mR a ete ee ee alate tt Rete Me Kd ete Be ss lon wa ‘ teria be met. They must have relatively limited temperature ranges and a good paleontologic record. This is made difficult by the differential preservation of coccolith structural types. Some of the best temperature indicators are found amongthe delicate caneoliths where fos- 60° sil recording is minimal. Eight species, Coccolithus pelagicus, Cyciococcolithus leptoperus, Heli- cosphaera carter, Rhabdosphaera stylifera, Syracosphaera pulchra, Umbellosphaera irreguiaris, Umbellosphae- ra tenuis, and Umbilicosphaera mirabilis, were selected on the basis of preservation and temperature range and their present distribution in the North Atlantic, compared with that of the midWisconsin (last glacial period}. Counts and identification of species were done 40° 20°- He with the electron microscope. Synchronous samples from a finite time in the Wisconsin glaciation can be obtained by C™ dates or, where these are inadequate, by use of sedimentation rates established by radioactive dating. The rates for each core, expressed in centimeters per 1000 years, can be multiplied by a unit time in years (24 = 24,000 years of the glacial period) that would represent the mid- Umbilicosphaera 10° Umbellosphaera “R= Recent mirabilis —— —- tenuis -— —-- G = Glacial B0° —— 70° 60° 50° 40° 30° 20° [0° Fig. 2. Species population boundaries for Recent and mid-Wisconsin time and percent values for Cyclococcolithus leptoporus in diamonds with the upper recent and the lower glacial values. Wisconsin. This gives a depth in centi- meters for each core from which synchronous samples are taken. While this technique is regularly used in geological oceanography,it is open to criticism SURFACE WATER ISOTHERMS, MID- WISCONSIN Since one must often assume that the sedimentation rate established for the upper portion of the glacial section in each core remains constant for the entire section. It is necessary to bear the limitations in mind since there is as yet no other means of obtaining the samples needed. The midglacial position of these samples was verified by planktonic foraminiferal stratigraphy. The data (Table 2) from eight spe- cies, when plotted on distributional maps (Figs. 1 and 2), indicate a marked change in biogeographic distribution between recent and glacial sediments. The ranges of each species are delineated in Figs. 1 and 2 by two lines marking the boundaries of the recent and glacial populations. These lines are drawn on the extreme northern or southern ap- Ppearances for warm and cold species, respectively. Coccolithus pelagicus, the only spe- cies limited to cold water presently rec- Ognized in the North Atlantic (4), is _Testricted to subarctic and transitional Waters. Its maximum southern limit coincides with the 14°C isotherm. At present this species extends to 35°N 8 DECEMBER 1967 act 70° 60° 50° ar 30 Atlantic erected with the use of North mid-Wisconsin the Fig. 3. Paleoisotherm map of coccolithophorid temperature data, The dark arrows indicate presumed position of the subtropical gyral based on coccolith boundaries of subtropical forms. The jagged line : oo represents pack ice [after Flint]. 1315