Environ. Sci. Technol. 2004, 38, 6507-6512
Plutonium from Global Fallout Recorded in an Ice Core from the Belukha Glacier, Siberian Altai S U S A N N E O L I V I E R , †,‡ S I X T O B A J O , † L. KEITH FIFIELD,§ H E I N Z W . G A¨ G G E L E R , † , ‡ TATYANA PAPINA,| PETER H. SANTSCHI,⊥ U L R I C H S C H O T T E R E R , ‡,∇ M A R G I T S C H W I K O W S K I , * ,†,‡ A N D L U K A S W A C K E R ‡,§,# Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland, Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, Australian National University, ACT 0200, Australia, Institute for Water and Environmental Problems, 656099 Barnaul, Russia, Texas A&M University, Galveston, Texas 77551, and Division of Climate and Environmental Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
Ice cores from glaciers situated near anthropogenic sources of air pollution provide important archives of the emissions of species with short atmospheric lifetimes. Here we present the history of atmospheric Pu fallout reconstructed from an ice core from the Belukha glacier in the Siberian Altai. Fourteen ice core samples covering the time period 1941-1986 were selected for Pu analysis, chemically processed, and measured using accelerator mass spectrometry. The Pu concentration peaks in 1963, coinciding with the maximum of the nuclear weapons tests and in concordance with the 3H activity concentration peak. The shapes of the 239Pu and 3H profiles reflect two main periods of atmospheric nuclear test activity: premoratorium testing before 1958 and postmoratorium testing in 1961 and 1962. Premoratorium tests contribute about 45% of the integrated Pu inventory. The average 240Pu/239Pu isotopic ratio is 0.18 ( 0.05, indicating that a large majority of the Pu in the Belukha glacier originates from global stratospheric fallout rather than from direct tropospheric input.
Introduction Anthropogenic Pu is present in the environment as a consequence of nuclear weapon testing and via releases from the nuclear industry during the second half of the 20th century. For environmental monitoring and radiological health protection, it is important to establish a picture of the Pu distribution. Pu is also of interest because it can serve as * Corresponding author phone: +41 56 310 4110; fax: +41 56 310 4435; e-mail:
[email protected]. † Paul Scherrer Institut. ‡ Department of Chemistry and Biochemistry, University of Bern. § Australian National University. | Institute for Water and Environmental Problems. ⊥ Texas A&M University. ∇ Division of Climate and Environmental Physics, University of Bern. # Present address: ETH-Ho ¨ nggerberg, CH-8093 Zu ¨ rich, Switzerland. 10.1021/es0492900 CCC: $27.50 Published on Web 10/27/2004
2004 American Chemical Society
a tracer for actinide transport mechanisms or even for air circulation and mixing-time studies (1). Approximately 6 t of 239Pu was released into the environment as a result of 541 atmospheric weapon tests with a total explosive yield of 440 Mt (TNT equivalent), though just 25 of these tests accounted for two-thirds of the yield (2, 3). Much of the radioactive debris was introduced to the stratosphere and, after a residence time of approximately one year (2), distributed globally with a ratio of approximately 3:1 between the Northern and Southern Hemispheres (4). However, significant quantities of radioactive debris from surface-based tests did not reach the stratosphere, and hence were removed rapidly from the troposphere, giving rise to enhanced local or regional pollution (5). Because the isotopic composition of Pu varies depending on its origin (6-8), measurements of the more abundant Pu isotopes 239Pu and 240Pu can provide information on the source. Several distinct periods of atmospheric weapon testing are identifiable, on the basis of the nuclear-test programs of one or more countries. High-yield thermonuclear tests were initially performed during 1952-1954 by the United States. They dominated the pollution of the atmosphere until the moratorium on nuclear testing in November 1958. This agreement continued until the USSR began a series of largescale atmospheric nuclear tests in September 1961, and thus became the primary testing country in 1961-1962. Since the 1962 Test Ban Treaty, smaller amounts of nuclear debris have been introduced to the atmosphere by French and Chinese activities (3, 9-11). Nuclear fallout, especially that of Pu, has been investigated in many different archives. Existing records have been derived from analyses of soil samples (4), marine and lacustrine sediments (12), dated corals (5), polar ice cores (13, 14), samples of stratospheric air obtained by aircraft (15), samples from a U.K. herbage archive (16), and samples from an Alpine ice core (16). Each of these archives has certain advantages, but often also limitations. For example, soil studies have been restricted to examinations of only the total Pu inventory because of their low accumulation rates. The evaluation of sediment records is complex because fallout nuclides enter the sediments through different pathways, such as direct deposition from the atmosphere to the water surface followed by sorption onto settling particles or, alternatively, erosion and subsequent deposition (12). Furthermore, typical lake sediments only accumulate at a rate of millimeters per year, restricting the time resolution of such archives (14). Last, stratospheric data and polar ice cores may not represent the ground-level fallout in temperate latitudes where the emission occurred. High-mountain glaciers at midlatitudes are especially attractive for studies of anthropogenic impact on the environment. These archives have the advantage of high annual accumulation rates, which are mandatory for obtaining highly resolved records, and in many cases are free from significant post-depositional alteration (1). However, Pu concentrations in glaciers are extremely low. Consequently, large samples have often been required, and therefore, only a few ice core records of Pu fallout exist to date. Accelerator mass spectrometry (AMS) enables quantitative determination of low levels of the long-lived Pu isotopes with detection limits that are superior to R spectrometry, high-resolution and multicollector inductively coupled plasma mass spectrometry (HR-ICP-MS and MC-ICP-MS), and thermal ionization mass spectrometry (TIMS) (6, 8, 17-19). Interfering molecules are eliminated efficiently during the stripping process, and detection limits in the subfemtogram VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map showing the location of the Belukha glacier (square) in the Siberian Altai, along with the two nuclear test sites Semipalatinsk and Lop Nor and the nuclear facilities of Tomsk and Krasnoyarsk. range have been reported (20). Only resonance ionization mass spectrometry (RIMS) has proven equally as sensitive, though RIMS has the drawbacks of low sample throughput and high instrumental expense (21-23). In addition, measurement of the 240Pu/239Pu ratio, unobtainable by R spectrometry, is straightforward via AMS even though less accurate than via MC-ICP-MS (8). In this paper we present the history of atmospheric Pu fallout reconstructed from an ice core from the Belukha glacier in the Siberian Altai. Pu was analyzed with AMS using an optimized sample preparation procedure. The 140 m long Belukha ice core was drilled to bedrock in July 2001 (24) with the aim of reconstructing air pollution levels and anthropogenic impacts on these remote environs and to study possible Pu contamination from nearby nuclear test sites. Parallel analysis of major ions in this ice core has shown that concentrations are comparable to those observed in European glaciers, except for NH4+ and HCOO-, where enhanced concentrations indicate biogenic emissions from Siberian forests. SO42-, NH4+, and NO3- records all show anthropogenic impacts despite the remoteness of the site (24).
Experimental Section Belukha Ice Core and Altai Archives. The 140 m long ice core used in this study was recovered in 2001 from the Belukha glacier (49°48′26′′ N, 86°34′43′′ E, 4062 m above sea level) in the Siberian Altai (Figure 1) (24). The englacial temperature of -17 °C indicates that this part of the glacier belongs to the cold infiltration recrystallization zone where meltwater immediately refreezes some centimeters below the surface when it has formed under the influence of solar radiation and high air temperature (25). The calculated 210Pb inventory was 0.4 Bq/cm2. The steadiness of the decrease of the 210Pb activity with depth (Figure 2) implicates a regular accumulation of snow over the investigated period. Thus, the glaciochemical record is well preserved and suitable for the reconstruction of air pollution levels. The chronology of the upper 86 m investigated in this study was established using the decay of 210Pb (Figure 2) and the 1963 peak in 3H fallout from nuclear weapon testing observed at a depth of 18 m water equivalent (Figure 3). The least-squares fit in Figure 2 takes into account radioactive decay and layer thinning (based on the model developed by Haefeli (26)). The 210Pb scale was adjusted to the 1963 3H maximum by subtracting one year. The estimated uncertainty of this dating increases from less than 1 year at the 3H peak to approximately (3 years at 1990 and (6 years at 1940. 6508
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FIGURE 2. 210Pb activity versus depth of the ice core, including a least-squares fit taking into account layer thinning (solid line), the 3 H reference horizon (square), and the calculated age (right-hand scale). The dashed lines correspond to the expanded uncertainty with a 95% level of confidence (52).
FIGURE 3. 3H activity concentration record from the Belukha ice core (decay-corrected, bold line) and 3H in precipitation at Ottawa (monthly values, thin line; 33). In addition to the ice core, two soil samples from the surface to 17 cm depth were taken near Ak-kem (49°57′ N, 86°32′ E, 2000 m above seal level), a meteorological station 7 km north of Belukha. The concentration of 137Cs in these samples was measured by γ counting. The 239+240Pu concentrations were determined by R spectrometric counting. Pu was extracted by leaching and, after addition of 236Pu yield tracer, was directly purified by passing the solution through a BIO-RAD AG 1-X2 anion exchange column (27, 28). The eluted Pu fraction was then quantitatively electrodeposited (29) and measured by R spectrometry. Finally, samples from a lake sediment core drilled in 1999 at Uzun Kol, about 90 km to the northeast from the Belukha glacier (50°29′ N, 87°6′30′′ E, 1985 m above sea level (30)), were analyzed. The 137Cs concentration in this core was measured by γ counting. Ice Sample Preparation for Pu Analyses. On the basis of the dating of the ice core, 14 samples (2-3 kg each) from the upper 40 m and covering the time period 1941-1986 were selected for Pu analysis. Samples were cut from the core in a -20 °C cold room, allowed to melt, immediately spiked with 1 pg of 242Pu yield tracer, and acidified to pH 0.3 using analytical grade 65% HNO3. The solution was left to stand overnight to ensure total leaching of the Pu from particles. The less reactive pentavalent (V) species of Pu are considered to be dominant in glacier ice (31) but are not coprecipitated quantitatively by the Ca3(PO4)2 coprecipitation often used for soil extracts. Therefore, preconcentration was
achieved by means of MnO2 coprecipitation (32), a procedure also suitable for the pentavalent species. After repetition of the coprecipitation to maximize the yield, the supernatant was set aside for 36Cl and 137Cs analyses, and the MnO2 precipitate was dissolved in 7 mL of 65% HNO3, 0.5 mL of 30% H2O2, and 160 µL of 40% HF. This solution was evaporated to dryness and the residue dissolved in 6.5 mL of 1 M HNO3. Then Pu was forced into oxidation state IV by first reducing it to state III with 130 mg of Fe(NH4)2(SO4)2‚6H2O and subsequent oxidation to state IV by adding 7 mL of 65% HNO3 and 200 mg of NaNO2. As high levels of uranium can interfere with the mass spectrometric measurement of 239Pu, uranium was separated from the Pu by ion exchange chromatography using Dowex 1X2 resin (100-200 mesh). The 8 M HNO3/Pu(IV) solution was filtered (0.45 µm) and transferred to a 1 cm diameter column that had been preconditioned with 8 M HNO3. After the column was rinsed with 8 M HNO3 and 37% HCl, the Pu was selectively eluted with a freshly made 0.1 M NH4I solution in 37% HCl. The iodide ion reduces Pu to state III, breaking the anionic complex and releasing Pu from the column. The eluted fraction was taken to dryness first with 65% HNO3 to drive off the iodine, then with 37% HCl to eliminate ammonium, and finally again with 65% HNO3. To prepare the target for the AMS measurement, the residue was dissolved in 2 mL of 1 M HNO3. Next, 0.37 mg of Fe and 4.5 mg of Ag, from 0.1 and 0.2 M nitrate solutions, respectively, were added, and the Pu was forced into oxidation state IV by subsequently adding 30 mg of NaNO2. Afterward the Pu was coprecipitated with iron hydroxide and silver oxide by raising the pH with NaOH to approximately 12. The precipitate was centrifuged and thoroughly rinsed twice with ultrapure water (18.2 MΩ quality) before being dried at ∼70 °C, transferred to a small glass ceramic crucible, and baked at 450 °C for 4.5 h. The resulting blend of plutonium, silver, and iron oxide was additionally mixed with high-purity Ag powder (∼2 mg), then firmly pressed into an aluminum AMS sample holder, and mounted onto a sample wheel. Accelerator Mass Spectrometry. AMS measurements were carried out using the 14 MV tandem accelerator at the Australian National University, Canberra (6). The 14 MV accelerator was operated at about 4 MV. PuO- ions from the negative ion source were injected into the instrument and accelerated to the high-voltage terminal. A low-pressure oxygen gas stripper was employed to dissociate the molecular ions and strip the Pu to high positive charge states, after which they were further accelerated. An analyzing magnet then selected specifically the 24 MeV Pu5+ ions. Background ions with a mass-to-charge (m/q) ratio different from that of the Pu ions were filtered out with a velocity filter. Pu ions were counted individually by a gas-filled ionization detector with an energy resolution of 3%. This resolution was more than adequate for separating Pu ions from those ions also arriving at the detector with the same m/q but lower charge states. Alternating between the different Pu isotopes was achieved by changing (i) the magnetic field after the ion source to inject the correct PuO- ions, (ii) the terminal voltage to adjust the magnetic rigidity of the Pu5+ ions to pass through the postacceleration beam transport system, and (iii) the electric field in the velocity filter. The three isotopes 242Pu, 240Pu, and 239Pu were counted sequentially using repeated cycles for each sample with counting times per isotope varying between 0.5 and 5 min, the exact time chosen according to the expected isotope ratios. On average, the total time for one analysis was 20 min. A certified Pu isotope ratio reference standard (AEA Technology), providing essentially equal amounts of 239Pu, 240Pu, and 242Pu, was included with each batch of samples for setting up and monitoring the integrity of the procedures. Blanks for target preparation and AMS measurement, all spiked with 1 pg of 242Pu, gave
TABLE 1. 239Pu Concentrations and Different Samples
240Pu/239Pu
datea
[239Pu]b, (atoms/kg) × 108
1983-1986 1981-1983 1978-1981 1975-1978 1972-1975 1969-1972 1966-1969 1962-1966 1959-1962 1955-1959 1952-1955 1949-1952 1945-1949 1941-1945
0.18 ( 0.02 0.28 ( 0.02 2.1 ( 0.16 3.8 ( 0.30 3.9 ( 0.22 12 ( 0.9 13 ( 1.0 79 ( 6.2 39 ( 3.0 43 ( 3.4 4.2 ( 0.33 0.18 ( 0.02 0.11 ( 0.01
