Geoscience Reference
In-Depth Information
describing lake size, form and specific features.
Among these parameters, three main groups can
be identified.
1
Size parameters: (i) different parameters in
length units, such as maximum length, water
depth, shoreline length, maximum depth and
maximum breadth, (ii) parameters expressed in
area units, e.g. two-dimensional water surface
area and three-dimensional bottom area, and
(iii) parameters expressed in volume units, such
as water volume and surface-water volume
(epilimnetic volume).
2
Form parameters: based on size parameters,
for example mean depth and shore development
(
This is an important process with relevance
to studies of sediments as historical archives,
because turbidity currents can cause older, pre-
viously deposited material to settle on top of
more recently deposited sediment.
4.2
SEDIMENT SOURCES AND SEDIMENT ACCUMULATION
PROCESSES
4.2.1 Sources and characteristics of lake sediments
Five major sources of matter forming lake sedi-
ments can be recognized:
1
Allochthonous materials (sometimes called
lithogenous materials) - particles and aggregates
transported from land to lakes by rivers (e.g.
sand, silt and clay).
2
Autochthonous materials (or biogenous
materials) - sediments produced in the lake by
organisms living in the lake (e.g. silica frustules
or dead phytoplankton).
3
Hydrogenous sediments - sediments that are
also produced in the lake but which emanate
from materials precipitated out of solution, for
example, carbonates (e.g. Pedley 1990; Pedley
et al. 1996) and evaporites (e.g. Trichet et al.
2001).
4
Wet and dry deposition of matter on the
lake surface - this is generally a relatively small
contributor to the total amount of suspended
matter found in lakes.
5
Direct point source emissions of matter, for
example from urban areas and industries.
The relative contributions of each source can be
assessed for a lake by means of mass-balance
calculations (Fig. 4.2), where the flux (which is
equivalent to the flow or transport) from each
source is calculated. This can be calculated on
a monthly basis (g month
−1
) to obtain seasonal
variations, or annually for overall budgets in
order to rank the relative importance of each flux.
The relative contribution of different sources
differs between lakes as a result of variations in
characteristics, lake morphometry and climato-
logical regime.
The sediment types within lakes can be clas-
sified in a number of ways. One system, widely
shoreline irregularity).
3
Special parameters: such as the dynamic ratio
(
DR
=
A
/
D
m
, where
A
is lake area in square
kilometres and
D
m
is the mean depth in metres)
and effective fetch (a measure in kilometres of
the free water surface over which the winds can
influence the waves and hence also the sediments).
=√
4.1.3 Lakes as sedimentary environments
The main sedimentary environments that occur
within lakes are:
1
Beach and nearshore areas dominated by wave
processes, on- and offshore movement of sedi-
ments and nearshore currents (see Teller 2001).
The sediments in these areas are generally coarse-
grained (sand, gravel, etc.).
2
Deltaic areas in lakes with relatively large
tributaries transporting coarse silt and sand.
3
Shallow areas above the wave base, where
winds and waves may exert an influence on sedi-
ments (the surface-water area approximates
to the area above the mean thermocline). Such
sediments are generally well oxygenated and
may be resuspended by storms.
4
Deep-water areas occur beneath the wave base
(below the thermocline). These areas provide a
relatively calm, less well oxygenated sedimento-
logical environment for fine materials to settle.
5
On slopes inclined at more than 4 m per
100 m length, slope processes dominate the sedi-
mentological conditions on lake bottoms. In
such areas turbidites may form and they can
influence the sediment record over large areas.
