Reliability of the 231Paj230Th Activity Ratio
405
mass flux of particulate matter. This is a result of
the strong latitudinal decrease in the scavenging
preference of230Th over 231Pa, probably related to
the high contents of biogenic opal of sinking particles. As a consequence, the xs231Palx,230Th ratio
cannot be applied as a reliable paleoproductivity
proxy in this region, as variations of the x,231PaJ
x,230Th o ratio through time, usually interpreted to
reflect changes in the total mass flux of particles,
could also be explained by changes in the content
of biogenic opal on sinking particles. The high affinity of231Pa to opal, however, could possibly be
used to trace fluxes of biogenic opal to the sediment that have been totally dissolved (e.g. throughout the Weddell Sea, Walter et al. 1997). Nearly
unpreferential scavenging 0[230Th and 231Pa by
manganese- and iron oxides further limits the use
of the x,231Pal x ,230Th ratio near mid-ocean ridges
and in areas of diagenetic remobilization of manganese.
In conclusion, the x,231Pal x ,230Th ratio can serve
as a proxy for relative changes in paleoproductivity
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 are not significant. The interpretation of the xs231PaI xs 230Th ratio
thus requires synoptic information on its basin-wide
distribution and on past changes in composition of
sedimenting particles. Reliable reconstructions of
changes in paleoproductivity, however, can only be
made in combination with other independent proxies, (like e.g. biogenic barium, authigenic uranium).
Recent improvements in mass spectrometric
techniques (e.g. Chen et al. 1992; Cheng et al.
1996) now allow the study of the distributions of
230Th and 231Pa in the ocean in more detail. Small
sample sizes of only 1-2 liters of seawater (compared to cubic meters with the traditionally alpha
spectrometry), sufficient for the high precision
determination of230Th (Moran et al. 1995; 1997;
Vogler et al. 1998) and 231Pa (Fran90is pers
comm ; Vogler pers comm; Edmonds et al. unpubl
data), promise the detailed investigation of scavenging of both radionuclides even in the surface
ocean, where their concentrations are extremely
low. This new analytical technique could thus provide the required answers to some of the remaining open questions regarding the oceanographic
behavior of 230Th and 231Pa.
Acknowledgments
Heike Hoeltzen and Walter Geibert is thanked for
their help with the radiochemical analyses. We are
grateful to Jan Scholten and Bradley Moran for
their constructive reviews. This research was
funded by the Deutsche F orschungsgemeinschaft
(Sonderforschungsbereich 261 at Bremen University, Contribution No. 178). This is A WI contribution No. 1496. Data are available under
www.pangaea.de/Projects/SFB261.
References
Anderson HL, Fran9Qis R, Moran SB (1992) Experimental evidence for differential adsorption ofTh and Pa
on different solid phases in seawater. EOS, Trans
Amer Geophys Union 73, S43:270
Anderson RF (1981) The marine geochemistry of thorium and protactinium. PhD Thesis, WHO!, Woods
Hole,MA,pp 1-287
Anderson RF, Bacon MP, Brewer PG (1983a) Removal
of2 3 0Th and 231Pa from the open ocean. Earth Planet
Sci Lett 62: 7-23
Anderson RF, Bacon MP, Brewer PG (1983b) Removal
of 23 0Th and 231Pa at ocean margins. Earth Planet Sci
Lett 66: 73-90
Anderson RF, Lao Y, Broecker WS, Trumore SE,
Hofmann HJ, Wolfi W (1990) Boundary scavenging
in the Pacific Ocean: a comparison opoBe and 231Pa.
Earth Planet Sci Lett 96: 287-304
Anderson RF, Fleisher MQ, Biscaye PE, Kumar N,
Dittrich B, Kubik P, Suter M (1994) Anomalous
boundary scavenging in the Middle Atlantic Bight:
evidence from 23 0 Th, 231Pa, lOBe and 210Pb. Deep-Sea
ResII41:537-561
Bacon MP (1988) Tracers of chemical scavenging in the
ocean: boundary effects and large scale chemical
fractionation. Philosoph. Trans Roy Soc London A,
320: 187-200
Bacon MP, Anderson RF (1982) Distribution of thorium
isotopes between dissolved and particulate forms
in the deep sea. J Geophys Res 87: 2045-2056
Bacon MP, Rosholt IN (1982) Accumulation rates ofTh230, Pa-23I, and some transition metals on the Bermuda Rise. Geochim Cosmochim Acta 46: 651-666
405
mass flux of particulate matter. This is a result of
the strong latitudinal decrease in the scavenging
preference of230Th over 231Pa, probably related to
the high contents of biogenic opal of sinking particles. As a consequence, the xs231Palx,230Th ratio
cannot be applied as a reliable paleoproductivity
proxy in this region, as variations of the x,231PaJ
x,230Th o ratio through time, usually interpreted to
reflect changes in the total mass flux of particles,
could also be explained by changes in the content
of biogenic opal on sinking particles. The high affinity of231Pa to opal, however, could possibly be
used to trace fluxes of biogenic opal to the sediment that have been totally dissolved (e.g. throughout the Weddell Sea, Walter et al. 1997). Nearly
unpreferential scavenging 0[230Th and 231Pa by
manganese- and iron oxides further limits the use
of the x,231Pal x ,230Th ratio near mid-ocean ridges
and in areas of diagenetic remobilization of manganese.
In conclusion, the x,231Pal x ,230Th ratio can serve
as a proxy for relative changes in paleoproductivity
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 are not significant. The interpretation of the xs231PaI xs 230Th ratio
thus requires synoptic information on its basin-wide
distribution and on past changes in composition of
sedimenting particles. Reliable reconstructions of
changes in paleoproductivity, however, can only be
made in combination with other independent proxies, (like e.g. biogenic barium, authigenic uranium).
Recent improvements in mass spectrometric
techniques (e.g. Chen et al. 1992; Cheng et al.
1996) now allow the study of the distributions of
230Th and 231Pa in the ocean in more detail. Small
sample sizes of only 1-2 liters of seawater (compared to cubic meters with the traditionally alpha
spectrometry), sufficient for the high precision
determination of230Th (Moran et al. 1995; 1997;
Vogler et al. 1998) and 231Pa (Fran90is pers
comm ; Vogler pers comm; Edmonds et al. unpubl
data), promise the detailed investigation of scavenging of both radionuclides even in the surface
ocean, where their concentrations are extremely
low. This new analytical technique could thus provide the required answers to some of the remaining open questions regarding the oceanographic
behavior of 230Th and 231Pa.
Acknowledgments
Heike Hoeltzen and Walter Geibert is thanked for
their help with the radiochemical analyses. We are
grateful to Jan Scholten and Bradley Moran for
their constructive reviews. This research was
funded by the Deutsche F orschungsgemeinschaft
(Sonderforschungsbereich 261 at Bremen University, Contribution No. 178). This is A WI contribution No. 1496. Data are available under
www.pangaea.de/Projects/SFB261.
References
Anderson HL, Fran9Qis R, Moran SB (1992) Experimental evidence for differential adsorption ofTh and Pa
on different solid phases in seawater. EOS, Trans
Amer Geophys Union 73, S43:270
Anderson RF (1981) The marine geochemistry of thorium and protactinium. PhD Thesis, WHO!, Woods
Hole,MA,pp 1-287
Anderson RF, Bacon MP, Brewer PG (1983a) Removal
of2 3 0Th and 231Pa from the open ocean. Earth Planet
Sci Lett 62: 7-23
Anderson RF, Bacon MP, Brewer PG (1983b) Removal
of 23 0Th and 231Pa at ocean margins. Earth Planet Sci
Lett 66: 73-90
Anderson RF, Lao Y, Broecker WS, Trumore SE,
Hofmann HJ, Wolfi W (1990) Boundary scavenging
in the Pacific Ocean: a comparison opoBe and 231Pa.
Earth Planet Sci Lett 96: 287-304
Anderson RF, Fleisher MQ, Biscaye PE, Kumar N,
Dittrich B, Kubik P, Suter M (1994) Anomalous
boundary scavenging in the Middle Atlantic Bight:
evidence from 23 0 Th, 231Pa, lOBe and 210Pb. Deep-Sea
ResII41:537-561
Bacon MP (1988) Tracers of chemical scavenging in the
ocean: boundary effects and large scale chemical
fractionation. Philosoph. Trans Roy Soc London A,
320: 187-200
Bacon MP, Anderson RF (1982) Distribution of thorium
isotopes between dissolved and particulate forms
in the deep sea. J Geophys Res 87: 2045-2056
Bacon MP, Rosholt IN (1982) Accumulation rates ofTh230, Pa-23I, and some transition metals on the Bermuda Rise. Geochim Cosmochim Acta 46: 651-666