Fjölrit RALA - 15.06.2004, Page 101

become frequent and below about 75 cm they are dominant. There are very large pores
associated with the woody fragments. They often occur between the bark and the shrinking
wood, which also has large discrete pores that reduce the active porosity for water movement.
Faecal material and living plant material predominate in the top 5 cm. The living plants
are mosses at the surface and roots below. Mediums to very small faecal pellets are found
within plant remains, occasionally clustered in the matrix and are being assimilated into the
fine organic material. Mineral grains diatoms and phytoliths occur within the pellets.
Plant residues. In the middle and lower part of the profile woody residues dominate. The
cork is the best preserved tissue with its cells mostly filled with dark brown to black material,
probably dense hydrocarbons formed from tannins and other ringed hydrocarbons. In some
cases the cork cells are breaking apart. The wood has in all cases separated from the bark
tissues due to decomposition of the cambium and phloem tissues but also due to shrinkage of
the wood. In some cases the wood has fully disappeared leaving an elliptical ring of bark
lying horizontally. Wood vessels fillings are light to yellowish brown indicating a different
alteration product than is found in the cork cells.
Of the herbaceous residues highly lignified rhizome tissues are the most conspicuous.
Rarely bundles of leaves occur but mostly the herbaceous residues are not identifiable. In root
residues the epidermis, endodermis and xylem are the most resistant tissues and the cells often
filled with yellow, light brown, red or black substances.
Organic matrix. The fine organic material can be divided into small plant residues,
individual cells, homogeneous substances often associated with other components acting as
cementing agent or highly humified material in soil layers, and very small microgranular
particles. The microgranular matrix is probably originates from broken pellets.
Bog iron. At about 5 cm a thin band of bog iron has precipitated indicating a change in the
redox conditions. The bog iron is dominantly black and opaque with massive to granular
structure. Some parts are dull red, microgranular breaking up in a blocky structure.
Pyrite. Below about 150 cm depth pyrite appears and increases with depth. The pyrite
occurs as small, black spheres 10-30 pm in diameter or as fused clusters up to a few hundred
pm in diameter. The spheres and clusters are mainly in cavities within plant residues, in cell
lumen but also in pores in the fine residues. The pyrite is of biological origin and the spheres
are colonies of sulfitic bacteria.
Coatings. These occur on both the small granular units as well as on linear pore surfaces at
about 30 - 120 cm depth. The pore coatings are bright yellow to yellowish-brown, weakly
anisotropic and have a dendritic structure or a fan-like form where the fans are formed from
needles growing at right angel to the pore wall. These coatings are mainly associated with
woody residues and have been observed in bog iron ores and identified as goethite (Stoops
1983).
Volcanic glasses and other primary minerals. Three types of volcanic glasses occur. Firstly,
there is light, greyish, fibrous, rhyolitic glass with large inner porosity. Secondly, there is
brown glass forming glass shards often with sharp edges, and thirdly black basaltic glass. In
the light coloured tephra layers the light coloured rhyolitic glass is dominant. Brown glasses
are the most frequent in organic layers together with light coloured in the top part of the
profile. An unidentified tephra layer at 168-171 cm depth consists dominantly of brown
coloured glasses. Other primary minerals including plagioclase, augite and olivine are rare.
References
FitzPatrick, E.A., and Th. Gudmundsson. 1978. The impregnation of wet peat for the
production of thin sections. Joumal of Soil Science 29: 585-7.
Stoops, G. 1983. SEM and light microscopic observations of minerals in bog-ores of the
Belgian Campine. Geoderma 30: 179-86.
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