Geoscience Reference
In-Depth Information
The first dating method uses soil sequences found on progressively
exhumed marine surfaces or on dated fluvial terraces. The soils seen
in Germany on Weichselian sediments (equivalent to the Würm in
northern Europe) are identified as Luvisols (Kühn 2003). Actually, they
are very special: firstly, they are differentiated in sandy parent materials
and their argic B contains less than 15 per cent clay; secondly, their E
horizon is limited to open, wedge-shaped cracks filled with sand inside
the B. In France, in the Rhône valley, Luvisols are found on Riss terraces
(between 110,000 and 128,000 years), while Albeluvisols and Planosols
occur on older surfaces. On the terraces of the Allier river, the soils
would have intermediate development (Jongmans
et al
. 1991): Eutric
Cambisols on surfaces dated 13,000 years, Luvisols between 13,000 and
30,000 years and, lastly, Planosols on the oldest surfaces.
Dated surfaces
Let us consider hydrolysed clay and calculate the time that would have
been necessary for elimination of the corresponding aluminium. The
reference to aluminium is justified by the fact that this element is less
soluble than silicon. It is therefore the former element that theoretically
determines the speed of the hydrolysis reaction. We calculated this
for the soils of the sands and clays of le Bourbonnais which we had
described then as 'degraded leached' (Albeluvisols or Planosols). They
correspond to the highest planation surfaces observed above the valley
of the Allier. We found an age of 2.2 million years (Legros 1982). The
next year, independently of our work, the planation surface was dated
at 2.4 million years by pollen analysis and various geomorphological
arguments (Le Griel 1983). The method has very low accuracy but it is
enough to show that the periods involved are long.
Rate of evacuation of aluminium
In Italy, traces of Acheulian and Mousterian activity buried in the loess
permit dating of the superposed soils (Busacca and Cremaschi 1998).
Overall, 400,000 years were necessary to get two cycles of evolution
resulting in Luvisols with very distinct B horizons with up to 50 per cent
clay. But evolution was certainly stopped during the glaciations. Thus
every profile obtained was formed in clearly less than 200,000 years.
Archaeology
Two factors mainly influence the duration of pedogenesis in the case of
Luvisols (WRB) or Alfisols (USA).
Attempt at synthesis
