How stable is the Mississippi Delta?

Large deltas are commonly believed to exhibit rapid rates of tectonic subsidence, largely due to sediment loading of the lithosphere. As a result, deltaic plains are prone to accelerated relative sea-level rise, coastal erosion, and wetland loss. Hurricane Katrina's devastation testifies to the severe threat that these processes pose to the Mississippi Delta, but the relative role of tectonics versus other mechanisms causing land subsidence remains elusive. Relative sea-level records derived from basal peat have the potential to quantify differential crustal movements over Holocene time scales with exceptionally high accuracy and precision. Here we present new sea-level index points from two study areas in the southwestern Mississippi Delta that essentially coincide with a recently published detailed relative sea-level record from the eastern part of the delta. Our results show that differential vertical movements among the three study areas have been only similar to 0.1 mm yr(-1). We compare our evidence with a recent sea-level compilation from the Caribbean, to a large extent based on data from areas that are tectonically stable. Our sea-level index points nearly coincide with the Caribbean data, showing surprising tectonic stability for considerable sections of the Mississippi Delta. However, the well-documented high subsidence rates in and near the birdfoot of the Mississippi Delta indicate that different conditions prevail there. The rapid wetland loss in coastal Louisiana is likely due, to a considerable extent, to the compaction of Holocene strata.