FUTURE SEA LEVEL CHANGES 

 ALONG THE LOUISANA COAST 



Dag Nummedal 



Department of Geology 



Louisiana State University 



Baton Rouge, LA 70803 



ABSTRACT 



The relative elevation of sea and land has been changing throughout time in 

 response to two fundamentally different groups of factors. Global factors include 

 changes in the volume of the ocean basins due to tectonic processes and changes in the 

 total amount of ocean water due to glaciation. Local factors include subsidence of 

 continental margins and the compaction of recent sediments. During this century, global 

 sea level (eustatic) appears to have been rising at a rate of 1.2 mm/yr. Along the south- 

 central Louisiana coast the land surface appears to be sinking at a rate of about 

 8 mm/yr. 



Recent global climatic modelling strongly suggests that we are about to enter a 

 period of rapid warming due to increased amounts of carbon dioxide (CO2) in the 

 atmosphere. As a consequence, eustatic sea-level rise is predicted to accelerate both 

 because of steric expansion of the ocean water and continued melting of polar ice caps. 

 For the next 40 years the eustatic sea-level rise may average 10 mm/yr. The local 

 relative sea level in coastal Louisiana would therefore rise at about twice its present 

 rate over this time period. At this rate local sea level will, in the year 2020, stand some 

 70 to 75 cm higher than now. 



INTRODUCTION 



Sea level, that universal elevation datum, is neither level nor constant. Spatial and 

 temporal fluctuations in sea level occur at all scales and frequencies. 



Global sea-level variations on the geologic time scale of tens of millions of years 

 occur in response to tectonically controlled changes in the volume of the ocean basins 

 (Hays and Pitman 1973). The actual change in location of the shoreline on a continental 

 margin becomes a function of the rate of global (eustatic) sea-level change relative to 

 the rate of margin subsidence, sedimentation (or erosion), and a number of local 

 factors. On passive continental margins (as the U.S. Atlantic and Gulf of Mexico coasts) 

 the rates of tectonically controlled changes in the relative elevation of sea and land are 

 quite slow, typically a few mm per 1,000 years (Pitman 1978, 1979). Furthermore, the 

 slow yet persistent subsidence of a continental margin geosyncline is generally 

 compensated by landward mantle flow and uplift of the coastal plain. Evidence for this 

 is seen in progressively older, uplifted strata in a landward successsion away from the 



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