Wetland Biogeochemistry Techniques - Wetland Techniques - page 404

material of choice because it is conducive to submerged systems, and its bright
white appearance is readily distinguishable from the surrounding sediment.
Typical plot size is 50 cm 50 cm, with a layer thickness of 5 mm. Care should

be taken during spreading the clay to ensure uniform horizon thickness. Plots are
marked with pipes or rods sufficient in height to rise above the water column or
vegetation to facilitate future sampling. Samples can be collected with a thin-walled
core tube. Collected cores are refrigerated and taken to the laboratory in a vertical
position. If processing is delayed, the cores are stored in the freezer. Melting
can compromise core integrity, especially those cores high in organic materials.
Therefore, the cores should remain frozen during processing. The frozen core is
sectioned to determine the thickness of the material above the feldspar marker.
The thickness of the sediment located above the feldspar marker is measured with
calibrated calipers. Sectioning the core facilitates sampling for bulk density
measurements and organic matter determinations.
A mass estimate of accretion can easily be determined by collecting a known
volume of the accreted material and determining bulk density (see Bulk Density
above). This estimate can be further refined into organic accretion and inorganic
accretion by subjecting the sample to LOI assay (See Soil Organic Matter: Loss
on Ignition above). Shallow compaction can be assessed by utilizing SET's data
(See Sedimentation - SETS below) with accretion data collected from short-term
accretion marker horizons as follows: compaction elevation

change. For this approach, feldspar plots must be established before taking the
SET baseline.

¼ sediment accretion

7.10.3 137Cs Dating

Marker integrity can be compromised by bioturbation, tidal action, resuspension, or
erosion. Therefore, the reliability of the artificial marker horizon technique suffers
in highly dynamic systems (DeLaune et al. 1989 ). An alternative approach to
assessing long-term sediment accumulation is to use cesium-137 tracer methods
(DeLaune et al. 1989 ; Milan et al. 1995 ). Production of 137Cs (half-life 30 year)
resulted from aboveground thermonuclear weapons testing. Atmospheric deposi-
tion of 137Cs began in 1954, but peak fallout occurred in 1964 (Ritchie and
McHenry 1990 ). Both dates are used to measure sediment accretion rates, but the
1954 sediment horizon is often difficult to discern (Ritchie and McHenry 1990 ), so
the 1964 peak is most often used as the marker layer in wetland investigations
(Reddy et al. 1993 ). In effect, the zone in the soil profile with the highest 137Cs
level represents a “marker horizon”, and all accretion above that zone occurred after
1964. The 137Cs signature may be compromised in areas with high erosion rates or
where large amounts of sediment washed-in from other areas (Milan et al. 1995 ).
In such situations, Milan et al. ( 1995 ) recommend that both 137Cs and 210Pb
methods be used (see DeLaune et al. ( 1989 )). The following 137Cs method was
presented by DeLaune et al. ( 1989 ). Sampling sites are established along a