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Long-term Accretion Rates on Tidal Salt Marshes
J. S. Pethick
Long-term variations in salt marsh accretion rates were estimated using data collected on the marshes of the North Norfolk coast, England. Fourteen discrete marsh areas were sampled all lying within a 20 km stretch of coast. The age of each marsh had previously been determined using radiocarbon or archival evidence for the date of vegetation inception on each marsh surface. Marsh ages ranged from 10 years to +2000 years.
The elevation of each marsh surface was determined in the field using levelling techniques. The relationship between age and surface elevation was assumed to be a function of the rate of marsh accretion within this area of coast. Statistical tests showed this to be a highly significant relationship while the accretion rates calculated from the age/height curve agree closely with field measurements on these marshes. Accretion rates were found to vary from 1.7 cm/yr on 10-year-old marshes to less than 0.002 cm on marshes older than 500 years.
The age/height relationship describes an asymptotic curve with the asymptote lying 0.8 m below the level of the highest spring tides, and appears to be controlled by the frequency of tidal maxima which peaks at 0.8 m below the maximum value and declines sharply above that height.
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