Pu Ratio Research Articles

Storage and distribution of plutonium, 241Am, 137Cs and 210Pb xs in North Sea sediments

The net storage of the radionuclides 238Pu (2.8 TBq), 239, 240Pu (75 TBq), 137Cs (730 TBq), 241Am (40 TBq) and 210Pb xs (2700 TBq) in North Sea sediments has been estimated from their concentrations in 40 boxcores. From comparison of 238Pu/ 239, 240 Pu ratios between those found in the sediments and those in atmospheric fallout and discharges from the nuclear fuel reprocessing plants at Sellafield (UK) and la Hague (France), it is concluded that the latter is the main supplier of plutonium to the Southern Bight, while the former is dominant in the remainder of the North Sea. About 7% of all Sellafield plutonium discharge is stored in North Sea sediments. The German Bight receives and stores plutonium and fine particulate matter mainly from the Elbe estuary. Of all 137Cs transported through the North Sea, about 2% is stored in the sediment. 241Am is scavenged faster than plutonium and is probably transported from Sellafield to the North Sea as 241Pu.

Sources of the transuranic elements plutonium and neptunium in arctic marine sediments

Abstract We report here thermal ionization mass spectrometry measurements of 239 Pu , 240 Pu , 241 Pu , 242 Pu , and 237 Np isolated from oceanic, estuarine, and riverine sediments from the Arctic Ocean Basin. 238 Pu/ 239+240 Pu activity ratios are also reported for alpha spectrometric analyses undertaken on a subset of these samples. Our results indicate that the Pu in sediments on the Alaskan shelf and slope, as well as that in the deep basins (Amerasian and Eurasian) of the Arctic Ocean, has its origin in stratospheric and tropospheric fallout. Sediments from the Ob and Yenisei Rivers show isotopic Pu signatures that are distinctly different from those of northern-hemisphere stratospheric fallout and indicate the presence of weapons-grade Pu originating from nuclear fuel reprocessing wastes generated at Russian facilities within these river catchments. Consequently, sediments of the Eurasian Arctic Ocean, particularly those in the Barents and Kara Seas, probably contain a mixture of Pu from stratospheric fallout, tropospheric fallout, and fuel-reprocessing wastes of riverine origin. In particular, the 241 Pu/ 239 Pu ratios observed in these sediments are inconsistent with significant contributions of Pu to the arctic sediments studied from western European reprocessing facilities, principally Sellafield in the UK. Several other potential sources of Pu to arctic sediments can also be excluded as significant based upon the transuranic isotope ratios presented.

Formula omitted] transport into the Arctic Ocean from underwater nuclear tests in Chernaya Bay, Novaya Zemlya

Radionuclide measurements have been conducted on sediment, seawater and biota samples collected in Chernaya Bay, on the southern coast of Novaya Zemlya, the site of two underwater nuclear tests conducted in the 1950s. 239,240 Pu levels in sediments from the central region of Chernaya Bay exceed concentrations of 15,000 Bq/kg, and are among the highest ever reported for the marine environment. It is estimated that approximately 11 TBq of 239,240 Pu from the tests has been retained in the sediments of Chernaya Bay. Plutonium from Chernaya Bay is distinguished by 240 Pu/ 239 Pu atom ratios of 0.03 that are much lower than ratios of 0.18 typical of global fallout. High levels of 137 Cs (Bq/kg) and 60 Co (Bq/kg) were also measured in surface sediments in the central regions of Chernaya Bay near the presumed epicentre of the explosions. Applications of a biodiffusion model to excess 210 Pb sediment depth profiles indicate that the distribution of 239,240 Pu is governed mainly by sediment mixing in this low sedimentation rate (<0.1 cm/yr) regime and, as a result, most of the 239,240 Pu has been retained in the upper 20 cm of the sediment column. Elevated levels of 239,240 Pu measured in Macoma (104 Bq/kg), Fucus (15 Bq/kg) and polychaete (1292 Bq/kg) from Chernaya Bay, indicate that 239,240 Pu levels in the benthos are comparatively high and that significant uptake has occurred in the food chain. Although levels of 239,240 Pu in bottom water from Chernaya Bay are high (4.2 Bq/m 3), restricted exchange over the fjord sill limits the present rates of 239,240 Pu transport from contaminated sites in Chernaya Bay into the eastern Barents Sea. However, low 240 Pu/ 239 Pu atom ratios measured in sediment cores collected throughout the eastern Barents Sea indicate that significant offshore transport of plutonium from Chernaya Bay has occurred in the past, probably at the time of the original nuclear tests. The large difference in end member 240 Pu/ 239 Pu atom ratios for Chernaya Bay fallout (0.03) and atmospheric fallout (0.18) has been exploited to estimate that 2 TBq of 239,240 Pu in Barents Sea sediments was originally derived from Chernaya Bay. Further, a plume of low 240 Pu/ 239 Pu ratio plutonium, distributed in a northwestward direction, is evident in sediments along the southern coastline of Novaya Zemlya, indicating that an additional quantity of Chernaya Bay plutonium may have been transported into the Arctic Ocean.

Plutonium Activity Ratios in Plankton: New Evidence of Hold-Up Time in Irish Sea Sediments

The determination of activity ratios for radioisotopes of different half-lives can be used to estimate transit times from a point source to locations further away. For conservative elements, this time is approximately equivalent to the net hydrological transport. However, for non-conservative elements such as plutonium, the additional influence of biogeochemical processes decreases the net transport time. In this study, 241Pu and 239,240Pu concentrations in Irish Sea plankton samples, collected in May 1994, were determined and the 241Pu/239,240Pu ratios calculated. Plutonium-239,240 was measured using a standard method by ion exchange chromatography and alpha counting, and 241Pu was determined by liquid scintillation counting using the disk-supported technique. The latter showed some methodological problems, which are briefly discussed. The 241Pu/239,240Pu ratios gave an estimate of the "transit time" from Sellafield to the different sampling points. In fact, this time represents the age of plutonium in plankton, i.e., the time lag between release from Sellafield and detection at the different sampling stations. The mean plutonium age was 17±2 years (n = 10) and 18.6±0.8 years (n = 13) in phytoplankton and zooplankton, respectively. The spatial distribution was reasonably homogeneous over the Irish Sea. The assimilation-elimination processes of plutonium in plankton are rather rapid. Therefore, it may be assumed that, in this time scale, the plutonium concentrations were in equilibrium with surrounding waters. Thus, it is concluded that plutonium was rather old because resuspension-sedimentation processes had occurred that delayed its transport within the Irish Sea. Therefore, the age of plutonium in plankton represented the hold-up time of plutonium in the sediments from the Irish Sea.

Study on the Horizontal Transport of Some Radionuclides in the Wieprz River Valley

A horizontal transport of radionuclides was studied by the analysis of the radioactivity of the surface soil samples from valleys of Wieprz river and its bottom sediments. Natural gamma-isotopes (40K, 238U and 232Th series) antropogenic (134Cs and 137Cs) and alpha-isotopes 238Pu and 239,240Pu were measured. The different kind of bed rock and terrain configuration, influenced the radionuclide transportation from the soil to river bottom sediments. Radioactivity of the sediment samples is definitely lower than the soils. Very strong adsorption of isotopes in soil hinders their horizontal migration. Calculated 238Pu/239,240Pu ratio is characteristic for global fallout and about 90% of the 137Cs comes from Chernobyl.

Natural and anthropogenic levels of tritium in a Canadian Arctic ice core, Agassiz Ice Cap, Ellesmere Island, and comparison with other radionuclides

AbstractNumerous studies of the ice caps in Greenland and Antarctica have observed accumulations of transuranic radionuclides and fission products from nuclear weapons testing, particularly during the period 1945–75. Recently, the concentrations of radionuclides in the annually deposited surface layers of Agassiz Ice Cap, Ellesmere Island, Canadian Arctic, from 1945 to the present have been measured and have demonstrated a continuous record of deposition of 137Cs and 239,240Pu in ice and snow. In this study, 3He-ingrowth mass spectrometry has been used to measure the low levels of tritium (3H) in some of these samples. Pre-nuclear-bomb tritium levels in ice-core samples were approximately 12 TU in high-latitude meteoric waters and 3–9 TU in mid-latitude meteoric waters. Comparisons of 3H levels and 3H/137Cs + 239,240Pu ratios, which were quite low during the earliest fission-bomb detonations (1946–51) and substantially higher during thermonuclear hydrogen-fusion bomb testing (1952–64), provide a clear indication of the type of nuclear device detonated. This finding accords with the results from other ice-core studies of the distribution of anthropogenic radionuclides from bomb fallout.

241Am concentration and241Pu/239(240)Pu ratios in soils contaminated by weapons-grade plutonium

241Am and238,239(240)Pu concentrations have been determined in soil samples from the Atolls of Mururoa and Fagataufa as well as from Palomares for comparison. Mean activity ratios of241Am/239(240)Pu for Mururoa, Fangataufa and Palomares with 0.017±0.005, 0.050±0.012 and 0.25±0.07 respectively, show, that also in the most unfavorable case the241Am contamination remains less than about one third of the239(240)Pu level. The estimated isotopic composition of the different types of weapons-grade Pu based on these data suggests generally a much lower241Pu/239(240)Pu ratio for Pu used in the first fisson bombs than for the material taken later on for construction of thermonuclear devies.241Pu/239(240)Pu activity ratios of 0.9±0.2 have been found at the safety shot area of the Colette motu and 8.4±0.8 at Palomares, with some hints to a lower one, viz. 5.7±1.5 for at least one of the accidentally involved bombs.

Distributions of nuclear fuel-reprocessing tracers in the Arctic Ocean: Indications of Russian river influence

Radionuclidesampling in 1986 and 1993 in the Canada Basin, and in 1993 in theAmundsen Basin and on the adjacent Laptev shelf, provides new insights into the origin, timing, pathways, and mechanisms for dispersal of non-fallout radioactive tracers in the Arctic Ocean. First, samples from the Beaufort Sea shelf, slope, and adjacentbasin show a four-fold increase in 129I concentrationsfrom 1986 to 1993. Second, anthropogenic non-fallout radionuclide concentrations in the Beaufort Sea increasewith proximity to slope boundarycurrents.Third, there is evidence for riverine contributions of anthropogenic radionuclides to surface waters of the Amundsen Basin and the Laptev continental shelf. This evidence includes high surface water burdens of 237Np and 129I, with the maximum in anthropogenic129I found in the least saline andmost 18O-depletedwaters, consistentwith an origin in high-latitude runoff.Additionally, the 237Np/129I atom ratios in the Laptev Sea andAmundsen Basin in 1993 were signiŽ cantly lower than observed elsewhere in the Arctic Ocean and can be reasonably explained by 129I added during transit of the Russian shelves. The 240Pu/239Pu ratios in the water column were mostly near 0.18, consistent both with stratospheric bomb fallout and with the discharged-weighted mean SellaŽ eld ratio during 1966–1985. In the least saline water samples collected at the most shallow Laptev shelf station, however, the Pu ratios were lower, consistentwith a non-Europeannuclear fuel reprocessingsource.There are clear secondarymaxima in 237Np and 129I 1. Environmental Science Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee, 37831-6038,U.S.A. email: jg9@ornl.gov 2. U.S. Department of Energy, 201 Varick Street, Fifth Floor, New York, New York, 10014,U.S.A. 3. Current address: 7 Pine Cone Lane, Sparta, New Jersey, 07871,U.S.A. 4. Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, 98195, U.S.A. 5. PaciŽ c Northwest National Laboratory, Battelle Boulevard, Richland,Washington, 99352,U.S.A. 6. Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee, 37996, U.S.A. Journal of Marine Research, 57, 715–738, 1999

Radionuclide concentrations in underground waters of Mururoa and Fangataufa Atolls

In 1997 an expedition to Mururoa and Fangataufa Atolls was carried out to sample underground waters from cavity-chimneys and carbonate monitoring wells. The aim of this study was to determine the prevailing concentration and distribution status of radionuclides. Elemental analysis of interstitial waters was carried out in the water fraction as well as in particles collected at 11 underground monitoring wells. 238Pu, 239,240Pu, 241Am, 137Cs, 90Sr, 3H, 125Sb, 155Eu and 60Co were analyzed in both fractions by alpha-, beta- and gamma-spectrometry. Measurements showed that at 60% of the sites, pH and Eh seemed to be related to tidal cycles; in contrast HTO was constant during the sampling time. Interstitial waters from carbonates and transition zones shared similar chemical composition that were not different from that of the surrounding seawater. Waters collected from basalt cavities left after nuclear tests, (Aristee and Ceto) have a different chemical signature characterized by a deficiency in Mg, K and SO 4 as well as enrichment in Sr, Si, Al and Cl compared to the rest of the stations. Radionuclide concentrations present in both, water and particulate fractions, were significantly higher at Ceto and Aristee than at any other monitoring wells, except for Fuseau and Mitre monitoring wells (Fangataufa) where values similar to Ceto were found (e.g. 239,240Pu: >20 mBq g −1). Considering that Pu isotopes showed high K d values compared to non-sorbing radionuclides such as 3H, 90Sr and 137Cs it is very unlikely that migration from cavities to monitoring wells accounts for the concentration of Pu isotopes and Am at Fuseau 30 and Mitre 27. Perhaps the contact of lagoon waters with the well before sealing could be a possible source of the transuranics found at these sites. The 238Pu/ 239,240Pu ratios measured in the particles were similar to that of the lagoon (0.38), thus supporting this hypothesis. The fact that transuranics were found only in the particle fraction, in the water (colloids included) these radionuclides were below detection limits, may be accounted for the conspicuous quantity of iron oxy-hydroxides present in the particulate fraction that under the appropriate redox conditions may be interacting selectively with elements in solution (scavenging) resulting in the enhanced transuranic signal. While transuranics have been found in places of their origin, radionuclides with low K d values ( 3H, 90Sr, 137Cs) have already been transported to monitoring wells, as well as to the atolls' lagoons and the open ocean.

Plutonium and americium in fish, shellfish and seaweed in the Irish environment and their contribution to dose

Plutonium and americium activity concentrations in fish and shellfish landed in Ireland in the period 1988 to 1997 are presented. Activity concentrations in fish are low and often below detection limits, while those in mussels and oysters sampled on the northeast coast show no significant signs of decline. The estimated doses to hypothetical typical and heavy seafood consumers remain below 1 μSv yr -1 (committed effective dose). Plutonium activity concentrations measured in Fucus vesiculosus around the Irish coastline have not fallen appreciably in the ten year period between 1986 and 1996. Furthermore, the mean 238Pu/ 239,240Pu ratio of 0.17±0.05 in Fucus vesiculosus from the west coast of Ireland demonstrates the increasing significance of Sellafield-derived plutonium in those waters.

Physico-Chemical Characterisation of Inhalable MOX Particles According to the Industrial Process

The physico-chemical properties of inhalable MOX powders made according to two different industrial processes (COCA and MIMAS) have been characterised. This was done using both a Scanning Transmission Electron Microscope (STEM) at 200 kV interfaced with Energy Dispersive X ray Spectrometry (EDS), and autoradiography. For particles containing more than 2% (w/w) Pu/U+Pu, EDS showed heterogeneity in the physico-chemical composition due to the presence of both single particles of 'pure' UO 2 , 'pure' PuO 2 , mixed oxides at different Pu/U+Pu ratios, and the presence of aggregates. The presence of Pu in particles containing less than 2% Pu/U+Pu, was estimated from quantitative autoradiography and shows more than 25% of the particles emitting α tracks contained less than 0.4% Pu.

Manzala lagoon, Nile delta, Egypt: modern sediment accumulation based on radioactive tracers

This study was undertaken to determine whether recent anthropogenic changes in the Nile basin have affected the modern rate of sediment accumulation in the Nile delta. Excess 210Pb, 137Cs, and 239,240Pu were used to develop a sediment chronology for a core from central Manzala lagoon, the delta sector which has had the highest average rate of sediment accumulation during the Holocene (to about 0.7 cm year–1). Excess 210Pb was detected in the top 32 cm of the core, yielding an accumulation rate of 1.2 cm year–1, higher than the mean rate for the Holocene. A high 137Cs/239,240Pu ratio requires a reactor source (possibly Chernobyl) for these nuclides. Low concentrations of excess 210Pb and weapons-fallout nuclides precluded recognition of changes in sediment accumulation rate in Manzala lagoon during this century and may limit the use of tracer radionuclides for modern sediment chronology in the Nile delta.

Radioactivity Levels in Kola Bay

Sediment samples were collected in May 1995 from 16 locations in Kola Bay, North-west Russia, during an expedition starting from Murmansk and ending at Kildin Island in the Barents Sea. The purpose was to study the contamination level in an area with several potential sources of civilian and military radioactive pollution. 137 Cs concentrations in the sediments, algae and benthic samples were low, but small particles containing 137 Cs were separated from the sediment samples. All the sediments between the nuclear icebreaker base Atomflot and the open Barents Sea contained 60 Co. Traces of 125 Sb, 134 Cs, 95 Zr, 154 Eu and 152 Eu were also detected in some of the samples. Plutonium levels were low, but the increased 238 Pu/ 239,240 Pu ratio at Atomflot indicated a fresh release from the facility or from the waste storage vessels, Lepse and Imandra, lying in front of it. An increased 238 Pu/ 239,240 Pu ratio was also found in sediment collected in the outlet of Kola Bay in the Barents Sea.

Plutonium in Arctic Surface and Sub-Surface Waters at the St. Anna and Voronin Troughs

The St Anna Trough has been identified as a key region for water-mass exchange between the Arctic Shelf and the Central Arctic Ocean. Here, the results are presented of a recent study of 239,240Pu (and 238Pu) concentrations in filtered surface and sub-surface water collected along a latitudinal section across the St Anna and Voronin Troughs in summer 1996. The data show no evidence of enhanced plutonium concentrations arising from nuclear waste dumping in the Kara Sea. Absolute 239,240Pu levels are extremely low, being in the range 2-8 mBq.m-3 (n=23), and are consistent with expected background levels arising mainly from global fallout. Further, the 238Pu/239,240Pu ratio was found to be <0.07 at each station, indicating that the contribution from European spent fuel reprocessing plants to the observed plutonium concentrations in this zone must be very small at the present time. However, comparison of the plutonium levels with temperature and salinity data confirms that the different water masses are characterised by differences in absolute 239,240Pu concentrations. Specifically, the intrusion of modified Atlantic water, labelled by higher levels of plutonium, into the Kara Sea via the Fram Strait and the continental shelf slope has been identified in the St Anna Trough.

Isotopic composition of plutonium immissions in Austria after the Chernobyl accident

Alpha-spectrometric Pu determinations in aerosol samples collected after the Chernobyl accident in Austria show a238Pu/239 (240)Pu ratio in the range of 0.33–0.76 with a most reliable value of 0.47. From241Am measurements in old Pu preparations and air filter analyses also241Pu activities have been calculated. The ratio241Pu/239 (240)Pu during the main contamination period is 74.6±5.7, when maximum241Pu concentrations in air achieved 4.5–6.0 mBq/m3.

Anthropogenic radionuclides in tide-washed pastures bordering the Irish Sea coast of England and Wales

The activity concentrations of 137Cs,238 Pu, 239,240Pu and 241Am were measured in root mat and vegetation samples collected from tide washed pastures in 17 estuaries spanning the eastern seaboard of the Irish Sea, extending from the Solway in north-west England to St. David's Head in south Wales. Some of these estuaries had been investigated in previous surveys, but this study is unique in that it covered a wide geographic range using the same sampling and analytical methodology and within a comparatively short time scale. This allows for a valid comparison within the data set of the contamination levels at the different areas. Spatial distributions of the radionuclides were consistent with transport of radionuclides discharged to the Irish Sea from the Sellafield Nuclear Reprocessing Plant, with the highest activities occurring in the Esk estuary (closest to Sellafield) and lowest at the Welsh sites. Measurable activity concentrations of238 Pu and 239,240Pu were found in root mat samples from the Solway estuary to as far south as the Gwyrfai in Wales and showed an average238 Pu/239,240Pu ratio of 0.2, consistent with Sellafield-derived Pu. The ratios of137 Cs/241Am increased with distance from the source, with values of 1:1 in estuaries near Sellafield to ratios between 2 and 5 in estuaries further south and in excess of 10:1 in Wales. This is probably due to the more rapid movement of dissolved137 Cs in Irish Sea waters compared with the actinides. In contrast, 241Am and 239,240Pu behave similarly with consistent ratios of between 2:1 and 3:1, as both are associated with particulates. Dose assessment calculations suggest that external exposure would be a maximum of 530 µSv at the most contaminated spot at the Esk estuary. Relatively lower doses arise from the ingestion of animal products (along the soil-vegetation-grazing animal pathway) due to the low availability of sediment-associated radionuclides for gut transfer. The dose assessment calculations highlight the importance of using the appropriate transfer parameters that take into account this low bioavailability.

Monitoring of environmental radiation and radionuclides around nuclear fuel cycle facilities from 1989 to 1995

Aomori Prefectural Govemment and Japan Nuclear Fuel Limited started environmental radiation monitoring around Nuclear Fuel Cycle Facilities in April 1989. External radiation is measured by NaI(TI) scintillator and Themoluminescence dosimeter. The level of external radiation is relatively low in the winter due to snow. We have collected terrestrial samples of drinking water, soil, agricultural products and marine samples of seawater, sea sediment, seafood, etc. periodically. We have measured many radionuclides and fluoride in these samples. In soil sample,239+240Pu ratio to137Cs was almost constant at all sampling points. A correlation was observed between salinity and concentration of tritium, uranium and fluoride in lake water of Lake Obuchinuma. The correlation between137Cs and239+240Pu in lake sediment was observed in each lake.

Concentration and vertical distribution of plutonium and americium in Italian mosses and lichens

The plutonium and americium concentration and vertical distribution in some Italian mosses and lichens have been determined. The239,240Pu,238Pu and241Am concentration ranges in tree trunk lichens 0.83–1.87, 0.052–0.154 and 0.180–0.770 Bq/kg, respectively. The corresponding values in tree mosses are higher and more scattered ranging from 0.321 to 4.96, from 0.029 to 0.171 and from 0.200 to 1.93 Bq/kg. The mean238Pu/239,240Pu and241Am/239,240Pu ratios are 0.088±0.037 and 0.38±0.13 in lichens and 0.091±0.072 and 0.54±0.16 in tree mosses. The Pu and Am concentrations are relatively low in terrestrial mosses. The239,240Pu,238Pu and241Am vertical distributions in a terrestrial moss core (Neckera Crispa) collected near Urbino (central Italy) show an exponential decrease with the height. On the contrary the241Am vertical distribution in another terrestrial moss core (Sphagnum Compactum) collected in the Alps (northern Italy) shows an interesting peak at 16 cm which corresponds to the deposition of fallout from the nuclear weapon tests in 1960's. The241Am movement upward and downward in the moss core is also studied. The results show once again that both mosses and lichens are very effective accumulators of Pu and Am and that they can be used as good biological indicators of the radionuclide airbome pollution from nuclear facilities and nuclear weapon tests. They can play a very impotant role in cycling naturally or artificially enhanced radionuclides in the atmosphere over long time scales.

Radiochemical contamination of bottom sediments of some natural and artificial lakes of Middle East Poland

An investigation of the presence of some radioactive isotopes in the bottom sediments of two lakes, Piasecmo and Magluchowskie (Middle East of Poland) has been performed. Similar measurements were done at the Zalew Zemborzycki artificial water reservoir, filled up few years after the most intensive period of atmospheric nuclear explosions. It was estimated, that the concentration of radionuclides, especially of aaificial origin was much greater in natural lakes than in examined artificial one. That testifies for an accumulation of radioactive substances on the bottom of the water tanks. The confirmation of this thesis may be the fact, that 85% of 137Cs detected in Zalew Zembor'zycki comes from Chemobyl fallout while in natural lakes this isotope amounts only to 56-60%. Other isotopes, such as 210pb, 226Ra, and 224Ra'were detected in significant amounts. The radioactivity of 239'240pu isotopes reaches 2.2-2.6 Bq/kg for the lakes and 0.7 for Zalew Zemborzycki. The 238pu to 239,240pu ratio suggests, that detected plutonium comes from world fallout.

240Pu/239Pu Ratios in Sun Moon Lake Sediments: Implications for Sources of Nuclear Fallout in Taiwan

The isotopic composition of plutonium in dated Sun Moon Lake sediments was determined by mass spectrometry in an effort to trace the source(s) and timing of nuclear fallout in Taiwan. The (superscript 240)Pu/(superscript 239)Pu ratios in the sediment core vary by a factor of＞5, with the highest ratio, 0.22, corresponding to the maximum level of nuclear fallout in 1963 and the lowest ratios, 0.04-0.06, being identified at two time horizons: 1982-83 and 1992-93. At other depth intervals, the (superscript 240)Pu/(superscript 239)Pu ratios fall within 0.15-0.19. The data indicate that global fallout from the stratosphere is the dominant source of Pu in the sediments, whereas the tropospheric transport of low burn-up debris from Lop Nor in NW China is mostly responsible for the unusually low (superscript 240)Pu/(superscript 239)Pu ratios.