CHAP. 34 J 



THE PLEISTOCENE RECORD 



907 



The relative abundances of the species listed above, in cores of Globigerina 

 ooze, afford a simple method by which past temperature variations at the 

 ocean surface can be estimated. This method was first proposed by Philippi 

 (1910) and was later applied by many authors to a large number of deep-sea 

 cores. In the absence of post-depositional solution (which would make quanti- 

 tative taxonomic studies impossible because shells of different species dissolve 

 at different rates), the method is sensitive enough to permit the detection of 

 glacial and interglacial stages not only in such areas as the Mediterranean 

 where the amplitude of the temperature change has been large, but also in 



cm 



100 



200 



300 - 



400 



500 



100 



200 



300 



400 



500 



Fig. 2. Temperature variations in a suite of four equatorial Atlantic cores estimated from 

 changes in the relative abundances of pelagic Foraminifera. W means "warin" and 

 C means "cold". (From Ericson and Wollin, 1956.) 



the equatorial Atlantic where the amplitude has probably not been larger than 

 6° or 7°C (Fig. 2). 



A much more precise method of analyzing the temperature record contained 

 in cores of Globigerina ooze involves mass-spectrometric measurement of the 

 isotopic ratio i^o/isQ in the calcite of the various species of pelagic Foramini- 

 fera. This method, devised by Urey (1947) and developed by Urey, Epstein and 

 co-workers (Urey et al., 1951; Epstein et al., 1951, 1953) is based on the fact 

 that the oxygen isotopes in CaCOs precipitating slowly in a water solution are 

 fractionated between water and the carbonate ions, and the fractionation factor 

 depends upon temperature. As a result, the i^O/i^O ratio changes by 0.00023 

 per degree centigrade within the normal temperature range of biological 



