Modeling shallow autocompaction in coastal marshes using cesium-137 fallout: Preliminary results from the Trinity River Estuary, Texas
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Accurate prediction of changes in the relative elevation of coastal marsh surfaces has gained considerable importance in light of global-warming-induced sea-level rise. Shallow autocompaction is an important component of elevation change that acts to decrease relative elevation. Unlike vertical accretion, which can be determined from repeated surveys of artificial marker horizons, autocompaction is difficult to measure. This report presents the results of a preliminary study that uses cesium-137 dating and down-core measurements of sediment bulk density to numerically model vertical accretion, shallow autocompaction and surface elevation change. The models produce vertical accretion rates that are comparable to rates derived from artificial marker horizons, and long-term surface elevation changes that are in close agreement with sedimentation rates based on cesium-137 dating of well-compacted sediment. The model results suggest that average annual vertical accretion may be higher than average annual surface elevation change by a factor of 1.6 to 21. Sedimentation rates derived from cesium-137 dating also are higher than average annual surface elevation change by a factor of 1.1 to 4.7. These findings suggest that sedimentation rates based on cesium-137 dating will tend to be lower than vertical accretion and higher than long-term surface elevation change. The results of the study re-emphasize the importance of fully accounting for shallow autocompaction when attempting to predict marsh surface elevation change
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