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tJNITED STATES NATIONAL MUSEUM BXILLETIN 215 



counter-currents and numerous bodies of water which 

 have been detached from the main water-mass. Cer- 

 tain sequences of cold- and warm-water planktonic 

 Foraminifera collected from this region may be sus- 

 pected of reflecting such water movements." 



For this reason, it would be necessary to show a similar 

 sequence of fluctuation over an area sufficiently broad 

 as to avoid control by minor current changes, in order 

 to correlate these faunal changes with world-wide 

 climatic changes. 



The actual cause of these faunal fluctuations is still 

 undetermined. As stated by Ovey (1950, p. 214), it is 

 certam that there are oscillations in the equatorial 

 Atlantic, and that "short-term fluctuations of tempera- 

 ture are unlikely to be traceable in deep-sea cores be- 

 cause sedimentation is slow and there is also the prob- 

 ability that the lag between temperature and faunal 

 change is considerable." 



Studies of ocean temperatures durmg the Tertiary by 

 Emiliani and Edwards (1953, p. 889) by means of 

 oxygen isotopes, showed "that greater mixing of the 

 oceanic waters occurred in non-glacial times ..." and 

 "adds weight to the point repeatedly stressed by geol- 

 ogists that the climate of the earth was much more 

 uniform in non-glacial times." 



This would suggest that perhaps planktonic Fo- 

 raminifera would be even more cosmopolitan in Cre- 

 taceous and Tertiary times than in the Recent seas, and 

 as a result would be of even greater time value, where 

 temperature control would be minimized. 



Depth and ecologic stratification: Only a few 

 studies of the distribution of living planktonic Fo- 

 raminifera have been made on the basis of plankton 

 tows. Early work established that there are approxi- 

 mately 20 or 30 living planktonic species, based on their 

 presence in plankton tows. The largest populations 

 are in the upper layers of water. Schott (1935) ob- 

 tained several hundred specimens per tow from the 

 upper 100 meters, and considerably less from greater 

 depths. Phleger (1951) found an average population 

 of 5 to 6 per cubic meter of water in the upper 50 meters 

 in the northwestern Gulf of Mexico. However, some 

 stations showed up to 73 living specimens per cubic 

 meter. 



Living specimens of planktonic species also were 

 found in sediment samples, and were either bottom- 

 dwelling or living in the 15 to 20 centimeters of water 

 directly above the bottom. According to Phleger 

 (1954, p. 3), "These data certainly suggest that whUe 

 planktonic Foraminifera appear to be most abundant 

 in the upper water layers they do live throughout the 

 water column all the way to the bottom." Many 

 plankton tows also contain empty tests of Foraminifera 

 which did not sink to the bottom immediately upon 

 death or reproduction of the animal. 



Phleger summarized his findings by stating (1954, p. 

 3): "The fauna in a sediment may represent environ- 

 mental conditions which existed throughout the entire 

 water colimm from the surface to the bottom. There 



may be several populations living in different depth 

 environments, or the same population may be variously 

 affected by environments at various depths. ... 



'Tlanktonic Forarmnifera do not sink immediately, 

 depending upon water turbulence conditions, and may 

 be deposited at some distance from where they actually 

 hved. The distance of such transport cannot be estab- 

 lished at the present time and must be variable." 



Studies of pelagic Foraminifera on the basis of 

 oxygen isotope ratios by Emiliani (1954, p. 149) 

 showed that different species from the same sample 

 registered different temperatures for their development. 

 They were, therefore, considered to occupy different 

 habitats with respect to temperature and water density 

 and therefore also with respect to depth. "The same 

 species may vary considerably in its depth habitat in 

 order to adjust itself to the proper temperature and 

 water density." Correlating the temperatures at 

 which these species lived with the variation in tempera- 

 ture with depth showed a well-defined stratification. 

 He stated (p. 152) that, "The species Olobigerinoides 

 conglobata, rubra and sacculifera appear to occupy the 

 shallower habitats, followed by Globigerina dubm, 

 PuLleniatina oUiquilocvlata and Globorotalia menardii, 

 while Globorotalia tumida and truncattdinoides occupy 

 the deeper habitats. . . . The stratification with re- 

 spect to temperature is, therefore, reproduced also with 

 respect to depth; however, as already well known, the 

 different species appear to be much less dependent upon 

 pressure than upon temperature." Further studies 

 showed that species appear to be adapted to waters of 

 the same densities in the different areas, even if this 

 involves considerable differences in pressure. None of 

 the pelagic specimens examined by Emiliani was found 

 to live at a depth greater than about 220 meters. 



Studies of specimens of different sizes of various 

 species by Emiliani showed that the majority main- 

 tained the same depth habitat during at least most of 

 their lives. The sole exception was Orbidina universa 

 which showed the larger specimens to Hve at progres- 

 sively shallower depths. This species was therefore 

 considered to change its depth habitat during its 

 development. 



The depths at which the planktonic Foraminifera 

 live and the modifications making this depth selection 

 possible were discussed more fully, above, in the section 

 on special characteristics of the planktonic Forami- 

 nifera. However, in determining climates, etc., on the 

 basis of planktonic assemblages, the effect of this 

 stratification of habitats should not be overlooked, as 

 colder water forms may well inhabit deeper layers of 

 the pelagial, whereas the surface layer may contain 

 species typical of warmer latitudes, and thus cause an 

 apparent mixing of faunas. 



Light: The primary effect of fight on the planktonic 

 assemblage would be that on the phytoplankton, to 

 which light is necessary for development. It would 

 have a secondary effect on the Foraminifera, as a result 

 of its effect on their source of food. There is also a 



