BIOLOGICAL PROCESSES CONTROLLING THE DEVELOPMENT OF MODERN PEAT-FORMING ECOSYSTEMS

Although no precise modern analogue exists for the peat-forming ecosys tems (mires) of Tertiary times, it is argued that the principles under lying peat formation are essentially unchanged in the sense that a pea t-forming ecosystem has an incomplete decomposition process (normally because of waterlogging) , leaving a residue of organic material in it s overall energy-Row budget. The study of modern mires is therefore of relevance to Tertiary coal geologists, The classification of modern m ires is best achieved by reference to their hydrological characteristi cs, particularly the source of water entering the system. Flow-fed (rh eotrophic) mires are often relatively rich in nutrients and elastic, w hile rainfed (ombrotrophic) mires are poor. The transition from one hy drological state to the other can occur during mire development and in volves a physical elevation of the mire surface by the growth of peat. Such mires have a two-layered (diplotelmic) structure in which the su rface layer (acrotelm) is periodically aerobic, has a loose structure and a high hydraulic conductivity, while the lower layer (catotelm) is compacted, anaerobic and has a low hydraulic conductivity. The hydrol ogical outcome is that precipitation landing upon a raised, ombrotroph ic mire drains from the elevated mire surface laterally through the ac rotelm. The water retention of the mire and its elevated water table i s dependent on the poor water conductivity of the catotelm. The transi tion to the ombrotrophic condition is accompanied by higher rates of p ear accumulation and increasing mire acidity. It begins with isolated colonization of hummock-forming plants that later converge into an ext ended cupola. This ecological transition may be a consequence of exter nal changes, such as climatic alteration (increased precipitation: eva poration ratio) or autogenic, internal successional developments. Basi n subsidence, however, or eustatic sea-level rise in the case of coast al systems, can operate in the reverse direction and prevent the perma nent establishment of an ombrotrophic system. The ombrotrophic mire is ultimately limited in the depth of its pear development by microbial respiration within the catotelm. As the catotelm extends with pear acc umulation, the total respiratory activity of the profile will equilibr ate with primary productivity and peat formation slows down to an even tual standstill. On the assumption that peatland hydrology operated in a similar way in former times, the deep peats (and eventually coals) of Tertiary times can best be explained, therefore, in terms of stacks of sequential mires rather than single, continuously forming ecosyste ms.