Geoscience Reference
In-Depth Information
element-bearing Fe and Mn oxide coatings on
other particles. Some metals, however, especially
under low pH conditions, can be transported in
solution (see Chapter 3). This dissolved portion
encompasses contaminants that are either truly
dissolved, or in colloid form. The partitioning
of contaminants between the dissolved and par-
ticulate load in aquatic systems depends on both
physical and chemical factors, including pH,
redox, sediment mineralogy, sediment texture,
suspended sediment concentration and sediment
grain size. Grain size is possibly the most signific-
ant factor controlling the concentration and
retention of contaminants in both suspended
and bottom sediment. Metals in particular have
been shown to be enriched in the fine silt and
clay fractions of sediments, as a result of their
large surface area, organic and clay contents,
surface charge and cation exchange capacity
(see Chapters 3 and 6).
caps is thought to be primarily associated with
suspension transport and these silica-rich sands
are commonly termed 'loess'.
1.4.3 Sediment transport in glacial environments
Sediment transport within glacial environments
occurs associated with a range of transport pro-
cesses, which include suspension, aqueous suspen-
sion and aqueous traction currents. These form
different types of deposits and are associated with
different glacial environments. Suspension trans-
port, as outlined above, results in the deposition
of loess deposits in glacial marginal areas. Aque-
ous suspension is associated with the deposition
of fine, laminated clay sequences (varves), whereas
aqueous traction currents are responsible for
extensive fluvioglacial sand and gravel trans-
port, and the development of extensive outwash
plains (see Chapter 2). Glacial ice also acts as
an important sediment transport medium and,
although the movement of ice is slow, it is
responsible for significant erosion of underlying
bedrock and sediment. The resultant debris is
transported under and within the ice, and is
deposited either along the flanks of glaciers or
at the terminal end after the ice starts to melt.
These deposits are typically structureless and
comprised of poorly sorted boulder to clay-sized
material. The descriptive term for the sediment
is diamict and when deposited directly by glacier
ice is termed till. Consequently till is a major
component of glacial landforms such as moraines
and drumlins. High-magnitude sediment trans-
port events in glacial environments can occur
associated with jökulhlaups - a flood caused
by the sudden drainage of a subglacial or ice-
dammed lake, commonly triggered by a volcanic
eruption (see Chapter 2). These events can trans-
port huge volumes of sediment, and result in
extensive deposition of outwash deposits.
1.4.2 Sediment transport in aeolian environments
Sediment transport within aeolian settings
occurs primarily associated with either traction
carpets or in suspension and is common in three
main environments:
1
arid deserts;
2
associated with shoreward areas of beaches
and barrier islands;
3
developed around ice caps.
Sediment transport in deserts (Chapter 5) and
coastal dunes (Chapter 8) is associated primarily
with rolling and saltation of grains in the traction
carpet. As in aqueous environments, the critical
velocities required to entrain sediment increase
with grain size, and high velocities are required
to entrain fine silt and clay-sized material. Once
entrained, however, such sediment may be trans-
ported long distances as dust clouds, a mechan-
ism that is known, for example, to transport
large volumes of Saharan dust across the Atlantic
and into the eastern Caribbean (Prospero et al.
1970). Controls on the development of aeolian
sediment bedforms are discussed in Chapter 5,
but are strongly influenced by wind direction
and its variability, and by the rate of sediment
supply. Aeolian sediment deposition around ice
1.4.4 Sediment transport associated with
gravitational processes
Within each of the settings described, an addi-
tional agent of sediment transport is gravity and
three main categories of gravitational sediment
