What is the Role of Organic Material in Delta Surface Elevation Gain?

Globally, mineral sediment supply to deltaic wetlands has generally decreased as a result of the construction of levees and other flood-control infrastructure. As a result, many wetland soils have become increasingly organic-rich. Because organic-rich soils, especially peats, are more compressible than their mineral-rich counterparts, many modern wetlands are vulnerable to soil compaction, subsidence, and conversion to open water.

Using data from sediment cores previously collected in the Mississippi Delta, we examine organic-rich facies to determine the nature of their contribution to land building and maintenance of deltaic wetlands. By comparing the bulk densities of organic-rich soils at the surface with similar facies buried at depth, we quantify the magnitude of soil volume lost through compaction and investigate whether organic-rich soils are able to maintain sufficient volume as they are buried for the overlying wetlands to keep pace with relative sea-level rise.

When soils are buried, compaction of organic material does not occur all at once. Although initial compaction may be rapid, we expect that soil compaction will continue for decades, centuries, or even millennia, although at progressively slower rates. As a result of this protracted time period of compaction, we expect to find a tipping point in soil organic content, above which deltaic wetlands are no longer sustainable over long (century to millennial) timescales. Highly organic-rich wetlands may initially keep pace with relative sea-level rise over annual to decadal timescales, but eventually compaction of the increasingly thick interval of organic material may outpace the accumulation rate of new material. If increased compaction causes relative sea-level rise to exceed accumulation, a wetland is no longer sustainable.