Calcareous Dinoflagellate Cysts as Paleo-Environmental Tools
153
IfTOC accumulation rates correspond to less than
10% of the variation in the calcareous cyst association it is considered as insignificant.
Oceanographic Background
The present upper-level current system of the
Equatorial Atlantic is mainly characterized by the
westward flow of the South Equatorial Current
(SEC; Fig. 3). The SEC consists of two branches,
a main southern branch flowing near and somewhat
south of 10° S and a faster flowing northern branch
between 2° and 4° S. These branches are divided
by the South Equatorial Counter Current (SECC;
cf. Richardson and Walsh 1986; Peterson and
Stramma 1991). The northern boundary of the SEC
is formed by the North Equatorial Counter Current
(NECC). The contact between the SEC and
NECC leads to convergence and downwelling of
surface waters and thereby depresses the
thermocline. Within the thermocline, water is transported equatorwards, where it supports the eastward flowing Equatorial Under Current (EUC).
The EUC is a fast flowing undercurrent which is
present in the Equatorial Atlantic, extending between 5°N and 5 0 S parallel to the equator. In the
eastern Atlantic, the contact between the EUC and
SEC forms the equatorial divergence zone, where
cooler water masses are upwelled from a depth
around the thermocline, leading to high nutrient concentrations and an enhanced productivity in the
surface waters. Core GeoB 1105-4 is situated below this divergence zone, whereas Core GeoB 22042 lies in the western Equatorial Atlantic, where the
thermocline is depressed and stratified, and where
oligotrophic conditions prevail in the upper water
layers.
The depth of the thermocline along the Equator varies considerably, geographically and seasonally. On average, it has a greater depth in the west
and shallows to the east. During boreal summer
(June-September) strong southern trade winds
invade the northern hemisphere and the
Intertropical Convergence Zone (ITCZ) is situated
furthest north. In the eastern part of the Equatorial Atlantic (position Core GeoB 1105-4)the SEC's
speed and divergence are at their maximum, resulting in a shallow thermocline, and a well mixed surface water layer with high productivity. The increased velocity of the SEC causes enhanced
transport of warm tropical surface water to the
west. The western Equatorial Atlantic thermocline
deepens, leading to stratified, oligotrophic surface
waters (position Core GeoB 2204-2). During boreal
winter (December-March) southern trade winds
are weak and part of the equatorial surface water,
piled up in the west, flows back to the eastern
Equatorial Atlantic in the form of counter currents
(e.g. Richardson and Reverdin 1987). The SEC
speed and divergence are at their minimum, resulting in less productivity and a relatively stratified
surface layer in the eastern Equatorial Atlantic. The
thermocline is relatively shallow in the west, but
remains deeper than in the eastern part of the
Equatorial Atlantic.
Paleoceanographic reconstructions suggest that
during glacial intervals, an increased SEC speed and
a strengthening of the equatorial upwelling resulted
in shallowing ofthe thermocline and enhanced primary production in the eastern Equatorial Atlantic
(e.g. McIntyre et al. 1989; Billups and Spero 1996;
Schneider et al. 1996; Wefer et al. I 996a). In the
western Equatorial Atlantic relative stable glaciall
interglacial oceano logic conditions have been reconstructed resulting in only small variations in primary production (Verardo and McIntyre 1994;
Rfihlemann 1996).
Results and Discussion
The global distribution patterns of calcareous cysts
are given in Figs. 4 - 7. Although sample covering
is sparse and is somewhat concentrated in neritic
regions of the North Atlantic, some trends can be
observed. It appears that calcareous cysts are
widely abundant in the seas and oceans and occur
in many different environments ranging from the
tropics to the sub-arctic and from inner neritic to
oceanic environments. Highest species diversity
can be observed in neritic environments.
Orthopithonella granifera (Ffitterer) Keupp and
Versteegh (Fig. 6b) and Sphaerodinella tuberosa
(Kamptner) Keupp and Versteegh (Fig. 6d) are
the only species which are exclusively documented from open oceanic environments.
Bicarinellum tricarinelloides Versteegh (Fig. 4a),
153
IfTOC accumulation rates correspond to less than
10% of the variation in the calcareous cyst association it is considered as insignificant.
Oceanographic Background
The present upper-level current system of the
Equatorial Atlantic is mainly characterized by the
westward flow of the South Equatorial Current
(SEC; Fig. 3). The SEC consists of two branches,
a main southern branch flowing near and somewhat
south of 10° S and a faster flowing northern branch
between 2° and 4° S. These branches are divided
by the South Equatorial Counter Current (SECC;
cf. Richardson and Walsh 1986; Peterson and
Stramma 1991). The northern boundary of the SEC
is formed by the North Equatorial Counter Current
(NECC). The contact between the SEC and
NECC leads to convergence and downwelling of
surface waters and thereby depresses the
thermocline. Within the thermocline, water is transported equatorwards, where it supports the eastward flowing Equatorial Under Current (EUC).
The EUC is a fast flowing undercurrent which is
present in the Equatorial Atlantic, extending between 5°N and 5 0 S parallel to the equator. In the
eastern Atlantic, the contact between the EUC and
SEC forms the equatorial divergence zone, where
cooler water masses are upwelled from a depth
around the thermocline, leading to high nutrient concentrations and an enhanced productivity in the
surface waters. Core GeoB 1105-4 is situated below this divergence zone, whereas Core GeoB 22042 lies in the western Equatorial Atlantic, where the
thermocline is depressed and stratified, and where
oligotrophic conditions prevail in the upper water
layers.
The depth of the thermocline along the Equator varies considerably, geographically and seasonally. On average, it has a greater depth in the west
and shallows to the east. During boreal summer
(June-September) strong southern trade winds
invade the northern hemisphere and the
Intertropical Convergence Zone (ITCZ) is situated
furthest north. In the eastern part of the Equatorial Atlantic (position Core GeoB 1105-4)the SEC's
speed and divergence are at their maximum, resulting in a shallow thermocline, and a well mixed surface water layer with high productivity. The increased velocity of the SEC causes enhanced
transport of warm tropical surface water to the
west. The western Equatorial Atlantic thermocline
deepens, leading to stratified, oligotrophic surface
waters (position Core GeoB 2204-2). During boreal
winter (December-March) southern trade winds
are weak and part of the equatorial surface water,
piled up in the west, flows back to the eastern
Equatorial Atlantic in the form of counter currents
(e.g. Richardson and Reverdin 1987). The SEC
speed and divergence are at their minimum, resulting in less productivity and a relatively stratified
surface layer in the eastern Equatorial Atlantic. The
thermocline is relatively shallow in the west, but
remains deeper than in the eastern part of the
Equatorial Atlantic.
Paleoceanographic reconstructions suggest that
during glacial intervals, an increased SEC speed and
a strengthening of the equatorial upwelling resulted
in shallowing ofthe thermocline and enhanced primary production in the eastern Equatorial Atlantic
(e.g. McIntyre et al. 1989; Billups and Spero 1996;
Schneider et al. 1996; Wefer et al. I 996a). In the
western Equatorial Atlantic relative stable glaciall
interglacial oceano logic conditions have been reconstructed resulting in only small variations in primary production (Verardo and McIntyre 1994;
Rfihlemann 1996).
Results and Discussion
The global distribution patterns of calcareous cysts
are given in Figs. 4 - 7. Although sample covering
is sparse and is somewhat concentrated in neritic
regions of the North Atlantic, some trends can be
observed. It appears that calcareous cysts are
widely abundant in the seas and oceans and occur
in many different environments ranging from the
tropics to the sub-arctic and from inner neritic to
oceanic environments. Highest species diversity
can be observed in neritic environments.
Orthopithonella granifera (Ffitterer) Keupp and
Versteegh (Fig. 6b) and Sphaerodinella tuberosa
(Kamptner) Keupp and Versteegh (Fig. 6d) are
the only species which are exclusively documented from open oceanic environments.
Bicarinellum tricarinelloides Versteegh (Fig. 4a),