SECT. 3J CROSS-CORRELATION OF DEEP-SEA SEDIMENT CORES 833 



vals involved most of the faunal zones cannot be defined by extinctions or by 

 first appearances of new species. Instead, the stratigrapher must learn to deal 

 with a series of oscillations between dominance by low latitude species and 

 dominance by high latitude species. The distinction between zones of low and 

 high latitude species in a single core is usually clear enough, but to estabhsh the 

 time equivalence of various zones in two or more cores from stations separated 

 by many kilometers is quite a different matter. For this purpose the mere 

 matching of species is hardly good enough. The need is rather for some variable 

 or variables of the foraminiferal assemblages which can be measured with a fair 

 degree of precision and which can be expressed numerically. 



The foraminiferal number of Schott (1937), defined by the number of tests 

 of planktonic Foraminifera in a gram of dry sediment, can be measured with 

 sufficient precision, but curves of variation of the foraminiferal number are not 

 very satisfactory in cross-correlation. Unfortunately, the foraminiferal number 

 is often strongly, sometimes overwhelmingly, influenced by the supply of ter- 

 rigenous and fine calcareous sediment, which may vary enormously from place 

 to place. The percentage of coarse fraction ( > 74 jx) determined by wet-sieving 

 the samples taken for foraminiferal analyses is essentially the same as the 

 foraminiferal number since in most deep-sea foraminiferal lutites the coarse 

 fraction is composed almost exclusively of the tests of planktonic Foraminifera. 

 It has been used a good deal at Lamont as an invaluable aid in distinguishing 

 different processes and environments of deposition, but it has not been found 

 to be particularly reliable in making cross-correlations. 



The frequency-to-weight ratio (Ericson and Wollin, 1956) is obtained by 

 counting the tests of a particular species in a weighted sample of coarse fraction. 

 It is therefore, in effect, a measure of the abundance of the particular species with 

 respect to the total number of tests of planktonic Foraminifera in the sample. 

 In this way the disturbing influence of variation in supply of fine terrigenous 

 material is eliminated. It has the further advantage that no time is wasted in 

 counting all of the eurythermal species which are important only as a back- 

 ground against which the good climatic indicators can be contrasted. As an index 

 of climatic change in individual cores the frequency-to- weight ratio is valuable, 

 and it is sometimes useful in cross-correlation. Its weakness lies in the well- 

 known fact that the species compositions of the thanatocoenoses of deep-sea 

 sediments are strongly influenced by differential solution. For this reason curves 

 of variation in the frequency-to-weight ratios of various species do not provide 

 a very satisfactory way of making firm correlations between cores from stations 

 differing considerably in depth and, therefore, subject to markedly different 

 degrees of solution of the tests. 



The most satisfactory method of establishing correlatable faunal zones 

 depends upon variations in the coiling directions of certain species of planktonic 

 Foraminifera. As yet no physical or chemical difference between the dextral and 

 sinistral tests has been found. It is, therefore, very probable that the two kinds 

 of a given species are equally subject to solution, and consequently coiling ratios 

 should be independent of solution. The species which have proven to be most 



