884 RiEDEL [chap. 33 



localities was higher during the middle Tertiary than at present, but this is not 

 paralleled by a higher content of siliceous microfossils in the Tertiary samples. 

 Presence or absence of calcareous microfossils in pelagic sediments depends 

 largely on whether the locality of accumulation is above or below the calcium 

 carbonate compensation depth, but the abundance of siliceous microfossils in 

 such sediments depends principally upon the rate of organic production in the 

 overlying waters (Riedel, 1959). Thus the middle Tertiary data seem to indicate 

 that the calcium carbonate compensation depth was greater at that time than at 

 present, presumably because of higher temperatures of the bottom water 

 (Emiliani and Edwards, 1953), and that the zone of high organic production 

 associated with the equatorial current system, for example, had limits in the 

 middle Tertiary not very different from those at the present day. Long cores 

 of slowly accumulating, non-fossiliferous clay from the north-central Pacific 

 probably penetrate to at least the late Tertiary, and provide evidence that the 

 North Pacific Central Water has maintained its stably stratified, relatively 

 unproductive character throughout the time represented by the deposits. 



Many of the most interesting aspects of paleoceanography which can be 

 elucidated by pelagic micropaleontological studies remain practically un- 

 touched. When samples are available from many widespread localities on each 

 of several isochronous levels through the Tertiary, investigations of the distri- 

 bution of species at each level may be expected to reveal the changes in the 

 water mass structure of the oceans throughout that epoch. If the lithologic 

 characters of the ancient sediments are investigated in relation to the associated 

 microfossils, more comprehensive interpretations of past oceanic conditions 

 can be made, as, for example, the rates of contribution of terrigenous minerals, 

 factors controlling preservation and dissolution of biogenous sediment com- 

 ponents, and so on. Many species of planktonic organisms have appeared and 

 become extinct during Tertiary time, and studies of pelagic microfossils will 

 very probably shed some light on problems of paleobiogeography and speciation 

 in the open oceans, which are of wide interest to marine biologists. Increased 

 knowledge of the evolutionary history of pelagic microfossils will almost 

 certainly result in improvements in the taxonomic systems of these planktonic 

 groups, which in turn will facilitate their further investigation and make 

 possible more meaningful interpretations of fossil assemblages. 



References 



Arrhenius, G., 1952. Sediment cores from the East Pacific. Reps. Sived. Deep-Sea Exped., 



5, 227 pp. 

 Be, A. W. H., 1959. Ecology of Recent planktonic foraminifera: areal distribution in the 



western North Atlantic. Micropaleontology, 5, 77-100. 

 Beckmann, J. P., 1953. Die Foraminiferen der Oceanic Formation (Eocaen-Oligocaen) 



von Barbados, Kl. Antillen. Eclogae. Geol. Helvet., 46, 301-412. 

 Blow, W. H., 1959. Age, correlation and biostratigraphy of the Upper Tocuyo (San 



Lorenzo) and Pozon formations, Eastern Falcon, Venezuela. Bull. Amer. Paleont., 39 



(178), 59-251. 



