Oxygen Isotope Values of Planktic Foraminifera
169
surface water temperatures (Hale and Ptlaumann
this volume), the surface water salinities (Wolff et
al. this volume) and the surface water productivity
(Ruhlemann et al. this volume) are the key for understanding past paleoceanographic and climatic
evolution.
Material and Methods
Fifty-nine sediment surface samples were recovered during six expeditions of the research vessel
POLARSTERN (Futterer 1988; Gersonde and
Hempel 1990; Bathmann et al. 1992, 1994;
Gersonde 1993,1995; Lemke 1994)(Table 1, Fig.
2). The samples were collected using a multicorer
(MUC) (Barnett et al. 1984), a minicorer (MIC)
(Kuhn and Dunker 1994), or a large box corer
(GKG) (Weaver and Schultheiss 1990). Especially
the MUC and MIC provide high-quality sample sets
with an undisturbed sediment surface. The samples were collected between 34°S to 56°S in the
South Atlantic Ocean and thus cover the area between the Subtropical Gyre (south-eastern part)
and the sea-ice free Antarctic Zone (Fig. 2). Additionally, one station atthe Walvis Ridge (20 0 S)
was sampled. The results of quantitative faunal
analysis ofthe planktic foraminiferal assemblages
in the 59 surface sediment samples (Niebler and
Gersonde 1998) document that these samples are
classified as belonging to the Holocene age. Reworked or inadequately sampled sediment surfaces
as well as non-modern age samples were excluded
from the study.
After separation of the fraction < 63 /lm, the
dried samples were sieved into five fractions (125
to 200 /lm, 200 to 250 /lm, 250 to 315 /lm, 315 to
400 /lm, > 400 /lm). The largest shells from each
fraction were picked to maximize comparability
between the 59 samples (Curry and Matthews
1981; Weferetal. 1983). From the twenty-four taxa
investigated, only few species covered all size fractions.
Clean shells were selected to be used in the isotopic measurements. Between 5 and 25 shells were
selected for each measurement, depending on the
particular species, size and morphotype. We did not
clean the specimens, as organic matrices or attached organic compounds do not significantly intluence the measurements (Ganssen 1983; Wefer
1985). The measurements were performed with a
Finnigan MAT 251 mass spectrometer equipped
with an automated carbonate preparation device.
The standard deviation ofthe oxygen isotope measurements was < 0.06 %0 (Hubberten and Meyer
1989). All oxygen isotope data are available from
the data server PANGAEA (www.pangaea.de).
At a given 8180wate<' the oxygen isotopic composition offoraminiferal shells increases with decreasing water temperature (Urey 1947; McCrea
1950;Ureyetal. 1951; Epsteinetal. 1953) as well
as with the size of the shells (eg. Vincent and
Berger 1981). Consequently, if8180water is known,
the calcification temperature of different size
fractions can be estimated. McCrea (1950) and
Epstein et al. (1953) developed an empirical
paleotemperature equation to reconstruct sea-water temperatures from oxygen isotope values. This
equation has later been modified to account for the
presence of 170, instrumental inaccuracies and
precisely estimated relationships between temperature and she1l8180ealeit, (Craig 1965; Shackleton
1974; Erez and Luz 1983).
Before calculating temperatures, the oxygen
isotope values must be corrected by the specific
"oxygen isotope disequilibrium" for each species.
The "oxygen isotope disequilibrium" is the difference between calcite formed in thermodynamic
equilibrium with sea-water and the isotopic composition of foraminiferal shell carbonate (cf. "vital
effect", Urey et al. 1951). Many studies have
shown that several factors can cause changes in
the "oxygen isotope disequilibrium": photosynthesis (Fairbanks et al. 1982, Spero and Williams 1988),
calcification and growth rate (McConnaghey 1989)
and carbonate chemistry (Spero et al. 1997).
The "oxygen isotope disequilibrium" depends
also on ontogeny (Spero and Lea 1996). In laboratory tests they proved, that the chamber 8180e.leit,
of Globigerina bulloides increased by 0.8 %0
between the smallest chambers and the final chamber, when temperature and 8 18 0 w • t e< were
kept constant. They pointed out that, for
paleoceanographic applications, a well defined
shell-size range is necessary (270 to 320 11m, as
suggested for G. bulloides). We recommend
smaller shell-sizes (230 to 250 11m) for the
169
surface water temperatures (Hale and Ptlaumann
this volume), the surface water salinities (Wolff et
al. this volume) and the surface water productivity
(Ruhlemann et al. this volume) are the key for understanding past paleoceanographic and climatic
evolution.
Material and Methods
Fifty-nine sediment surface samples were recovered during six expeditions of the research vessel
POLARSTERN (Futterer 1988; Gersonde and
Hempel 1990; Bathmann et al. 1992, 1994;
Gersonde 1993,1995; Lemke 1994)(Table 1, Fig.
2). The samples were collected using a multicorer
(MUC) (Barnett et al. 1984), a minicorer (MIC)
(Kuhn and Dunker 1994), or a large box corer
(GKG) (Weaver and Schultheiss 1990). Especially
the MUC and MIC provide high-quality sample sets
with an undisturbed sediment surface. The samples were collected between 34°S to 56°S in the
South Atlantic Ocean and thus cover the area between the Subtropical Gyre (south-eastern part)
and the sea-ice free Antarctic Zone (Fig. 2). Additionally, one station atthe Walvis Ridge (20 0 S)
was sampled. The results of quantitative faunal
analysis ofthe planktic foraminiferal assemblages
in the 59 surface sediment samples (Niebler and
Gersonde 1998) document that these samples are
classified as belonging to the Holocene age. Reworked or inadequately sampled sediment surfaces
as well as non-modern age samples were excluded
from the study.
After separation of the fraction < 63 /lm, the
dried samples were sieved into five fractions (125
to 200 /lm, 200 to 250 /lm, 250 to 315 /lm, 315 to
400 /lm, > 400 /lm). The largest shells from each
fraction were picked to maximize comparability
between the 59 samples (Curry and Matthews
1981; Weferetal. 1983). From the twenty-four taxa
investigated, only few species covered all size fractions.
Clean shells were selected to be used in the isotopic measurements. Between 5 and 25 shells were
selected for each measurement, depending on the
particular species, size and morphotype. We did not
clean the specimens, as organic matrices or attached organic compounds do not significantly intluence the measurements (Ganssen 1983; Wefer
1985). The measurements were performed with a
Finnigan MAT 251 mass spectrometer equipped
with an automated carbonate preparation device.
The standard deviation ofthe oxygen isotope measurements was < 0.06 %0 (Hubberten and Meyer
1989). All oxygen isotope data are available from
the data server PANGAEA (www.pangaea.de).
At a given 8180wate<' the oxygen isotopic composition offoraminiferal shells increases with decreasing water temperature (Urey 1947; McCrea
1950;Ureyetal. 1951; Epsteinetal. 1953) as well
as with the size of the shells (eg. Vincent and
Berger 1981). Consequently, if8180water is known,
the calcification temperature of different size
fractions can be estimated. McCrea (1950) and
Epstein et al. (1953) developed an empirical
paleotemperature equation to reconstruct sea-water temperatures from oxygen isotope values. This
equation has later been modified to account for the
presence of 170, instrumental inaccuracies and
precisely estimated relationships between temperature and she1l8180ealeit, (Craig 1965; Shackleton
1974; Erez and Luz 1983).
Before calculating temperatures, the oxygen
isotope values must be corrected by the specific
"oxygen isotope disequilibrium" for each species.
The "oxygen isotope disequilibrium" is the difference between calcite formed in thermodynamic
equilibrium with sea-water and the isotopic composition of foraminiferal shell carbonate (cf. "vital
effect", Urey et al. 1951). Many studies have
shown that several factors can cause changes in
the "oxygen isotope disequilibrium": photosynthesis (Fairbanks et al. 1982, Spero and Williams 1988),
calcification and growth rate (McConnaghey 1989)
and carbonate chemistry (Spero et al. 1997).
The "oxygen isotope disequilibrium" depends
also on ontogeny (Spero and Lea 1996). In laboratory tests they proved, that the chamber 8180e.leit,
of Globigerina bulloides increased by 0.8 %0
between the smallest chambers and the final chamber, when temperature and 8 18 0 w • t e< were
kept constant. They pointed out that, for
paleoceanographic applications, a well defined
shell-size range is necessary (270 to 320 11m, as
suggested for G. bulloides). We recommend
smaller shell-sizes (230 to 250 11m) for the