Geoscience Reference
In-Depth Information
Sedimentation (cm/yr)
Sediment
depth (cm)
x
0
Upward
biotransport
(
B
u
)
Downward
biotransport
(
B
d
)
Water content;
compaction
W
0
x
1
W
1
x
2
W
2
x
3
W
3
x
4
W
4
x
5
W
5
Bioturbation limit
x
6
W
6
B
u
>
B
d
at all
sediment depths
Substrate
decomposition
= mineralization
W
1
>
W
2
>
W
3
, etc.
Fig. 4.10
Illustration of key processes related to bioturbated sediments (sedimentation, upward and downward biotransport, substrate
decomposition and compaction). (Modified from Håkanson & Jansson 1983.)
Laminated
sediments
(generally thinner,
darker winter layers;
thicker lighter
summer layers)
Bioturbation
(showing areal, vertical, temporal and
species specific patchiness)
Fig. 4.11
Bioturbation and laminated
sediments. Under aerobic (
=
oxic)
conditions zoobenthos may create a
biological mixing of sediments down to
about 15 cm sediment depth (the
bioturbation limit). If the deposition of
organic materials increases and hence
also the oxygen consumption from
bacterial degradation of organic
materials, the oxygen concentration
may reach the critical limit of 2 mg L
−
1
,
when zoobenthos die and bioturbation
ceases and laminated sediments
appear. (Modified from Pearson &
Rosenberg 1976).
0
3
Aerobic sediments
6
Anaerobic sediments
9
(cm)
Bioturbatio
n
limit
(at 5-15 cm)
Normal conditions Transition
Polluted zone Very polluted
zone
zone
Increased contamination of organic materials
Decreased oxygen concentration
4.2.2.3.2 Compactional and diagenetic processes
A
number of important processes influence sedi-
ment accumulation following deposition. These
include diffusion, mineralization, compaction
and burial. Diffusion is the chemical transport
of dissolved substances from sediment inter-
stitial water back to lake water regulated by
concentration gradients. For many substances
(e.g. phosphorus and caesium), the diffusive
transport is highly dependent on sediment
