USGS National Wetlands Research Center: Accretion, Subsidence, and Sea-level Rise

Accretion, Subsidence, and Sea-level Rise

Sediment deposition, plant growth, decomposition, and subsidence are measured in region-wide field studies to complement laboratory experiments. Data are compared to local rates of sea-level rise to determine the potential for coastal marsh submergence.

cryo unit Several innovative field methods are employed for this program. Surface accretionary processes are measured from artificial marker horizons. To minimize disruption of these horizons during coring, we use a cryogenic coring apparatus that freezes the marsh, allowing the removal of only a small segment of the substrate while still maintaining soil stratigraphy. Surface elevation change is measured with a Surface Elevation Table (SET). A benchmark pipe is driven into the marsh soil to a depth of 3-6 m and forms the base for the SET. Changes in elevation of the marsh surface relative to the bottom of the benchmark pipe are measured by lowering pins from the SET to the marsh surface. This technique incorporates both surface and subsurface processes occurring between the surface and the bottom of the pipe. The difference in rates between the SET and the marker horizon provides an estimate of the rate of subsurface processes or what we call shallow subsidence.

Our findings indicate that the assumption of a 1:1 relationship between vertical accretion and surface elevation change is too simplistic a generalization of the complex interactions between accretionary and substrate processes. In many marsh types, particularly those with highly organic or deteriorating mineral substrates, surface elevation change is lower than the vertical accretion rate. In some cases, the two processes appear to be completely decoupled. Consequently, the potential for coastal marsh submergence is often being underestimated and should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits. These findings also indicate the need for greater understanding of the influence of subsurface and small-scale hydrologic processes on marsh surface elevation.

Research

Interaction Between Sea-level Rise and Vertical Accretion of Marshes in the Southeastern United States (Dr. Karen L. Mckee)
Response of Coastal Ecosystems to Sea-level Rise, Subsidence and Accretion: Assessing Relevative Elevation Changes and Management Options (Dr. James B. Grace)
Vulnerability of Managed Federal Lands to Sea-level Rise (Dr. Thomas W. Doyle)

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