Geoscience Reference
In-Depth Information
cooled more slowly, with a time-lag of about 1000 years (RDT 2005). For
the global climate, the Southern hemisphere mainly occupied by oceans
does not play the same driving role as the Northern one.
But for a few details, the Milankovitch theory has been validated
by observations. In the ice of the Antarctic Ocean, the variations in the
18
O/
16
O and
2
D/
1
H ratios have enabled determination of temperature
changes undergone by the Earth. As predicted by the theory, a glaciation
every 100,000 years has been observed in the Recent period. Thus,
more than 20 glaciations followed each other during the Quaternary,
although only traces of the largest are seen. However, the cosmographic
phenomena reviewed above do not coincide at regular intervals if things
are examined carefully. Thus, the Earth never returns to exactly the
same conditions as before. The last interglacial period was particularly
warm (+2º) and it is not known very well when the current one, which
should be near its end, will terminate. The cosmographic theories are
also confirmed by Paleosols, which are found buried under Quaternary
loesses. On the continental scale, they belong to about twenty different
epochs presenting as many interglacial climates.
The glacier, during its advance, efficaciously scours the land by means
of the boulders that it embeds on its bottom and which act as a sort
of gigantic sandpaper. The soils are eliminated! Soil formation then
recommences from time zero, by weathering of materials
in situ
or by
starting afresh from aeolian or colluvial deposits. Thus, if the date of
passing of the glacier or of the glaciation is known, we have by the same
token the maximum age of the soil that has been established following
it. In the Alps, the very end of the last glaciation (Würm) is dated at
about 10,000 years. But for rare exceptions, the soils on the moraines
and terraces cannot be older than this period of time. This observation
can be compared to the fact that Luvisols (with marked enrichment of
clay in the B horizon) are not found in those locations. We can consider
the example of the Swiss plateau. Obviously, more than 10,000 years are
necessary for a soil of this type to be formed, actually 50,000 to 100,000
years, as we shall see later.
Return to time zero through glacial ablation
2.3.4 Chronosequences Related to Oceanic Movements
It is known that the northern part of Europe, liberated of its ice cover,
rises vertically and continuously by isostatic movement. The rate of this
rise has been calculated. In these conditions, the soils located at the sea
