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Measuring Elevation Change

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A critical component of this study involves high-resolution measures of marsh surface elevation change. The surface elevation table-marker horizon (SET-MH) approach (Fig. 9) makes it possible to:

  1. quantitatively determine with high precision changes in marsh surface elevation,
  2. determine the separate contributions of surface and subsurface processes to surface elevation,
  3. calculate shallow subsidence (accretion minus elevation, Cahoon et al. 1995), and to
  4. partition shallow subsidence between shallow (root zone) and deeper (below the root zone to > 10 m) portions of the soil profile (Cahoon et al., 2002a, b) using benchmarks of different depths.

Vertical accretion (i.e., sediment deposition and erosion) was measured to the nearest millimeter from cores taken through soil marker horizons laid on the marsh surface (Cahoon and Turner 1989) (Figs. 9 & 10). Surface elevation change was measured from a deep-rod Surface Elevation Table (SET) (Cahoon et al. 2002a, b), a highly precise (1-2 mm) mechanical leveling device (Figs. 9 & 10) that attaches to a benchmark driven into the substrate to refusal (~40-60’). SET measurements incorporate the surface processes measured from the marker horizon plus the subsurface processes occurring between the marker horizon and the base of the SET benchmark. The collective influence on surface elevation of these subsurface processes (i.e., root growth and decomposition, sediment compaction, and shrink/swell from water flux), which is called shallow subsidence, is calculated by subtracting elevation from accretion (Cahoon et al. 1995). Measurements were also taken from the surface of a shallow benchmark pipe driven to the depth of the root zone (~0.3 m) in order to determine the relative effect of root processes on surface elevation change. A full and detailed explanation of the SET-marker horizon approach is provided on Dr. Cahoon’s web site. Figure 11 shows the experimental plot layout for measuring elevation change and other variables.

Fig. 9. Conceptual diagram depicting SET-Marker Horizon technique and processes contributing to elevation change.
 

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Fig. 10. Soil core depicting marker horizon and accreted sediment (above) and reading the SET (below)
Fig. 11. Experimental plot layout at Blackwater NWR for measuring elevation change and other variables.

 

Literature Cited
Cahoon, D. R. and R. E. Turner. 1989. Accretion and canal impacts in a rapidly subsiding wetland II. Feldspar marker horizon technique. Estuaries 12 (4): 260-268.
Cahoon, D. R., D.J. Reed, and J.W. Day, Jr. 1995. Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited. Marine Geology 128:1-9.
Cahoon, D.R., Lynch, J.C., Hensel, P., Boumans, R., Perez, B.C., Segura, B. & Day, Jr., J.W. 2002a. High precision measurement of wetland sediment elevation: I. recent improvements to the sedimentation-erosion table. J. Sed. Res., 72, 730-733.
Cahoon, D.R., J. C. Lynch, B. C. Perez, B. Segura, R. Holland, C. Stelly, G. Stephenson, and P. Hensel. 2002b. High precision measurement of wetland sediment elevation: II. the rod surface elevation table. J. Sed. Res., 72, 734-739.

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