Reliability of the 231Pa / 230 Th Activity Ratio as a Tracer
for Bioproductivity of the Ocean
H.-J. Walter 1 *, M.M. Rutgers van der Loeffl and R. Franyois 2
1 Alfred-Wegener-Institut fur Polar- und Meeresforschung, Postfach 12 01 61,
D-27515 Bremerhaven, Germany
2 Department of Marine Chemistry, Woods Hole Oceanographic Institution,
Woods Hole, Massachusetts, USA
• corresponding author (e-mail):hjwalter@awi-bremerhaven.de
Abstract: In large areas of the world's oceans, there is a relationship between the mass flux of
particulate matter and the unsupported 231Pal23°Th (,,23IPalx/JOTh) activity ratio of recent sediments.
This observation forms the basis for using the 2J1PaI 230Th ratio as a proxy for past changes in
export productivity. However, a simple relatio;~hip b~tween ,,23IPaI,,230Th ratio and particle flux
requires that the water residence time in an ocean basin is far in excess ofthe scavenging residence
time of 2J1Pa, and that the composition of sinking particles maintains a strong preference for the
adsorption of 2J OTh over 2JlPa with a constant 2JOTh/2J1Pa fractionation factor (F). The best correlation
between 2JlPai 2JOTh ratio and mass flux is found in the Pacific Ocean. In the Atlantic, the contrast
in the 2iipal 2i6Th ratios between open ocean (low flux regions) and ocean margins (high flux
region~) is m~ch less pronounced due to the shorter residence time of deep water, resulting in less
effective boundary scavenging of mPa. In the Southern Ocean, south of the Polar Front, there is no
more a simple relationship between 2JlPai 2JoTh and particle flux. This is a result of a southward
decrease in F, probably reflecting th~ incr~ased opal content of sinking particles. Opal does not
fractionate 2JlPa and 2JOTh significantly. This lack of fractionation results in high ",2JIPaI", 2J OTh ratios
in opal-dominated regions, even in areas of very low particle fluxes such as the Weddell Sea. The
23IPaI 2JOTh ratio can therefore only be used as a paleoproductivity proxy if, in the time interval of
interest: changes in the basin ventilation rate and differential scavenging of both radionuclides due
to changes in the chemical composition of particulate matter can be excluded.
Introduction
The 23IPal 23 °Th activity ratio in sediments has been
proposed and used (Lao et al. 1992a,b; Kumar et
al. 1993; Frans;ois et a!. 1993; Kumar 1994; Yu
1994; Boyle 1996; Kumar et al. 1995; Yang et al.
1995) as a tracer to assess changes in
bioproductivity ofthe ocean during the last 150.000
years. The major advantage of the 231Pal23°Th ratio compared to other productivity tracers (e.g.
Corg, biogenic opal, biogenic Ba) is its insensitivity
to remineralization ofthe biogenic detritus both in
the water column and in surface sediments
(Anderson et a!. 1983a, b; Lao et al. I 992a,b; 1993;
Kumar et a!. 1993).
2JIPaf2 3 0Th ratios are also affected by secondary processes, which complicate their interpretation
in tenns of export production. Its reliability as a
paleoproductivity proxy can be tested by comparing unsupported 2J1PaP30Th ratios C,23IPaI",2JOTh,
corrected for detrital, U-supported activities of2 3l Pa
and 23°Th) in modem oceanic sediments with primary production in surface waters. Using this approach, we outline the conditions required for producing a linear relationship between the particle flux
(as an index for export production) and the ",'JlPai
,, 230 Th ratio in the sediment. We also establish the
circumstances under which the influence of other
factors, such as the advective transport of 231Pa
From FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 393-408
for Bioproductivity of the Ocean
H.-J. Walter 1 *, M.M. Rutgers van der Loeffl and R. Franyois 2
1 Alfred-Wegener-Institut fur Polar- und Meeresforschung, Postfach 12 01 61,
D-27515 Bremerhaven, Germany
2 Department of Marine Chemistry, Woods Hole Oceanographic Institution,
Woods Hole, Massachusetts, USA
• corresponding author (e-mail):hjwalter@awi-bremerhaven.de
Abstract: In large areas of the world's oceans, there is a relationship between the mass flux of
particulate matter and the unsupported 231Pal23°Th (,,23IPalx/JOTh) activity ratio of recent sediments.
This observation forms the basis for using the 2J1PaI 230Th ratio as a proxy for past changes in
export productivity. However, a simple relatio;~hip b~tween ,,23IPaI,,230Th ratio and particle flux
requires that the water residence time in an ocean basin is far in excess ofthe scavenging residence
time of 2J1Pa, and that the composition of sinking particles maintains a strong preference for the
adsorption of 2J OTh over 2JlPa with a constant 2JOTh/2J1Pa fractionation factor (F). The best correlation
between 2JlPai 2JOTh ratio and mass flux is found in the Pacific Ocean. In the Atlantic, the contrast
in the 2iipal 2i6Th ratios between open ocean (low flux regions) and ocean margins (high flux
region~) is m~ch less pronounced due to the shorter residence time of deep water, resulting in less
effective boundary scavenging of mPa. In the Southern Ocean, south of the Polar Front, there is no
more a simple relationship between 2JlPai 2JoTh and particle flux. This is a result of a southward
decrease in F, probably reflecting th~ incr~ased opal content of sinking particles. Opal does not
fractionate 2JlPa and 2JOTh significantly. This lack of fractionation results in high ",2JIPaI", 2J OTh ratios
in opal-dominated regions, even in areas of very low particle fluxes such as the Weddell Sea. The
23IPaI 2JOTh ratio can therefore only be used as a paleoproductivity proxy if, in the time interval of
interest: changes in the basin ventilation rate and differential scavenging of both radionuclides due
to changes in the chemical composition of particulate matter can be excluded.
Introduction
The 23IPal 23 °Th activity ratio in sediments has been
proposed and used (Lao et al. 1992a,b; Kumar et
al. 1993; Frans;ois et a!. 1993; Kumar 1994; Yu
1994; Boyle 1996; Kumar et al. 1995; Yang et al.
1995) as a tracer to assess changes in
bioproductivity ofthe ocean during the last 150.000
years. The major advantage of the 231Pal23°Th ratio compared to other productivity tracers (e.g.
Corg, biogenic opal, biogenic Ba) is its insensitivity
to remineralization ofthe biogenic detritus both in
the water column and in surface sediments
(Anderson et a!. 1983a, b; Lao et al. I 992a,b; 1993;
Kumar et a!. 1993).
2JIPaf2 3 0Th ratios are also affected by secondary processes, which complicate their interpretation
in tenns of export production. Its reliability as a
paleoproductivity proxy can be tested by comparing unsupported 2J1PaP30Th ratios C,23IPaI",2JOTh,
corrected for detrital, U-supported activities of2 3l Pa
and 23°Th) in modem oceanic sediments with primary production in surface waters. Using this approach, we outline the conditions required for producing a linear relationship between the particle flux
(as an index for export production) and the ",'JlPai
,, 230 Th ratio in the sediment. We also establish the
circumstances under which the influence of other
factors, such as the advective transport of 231Pa
From FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 393-408