Recent volumetric changes in salt marsh soils

Salt marsh sediment volume decreases from organic decomposition, compaction of solids, and de-watering, and each of these processes may change with age. Variability in the vertical accretion rate within the upper 2 m was determined by assembling results from concurrent application of the 137Cs and 210Pb dating techniques used to estimate sediment age since 1963/1964, and 0 to ca 100þ years before present (yBP), respectively. The relationship between 210Pb and the 137Cs dated accretion rates (Sed210 and Sed137, respectively) was linear for 45 salt marsh and mangrove environments. Sed210 averaged 75% of Sed137 suggesting that vertical accretion over the last 100þ years is driven by soil organic matter accumulation, as shown for the pre 137Cs dated horizon. The ratio of Sed210/Sed137 declines with increasing mineral content. A linear multiple regression equation that includes bulk density and Sed137 to predict Sed210 described 97% of the variance in Sed210. Sediments from Connecticut, Delaware and Louisiana coastal environments dated with 14C indicate a relatively constant sediment accretion rate of 0.13 cm year 1 for 1000e7000 yBP, which occurs within 2 m of today’s marsh surface and equals modern sea level rise rates. Soil subsidence is not shown to be distinctly different in these vastly different coastal settings. The major reason why the Sed137 measurements indicate higher accretion rates than do the Sed210 measurements is because the former apply to younger sediments where the effects of root growth and decomposition are greater than in the latter. The most intense rates of change in soil volume in organic-rich salt marshes sediments is, therefore, neither in deep or old sediments (>4 m; >1000 years), but within the first several hundreds of years after accumulation. The average changes in organic and inorganic constituents downcore are nearly equal for 58 dated sediment cores from the northern Gulf of Mexico. These parallel changes downcore are best described as resulting from compaction, rather than from organic matter decomposition. Thus most of the volumetric changes in these salt marsh sediments occurs in the upper 2 m, and declines quickly with depth. Extrapolation forwards or backwards, using results from the 210Pb and the 137Cs dating technique appear to be warranted for the types of samples from the environments described here.