Major subsidence of the south-central United States of America and future inundation of coastal areas

The northern shore of the Gulf of Mexico is the site of America's greatest wetland, the gateway to vast energy resources, and home to over 10 million people. This critical area is being increasingly threatened by progressive inundation by the relative rise of the Gulf of Mexico. This slow inundation was detected several decades ago and has been generally attributed to eustatic sea level rise, sediment starvation of the delta due to construction of flood control levees along the Mississippi River, and subsidence of the land relative to sea level. Although the former two effects are reasonably well understood, the lack of precise quantitative spatial data on the later related to a well defined, common datum has prevented the development of a satisfactory theory to explain modern surface motions. Analysis of National Geodetic Survey (NGS) 1st order leveling data produced vertical velocities for over 2700 benchmarks in Louisiana, Mississippi, Alabama, Texas, Arkansas, Florida, and Tennessee. All motions were related to NAVD88 and show that subsidence is not limited to coastal wetland areas, but rather includes the entire coastal zone as well as inland areas several hundred km from the shore. Subsidence can also be tracked to the north and follows the trend of the alluvial valley of the Mississippi River. Regionally, vertical velocities range from less than -30 mm/yr along the coast to over +5 mm/yr in peripheral areas of eastern Mississippi-Alabama. The mean rate is ~11 mm/yr in most coastal parishes of Louisiana. In the Mississippi River deltaic plain, subsidence was significantly higher than previous estimates based on long-term geologic measurements. The data also indicate that adjacent alluvial ridges where the population is concentrated have been similarly affected. In the Chenier plain of southwest Louisiana, a region previously thought to be subsiding at slowly, rates of sinking are similar to those of the deltaic plain. Demonstration that all areas of the coastal landscape as well as inland areas are affected implies that subsidence recorded by benchmarks is not solely due to local sedimentary processes and/or the activities of humans. Instead, geodetic data when integrated with subsurface geologic information suggest that subsidence includes a strong regional component that is the product of lithospheric flexure and normal faulting. This component is mainly due to the derivative effects of late Quaternary sediment loads such as the modern Mississippi River delta and Pleistocene deposits offshore. Models of simple flexure are inadequate, however, to explain the regional component of subsidence. Instead, it is proposed that active faulting plays a key role in regional subsidence throughout the coast by episodically weakening the lithosphere, which in turn changes the way that the lithosphere bears the load of sediments over time. Salt intrusion/evacuation induced by loading is a major cause of subsidence in southwest Louisiana. If subsidence continues at similar rates and construction efforts fail to build protection levees to appropriate heights, substantial portions of the gulf coast (primarily Louisiana) will lie below sea level and be inundated by end of this century. In Louisiana, this will result in a loss of ~$140B of land and property, as well as the land, livelihoods, and cultural heritage of over 2 million people.