410
Frank eta!.
paleoproductivity, however, arise from the low degree of preservation and possibly varying amounts
of refractory organic carbon, variations of the preservation efficiency and early diagenetic effects
within the sediments which mainly are a function
of deep-water oxygenation and bulk sediment accumulation rate (e.g. Rosenthal et al. 1995;
Thomson et al. 1995; Francois et al. 1997; Jung et
al. 1997). In addition, in areas of high bottom water dynamics sediment redistribution may influence
the accumulation rate record.
The above described uncertainties associated
with organic carbon preservation led to the search
for alternative proxies for paleoproductivity in marine sediments which are more resistant to early
diagenetic processes than organic carbon itself.
Many of these are discussed extensively in other
contributions to this volume and will not be considered in detail here. This article will focus on the
application ofthe natural radionuclide 2Jo Th for the
assessment of realistic (with respect to sediment
redistribution) accumulation rates for Late Quaternary sediments of approximately the last 300 kyr
as the basis of paleoproductivity reconstruction from
proxy accumulation rates. The knowledge of redistribution-corrected accumulation rates (furtheron
referred to as "rain rates") is the basis for realistic
estimation of fluxes of any sedimentary component.
We wi II present and summarize case studies on
applications of sedimentary2J O Th" (ex standing for
230Th derived from the water column, which is corrected for 23 0 Th in secular radioactive equilibrium
with 234U in the lithogenic particles and also
corrected for diagenetic diffusive deposition ofU
(Francois et al. 1993)) from two different areas:
The increasing demand for high time resolution
studies also from low resolution sediment cores
may cause problems. Calcareous sediments of the
Ceara Rise in the western equatorial Atlantic, an
area of low surface water productivity, are chosen
to demonstrate the applicability of 2J OTh for the
calculation of mass accumulation rates in relatively
low resolution sediments. In this area the method
of 2JOTh -normalization for calculating correct
mass ac~umulation rates was shown to be applicable without restrictions in six sediment cores altogether which uniformly yielded a ratio close to 1
between expected and measured flux o[2 JO Th" to
the sediments (Francois et al. 1990). The use of the
2JOTh ex -normalization avoids biases due to uncertainties originating from the application ofthe high
resolution 8 18 0 event stratigraphy (Imbrie et al.
1984; Martinson et al. 1987) to low resolution
sediments, because the ages of the 8 18 0 stage
boundaries have certain errors which in this case
become important (Riihlemann et al. 1996).
In contrast, the biologically high productive regime of the Antarctic Circumpolar Current (ACC)
in the Atlantic section of the Southern Ocean was
selected as an example of an area where current
activity is high today and probably has been so during the Late Pleistocene. Sediment redistribution
processes have resulted, locally and regionally, in
the accumulation of thickened Late Quaternary
sediment sections (Francois et al. 1993; Kumar
1994; Frank 1996; Frank et al. 1996). The results
of a recent study on two sediment cores from the
vicinity of the present day position of the
Subantarctic Front (SAF) are presented to show the
importance ofthe correction for sediment redistribution in such an environment of high current
activities.
Finally, an outlook will be given to further potential applications of2JOThex-normalization and to
possible alternatives for determining sediment redistribution effects in sedimentary records older than
300 kyr.
Lateral Sediment Redistribution
Currents occur in most marine sedimentary environments and particles sinking through the water
column experience a dislocation depending on their
respective grain sizes as well as sinking velocities
(Scholten et al. 1994). This may lead to a preferential deposition of particles in the lower energy
areas of currents behind barriers or elevations. In
addition, an increase of bottom current speed or
slight relocations ofthe currents' pathways may lead
to a resuspension and subsequent redeposition of
already deposited particles. However, as facilitated
by the development of modern sonar systems on
board of most research ships (cf. Gersonde and
Hempel 1990), such sediments, thickened by laterally supplied material, may be preferentially cored
during research cruises because of their expected
Frank eta!.
paleoproductivity, however, arise from the low degree of preservation and possibly varying amounts
of refractory organic carbon, variations of the preservation efficiency and early diagenetic effects
within the sediments which mainly are a function
of deep-water oxygenation and bulk sediment accumulation rate (e.g. Rosenthal et al. 1995;
Thomson et al. 1995; Francois et al. 1997; Jung et
al. 1997). In addition, in areas of high bottom water dynamics sediment redistribution may influence
the accumulation rate record.
The above described uncertainties associated
with organic carbon preservation led to the search
for alternative proxies for paleoproductivity in marine sediments which are more resistant to early
diagenetic processes than organic carbon itself.
Many of these are discussed extensively in other
contributions to this volume and will not be considered in detail here. This article will focus on the
application ofthe natural radionuclide 2Jo Th for the
assessment of realistic (with respect to sediment
redistribution) accumulation rates for Late Quaternary sediments of approximately the last 300 kyr
as the basis of paleoproductivity reconstruction from
proxy accumulation rates. The knowledge of redistribution-corrected accumulation rates (furtheron
referred to as "rain rates") is the basis for realistic
estimation of fluxes of any sedimentary component.
We wi II present and summarize case studies on
applications of sedimentary2J O Th" (ex standing for
230Th derived from the water column, which is corrected for 23 0 Th in secular radioactive equilibrium
with 234U in the lithogenic particles and also
corrected for diagenetic diffusive deposition ofU
(Francois et al. 1993)) from two different areas:
The increasing demand for high time resolution
studies also from low resolution sediment cores
may cause problems. Calcareous sediments of the
Ceara Rise in the western equatorial Atlantic, an
area of low surface water productivity, are chosen
to demonstrate the applicability of 2J OTh for the
calculation of mass accumulation rates in relatively
low resolution sediments. In this area the method
of 2JOTh -normalization for calculating correct
mass ac~umulation rates was shown to be applicable without restrictions in six sediment cores altogether which uniformly yielded a ratio close to 1
between expected and measured flux o[2 JO Th" to
the sediments (Francois et al. 1990). The use of the
2JOTh ex -normalization avoids biases due to uncertainties originating from the application ofthe high
resolution 8 18 0 event stratigraphy (Imbrie et al.
1984; Martinson et al. 1987) to low resolution
sediments, because the ages of the 8 18 0 stage
boundaries have certain errors which in this case
become important (Riihlemann et al. 1996).
In contrast, the biologically high productive regime of the Antarctic Circumpolar Current (ACC)
in the Atlantic section of the Southern Ocean was
selected as an example of an area where current
activity is high today and probably has been so during the Late Pleistocene. Sediment redistribution
processes have resulted, locally and regionally, in
the accumulation of thickened Late Quaternary
sediment sections (Francois et al. 1993; Kumar
1994; Frank 1996; Frank et al. 1996). The results
of a recent study on two sediment cores from the
vicinity of the present day position of the
Subantarctic Front (SAF) are presented to show the
importance ofthe correction for sediment redistribution in such an environment of high current
activities.
Finally, an outlook will be given to further potential applications of2JOThex-normalization and to
possible alternatives for determining sediment redistribution effects in sedimentary records older than
300 kyr.
Lateral Sediment Redistribution
Currents occur in most marine sedimentary environments and particles sinking through the water
column experience a dislocation depending on their
respective grain sizes as well as sinking velocities
(Scholten et al. 1994). This may lead to a preferential deposition of particles in the lower energy
areas of currents behind barriers or elevations. In
addition, an increase of bottom current speed or
slight relocations ofthe currents' pathways may lead
to a resuspension and subsequent redeposition of
already deposited particles. However, as facilitated
by the development of modern sonar systems on
board of most research ships (cf. Gersonde and
Hempel 1990), such sediments, thickened by laterally supplied material, may be preferentially cored
during research cruises because of their expected