Reliability of the 231Pa j230Th Activity Ratio
401
tions through time of the ,,231Pa/,,230Th o signal,
rather then being due to changes in the
bioproductivity of the ocean, could reflect changes
in the magnitude of the conveyer belt circulation.
For the last glacial, there are indications of a similar export rate of deep water from the Atlantic to
the Southern Ocean, based on the similarity of the
,,231Pa/ xs 23 °Th distribution patterns in modem
Atlantic and Last Glacial Maximum (LGM) Atlantic sediments (Yu et al. 1996). This observation implies that, neglecting the effects of the chemical
composition of particles which becomes very important south of 50 0 S (see chapter Southern
Ocean), the xs231Pa/xs230Tho ratio should provide
reasonable results about changes in bioproductivity
in the LGM (Kumar et al. 1995; Franyois et al.
1997).
Southern Ocean
A further complication of the relationship between
xs231Palxs230Th ratio and particle flux is apparent in
results from the Southern Ocean. The first investigation of231Pa and 230Th in this region was carried out by DeMaster (1979). In rapidly accumulating (up to 100 cmlka) siliceous sediments within
the Antarctic Circumpolar Current (ACC) between
48°S and 55°S, DeMaster found evidence for preferential deposition of231Pa relative to 23°Th, with
inventories of2 3 0Th and 231Pa exceeding the production rate in the water column by up to 6 and 14
times, respectively. Moreover, he showed that the
231Paf230Th deposition ratio was up to five times
higher than the production ratio of 0.093 . DeMaster
explained his findings by a boundary scavenging
effect and hypothesized that, due to the high particle flux within the ACC, scavenging of231Pa and
230Th is so effective that both radionuclides are
quantitatively stripped from the water column. This
would imply that the high ,,23JPaI xs230Th ratios in
sediments underlying the ACC reflect both the local production of the two radionuclides in the water column and the remaining fractions of dissolved
231Pa and (less important) of2 3 0Th imported from
the Atlantic Ocean by advection (Yu et al. 1996).
Several recent findings indicate that a simple
boundary scavenging model cannot be directly
applied in the Southern Ocean. First, total mass
fluxes measured in sediment traps located at 50 0 S
(PF) and 55°S (B01/2) within the ACC are 38.3
gm- 2 ·a- 1 (Wefer and Fischer 1991) and 53 gm- 2 a- 1
(Fischer and Wefer unpublished), respectively.
Although not corrected for trapping efficiency,
these are much lower fluxes than measured in
upwelling regions such as the Panama Basin or the
California margin (> 100 gm- 2 a- l ; Honjo 1982; Yu
1994). These low fluxes are also in agreement with
primary production being much lower (in general
less than 100 gCm- 2 a- I ), than in upwelling regions
(sometimes exceeding 200 gCm- 2 a- l , Fig. 4a,b;
5a,b). According to Fig. 3, such mass flux would
be insufficient to enhance 231Pa scavenging above
its rate of production in the water column. Second,
according to the reversible scavenging model of
Bacon and Anderson (1982), the concentrations of
231Pa and 230Th in the water column should adjust
to the flux of particulate matter. Consequently, the
high particle fluxes postulated by DeMaster(1979)
should lead to a depletion oftheir dissolved activities relative to regions with low particle fluxes.
Actually, there are no indications of a depletion of
231Pa and 230Th in the water column within the
ACC relative to the less productive regions to the
north and south (Rutgers van der Loeff and Berger
1993; Walter et al. 1997). Third, a recent study of
surface sediments in the Atlantic sector of the
Southern Ocean (Fig. 5) has revealed that the region of high xs231Palxs230Th ratios is not restricted
to the area ofthe ACC, but persists throughout the
Weddell Sea (Walter et al. 1997). This is in contrast to the low particle fluxes of this region compared to the ACC, inferred from sediment trap data
« 50 gm- 2 a- l ; Fischer et al. 1988; Wefer and
Fischer 1991) and fluxes of biogenic opal through
the sediment-water interface deduced from pore
water measurements of silicate (Schluter et al.
submitted). The latter observation unequivocally
illustrates the decoupling between xs231Palxs230Th
ratio and the mass flux of particulate matter in this
region. As boundary scavenging cannot explain the
observations above, another factor than the mass
flux must be responsible for the high xs23J Pal" 230Th
ratios in the sediments within and south of the ACC.
As discussed above, the application of the
xs231Palxs230Th ratio as a paleoproductivity proxy
requires a constant 230Thf231Pa fractionation factor (F) with a strong preference for the adsorption
401
tions through time of the ,,231Pa/,,230Th o signal,
rather then being due to changes in the
bioproductivity of the ocean, could reflect changes
in the magnitude of the conveyer belt circulation.
For the last glacial, there are indications of a similar export rate of deep water from the Atlantic to
the Southern Ocean, based on the similarity of the
,,231Pa/ xs 23 °Th distribution patterns in modem
Atlantic and Last Glacial Maximum (LGM) Atlantic sediments (Yu et al. 1996). This observation implies that, neglecting the effects of the chemical
composition of particles which becomes very important south of 50 0 S (see chapter Southern
Ocean), the xs231Pa/xs230Tho ratio should provide
reasonable results about changes in bioproductivity
in the LGM (Kumar et al. 1995; Franyois et al.
1997).
Southern Ocean
A further complication of the relationship between
xs231Palxs230Th ratio and particle flux is apparent in
results from the Southern Ocean. The first investigation of231Pa and 230Th in this region was carried out by DeMaster (1979). In rapidly accumulating (up to 100 cmlka) siliceous sediments within
the Antarctic Circumpolar Current (ACC) between
48°S and 55°S, DeMaster found evidence for preferential deposition of231Pa relative to 23°Th, with
inventories of2 3 0Th and 231Pa exceeding the production rate in the water column by up to 6 and 14
times, respectively. Moreover, he showed that the
231Paf230Th deposition ratio was up to five times
higher than the production ratio of 0.093 . DeMaster
explained his findings by a boundary scavenging
effect and hypothesized that, due to the high particle flux within the ACC, scavenging of231Pa and
230Th is so effective that both radionuclides are
quantitatively stripped from the water column. This
would imply that the high ,,23JPaI xs230Th ratios in
sediments underlying the ACC reflect both the local production of the two radionuclides in the water column and the remaining fractions of dissolved
231Pa and (less important) of2 3 0Th imported from
the Atlantic Ocean by advection (Yu et al. 1996).
Several recent findings indicate that a simple
boundary scavenging model cannot be directly
applied in the Southern Ocean. First, total mass
fluxes measured in sediment traps located at 50 0 S
(PF) and 55°S (B01/2) within the ACC are 38.3
gm- 2 ·a- 1 (Wefer and Fischer 1991) and 53 gm- 2 a- 1
(Fischer and Wefer unpublished), respectively.
Although not corrected for trapping efficiency,
these are much lower fluxes than measured in
upwelling regions such as the Panama Basin or the
California margin (> 100 gm- 2 a- l ; Honjo 1982; Yu
1994). These low fluxes are also in agreement with
primary production being much lower (in general
less than 100 gCm- 2 a- I ), than in upwelling regions
(sometimes exceeding 200 gCm- 2 a- l , Fig. 4a,b;
5a,b). According to Fig. 3, such mass flux would
be insufficient to enhance 231Pa scavenging above
its rate of production in the water column. Second,
according to the reversible scavenging model of
Bacon and Anderson (1982), the concentrations of
231Pa and 230Th in the water column should adjust
to the flux of particulate matter. Consequently, the
high particle fluxes postulated by DeMaster(1979)
should lead to a depletion oftheir dissolved activities relative to regions with low particle fluxes.
Actually, there are no indications of a depletion of
231Pa and 230Th in the water column within the
ACC relative to the less productive regions to the
north and south (Rutgers van der Loeff and Berger
1993; Walter et al. 1997). Third, a recent study of
surface sediments in the Atlantic sector of the
Southern Ocean (Fig. 5) has revealed that the region of high xs231Palxs230Th ratios is not restricted
to the area ofthe ACC, but persists throughout the
Weddell Sea (Walter et al. 1997). This is in contrast to the low particle fluxes of this region compared to the ACC, inferred from sediment trap data
« 50 gm- 2 a- l ; Fischer et al. 1988; Wefer and
Fischer 1991) and fluxes of biogenic opal through
the sediment-water interface deduced from pore
water measurements of silicate (Schluter et al.
submitted). The latter observation unequivocally
illustrates the decoupling between xs231Palxs230Th
ratio and the mass flux of particulate matter in this
region. As boundary scavenging cannot explain the
observations above, another factor than the mass
flux must be responsible for the high xs23J Pal" 230Th
ratios in the sediments within and south of the ACC.
As discussed above, the application of the
xs231Palxs230Th ratio as a paleoproductivity proxy
requires a constant 230Thf231Pa fractionation factor (F) with a strong preference for the adsorption