Oxygen Isotope Values of Planktic Foraminifera
167
chamber-oI80c.lcit. of Globigerina bulloides increases by 0.8 %0 between the smallest chambers
and the final chamber, when temperature and
0180 w • t ., are kept constant.
The foraminiferal flux to the sea-floor is seasonal in most regions, inasmuch as the oxygen isotope composition of the shell reflects the 0 18 0 wate,
and sea-water temperature of the season in which
the individual has prospered (Deuser et al. 1981;
Deuser 1987; Sautter and ThunellI991). Investigations of foraminiferal population dynamics in high
latitudes indicate, that the foraminifera did have their
maximum occurrence in summer (Donner and
Wefer 1994; Kohfeld 1998).
Using vertically-stratified tows, planktic
foraminifera were categorized in species dwelling
at different water depths (Be 1977; Oberhansli et
al. 1992; Kemle v. Miicke and Oberhansli this volume). The depth habitat of many species varies
depending on the location, regional factors, and also
in the course of the reproductive life cycle
(Fairbanks et al. 1982; Bijma et al. 1990; Erez et
aI.1991). While smaller, juvenile foraminifera were
primarily found in the surface water, the larger, adult
stages descend to greater depths of the upper water column. Additionally, encrustation (down to 800
m water depth) can effect the oxygen isotope composition of foraminiferal shells (Duplessy et al.
1981; Lohmann 1995; Kohfeld 1998).
The fact that some foraminiferal species live and
calcify in deeper waters than others and that therefore the difference of oxygen isotope values between the species could serve as an effective monitor of vertical temperature gradients in the upper
water-column is the aim of this paper. Previous
paleoceanographic studies ofthe vertical distribution of planktic foraminifera and surface water
stratification during the Pleistocene inferred from
0180calcite have been undertaken in the Indian Ocean
(Williams and Hea1y-Williams 1980), the Equatorial
Pacific Ocean (Whitman and Berger 1992; Farrell
et al. 1994), and the South Atlantic Ocean (Mulitza
et al. 1997). Ravelo et al. (1990) used a principal
components analysis (EOF) of both foraminiferal
assemblages and hydrographic conditions, as well
as an ocean model to estimate changes in vertical
water-column hydrography in the tropical Atlantic
Ocean. These aspects of paleoclimatological research are important for the reconstruction of the
past surface water stratification, a parameter that
is crucial for the evaluation of surface water mass
stability. Here we present oxygen isotope data from
planktic foraminiferal shells that have been precipitated under different stratification conditions in the
southern South Atlantic and the adjacent Southern
Ocean. Subsequently, the temperatures derived
from oxygen isotope values of different species
were related to temperature atlas data (Olbers et
al. 1992; Levitus and Boyer 1994), in order to estimate the mean calcification depth of the species.
Following this procedure, species were identified
that should be used for reconstructing the stratification pattern of surface waters, particularly in high
southern latitudes.
General Oceanographic Background
The Southern Ocean represents the most important
junction in the global ocean circulation system. The
Antarctic Circumpolar Current (ACC) controls the
exchange of nutrients, salt and heat in the World
Ocean by connecting the Atlantic, the Indian and
the Pacific Ocean, and thus plays an important role
in the development of Earth's climate (Broecker
1982a, 1982b; Keir 1988; Whitworth III 1988;
Olbers 1989; Peterson and Stramma 1991). The
eastward flowing ACC is composed of zonal jets,
confined by three oceanographic frontal systems,
the Antarctic Polar Front, the Subantarctic Front
and the Subtropical Front (Fig. 2). Each of these
three fronts constitute the border of distinct regions
of temperature, salinity and nutrient distribution
(Lutjeharms and Valentine 1984). These regions,
from south to north, are the following: Antarctic
Zone, Polar Front Zone, Subantarctic Zone. North
of the Subtropical Front, which marks the northern
boundary of the ACC, the anticyclonic Subtropical
Gyre (SG) is the main circulation system (Peterson
and Stramma 1991).
Due to the outstanding role of the ACC and its
interaction with other oceanographic systems, such
as the anticyclonic Subtropical Gyre in the South
Atlantic Ocean, it is crucial to establish firm tools
for reconstructing past hydrographic parameters
within this regime. This is the reason why reconstructions of the surface water stratification, the
167
chamber-oI80c.lcit. of Globigerina bulloides increases by 0.8 %0 between the smallest chambers
and the final chamber, when temperature and
0180 w • t ., are kept constant.
The foraminiferal flux to the sea-floor is seasonal in most regions, inasmuch as the oxygen isotope composition of the shell reflects the 0 18 0 wate,
and sea-water temperature of the season in which
the individual has prospered (Deuser et al. 1981;
Deuser 1987; Sautter and ThunellI991). Investigations of foraminiferal population dynamics in high
latitudes indicate, that the foraminifera did have their
maximum occurrence in summer (Donner and
Wefer 1994; Kohfeld 1998).
Using vertically-stratified tows, planktic
foraminifera were categorized in species dwelling
at different water depths (Be 1977; Oberhansli et
al. 1992; Kemle v. Miicke and Oberhansli this volume). The depth habitat of many species varies
depending on the location, regional factors, and also
in the course of the reproductive life cycle
(Fairbanks et al. 1982; Bijma et al. 1990; Erez et
aI.1991). While smaller, juvenile foraminifera were
primarily found in the surface water, the larger, adult
stages descend to greater depths of the upper water column. Additionally, encrustation (down to 800
m water depth) can effect the oxygen isotope composition of foraminiferal shells (Duplessy et al.
1981; Lohmann 1995; Kohfeld 1998).
The fact that some foraminiferal species live and
calcify in deeper waters than others and that therefore the difference of oxygen isotope values between the species could serve as an effective monitor of vertical temperature gradients in the upper
water-column is the aim of this paper. Previous
paleoceanographic studies ofthe vertical distribution of planktic foraminifera and surface water
stratification during the Pleistocene inferred from
0180calcite have been undertaken in the Indian Ocean
(Williams and Hea1y-Williams 1980), the Equatorial
Pacific Ocean (Whitman and Berger 1992; Farrell
et al. 1994), and the South Atlantic Ocean (Mulitza
et al. 1997). Ravelo et al. (1990) used a principal
components analysis (EOF) of both foraminiferal
assemblages and hydrographic conditions, as well
as an ocean model to estimate changes in vertical
water-column hydrography in the tropical Atlantic
Ocean. These aspects of paleoclimatological research are important for the reconstruction of the
past surface water stratification, a parameter that
is crucial for the evaluation of surface water mass
stability. Here we present oxygen isotope data from
planktic foraminiferal shells that have been precipitated under different stratification conditions in the
southern South Atlantic and the adjacent Southern
Ocean. Subsequently, the temperatures derived
from oxygen isotope values of different species
were related to temperature atlas data (Olbers et
al. 1992; Levitus and Boyer 1994), in order to estimate the mean calcification depth of the species.
Following this procedure, species were identified
that should be used for reconstructing the stratification pattern of surface waters, particularly in high
southern latitudes.
General Oceanographic Background
The Southern Ocean represents the most important
junction in the global ocean circulation system. The
Antarctic Circumpolar Current (ACC) controls the
exchange of nutrients, salt and heat in the World
Ocean by connecting the Atlantic, the Indian and
the Pacific Ocean, and thus plays an important role
in the development of Earth's climate (Broecker
1982a, 1982b; Keir 1988; Whitworth III 1988;
Olbers 1989; Peterson and Stramma 1991). The
eastward flowing ACC is composed of zonal jets,
confined by three oceanographic frontal systems,
the Antarctic Polar Front, the Subantarctic Front
and the Subtropical Front (Fig. 2). Each of these
three fronts constitute the border of distinct regions
of temperature, salinity and nutrient distribution
(Lutjeharms and Valentine 1984). These regions,
from south to north, are the following: Antarctic
Zone, Polar Front Zone, Subantarctic Zone. North
of the Subtropical Front, which marks the northern
boundary of the ACC, the anticyclonic Subtropical
Gyre (SG) is the main circulation system (Peterson
and Stramma 1991).
Due to the outstanding role of the ACC and its
interaction with other oceanographic systems, such
as the anticyclonic Subtropical Gyre in the South
Atlantic Ocean, it is crucial to establish firm tools
for reconstructing past hydrographic parameters
within this regime. This is the reason why reconstructions of the surface water stratification, the