A method for measuring vertical accretion, elevation, and compaction of soft, shallow-water sediments

High-resolution measures of vertical accretion, elevation, and compaction o f shallow-water sediments are fundamental to understanding the processes th at control elevation change and the mechanisms of progradation (e.g., devel opment of mudflats and intertidal wetlands) in coastal systems. Yet, measur ements of elevation by traditional survey methods often are of low accuracy because of the compressible nature of the substrates, Nor do they provide measures of vertical accretion or sediment compaction. This paper evaluates the use in shallow-water systems of an approach designed to measure these variables in vegetated wetlands, The approach employs simultaneous measures of elevation from temporary benchmarks using a sedimentation-erosion table (SET) and vertical accretion from marker horizons with sediment cores coll ected with a cryogenic coring apparatus. The measures are made with a level of resolution sufficient to distinguish between the influence of surface a nd subsurface processes on elevation, thus providing quantitative estimates of shallow subsidence. The SET-marker horizon approach was evaluated on a developing splay created by an artificial crevasse of a distributary in the Mississippi River delta. The approach provided high-resolution measures of vertical accretion (48.3 +/- 2.0 cm) and elevation (36.7 +/- 1.6 cm) over a 4-year period, with the difference between the two indicating the amount of shallow subsidence. In addition, by laying new marker horizons in later years, the approach provided rates not only of shallow subsidence (3.9 +/- 0.5 cm y(-1)) but also compaction of newly deposited sediments (2.1 +/- 0.6 cm y(-1)) and compaction of underlying sediments (1.8 +/- 0.20 cm y(-1)) o ver a two-year period. Hence, the SET-marker horizon approach has widesprea d applicability in both emergent wetland and shallow water environments for providing high resolution measures of the processes controlling elevation change.