Geoscience Reference
In-Depth Information
climate about 3,500 year ago. Because of poor drainage on flat or gently sloping land,
such as old glacial lakebeds, peat begins to accumulate.
In tropical areas, such as Malaysia and Indonesia, peat deposits also occur in
both highlands and lowland areas. They are generally termed basin and valley peat
respectively. However, lowland or valley peat is more extensive and occurs in low-lying,
poorly drained depressions or basins in coastal areas. Basin peat is usually found on the
inward edge of the mangrove swamps along a coast. The individual peat bodies may
range from a few hectares to 100,000 hectares, and they generally have a dome-shaped
surface. The peat is generally classified as ombrogenous or rain-fed peat, and is poor
in nutrients (oligotrophic). Due to coastal and alluvial geomorphology they are often
elongated and irregular, rather than having the ideal round bog shape. The depth of the
peat is generally shallower near the coast and increases inland, locally exceeding more
than 20m. The coastal peat land is generally elevated well above adjacent river courses.
Steep gradients are found at the periphery, while the central peat plain is almost flat.
Water plays a fundamental role in the development and maintenance of tropical peat.
A balance of rainfall and evapotranspiration is critical to their sustainability. Rainfall
and surface topography regulate the overall hydrological characteristics of the peat
land. Peat land is also generally known as wetland or peat swamp because of its water
table, which is close to, or above, the peat surface throughout the year and fluctuates
with the intensity and frequency of rainfall. Peat swamps are an important component
of the world's wetlands - the dynamic link between land and water, a transition zone
where the flow of water, the cycling of nutrients and the energy of the sun combine to
produce a unique ecosystem of hydrology, soils and vegetation. The build-up of layers
of peat and degree of decomposition depends principally on the local composition of
the peat and the degree of waterlogging (Figure 2.10).
Peat formed in very wet conditions accumulates considerably faster and is less
decomposed than peat accumulating in drier places. The peat acts as a natural sponge,
retaining moisture at times of low rainfall but, because it is normally waterlogged
already, with a very limited capacity to absorb additional heavy rainfall during periods
such as a tropical monsoon. Peat swamp forests develop on these sites where dead
vegetation has become waterlogged and is accumulating as peat. Water in peat swamps
is generally high in humic substances (humus and humic acids) that give a typically
dark brown to black colour to the water. Peat swamps are characterized by diverse
features that relate to the nature of the water supply, such as flooding by surface or
groundwater, or solely from rainfall; the type of landscape in which the peat swamp
occurs, such as shallow depressions close to rivers; and the type of landscape that
the swamp creates, such as accumulation of peat above groundwater level so that
vegetation, often with prominent aerial roots, becomes wholly dependent on rainfall.
Basin peat forms domes, which according to Mutalib
et al
. (1991) are up to 15m
high whilst valley peats are flat or interlayered with river deposits. Normally, sandy
ridges bound basin peat at their seaward side or they gradually merge into muddy
coastal flats. Low-lying levees flank these domes along the rivers. The complexity of
the domes becomes more pronounced as the distance from the sea increases, as shown
in Figure 2.11.
Tropical (basin) peat domes were found to have typically well-developed internal
stratification. An example is shown in Figure 2.12. The peat deposit is shown to be
lenticular and dome surfaced, with a typical concave base. The centre of the dome,
