The influence of rising sea level on Salt Marsh Accretion: A geochemical and Radiochemical investigation

Abstract

The Westerschelde estuary has undergone two major phases of channel dredging, one in the mid 1970s and one in the late 1990s. Local sea level rise induced by such engineering modification can have an effect on salt marsh accretion rates, due to an increase in inundation frequencies arising from reduced tidal friction. This study applied geochemistry and radiochronology to determine the effect of the channel modification on the salt marshes in the Westerschelde. A series of cores were taken from three of the salt marshes (Waarde, Zuidgors and Ritthem) in this estuary, and accretion rates were determined from the position of the ¹³⁷Cs and ²⁴¹Am bomb fallout maxima, and the ¹³⁷Cs Chernobyl maximum. These data showed that the sediment accretion rate of the marshes had increased in ~ 1980. The depth at which this occurred corresponded with the deposition of sediment of low ²¹⁰Pb_xS specific activity, which was caused by a combination of increased sediment accretion rate, and also a change in the grain size of the accumulating material. The source of this coarse, heavy-mineral rich, material is inferred to be reworked dredge spoil (Waarde), or redeposited marsh material (Zuidgors). Erosion at the eastern ends of Waarde and Zuidgors marshes commenced at the same time as the enhanced accretion rates, implying that the eroding marsh was a source of additional material for marsh accretion. The channel dredging was not the only possible cause of these marsh changes, as a greater number of storm force winds from the SW quarter were recorded in the period 1974 - 1979, causing increased wave activity in the Westerschelde, which could have contributed to the erosion of the marshes. The Westerschelde salt marshes are relatively unpolluted, despite the riverine end-member particulate material displaying greatly enhanced concentrations of pollutant elements (As, Cu, Cr, Ni, Pb, Zn) relative to the marine end member. Erosion of the salt marshes is not, therefore, likely to be a large source of pollutants to the Westerschelde.

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