Geology Reference
In-Depth Information
1000
900
800
700
600
500
400
300
200
0
0.02
0.04
0.06 0.08
Level below surface (m)
0.1
0.12
0.14
0.16 0.18
0.2
0.22
Fig. 8.13
Bulk dry density as a function of depth beneath the salt
marsh surface at Skallingen. The diamonds represent measurements
of ½ cm slices from the
top down
to
just above
the sand fl at beneath
the clay. The
broken black line
represents the running mean of
3 measurements (1½ cm); the
full black line
is the logarithmic best
fi t curve:
BDD
= 113 ln(z) +935;
R
2
= 0.67; P < 0.01; the
stippled
grey line
(
BDD
= 100 ln (z) + 897) represents the suggested method
(see the text) (From Bartholdy et al.
2010b
)
8.5
Salt Marsh Accretion Models
or LOI and S in Eq.
8.5
. The stippled grey line in
Fig.
8.13
, which in praxis is identical with the regres-
sion line, is constructed on the basis of this method.
The method is based on empirical relations derived
from silty salt marsh types like those present in the
Wadden Sea.
The importance of adjusting accretion measure-
ments to autocompaction is evident from Fig.
8.14
.
Here, a constant rate of deposition of 1.7 kg m
−2
year
−1
of the same sediment type as that from Fig.
8.13
is modelled to show the position of any given yearly
surface relative to an incompressible base each year
over a 75-year period. If a marker horizon is estab-
lished on the surface, it will, with the given conditions
after a 5-year period, be located exactly 2.19 cm below
the surface no matter when it is established. Because
of autocompaction, however, the correct accretion in
relation to the fi xed level of the base will decrease to
half its start value after approximately 75 years
(~20 cm). It is therefore crucial for any studies of
accretion with marker horizons to be aware of both
position of the marker below the surface and the degree
of autocompaction. Accretion rates based on markers
spread on the surface will, if these matters are not
addressed, overestimate the correct accretion rates in
relation to, for example, a measured sea-level rise.
8.5.1
Model Formulation
Vertical salt marsh accretion consists of elevation
change as a function of net accumulation due to sedi-
ment supply plus the net accumulation of internally
derived plant detritus. Furthermore, it is necessary to
take into consideration the rate of deposit thinning due
to autocompaction and possible isostatic changes in
order to keep track with the absolute level variations. If
the level is to be evaluated in relation to the relative sea
level at a given location, eustatic changes also need to
be considered (Fig.
8.15
).
The continuity equation for salt marsh formation
can be expressed as:
Δ=Δ
es
+Δ
s
(8.6)
sed
org
where D
e
is the change in salt marsh mass per unit area
and unit time, D
s
sed
is the sediment supply per unit area
and unit time and D
s
org
is the change in organic matter
due to biological production per unit area and unit
time. In order to translate this equation into levels, it is
necessary to incorporate autocompaction and isostatic
movements:
