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