Geoscience Reference
In-Depth Information
climate but are also linked to other factors. For example, topography
concentrates water locally. It is said that the rate of chemical reactions
doubles for every 10-degree rise in temperature. Water is totally
indispensable. There is no chemical weathering without it (consider the
case of the moon).
Large seasonal contrasts act on the soil through biological activity.
Transformation of humus and mineralization of organic compounds
(organic carbon Æ CO
2
) are affected. Certain mineral constituents, iron
and clay minerals in particular, are also transformed with alternate
wetting and drying. Lastly, seasonal contrasts facilitate the phenomena
of redistribution (mobilization of salts in the wet season and deposition
in the dry period).
Seasonal climatic variability
Freezing acts in many ways in the cold regions of the world:
it fragments the parent materials,
it removes liquid water and considerably slows down the weath-
ering phenomena; for example, it has been observed that some
soils of Antarctica are saline because of ocean spray and absence
of liquid water to wash them,
it induces presence of soils permanently frozen at depth called,
according to country and classification, pergélisol, permagel or
permafrost,
alternate freezing and thawing cause swelling and shrinking that
affect detailed morphology: frost boils, polygonal soils…
In the temperate zone, the role of freezing appears to be rather small,
but many rocks were fragmented or pulverized by it during the cold
periods of the Quaternary. Under those conditions, physical weathering
preceded chemical weathering, which was accelerated because the
specific surface area exposed to attack was larger (everyone knows it
is enough to crush a lump of sugar into smaller pieces to speed up its
dissolution in coffee!).
Freezing
2.1.3 Reconstruction of Ancient Climates
There are all sorts of methods available for working out climatic
chronology. We only shall mention them.
