Coccolithophores as Indicators of Ocean Water Masses
119
these studies were performed in the NorwegianGreenland Sea, the North Atlantic, and in the eastern Pacific Ocean, whereas only very scarce information on coccolithophores from sediment traps
in the South Atlantic exists (Cepek and Wefer in
press).
Recent coccoliths in sediments of the South
Atlantic are also only scarcely studied, and most of
these studies were limited to the shelf/upper slope
region around southern Africa (Fincham and Winter 1989; Giraudeau 1992; Giraudeau and Bailey
1994). This is surprising since both the equatorial
Atlantic and the eastern South Atlantic have been
the focus of numerous paleoceanographic studies.
Many of the previous reconstructions of Late
Quaternary variations of South Atlantic surface circulation using sea-surface temperature proxies have
concentrated on these regions (Gardner and Hays
1976; Molfino et al. 1982; Mix et al. 1986; L
McIntyre et al. 1989; Schneider et al. 1996). In
addition, the potential of coccolithophores for paleoenvironmental studies in the area off southwest
Africa has already been shown by Giraudeau
(1992) and Winter and Martin (1990).
The present study deals with coccolithophores
and coccoliths from surface-waters, surface
sediments, and sediment cores recovered from both
areas, the equatorial Atlantic and the eastern
South Atlantic off Namibia. Selected examples
are presented to document the significance of
coccolithophores as indicators of oceanic surfacewaters, sea-surface temperatures, and paleoproductivity. The composition and concentration of
coccolithophores in the plankton are determined
along transects crossing the main water masses
and the data are compared with the distribution of
coccoliths ofthe surface sediments. Knowledge of
their living occurrences as well as their distribution
in surface sediments is still a prerequisite for paleoecological and paleoceanographi-cal studies of
coccoliths in Quaternary sediments. These data are
used to demonstrate the varying significance of
coccoliths for distinct environments. The presence
of distinct coccolithophore assemblages associated
with equatorial upwelling and the subtopical oligotrophic gyres suggest control by nutrients/trophic
level rather than temperature alone. Thus, coccolith
assemblages in a sediment core of the equatorial
Atlantic will be presented as an example for the
significance of coccolith studies in paleoproductivity
estimates, whereas variations in sea-surface temperatures derived from coccoliths and alkenones
will be presented for the eastern South Atlantic.
Hydrography
In general, surface-waters of the South Atlantic
exhibit a complex system of currents. The oceanic
upper-layer circulation of the South Atlantic has
been summarized by Peterson and Stramma (1991)
and only a brief summary will be given here for the
equatorial and eastern South Atlantic (see Fig.1).
The surface current system in the South
Atlantic is dominated by a subtropical anticyclonic
gyre, and is closely coupled to lowered atmospheric
wind stress. In the eastern South Atlantic off Southwest Africa, the surface-water circulation is dominated by the northward-directed Benguela Coastal
Current (BCC), the coastal tongue ofthe Benguela
Current (BC), and the warmer southward-flowing
Angola Current (AC) (Fig. 2a). BCC and AC converge between 14° and 16°S building a marked
front (Angola-Benguela Front) which is well defmed
in terms of both temperature and salinity. A horizontal gradient of 4°C per 1 ° oflatitude is typically
observed on the shelf(Shannon and Nelson 1996).
In addition, the prevailing winds in this region in tum
drive an offshore surface drift and cause a coastal
upwelling of cold, nutrient-rich water especially
during austral winter. Upwelling occurs in a number
of cells south of about 18°S with a major, semipermanent cell at 27°S (Fig. 2a; Shannon and
Nelson 1996). This upwelling leads to an enhanced
biological productivity off Namibia. The typical
westward extent of the upwelling is between 150
and 250 km off the coast.
Further offshore the northwestward-flowing
Benguela Oceanic Current (BOC), the oceanic
portion of the Benguela Current, is characteristic
for the upper-layer waters. This flow feeds into a
broad, northwestward-flowing South Equatorial
Current (SEC), forming the eastern limb of the
subtropical gyre (Fig. 1). The SEC consists of two
branches, a mainstream flowing south of 1 ooS, and
a smaller, tradewind-forced, faster flowing branch
between 2° and4°S (Peterson and Stramma 1991).
119
these studies were performed in the NorwegianGreenland Sea, the North Atlantic, and in the eastern Pacific Ocean, whereas only very scarce information on coccolithophores from sediment traps
in the South Atlantic exists (Cepek and Wefer in
press).
Recent coccoliths in sediments of the South
Atlantic are also only scarcely studied, and most of
these studies were limited to the shelf/upper slope
region around southern Africa (Fincham and Winter 1989; Giraudeau 1992; Giraudeau and Bailey
1994). This is surprising since both the equatorial
Atlantic and the eastern South Atlantic have been
the focus of numerous paleoceanographic studies.
Many of the previous reconstructions of Late
Quaternary variations of South Atlantic surface circulation using sea-surface temperature proxies have
concentrated on these regions (Gardner and Hays
1976; Molfino et al. 1982; Mix et al. 1986; L
McIntyre et al. 1989; Schneider et al. 1996). In
addition, the potential of coccolithophores for paleoenvironmental studies in the area off southwest
Africa has already been shown by Giraudeau
(1992) and Winter and Martin (1990).
The present study deals with coccolithophores
and coccoliths from surface-waters, surface
sediments, and sediment cores recovered from both
areas, the equatorial Atlantic and the eastern
South Atlantic off Namibia. Selected examples
are presented to document the significance of
coccolithophores as indicators of oceanic surfacewaters, sea-surface temperatures, and paleoproductivity. The composition and concentration of
coccolithophores in the plankton are determined
along transects crossing the main water masses
and the data are compared with the distribution of
coccoliths ofthe surface sediments. Knowledge of
their living occurrences as well as their distribution
in surface sediments is still a prerequisite for paleoecological and paleoceanographi-cal studies of
coccoliths in Quaternary sediments. These data are
used to demonstrate the varying significance of
coccoliths for distinct environments. The presence
of distinct coccolithophore assemblages associated
with equatorial upwelling and the subtopical oligotrophic gyres suggest control by nutrients/trophic
level rather than temperature alone. Thus, coccolith
assemblages in a sediment core of the equatorial
Atlantic will be presented as an example for the
significance of coccolith studies in paleoproductivity
estimates, whereas variations in sea-surface temperatures derived from coccoliths and alkenones
will be presented for the eastern South Atlantic.
Hydrography
In general, surface-waters of the South Atlantic
exhibit a complex system of currents. The oceanic
upper-layer circulation of the South Atlantic has
been summarized by Peterson and Stramma (1991)
and only a brief summary will be given here for the
equatorial and eastern South Atlantic (see Fig.1).
The surface current system in the South
Atlantic is dominated by a subtropical anticyclonic
gyre, and is closely coupled to lowered atmospheric
wind stress. In the eastern South Atlantic off Southwest Africa, the surface-water circulation is dominated by the northward-directed Benguela Coastal
Current (BCC), the coastal tongue ofthe Benguela
Current (BC), and the warmer southward-flowing
Angola Current (AC) (Fig. 2a). BCC and AC converge between 14° and 16°S building a marked
front (Angola-Benguela Front) which is well defmed
in terms of both temperature and salinity. A horizontal gradient of 4°C per 1 ° oflatitude is typically
observed on the shelf(Shannon and Nelson 1996).
In addition, the prevailing winds in this region in tum
drive an offshore surface drift and cause a coastal
upwelling of cold, nutrient-rich water especially
during austral winter. Upwelling occurs in a number
of cells south of about 18°S with a major, semipermanent cell at 27°S (Fig. 2a; Shannon and
Nelson 1996). This upwelling leads to an enhanced
biological productivity off Namibia. The typical
westward extent of the upwelling is between 150
and 250 km off the coast.
Further offshore the northwestward-flowing
Benguela Oceanic Current (BOC), the oceanic
portion of the Benguela Current, is characteristic
for the upper-layer waters. This flow feeds into a
broad, northwestward-flowing South Equatorial
Current (SEC), forming the eastern limb of the
subtropical gyre (Fig. 1). The SEC consists of two
branches, a mainstream flowing south of 1 ooS, and
a smaller, tradewind-forced, faster flowing branch
between 2° and4°S (Peterson and Stramma 1991).