Geoscience Reference
In-Depth Information
The reactions producing methane (Cleemput and El Sebaay 1985)
are:
￿ CO 2 + 4 H 2 Æ CH 4 + 2 H 2 O
This reaction provides a lot of energy. It has the advantage of
consuming hydrogen produced by fermentation because it is toxic to
microorganisms. The methanogenic bacteria are autotrophic since they
utilize CO 2 . They are strict anaerobes. They also exist in the stomach
of ruminants, where they reduce the gases present to one-fifth.
￿ CH 3 COOH ( acetic acid ) Æ CH 4 + CO 2 ( decarboxylation )
This reaction alone will be responsible for two-thirds of the methane
produced in Nature.
￿ HCOOH ( formic acid ) + 3 H 2 Æ CH 4 + 2 H 2 O
￿ CH 3 OH ( methanol ) + H 2 Æ CH 4 + H 2 O
￿ CH 3 NH 2 ( methylamine: toxic, rare ) + H 2 Æ CH 4 + NH 3
The average flux of methane produced and released to the
atmosphere is of the order of 10 mg CH 4 per m -2 h -1 . But this can turn
into a catastrophe. In Cameroon, the night of 21 August 1986, Lake
Nyos suddenly degassed (CO 2 + CH 4 ), killing 1800 people on its banks,
not to mention head of cattle numbering tens of thousands. Sudden
degassing occurs when the pressure of accumulated gases exceeds the
pressure of the water column above. Lake Kivu, between Rwanda and
the Democratic Republic of the Congo is threatening to degas. The lake
holds several thousand million cubic metres of methane!
The sparks thrown by a studded shoe are enough to ignite the
methane that sometimes stagnates on the surface of marshes. This
results in will-o'-the-wisps that have scared people.
Methane is 21 times more efficient than CO 2 in absorbing
infrared radiation. In spite of its low concentration in the atmosphere,
it is counted among the gases responsible for the greenhouse effect.
Peats are exceptional soils. Firstly, the most recent horizons are on top;
secondly, the soil surface rises instead of descending. This has important
consequences. During its formation, peat traps the pollens that fall on its
surface. They are preserved because of the low biological activity, and
are arranged in sequence according to the date of deposition. According
to Davidson et al. (1999), only peatlands and Podzols have a biological
activity low enough to permit serious palynological investigations. Such
studies enable us to reconstruct roughly the succession of past climates
Preservation of carbonaceous materials
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