Eustatic sea level change is caused by change in the relative volumes of 

 the world's ocean basins and the total amount of ocean water. It can be 

 measured by recording the movement in sea-surface elevation relative to some 

 universally adopted reference frame. This is an exceptionally difficult 

 problem because it is essential that eustatic measurements be obtained from 

 the use of a reference frame that is sensitive only to ocean water and ocean 

 basin volumes. Sahagian and Holland (1991) have recently used the 

 extensive, undeformed Russian platform to generate a Mesozoic-Cenozoic 

 eustatic sea level curve. 



Changes in water level include: 



a. Slow absolute secular sea and land level changes (time spans of thou- 

 sands or millions of years). These have been caused by glacioeustatic, 

 tectonic, climatologic, and oceanographic factors (to be discussed in 

 more detail in Chapter 5). Sea level was about 100 to 130 m lower 

 during the last glacial epoch (Figure 6), about 15,000 years before 

 present. Ancient shorelines and deltas can be found at such depths 

 along the edge of the continental shelf. Other changes of this magni- 

 tude have been recorded during other geological epochs (Payton 

 1977). 



b. Short-term sea level changes caused by seasonal oceanographic 

 factors. These may be due to movements of ocean currents, runoff, 

 melting ice, and regional atmospheric variations. Figures 7 and 8 plot 

 monthly mean water levels from Juneau and Galveston, showing how 

 sea level, averaged over decades, is higher during certain months. The 

 1985 Juneau curve (Figure 7), however, shows that during any one 

 year, the average trend may not be followed. 



c. Land level changes. These may be slow, occurring over centuries, 

 (for example, the compaction and dewatering of sediments in deltas) 

 or may be abrupt, the result of volcanic activity or earthquakes. A 

 notable example of rapid change was caused by the Great Alaskan 

 Earthquake of 1964, when shoreline elevations ranged from 10 m 

 uplift to 2 m downdrop (Hicks 1972; Hicks, Debaugh, and Hickman 

 1983). Vertical crustal displacements may be reflected in sea level 

 curves from localized areas. Figure 9 shows how the mean sea level 

 at Juneau is falling because of isostatic rebound of the land. In 

 Galveston (Figure 10), a rapid rise is recorded because the land is 

 subsiding (causing the tide gage to subside, too). 



Variations in sea level, both long-term (geologic scale) and historic, do not 

 have a direct effect on most shorelines in the same manner that waves or 

 storm surges do. But storms have more devastating effects on a shore over 

 time if relative sea level in the area is rising. Data on water levels can be 

 important in predicting erosion or accretion and changes in shoreline response 

 (Wells and Coleman 1981; Hands 1980). 



Chapter 2 Secondary Sources of Coastal Information 



15 



