Sediment Redistribution, 230Th - Normalization and Implications for the
ex
Reconstruction of Particle Flux and Export Paleoproductivity
M. Frank,I,3' R. Gersonde 2 and A. ManginP
I Department of Earth Sciences, University of Oxford, Parks Road,
Oxford OX1 3PR, UK.
2 Alfred-Wegener-Institutfor Polar- und Meeresforschung, Postfach 12 0161,
D-27515 Bremerhaven, Germany
3 Heidelberger Akademie der Wissenschaften, INF 366, D-69120 Heidelberg, Germany
* corresponding author (e-mail):martirif@earth.ox.ac. uk
Abstract: The reconstruction of biogenic particle fluxes and export paleoproductivity from
accumulation rates of biogenic proxies in late Quaternary marine sediments, besides other factors,
greatly depends on the quantification of the amount of laterally redistributed sediment particles.
This is particularly crucial in areas of highly dynamic bottom currents such as in the vicinity ofthe
Antarctic Circumpolar Current System of the Southern Ocean but may also be important in
environments with lower current speeds. The flux of the natural radionuclide 230Th to the sediments
is expected to match its local production rate because of its very low oceanic residence time and high
particle reactivity. Some uncertainties possibly affecting this assumed constant flux in areas of
highly dynamic water masses and strong spacial contrasts of particle flux intensity have to be
considered. Advective 230Th transport and particle size dependent relocation of particles may occur
there. Sediment redistribution can therefore in these areas only be evaluated applying a normalization
to 230Th", if its fluxes deviate significantly (>50%) from the expected values. It has been shown using
this method that up to twelve times the vertical particle flux has been supplied laterally in areas such
as the ACC. It is in tum possible to calculate realistic bulk accumulation rates (rain rates) as the basis
for reconstructing proxy fluxes from marine sediments. A high variability of sediment redistribution
intensity, and thus uncorrected proxy accumulation rates from one location to the other, but also
within single cores, demonstrates the necessity of its quantification and correction before any
paleoceanographic interpretations applying these accumulation rates may be carried out.
Introduction
The amount of organic carbon sinking through a cribes the amount of organic carbon produced in the
certain area per unit of time defmes the flux ofbio- surface water of the ocean and the export producgenic matter and thus productivity (e.g. MUller and tivity defines the amount of primarily produced orSuess 1979). Global and regional variations ofbio- ganic carbon which survives the very efficient
logical productivity in the past have been linked to remineralization within the surface waters and is
changes in atmospheric CO 2 , and thus the knowl- exported into the deeper water. The amount of oredge of paleoproductivity is important for the un- ganic carbon finally reaching the sediments is only
derstanding of the mechanisms which have con- a very small fraction (about 1 %) of the primary
trolled the climate in the past and will do so in the production (Berger et al. 1989). Difficulties for
future (Broecker 1982; Sarmiento and Toggweiler using variations of the organic carbon flux into
1984; Keir 1988). The primary productivity des- marine sediments as a measure of export
From FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 409-426
ex
Reconstruction of Particle Flux and Export Paleoproductivity
M. Frank,I,3' R. Gersonde 2 and A. ManginP
I Department of Earth Sciences, University of Oxford, Parks Road,
Oxford OX1 3PR, UK.
2 Alfred-Wegener-Institutfor Polar- und Meeresforschung, Postfach 12 0161,
D-27515 Bremerhaven, Germany
3 Heidelberger Akademie der Wissenschaften, INF 366, D-69120 Heidelberg, Germany
* corresponding author (e-mail):martirif@earth.ox.ac. uk
Abstract: The reconstruction of biogenic particle fluxes and export paleoproductivity from
accumulation rates of biogenic proxies in late Quaternary marine sediments, besides other factors,
greatly depends on the quantification of the amount of laterally redistributed sediment particles.
This is particularly crucial in areas of highly dynamic bottom currents such as in the vicinity ofthe
Antarctic Circumpolar Current System of the Southern Ocean but may also be important in
environments with lower current speeds. The flux of the natural radionuclide 230Th to the sediments
is expected to match its local production rate because of its very low oceanic residence time and high
particle reactivity. Some uncertainties possibly affecting this assumed constant flux in areas of
highly dynamic water masses and strong spacial contrasts of particle flux intensity have to be
considered. Advective 230Th transport and particle size dependent relocation of particles may occur
there. Sediment redistribution can therefore in these areas only be evaluated applying a normalization
to 230Th", if its fluxes deviate significantly (>50%) from the expected values. It has been shown using
this method that up to twelve times the vertical particle flux has been supplied laterally in areas such
as the ACC. It is in tum possible to calculate realistic bulk accumulation rates (rain rates) as the basis
for reconstructing proxy fluxes from marine sediments. A high variability of sediment redistribution
intensity, and thus uncorrected proxy accumulation rates from one location to the other, but also
within single cores, demonstrates the necessity of its quantification and correction before any
paleoceanographic interpretations applying these accumulation rates may be carried out.
Introduction
The amount of organic carbon sinking through a cribes the amount of organic carbon produced in the
certain area per unit of time defmes the flux ofbio- surface water of the ocean and the export producgenic matter and thus productivity (e.g. MUller and tivity defines the amount of primarily produced orSuess 1979). Global and regional variations ofbio- ganic carbon which survives the very efficient
logical productivity in the past have been linked to remineralization within the surface waters and is
changes in atmospheric CO 2 , and thus the knowl- exported into the deeper water. The amount of oredge of paleoproductivity is important for the un- ganic carbon finally reaching the sediments is only
derstanding of the mechanisms which have con- a very small fraction (about 1 %) of the primary
trolled the climate in the past and will do so in the production (Berger et al. 1989). Difficulties for
future (Broecker 1982; Sarmiento and Toggweiler using variations of the organic carbon flux into
1984; Keir 1988). The primary productivity des- marine sediments as a measure of export
From FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 409-426