surements of the isotope ratios in the shells gives an indication of 

 the temperature at which the forams lived. In relating isotope ratios 

 to temperature, however, it is essential to know what the isotopic 

 composition of the seawater itself was, and this is complicated by 

 the so-called ice-volume effect. Its basis is that when water is va- 

 porized from the surface of the ocean, the lighter of the two isotopes 

 is preferentially taken up into the water vapor, leaving the sea en- 

 riched in the heavier one. When the water vapor is deposited on a 

 continent as snow, for instance, this land-based ice will remain en- 

 riched in lighter isotopes at the expense of the surrounding ocean. 

 This large-scale build-up of terrestrial ice caps will result in an en- 

 richment of seawater with oxygen 18. Clearly, the two signals are 

 closely related, but there has been a great deal of discussion as to 

 which would provide the more reliable temperature record. Only 

 very recently has it become possible to separate the global ice-vol- 

 ume effect from the pure temperature effect in benthic foraminiferal 

 tests by using the magnesium/calcium ratio in the calcite of these 

 tests (Lear et al., 2000). The new curve showing the decline in deep 

 sea temperatures during the last 50 million years is surprisingly sim- 

 ilar to that published by Shackleton and Kennett in 1975 indicating 

 an overall temperature drop during this period of about 12° Celsius. 

 Of particular interest to this discussion is the conclusion that an ice 

 cap first developed in east Antarctica about 34 million years ago. 



Based on other evidence, it seems likely that the cooling trend 

 may be attributed largely to continental drift following the breakup 

 of the supercontinent Gondwanaland, which initially embraced 

 South America, Africa, Antarctica, and Australia. When Antarctica 

 took up its south polar position, other continents drifted away from 

 it to the north, and an open seaway was created around its periphery. 

 This started the Circum-Antarctic Current driven by the rotation of 

 the earth and the resultant westerly winds. Once this happened, the 

 thermal isolation and refrigeration of Antarctica began. 



As far as African habitats go, the global cooling trend seems to 

 have crossed a critical threshold about 6.5 million years ago with 

 the "Terminal Miocene Event." At about this time a widespread 

 sea-level drop has been recorded and this, in addition to tectonic 

 movements in the Gibraltar area, resulted in the isolation of the 



