Sediment Redistribution, 230Th", - Normalization and Implications for the Reconstruction of Particle Flux 415
however, statistical uncertainties pertaining to the
230Th ex measurements are already relatively high
due to their respective ages.
In Fig. 2d, e the accumulation rates and rain
rates of carbonate for this core are shown to illustrate the differences between the conventional and
the 230Th ex -normalized approaches. Carbonate rain
rates and export paleoproductivites reconstructed
on the basis of the carbonate rain rates showed that
the carbonate productivity was generally increased
during the interglacial stages and substages of
paleoclimate within the last 190 kyr (Riihlemann et
al. 1996). This is in accordance with earlier investigations from the Ceara Rise (Francois et al. 1990).
Again, the only exception is the fIrst part of glacial
stage 6, where carbonate productivity apparently
was as high as during the warmer periods afterwards.
In constrast to the carbonate accumulation rate,
the carbonate rain rate does not show any peak at
the beginning of stage 2 and no lowered values at
the begining of stage 3. The maximum values of
the carbonate rain rate during stage 5 decrease from
stage 5e to stage Sa. As mentioned before,
enhanced dissolution of carbonate during the glacial stages (Yang et al. 1990) could also have produced the observed pattern, but the fragmentation
index produced by Riih1emann et aI. (1996) showed
that over the last 190 kyr a signifIcant deepening
of the lysocline had occurred only during stage 4
and had led to increased dissolution. The stage 4
carbonate rain rate, therefore, only represents a
minimum estimation. In conclusion, the
paleoproductivity reconstruction based on the carbonate rain rates yields a more consistent picture
than the one based on carbonate accumulation
rates.
Sediment Redistribution and Biogenic
Particle Flux in the Southern Ocean
The 230Thex method of quantifIcation of laterally
supplied sediment material was applied to a number
of sediment cores in the Atlantic sector of the ACC,
the results of which are briefly summarized here
(Francois et al. 1993; Kumar 1994; Kumar et al.
1995; Frank 1996; Frank et al. 1996) (Fig. 3). In
addition, new results from two locations close to
the present day position of the SAF in the area of
the Mid-Atlantic Ridge in the Atlantic section of the
Southern Ocean are presented (Asmus et al. 1999).
In Fig. 4, data for fIve sediment cores which
form a transect from 43°S to 56°S covering all zonal
bands of the ACC in the eastern Atlantic sector
are shown (Peterson and Stramma 1991; see Frank
(1996) and Frank et al. (1996) for detailed description of stratigraphies). On the transect, the
southernmost core has only slightly been influenced
by sediment redistribution (\jJ between 0.5 and 2,
Table 1) which, however, as discussed in the previous chapter, may also be the result of a
stratigraphic artifact due to the low sedimentation
rate of this core for the period of the last 100 kyr.
Further north in the Antarctic Zone, at the location
of core PS1768-8 a huge contribution oflaterally
supplied sediment has led to a focussing factor between 4.5 and 6 during the last 74 kyr, while only a
factor of2 was calculated for isotope stage 5. The
locations ofthe two cores recovered from the Polar
Frontal Zone (PS1756-5 and PS1754-1) experienced some periods of significant sediment export,
probably due to increased bottom water current
speeds. The location ofPS1756-5 was subject to
winnowing during oxygen isotope stage 3 and, depending on the stratigraphy, also during stage 5,
while PS 1754-1 shows winnowing from stage 3 to
6. At the location of Core PS2082-1 retrieved from
the Subantarctic Zone, a large contribution of
focussed sediment is observed during the glacial
stages 2-4 and 6.
The importance of sediment redistribution processes for relevant conclusions on paleoproductivity
or paleoclimate can be demonstrated by comparing (uncorrected) biogenic Ba accumulation rates
with (corrected) rain rates along the described
transect over the ACC (Kumar 1994; Kumar et al.
1995; Frank 1996; Frank et al. 1996; Niirnberg et
al. 1997). The rate of biogenic Ba deposition may
serve as a basis for the evaluation of export
paleoproductivity (Dymond et al. 1992; Niirnberg
et al. 1997). The B~io rain rate is applied to reconstruct the biogenic particle fluxes, originating
from the surface waters above, at a particular location (Fig. 5). The development of a transfer
function is in progress for the quantitative reconstruction of export paleoproductivity from marine
however, statistical uncertainties pertaining to the
230Th ex measurements are already relatively high
due to their respective ages.
In Fig. 2d, e the accumulation rates and rain
rates of carbonate for this core are shown to illustrate the differences between the conventional and
the 230Th ex -normalized approaches. Carbonate rain
rates and export paleoproductivites reconstructed
on the basis of the carbonate rain rates showed that
the carbonate productivity was generally increased
during the interglacial stages and substages of
paleoclimate within the last 190 kyr (Riihlemann et
al. 1996). This is in accordance with earlier investigations from the Ceara Rise (Francois et al. 1990).
Again, the only exception is the fIrst part of glacial
stage 6, where carbonate productivity apparently
was as high as during the warmer periods afterwards.
In constrast to the carbonate accumulation rate,
the carbonate rain rate does not show any peak at
the beginning of stage 2 and no lowered values at
the begining of stage 3. The maximum values of
the carbonate rain rate during stage 5 decrease from
stage 5e to stage Sa. As mentioned before,
enhanced dissolution of carbonate during the glacial stages (Yang et al. 1990) could also have produced the observed pattern, but the fragmentation
index produced by Riih1emann et aI. (1996) showed
that over the last 190 kyr a signifIcant deepening
of the lysocline had occurred only during stage 4
and had led to increased dissolution. The stage 4
carbonate rain rate, therefore, only represents a
minimum estimation. In conclusion, the
paleoproductivity reconstruction based on the carbonate rain rates yields a more consistent picture
than the one based on carbonate accumulation
rates.
Sediment Redistribution and Biogenic
Particle Flux in the Southern Ocean
The 230Thex method of quantifIcation of laterally
supplied sediment material was applied to a number
of sediment cores in the Atlantic sector of the ACC,
the results of which are briefly summarized here
(Francois et al. 1993; Kumar 1994; Kumar et al.
1995; Frank 1996; Frank et al. 1996) (Fig. 3). In
addition, new results from two locations close to
the present day position of the SAF in the area of
the Mid-Atlantic Ridge in the Atlantic section of the
Southern Ocean are presented (Asmus et al. 1999).
In Fig. 4, data for fIve sediment cores which
form a transect from 43°S to 56°S covering all zonal
bands of the ACC in the eastern Atlantic sector
are shown (Peterson and Stramma 1991; see Frank
(1996) and Frank et al. (1996) for detailed description of stratigraphies). On the transect, the
southernmost core has only slightly been influenced
by sediment redistribution (\jJ between 0.5 and 2,
Table 1) which, however, as discussed in the previous chapter, may also be the result of a
stratigraphic artifact due to the low sedimentation
rate of this core for the period of the last 100 kyr.
Further north in the Antarctic Zone, at the location
of core PS1768-8 a huge contribution oflaterally
supplied sediment has led to a focussing factor between 4.5 and 6 during the last 74 kyr, while only a
factor of2 was calculated for isotope stage 5. The
locations ofthe two cores recovered from the Polar
Frontal Zone (PS1756-5 and PS1754-1) experienced some periods of significant sediment export,
probably due to increased bottom water current
speeds. The location ofPS1756-5 was subject to
winnowing during oxygen isotope stage 3 and, depending on the stratigraphy, also during stage 5,
while PS 1754-1 shows winnowing from stage 3 to
6. At the location of Core PS2082-1 retrieved from
the Subantarctic Zone, a large contribution of
focussed sediment is observed during the glacial
stages 2-4 and 6.
The importance of sediment redistribution processes for relevant conclusions on paleoproductivity
or paleoclimate can be demonstrated by comparing (uncorrected) biogenic Ba accumulation rates
with (corrected) rain rates along the described
transect over the ACC (Kumar 1994; Kumar et al.
1995; Frank 1996; Frank et al. 1996; Niirnberg et
al. 1997). The rate of biogenic Ba deposition may
serve as a basis for the evaluation of export
paleoproductivity (Dymond et al. 1992; Niirnberg
et al. 1997). The B~io rain rate is applied to reconstruct the biogenic particle fluxes, originating
from the surface waters above, at a particular location (Fig. 5). The development of a transfer
function is in progress for the quantitative reconstruction of export paleoproductivity from marine