Geology Reference
In-Depth Information
autocompaction. Esserlink et al. (1998) used detailed
surveyed lines in the Dollard Estuary, the Netherlands,
to determine vertical salt marsh accretion. Pethick
(
1980,
1981
) used maps and historical data to deter-
mine the time of marsh inception at spots where he,
through standard fi eld levelling techniques, also deter-
mined the actual salt marsh level. Based on data from
14 salt marsh areas in North Norfolk (UK), he found
an asymptotic age-surface elevation relationship.
Lately, the incorporation of LIDAR data in such
studies (e.g. Van der Wall et al.
2002
) represents a new
and powerful tool to detect and examine detailed
morphology and morphological changes through time.
Direct measurements of elevation change at carefully
selected points have also been made by many researchers.
Boumans and Day (
1993
) presented a so-called sedi-
mentation-erosion table (SET) which was used by
Cahoon et al. (
2000
) together with measurements based
on marker horizons to evaluate the role of autocompac-
tion. Lately, Charman et al. (
2007
) have come up with
an automatic devise, primarily developed for measuring
downwearing rates (TEB) that can be seen as a modi-
fi ed modern version of the SET.
Modern dating methods represent a vital tool for
measurements of salt marsh accretion over longer tim-
escales.
14
C dating is widely used in salt marsh envi-
ronments (e.g. Gehrelds et al.
2006
) . In many instances,
however, it fails to give useful results concerning accu-
rate salt marsh accretion rates, partly because of the
upper time limitations and accuracy of the method, and
partly because of problems with a comprehensive way
of treating autocompaction. Following Goldberg
(
1963
) who developed the
210
Pb dating technique for
snow accumulation studies in Greenland, this method
was used to date sediments (Krishnaswami et al.
1971
) .
A combination of
210
Pb dating and the use of the fallout
product of nuclear testing,
137
Cs, became widely used
since the 1970s as a tool for measuring sedimentation
rates in lakes (Robbins and Edington
1975
; Oldfi eld
et al.
1978
) . Koide et al. (
1972
) were the fi rst to use
210
Pb dating of sediments in the marine environment.
Two different assumptions can be used in the process
of
210
Pb dating of sediments: (1) the constant net rate of
supply (c.r.s.) and (2) the constant initial concentration
(c.i.c.). The use of one or the other was strongly
debated in the late 1970s (e.g. Megumi
1978
; Oldfi eld
et al.
1978
; Appleby et al.
1979
) . Appleby and Oldfi eld
(
1978
) suggested dating by means of the c.r.s. method.
In salt marsh environments, the c.i.c. method has been
preferred (Kirchner and Ehlers
1998
; Bartholdy et al.
2004
; Pedersen and Bartholdy
2006
) and verifi ed by
means of marker horizons (Madsen
1981
; Pedersen
et al.
2007
) .
210
Pb dating was quickly implemented in
sediment budget studies of tidal areas (Bartholdy and
Madsen
1985
) and represents today an important tool
for evaluating a vast majority of aspects related to salt
marsh accretion such as heavy metal accumulation
(Christiansen et al.
2001
), and accumulation of other
pollutants (French et al.
1994
). An important aspect of
210
Pb dating is that not all sediment types have the same
ability to retain this radionuclide (Ackermann
1980
) .
Studying heavy metal concentrations in sediment,
Ackermann et al. (
1983
) advocate the use of grain
sizes <20 mm. Other authors (e.g. Olsen et al.
1982
)
have argued that also organic matter is able to bind
210
Pb. So, if a sediment core is not homogeneous in
terms of organic matter and/or grain size, it is impor-
tant to normalize the unsupported
210
Pb, in order to
avoid unwanted effects of sediment-type variation.
Kirchner and Ehlers (
1998
) suggested the unsupported
210
Pb to be normalized by the sum of loss-on-ignition
and grain-size content of <20 mm. Lately, also the
OSL method (optical stimulated luminescence) has
been used to assess salt marsh accretion rates (Madsen
et al.
2007
). This type of sediment dating no doubt
will be of great importance in future salt marsh
research, as it can be used to date sediments over small
(decades) as well as large (millennia) timescales and
is found to work well in the estuarine environment
(Madsen et al.
2010
) .
8.3
Morphodynamics of Salt Marshes
Salt marsh sedimentation can be separated into three
main types: (1) sedimentation associated with channel
fl ow in the vicinity of salt marsh creeks, (2) sedimenta-
tion associated with sheet fl ow over vegetated salt
marsh surfaces, and (3) sedimentation associated with
exposed salt marsh edges. The two former has large
resemblance with sedimentation in fl uvial systems.
There are, however, some obvious differences between
the fl uvial and the tidal dominated environment. First
of all, the bidirectional fl ow in tidal areas can alter the
channel-related morphology compared to the unidirec-
tional fl ow in fl uvial systems. Furthermore, and as a
consequence of this, the fl ow velocity is at a minimum
in the tide-dominated environment at the time of both
