Geology Reference
In-Depth Information
the lower limit for the so-called sortable silt parti-
cles with arguments similar to those stated above
(McCave et al.
1995
; Chang et al.
2006b
) . It is
important to stress that these limits are based on set-
tling diameters. The increasing use of grain-size
measuring devices based on lacer diffraction opens
for possible misinterpretations as these devices tend
to overestimate the size of especially the fi nest silt
fractions (e.g. Konert and Vandenberghe
1997
;
McCave et al.
2006
; Ramaswamy and Rao
2006
) .
The primary reason for this is that the laser diffrac-
tion technique has a tendency of measuring platey
particles as their large projected grain area, whereas
the same platey particle's settling diameter is much
smaller. McCave et al. (
2006
) found that settling
diameters of 2 and 16 mm are equivalent to ~8 and
22 mm sizes, respectively, when measured by a laser
particle sizer. In F units, this corresponds to a
change for a clay particle of 9 F to be seen as a silt
particle of 7 F and in the coarse end to a more mod-
est but similar change from 6 to 5.5 F .
According to Allen (
2000
) , Skempton's empirical
fi ndings suggest that:
(
)
kH
TTTe
−
=−
+
T
(8.1)
0
in
in
where
T
is the actual thickness of a layer which origi-
nally right after deposition had a thickness of
T
0
and
which limiting thickness ('zero' porosity) is
T
min
. H is
the depth below the surface and k (m
−1
) is an empirical
coeffi cient describing the compressibility of the layer.
However, this attempt to describe the natural compact-
ing behaviour of shallow silty salt marsh sediments
fails to describe autocompaction in the uppermost lay-
ers as pointed out by Bartholdy et al. (
2010b
) . They
designed a method by which the down core bulk dry
density
BDD
z
(kg m
−3
) at level z (m) beneath the sur-
face can be directly related to the bulk dry density of
the uppermost 5 cm (
BDD
0−0.05
) in uniform silty salt
marsh clay. The basis for this method is the fi nding that
bulk dry density varies down core as a logarithmic
function of depth (Fig.
8.13
):
BDD
=
A
ln( )
z
+
B
(8.2)
z
8.4.2
Autocompaction
and that the two empirical constants A and B can be
directly related to the bulk dry density of the upper-
most 5 cm by the following two empirically derived
equations:
Any interpretation of salt marsh sedimentation in
relation to dynamics is restricted to deal with the
amount of sediment in form of concentration (mass
per unit volume) in the water. When evaluating sedi-
mentation as a result of salt marsh dynamics, the most
appropriate measure is therefore weight per unit area
and time. Such data can be obtained by means of sed-
iment traps as in French and Spencer (
1993
) and van
Proosdij et al. (
2006b
). The most common and easy
measure of deposition, however, is the accretion rate
measured in length (level or thickness) per unit time.
This is also the measure that is relevant in order to
compare salt marsh growth to sea-level rise. In order
to evaluate accretion rates in relation to dynamically
controlled deposition rates, the bulk dry density and
its variation in the uppermost layers, therefore,
become of vital importance. The bulk dry density
depends on grain-size parameters, organic content,
and level of compaction. For the same location in a
salt marsh environment, the sediment type might be
regarded as constant with depth. Autocompaction is
then usually related to the sediment deposited above
the observed layer. Models relating this overburden
to autocompaction are described by Skempton (
1970
) .
A
=
0.16
B
D
+
00.05
20.63
(8.3)
−
B
=
1.64
B
D
+
00.05
82.44
(8.4)
−
In case
BDD
0−0.05
is unknown, Bartholdy et al.
(
2010b
) suggested the following relationship between
BDD
0−0.05
and the loss on ignition (
LOI
, %) and sand
content (
S
, %):
−
0.73
0.036
BDD
=
4478·
LOI
· 0.98
S
(8.5)
00.05
−
Integration of Eq.
8.2
from the surface and down
to the level, z under the surface gives the mass depth
of
z
:
MSD
=
A z
·
ln( )
z
+
z B
(
−
A
)
(8.2¢)
z
By means of the above equations, it is possible to
calculate the most likely variation of
BDD
z
with depth
under the salt marsh surface based on either
BDD
0−0.05
