06/01496 Sequential fatty acid analysis of a peat core covering the last two millennia (Tritrivakely lake, Madagascar): Diagenesis appraisal and consequences for palaeoenvironmental reconstruction: Disnar, J.-R. et al. Organic Geochemistry, 2005, 36, (10), 1391–1404

Abstract

The genesis of the microscopic constituents of coal (macerals) and of maceral associations (microlithotypes) representing various coal facies is discussed in the light of recent advances that have been made in the study of coals and modern peats. Peat microscopy especially promoted the understanding of humification and biological gelification, both of which are decisive for the genesis of huminites/vitrinites in brown coals and hard coals, respectively. Microbial activity during peatification has been documented by chemists, microbiologists and petrologists and, recently, by electron-microscopical observations in coals. This activity leads to the incorporation of lipid substances into the humic matter and, later, influences the chemical, physical and technological properties of vitrinites. Geochemical gelification transforms the huminites of brown coals into the vitrinites of bituminous coals and is probably caused by the onset of bitumen generation in coal. In this process, the hydrogel brown coal is transformed to the bitumogel bituminous coal. Coal as a source rock for oil has become an important research subject for coal petrologists and oil exploration. The formation of more or less liquid bitumen and its later cracking to gaseous hydrocarbons leads to the “coalification jumps” of the macerals, especially liptinites and vitrinites and to the generation of secondary macerals like exsudatinite (bitumen) and micrinite (dead carbon). Bituminization in coal has been revealed by the use of fluorescence microscopy which also found the presence of new liptinite macerals (bituminite, fluorinite and exsudatinite) and suggests a higher proportion of algal material in coals than was thought earlier. The genesis of various types of inertinite macerals will probably become an increasingly important subject of research, especially with regard to “rank-inertinites” that seem to attain their high inertinitic reflectance initially during coalification.Besides the origin of macerals, the origin of various coal facies in terms of maceral associations is considered in relation to the palaeogeographic depositional milieu, the peat-forming vegetation, palaeoclimate, water and nutrient supply, acidity, marine and calcareous influence and fire, with examples from Euramerican Carboniferous coals, Gondwana coals, and Tertiary brown coals. Vegetation, water and oxygen supply (Eh-conditions) are most important for the generation of coal facies.Vegetation has been reconstructed with the help of palaeobotany, including palynology, cuticular analysis and wood anatomy (in brown coals) and with the study of petrified (permineralized) peats preserved in coal balls. The microscopical investigations of botanically recognizable plant remains isolated from coals or accompanying rocks has been especially informative. However, the results of palynological and cuticular analyses should be evaluated with caution, considering the varying resistance of palynomorphs and cuticles and, in the case of palynomorphs, their possible extrapaludal origin.Microscopical studies of modern peats that were derived from different types of vegetation growing in different hydrological environments have been very helpful in interpreting coal facies, especially in brown coals. The much disputed origin of brown-coal lithotypes is considered in detail. For hard coals there seems to be agreement that coal layers rich in vitrites and liptinite-poor clarite were deposited in wet-forest swamps, that microlayered liptinite-rich clarites are sub-aquatic humic muds, and that inertinite-rich durites represent oxidized peats. The origin of liptinite-rich durites is controversial (either subaquatic muds or oxidized peats, the latter perhaps from raised bogs).A recent trend in the literature is the assumption that many if not all mineable coal seams have been deposited in raised bogs under ombrogenous, oligotrophic conditions. Although raised bogs with a dense arborescent vegetation are known from Borneo, it seems not probable that most seams (which were deposited in foredeeps or steadily sinking graben zones) are of ombrogenous origin although a succession from relatively wet and eutrophic to relatively dry and oligotrophic conditions may be true for occasional seams. But, as in modern peats, striking changes of coal facies are commonly caused by hydrological events, e.g., inundations as a consequences of crevasses or of fires that burned the peat.The origin of the inertinite-rich Gondwana coals is discussed in relation to their specific climatic conditions and a comparison with sub-arctic peats is recommended.Coals influenced by marine and calcareous conditions are characterized by high contents of sulfur and pyrite, as well as by hydrogen-rich vitrinites with a relatively low reflectance and high fluorescence, all due to a strong bacterial activity in peats of low acidity.Proposals for further research are made in the conclusions.