CYCLICAL BEHAVIOR OF CLIMATE 



sand years, a time during which ice- 

 sheets repeatedly formed and ad- 

 vanced to cover North America as far 

 south as a front running from Seattle 

 to New York, and Europe as far south 

 as a front running from London to 

 east of Moscow. A substantial ice- 

 sheet also repeatedly covered Pata- 

 gonia, and mountain glaciers formed 

 wherever high mountains and moun- 

 tain ranges were available. The re- 

 peated advances of the ice-sheets were 

 separated by interglacial times during 

 which all continental ice-sheets dis- 

 appeared except those of Greenland 

 and Antarctica. We are presently in 

 the middle of one of these interglacial 

 times. 



The advances and retreats of con- 

 tinental ice have left the glaciated 

 lands littered with glacial debris, 

 ranging in size from fine sand and 

 clays to boulders as large as a house. 

 From the study of these sediments, 

 geologists have concluded that the 

 ice-sheets formed and swept across 

 the northern continents at least five 

 times during the recent past. The 

 sediments are so mangled, however, 

 that it is difficult to reconstruct a 

 complete history of the glacial events. 



"Globigerina Ooze" — For a more 

 complete record one must turn to 

 the deep sea. About 40 percent of 

 the deep ocean floor is covered with 

 a sediment known as "Globigerina. 

 ooze." This sediment is rich with the 

 empty shells of planktonic Foramini- 

 fera, microscopic protozoans freely 

 floating near the surface when alive. 

 Of the fifteen common species of 

 planktonic Foraminifera, several are 

 restricted to equatorial and tropical 

 waters, several to temperate waters, 

 and one to polar waters. When cli- 

 mate changes, the foraminiferal spe- 

 cies move north or south, and these 

 movements are recorded in the sedi- 

 ment on the ocean floor by alter- 

 nating layers of empty shells belong- 

 ing to warm, temperate, and cold 

 species. Sediment core samples up to 

 20 meters long have been recovered. 

 Paleontological analysis of the chang- 



ing foraminiferal faunas through these 

 cores reveals the climatic changes that 

 occurred while the sediment was being 

 deposited. In addition, it is known 

 that foraminiferal shells formed dur- 

 ing cold intervals contain a greater 

 amount of the rare oxygen isotope 1S O 

 than shells formed during warm in- 

 tervals. Thus, oxygen isotopic analy- 

 sis of the foraminiferal shells yields 

 accurate information on the actual 

 temperature of the ocean surface and 

 its variations through time. The re- 

 sults given by micropaleontological 

 and isotopic analysis are essentially 

 identical. 



Because Globigerina ooze accumu- 

 lates at the rate of a few centimeters 

 per thousand years, a deep-sea core 

 20 meters long reaches sediments half 

 a million years old. Deep-sea cores 

 can be dated by various radioactive 

 methods, including radiocarbon and 

 the ratio of thorium-230 to protac- 

 tinium-231. Thus, climatic changes 

 can not only be followed in continuity 

 by studying deep-sea sediments but 

 can also be dated. 



The study of many deep-sea cores 

 from the Atlantic and the Caribbean 

 has made it possible to reconstruct a 

 continuous curve showing the tem- 

 perature changes of the surface ocean 

 water at low latitudes over the past 

 425,000 years. This curve, shown in 

 Figure III-2, exhibits a number of 



alternating high- and low-temperature 

 intervals, with a gross periodicity of 

 about 40,000 years. A comparison of 

 this curve with the chronology of con- 

 tinental glaciation, based largely on 

 radiocarbon dating, shows that the 

 most recent low-temperature interval 

 (70,000 to 15,000 years ago) repre- 

 sents the last major glaciation. One 

 may safely infer that earlier low- 

 temperature intervals of the oceanic 

 curve represent earlier continental 

 glaciations. 



Sediments older than the oldest 

 ones represented in Figure III-2 have 

 been recovered recently from the 

 ocean floor by the drilling vessel 

 Clomar Challenger; analysis of these 

 sediments, yet to be performed, 

 should show how far back in the 

 past temperature variations as large 

 as those of Figure III-2 continue. 

 For the time being, sections of older 

 marine sediments now occurring on 

 land have been used. One of these 

 sections, representing sediment de- 

 posited about 1.8 million years ago in 

 southern Italy, shows that climatic 

 variations as large as the most recent 

 ones were already occurring at that 

 time. 



The General Temperature Curve — 

 The apparent periodicity of 40,000 

 years is intriguing. No terrestrial 

 phenomenon of the type described 

 before is believed to take place with 



Figure 111-2 — CHANGES IN THE TEMPERATURE OF THE OCEAN SURFACE 



Changes in the ocean surface temperature over the past 425,000 years have been 

 reconstructed from deep-sea cores. Present time is at the left of the graph. The 

 numbers above the time axis are for reference, indicating the peak of the long-term 

 cycles. 



53 



