Investigating Modern and Historical Sediment Accretion in a Deltaic Marsh (Louisiana) Using Radiocarbon Geochronology

Coastal land loss in the northern Gulf of Mexico, especially Louisiana marshes, has been estimated at close to 10,000 ha annually. This loss has been attributed to a variety of natural and human disturbances, including sea level rise, erosion, levee construction, and canal building. Breton Sound Basin (BSB), Louisiana, located south of New Orleans, is one such marsh area that has been separated from Mississippi River inputs by levees (between ~1890s and 1991) and is the site of long-term physical and ecological studies. A freshwater diversion was built in 1991 which releases river water to the estuary annually on the spring cycle, and during the rest of the year, modest discharge through the diversion may occur when the river stage is above ~1.1 m. The diversion has been credited with reversing or slowing land loss trends in BSB estuary. In 2005, BSB received a major component of the storm surge force associated with Hurricane Katrina and the recent sedimentary record was greatly disturbed. We examined the potential for sustainability of a Louisiana coastal wetland based on its capacity to maintain elevation in the face of relative sea level rise and changing deposition rates through time. Using a combination of 137Cs, 210Pb, and 14C geochronology as well as stratigraphic analysis of six 3 to 4 m vibracores, we quantified historical sediment accretion rates in Breton Sound. Event deposits range in age between 145 and 3524 years old. Modern sedimentation rates measured using 210Pb dating are less than 0.2 cm/yr. Within this context, we evaluate the efficacy of modern pulsed sedimentation schemes that reconnect wetlands with the Mississippi River.