Environment peat-forming

Figure 4 Schematic diagrams of different types of peat-forming environments (a) a planar ortopogenous peat, such as the Okefenokee Swamp, Georgia, USA and (b) a domed or ombrogenous peat deposit, sucb as in Kalimantan, Indonesia. Note the much greater peat depth in the domed peat compared to the topogenous peat.

Casagrande et al. (1977) found pyrite in peat formed in a marine environment to constitute over 15% of the sulfur present while the pyrite level in freshwater peat was an order of magnitude lower. Carbon-bonded sulfur accounted for 50% of the total sulfur in marine peat, but 70% in freshwater peat. Ester-sulfate constituted 25% of the sulfur in both cases. These authors concluded that the total sulfur found in coal can be incorporated in the peatforming stage. 

The formation of humic substances in a peatland environment is a complex humification process which is principally due to certain enzymatic and microbial activities. These organic matter transformation processes are influenced by the nature of the peat-forming plants and certain physical and chemical properties within a particular peatland. In very acidic or low nutrient peatlands a very different microflora may exist than in a more eutrophic and less wet situation. In the former situation humification may be retarded and the peatland plants will be preserved and thus accumulate. On the other hand, in less acid environments with moderate amounts of nutrients and periodic water-table fluctuations humification proceeds relatively rapidly and leads to decomposed organic soils such is the case in drained and cultivated organic soils. 

Studies of modern peat-forming environments have emphasized the importance of detrital influx (, syngenetic formation of pyrite (8) and biogenic silica (7,9), and in-situ mixing with underlying sediments (7,10) to account for mineral constituents in peat. Within... 

Table 3.9 Comparison of peat-forming environments (after Bend 1992)...

This concept of sulphur fractionation in soils has been successfully applied to sewage sludge (Sommers et al., 1977) and to freshwater and marine derived peat-forming systems (Cas ande et al., 1977). In the marine peat, carbon-bonded sulphiu" accounted for 50% of total sulphur while ester sulphate constituted only ca. 25%. These authors noted an overall increase in sulphur in going from plant samples to peat in the marine environment, and concluded that plants were not the dominant sulphur-concentrating mechanism. The sulphur was probably delivered to a large extent by sulphate diffusion and microbial reduction, whereby carbon-bonded sulphur acted as a sink for sulphur in the peat. 

The debate over the origin of coal seams appears to have been decided in favor of in situ (or autochthonous) formation from peats formed slowly in swamps of various descriptions (Diessel, 1980 Stach et al., 1982). One of the key factors in this ascendancy of the peat swamp model over the various allochthonous (or transported) models was the recognition of so-called fossil forests—tree stumps with roots and logs in apparent growth positions on top of coal seams. The peat swamp model has not only become the basis of virtually all studies on coal seam formation but is now also the basis of studies on the coalification of the plant constituents to produce the various coal macerals. For this reason, considerable effort has been directed toward the study of modern peat-forming environments. 

Other potential monitors of historical trace elements are core samples of ice, peat, and lacustrine deposits. Each individual stratus reflects the environment in which it formed. Analyses of these core samples could reconstruct the past local and regional environmental history. 

Peat. Peat is the layer of vegetable material directly underlying the growing zone of a coal-forming environment. The vegetable material shows very little alteration and contains the roots of living plants. Peat is widely used as a domestic fuel in rural parts of the world. 

The plant precursors that eventually formed coal were compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. It is suspected that coal was formed from prehistoric plants that grew in swamp ecosystems. When such plants died, their biomass was deposited in anaerobic, aquatic environments where low oxygen levels prevented their reduction (rotting and release of carbon dioxide). Successive generations of this type of plant growth and death formed deep deposits of unoxidized organic matter that were subsequently covered by sediments and compacted into carboniferous deposits such as peat or bituminous or anthracite coal. Evidence of the types of plants that contributed to carboniferous deposits can occasionally be found in the shale and sandstone sediments that overlie coal deposits. 

Formation of Pyrite. Iron is carried to the peat swamp, before seawater transgression, as ferric oxide and hydroxides adsorbed on fluvial clays (123). During early diagenesis in a reducing environment, ferric iron is reduced to ferrous, which reacts with hydrogen sulfide to form iron monosulfide. If the basic mechanism of pyrite formation is similar to that in marine sediments... 

Supardi A. D. and Subekty S. S. (1993) General geology and peat resources of the Siak Kanan and Bengkalis Island peat deposits, Sumatra, Indonesia. In Modem and Ancient Coal Forming Environments, (eds. J. C. Cobb and C. B. Cecil). Geological Society of America, Washington, DC, vol. 86, pp. 45-61. 

Occurrences of elemental sulfur in peat, coal, and petroleum are described in Chapter 6.4. The role of sulfate reducers in these environments is suggested by the fact that fossil fuels formed in marine environments, where sulfate is in abundant supply, have significantly more sulfide and native sulfur than those formed under freshwater conditions. In fact, a general geological feature of native sedimentary sulfur deposits is their location in sulfate-carbonate rocks and proximity to oil-gas-bearing strata and hydrologic zones where sulfate waters mix with chloride brines (Ivanov, 1964). 

Sediments are enriched in organic matter forming calcareous peat close to the shore, e.g. to the palustrine environment. 

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