspian Sea, it met the Aral Sea, generating the largest "pluvial" lake in
the world, flooding an area of 1,100,000 klTl 2 and penetrating 1300 km into the river Volga
(Goudie, 1992). The Caspian Sea also reached the Black Sea through the Mantych
depression.
Despite the difficulties of interpreting climate on the basis of geomorphologic studies of
paleolakes, these are crucial for paleoclimatic reconstructions, especially if they are based
on multidisciplinary paleoenvironmental studies.
1.3. Dune systems
1.3.1. Introduction
Large aeolian accumulations or ergs, together with the erosive landforms existing in
intertropical deserts, occupy 20 to 25% of the Earth's surface (Livingstone and Warren,
1996). The extension of these dune fields is well known in most of the deserts in the world
(Figure 18.2) (Thomas, 1997b), from aerialphotos and satellite images. Dunes have been
generated in periods of 1000 to 100,000 years (Lancaster, 1995). Expansions and
contractions of the principal ice sheets, which considerably affected lower latitudes,
changing climate and sea level, occurred during dune-field formation. The most significant
variations become evident through changes of the boundaries of great sand deserts, in
sand-particle supply and in the mobility of the dunes. These sand deserts are not
exclusively Pleistocene, but many of them also became larger during the global climate
cooling (Figure 25.26) (Shackleton and Kennet, 1975; Goudie, 1992); this is what
Williams (1994) call "upper Cenozoic desiccation."
Many periods of dryness can be identified in the course of the Quaternary. The study of
the ice cores dated in the upper part of the Greenland ice sheet (GISP2: Greenland Ice
Sheet Project 2), show fluctuations in the electrical conductivity of the ice, at scales of < 5
to 20 years. These reveal rapid oscillations in the atmospheric dust content (Figure 25.27)
in periods in which the atmosphere was very dusty, separated by others without dust. The
analyses of continental sediments within deep oceanic cores can also be used to interpret
the extension, intensity and duration of the arid zones during the Quaternary. Continents in
intertropical regions provide fluvial and aeolian sediments to the marine bottom.
Furthermore, there is a relatively constant supply of organic carbonates, and thus, those
cores with a high content of carbonates indicate a scarce supply from continents. Aeolian
supplies are represented by high percentage of silt-sized quartz grains, which can be
differentiated from fluvial ones because of their tendency for reddish staining (DiesterHaas, 1996). Increments of aeolian dust took place during Quaternary cold periods, in
which the quartz content at the bottom of the Atlantic Ocean moved toward the south,