Geoscience Reference
In-Depth Information
the sphagnum gradually decomposes and gives peat (Gobat
et al
. 2003).
Therefore, it is at the base of the soil that carbon compounds are the
oldest. The typical organization is the following (Fig. 12.12).
Sphagnum can live only just above water. If it is submerged for
whatever reason, it disappears completely. The water holes that are
dotted with peat are therefore very deep although they are small across
(a few dm) and appear insignificant. In general they go down to hard
rock, which could be several metres below. However, the structural
scheme proposed is only statistically true. Bogs are very heterogeneous
environments. It is enough that the drainage axis be displaced a few
metres for the profile to change and become complex with, for example,
many levels of humified peat.
Mound with heather and blueberry
Acrotelm
Free water
Sphagnum
Light-coloured
fibrous peat
Non-fibrous brown
peat
Catotelm
Humified black
peat
Grey (reduced iron) sandy arène
Rock (e.g., schist)
Fig. 12.12
Typical vertical organization of a fully developed peat. Left: photograph in which
we can clearly distinguish sphagnum and humifi ed peat below, but not the colour changing
from black to brown from the bottom up. Right: Corresponding sketch.
Gobat
et al
. (2003) emphasize the distinction between the upper part
of the profile where living sphagnum generates the organic matter by
photosynthesis and the lower part where fermentation, humification and
slow mineralization occur. The former is called
acrotelm
and the latter,
catotelm
.
12.5.3 Functioning
In marshes and at the bottom of lakes, anoxia is often strong. If the
potential drops below -330 mV, it can lead to production of methane,
CH
4
. This is
methanization
.
Methanization
