eevee damellmim 6 nm
latitude in the cold Labrador water
found between the Gulf Stream and
the coast of North America. In the
mid-Wisconsin it ranged as far south
as 13°N latitude along the African
coast (Fig. 1). The average latitude
shift between recent and glacial is approximately 15° latitude and even greater between living material and glacial
(4).
At the opposite extreme in temperature tolerance is Umbellosphaera irregularis (Fig. 1), a tropical species.
The minimum shift between recent and
glacial boundaries for this form is approximately 20° latitude. The accuracy
to which such shifts can be determined
reflects the spacing of core samples. Between the glacial presence (core Nos.
V16-200 and A180-72) and absence
(core No. A180-56) there is a gap of
10° latitude (see Table 1).
The temperature ranges of Helico-
ee me ee
water forms. The distribution of U.
mirabilis is today bounded by the 18°C
isotherm; this may be too high a value
since the colder-stage coccoliths are
present at higher latitudes in sediment
samples from this study than was ob-
with the higher percentage (which today
occurs in this species in transitional
waters) indicative of the optimum range.
served with either living or surface sedi-
consin. This type of subtle biogeograph-
ment material in the survey of modern
forms. It is possible that the colder
stage of U. mirabilis has a distribution
similar to that of R. stylifera and S.
pulchra. Nevertheless, the species shows
a definite shift between recent and
glacial sediments which averages 15°
latitude (Fig. 2). Further evidence indicative of a cooling of the Atlantic in
glacial times is the preponderance of
cold-form coccoliths in glacial-age sediments from the mid-North Atlantic, a
situation reversed in recent surface sediments.
Cyclococcolithus leptoporus (Fig. 2)
is among the most eurythermal of the
1). They are presently
Coccolithophoridae. Today it ranges
from the equator to Arctic waters.
There is no apparent difference in
maximum range between recent and
distribution during mid-Wisconsin time.
to the lack of core material further
north than the line Sp 10-1 through
sphaera carteri, Rhabdosphaera stylifera, and Syracosphaera pulchra are not
as limited as those of the two preceding
species (Fig.
found in subtropical to transitional waters. All show a much more limited
Helicosphaera carteri has an average
distributional difference of 10° latitude
while Rhabdosphaera
stylifera and
Syracosphaera pulchra both have an
glacial, although this is probably due
R 10-2 (see Table 1). There is a marked
change in percentage distribution between recent and glacial populations,
approximate shift of 17° latitude. Note
that in these three species the recent
maximum distributional lines follow the
northern border of the subtropical gyral
(Gulf
Stream)
across
the
Atlantic
Ocean. This agrees with plankton data
from the North Atlantic, where the
boundary between subtropical and subarctic species approximates the northern
border of the Gulf Stream (4).
Umbellosphaera tenuis and Umbili-
cosphaera mirabilis, while having a sat-
isfactory preservation record, do not
show as distinctive a difference in their
recent and glacial distributions as the
preceding species.
Umbellosphaera tenuis is found at
higher latitudes and is limited today
by the 16°C isotherm; it is typical of
subtropical waters. Its glacial to recent
shift is from 10° to 15° latitude (Fig.
2). It is not as abundant in glacial as
in recent sediments. This may be due
to the fragility of its macrococcoliths.
The coccoliths of Umbilicosphaera
mirabilis, like Coccolithus huxleyi, have
temperature-dependent structural variations (4, 6). While these changes are gra-
dational in C. huxleyi, U. mirabilis appears to have separate cold- and warm1316
Table 1. Core locations in the North Atlantic and the depth of the glacial sample in
each core.
Depth
Core
No
,
Location
Latitude
Longitude
of
glacial
sample
(cm)
A153-146
Al56-4
Al156-5
Al64-59
A167-13
A167-14
A179-13
A1T79-17
AT80-9
A180-16
A180-32
A180-48
33°43'N
34°49 'N
37°07'N
38°42'N
31°39'N
31°28'N
23°56'N
28°00'N
39°92TN
38°21N
29°O7'N
15°19’'N
44°45'W
T4°41'W
73°37W
67°52'W
75°21'W
76°28'W
78°45W
73°47 W
45°STW
32°29W
26°15'W
18°06'W
80
844
95
245
300
300
97
280
115
140
59
488
A180-72
R5-36
R5-54
R5-57
R10-2
SP8-4
SP9-3
SP9-4
SP10-1
V16-200
00°36'N
46°55'N
25°52'’N
19°40'N
56°59'N
32°50'N
53°53'N
§0°02'’N
51°23'N
01°58'N
21°47'W
18°35'W
19°03'W
19°06'W
12°238'W
18°32'W
21°06 W
14°46'W
38°04"W
37°04 W
120
162
35
270
100
65
220
200
150
120
A180-56
12°15'N
17°46'W
207
Thus the line representing percentage
change may be a rough indicator of
the subtropical boundary in the Wis-
ic change will require further work
before definite conclusions can be
drawn.
A number of other species showed
some degree of biogeographic change
by a shift in maximum boundaries:
however, they are not plotted because
of relative rarity in the core material.
In the case of the ubiquitous eurythermal species (Coccolithus huxleyi and
Gephyrocapsa oceanica), their distri-
bution is similar to that of Cyclococcolithus leptoporus.
One interesting change, presently not
usable for paleoclimatic work but im-
portant in systematics, is the reversal
in dominance of Coccolithus huxleyi
and Gephyrocapsa oceanica from glacial to recent (Table 2).
In today’s ocean Coccolithus huxleyi usually constitutes over 50 percent
of the flora, but in the mid-Wisconsin
it shared and in some latitudes was
dominated by Gephyrocapsa oceanica.
I consider that Coccolithus huxleyi is
a relatively recent form, none being
found before the Pleistocene, and that
it evolved from the Gephyrocapsa
oceanica complex during the late Pleis-
tocene. Similarities in form and ecology.
combined with the finding of intermedi-
ate forms in Pleistocene core samples
that T am now investigating, support
this theory.
A comparison of the latitudinal
change in flora from northern cold wa-
ters to southern warm waters in recent
and glacial sediments indicates the
dominance of cooler water forms in
lower latitudes during the mid-Wiscon-
sin. Umbilicosphaera mirabilis and Syra-
cosphaera pulchra constitute a much
larger percentage of the flora in glacial
times in this area, while Cyclococcolithus fragilis and Umbellosphaera ir-
regularis, subtropical to tropical forms.
are nearly absent during glacial time
from the North Atlantic.
The biogeographic boundaries of Coc-
colithophoridae species in today’s seas
can be correlated with surface water
isotherms (4). If we assume that the
present temperature ranges of the spe-
cies held for the last glacial period.
then the paleogeographic boundaries of
species can also be assigned temperature values. If all these boundaries are
SCIENCE, VOL. 158