Scavenging and fractionation of thorium vs. protactinium in the ocean, as determined from particle–water partitioning experiments with sediment trap material from the Gulf of Mexico and Sargasso Sea
Introduction
Thorium and protactinium isotopes have long been used as tracers of oceanic processes. In particular, the ratio of the two longer-lived 231Pa and 230Th, have been used to assess boundary scavenging, ocean circulation, and paleoproductivity (e.g., Anderson et al., 1983a, Anderson et al., 1983b, Kumar et al., 1995, Walter et al., 1997). However, thorium to protactinium ratios in suspended or sinking particles can vary as a function of location, depth and size (Walter et al. 2001). Based on field or laboratory experiments, there are several mutually exclusive theories as to what causes the fractionation between these two isotopes. Most maintain that the cause of Th/Pa fractionation lies in the type of major inorganic sorbents exhibiting different affinities to Pa vs. Th. Anderson et al., 1983a, Anderson et al., 1983b attributed Th/Pa fractionation effects to the role of MnO2. Some, like Taguchi et al., 1989, Rutgers van der Loeff and Berger, 1993, Walter et al., 1997 attribute fractionation effects to preferential removal of Pa by opal, while Luo and Ku (1999) proposed that they are due to changing biogenic opal to clay ratios, or to the clay content (Luo and Ku, 2004). The work by Chase et al., 2002, Chase et al., 2003, on the other hand, suggested that Th/Pa fractionation is due to changing opal to carbonate ratios. Finally, Li (2005) pointed out that one cannot ignore the role of organic matter in controlling Th/Pa fractionation. As a consequence of Li's suggestion, one might expect that Pa(V) is first reduced to the more particle reactive Pa(IV) by redox-active moieties in natural organic matter, e.g., hydroquinones (e.g., Lovely and Blunt-Harris, 1999), allowing it then to be effectively scavenged from the water column.
Work by our group has investigated the importance of different types of macromolecular organic matter on the sorption of actinides onto organic or inorganic particles and have come to the conclusion that it is not only the presence of organic moieties that control the extent of adsorption onto particles, but especially the type of organic matter. For example, laboratory experiments with varying types of organic matter showed that the polysaccharide-enriched fraction of marine colloidal organic matter (COM) had a higher partition coefficient (Kdc) for Th than that of the bulk COM or other known mineral sorbent. Interestingly, the log Kc value for Th of this natural polysaccharide-rich organic material is very similar to the highest value obtained from model acid polysaccharides (Quigley et al., 2002). Further evidence for the importance of organic moieties in controlling Th removal from the ocean is reviewed in Santschi et al. (2006).
Like thorium, protactinium is also a highly particle reactive element. However, the chemistry of protactinium is more complex in large part because it has three possible oxidation states (IV, V, VI) in natural waters (Alhassanieh et al, 1999). The predominant form is likely Pa(V) in oceanic systems (Choppin, 1983, Choppin, 2003, Choppin, 2007), which, as a PaO2+ ion, is quite soluble, can be complexed by F− in seawater, and sorbs to particle surfaces to a lesser extent than the four-valent oxidation state ion, Pa(IV). It is likely that Pa(V) must first be reduced to Pa(IV) before stronger adsorption and removal from natural waters can take place, as has been seen for Np(V), which is reduced in the presence of natural organic matter such as humics (Zeh et al., 1999). Organic ligands such as extracellular polymeric substances (EPS) produced by bacteria and phytoplankton are another possible source of reductants. Very little is known not just about the complexation of Pa with EPS, but also about their redox behavior in aquatic systems. What is known is the redox potential for Pa(V) to Pa(IV) reduction is −0.05 V, as shown below.Pa(V)O2+ + 2H2O + e− = Pa(IV)(OH)4; Eho = − 0.05 V (Greenwood, N.N., and Earnshaw, A. 1998.)Biologically mediated redox reactions, as well as abiotic reactions by simple organic molecules, such as aldehydes, ferridoxin, and semiquinones, have Eho(w) values below −0.05 V at pH of 7 (Stumm and Morgan, 1996), and thus, could be capable of reducing Pa(V) to Pa(IV).
In order to determine the possible carrier phases that cause differential scavenging of thorium and protactinium, i.e., their fractionation, this study compared log Kd values of these radionuclides that were determined from laboratory tracer experiments on board ship a using freshly collected sediment trap material from the upper 150 m in the Gulf of Mexico or in the laboratory using archived Ocean Flux Program (OFP) sediment trap material collected at mesopelagic and bathypelagic depths (500, 1500, and 3200 m) in the Sargasso Sea offshore Bermuda. We then compare our data with those from the literature.
Section snippets
Gulf of Mexico (GOM)
In May 2006, surface sediment traps (65, 90, 120, and 140 m water depth) were deployed for a 48 hour period at three stations in the Gulf of Mexico (Table 1). Station 1 was in a Cold Core Eddy (25° 52.31′N, 92° 31.10′W), Station 2 was in a Warm Core Eddy (26° 54.59′N, 89° 59.45′W and Station 3 in a Cold Core Eddy with coastal upwelling features (27° 52.11′N, 88° 38.448′W). After recovery, material in the plastic tubes was allowed to settle before draining off the surface water. The remaining
Gulf of Mexico (GOM)
Log Kd values in the GOM ranged from 5.22 to 6.62 for 234Th and 3.96 to 4.66 for 231Pa. Log Kd values of Th and Pa increased with depth, except at Station 3 (Fig. 1), which exhibited coastal upwelling and lateral transport features. The log Kd values of the radioisotopes are significantly correlated with each other (p < 0.05 for 233Pa vs. 234Th), with log Kd(Th) > log Kd(Pa).
When the colloidal fraction is included with the particulate (log Kdc), the log Kdc values for 233Pa become closer, but not
Discussion
As can be seen from Fig. 2, log Kd values for both isotopes and from both GOM and OFP agree well with previously determined data from sediment trap material from the Atlantic Ocean (Nyffeler et al., 1984). When all log Kd(Th) values are plotted against log Kd(Pa) values, the F(Th/Pa) values cluster around 1 at higher Kd values, indicating little fractionation between the two radionuclides, while at lower Kd values, F(Th/Pa) values increase to a maximum of 42 (Fig. 2). The fact that the GOM
Conclusions
Potential carrier phases for Th and Pa nuclides in the ocean that had previously been proposed include calcium carbonate, Fe and Mn oxides, lithogenic and organic carbon phases. Using correlations with data from radiotracer partitioning studies to sediment trap material from the Gulf of Mexico and the Ocean Flux Program off Bermuda, only an organic carbon phase, i.e., the polysaccharide fraction of organic carbon, correlated with Log Kd values of Th and Pa radioisotopes. Both log Kd(Th) and log
Acknowledgements
This work was funded, in parts, by NSF project OCE-0351559 to PHS, KAR and CCH, the Welch Foundation (Grant number BD-0046) to XC, and NSF projects OCE-0325627, OCE-0509602 and OCE-0623505 to MHC. We wish to thank the crew of the R/V Seward Johnson for all of their help in the Gulf of Mexico.
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Present address: Savannah River National Laboratory, Aiken, SC 29808, United States.