Sources of rare earth elements in shells and soft-tissues of bivalves from Tokyo Bay

Accepted Manuscript Sources of rare earth elements in shells and soft-tissues of bivalves from Tokyo Bay

Tasuku Akagi, Keisuke Edanami PII: DOI: Reference:

S0304-4203(16)30125-6 doi: 10.1016/j.marchem.2017.02.009 MARCHE 3444

To appear in:

Marine Chemistry

Received date: Revised date: Accepted date:

12 September 2016 22 February 2017 28 February 2017

Please cite this article as: Tasuku Akagi, Keisuke Edanami , Sources of rare earth elements in shells and soft-tissues of bivalves from Tokyo Bay. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Marche(2017), doi: 10.1016/j.marchem.2017.02.009

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Sources of Rare earth elements in shells and soft-tissues of bivalves from Tokyo Bay. Tasuku Akagia and Keisuke Edanamib a

Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 744

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Motooka, Nishi-ku, Fukuoka 819-0395, Japan Environmental Science on Biosphere, Tokyo University of Agriculture and Technology,

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3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan

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* Corresponding author E-mail address: [email protected] Keywords: bivalves, rare earth elements, shells, soft tissue, seawater, sediments,

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suspended particles, distribution coefficients

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Abstract

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Five species of bivalves (Japanese littleneck, blue mussel, trough shell, Stimpson’s quahog, and Japanese dosinia) were collected from three different sites in Tokyo Bay and their shells and soft tissues were analyzed for rare earth element (REE)

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concentrations. Their REE compositions were compared with those of sediment and seawater at corresponding sites. To estimate the digestive strength of the bivalves,

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different acids were used to extract the elements in sediment for comparison of REE compositions. The REE concentrations of soft tissues were higher than those of shells. The REE abundance patterns of both shells and soft tissues resembled those of sediments or suspended particles rather than those dissolved in seawater, except for mussels. REE composites calculated by mixing particulate and dissolved REEs at various ratios can explain the REE composition of both shells and soft-tissues. In the case of mussel shells, the contribution of REEs dissolved in seawater is important, but despite this most of mussel REEs similarly originate from suspended particles in seawater. This implies that a simple projection to seawater REEs from the REE composition of shells would be difficult. The apparent partitioning coefficients of REEs over Ca in shells ranged from 0.1

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to 10 against REEs/Ca of seawater. 1. Introduction Rare earth elements form a geochemically interesting group. Due to the similar chemical property of their trivalent ions, they act coherently in geochemical processes,

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providing REEs with a basis for a set of geochemical tracers. Their abundance relative to shale or chondrite is often expressed on a logarithmic scale known as a “Masuda-Coryell

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plot” and has been used as a fingerprint inherited from an REE source (Coryell et al., 1963; Masuda, 1962). The usefulness may not be limited to geological problems and REE

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fingerprints may be applicable to ecological samples (Akagi et al., 2004; Almeida and Vasconcelos, 2003; Costas-Rodríguez et al., 2010; Fu et al., 2004; Fu et al., 2000;

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Ichihashi et al., 1992; Laveuf and Cornu, 2009). However, only few reports have been made for the application of the REE fingerprint to ecological or biological problems. One of the primary reasons for the limited application to those problems may lie in unknown

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mechanism of the incorporation of REEs by plants or animals and, thus, in the absence of knowledge of fractionation across REEs. The sources of REEs in biocarbonates have been considered to be those dissolved in seawater. REEs in coral samples have been

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studied and partitioning of REEs between coral and seawater were reported (Akagi et al.,

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2004; Sholkovitz and Shen, 1995). REE composition of reefal microbialites are suggested to be a proxy of surface seawater (Webb and Kamber, 2000), but the resemblance in REE compositions between surface seawater and groundwater poses some questions on the

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proxy (Johannesson et al., 2006). Mussel shells were analyzed for REEs to estimate REE composition or condition (such as pH or temperature), which affects the activities of

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dissolved REE species, of ancient seawater, assuming that dissolved REEs in seawater are the exclusive source of REEs for mussel (Bau et al., 2010; Ponnurangam et al., 2016). Diagenetic modification may degrade the potential of biocarbonate REE composition as a proxy of seawater REEs (Scherer and Seitz, 1980). The sources of REEs in shells need to be understood to correctly apply REE compositions to solve ecological or geochemical problems. In this study REEs in bivalves have been closely studied together with their environmental media such as seawater, suspended particles, and sediment. Bivalves are known to incorporate nutrients from particles/solids by filtering/percolating water through their siphons. The extent to which REEs are released from solid matter in their

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digestive organs should influence the REE composition of bivalves. Therefore, close comparison between REEs in bivalves and those of different extractabilities in surface sediment using different acids was implemented together with those dissolved/suspended in seawater to roughly estimate the digestive strength of bivalves and to understand the

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sources of REEs in bivalves. 2. Method

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2.1 Samples

Bivalve specimens were collected from three sites in Tokyo Bay, whose

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sediments differ from each other in substrate properties (Fig.1). At Daiba site (referred to as Site D hereafter) two species, Japanese littleneck (Ruditapes philippinarum),

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Stimpson’s quahog (Mercenaria stimpsoni) were sampled form the muddy floor in July, 2004. At Ogijima site (Site O) Japanese littleneck and blue mussel (Mytilus galloprovincialis) were sampled in July, 2004. Japanese littleneck was collected from the

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white calcic sandy floor, whereas mussel was sampled from floating nets over the seafloor. At Kanazawa-hakkei site (Site K), Japanese littleneck, trough shell (Mactra veneriformis), and Japanese dosinia (Phacosoma japonicum) were sampled from the

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sandy floor in May 2005. Shells and soft-tissues were separated from the specimens and

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were washed with Milli-Q water before being dried at a room temperature. At the same time of bivalve sampling, seawater and sediment samples were collected from the three sites: seawater from 10 cm above the seafloor and sediments

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from the depth of 0 and 10 cm below the seafloor.

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2.2 Pretreatment of shells

Throughout the method section, all acid solutions were purified by sub-boiling

3 times from corresponding super analytical grade reagents purchased from Wako Chemical Co.Ltd. Ammonium solution was once purified by evaporation-condensation from its analytical reagent purchased from Wako Chemical Co. Ltd. Each shell was broken to pieces with a hammer and was washed with 0.1 M HCl to remove metals adsorbed on the surface. Crushed shell was then dissolved in a 10 M HNO3 and 6M HCl mixture at ca. 80˚C on a hot plate. The solution was subject to centrifugation and only the supernatant solution recovered. The supernatant solution was evaporated to dryness and was dissolved with HNO3 solution to adjust the pH of the

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solution to 3.0 with an NH3 solution before subsequent solvent extraction of REEs. 2.3 Pretreatment of soft tissues Each soft tissue sample was combusted in a Ni crucible at 300 ˚C for 1 hour, 550 ˚C for 2 hours, and 700 ˚C for 1 hour sequentially. This ashed sample was then

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dissolved with a 10 M HNO3 and 6 M HCl mixture at ca. 100˚C on a hot plate. The solution was centrifuged with only the supernatant solution recovered which was then

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evaporated to dryness. This dry residue was dissolved with HNO3 solution to adjust pH of

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the solution to 3.0 with an NH3 solution before subsequent solvent extraction of REEs. 2.4 Pretreatment of sediments

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Sediment samples were dried at room temperature. 20 g of a dried sediment sample was placed in a polyethylene bottle and was shaken well with 30 ml Milli-Q water for 1 minute. The bottle was kept stationary for 5 minutes. The solid in the supernatant

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was collected as suspended solids in sediment and the separated solid was collected as non-suspended solids in sediment. Both the solids were dried and ashed by heating successively at 300 ˚C for 1 hour, 550 ˚C for 2 hours, and 700 ˚C for 1 hour in a Ni

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crucible. Each of ashed samples (suspended and non-suspended) was treated with four

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different acids (5 ml of 5 M HF, 10 M HNO3 + 6M HCl, 1 M HNO3 and 50% acetic acid (HAc)) + 10 ml of water to 1 g of non-suspended solids and to the collected suspended solid samples, separately on a hot plate. This treatment with different acids was intended

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to gain an estimate of the digestive strength of bivalves during percolation of solid matter. The leaching with HF facilitates the release of all REEs in solids, that with HCl+HNO3

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and HNO3 to release REEs from carbonate, oxide and some silicates with different acidities, and that with HAc to release REEs only from carbonates. Because the sediments had been ashed, any REEs in organic matter should be dissolved in HCl+HNO3 leaching. Each supernatant acid solution was separated with centrifugation and dried again on a hot plate and prepared as a pH 3 HNO3-NH3 solutions for further solvent extraction. The mass of the residue in each leaching treatment was measured after washing the residue with Milli-Q water and drying. The weight reduction was used as the basis of REE concentration. Site O sediment, which mostly consisted of broken shells, was firstly treated with 1 M HNO3 solution to remove shell-derived matter. The residue was dissolved by 5

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M HF acid solution, dried again and finally dissolved to prepare a pH3 HNO3 solution. 2.5 Pretreatment of seawater samples Seawater samples were filtered with Nuclepore filters (pore size 0.4 m) immediately after sampling and the filtered seawaters were spiked with 10M HNO3 to

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adjust pH of water just below 3 for short storage. The filters, on which suspended particles were collected, were ashed at 300 ˚C

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for 1 hour, 550 ˚C for 2 hours, and 700 ˚C for 1 hour in a Ni crucible. The solids after ashing were further dissolved using 5M HF, 10 M HNO3, and 6 M HCl solutions on a hot

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plate. After being evaporated to dryness, the solution was prepared at pH 3 with HNO3

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and NH3 solutions

2.6 REE preconcentration and ICP-MS determination

The basic method employed here was the C18 mini-column method that was

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originally developed for seawater analysis (Shabani et al., 1992) and further modified for coral analysis (Akagi et al., 2004). It was further modified in this study. Each C18 cartridge was pretreated with 20 ml of 6 M HCl acid and then loaded with a 0.2 g mixture

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of 65% 2-ethylhexyl hydrogen phosphate and 35% 2-ethylhexyl dihydrogen phosphate.

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Two of the loaded C18 cartridges were connected in a series to avoid a loss of REEs in preconcentration. Then 10 mL of 6 M HCl acid and 200 mL of Milli-Q water were passed through the cartridges using a peristaltic pump. Subsequently the sample solution was

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then passed at a flow rate of 5 ml/min. Matrix ions were washed off by passing 40 ml of 0.1 M HCl solution at the rate of 6.5 ml/min. REEs were then eluted into a Teflon beaker

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with 50 ml of 10 M HCl solution at a flow rate of 4.0 mL/min. The solution was evaporated nearly to dryness to remove HCl. A small amount of nitric acid was then added and the solution was again evaporated to dryness. The residue was dissolved in 6 mL of Milli-Q water with 0.5 ml 10 M HNO3 solution and a known amount of indium was added as an internal standard for inductively coupled plasma mass spectrometry (ICP-MS) measurements. By this procedure the recovery exceeded 90% for REEs except La, Yb and Lu, with 99.9% removal of Ba. The recoveries of La, Yb and Lu are around 80%. The ICP-MS used was an ICP-MS 7500 (Agilent Co. Ltd.). A water-cooled chamber was used to suppress oxide formation. Correction for oxide interference

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(although quite small) was carried out using oxide interference-factors (typically MO+/M+ < 0.002), which were determined by measuring several REE solutions on the same day of sample measurement. The presence of 135Ba and 137Ba in the final solutions was depressed to a level comparable to 151Eu and 153Eu, so that the oxide interference of BaO is less than 1% of Eu. The relative standard deviation of measurement is typically

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less than two percent when counts per second (CPS) > 3000. For each sample, a separate column separation was carried out with known amounts of REEs being added to an

3. Results

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3.1 Concentration of REEs in shells of bivalves

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for report of the concentration data in Tables 1, 2 and 3.

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aliquot of the sample to measure the procedure recoveries. The recoveries were corrected

The analytical results of shells of bivalves are summarized in Table 1. The concentration levels vary among species and sites mostly within a factor of ten except for

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blue mussel at Site O. The higher concentration of REEs was seen in the shell of Japanese dosinia at Site K than in other species. Mussel showed the lowest. Japanese littleneck tended to show lower concentration, but it showed the highest one in Site D. Their

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shale-normalized patterns are shown in Fig. 2a. The REE composition of Post-Archean

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Australian Shale (PAAS) (McLennan, 1989) are used for the normalization. The patterns are generally similar to each other except for a mussel from Site O. All shells but the mussel show an increase from La to Gd, a decrease at Tb, a plateauing or slight increase

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from Tb to Er and then a decrease from Er to Yb or Lu. In the case of the mussel the pattern is very different from those of other shells and displays lower overall

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concentrations which decrease from La to Eu, spike at Gd, decline at Tb and the gradually increase to Tm, dip at Yb and increase to Lu. There seems to be no correlation of shell colour with the concentration. 3.2 Concentration of REEs in soft tissues of bivalves The analytical results of soft-tissues of bivalves are summarized in Table 2. Compared with corresponding shells, the concentrations in soft tissues are about 10 times higher. The concentration levels were slightly higher at Site K than at Site D. The shale-normalized patterns of soft-tissues (Fig. 2b) are very similar irrespective of species and sites. They are featured with an inverse-V shape with a maximum at Gd and typically

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with a negative Ce anomaly. At both the end of the REE span, La and Lu tended to be greater than or similar to next Ce and Yb, respectively. Soft-tissue of mussel was not analyzed. 3.3 Concentration of REEs in sediment and seawater

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The analytical results of sediment and seawater samples are summarized in Table 3. Their shale-normalized REE patterns are shown in Fig. 3. Generally speaking the

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patterns of any phases in sediments are stereotypical and are featured with an inverse-V shape except for Ce. Scrutiny of the patterns identifies the appearance of positive Eu

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anomaly in the HF leachate (bulk) of non-suspended solids and the greater bend of the inverse-V shape in the acetic acid leachate of non-suspended solids. The non-suspended

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solids of sediment showed widely-varying concentration among the REEs leachable with different acids, with leachates of weaker acid (HAc) being more enriched in REEs. In contrast the suspended solids of sediment tended to show a rather similar concentration

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range within a factor of 10. The two stronger acids (HCl+HNO3 and HNO3) gave more or less similar REE patterns. The maxima usually occur at Eu, but in some patterns at Gd. At Site D, depletion of HREEs is more conspicuous than at Site K. Ce anomalies are not

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consistently seen, but stronger acid leacheates such as HCl+HNO3 tended to show smaller

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Ce anomalies.

The pattern of dissolved REEs in seawater (Fig. 4) was markedly distinct from those of sediments. HREE enrichment and a strong Gd anomaly are general features

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commonly seen in seawater at all sites. Both the features are most conspicuously seen at Site D. At Sites O and K, a marked positive Ce anomaly is observed. On the contrary to

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dissolve REEs in seawater, the patterns of suspended particles in seawater are featured with the inverse-V shape and small positive Eu anomalies, and are generally similar to those of the suspended solids in sediments (Fig. 3) with the exception of some positive Ce anomalies. 4. Discussion 4.1 Inverse-V-shaped REE abundance pattern of sediment and bivalves It should be noted that our leaching experiment is not sequential leaching, and the all leachates (HAc, HNO3, HCl+HNO3 and HF leachates) contain REEs in the most soluble carbonate. Most of the samples analyzed in this study showed distinct

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inverse-V-shaped patterns (Figs. 2, 3 and 4). The acetic acid leachate of non-suspended solids gave one of the most typical inverse-V-shaped patterns with a distinct break at Gd (Fig. 3). The acetic acid dissolution also gave the highest concentration of REEs per reduced weight (Table 2). On the contrary HF leachate (bulk) samples tended to show the slightest and least defined inverse-V-shaped patterns (Fig. 3 and Table 2). These imply

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that carbonate phases have the distinct inverse-V-shaped pattern and also concentrated REEs much more than the other phases. It is known that carbonate accumulates REEs

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abiotically during diagenesis (Scherer and Seitz, 1980) and from experimental solutions (Lakshtanov and Stipp, 2004; Tanaka and Kawabe, 2006; Zhong and Mucci, 1995). The

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typical adsorption/partitioning pattern (Lerche and Nozaki, 1998; Sholkovitz et al., 1994; Tanaka and Kawabe, 2006; Zhong and Mucci, 1995) exhibits a slight increase or plateau

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from La to Sm or Gd and a sudden decrease from Gd to Lu. The inverse-V shape is a result of the accumulation of LREEs and MREEs by carbonate from seawater. Because seawater is depleted with LREEs compared with shale, the accumulated REEs in

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carbonate, when they are normalized to shale, assume an inverse-V shaped pattern. Carbonate fractions of settling particles and suspended particles in open seas are the examples (Akagi et al., 2011; Lerche and Nozaki, 1998; Sholkovitz et al., 1994). It is

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likely that REEs accumulated in carbonate within sediment or suspended particles by

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abiotic processes are almost unselectively incorporated by bivalves, whereby the inverse-V-shaped finger print is copied to shells and soft-tissues of bivalves. The minor features observed in the inverse V-shaped patterns are increases at

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both the ends of the REE span (La and Lu). The features are more pronounced in the soft-tissues than in shells. The recoveries of the two elements during the column

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separation are slightly poorer than other REEs. These can be due to analytical artifacts, but possibility of W-type tetrad/octad effects + negative Ce anomaly may remain. Here, a W-type tetrad effect assumes five apices at La, Nd-(Pm), Gd, Ho-Er, Lu and octad effect three apices at La, Gd, Lu (Masuda et al., 1987). Tetrad effect emerges in the partitioning pattern between complex with organic or carbonate chelates and aquo-complex (Kawabe et al., 1998) and association of water in the REE assimilation of bivalves can be considered. 4.2 Comparison of REE abundance patterns of bivalves and those of their environment (sediment and seawater)

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The comparison of the REE patterns of bivalves (Fig 2 a and b) with those of sediment (Fig. 3) and seawater (Fig. 4) leads to the following remarks: 1) All the bivalves dwelling on the seafloor exhibit only small Gd anomaly (either shells or soft tissue analyses), indicating that the dissolved REEs in seawater are not the major source of REEs.

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2) All the bivalves dwelling on the seafloor exhibit slight Eu anomalies (either in shells or soft tissues). This indicates that HF leachate of sediment (mostly as non-suspended

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solids) does not comprise the major REE sources.

3) Unlike dissolved REEs in seawater with positive Ce anomaly, soft-tissues show

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negative Ce anomalies and Ce anomaly is absent in shells, which indicates that dissolved REEs may not be major sources and that some REEs leached by stronger

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acids such as HCl or HNO3 may be the sources.

The first remark presents the important implication that REEs in bivalves cannot be used as a proxy of REEs dissolved in seawater. The second remark means either that the

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acid bivalves secrete is too weak to dissolve the bulk of sediments and/or that bivalves incorporate most of REEs from suspended particles in seawater. The third remark implies that the acid bivalves secrete may be stronger than acetic acid. Because Ce can assume

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tetra-valency, it is likely that Ce has been excluded during the assimilation. Therefore, the

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Ce anomaly may not be important. The sheer absence of a Ce anomaly in any shells, however, is likely to imply that REEs in particulate matter or a not-readily-leachable phase of sediment are their sources, and a negative Ce anomaly in soft tissues would

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imply that Ce of the REEs thus taken may further have segregated. Again in our leaching scheme all treatments (acetic acid, HCl+HNO3 and HF

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treatments) contain REEs in the most soluble and influential carbonate with an inverse-V feature. The REE abundance patterns of acid leachates other than HF leachate of the non-suspended solids in sediments are similar to each other. Therefore, it is difficult to further specify which fraction of particulate matter or sediment beside carbonate is selectively incorporated by bivalves. There seems to be no consistent phase of sediment, whose concentration levels vary in parallel with the concentration of shells or soft-tissues. Hereafter, the possibility of two sources (sediment and seawater) separately examined with respect to REE composition. (sediment) The REE patterns of shells seem slightly more similar to those of bulk (HF leachate) suspended solids of sediments than, and those of soft tissues to those of acetic

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acid leachate non-suspended solids of sediments, than the others, with respect to break around MREEs and Eu anomaly. In Fig. 5a and 5b, the relative abundance (shells/bulk suspended sediment and soft tissues/carbonate of non-suspended sediment) is shown in a logarithmic scale. Anomalies of Eu and Gd and marked bend in the middle of the REE span are somewhat subdued. Ce is apparently anomalous. However, a Ce anomaly is not

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considered as a significant indicator unless it is positive, since bivalves can reject Ce(IV) during assimilation, creating negative Ce anomalies. The rather uniform trends with or

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without a tetrad/octad feature can be seen.

(seawater) Because for bivalves food sources are suctioned with seawater through their

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siphons, it is reasonable to consider that REEs in suspended particles or dissolved REEs are the sources of REEs in the shells. Dissolved REEs in seawater show a strong Gd

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anomaly (Fig. 4). It has been reported that Gd is widely released into rivers via human urine in organic complexes, which are derived from medical doses used in contrasting agents for magnetic resonance imaging (Bau and Dulski, 1996). In this interpretation, a

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Gd anomaly is defined as the aberration of Gd from its neighbors (Eu and Tb) in the PAAS normalized REE patterns, most of the shells and soft tissues in the studies gave small or distinct (only for mussel shells in Site O) positive values. The Gd anomaly may

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not be a part of the tetrad/octad effect, but may be inherited from seawater in this

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interpretation. Assuming that the Gd anomaly is taken as an exclusive signal of seawater, we can calculate the contribution of REEs in seawater to the diet. This discussion should definitely be applied to the mussel specimens, which were taken from nets above the

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seafloor at Site O. The comparison of shells and soft-tissues with composite mixtures is shown in Fig. 5c and 5d. Composites were made by mixing REEs dissolved in seawater

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and those in suspended particles collected each site to eliminate the Gd anomaly in sample/composite patterns. Although seawater concentrations and, thus, its extent of the Gd anomaly are only a transient measurement and may not be representative for the long-term growth, it is interesting to note that the same mixture almost eliminates the Eu anomaly, which may be inherited from suspended particles (Fig. 4). The partitioning patterns of shells look fairly consistent irrespective of sites and species, except for mussel at Site O (Fig. 5c). They show rather flat patterns around MREEs, although the mixing ratios of particulate : dissolved REEs varied from 200:1 to 3:1 on Nd basis. Ce is again anomalously negative. The REE composition of soft-tissues also gave consistent partitioning patterns against composites irrespective of sites and species, and display

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marked LREE enrichment and negative Ce anomalies (Fig. 5d) again with mixing ratios being varied from 100:1 to 1.5:1. The ratios are not explained by the concentrations of particulate matter in seawater (Table 3), although the concentrations may not be representative for the long-term growth. The composites for site K were composed with much higher ratios of seawater than those for sites O and D (Fig. 5c and d). Site K, with

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sandy floor, is less polluted than other sites and the suspended particles at the site contains the highest concentrations of REEs.

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Exchange of particles between floor and seawater should be very active and the suspended particles may be almost identical to suspended solids of sediments in

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composition. Such particles, which may not be represented merely by carbonate and may contain silicate or oxide, are major sources of REEs to most bivalves together with

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dissolved REEs in seawater as subsidiary sources.

The results of this study indicates considerable amounts of REEs (1/8 on Nd basis) are taken from dissolved REEs in seawater by mussels (Fig. 5c). Mussels are

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widely used to monitor the chemical environment (Farrington et al., 1983; Goldberg, 1986). One of the reasons for this is their specific dietary needs from seawater. However, mussels intake REEs not only from seawater but most of them from suspended particles.

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Therefore, a simple projection to seawater REEs from the REE composition of shells

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would be difficult, unless (i) suspended particles consist mostly of carbonate matter, (ii) that this carbonate has accumulated REEs in equilibrium with REEs dissolved in seawater, and (iii) the bivalves would incorporate Ca and REEs indiscriminately from the particles

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at the same time. Meeting the three requirements would seem rather unlikely in view of

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the REE distribution coefficient discussed next. 4.3 REE distribution coefficients of bivalve shells against seawater We concluded that most REEs in bivalves are not from seawater, but rather

from solid matter. We have calculated the distribution coefficients of REEs with respect to Ca in bivalve shells against local seawater, though they may simply be a guideline to show the extent to which bivalves accumulate REEs compared with those dissolved in seawater. The distribution patterns are shown in Fig. 6. The distribution coefficients vary widely among REEs, species and sites in a range from 10-1 to 101. All bivalves regularly accumulate LREEs much more than HREEs compared with seawater. They apparently develop a negative Ce and Gd anomaly compared to seawater. Some specimens gave the

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greatest accumulation at Sm or Nd, which can be seen in the pattern of experimental partitioning or adsorption between carbonate and dissolved in seawater (Tanaka and Kawabe, 2006; Toyama and Terakado, 2014; Zhong and Mucci, 1995). However, it should be noted that this partitioning is ostensible, reflecting REEs in carbonate, a primary storage of REEs, in sediment, which bivalves incorporate, vs. those dissolved in

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seawater, and should not be understood as biological fractionation from seawater. The strong negative Gd anomalies seen in all specimens imply that requirements (i) and (ii)

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(Section 4.2) may not be met and the distribution coefficients around unity imply that requirements (ii) and (iii) are not satisfied.

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In the case of coral, the distribution coefficients of REEs are almost unity or slightly higher than unity (Akagi et al., 2004; Sholkovitz and Shen, 1995). It is interesting REEs in shells are not from seawater.

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5. Conclusions

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to note that the apparent partitioning of shell distributes not far from unity, although most

This study compares the REE abundance of shells and soft tissues of bivalves with those of several different phases in sediments and seawater. The results of this study

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are:

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1) REEs in most bivalves (both shells and soft tissues) are not exclusively from those dissolved in seawater, but REEs in sediment or particulate REEs extractable by a strong acid may comprise major sources. in shells.

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2) Concentrations of REEs in soft tissues are higher by an order of magnitude than those

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3) Considerable amounts of REEs in mussels are from those dissolved in seawater, but most REEs in mussel shell are from particulate matter. 4) Apparent distribution coefficients of REEs in shells against seawater are from 10-1 to 10 and are not far from unity. 5) Prediction of seawater REEs from the REE composition of shells is very difficult. In the environment REEs are contained in various components of inorganic and organic matter. This study demonstrates the usefulness of a suite of REEs as tracers of substances which potentially supply REEs, in ecological studies.

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Figure captions Fig. 1 Map showing three sampling sites of bivalves in Tokyo Bay. The map of Tokyo Bay used is from http://www.d-maps.com/pays.php?num_pay=294&lang=en. Fig. 2 Shale normalized REE patterns of shells (a) and soft-tissues (b) of bivalves. The

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normalizing values used are PAAS (McLennan, 1989).

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Fig. 3 Shale normalized REE patterns of acid leachates in suspended and non-suspended

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solids in sediment. The normalizing values used are PAAS (McLennan, 1989). Fig. 4 Shale normalized patterns of dissolved and particulate REEs in seawater. The

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normalizing values used are PAAS (McLennan, 1989).

Fig. 5 Relative abundance pattern of shells/HF leacheate of suspended matter (a), soft

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tissues/HAc leacheate of non-suspended matter (b), shells/seawater composites (c), and soft-tissues/seawater composites (d). Seawater composites were prescribed by mixing particulate and dissolved REEs in the local seawaters. The figures in (c) and (d) are the

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mixing ratios of particulate and dissolved REEs on Nd basis. Fig. 6 Apparent partitioning coefficients of bivalve shells against seawater (REE/Ca)shell/(REE/Ca)seawater. This is a guideline only, as REEs in shells are concluded to

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be practically from suspended solids of sediment or suspended particles, not from those

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dissolved in seawater in this study.

References Akagi, T., Fu, F.-f., Hongo, Y. and Takahashi, K., 2011. Composition of rare earth elements in settling particles collected in the highly productive North Pacific Ocean and Bering Sea: Implications for siliceous-matter dissolution kinetics and

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Lerche, D. and Nozaki, Y., 1998. Rare earth elements of sinking particulate matter in the Japan Trench. Earth and planetary science letters, 159(1): 71-86. Masuda, A., 1962. Regularities in variation of relative abundances of lanthanide elements

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Geochemistry, 21(1): 169-200. Ponnurangam, A., Bau, M., Brenner, M. and Koschinsky, A., 2016. Mussel shells of Mytilus edulis as bioarchives of the distribution of rare earth elements and yttrium in seawater and the potential impact of pH and temperature on their partitioning behavior. Biogeosciences, 13(3): 751-760. Scherer, M. and Seitz, H., 1980. Rare-earth element distribution in Holocene and Pleistocene corals and their redistribution during diagenesis. Chemical Geology, 28: 279-289. Shabani, M.B., Akagi, T. and Masuda, A., 1992. Preconcentration of trace rare-earth

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elements in seawater by complexation with bis (2-ethylhexyl) hydrogen phosphate and 2-ethylhexyl dihydrogen phosphate adsorbed on a C18 cartridge and determination by inductively coupled plasma mass spectrometry. Analytical Chemistry, 64(7): 737-743. Sholkovitz, E. and Shen, G.T., 1995. The incorporation of rare earth elements in modern

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coral. Geochimica et Cosmochimica Acta, 59(13): 2749-2756. Sholkovitz, E.R., Landing, W.M. and Lewis, B.L., 1994. Ocean particle chemistry: the

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Tanaka, K. and Kawabe, I., 2006. REE abundances in ancient seawater inferred from marine limestone and experimental REE partition coefficients between calcite

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and aqueous solution. Geochemical Journal, 40(5): 425-435. Toyama, K. and Terakado, Y., 2014. Experimental study of rare earth element partitioning between calcite and sodium chloride solution at room temperature and pressure.

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Webb, G.E. and Kamber, B.S., 2000. Rare earth elements in Holocene reefal 64(9): 1557-1565.

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microbialites: a new shallow seawater proxy. Geochimica et Cosmochimica Acta,

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Zhong, S. and Mucci, A., 1995. Partitioning of rare earth elements (REEs) between calcite and seawater solutions at 25 C and 1 atm, and high dissolved REE

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concentrations. Geochimica et Cosmochimica Acta, 59(3): 443-453.

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Table 1 Concentration of REEs in shells of bivalves (unit: g/g). ---------------------------------------------------------------------------------------------------------------------------------------------------------Site O Site K Site D ---------------------------------------------------------------------------------------------------------------------------------------

T P

I R

Jp. Littleneck blue mussel Jp. Littleneck Jp. dosinia trough shell Jp. littleneck Stimpson’s quahog n=3 n=3 n=6 n=9 n=3 n=3 n=2 ---------------------------------------------------------------------------------------------------------------------------------------------------------La 35.1 ±1.3 19.6 ±2.5 32.0 ±4.5 103 ±33 24.1 ±5.4 104.3 ±9.7 96.0 ±4.4 Ce 70.7 ±1.9 27.7 ±4.2 63 ±10 209 ±36 48.6 ±2.0 186 ±13 180 ±15 Pr 7.61 ±0.26 2.07 ±0.42 7.14 ±1.00 27.2 ±3.7 5.23 ±0.84 24.0 ±1.7 22.57 ±0.16

C S U

Nd Sm Eu Gd Tb Dy Ho

27.2 ±1.3 5.70 ±0.29 1.35 ±0.11 6.48 ±0.32 0.906 ±0.046 5.41 ±0.32 1.119 ±0.043

6.5 ±1.4 0.750 ±0.083 0.135 ±0.006 0.97 ±0.13 0.114 ±0.001 0.790 ±0.073 0.191 ±0.017

Er Tm Yb

3.40 ±0.22 0.404 ±0.053 2.65 ±0.31

0.692 ±0.027 0.109 ±0.005 0.537 ±0.043

30.3 6.40 1.57 8.0 1.13 6.99 1.48

4.63 ±0.61 0.590 ±0.076 3.74 ±0.39

M

121 ±17 27.0 ±3.6 6.35 ±0.80 31.5 ±3.7 4.44 ±0.54 28.0 ±3.8 6.03 ±0.79

D E

T P E

C C

A

±3.9 ±0.82 ±0.18 ±1.0 ±0.12 ±0.87 ±0.20

N A

18.1 ±2.1 2.20 ±0.23 12.7 ±1.5

21.6 4.93 1.39 6.45 0.91 5.58 1.18

±3.8 ±0.80 ±0.20 ±0.67 ±0.12 ±0.68 ±0.14

3.53 ±0.34 0.444 ±0.049 2.73 ±0.27

89.6 18.4 4.62 21.5 3.00 17.7 3.76

±6.6 ±1.6 ±0.30 ±1.3 ±0.22 ±1.3 ±0.33

97.6 ±3.5 19.30 ±0.02 4.597 ±0.003 21.38 ±0.08 3.092 ±0.002 18.18 ±0.04 3.957 ±0.006

12.3 ±1.1 1.48 ±0.14 9.54 ±0.77

11.790 ±0.049 1.480 ±0.001 9.383 ±0.021

Lu 0.424 ±0.060 0.102 ±0.014 0.615 ±0.090 1.83 ±0.20 0.389 ±0.037 1.35 ±0.11 1.363 ±0.001 ----------------------------------------------------------------------------------------------------------------------------------------------------------

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Jp. Littleneck

Jp. dosinia

trough shell

Jp. littleneck

Stimpson’s

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quahog

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Table 2 Concentration of REEs in soft-tissues of bivalves (unit: ng/g). -----------------------------------------------------------------------------------------------------------------------Site K Site D ----------------------------------------------------------------------------------------------------

100.8 25.13 133.5 15.60 99.8 19.65 54.51 6.63 43.51

137.8 ±1.1 35.12 ±0.13 167.5 ±1.4 22.46 ±0.44 130.4 ±1.0 26.24 ±0.14 77.5 ±3.2 9.84 ±0.27 59.5 ±1.0

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±2.2 ±0.42 ±2.6 ±0.32 ±1.5 ±0.37 ±0.57 ±0.11 ±0.29

532 ±14 1100 ±160 176.5 ±2.5 854.8 ±6.4

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Sm Eu Gd Tb Dy Ho Er Tm Yb

522 ±86 1156 ±15 169.4 ±1.6 653.9 ±9.4

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317 ±19 471 ±32 74.1 ±1.7 367.2 ±5.7

D

La Ce Pr Nd

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n=3 n=3 n=3 n=3 n=3 ------------------------------------------------------------------------------------------------------------------------

203.2 ±1.6 52.0 ±1.3 229.5 ±7.3 27.63 ±0.68 144.8 ±1.5 27.28 ±0.37 70.78 ±0.72 7.85 ±0.08 44.49 ±0.44

643 ±47 1161 ±34 99.4 ±3.2 411 ±20 89.0 ±4.8 23.4 ±1.0 112.5 ±5.7 13.56 ±0.64 74.7 ±2.8 14.46 ±0.32 38.67 ±0.97 4.48 ±0.16 24.6 ±1.5

353 ±17 337 ±75 52.8 ±2.0 222.5 ±5.8 45.06 11.53 58.5 6.59 32.7 6.12 15.63 1.62 9.26

±0.68 ±0.44 ±1.7 ±0.17 ±1.3 ±0.19 ±0.22 ±0.05 ±0.35

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Lu 8.86 ±0.29 10.24 ±0.42 6.43 ±0.18 3.82 ±0.31 1.45 ±0.06 --------------------------------------------------------------------------------------------------------------------------

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Table 3 Concentration of REEs dissolved in seawater and in suspended particles. --------------------------------------------------------------------------------------------Dissolved (ng/kg)

Suspended particles (g/g)

---------------------------------

----------------------------------

O

K

D

a

Weight (mg)

O

K

D

27.5

32.5

42.4

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Site

15

12.7

Ce

109

74.9 4.6

2.92 14

Pr

5.56

Nd

8.44

Sm

1.53

3.42

1.83

Eu

0.4

1.05

0.609

Gd

11.4

8.89

5.82

3.91 0.319

7.63

1.13

27.1

2.89

11.3

6.93

25.4 2.27 8.71

2.84

1.97

0.0614

0.758

0.502

0.285

3.07

2.06

0.041

0.469

0.282

3.96

0.245

2.81

1.64

1.18

0.0495

0.582

0.322

28.4

Tb

0.404

0.863

Dy

2.64

5.8

Ho

0.764

1.52

Er

3.3

5.7

4.99

0.155

1.75

0.936

Tm

0.59

0.84

0.886

0.0225

0.236

0.123

Yb

4.48

5.85

8.06

0.115

1.44

0.721

Lu

0.973

1.11

1.78

0.0172

0.219

0.10

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D

MA

0.558

0.258

NU

17

1.86

1.25

SC

La

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---------------------------------------------------------------------------------------------

CE

---------------------------------------------------------------------------------------------

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a) Weight of particulate matter in 1 kg of seawater.

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Table 4 Concentration of REEs in non-suspended solids and suspended solids in sediment (unit: μg/g). ----------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------Site O Site K Site D -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

T P

Non-sus.

a

Sus.

a

Non-sus.

a

Sus

I R

.a

Non-sus.a

b

Sus.a

C S U

b

0.91% 0.45% 0.23%b ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------acidc HF HNO3 HF HC+HN HNO3 HAc HF HCl+HN HNO3 HAc HF HCl+HN HNO3 HAc HF HC+HN HNO3 HAc ----------------------------------------------------------------------------------------------------------------------------- -----------------------------------------------------------------------------

N A

La

19.6

4.46

13.3

120

30.6

4980

Ce

34.4

9.93

28.6

234

58.8

8400

Pr Nd

4.03 14.0

1.13 3.62

4.00 16.3

Sm

2.47

0.693

4.10

Eu

0.830

0.176

1.66

Gd

2.49

0.759

4.29

Tb

0.412

0.121

0.681

Dy

2.40

0.741

4.16

Ho

0.513

0.162

0.833

Er

1.58

0.581

2.58

30.8

1.67

2.36

1.53

PT

5.97

4.83

4.65

3.54

0.726

0.507

0.631

0.473

0.670

25.1

21.0

5580

2.83

2.06

2.39

1.96

2.62

97.4

80.7

1280

0.653

0.543

0.558

0.492

0.599

21.93

16.7

1.63

267

0.173

0.131

0.127

0.110

0.161

4.92

7.91

1320

0.722

0.572

0.605

0.523

0.589

1.19

200

0.115

0.092

0.094

0.082

0.083

6.90

1130

0.712

0.542

0.554

0.520

0.479

1.31

215

0.144

0.107

0.108

0.104

0.090

3.09

2.41

3.87

608

0.455

0.334

0.320

0.334

0.260

8.53

6.71

E C

27.9

C A

6.10

27.9

4.42

22.8 4.05 12.5

2.69

8.56

119

D E

M

34.4

8.02

1440

2.32 10.9

86.0 184

21.2 2.98 16.8

79.0 168

3.88 16.8 2.37 13.3

154 335 42.0 164 39.7 8.97 41.6 6.04 34.2 6.43 19.2

3.57 10.1 1.11 4.16

13.1 30.8 3.26 11.4

4.18 13.1

8.98 13.1

1.45

2.12

5.81

7.68

1.31

1.61

0.861

2.33

0.208

0.558 0.313

0.369

0.866

2.41

1.58

0.133

0.350 0.203

0.227

0.756

2.00

1.39

0.145

0.381 0.215

0.254

0.440

1.11

0.783

1.29

1.18

0.671

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Tm

0.232

0.075

0.368

Yb

1.50

0.578

2.50

Lu

0.238

0.083

0.388

1.64 11.2 1.58

0.504 3.33 0.512

78.1 0.063

0.047

0.042

0.044

0.031

1.02

0.786

0.428

0.315

0.246

0.314

0.186

5.74

4.50

62.8 0.072

0.049

0.042

0.049

0.025

0.798

0.667

468

2.44 15.3 2.20

0.064

0.133 0.083

0.101

0.410

0.801 0.480

0.608

0.064

0.143 0.074

0.096

T P

a) Non-sus. (non-suspended solids) and Sus. (suspended solids), which sedimented and remained in suspension, respectively, after 5 min.

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sedimentation. b) Proportion of weight collected as suspended solids to total weight.

C S U

c) Acids used in leaching. HF: hydrofluoric acid , HC+HN: nitric acid and hydrochloric acid , HNO3: dil. nitric acid, HAc: acetic acid

N A

D E

T P E

A

C C

M

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Highlight The sources of rare earth elements in marine bivalves have been identified for the first time. Shells and soft tissues of bivalves collected from Tokyo Bay, together with various components of sediment, seawater and suspended particles were analyzed for rare earth

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element composition. Rare earth elements were shown to be sourced mainly from a fairly readily-extractable

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phase of sediment or suspended particles in seawater and only at trace levels from those

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dissolved in seawater.