deposition can offset this tendency (Curray 1964). Regression, the seaward 

 migration of the shoreline, typically occurs with falling relative sea level, 

 unless high rates of erosion offset this tendency (Payton 1977). 



Eustatic changes are primarily the result of changes in the volume of water 

 in the oceans or changes in the dimensions of ocean basins. Although there 

 are many reasons for variations in ocean volume, the growth, or waxing, and 

 decay, or waning, of large continental and alpine glaciers have been the 

 primary causes of fluctuations in the past 2 million years (the Pleistocene and 

 Holocene Epochs) (Emery and Aubrey 1991). During glacial episodes, 

 enormous amounts of water were locked up in the massive continental and 

 widespread alpine glaciers, causing a substantial reduction in the volume of 

 oceanic water and a reduction of sea level to more than 100 m below the 

 present level. During interglacial episodes, the amount of meltwater that 

 returned to the ocean caused a rise in sea level that at times exceeded the 

 present elevation by more than 80 m. 



The last major glaciation, known as the late Wisconsin age in North 

 America, reduced sea level to over 100 m below the present. The waning of 

 that glaciation precipitated a rapid rise of sea level commencing about 

 18,000 years ago, reaching near the present level about 4,000 years ago, 

 when sea level became more or less stable. Because large glaciers still exist 

 in Antarctica, Greenland, and some mountainous areas, the present sea level is 

 below that of some prior interglacial periods when a more extensive melting 

 of glacial ice occurred. 



The fluctuations of sea level during the Pleistocene and Holocene Epochs 

 have caused the shoreline to migrate back and forth, alternately exposing and 

 submerging large parts of the present continental shelves and areas inland of 

 the present shoreline. These transgressive and regressive events have left 

 depositional features of terrestrial origin in marine environments, and features 

 of marine origin in terrestrial environments. 



Studies of historical tide records in recent years indicate that a rise in sea 

 level is taking place in many locations (Hicks 1978; 1983; Gornitz, Lebedeff, 

 and Hansen 1982). This rising trend, which some scientists believe may 

 accelerate in the future, has been attributed by some to the general warming of 

 the planet accompanied by glacial melting, possibly caused by increased 

 concentrations of carbon dioxide (CO2), methane (CHJ, and other gases in the 

 earth's atmosphere which trap longwave-electromagnetic radiation. Projection 

 of future trends based on past sea level history is difficult because of the 

 relatively short time that widespread accurate data have been systematically 

 collected, because of problems in correcting for vertical earth movements, and 

 because of difficulties in projecting trends. 



Local and regional changes in relative sea level also occur where coastal 

 landmasses are uplifted or depressed as a result of tectonic activity, sediment 

 compaction, or unloading and loading by glacial ice or water. These move- 

 ments may increase or decrease the effects of eustatic sea level fluctuations, 



Chapter 2 Relevant Processes and Factors 



21 



