Environ. Sci. Technol. 2006, 40, 4103-4108
Plutonium Isotopes in Settling Particles: Transport and Scavenging of Pu in the Western Northwest Pacific JIAN ZHENG* AND MASATOSHI YAMADA Nakaminato Laboratory for Marine Radioecology, National Institute of Radiological Sciences, 3609 Isozaki-cho, Hitachinaka, Ibaraki, 311-1202 Japan
We examined the vertical distributions of 239+240Pu activity and 240Pu/239Pu atom ratio in settling particles and quantified the particulate 239+240Pu fluxes in the water column in the western Northwest Pacific. Settling particle samples were collected using sediment traps. Plutonium isotopes were analyzed using a sector field high-resolution ICP-MS. To the best of our knowledge, this is the first time that both Pu activity and Pu isotope ratio data have been obtained for settling particles in the Pacific Ocean. The high (>0.18) 240Pu/239Pu atom ratios in settling particles indicate that plutonium from the Pacific Proving Grounds (PPG) source in the central Pacific is transported toward the western Northwest Pacific. Evidence indicates that Pu scavenging onto the settling particles is strongly dependent upon the bulk mass flux. The results suggest that advective lateral transport of dissolved Pu from the open ocean to the ocean margin and removal of Pu into the margin sediments by particle scavenging is a common phenomenon in the Pacific Ocean. Plutonium can be considered as a useful tracer to study the transport and fate of other contaminants that readily adsorb to particles in marine environments.
Introduction Recently, studies on Pu inventories and 240Pu/239Pu atom ratios in the water column and sediments have revealed the wide presence of Pacific Proving Grounds (PPG) close-in fallout Pu in the western North Pacific and its marginal seas (1-7). We previously observed PPG source Pu in sediments in the Okinawa Trough (3), in the Japan Sea (6), and in Sagami Bay (2). Lee et al. (4) also found elevated 240Pu/ 239Pu ratios in sediments from the southern Okinawa Trough. Kim et al. (1, 7) further reported the presence of PPG-source Pu in seawater and sediments near the Korean Peninsula. The oceanic current transportation of Pu from the PPG and the early direct tropospheric fallout have been proposed to be responsible for the wide presence of PPG close-in fallout Pu signature (1, 2, 4, 8); however, details for the Pu transportation are not well understood. Studying the Pu scavenging process by settling particles may provide useful information to elucidate the details of the oceanic Pu transportation and deposition processes because Pu is particle-reactive and thus can be transported either in solution or in association with particles. Settling * Corresponding author phone: 0081-29-265-7130; fax: 0081-29265-9883; e-mail:
[email protected]. 10.1021/es0601179 CCC: $33.50 Published on Web 05/28/2006
2006 American Chemical Society
particles have been regarded as the primary means by which surface-introduced transuranics, for instance, Pu, are transported to the sediments (9-11). Direct particle Pu flux measurements and Pu isotope ratio (240Pu/239Pu) measurements of trapped suspended sediment from the Pacific Ocean are rare. Livingston and Anderson (12) reported the large particle transport of Pu to the deep ocean in the Equatorial North Pacific by measuring 239+240Pu activity and its flux. They also estimated the apparent residence time of Pu as 462 yr. Hong et al. (13) studied the particulate 239+240Pu activities and fluxes in the Sea of Japan and suggested that Pu scavenging onto the settling particulate matter may be more dependent upon the bulk mass flux rather than the chemical composition of the particulate matter. Yamada and Aono (14) reported large particle flux of 239+240Pu on the continental margin of the East China Sea and indicated that the episodic lateral transport of particles was significant for 239+240Pu transport on the continental slope in the East China Sea. However, none of these studies provided Pu isotopic ratio data in settling particles, which are very important in order to have a better understanding of the sources and the basinwide scavenging feature of plutonium. In this work, we carried out a sediment-trap experiment in the western Northwest Pacific to investigate Pu isotopes in settling particles. The objectives of this study were as follows: (1) to measure the activities of Pu isotopes and the 240Pu/239Pu atom ratios in settling particles collected in the East China Sea continental margin, in the Okinawa Trough, and in the open ocean east of Okinawa Island; (2) to quantify the fluxes of 239+240Pu; (3) to understand the transport and particle scavenging processes of Pu-containing particles; and (4) to discuss the source and sink of Pu in these regions.
Materials and Methods Sediment Trap Sampling. Settling particle samples were collected at seven stations located as shown in Figure 1. Three moorings of sediment traps (Stns. F-4, F-6, and F-8) were deployed on the continental margin of the East China Sea (14). Two moorings of sediment traps (Stns. SST-1 and SST2) were deployed in the Okinawa Trough. Another two moorings of sediment traps (Stns. SST-3 and SST-4) were deployed for time-series settling particle sampling on the slope east of Okinawa Island. The sediment trap mooring strategy is shown in the Supporting Information (Figure 1S). Details for the sediment trap locations and sampling periods are summarized in Table 1S and Table 2S (see Supporting Information). Two types of traps were used, cylindrical traps and conical time-series traps. The trapping efficiency of cylindrical and conical traps has been evaluated in previous work (15). In general, conical traps provided lower trapping efficiency compared to the cylindrical trap. Two sediment cores were also collected at Stn. SST-3 (28 July 1994, Cruise K94-04) and Stn. SST-4 (14 Oct 1993, Cruise K93-05). These cores were cut on board into 2-cm segments from surface to the bottom. Sediment core samples were then stored in a freezer until analyzed. Analytical Procedure. After recovery of the sediment traps, sample bottles were stored in a refrigerator (5 °C). In the laboratory, the sediment trap samples were first wet-sieved through a 1-mm nylon mesh to exclude large zooplankton and other swimmers and then filtered onto preweighed Nuclepore polycarbonate membrane filters (0.6-µm pore size, 47-mm diameter) and rinsed with deionized water to remove salts (16). The dried settling particle samples were analyzed for 239+240Pu activity and 240Pu/239Pu atom ratio by SF-ICPMS with a high-efficiency sample introduction system after VOL. 40, NO. 13, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4103
FIGURE 1. Map showing the sediment trap sampling locations. the leaching of Pu using 8 M HNO3 and Pu purification using anion-exchange chromatography (17). The yield from the acid leaching and chromatographic purification steps was evaluated using 242Pu tracer and ranged from 70% to 90%. For the time-series samples collected at Stns. SST-3 and SST4, due to the small sample size, several samples were combined for analysis when the sample size obtained in a single sampling period was too small to allow the Pu isotope measurement. Sediment samples were analyzed using the procedure described in the previous work (18).
Results and Discussion 240Pu/239Pu
Atom Ratios in Settling Particles. The 240Pu/ atom ratios in the settling particles ranged from 0.225 to 0.268 in the East China Sea and Okinawa Trough (Table 1) and from 0.207 to 0.272 in the open ocean stations, SST-3 and SST-4 (Figure 2 and Table 3S (Supporting Information)). These ratios are significantly higher than that of the global fallout ratio of 0.18 (19, 20) and are comparable with those in the underlying surface sediments (0.21-0.26) reported by Wang and Yamada (3), confirming the presence of the PPG close-in fallout Pu in these regions. Overall, a mean of 0.24 for atom ratios of 240Pu/239Pu in settling particles is obtained in the studied region. Assuming the global fallout 240Pu/239Pu atom ratio of 0.18 (19) and 0.30-0.36 for the PPG close-in fallout (5, 21), the application of the two-end member fallout mixing model (2, 20, 22) indicates that the contribution of the PPG close-in fallout Pu is ca. 50-60%, which is close to the previous estimation made 20 years ago that up to 60% of the Pu inventory in the North Pacific originated from local sources (10). Although Buesseler (5) reported similar 240Pu/ 239Pu atom ratios (0.22-0.25) in seawater collected in 1980 in the North Pacific, an accurate record on the variation of 240Pu/239Pu atom ratios in the Pacific in the past 4 decades since 1963 is not available; therefore, the question on the long-term variation of PPG close-in fallout Pu contribution in the Pacific remains open. As shown in Figure 2, the 240Pu/239Pu atom ratios at the open ocean stations, SST-3 and SST-4, are quite constant; no significant seasonal variation can be observed during the sampling period from Oct 1993 to July 1994 (Table 2S, Supporting Information). The change of total mass flux in 239Pu
4104
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 13, 2006
the different seasons does not result in a variation of 240Pu/ 239Pu atom ratios in the settling particles. The flux-weighted means of 240Pu/239Pu atom ratios on an annual basis are presented in Table 2. At SST-3, the 240Pu/239Pu atom ratios at the upper, medium, and near-bottom traps are 0.233, 0.240, and 0.244, respectively. At SST-4, those values are 0.238, 0.226, and 0.227 in the upper, medium, and near-bottom traps. The 240Pu/239Pu atom ratios are quite similar in the settling particles at different depths in the water column; similar results are also obtained in the East China Sea continental margin and Okinawa Trough (Table 1). These results indicate that the Pu derived from the global fallout (0.18) and that from the PPG close-in fallout (0.30) has been well-mixed in the water column prior to their deposition into the bottom sediments. In our previous work on Pu isotopes in Sagami Bay sediments, we observed that 240Pu/239Pu atom ratios have hardly changed from a value of 0.22-0.24 between the global fallout deposition maximum of 1963 and the present time while the Pu level has been continuously decreasing. Several possibilities have been considered but remain to be investigated (2). Now we believe that this constant distribution of high 240Pu/239Pu atom ratio in the bottom sediments was not a consequence of mixing in sediments by bioturbation during and after deposition, but the mixing of global fallout and PPG close-in fallout Pu in the water column, followed by the continuous particle scavenging to get removal from the water column and deposition in the sediments. 239+240Pu Activities and Particulate 239+240Pu Fluxes. 239+240Pu activities in settling particles have been reported to have a wide variation spatially and temporally, i.e., more than 1 order of magnitude (12). As shown in Table 1 and Table 3S (Supporting Information), the 239+240Pu activities in the settling particles observed in this study ranged from 1.54 mBq/g (SST3, 707 m, sampling period 8, 1994/03/04-1994/04/03) to 5.96 mBq/g (SST-2, 813 m, sampling period of 1993/12/24-1994/ 01/03), which are comparable to those collected from the VERTEX I station off central California (generally
