404
Walter et al.
observation holds for the entire Weddell Sea, this
would suggest that most of the 23 0 Th and 231Pa
produced in this basin must be advected to other
regions of the Southern Ocean prior to scavenging, such as the ACC where particle fluxes are
higher. Hence, the Weddell Sea would not be a sink
for 231Pa, and the high average xs23lPalxs230Th ratios of 0.1 5 found in surface sediments (Fig. 5a)
reflect an increase in the scavenging efficiency of
231Pa relative to 230Th, probably related to the high
opal contents of sedimenting particles.
Further evidence for an advective export of
230Th of up to 50% ofthe production comes from
other ocean basins with short residence times of
deep waters «100 years), such as the North
Atlantic and the Arctic Seas (Moran et al. 1995;
1997; Vogler et al. 1998). Theexportof2 30 Thhas
to be taken into account when using its flux to the
sediment as a reference to predict depositional
fluxes of 231Pa (e.g. Anderson et al. 1994; Yu et
al. 1996) in such regions.
Enhanced Scavenging of23lPa by Mn- and
Fe-oxides
In our simple box model illustration, we have shown
that the large-scale distributions of231Pa and 230Th
in oceanic surface sediments can be explained by
assuming a homogenous particle composition with
a strong preference for the adsorption of 230Th
relative to 231Pa. This assumption is reasonable,
provided that scavenging of both radionuclides is
primarily determined by the mass flux of particles.
There are exceptions, however, as shown for the
Southern Ocean. There is experimental evidence
that as for opal, fractionation between 231Pa and
23 0 Th is also limited on manganese oxides and
hematite, with 230Thf2 31 Pa fractionation factors of
0.8 and 2.1, respectively (Fig. 7). Hydrothermal
plumes emanating in the vicinity of mid-ocean
ridges are an important localized source for Mnand Fe-oxides. They are effective scavengers for
231Pa and, to a lesser extent, for 23"Th relative to
the surrounding area (German et al. 1991, Frank
et al. 1994). This so-called hydrothermal scavenging leads to high xs23lPaI,,230Th ratios in ridge
sediments, as found in the vicinity of the East
Pacific Rise, (Shimmield and Price 1988, see also
Fig. 4a,b). Further evidence for enhanced scavenging of 231Pa relative to 23 0 Th is reported from the
Panama and Guatemala basins and has been attributed to the presence of Mn0 2 coated particles,
originating from recycling of reduced Mn from
suboxic sediments (Anderson et al. 1983b). The
use of the xs231Palxs23°Th ratio as a paleoproductivity
index is thus further limited near mid-ocean ridges
and possibly in regions with diagenetic
remobilization ofMn and near mid-ocean ridges.
Evaluation of the Quality as a Paleoproductivity Proxy
As we have shown in the discussion above, the
231PaI 230Th ratio of oceanic surface sediments
xs
xs
is determined by three different parameters: the
mass flux of particles, their chemical composition
and the hydrography. Consequently, it can only be
applied as a reliable tracer for bioproductivity in the
past, ifthe influence of the chemical composition
and the hydrography is small. This condition appears to be met in the Pacific Ocean, where the
xs231Palxs230Th ratio is well correlated with particle
flux and primary production. This region is an appropriate study area to apply the xs231Pa/xs230Tho
ratio as an index for oceanic productivity in the past.
Even under such circumstances, however, the
xs231Pa/xs230Th ratio can only provide relative
intensities of export flux between low and high
productivity regions.
Throughout the modem Atlantic Ocean, there
is still a relationship between xs231Palxs230Th ratio
and mass flux of particles, but the overall values
are lower due to the low residence time of deep
water in this ocean basin. Nevertheless, during time
periods with similar hydrographic conditions, the
xs231Pa/xs230Tho ratio should reflect relative
changes in paleoproductivity. Conversely, mean
basin-wide xs231Pa/xs230Tho ratios can be used as
a paleocirculation tracer (Yu et al. 1996). Periods
of reduced global thermohaline circulation would
be characterized by higher mean xs231Pa/xs230Tho
ratios in Atlantic sediments and higher contrasts in
the x,231Pa/ ",23°Th o ratios between open ocean and
ocean margin (as documented in today's Pacific).
In the Southern Ocean there is no more a simple relationship between xs231Palxs230Th ratio and
Walter et al.
observation holds for the entire Weddell Sea, this
would suggest that most of the 23 0 Th and 231Pa
produced in this basin must be advected to other
regions of the Southern Ocean prior to scavenging, such as the ACC where particle fluxes are
higher. Hence, the Weddell Sea would not be a sink
for 231Pa, and the high average xs23lPalxs230Th ratios of 0.1 5 found in surface sediments (Fig. 5a)
reflect an increase in the scavenging efficiency of
231Pa relative to 230Th, probably related to the high
opal contents of sedimenting particles.
Further evidence for an advective export of
230Th of up to 50% ofthe production comes from
other ocean basins with short residence times of
deep waters «100 years), such as the North
Atlantic and the Arctic Seas (Moran et al. 1995;
1997; Vogler et al. 1998). Theexportof2 30 Thhas
to be taken into account when using its flux to the
sediment as a reference to predict depositional
fluxes of 231Pa (e.g. Anderson et al. 1994; Yu et
al. 1996) in such regions.
Enhanced Scavenging of23lPa by Mn- and
Fe-oxides
In our simple box model illustration, we have shown
that the large-scale distributions of231Pa and 230Th
in oceanic surface sediments can be explained by
assuming a homogenous particle composition with
a strong preference for the adsorption of 230Th
relative to 231Pa. This assumption is reasonable,
provided that scavenging of both radionuclides is
primarily determined by the mass flux of particles.
There are exceptions, however, as shown for the
Southern Ocean. There is experimental evidence
that as for opal, fractionation between 231Pa and
23 0 Th is also limited on manganese oxides and
hematite, with 230Thf2 31 Pa fractionation factors of
0.8 and 2.1, respectively (Fig. 7). Hydrothermal
plumes emanating in the vicinity of mid-ocean
ridges are an important localized source for Mnand Fe-oxides. They are effective scavengers for
231Pa and, to a lesser extent, for 23"Th relative to
the surrounding area (German et al. 1991, Frank
et al. 1994). This so-called hydrothermal scavenging leads to high xs23lPaI,,230Th ratios in ridge
sediments, as found in the vicinity of the East
Pacific Rise, (Shimmield and Price 1988, see also
Fig. 4a,b). Further evidence for enhanced scavenging of 231Pa relative to 23 0 Th is reported from the
Panama and Guatemala basins and has been attributed to the presence of Mn0 2 coated particles,
originating from recycling of reduced Mn from
suboxic sediments (Anderson et al. 1983b). The
use of the xs231Palxs23°Th ratio as a paleoproductivity
index is thus further limited near mid-ocean ridges
and possibly in regions with diagenetic
remobilization ofMn and near mid-ocean ridges.
Evaluation of the Quality as a Paleoproductivity Proxy
As we have shown in the discussion above, the
231PaI 230Th ratio of oceanic surface sediments
xs
xs
is determined by three different parameters: the
mass flux of particles, their chemical composition
and the hydrography. Consequently, it can only be
applied as a reliable tracer for bioproductivity in the
past, ifthe influence of the chemical composition
and the hydrography is small. This condition appears to be met in the Pacific Ocean, where the
xs231Palxs230Th ratio is well correlated with particle
flux and primary production. This region is an appropriate study area to apply the xs231Pa/xs230Tho
ratio as an index for oceanic productivity in the past.
Even under such circumstances, however, the
xs231Pa/xs230Th ratio can only provide relative
intensities of export flux between low and high
productivity regions.
Throughout the modem Atlantic Ocean, there
is still a relationship between xs231Palxs230Th ratio
and mass flux of particles, but the overall values
are lower due to the low residence time of deep
water in this ocean basin. Nevertheless, during time
periods with similar hydrographic conditions, the
xs231Pa/xs230Tho ratio should reflect relative
changes in paleoproductivity. Conversely, mean
basin-wide xs231Pa/xs230Tho ratios can be used as
a paleocirculation tracer (Yu et al. 1996). Periods
of reduced global thermohaline circulation would
be characterized by higher mean xs231Pa/xs230Tho
ratios in Atlantic sediments and higher contrasts in
the x,231Pa/ ",23°Th o ratios between open ocean and
ocean margin (as documented in today's Pacific).
In the Southern Ocean there is no more a simple relationship between xs231Palxs230Th ratio and