f'HAP. 33] THE PRESERVED RECORD: PALEONTOLOGY OF PELAGIC SEDIMENTS 877 



the northeast Pacific area (Fig. 10, I-V). The southern hmit of the assemblages 

 characterized by the cold-water diatom Denticula marina Semina corresponds 

 rather well to the southern limit of subarctic water, and the fact that the 

 other boundaries between the different thanatocoenoses remain distinct argues 

 against much lateral transport of the sinking frustules by subsurface currents. 

 Lohman (1941) estimated that isolated diatom frustules would require several 

 hundred years to settle through 3000-4000 m of ocean water, and currents are 

 known to exist in intermediate and deep waters. It is, therefore, at first sight 

 remarkable that patterns of diatom assemblages are discernible in Recent 

 pelagic sediments. There appears to be httle doubt that a high proportion of at 

 least the smaller microfossils settle through the water column as aggregates, 

 and therefore sink more rapidly than would isolated skeletons. Large numbers 



20' 



_ Sth. limit of common Denliculo merino 



— Nth. limit of common CoscinocJisc 



160° 



140° 



120° 



Fig. 10. Diatom assemblages (I-V) tentatively recognized by T. Kanaya in Recent sedi- 

 ments of the NE Pacific, x marks positions of samples forming the basis of assemblage 

 definitions. 



of diatoms and silicoflagellates, for example, are observed in the stomachs of 

 planktonic crustaceans and tunicates, and faecal pellets and decaying intestines 

 would provide a source of rapidly setthng aggregates. 



Havins estabhshed that the microfossils of Recent sediments reflect condi- 

 tions in the overlying waters, it is then justifiable to attempt to deduce past 

 oceanic conditions on the basis of changes in microfossil assemblages of sediment 

 cores. Many workers investigating Foraminifera in deep-sea sediment cores 

 have been able to trace successions of alternating assemblages representing 

 warmer and colder conditions in Quaternary sequences: examples are those 

 described by Schott (1935, 1952), Cushman and Henbest (1940), Phleger et al. 

 (1953) and Ericson and Wolhn (1956) in the Atlantic, by Todd (1958) and Parker 



