| Summary: | Deltaic marshes of the Mississippi River in Louisiana disappeared at a rate of 88 km2 annually from 1956 to 2000 (Barras et al. 2003) as marshes become inundated by sea water. Marsh surface elevation varies spatially and temporally due to fluvial sediment deposition, resuspension, erosion, compaction, sea level rise, and organic matter accumulation and decomposition. If net accretion from sediment deposition and/or peat production is insufficient, marshes respond to sea level rise by migrating landward. Since human development prevents landward migration of marsh in Breton Sound Basin, Louisiana, marsh sustainability can only be achieved if vertical accretion keeps pace with a relative sea level rise of 10 mm/yr so that marsh surface elevation is maintained within the tidal range.
Measurement time scale and changing influences on marsh sediment were considered in an assessment of the long-term sustainability of Breton Sound marsh based on comparison of the rate of relative sea level rise to measured accretion rates. Six cores (~4 m long each) were collected in Breton Sound and a combination of three radioisotopes, as well as stratigraphic analysis were used to measure accretion rates and identify evidence of historical river effects and storms.
Net accretion rates over recent short-term (decadal) and long-term (centennial/millennial) time scales were measured using 210Pb, 137Cs, and 14C dating. Long-term mean accretion based on 14C dating was highly variable (3.5 mm/yr, σ=4.5). Three 210Pb rates were recovered, averaging 4.3 mm/yr (σ=1.9). Accretion rates measured using 137Cs averaged 7.7 mm/yr (σ=2.3). Rates of sediment accretion are ultimately insufficient to offset relative sea level rise, especially after allowing for sediment volume reduction encountered over the long term. The combined effects of reduced fluvial input, rising sea level, and prevention of landward marsh migration create an environment that is inherently unstable.
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