54° i 



—I Jb 



be dredged up and studied by the oceanographer, it becomes a 

 book to the past, providing a record of twenty-million-year-old 

 volcanic eruptions, or spasms of prolific animal life. But recently 

 an even more subtle way has been devised to trace back into geolog- 

 ical history and reveal past surface temperatures of the oceans. 

 When water evaporates, those water molecules that contain the 

 lighter isotope of oxygen are lost more rapidly to the atmosphere 

 than are the heavier water molecules. During warm conditions the 

 surface water contains an extra abundance of heavy oxygen isotopes, 

 and this is reflected in the composition of animals which grow and 

 live near the surface. When the animals die and sink to the sea floor 

 their skeletons have locked up inside them the oxygen isotope ratio 

 from which the surface temperature can be calculated. So besides 

 the qualitative evidence of cycles of activity in geological history, 

 the sediments of the ocean floor contain a history of the environ- 

 mental conditions. 



The oceanographer must take great care when he collects sam- 

 ples of these ocean sediments. Deposits from turbidity currents can 

 mislead him ; also, there are burrowing animals that live in the top 

 few feet of the sea floor. These creatures have been brought up in 

 core samples, and they have been seen by oceanographers who 

 have penetrated the deeps in bathyscaphes. The difficulty of select- 

 ing an undisturbed sediment hunting ground is one with which 

 geologists are familiar in their work on land. 



Seismic measurements in the oceans show that more than a 

 thousand feet of clayUke material covers the deep, flat parts of the 

 oceans. Compared with the rate of sedimentation in shallow water 

 - 40,000 feet have accumulated in the Persian Gulf in the last 

 hundred million years alone - this is only a thin veneer of material. 

 Until we can study cores brought up from the bottom part of the 



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