712 



ARKHENIUS 



[chap. 25 



characterized by less variability with time of the carbonate content. The 

 Pliocene-Pleistocene transition is indicated by a drop in the temperature of the 

 bottom water, shown by the oxygen isotope distribution in benthonic Fora- 

 minifera (Emiliani and Edwards, 1953), and by a marked change in the fossil 



CaC03 



90% 0_ 



Depth 



lOOOi 

 cm 



Core SW 60 

 1°35'N,134°57'W 



30 60% 



-i — \ — \ — I — \ — r 



Core SW 45 

 1°40'N,106°21'W 



Fig. 38. Age relations of strata in Pacific equatorial carbonate facies as shown by two 

 cores, 3600 km apart. The 95,000-year level in core 60 is determined by the prot- 

 actinium/ionium ratio at this level, as compared to the sediment surface. The exten- 

 sion of the time scale beyond this age is based on extrapolation assuming a constant 

 rate of deposition of inorganic components but allowing for the wide variability in 

 accumulation of biotic compounds. Strata interpreted as isochronous on the basis of 

 stratigraphic correlation are marked by numbers in the graph (cf. Fig. 37). 



assemblages of coccolithophorids (Bramlette, unpublished). In this profile, the 

 transition is marked by a dashed line crossing over the carbonate compensation 

 surface. 



From measurements in stratum 4 the full line curve marked "Pleistocene 



