+ 4 "^ o- 



+ 3 » Worm 



-^ ^ ^ ^ f55 ioO 50 b Depth below sea floor (m) 



4n..y. 2.4 my. 2 m.y. 1 m.y. 



Approximale Age 



Pliocene Plelilocene 



Figure 2. Oxygen isotopic record for the Pliocene and Pleistocene in Deep 

 Sea Drilling Project Site no. 397 in the Atlantic Ocean off northwest 

 Africa (modified from Shackleton and Cita 1979). Highe^^O values indicate 

 periods of global cooling and formation of continental ice sheets. 



RECENT GLOBAL SEA LEVEL CHANGES 



Sea-level fluctuations on the time scales discussed have controlled the time- 

 stratigraphic evolution of continental margin sedimentary sequences. They also provide 

 some hints of what factors should be considered in trying to explain present short-term 

 sea level fluctuations (lO's of years) and they may guide our modelling efforts in 

 attempts to predict the future, in the following discussion of present and near-future 

 sea-level changes, a clear distinction has been made between eustatic factors (i.e., 

 factors which affect the global sea level) and local factors (which include subsidence and 

 local oceanographic effects). 



Recent data increasingly support the view that sea level did not rise in a smooth 

 and continuous fashion during the Holocene transgression. The rise appears to have been 

 characterized by a series of oscillations with an amplitude of a few meters on a typical 

 time scale of lOO's of years. Data supporting this view are mostly archaeological (Figure 

 3A, Brooks et al. 1979), yet historical data in Europe suggest that there has been a one- 

 meter sea-level fluctuation within the last millenium (Figure 3B, Rhode 1978). The most 

 recent sea-level minimum coincides with the peak of the "little ice age" at the end of 

 medieval time. 



Over the last century an increasing number of tide gauges have been installed in 

 harbors around the world. Records from such gauges yield information about the local 

 relative change in level of the sea and the land upon which the gauge is placed. All such 

 records demonstrate large fluctuations in mean annual sea level; generally, however, 

 these fluctuations are superimposed on a secular trend. The annual fluctuations derive 

 from long-term meteorological tides (atmospheric pressure variations), continental run- 

 off and winds (Fairbridge and Krebs 1962). The longer term (decades) trend is more 

 controversial yet of paramount significance to human efforts at developing the coastal 

 zone. 



Attempts to derive the rate of global sea-level rise from such tide gauge records 

 have generally been based on various trend analysis techniques applied to the "average" 

 of records from a number of stations. Records from stations known to be subject to 

 rapid sinking (Galveston, Texas; Louisiana coast) or rising due to glacioisostatic rebound 

 (Scandinavia, parts of Canada) or underthrusting of an oceanic plate (Oregon, 



166 



