Geoscience Reference
In-Depth Information
But the pedogenesis here does not lead, as postulated more or less
by the weathering-front model, to establishment of a stable profile
collapsing slowly on itself and being unchanged in morphology over
time. In fact, the soils of the various terraces are not identical! On this
point, our model is invalidated or, more precisely, cannot be extended
up to this consequence. Indeed, one could object that establishment of
a stable equilibrium has not yet been achieved and that, for this, more
than the entire duration of the Quaternary would be necessary. But then
this hypothetical equilibrium will very much resemble the last profile
in the sequence (Villafranchian). This is another way of stating that the
rubefied state is transitory and that it tends to self-destruct after being
formed, at least in a climate with a wet season.
9.2
ECOLOGY OF THE RED SOILS
9.2.1 The Time Factor
The periods of pedogenesis in the Rhône valley can be established
on the basis of the researches of M. Bornand and consideration of
the most recent data on the chronology of the Quaternary glaciations
(Table 9.2). In fact, it is known that there has been a glaciation once
every 100,000 years in the very recent history of the Earth. But different
factors lead us to think that the terraces observed do not correspond to
the last four of these glaciations. Starting from the current epoch, the
Günz seems to be the fifth or sixth period of great cooling. Also, the
glaciers are localized in space, so much so the glaciations did not last the
same time everywhere. In these circumstances, the data in Table 9.2 are
approximate. They show, however, three things about the Rhône region:
(i) more than 13,000 years are necessary (present interglacial period) for
a Rhodic soil to form; (ii) the red colour disappears after 540,000 years
as a result of 'self-asphyxia'; and (iii) the Luvic feature requires more
than 120,000 years to become very distinct.
Age of the red soils
Table 9.2
Data on the age of soils of the Rhône valley (Wikipedia).
Interglacial period
Duration (×1000 y)
Corresponding soil
Present
Less than 12.7
Regosol/Cambisol Calcaric
Riss-Würm
180-120
Cambisol Rhodic
Mindel-Riss
430-240
Luvisol Rhodic
Günz-Mindel
540-480
Alisol Rhodic
Before Günz
more than 600
Acrisol Gleyic
