Sustainability of coastal marshes under accelerated relative sea-level rise

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Coastal marshes rank as some of the world's most valuable ecosystems in terms of goods and services; their fate will have major consequences for coastal fisheries, nutrient cycling, storm-surge protection, and related processes. Coastal marshes are also rapidly degrading due to accelerated sea-level rise and other human-induced stresses. Louisiana experiences some of the world's most rapid rates of coastal wetland loss, constituting a bellwether for other coastal regions around the nation. Multi-billion dollar coastal restoration projects are currently being initiated in this area, and it is paramount that these investments be guided by the best available scientific information. Considerable effort has been expended to determine the resiliency of coastal environments in the face of relative sea-level rise, but these studies have largely been limited to observations over short (annual to decadal) timescales. This project will fill this important gap through an analysis of the sedimentary deposits of the Mississippi Delta over the past few thousand years.The project involves close interaction with the Louisiana Coastal Protection and Restoration Agency within the context of the Master Plan for a coast that in all likelihood can only be partially sustained. Furthermore, the project will be used to enhance the curriculum at a minority-serving New Orleans science and math high school.The following two hypotheses will be tested: (1) coastal marshes in the Mississippi Delta can keep pace with rates of relative sea-level rise up to about 10 mm/yr; and (2) coastal marshes in the Mississippi Delta keep up with relative sea-level rise to a significant extent by means of the accumulation of organic matter. The methods will involve a reconstruction of sedimentary environments in the Delta based upon data from hundreds of boreholes that have been collected in previous years. The response of ancient marshes to contemporary rates of relative sea-level rise of ~0.5 to 10 mm/yr (i.e., the full range from the pre-industrial period to rates predicted for the end of the 21st century) will be determined from a direct comparison of the sedimentary patterns with the rates. In order to apply the stratigraphic analysis to present-day marsh dynamics, a compilation of measurements of modern marsh surface-elevation changes and soil properties from over100 sites in coastal Louisiana will be carried out. Collectively, this will enable a rigorous comparison of marsh sustainability over a wide range of time scales.