Geology Reference
In-Depth Information
a
b
1.6
0.6
1.2
Total sediment
(sand + mud)
~0.41 g cm
-3
0.4
0.8
Sand
~0.29 g cm
-3
0.2
0.4
Mud
0.0
0.0
0
20 40 60
Mud content (dry weight-%)
80
100
0
20
40
60
80
100
Mud content (dry weight-%)
Fig. 10.12
Dry mass concentrations of sand and mud relative
to the total sediment (
a
). Note the counter-intuitive trend in the
progression of the mud component (
b
) which reverses after
peaking around 60% in this dataset from the Wadden Sea (Based
on Flemming and Delafontaine
2000
)
regression curve, the network structure is very loose,
being merely supported by flocs and aggregates. As
sand is added, the grains initially fill the voids by
expelling water without breaking down the network
structure that results in a proportional increase in the
mass concentration of mud. This continues up to the
apex point where the sand content in this case is about
40%. As this limit is approached, the network structure
begins to break down and the sediment is increasingly
grain supported, water and mud now filling the voids
between the sand grains. In effect this means that, in a
unit volume of intertidal sediment, the mass concentra-
tion of mud in sediment consisting of pure mud (>95%
mud content) is equal to that at mud contents as low as
25%, while the highest mass concentration of mud is
registered at the apex of the regression curve. As in the
case of bulk density, other tidal flat environments may
have slightly different trends to the Wadden Sea exam-
ple shown here. If required, corresponding calibration
curves should therefore be established for other tidal
flat environments.
The unexpected trend observed in mud mass con-
centration has far-reaching implications because any
other parameter linked to the mud fraction (e.g.,
organic matter, trace elements, pollutants) will by
necessity follow a similar trend. Contrary to common
perception, highest mass concentrations of mud, and
hence of any substances linked to the mud fraction, are
found in mixed sediments (muddy sand and sandy
mud) and not in pure mud. This potentially confusing
issue and its pitfalls are discussed in detail by Flemming
and Delafontaine (
2000
). A particularly common mis-
take is to relate measures of concentration, i.e. masses
per unit volume or area, e.g., animal density per m
2
, to
measures of content, i.e. masses per unit mass, e.g.
weight-% organic matter. By ignoring the dimensional
incompatibility between contents and concentrations,
it goes unnoticed that corresponding masses occupy
increasingly larger volumes as the water content and
the mud content increases. Thus, the volume occupied
by a unit mass of pure mud with a dry bulk density
~0.3 g cm
−3
is more than five times larger than that
occupied by the same mass of pure sand having a dry
bulk density ~1.6 g cm
−3
. For organic carbon, which is
a measure commonly associated with the amount of
food available to organisms, this disparity is illustrated
in Fig.
10.13
. The positive correlation between POC
content and mud content (Fig.
10.13a
) is commonly
assumed to indicate that the largest amount of food is
contained in pure mud as reflected by the highest POC
content. However, as in the case of mud mass concen-
tration (Fig.
10.12b
), the relationship between POC
mass concentration and mud content (Fig.
10.13b
)
clearly demonstrates that this assumption is wrong, the
amount of POC per unit volume of sediment that
corresponds to the dimensional measure for animal
