Journal of Oceanology and Limnology   2021, Vol. 39 issue(6): 2209-2219     PDF
Institute of Oceanology, Chinese Academy of Sciences

Article Information

ZHANG Xinxing, YANG Weifeng, QIU Yusheng, ZHENG Minfang
Adsorption of Th and Pa onto particles and the effect of organic compounds in natural seawater
Journal of Oceanology and Limnology, 39(6): 2209-2219

Article History

Received Aug. 1, 2020
accepted in principle Nov. 25, 2020
accepted for publication Dec. 24, 2020
Adsorption of Th and Pa onto particles and the effect of organic compounds in natural seawater
Xinxing ZHANG1,2, Weifeng YANG1,3, Yusheng QIU1, Minfang ZHENG1     
1 College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China;
2 North China Sea Environmental Monitoring Center, State Oceanic Administration, Qingdao 266033, China;
3 State Key Laboratory of Marine Environmental Science, Xiamen 361102, China
Abstract: 231Pa and 230Th are two crucial isotopes in the ongoing GEOTRACES Project. However, the controversy on 231Pa/230Th proxy pertaining to archiving ocean circulation or recording paleoproductivity, is still unresolved, partly owing to the unclear understanding of fractionation between 231Pa and 230Th during adsorption. In this study, controlled experiments were conducted to examine the adsorption of 234Th and 233Pa onto biogenic particles (SiO2 and CaCO3), authigenic minerals (MnO2 and Fe2O3), and lithogenic minerals (kaolinite, attapulgite, montmorillonite, and aluminum oxyhydroxides), and the role of organic compounds in regulating the adsorption of 234Th and 233Pa in natural seawater was evaluated. The distribution coefficients (Kd, presented as logKd) varied from 3.56 to 6.05 and from 3.27 to 5.82 for 234Th and 233Pa, respectively. Fe2O3 is the strongest sorbent for both 234Th and 233Pa. Most of the particles showed comparable logKd values for either 234Th (~4.8) or 233Pa (~3.9) in the presence of dextran, indicating that the adsorption of Th and Pa is likely controlled by organic coating on particle surfaces. The fractionation factors (FTh/Pa) of SiO2 (3±1) and CaCO3 (33±1) suggest in situ observed preferential scavenging of 230Th to 231Pa in the surface water of low- to mid-latitude regions and the nearly equal removal in the Antarctic Ocean where biogenic silica dominates the particle regime. The FTh/Pa values of the lithogenic and biogenic particles indicate that 230Th is scavenged prior to 231Pa in the particle-scarce ocean interior. The equal scavenging of 230Th and 231Pa at the ocean margins and the ridge crests is dominated by high particle fluxes instead of particle composition control. These results imply that 230Th/231Pa can be used as different proxies in different oceanic settings.
Keywords: thorium    protactinium    paleoproductivity    circulation    particle dynamics    

230Th and 231Pa are important radionuclides in the ongoing GEOTRACES Project (Pavia et al., 2018; Lerner et al., 2020). In the ocean, the two radionuclides are produced from uranium isotopes. Owing to the even distribution (Lippold et al., 2011) and stable decay of 235U and 234U, the production ratio of 231Pa and 230Th is 0.093 (Lippold et al., 2012). Both Th and Pa are particle-reactive in seawater (Ma et al., 2008; Gdaniec et al., 2018), with Th exhibiting somewhat stronger particle reactivity than Pa (Deng et al., 2018). The removal rate of 231Pa is usually lower than that of 230Th, leading to a longer residence time of 231Pa in the water column (Anderson et al., 1983a, b; Hayes et al., 2013). Therefore, 231Pa is widely distributed along with ocean circulation (Lippold et al., 2011), whereas 230Th is mostly confined within its production location (Gdaniec et al., 2018; Costa et al., 2020). Based on this fractionation during their scavenging, the 231Pa/230Th signal unsupported by the decay of parent U nuclides in sediments was proposed to constrain the ocean circulation (Yu et al., 1996). This signal was used to evaluate the Atlantic meridional overturning circulation strength between the Last Glacial Maximum and the present (Lippold et al., 2012), changes in the deep circulation in the northwest and central South Atlantic (Jonkers et al., 2015; Rempfer et al., 2017), and deep-water masses in the southwest Indian Ocean (Thomas et al., 2006) and the Arctic Ocean (Luo and Lippold, 2015). Some researchers have reported that sedimentary 231Pa/230Th signals record the opal-dominant paleoproductivity in the Southern Ocean (Anderson et al., 2009; Bradtmiller et al., 2009; Kretschmer et al., 2011) and the Equatorial Pacific (Bradtmiller et al., 2006; Costa et al., 2017). These studies suggest that sedimentary 230Th/231Pa contains mixed signals of multiple oceanic processes (Grenier et al., 2019). A controversy regarding the application of 231Pa/230Th has emerged in the last decades (Lippold et al., 2011). Hence, a thorough understanding of the differences in 230Th and 231Pa adsorption onto particulate matter is crucial for determining the use of sedimentary 231Pa/230Th as a paleo proxy (Hayes et al., 2015).

Previous in situ studies on the role of different particulate components in scavenging 230Th and 231Pa showed contrasting conclusions (Luo and Ku, 1999; Chase et al., 2002), probably because of the complex of particle composition that hinders our understanding. Controlled laboratory experiments revealed that different particle types have very different affinity to 230Th and 231Pa, resulting in fractionation, to varying degrees during their adsorption on particles (Guo et al., 2002). A similar scenario was observed in natural seawater simulation experiments (Geibert and Usbeck, 2004). Later, some experiments proved that colloidal particles play an important role in the adsorption and fractionation of Th and Pa in seawater (Lin et al., 2014, 2015). Moreover, phytoplankton-associated biopolymers bind both Th and Pa (Chuang et al., 2013, 2015b). These available studies suggest that both microparticles and nanoparticles (i.e., colloids) affect the adsorption of 230Th and 231Pa in natural seawater, thereby leading to complex interactions between particulate matter and Th and Pa than previously expected.

In this study, we have examined the adsorption of 234Th and 233Pa on different types of microparticles in seawater, including biogenic (amorphous SiO2 and CaCO3), lithogenic (kaolinite, attapulgite, montmorillonite, α-Al2O3, β-Al2O3, and Al(OH)3), and authigenic (MnO2 and Fe2O3) particles. Furthermore, model macromolecular organic compounds (chitin, carrageenan, humic acid, and dextran) were used to determine the binding of organic compounds with 234Th and 233Pa. Dextran (polysaccharide) was used to examine the influence of organic coating on the adsorption of 234Th and 233Pa onto microparticle surfaces. The findings of these controlled experiments further enhance our understanding of the available in situ sedimentary 231Pa/230Th datasets.

2 MATERIAL AND METHOD 2.1 Particle selection and seawater preparation

Ten types of inorganic microparticles and four types of organic compounds were used in the sorption experiments (Table 1). Amorphous SiO2 and CaCO3 represent biogenic silica (SiO2·xH2O) and carbonate, respectively (Yang et al., 2013, 2015b). MnO2 and Fe2O3 were used because iron and manganese can form authigenic minerals in oxidative seawater. Kaolinite, attapulgite, and montmorillonite represent typical lithogenic clay particles, and α-Al2O3, β-Al2O3, and Al(OH)3 were used to determine whether particles with nearly identical chemical compositions but different structures have distinct affinities to a specific particle-reactive nuclide. All the inorganic particles used in the experiments had comparable sizes (mainly 2–5 μm) to avoid the size effect and enable easy comparison of the composition effect. Chitin, carrageenan, and dextran represent polysaccharides and have been previously used to study the adsorption of Th in seawater (Guo et al., 2002; Chuang et al., 2014, 2015a; Lin et al., 2015). Humic acid represents humic substances (Guo et al., 2002).

Table 1 List of particle types and basic parameters used in the adsorption experiments

De-particle and organic seawater was used in the controlled experiments. Natural seawater collected from the South China Sea (salinity 35) was filtered using a precombusted GF/F membrane (Whatman), followed by ultrafiltration using a 1-kDa membrane to remove colloidal particles (Guo et al., 1995). Finally, the seawater was exposed to ultraviolet irradiation for 48 h to decompose residual organic compounds (Lin et al., 2014). Thus, the prepared seawater exhibited characteristics similar to those of natural seawater, except particulate matter was excluded.

2.2 Sorption experiment

The experiments are schematically shown in Fig. 1. First, 50 mL of de-particle seawater, including 1 mL of noncomplexing Tris-HCl buffer solution, was added to a stirrer-cell unit to maintain a stable pH and prevent pseudocolloid formation during the addition of spike nuclides (Roberts et al., 2009). For single-sorbent experiments, 2.0 mg of particles were added, resulting in a particle concentration of 40 mg/L, which is comparable to the particle concentrations in some coastal seawater (Chuang et al., 2013; Yang et al., 2013). Then, 400 Bq of 234Th (milked from 238U) or 233Pa (milked from 237Np) were added dropwise while stirring. The pH value was checked before and after spike addition to ensure it remained constant. 234Th or 233Pa were adsorbed on particles for 2 h while stirring to compare our results with previously reported results (Guo et al., 2002; Yang et al., 2013, 2015b; Lin et al., 2014). For inorganic single-sorbent experiments, 234Th or 233Pa in the particulate phase was separated from the dissolved phase via filtration using a 0.2-μm polycarbonate membrane. All the inorganic particles were larger than 1 μm; hence, a 0.2-μm pore size was sufficient for separating the particles from the seawater. Because carrageenan, humic acid, and dextran generally dissolve in seawater, ultrafiltration is often used to separate them from seawater (Chuang et al., 2014, 2015b). In our study, particulate 234Th or 233Pa in the organic single-sorbent experiments was separated via ultrafiltration using a 1-kDa membrane, as performed by Lin et al.(2014, 2015).

Fig.1 Schematic of the adsorption experiments, including inorganic single-sorbent, organic single-sorbent, and binary-sorbent experiments in natural seawater LSC denotes liquid scintillation counter.

For binary-sorbent experiments, after the addition of a type of inorganic particle, 1 mL of dextran solution was added to reach a final concentration of 5 mg/L (Fig. 1). The seawater was stirred for 20 min to allow inorganic particles to interact with dextran (Yang et al., 2015b). The procedures were similar to those adopted in inorganic single-sorbent experiments. Since 0.2-μm polycarbonate membrane was used to separate particles from the dissolved phase in binary-sorbent experiments, nuclides left in the dissolved phase included both colloidal and truly soluble species which was similar to natural seawater environment. The obtained partition of nuclides between particles and seawater in the presence of dextran would simulate in situ scenario. In our study, both single- and binary-sorbent experiments were performed in duplicate.

2.3 Measurements of 234Th and 233Pa

The activities of 234Th and 233Pa were counted using a liquid scintillation counter (LSC, Tris-Carb 2900TR, PerkinElmer), as performed by Lin et al.(2014, 2015). All particulate and dissolved samples were dried at 100 ℃. Then, 10 mL of the cocktail was added to the samples, and the samples were measured. Within an activity scope of 0–600 Bq, the constant counting efficiency of LSC is 93%. The blank of the scintillation solution was also determined to correct the influence of the background. Samples were counted until a counting error of less than 5% was achieved, depending on their specific activities.

2.4 Distribution coefficient and fractionation factor

The distribution coefficient (Kd) between the particulate and dissolved phases (Honeyman and Santschi, 1989) was used to represent the adsorption of nuclide on various particles. Kd is defined as


where AP and AD denote the particulate and dissolved activities of 234Th and 233Pa in Bq/L, respectively. SPM denotes the suspended particulate matter content in kg/L. Thus, Kd is expressed in units of L/kg. To facilitate discussion and comparison with published datasets, logarithmic Kd i.e. logKd (Table 2) was adopted herein.

Table 2 LogKd values (mean±SD) for 234Th and 233Pa of different particles in the absence (single-sorbent experiment) and presence (binary-sorbent experiment) of dextran, and the fractionation factor (FTh/Pa) between 234Th and 233Pa during adsorption

Fractionation factor (i.e. FTh/Pa) was used to quantify the difference in the affinity of particle types to 234Th and 233Pa, it is defined as


where Kd, Th and Kd, Pa are the distribution coefficients of 234Th and 233Pa, respectively. When FTh/Pa equals 1.0 for a type of particle, this particle has the same affinity to both 234Th and 233Pa; then, no fractionation effect is observed during the adsorption of the two nuclides onto this particle type. Particles with FTh/Pa > 1 tend to preferentially adsorb 234Th than 233Pa. Particles with FTh/Pa < 1.0 exhibit greater affinity to 233Pa than to 234Th.


In the single-sorbent experiments, the logKd, Th values, spanning two to three orders of magnitude, varied from 3.56±0.08 (mean±SD) to 6.05±0.05, on average of 4.98±0.58 (Table 2). The organic matter showed similar logKd, Th values, ranging from 4.96±0.23 to 5.46±0.13 (Fig. 2a). Lithogenic particles showed relatively low logKd, Th values of 3.56–4.97 but larger variability of one to two orders of magnitude. The logKd, Th values of biogenic SiO2, CaCO3, and authigenic MnO2 and Fe2O3 were in the range of 4.64–6.05. In all the experiments, logKd, Pa varied from 3.27 to 5.82 on average of 4.14±0.78 (Fig. 2b). Both organic and lithogenic particles exhibited lower variable logKd, Pa values, ranging from 3.27±0.04 to 4.28±0.05. Conversely, biogenic and authigenic particles showed higher logKd, Pa values, varying from 4.61±0.01 to 5.82±0.01. The FTh/Pa values spanned more than two orders of magnitude, varying from 0.2 to 62 on average of 16±20 (Fig. 2c). Only for SiO2, FTh/Pa was less than 1.0. For all other particles, FTh/Pa was greater than 1.0 (Table 2).

Fig.2 Variations in the logKd values of 234Th (a) and 233Pa (b) and FTh/Pa (c) values of different particle types in natural seawater

In the binary-sorbent experiments, the logKd, Th values varied from 4.30±0.04 to 5.30±0.11, on average of 4.83±0.28 (Table 2), thus showing considerably lesser variability compared with those of pure inorganic particles (i.e., single-sorbent experiments). The logKd, Th values of biogenic and authigenic particles were similar to those of lithogenic particles (4.96±0.29 vs. 4.74±0.25) in the presence of dextran (Fig. 3a). For all inorganic particles with dextran, the logKd, Pa values varied from 3.39 to 4.17, on average of 3.91±0.24 (Fig. 3b). CaCO3 delivered the lowest logKd, Pa value of 3.39±0.16. The logKd, Pa value of all other particles was approximately 4.0. The FTh/Pa values varied from 2 to 33, on average of 11±9, showing an overall narrow range than that in the single-sorbent experiments (Fig. 3c). Notably, FTh/Pa > 1.0 for all particle types, including SiO2, in the presence of dextran.

Fig.3 Variations in the logKd values of 234Th (a) and 233Pa (b) and FTh/Pa (c) values of different particle types in the presence of dextran
4 DISCUSSION 4.1 Adsorption of organic compounds, SiO2, and CaCO3

For the four organic compounds considered in this study, the logKd, Th values were nearly consistent, varying from 4.96 to 5.46 (Fig. 2a). The main components of chitin, carrageenan, and dextran are polysaccharides (Guo et al., 2002; Yang et al., 2013). Polysaccharides represent the main component of organic matter in seawater (Engel et al., 2004; Chuang et al., 2014, 2015b). Probably, owing to their similar ligands, polysaccharides show comparable affinities to 234Th. The narrow logKd, Pa range (3.46–4.28) also implied the similar affinities of the four organic compounds to 233Pa (Fig. 2b). The Martin curve (Martin et al., 1987) indicates that most of the particulate organic matter is confined to the euphotic zone (Yang et al., 2015a, 2016) and the upper mesopelagic water; hence, the microparticulate organic matter absorbing Th and Pa isotopes mainly occurs in the upper oceans. In the deep oceans, organic matter probably affects the adsorption of Th and Pa by means of coating their form on inorganic particles, as observed from the experimental results obtained in the presence of dextran (Fig. 3). All types of organic compounds exhibited stronger affinities to Th than to Pa, with FTh/Pa > 1.0 (Fig. 2c).

The logKd, Th value of CaCO3 was higher than that of SiO2 (Fig. 2a); the relation is reversed in the case of 233Pa (Fig. 2b). Similar results were reported by Guo et al. (2002). The FTh/Pa values of SiO2 and CaCO3 were 0.2 and 2.9, respectively. Controlled experiments on nanoparticles also revealed a similar scenario (Lin et al., 2014, 2015). These results indicate that biogenic silica preferentially scavenged Pa over Th, whereas carbonates tended to adsorb Th prior to Pa. Notably, biogenic silica was the only type of particle that preferentially adsorbed Pa in our study (Fig. 2c). In the presence of dextran, both SiO2 and CaCO3 exhibited logKd, Th values comparable to those of the single-sorbent systems (Figs. 2 & 3). However, dextran significantly weakened the adsorption of 233Pa on both SiO2 and CaCO3 particles (Fig. 3). The logKd, Pa values decreased from 5.42 to 4.11 and from 4.90 to 3.39 for SiO2 and CaCO3, respectively (Table 2). Consequently, SiO2 and CaCO3 showed higher FTh/Pa values than the single-sorbent systems (Fig. 3c). Thus, dextran changed the priority of Pa and Th adsorption on SiO2. Lin et al. (2015) observed similar changes in the case of SiO2 in the presence of humic acid and acid polysaccharide. Dextran was probably coated on the SiO2 particle surface, thereby largely changing the surface characteristics of SiO2 as revealed by the adsorption of 210Po, 210Pb, and 7Be onto SiO2 particles in the presence of organic compounds (Yang et al., 2013, 2015b). Because of the coating effect, SiO2 adsorbed Pa in a manner more similar to dextran; this inference was supported by the comparable logKd values in the single and binary SiO2 experiments (Figs. 2 & 3). Thus, these results indicate that organic compounds influence the adsorption of Th and Pa onto inorganic particles. Conversely, Chuang et al. (2014) reported that biomolecules from diatoms significantly increase the Kd values by one to two orders of magnitude compared with inorganic particles using biogenic SiO2. The biomolecules are obtained from cultured diatoms, representing fresh organics with abundant ligands (Chuang et al., 2014). In our study, the organic compounds were commercial products, which generally have less ligands. Abundant binding sites in the fresh organics may enhance the sorption of Th and Pa onto inorganic particles. Thus, the contrasting results support the conclusion that functional groups or ligands essentially determine the radionuclide sorption ability of particles (Chuang et al., 2014).

The particle concentrations used in our experiments were considerably higher than those in the open oceans. Hence, our results can mainly be used to represent the coastal and estuary scenarios. Owing to the linear decrease in Kd with an increase in particle concentration (i.e., particle concentration effect, Honeyman and Santschi, 1989), our logKd values cannot be compared with the in situ measured values in the open oceans or directly used to discuss the roles of specific particle types in Th and Pa removal. However, the same particle concentration effects were observed for 210Po and 210Pb with the same particle composition (Yang, 2005), indicating that the particle concentration may have similar influences on the adsorption of nuclides with similar particle reactivity. In this study, the Kd ratio between Th and Pa (i.e., FTh/Pa) would be little influenced by the particle concentration. Thus, in our study, FTh/Pa was used to determine the fractionation between 230Th and 231Pa in the upper open oceans where diatoms, dinoflagellates, and coccolithophores dominate the particle regime (Chust et al., 2013). The percentage of coccolithophores in the phytoplankton community usually decreases from tropical and subtropical oceans to high-latitudinal oceans (Chust et al., 2013; Tréguer et al., 2018), as reflected by changes in the biogenic silica content from 40% at the Polar Front to 80% in the southeast Weddell Sea (Walter et al., 2001). According to our results (Fig. 3c), FTh/Pa > 30 for CaCO3, whereas FTh/Pa=3 for SiO2 in the presence of organic compounds. Near the Polar Front, both CaCO3 and SiO2 dominate the scavenging of 230Th and 231Pa; here, the FTh/Pa value reaches 14 (Walter et al., 1997). To the south, the FTh/Pa values decrease with an increase in the biogenic silica content. In the Weddell Sea, biogenic opal dominates the particulate matter (Walter et al., 2001) and the corresponding FTh/Pa value can be very close to that of SiO2 endmember (i.e., 3±1, Table 2). In fact, the measured in situ FTh/Pa value from the euphotic zone of the surface of the Weddell Sea is approximately 1.7 (Walter et al., 2001). Moran et al. (2002) also reported a decreasing FTh/Pa trend from ~11 near the equator and South Atlantic gyre to ~2 in the Southern Ocean. Thus, our results, based on the interactions between particles and nuclides, suggest composition dominance over the fractionation of 231Pa and 230Th in the surface water of the open oceans and the Antarctic Ocean.

4.2 Lithogenic particle adsorption

In the single-sorbent experiments, the three clay minerals (i.e., kaolinite, attapulgite, and montmorillonite) exhibited comparable logKd, Th (4.64–4.97) and logKd, Pa (3.72–4.06) values (Table 2), implying that various clay particles show similar affinities for Th and Pa isotopes. Notably, both logKd, Th and logKd, Pa of α-Al2O3 were discernibly lower, to varying degrees, than those of β-Al2O3 and Al(OH)3; this result agrees with the more active nature of β-Al2O3 (Table 1). Thus, in addition to the chemical composition, the mineral structure also influences the adsorption characteristics of nuclides on particles. The FTh/Pa values of lithogenic particles varied from 2 to 25 (Fig. 2c), indicating their priority for scavenging Th prior to Pa.

In the binary-sorbent experiments, most of the logKd, Th values were approximately 4.8 (Fig. 3a), with an exception of kaolinite, which was characterized by a low value of 4.30±0.04. All the logKd, Pa values were comparable and approximately 4. Overall, in the presence of dextran, the logKd, Th values were an order of magnitude higher than the logKd, Pa values (Fig. 3a & b), yielding FTh/Pa values of 2–14 (average: 7.4). Overall, FTh/Pa values showed a larger range for the single-sorbent lithogenic particles than for binary-sorbent systems; this large range of values may be ascribed to the differences in either composition or structure (Table 1). In the presence of dextran, the coating effects probably diminished the influence of various lithogenic particles, resulting in synergistic adsorption effects. The FTh/Pa range coincided with the range of the typical fractionation factor of 10 in deep oceans (Anderson et al., 1983a; Walter et al., 1997), implying that the binary-sorbent experiments probably revealed the interaction between the particles and Th and Pa nuclides in the ocean interior. Unlike biogenic silica, lithogenic particles are usually refractory and their percentages in the total particulate matter increase with depth (Brewer et al., 1980). The available report on Th adsorption onto lithogenic particles indicates that these particles have considerably less important roles than particulate organic matter and carbonates in the upper ocean (Yang et al., 2009). Conversely, lithogenic particles can be the crucial components for the adsorption of Th and Pa in the ocean interior (Nozaki and Yamada, 1987). Additionally, most 230Th and 231Pa are produced below the euphotic zone (Bacon et al., 1985; Pavia et al., 2018). Thus, generally, the scavenging and fractionation of 230Th and 231Pa in ocean interior can be closely related to lithogenic particles. Most 230Th produced by 234U decay in the open ocean interior is locally removed by adsorption, whereas less than 50% of 231Pa is locally removed, supporting inference of the preferential scavenging of Th over Pa (Anderson et al., 1983b). Using the limited in situ data from the equatorial Pacific Ocean and Atlantic Ocean (Anderson et al., 1983b), the estimated FTh/Pa varies from 3 to 12, which is close to our results. The FTh/Pa values of approximately 10 in the deep central Pacific Ocean (Anderson et al., 1983a) also support the preferential adsorption of 230Th. In the ordinary deep Pacific Ocean and Atlantic Ocean, the high unsupported 230Th/231Pa ratios in sediments are attributed to the less efficient removal of 231Pa over 230Th (Yang et al., 1986). All these in situ datasets agree with our results. They support the expectation that inorganic particles and organic compounds jointly determine the fractionation between 230Th and 231Pa rather than inorganic components alone (Li, 2005).

4.3 Authigenic Fe and Mn oxyhydroxide adsorption

In the single-sorbent experiments, Fe2O3 showed the highest logKd, Th (6.05) and logKd, Pa (5.82) values among all the studied particle types (Fig. 2). Further, Fe2O3 and MnO2 exhibited higher logKd, Th and logKd, Pa values than other clay minerals and Al oxyhydroxides. Under similar experimental conditions, both Fe2O3 and MnO2 showed stronger affinities to particle-reactive 210Pb and 210Po than kaolinite and α-Al2O3 (Fig. 4). A similar scenario was observed in the case of Fe2O3 nanoparticles (Lin et al., 2014; Yang et al., 2015b). Thus, Fe and Mn oxyhydroxides seem to be the most effective nonbiogenic sorbents for Th, Pa, Pb, and Po in natural seawater. However, logKd, Pa of SiO2 and CaCO3 were somewhat higher than that of MnO2 (Fig. 2b), in contrast to that of 210Pb and 210Po (Yang et al., 2013). By considering all the results obtained under similar experimental conditions, it is clear that various particles (i.e., biogenic, lithogenic, and authigenic particles) showed very complex affinities to Th, Pa, Pb, Po, and Be, although these radionuclides were all particle-reactive (Fig. 4). The FTh/Pa values of pure Fe2O3 and MnO2 were 2±1 and 7±1, respectively, indicating that Fe2O3 adsorbed Th and Pa almost equally and that MnO2 preferentially scavenged Th prior to Pa. Anderson et al. (1983b) reported that Fe(OH)3 adsorbs 234Th and 233Pa with a Th/Pa ratio of 0.95–1.25 and MnO2 shows a slight preference for 234Th prior to 233Pa with a Th/Pa ratio of 1.13–1.64; these observations are consistent with our results.

Fig.4 Affinities of various particles to 234Th, 233Pa, 7Be, 210Pb, and 210Po in natural seawater 7Be, 210Po, and 210Pb data are from Yang et al. (2013).

In the presence of dextran, the adsorption ability of both Fe2O3 and MnO2 was reduced (Fig. 3). The affinities of MnO2 to Th and Pa were only slightly lower than that of Fe2O3. Consequently, the FTh/Pa values (17±1) for Fe2O3 and MnO2 were comparable (Fig. 3c). In the presence of dextran, the fractionation between Th and Pa during their adsorption was enhanced compared with that observed for pure Fe2O3 and MnO2 particles (Fig. 2). A similar result was observed for Fe2O3 in the presence of acid polysaccharide (Lin et al., 2015). However, a weakened fractionation was observed for Fe2O3 in the presence of humic acid and protein (Lin et al., 2015). A study reported that the adsorption of Th onto Fe oxides closely depends on the ratio of humic acid to the combined sites of Fe oxides (Reiller et al., 2002). Thus, the synergistic interactions between Fe2O3, MnO2, and organic compounds regulate the fractionation between Th and Pa. Additionally, different organic compounds have different fractionation effects; however, further investigations are required to understand the detailed mechanism. Together with lithogenic particles, the nonbiogenic particles preferentially scavenged Th prior to Pa, with FTh/Pa in the range of 2–17 (Table 2).

Because they are present in trace amounts, generally, very little Fe and Mn oxyhydroxides are observed in seawater, except in the case of seawater over ridge crests near the hydrothermal emanation, e.g., at the East Pacific Rise at 20°S (Shimmield and Price, 1988) and the Mid-Atlantic Ridge (Hayes et al., 2015). Because FTh/Pa > 1.0 for Fe2O3 and MnO2 in the presence of organic compounds, and other particles also show priority adsorption for Th (Fig. 3c), significant fractionation was observed during 230Th and 231Pa scavenging in the presence of Fe and Mn oxyhydroxides. However, the scavenging of 230Th and 231Pa was equally effective during the precipitation of Fe-Mn oxyhydroxide-rich sediments at 20°S, i.e., the East Pacific Rise; this behavior remains unexplained (Shimmield and Price, 1988). Further, fractionation between 230Th and 231Pa during scavenging has not been observed near continental margins (Anderson et al., 1983a; Shimmield et al., 1986). For example, the unsupported 231Pa/230Th ratios of approximately 0.5 in the water column (Nozaki and Yamada, 1987) are similar to the ratios of 0.5 observed in the surface sediment of the Japan Sea (Yang et al., 1986). Interestingly, these regions without the fractionation of 230Th and 231Pa also show high sediment accumulation rates (Boström et al., 1973), thus supporting the inference that in addition to particle composition, particle flux is a principle factor influencing 230Th and 231Pa scavenging from the water column (Lao et al., 1993; Lippold et al., 2011; Pavia et al., 2018). Probably, the adsorption capacity of particles for nuclides depends on both their affinity to nuclides and the abundance. Abundance is a dominant factor for scavenging when particles are sufficiently abundant for scavenging nearly all the nuclides, as observed at the ocean margins and ridge crests (i.e., flux domain) (Gdaniec et al., 2018). Under these circumstances, the scavenging of 230Th and 231Pa is only slightly influenced by the particle composition (Nozaki and Nakanishi, 1985). Conversely, affinity can play a predominant role when particles are too scarce for removing all nuclides, as observed in the open ocean (i.e., the composition domain). Thus, the fractionation induced by the particle composition drivers the preferential scavenging of Th prior to Pa.


Controlled experiments provide valuable information for understanding the adsorption and fractionation of Th and Pa in natural seawater. The comparability between our results and the available in situ FTh/Pa values indicated that the experiments largely mimic the adsorption of Th and Pa onto particulate matter. With the addition of organic compounds, most of the particle types (i.e., biogenic silica and carbonate, authigenic Fe and Mn oxyhydroxides, and lithogenic minerals) tend to show similar adsorption characteristics for Th or Pa, although these particles have significantly different affinities to Th or Pa in the absence of organic compounds. Thus, the synergistic interactions between inorganic particles and organic compounds jointly determine the adsorption of Th and Pa in natural seawater rather than inorganic particles. Our results support the fractionation between 230Th and 231Pa observed in different oceanic settings. In the surface ocean with abundant biogenic particles, silica and carbonate preferentially or equally scavenge 230Th prior to 231Pa depending on their abundance. Generally, the ocean interior with scarce particles, lithogenic, probably together with residual biogenic particles, results in preferential scavenging of 230Th prior to 231Pa. At the ocean margins and ridge crests with sufficient particles, the total particle flux dominates the equal scavenging of both 230Th and 231Pa rather than the particle composition. These conclusions imply that 230Th/231Pa can be used to constrain different oceanic processes based on the different fractionation mechanisms.


The datasets generated in the current study are presented in Tables 1 & 2.


The authors thank two anonymous reviewers for their insightful suggestions and Dr. Peng LIN for his help during the de-particle seawater preparation.

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