Reliability of the 231Pa p30Th Activity Ratio
403
Mn02 Sed. Sed. Fe~3 Fe20a AI~3 Silica
(FSW)
(FSW)
Particle Type
Fig. 7. Kl30ThlKl31Paratios or 23"Th1"'Pa fractionation
factors for different particle types, determined in artificial seawater, as well as for Fe 2 0 3 and natural sediments
in filtered seawater (FSW). Reproduced with permission
from Anderson et al. (1992).
to advection of dissolved 230Th from the Atlantic),
the mass balance for 23lPa can be calculated (Yu
et al. 1996). By taking a mean x,23'Pal x ,230Th rain
ratio of 0.165 for the region south of 50 0 S, they
estimated that the depositional flux of 231Pa
throughout the entire Southern Ocean balances insitu production plus 23'Pa import from the Atlantic
Ocean, suggesting this region to be an important
sink for 23lPa.
The assumption of a constant flux/production
ratio of230Th around 1 does not appear to be valid
throughout the entire Southern Ocean. Based on
trap
latitude
longitude
depth mass flux
(m) g·m-2·a-1
PF 1-4 50' 09.0' S
05' 46.4' E
700
38.3
BO 1+2 54' 20.2' S 03'20.2' W 450
53.1
WS 3 64' 54.1' S 02' 33.8' W 360
33.7
measurements of the dissolved and particulate
concentrations of23% in the water column of the
South Atlantic, Rutgers van der Loeff and Berger
(1993) hypothesized that in the Central Weddell
Sea south of the Antarctic Weddell boundary
(A WB), scavenging rate of2 3 0Th might be strongly
reduced relative to the regions to the north (ACC).
They based their suggestion on both the extremely
low particle fluxes (Fischer et al. 1988; SchlUter et
al. subm) and the short residence time of water in
the Weddell Sea (35 years; Rutgers van der Loeff
and Berger (1993» comparable to the scavenging
residence time of 230Th. Further support for low
scavenging rates in the Central Weddell Sea comes
from the sedimentary record, with extremely low
2lOPb inventories of only 15% of its production rate
(Rutgers van der Loeff and Berger 1991). These
findings are confirmed by an investigation of the
inventories of 23'Pa and 23°Th, in three sediment
cores from the Western Central Weddell Sea. The
selected area is characterized by slow bottom currents, so that effects of lateral sediment redistribution are only small, which is a reflection of sluggish circulation ofthe center of the Weddell Gyre
compared to the outer parts such as the northern
Weddell (Pudsey et al. 1988). Therefore, it is not
to be expected that the core locations are influenced
by sediment winnowing (Pudsey et al. 1988). Results from the core PS1508 (67°00,1 'S/32°23,5'W)
have shown that over the last 150ka only 37% and
52% of the production of 23 0Th and 23'Pa, respectively, are found in the sediment. These observations imply a N-S change in the scavenging efficiency not only for 23'Pa, but also for 230Th. If this
opat xs231 Palxs230Jb. *(231 PaJ23OTh)diss F-factor
(%)
40
0.322±0.066
0.599 ± 0.064
1.8 ±0.43
62
0.344± 0.034
0.465 ± 0.036
1.3 ±0.17
70
0.278 ± 0.016
0.280 ± 0.058
1.0 ±0.23
Table 2. Opal contents and }31Pa/}30Th ratios of sinking particles, and estimated F-factors (from Walter et al.
1997) Data on mass flux and opal content ofPF 1-4 and WS-3 are from Wefer and Fischer (1991), and ofBO 1 +2 from
Wefer and Fischer (unpublished).
403
Mn02 Sed. Sed. Fe~3 Fe20a AI~3 Silica
(FSW)
(FSW)
Particle Type
Fig. 7. Kl30ThlKl31Paratios or 23"Th1"'Pa fractionation
factors for different particle types, determined in artificial seawater, as well as for Fe 2 0 3 and natural sediments
in filtered seawater (FSW). Reproduced with permission
from Anderson et al. (1992).
to advection of dissolved 230Th from the Atlantic),
the mass balance for 23lPa can be calculated (Yu
et al. 1996). By taking a mean x,23'Pal x ,230Th rain
ratio of 0.165 for the region south of 50 0 S, they
estimated that the depositional flux of 231Pa
throughout the entire Southern Ocean balances insitu production plus 23'Pa import from the Atlantic
Ocean, suggesting this region to be an important
sink for 23lPa.
The assumption of a constant flux/production
ratio of230Th around 1 does not appear to be valid
throughout the entire Southern Ocean. Based on
trap
latitude
longitude
depth mass flux
(m) g·m-2·a-1
PF 1-4 50' 09.0' S
05' 46.4' E
700
38.3
BO 1+2 54' 20.2' S 03'20.2' W 450
53.1
WS 3 64' 54.1' S 02' 33.8' W 360
33.7
measurements of the dissolved and particulate
concentrations of23% in the water column of the
South Atlantic, Rutgers van der Loeff and Berger
(1993) hypothesized that in the Central Weddell
Sea south of the Antarctic Weddell boundary
(A WB), scavenging rate of2 3 0Th might be strongly
reduced relative to the regions to the north (ACC).
They based their suggestion on both the extremely
low particle fluxes (Fischer et al. 1988; SchlUter et
al. subm) and the short residence time of water in
the Weddell Sea (35 years; Rutgers van der Loeff
and Berger (1993» comparable to the scavenging
residence time of 230Th. Further support for low
scavenging rates in the Central Weddell Sea comes
from the sedimentary record, with extremely low
2lOPb inventories of only 15% of its production rate
(Rutgers van der Loeff and Berger 1991). These
findings are confirmed by an investigation of the
inventories of 23'Pa and 23°Th, in three sediment
cores from the Western Central Weddell Sea. The
selected area is characterized by slow bottom currents, so that effects of lateral sediment redistribution are only small, which is a reflection of sluggish circulation ofthe center of the Weddell Gyre
compared to the outer parts such as the northern
Weddell (Pudsey et al. 1988). Therefore, it is not
to be expected that the core locations are influenced
by sediment winnowing (Pudsey et al. 1988). Results from the core PS1508 (67°00,1 'S/32°23,5'W)
have shown that over the last 150ka only 37% and
52% of the production of 23 0Th and 23'Pa, respectively, are found in the sediment. These observations imply a N-S change in the scavenging efficiency not only for 23'Pa, but also for 230Th. If this
opat xs231 Palxs230Jb. *(231 PaJ23OTh)diss F-factor
(%)
40
0.322±0.066
0.599 ± 0.064
1.8 ±0.43
62
0.344± 0.034
0.465 ± 0.036
1.3 ±0.17
70
0.278 ± 0.016
0.280 ± 0.058
1.0 ±0.23
Table 2. Opal contents and }31Pa/}30Th ratios of sinking particles, and estimated F-factors (from Walter et al.
1997) Data on mass flux and opal content ofPF 1-4 and WS-3 are from Wefer and Fischer (1991), and ofBO 1 +2 from
Wefer and Fischer (unpublished).