Oxygen Isotope Values of Planktic Foraminifera
185
Summary and Conclusions
Based on the 8 18 0 e ,lei'. composition of planktic
foraminiferal shells, the preferred mean calcification depths of the different size fractions belonging to 24 foraminiferal taxa in the South Atlantic
Ocean and the adjacent Southern Ocean are presented. The vertical separation of species is more
distinct in the warm and temperate Suptropical Gyre
than in the colder waters of the Antarctic
Circumpolar Current. The data suggest that the
investigated species can be roughly divided into four
groups, calcifying in water depths between 0 and
50 m, 0 and 200 m, 100 and 250 m, below 250 m.
Only for G/obigerinita g/utinata, could a narrow
defined calcification depth not be given, as their
8 18 0 e ,leite values are in equilibrium with calculated
8180 values between 0 and 300 m water depth.
The wa;~sults suggest that for quantitative
paleotemperature estimations (using the Transfer
Function or the Modem Analogue Technique), species displaying a mean calcification depth below
250 m should not be applied to reconstruct seasurface conditions (Niebler and Gersonde 1998).
In the temperate southern South Atlantic Ocean
as well as in the Antarctic Circumpolar Current,
8 18 0 e ,lei,. values of the shallow-calcifying
G. bulloides and the deep-calcifying G. truncatulinoides as well as the deep- / intermediate-calcifying G. inflata are most applicable for the reconstruction of past surface water stratification.
We have demonstrated that 8180eweite differences
between shallow and deep-calcifying taxa are suitable to calculate the vertical temperature gradient
in the upper 250 m of the water column. The estimated gradients have a standard deviation of
±1.3° C.
To improve such studies in the future it is necessary to learn more about physiological and ecological effects which control the signal in
foraminiferal she1l8 18 0 e ,lei,. of the different species. A better knowledge of ecology, life cycle, and
shell calcification processes of the individual
foraminiferal species, especially in the cold water
realm of the oceans, is essential for a correct interpretation of the fossil record. Plankton-tow investigations in these areas are necessary to expand
our knowledge of the main flux seasons for the different species. Furthermore, exploring the exact
nature of the "oxygen isotope disequilibrium" range
for each species in transitional and polar regions is
very important, because it contributes in studies on
the surface water stratification directly to the essential conclusion.
Within the examined region, future work will
have to focus on the integration of samples between
the Walvis Ridge and the Cape of Good Hope to
continue the transect northwards. In the next step,
it seems to be necessary to investigate samples
from the western Atlantic sector of the ACC as
well as from the southwestern Subtropical Gyre.
The 8180eweite composition in the shell of the three
selected species should be measured. Indeed, subtropical and tropical species become more relevant
in the central Subtropical Gyre (Mulitza et al. 1997),
consequently more frequent appearing subtropical
species should also be measured in these samples.
Nevertheless, it was demonstrated that no combination of single species (one species for the seasurface, one species for the 250 m layer) can be
used for the whole temperature range in the South
Atlantic Ocean. Besides the well established
micropaleontological and isotope-geochemical
methods, the evaluation of surface water mass stability is highly important for paleoceanographic interpretations.
Acknowledgments
Reviews by C. Charles and K. Kohfeld significantly
improved the manuscript. We would like to thank
1. Bijma, G. Fischer, S. Mulitza and G. Wefer for
comments on an earlier version of the manuscript
and A. Mackensen and N. Scheele for helpful discussions. We acknowledge the assistance of
R. V. POLARSTERN's crew of six cruises and
are grateful to U. Bock, R. Frohlking, N. Lensch
and G. Meyer for technical assistance.
This research was funded by the Deutsche
F orschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution
No. 201). This is publication No. 1560 of the
Alfred Wegener Institute for Polar and Marine Research. Data available under www.pangaea.de/
Projects/SFB261.
185
Summary and Conclusions
Based on the 8 18 0 e ,lei'. composition of planktic
foraminiferal shells, the preferred mean calcification depths of the different size fractions belonging to 24 foraminiferal taxa in the South Atlantic
Ocean and the adjacent Southern Ocean are presented. The vertical separation of species is more
distinct in the warm and temperate Suptropical Gyre
than in the colder waters of the Antarctic
Circumpolar Current. The data suggest that the
investigated species can be roughly divided into four
groups, calcifying in water depths between 0 and
50 m, 0 and 200 m, 100 and 250 m, below 250 m.
Only for G/obigerinita g/utinata, could a narrow
defined calcification depth not be given, as their
8 18 0 e ,leite values are in equilibrium with calculated
8180 values between 0 and 300 m water depth.
The wa;~sults suggest that for quantitative
paleotemperature estimations (using the Transfer
Function or the Modem Analogue Technique), species displaying a mean calcification depth below
250 m should not be applied to reconstruct seasurface conditions (Niebler and Gersonde 1998).
In the temperate southern South Atlantic Ocean
as well as in the Antarctic Circumpolar Current,
8 18 0 e ,lei,. values of the shallow-calcifying
G. bulloides and the deep-calcifying G. truncatulinoides as well as the deep- / intermediate-calcifying G. inflata are most applicable for the reconstruction of past surface water stratification.
We have demonstrated that 8180eweite differences
between shallow and deep-calcifying taxa are suitable to calculate the vertical temperature gradient
in the upper 250 m of the water column. The estimated gradients have a standard deviation of
±1.3° C.
To improve such studies in the future it is necessary to learn more about physiological and ecological effects which control the signal in
foraminiferal she1l8 18 0 e ,lei,. of the different species. A better knowledge of ecology, life cycle, and
shell calcification processes of the individual
foraminiferal species, especially in the cold water
realm of the oceans, is essential for a correct interpretation of the fossil record. Plankton-tow investigations in these areas are necessary to expand
our knowledge of the main flux seasons for the different species. Furthermore, exploring the exact
nature of the "oxygen isotope disequilibrium" range
for each species in transitional and polar regions is
very important, because it contributes in studies on
the surface water stratification directly to the essential conclusion.
Within the examined region, future work will
have to focus on the integration of samples between
the Walvis Ridge and the Cape of Good Hope to
continue the transect northwards. In the next step,
it seems to be necessary to investigate samples
from the western Atlantic sector of the ACC as
well as from the southwestern Subtropical Gyre.
The 8180eweite composition in the shell of the three
selected species should be measured. Indeed, subtropical and tropical species become more relevant
in the central Subtropical Gyre (Mulitza et al. 1997),
consequently more frequent appearing subtropical
species should also be measured in these samples.
Nevertheless, it was demonstrated that no combination of single species (one species for the seasurface, one species for the 250 m layer) can be
used for the whole temperature range in the South
Atlantic Ocean. Besides the well established
micropaleontological and isotope-geochemical
methods, the evaluation of surface water mass stability is highly important for paleoceanographic interpretations.
Acknowledgments
Reviews by C. Charles and K. Kohfeld significantly
improved the manuscript. We would like to thank
1. Bijma, G. Fischer, S. Mulitza and G. Wefer for
comments on an earlier version of the manuscript
and A. Mackensen and N. Scheele for helpful discussions. We acknowledge the assistance of
R. V. POLARSTERN's crew of six cruises and
are grateful to U. Bock, R. Frohlking, N. Lensch
and G. Meyer for technical assistance.
This research was funded by the Deutsche
F orschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution
No. 201). This is publication No. 1560 of the
Alfred Wegener Institute for Polar and Marine Research. Data available under www.pangaea.de/
Projects/SFB261.