22
The Historical Background
cial, surface water temperature probably varied between 21°-30° C, similar to
present-day values in the area. In the two successive periods temperatures decreased until a minimum of 13°-14° C was reached for a short period between
23,000-13,000 BP (Berggren, 1969). About 11-12,000 BP, an abrupt change in
climate occurred, which led to "the general type of climate that persisted to the
present time" (Deuser and Degens, 1969).
The low temperature periods coincided with periods of lowered sea level or
tectonic uplifts in the Southern Red Sea and during these periods the water
exchange with the Indian Ocean diminished or even occasionally stopped. The
response to that was increased salinity (measured by higher 0 18 content in the
water) which, because of the lack of watersheds draining into the Red Sea, could
not be diluted by the eventual increase in the amount of rains during the glacials.
From the data of Butzer (1966) it appears that the glacial periods of the high
latitudes were not necessarily accompanied by increased precipitations in the
subtropical zone. The case of the present-day Persian Gulf, amply supplied with
the fresh water of three big rivers at its far end-the Euphrates, Tigris, and
Karun-but with open-sea salinity of around 40%0, shows that high evaporation
and isolation from the open ocean are of overwhelming importance.
There are no indications as to the amount of salinity increase. This has to be
extrapolated from the alternation of stenohaline and euryhaline planktonic organisms. It seems likely that euryhalinity and eurythermy were the important
survival factors during the changing oceanographic conditions ofthe Upper Pleistocene. There are few indications that a true cold-water assemblage ever appeared. Only for the coccolithophorids is there an indication (McIntyre, 1969) that
during the height of the Pleniglacial (20-10,000 BP) cool-water species like U mbilicosphaera mirabilis and Cyclolithella annulus appeared. As for the salinity it
reached such high values in late Würm that even the most euryhaline foraminiferid Globigerinoides ruber disappeared and the sediments were for a short period
devoid of foraminiferida. However, the euryhaline pteropod Cresseis acicula survived even under these extreme conditions (Berggren and Boersma, 1969) indicating that conditions remained marine and did not deteriorate to those of a saline
brine water (Fig. 8).
Deuser and Degens (1969) emphasize the fact that during the brief disappearance of the foraminiferids from the hot brine area, they did survive in the area of
core 154 P, about 100 miles south of the hot brine hole. They assume that the
increase in the salinity "was probably stronger the greater the distance from the
Gulf of Aden". But since the periods of extreme environmental deviation favored
the eurytopic neritic species, the distance from the shoreline also had a marked
influence on the biological populations.
It is interesting to remark (see Berggren, 1969) that the uplift of the coral reefs
on the Island ofMuseri, dated at about 17,000 BP by Horowitz (1967), might be
corroborated by the core data to show that tectonic movement was also responsible for the extreme isolation of the Red Sea about 20-15,000 years ago.
The present situation was established about 12-11,000 years ago and since
then the fauna has not shown show major oscillations (Fig.9).
One should mention briefly that the relatively high percentage of endemic
species of the Red Sea-siblings of ancestral Indian Ocean forms~an be ex-
The Historical Background
cial, surface water temperature probably varied between 21°-30° C, similar to
present-day values in the area. In the two successive periods temperatures decreased until a minimum of 13°-14° C was reached for a short period between
23,000-13,000 BP (Berggren, 1969). About 11-12,000 BP, an abrupt change in
climate occurred, which led to "the general type of climate that persisted to the
present time" (Deuser and Degens, 1969).
The low temperature periods coincided with periods of lowered sea level or
tectonic uplifts in the Southern Red Sea and during these periods the water
exchange with the Indian Ocean diminished or even occasionally stopped. The
response to that was increased salinity (measured by higher 0 18 content in the
water) which, because of the lack of watersheds draining into the Red Sea, could
not be diluted by the eventual increase in the amount of rains during the glacials.
From the data of Butzer (1966) it appears that the glacial periods of the high
latitudes were not necessarily accompanied by increased precipitations in the
subtropical zone. The case of the present-day Persian Gulf, amply supplied with
the fresh water of three big rivers at its far end-the Euphrates, Tigris, and
Karun-but with open-sea salinity of around 40%0, shows that high evaporation
and isolation from the open ocean are of overwhelming importance.
There are no indications as to the amount of salinity increase. This has to be
extrapolated from the alternation of stenohaline and euryhaline planktonic organisms. It seems likely that euryhalinity and eurythermy were the important
survival factors during the changing oceanographic conditions ofthe Upper Pleistocene. There are few indications that a true cold-water assemblage ever appeared. Only for the coccolithophorids is there an indication (McIntyre, 1969) that
during the height of the Pleniglacial (20-10,000 BP) cool-water species like U mbilicosphaera mirabilis and Cyclolithella annulus appeared. As for the salinity it
reached such high values in late Würm that even the most euryhaline foraminiferid Globigerinoides ruber disappeared and the sediments were for a short period
devoid of foraminiferida. However, the euryhaline pteropod Cresseis acicula survived even under these extreme conditions (Berggren and Boersma, 1969) indicating that conditions remained marine and did not deteriorate to those of a saline
brine water (Fig. 8).
Deuser and Degens (1969) emphasize the fact that during the brief disappearance of the foraminiferids from the hot brine area, they did survive in the area of
core 154 P, about 100 miles south of the hot brine hole. They assume that the
increase in the salinity "was probably stronger the greater the distance from the
Gulf of Aden". But since the periods of extreme environmental deviation favored
the eurytopic neritic species, the distance from the shoreline also had a marked
influence on the biological populations.
It is interesting to remark (see Berggren, 1969) that the uplift of the coral reefs
on the Island ofMuseri, dated at about 17,000 BP by Horowitz (1967), might be
corroborated by the core data to show that tectonic movement was also responsible for the extreme isolation of the Red Sea about 20-15,000 years ago.
The present situation was established about 12-11,000 years ago and since
then the fauna has not shown show major oscillations (Fig.9).
One should mention briefly that the relatively high percentage of endemic
species of the Red Sea-siblings of ancestral Indian Ocean forms~an be ex-