Rainwater-fed peat as a precursor of coal

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Abstract
About 3% of the Earth’s land surface is peat-covered at present. Most peat forms where there is waterlogging, because oxygen diffuses down through water more slowly than it is consumed by micro-organisms. The anaerobic micro-organisms in peat continue the decay process, but only slowly. A great variety of rheotrophic (usually minerotrophic and eutrophic) peats form if the waterlogging is caused by groundwater. Of greater extent, but less varied, are the ombrotrophic peats which are dependent on precipitation. Most of these peatlands are in Russia, Canada, Fennoscandia and the British Isles. The main peat-forming plants in such places are bogmosses ( Sphagnum ), sedges and heaths, with pines ( Pinus ) and larch ( Larix ) in some places. These peats may be analogous to those which formed the Permian coals of Australia. In western Malesia are large areas of coastal ombrotrophic forest-bogs. These tropical forest peats may be analogous to those which formed the Carboniferous coals. The organic chemistry of newly dead plants differs considerably amongst all these plant groups, and they in turn may be very different from the majority of plants which formed the starting materials for the formation of coals in the past. In spite of these differences the study of present-day moss and sedge bogs may be helpful and particularly in the study of Permian coals. There is usually a two-layered structure. In the 5–50 cm thick surface layer aerobic decay may remove as much as 90% of the original matter and do so selectively. Below is the much thicker waterlogged anaerobic layer in which microbial decay is of a different sort and is much slower. But there is evidence that it continues for thousands of years at least. The extent to which peat is a product of microbial activity operating on a slowly changing substrate is unknown. Equally unknown is the balance between microbially mediated and chemical changes. One consequence of bacterial decay is that there seems to be an inherent limit to the depth of such peat-forming systems. At least one (unusual) case is known which has resulted in the production of a lignitic peat, which breaks with a conchoidal fracture, and which is barely 12 000 years old. The limiting depth, assessed from growth during the last 10 000 years, seems to be too little to account for the observed thickness of coals.