440
Muiitza et al.
Schneider et al. 1994). This has been attributed to
the late seasonal occurrence ofG. bulla ides when
primary production has already decreased the nutrient concentrations and increased the (il3C of
surface waters. Another factor increasing the(il3C
of G. bulla ides in upwelling regions is temperature. Culturing experiments indicated that the(il3C
of G. bulla ides increases by about 0.1 %0 per degree of temperature decrease (Bemis and Spero
1997). Hence, increased upwelling might also increase the (il3C of G. bulla ides by cooling SSTs.
Core GeoB 1023-5 is located in the Benguela
Upwelling Region off Namibia and has been extensively studied by Schneider et al. (1992). In
sharp contrastto the high-latitude core PS I 768-8,
the glacial-interglacial (i13C difference of G.
bullaides is close to zero (Fig. 9b). Taking into
account a negative global(i l3 C shift of 0.3-0.4 %0
from the Holocene to the last glacial, there must
have been a local increase in glacial surface water (il3C relative to the global ocean, or a surface
water cooling of about 3-4 DC. A cooling of about
4°C has been observed for the nearby core GeoB
1710-3 (Kirst et al. 1998). Hence, the negative global (il3C shift during the LGM is nearly compensated by a positive (il3C shift due to a cooling of
ambient surface waters, and no additional effects
are necessary to explain the glacial carbon isotope
values of G. bullaides.
The deglaciation in Core GeoB 1023-5 is
marked by a strong (iI3C-minimum at about 13 ka.
According to Oppo and Fairbanks (1989) and Schneider et al. (1992), the deglacial minimum might
either reflect an increased upwelling of subsurface
waters, or a change in the preformed isotopic composition of source waters. Since there were no indications of higher upwelling during deglaciation,
Schneider et al. (1992) suggested that the minimum
is due to a change in the isotopic composition of
the upwelled source waters. Today, waters
upwelled off Namibia originate from the South
Atlantic Central Water level at 200-400 m depth.
The modem source area of this water type is near
the Subtropical Front (e.g. Gordon 1981). We have
already shown that Southern Ocean surface waters exhibit a strong deglacial minimum with comparable timing (Fig. 9a). Therefore, it seems to be
reasonable that the deglacial minimum is due to a
change in the preformed isotopic composition of the
upwelled waters. Problems with this scenario are
that cores closer to the potential source area (e.g.
Ninneman and Charles 1997) show a much smaller
de glacial minimum than observed in core GeoB
1023-5. However, future work might show that
upwelled central waters originated further south,
or that the deglacial (iI3C depletion was actually
larger at the Subtropical Front.
Oligotrophic Tropical Atlantic. In contrast to
the high latitudes and the upwelling regions, cores
recovered from the tropical Atlantic provide the opportunity to study carbon isotope records from
nutrient-depleted surface waters. Changes in the
carbon isotope composition of tropical surface
waters should be mainly due to processes unrelated
to nutrient cycling. Core GeoB 3104-1 comes from
the western equatorial Atlantic off Brazil, from a
water depth of about 767 m. The isotope records
and 14C-datings were measured on the shallowdwelling species G. sacculifer (Fig. lOa, Arz
et al. 1998). As already shown in Fig. 5,
G. sacculifer exhibits a strong size-trend in (i13e.
By using a narrow size range (350-400 ~m in this
case) the effects of ontogeny should be minimized.
Generally, the variability of tropical (iI3C_records
is lower than in high- and mid-latitudes. Glacial values in core GeoB 3104-1 are about 0.8 %0 lower
than Holocene values. Like all other cores presented in this study, GeoB 3104-1 contains a strong
deglacial minimum dated to about 13 ka. Although,
the magnitude of the observed changes in core
3104-1 is much lower than in the high and mid-latitude cores, the changes are too great to be solely
explained with global changes in carbon inventories. Since tropical surface waters are always depleted in nutrients, changes in the thermodynamic
imprint must be responsible for the glacial and
deglacial depletion in (il3C. As benthic foraminifera
from cores located in the Antarctic Intermediate
Water contain a synchronous deglacial(iI3C-minimum, Oppo and Fairbanks (1989), and later LynchStiegltz et al. (1994), suggested that the de glacial
minimum might be due to changes in the preformed
(iI3C signal of intermediate waters and transferred
to surface waters by upwelling. We have presented
indications for both mechanisms, changes in preformed (iI3C of AAIW source waters (PS 1768-8)
Muiitza et al.
Schneider et al. 1994). This has been attributed to
the late seasonal occurrence ofG. bulla ides when
primary production has already decreased the nutrient concentrations and increased the (il3C of
surface waters. Another factor increasing the(il3C
of G. bulla ides in upwelling regions is temperature. Culturing experiments indicated that the(il3C
of G. bulla ides increases by about 0.1 %0 per degree of temperature decrease (Bemis and Spero
1997). Hence, increased upwelling might also increase the (il3C of G. bulla ides by cooling SSTs.
Core GeoB 1023-5 is located in the Benguela
Upwelling Region off Namibia and has been extensively studied by Schneider et al. (1992). In
sharp contrastto the high-latitude core PS I 768-8,
the glacial-interglacial (i13C difference of G.
bullaides is close to zero (Fig. 9b). Taking into
account a negative global(i l3 C shift of 0.3-0.4 %0
from the Holocene to the last glacial, there must
have been a local increase in glacial surface water (il3C relative to the global ocean, or a surface
water cooling of about 3-4 DC. A cooling of about
4°C has been observed for the nearby core GeoB
1710-3 (Kirst et al. 1998). Hence, the negative global (il3C shift during the LGM is nearly compensated by a positive (il3C shift due to a cooling of
ambient surface waters, and no additional effects
are necessary to explain the glacial carbon isotope
values of G. bullaides.
The deglaciation in Core GeoB 1023-5 is
marked by a strong (iI3C-minimum at about 13 ka.
According to Oppo and Fairbanks (1989) and Schneider et al. (1992), the deglacial minimum might
either reflect an increased upwelling of subsurface
waters, or a change in the preformed isotopic composition of source waters. Since there were no indications of higher upwelling during deglaciation,
Schneider et al. (1992) suggested that the minimum
is due to a change in the isotopic composition of
the upwelled source waters. Today, waters
upwelled off Namibia originate from the South
Atlantic Central Water level at 200-400 m depth.
The modem source area of this water type is near
the Subtropical Front (e.g. Gordon 1981). We have
already shown that Southern Ocean surface waters exhibit a strong deglacial minimum with comparable timing (Fig. 9a). Therefore, it seems to be
reasonable that the deglacial minimum is due to a
change in the preformed isotopic composition of the
upwelled waters. Problems with this scenario are
that cores closer to the potential source area (e.g.
Ninneman and Charles 1997) show a much smaller
de glacial minimum than observed in core GeoB
1023-5. However, future work might show that
upwelled central waters originated further south,
or that the deglacial (iI3C depletion was actually
larger at the Subtropical Front.
Oligotrophic Tropical Atlantic. In contrast to
the high latitudes and the upwelling regions, cores
recovered from the tropical Atlantic provide the opportunity to study carbon isotope records from
nutrient-depleted surface waters. Changes in the
carbon isotope composition of tropical surface
waters should be mainly due to processes unrelated
to nutrient cycling. Core GeoB 3104-1 comes from
the western equatorial Atlantic off Brazil, from a
water depth of about 767 m. The isotope records
and 14C-datings were measured on the shallowdwelling species G. sacculifer (Fig. lOa, Arz
et al. 1998). As already shown in Fig. 5,
G. sacculifer exhibits a strong size-trend in (i13e.
By using a narrow size range (350-400 ~m in this
case) the effects of ontogeny should be minimized.
Generally, the variability of tropical (iI3C_records
is lower than in high- and mid-latitudes. Glacial values in core GeoB 3104-1 are about 0.8 %0 lower
than Holocene values. Like all other cores presented in this study, GeoB 3104-1 contains a strong
deglacial minimum dated to about 13 ka. Although,
the magnitude of the observed changes in core
3104-1 is much lower than in the high and mid-latitude cores, the changes are too great to be solely
explained with global changes in carbon inventories. Since tropical surface waters are always depleted in nutrients, changes in the thermodynamic
imprint must be responsible for the glacial and
deglacial depletion in (il3C. As benthic foraminifera
from cores located in the Antarctic Intermediate
Water contain a synchronous deglacial(iI3C-minimum, Oppo and Fairbanks (1989), and later LynchStiegltz et al. (1994), suggested that the de glacial
minimum might be due to changes in the preformed
(iI3C signal of intermediate waters and transferred
to surface waters by upwelling. We have presented
indications for both mechanisms, changes in preformed (iI3C of AAIW source waters (PS 1768-8)