Geology Reference
In-Depth Information
Fig. 10.13
Comparison of the trends in organic carbon con-
tent (
a
) and organic carbon concentration (
b
) as a function of
mud content (data points omitted for clarity) (Based on
Flemming and Delafontaine
2000
). Note the similarity of the
POC concentration curve to that of mud mass concentration in
Fig.
10.12b
density being identical at 18% mud content, and up to
50% larger at intermediate mud content, in comparison
to that at 100%. As pointed out earlier, this would also
apply to any other sediment component linked to the
mud fraction, e.g. heavy metals, trace elements, organic
pollutants, toxic substances.
A mass physical property of intertidal sediment that
plays an important role in the mobility or stability, i.e.
the erosion resistance, of sediment, is the shear strength
of the substrate. This parameter is conveniently deter-
mined in the field by a so-called vane shear apparatus
where cross-vanes of different dimensions are cali-
brated such as to provide the shear strength upon yield
after being inserted into the sediment and twisted
against the resistance of a spring. Shear strength of
intertidal sediment in relation to mud content, porosity,
wet bulk density, and dry bulk density is illustrated in
Fig.
10.14
. Overall, the shear strength of intertidal
sediment decreases with increasing mud content and
porosity, and consequently increases with increasing
wet and dry bulk density. Two features in the illustrated
trends are of particular interest here. First, in all four
cases, the highest and lowest shear strength for any
value of the other parameter is well defined in what
could be called an upper and lower boundary criterion.
Both have a similar positive or negative trend as the
mean trend line that would be defined by a regression
analysis. In the case of mud content this means that for
each of the criteria (i.e. upper boundary, mean, lower
boundary) the shear strength progressively decreases
as mud content increases, the reverse being true for
bulk density. This applies in corresponding manner to
any other correlating parameter.
The other interesting feature is the increasing range
in shear strength (increasing standard deviation)
toward lower mud content and porosity, and higher
bulk density. The increasing scatter of the data points
reflects an increasing variability in the degree of com-
paction (grain packing density) toward more sandy
sediments. This is not unexpected as the hydrody-
namic energy also increases toward higher sand con-
tent. The trends therefore trace the shoreward energy
gradient together with its local variability, which is
highest in sand. Thus, wave-compacted sands will
have relatively high shear strength, whereas water-
logged sand will display a shear strength that may be
as low as that at intermediate to high mud content.
Excluded from these examples is dewatered mud
commonly found in the subsurface of mixed flats, in
channel-fill sequences, and between the neap and
spring high-tide level where desiccation over the
neap-tide period results in compaction and corre-
sponding higher bulk density associated with lower
porosity and water content.
