1. Explanation of Sea Level Trends. 



The worldwide increase in sea level, occuring between 16,000 and 6,000 years ago, is 

 generally attributed to the increase in the volume of water in the oceans as a result of 

 melting of the ice sheet from the last glacial stage. Much of the evidence for this explanation 

 is summarized by Walcott (1975). The concept of worldwide changes in sea level due to 

 melting ice is generally referred to as the glacioeustatic rise. 



The trend toward faUing sea levels, relative to high latitude landmasses, is generally 

 explained as a regional glacioeustatic adjustment of the Earth's crust to the removal of the 

 ice overburden. As the ice sheet accumulated, its increased weight caused a downward 

 deformation of the Earth's crust in the glaciated area and a compensatory rise in peripheral 

 zones. With the removal of the excess weight, tlie glaciated areas gradually rebounded 

 toward their former shape, leading to emergence of these land surfaces relative to the water. 

 Walcott (1972) summarizes much of the evidence of the glacioeustatic adjustment. 



The downwarping was a maximum near the centers of the ice sheets and decreased 

 toward the edges. Because of upwarping beyond the edge of the ice sheet, the rebounding of 

 the Earth's crust in some regions may be associated with subsidence in areas not covered by 

 the ice. The glacioeuostatic adjustment during and after tlie period of glaciation, was 

 therefore nonuniform and this factor may explain much of the nonunif ormity of recent sea 

 level changes. 



Other factors, however, must also be considered. Rather dramatic vertical movements of 

 the Earth's surface may result from earthquakes and volcanic eruptions. These changes are 

 called tectonic adjustments. 



Continued slow increase in sea level during the last few thousand years may be partially 

 attributable to delayed isostatic response to the loading by higher sea levels on continental 

 shelves (Weliman, 1964; Bloom, 1967). Some of this rise in sea level may also be due to 

 retreat of coastal glaciers. According to Donn, Farrand, and Ewing (1962) an additional 

 increase of about 60 meters (200 feet) could stiU occur if the Greenland and Antartica ice 

 sheets were to melt. 



A more subtle and, in certain cases, a more important factor affecting modern sea level 

 relative to the land, is subsidence of the Earth's surface, often as a result of the withdrawal 

 of subsurface water, petroleum, gas, minerals, or the imposition of excessively heavy loads, 

 such as buildings, dams, and occasionally water. Swanson and Thurlow (1973) discuss the 

 evidence of subsidence along the Gulf of Mexico coast as determined from tide 

 measurements. Poland and Davis (1969) present examples of subsidence due to the 

 withdrawal of fluids in many parts of the world, and provide an extensive bibliography of 

 case studies where coastal land fringes are flat. Small changes in relative sea level can 

 inundate broad expanses of low-lying coasts. Even where steeper backshore gradients limit 

 inundation, measurements have indicated that small changes in relative water level can 

 accelerate erosion of coastal dunes, and over a period of years be responsible for a major 



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