Paleoceanography 



133 



term variations. In southern California an 

 annual cycle of about 21 cm was observed 

 in 1937-1938 with low sea level in the spring 

 and high in the late summer and fall (Lafond, 

 1939a). The same cycle characterizes later 

 years also (Stewart, Zetler, and Taylor, 1958), 

 and it has been found to be typical of most 

 other areas of the ocean where adequate 

 data are available (Pattullo, Munk, Revelle, 

 and Strong, 1955). The chief cause of vari- 

 ation appears to be that of density of sea 

 water, a function mostly of temperature but 

 partly also of salinity, which in turn is con- 

 trolled by relative amounts of rainfall and 

 evaporation. Winds exert only a minor and 

 irregular effect on sea level in this region. It 

 is possible, of course, that parallel variations 

 of sea level, temperature, salinity, and baro- 

 metric pressure may all be results of a more 

 general cause not yet known. 



A smooth curve drawn through a plot of 

 yearly average sea level for California tide 

 gauge stations (Fig. 120) appears to show a 

 general rise relative to land since good records 

 were begun about 1900, although the year-to- 

 year variations are nearly as great as the long- 

 term rise. For San Diego the rise averages 

 about 23 cm per century, similar to but a 

 little greater than rates observed at San Fran- 

 cisco and Seattle (Marmer, 1949). A rise of 

 about 12 cm per century was obtained by 

 Gutenberg (1941 fl) and Kuenen (1950, p. 534) 

 based on tide gauges of the world. This rate 

 of rise is about the same as might be expected 

 from the return of water to the ocean by 

 melting glaciers of the world during recent 

 years, and it suggests that the relative rise of 



sea level at San Diego is probably real. In 

 contrast, the relative rise at stations on the 

 United States coasts of the Atlantic and 

 Gulf of Mexico is three times as great as at 

 San Diego, suggesting that land subsidence 

 and compaction in these areas may have 

 been twice as great as the rise of sea level. A 

 much lower rate found in Scandanavia indi- 

 cates continuing emergence of the land by re- 

 bound after melting away of its former ice cap. 

 It has been suggested that the rate of melt- 

 ing of glaciers and the corresponding rise of 

 sea level have been accelerated by a small in- 

 crease of temperature of the atmosphere 

 resulting from the greenhouse effect of car- 

 bon dioxide liberated by the burning of fossil 

 fuels since the beginning of the industrial 

 revolution (Plass, 1956; Revelle and Suess, 

 1 957). If the rise of sea level continues and 

 increases in rate, a rather damp future awaits 

 some of southern California's resort cities 

 which are built upon low sand spits and 

 beaches. Another result of rise of sea level 

 was pointed out by Munk and Revelle (1952), 

 who showed that a 10-cm (0.3 foot) rise can 

 lead to a 0.6-millisecond lengthening of the 

 day and a 3-meter shift of the pole of rotation 

 of the earth. 



Paleoceanography 



The chief remaining evidence of the nature 

 of ancient seas is that preserved in their sedi- 

 ments. Distribution and composition of 

 these sediments provide a more or less com- 

 plete record of the outline of the seas, or the 



Figure 1 20. Yearly average sea 

 level at San Diego (1915-1924) 

 and La Jolla (1925-1957). 

 Fluctuations are to some de- 

 gree related to annual water 

 surface temperature (La Jolla) 

 which, in turn, is inversely 

 proportional to water density. 

 The long-term rise of sea level 

 is ascribed to the return of 

 water to the ocean by melting 

 ice caps. Data from UGGI 

 and USC&GS (Anonymous, 

 1940, 1950, 1956a). 



