CHAP. 34] THE PLEISTOCENE RECORD 919 



are made questionable by Opik's conclusion, based on elaborate, quantitative 

 analysis (Opik, 1953), that the reduction of solar emission required to produce 

 the observed ice sheets of the glacial ages would have reduced the equatorial 

 temperatures to 8°C. Theories based on variations of CO2 content in the atmos- 

 phere (Plass, 1956; cf. also Flint, 1957, p. 504) are made improbable by the 

 rapid exchange of CO 2 between atmosphere and ocean, including the deep sea 

 (Craig, 1957; Revelle and Suess, 1957). A recent theory claiming that the 

 Arctic Ocean was open during glacial ages and that it provided the moisture 

 for the northern ice caps (Ewing and Donn, 1956, 1958) has been questioned on 

 theoretical grounds as incapable of maintaining sustained oscillations (Living- 

 stone, 1959) and is apparently contradicted by the pattern of glaciation in 

 Alaska and Siberia (cf. Flint, 1957) and by the stratigraphy of Arctic deep-sea 

 cores (Ericson and Wollin, 1959). 



According to a model of glaciation developed by Emiliani and Geiss (1959) 

 and largely based on ideas of earlier authors (esp. Milankovitch, 1938; Koppen 

 and Wegener, 1924; Flint, 1947), glaciation was made possible by Tertiary 

 cooling caused by albedo increase associated with the Alpine orogenesis; the 

 successive Pleistocene glaciations were started by quasi -periodic reductions of 

 summer insolation in northern latitudes (the so-called Milankovitch mechanism, 

 see Zeuner, 1945, 1952, 1959) known to have occurred at intervals of about 

 40,000 years; the further development of glaciation was largely a self-sustaining 

 process; and complete deglaciations outside of Antarctica and Greenland 

 resulted from surface freezing of the northern North Atlantic and time-delay 

 effects introduced by plastic flow of glaciers, heat absorption by ice melting and 

 crustal warping. 



Most of the very numerous theories of glaciation which have been proposed to 

 date have been disproved or made questionable by subsequent discoveries. Of 

 the very few which survive, none exceed the status of theory. The cause of 

 glaciation and the mechanism regulating the repeated advances and retreats of 

 glaciers during the Pleistocene EjDOch undoubtedly will become clear in the 

 future, but only through an intensive study of both continental and deep-sea 

 deposits directed to establish standard sections, and only with time calibration 

 by absolute dating methods. 



4. Summary 



In the foregoing discussion we have summarized what we believe are some of 

 the basic facts in the record of the Pleistocene, gathered from continental and 

 sea-floor sources. The facts concern three chief parameters: (1) stratigraphic 

 sequence, (2) paleotemperatures, and (3) chronology. The sequence displayed 

 beneath both lands and oceans clearly shows both major and minor fluctuations 

 of temperature. The fluctuation is secular, but its amplitude varies with geo- 

 graphic location. Repeated changes of temperature characterize the Pleistocene 

 but have not been discerned in the Tertiary, which is marked by slow, general 

 decrease in temperature. 



