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Vertical accretion rates in microtidal wetlands and sea-level rise: Mustang Island, Texas

November, 2011

Abstract

This study determined vertical accretion (VA) rates in microtidal estuarine wetlands on Mustang Island, Texas. These environments exist within a very narrow range of elevations (<0.5 m) relative to the tidal frame and are threatened by a high rate of relative sea-level rise (~ 5 mm yr-1). Wetland survival and the effect of sea level rise depend to what extent the marsh surface is able to accrete vertically, due to the accumulation of organic and inorganic material, and maintain the relative elevation and stem the effect of sea level rise. Shallow sediment cores were analyzed to determine whether the net accumulation of organic and inorganic material is keeping pace with sea-level rise (SLR).

Shallow sediment cores were collected along transects in high-marsh, high-flat, low-marsh and low-flat environments on Mustang Island. Cores were sliced at 1 cm intervals and samples analyzed for gamma radiation, organic content, bulk density and grain size. Determination of VA was based on radioactivity of Cesium-137, an artificial radionuclide introduced into the environment through atmospheric nuclear weapons tests. Maximum fallout occurred in 1963, thus the peak activity can serve as a temporal marker in sediments. Three different approaches were used to address core compaction and shallow autocompaction, so that three accretion rates per core were calculated.

The results indicate an accretionary deficit is present in all estuarine wetland environments in the study area, with the possible exception of low marsh. Average accretion was on the order of 1.30±0.09 mm yr-1 in high-marsh environments; 1.29±0.01 mm yr-1 in high-flat environments; 3.81±0.37 mm yr-1 in low marsh environments; and 0.69±0.1 mm yr-1 in low-flat environments. Seventeen cores were analyzed for 137Cs, but only thirteen accretion rates could be determined. Reworking of sediment through bioturbation and/or erosion obscured the Cesium peak and complicated the determination of accretion in low-marsh and low-flat environments. No statistical difference was found among the different approaches to calculate accretion.

It is clear, however, that VA rates in marsh habitats is negatively correlated with elevation. In comparing relative dry weight contributions, VA is dominated by mineral matter (MM) since 1963. However this does not adequately describe in-situ conditions as the organic matter (OM) present is correlated with water holding capacity and porosity. The relative contributions of MM and OM were approximately equal when pore space was combined with the organic portion.

Lastly, stratigraphic evidence showed that much of the sediments underlying modern wetlands are hurricane washover deposits. Therefore, a comprehensive natural resource management strategy should include development set-backs that protect both gulf- and bay margin environments.

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