634
Climatic Geomorphology
These data obtained from the studies of glacioisostasy have been used to obtain
information on the viscous nature of the mantle. Two models have been proposed; the first
indicates that the viscosity of the mantle is relatively constant with depth, and the second
suggests an increment of the viscosity on the top of the upper mantle. Values of the second
model fit well with the loss of load of the Laurentide ice sheet (Peltier and Andrews, 1983)
and for Fennoscandia, the first model fits better (M6rner, 1980). This implies that the
processes of the icecap construction and thaw are associated with plastic deformation
within the asthenosphere. As a consequence of the increments and decreases of the ice
masses existing on the continents, important variations of sea level are produced, called
glacioeustatic changes. These fluctuations are recorded by means of the study of isotopes
of oxygen in foraminifera from deep sea cores (Figure 24.3).
Other mechanisms coexist with glacioeustasy: the tectonic factors related to the
capacity of the oceans; isostasy; hydrostasy, which represents the response of the lithosphere to the load of water in the oceans, and the geoidal effect. The most important are
the glacial and tectonic factors, although during upper Quaternary, the glacioeustatic
variations were more significant. Without doubt the reconstruction of the ancient sea levels
for some areas is quite complex, because several factors are involved.
Fairbanks (1989) carried out a study on the corals of the Barbados Islands. Corals die
when they are exposed above sea level. If the sea level rises, new corals grow on older
ones. Thus, by means of radiometric dating, it is possible to know the date of the corals,
and so, a record of the variations of the sea levels in the postglacial period. This record,
together with that carried out for the Indian Ocean (Figure 24.3), can represent the
fluctuations of the sea level for a great part of the Earth' s surface, because both of them are
similar, but cannot be applied in areas close to ice sheets (Boulton, 1993).
The importance of eustasy and isostasy depends on the location in relation to the
icecaps. At the boundaries of the caps of ice, isostasy is the principal effect. According
to the characteristics of the postglacial contour lines, the surface of the Earth has been
divided into six sea level zones (Clark et al., 1978, 1980) (Figure 24.16). Zone 1 is
located on the ice sheet margins, experiencing constant uplift due to isostatic recovery.
Zone 1 to 2 constitutes a transition area, with an initial rise and a later subsidence due
to the marginal rebound of the ice sheet. Zone 2 is far from ice sheets and sea level
continuously rises, as in Zone 3, but an emerging beach 1000 years ago formed the
latter. Zone 4 corresponds to tropical areas where sea has been rising since the
glaciation, while in Zone 5 a rise occurred when the large ice sheets melted and then
surfaced due to a hydrostatic effect. Zone 6 includes all the continental platforms,
except those of Zone 2, and is characterized by a slight rise after the thaw, as a result
of the isostatic uplift of the coastal margins. These sea level lines can be affected by
tectonic movements.
6. Reconstruction of periglacial environments
Relict periglacial landforms play an important role in the reconstruction of the climates
of the upper Quaternary. The degree of modification of the landscape depends on the
location and persistence of the periglacial areas, which are more pronounced in the
proximity of the ice sheets. Lithology has a considerable influence on the preservation
Climatic Geomorphology
These data obtained from the studies of glacioisostasy have been used to obtain
information on the viscous nature of the mantle. Two models have been proposed; the first
indicates that the viscosity of the mantle is relatively constant with depth, and the second
suggests an increment of the viscosity on the top of the upper mantle. Values of the second
model fit well with the loss of load of the Laurentide ice sheet (Peltier and Andrews, 1983)
and for Fennoscandia, the first model fits better (M6rner, 1980). This implies that the
processes of the icecap construction and thaw are associated with plastic deformation
within the asthenosphere. As a consequence of the increments and decreases of the ice
masses existing on the continents, important variations of sea level are produced, called
glacioeustatic changes. These fluctuations are recorded by means of the study of isotopes
of oxygen in foraminifera from deep sea cores (Figure 24.3).
Other mechanisms coexist with glacioeustasy: the tectonic factors related to the
capacity of the oceans; isostasy; hydrostasy, which represents the response of the lithosphere to the load of water in the oceans, and the geoidal effect. The most important are
the glacial and tectonic factors, although during upper Quaternary, the glacioeustatic
variations were more significant. Without doubt the reconstruction of the ancient sea levels
for some areas is quite complex, because several factors are involved.
Fairbanks (1989) carried out a study on the corals of the Barbados Islands. Corals die
when they are exposed above sea level. If the sea level rises, new corals grow on older
ones. Thus, by means of radiometric dating, it is possible to know the date of the corals,
and so, a record of the variations of the sea levels in the postglacial period. This record,
together with that carried out for the Indian Ocean (Figure 24.3), can represent the
fluctuations of the sea level for a great part of the Earth' s surface, because both of them are
similar, but cannot be applied in areas close to ice sheets (Boulton, 1993).
The importance of eustasy and isostasy depends on the location in relation to the
icecaps. At the boundaries of the caps of ice, isostasy is the principal effect. According
to the characteristics of the postglacial contour lines, the surface of the Earth has been
divided into six sea level zones (Clark et al., 1978, 1980) (Figure 24.16). Zone 1 is
located on the ice sheet margins, experiencing constant uplift due to isostatic recovery.
Zone 1 to 2 constitutes a transition area, with an initial rise and a later subsidence due
to the marginal rebound of the ice sheet. Zone 2 is far from ice sheets and sea level
continuously rises, as in Zone 3, but an emerging beach 1000 years ago formed the
latter. Zone 4 corresponds to tropical areas where sea has been rising since the
glaciation, while in Zone 5 a rise occurred when the large ice sheets melted and then
surfaced due to a hydrostatic effect. Zone 6 includes all the continental platforms,
except those of Zone 2, and is characterized by a slight rise after the thaw, as a result
of the isostatic uplift of the coastal margins. These sea level lines can be affected by
tectonic movements.
6. Reconstruction of periglacial environments
Relict periglacial landforms play an important role in the reconstruction of the climates
of the upper Quaternary. The degree of modification of the landscape depends on the
location and persistence of the periglacial areas, which are more pronounced in the
proximity of the ice sheets. Lithology has a considerable influence on the preservation