首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
《Marine Geology》2005,216(4):249-263
Bottom sediments collected in the Northwest (NW) Pacific Ocean in 1997 were analysed for 90Sr, 137Cs, 239,240Pu and 241Am contents to determine their distribution patterns, inventories and sources. Enhanced inventories of 239,240Pu and 241Am were observed in the latitudinal belts of 10–20°N and 30–40°N, which correspond to major areas of local (tropospheric) and global (stratospheric) fallout (with a contribution from local fallout), respectively. The sediment inventory of 239,240Pu near the Bikini Atoll exceeded its overlying water inventory, however, in the mid-latitudes, more than 70% of 239,240Pu still remains in the water column. 241Am inventories in sediments exceeded that of the water column for the entire NW Pacific Ocean. Higher 137Cs and 90Sr sediment inventories in the latitudinal belt of 30–40°N are due to global fallout, and they account for about 10% and less than 5% of the water column inventories, respectively. The observed activity ratios of 137Cs/90Sr, 238Pu/239,240Pu and 241Am/239,240Pu in sediment were at some stations higher than the global fallout ratios due to contributions from local fallout and due to specific processes in the water column. Two end-member mixing model based on the 240Pu/239Pu atom ratios observed in global and local fallout yielded ∼60% contribution of the local fallout in the bottom sediments near the Bikini Atoll. The upward decrease in the 240Pu/239Pu atom ratios in the sediment column indicates a decrease in the contribution of local fallout to the Pu inventory with time. 241Am and 241Pu dating of sediment layers was utilized to explain a hiatus in sediment accumulation in the deep seafloor.  相似文献   

2.
The distribution and inventory of artificial radionuclides,239,240Pu and137Cs were determined in the East China and the Yellow Seas in 1987. Almost all of239,240Pu and 50 to 80% of137Cs in the continental shelf area are retained in the sediment column.239,240Pu sediment inventory in the sea area is larger than the fallout input and tends to increase southwardly. This excess239,240Pu and the lateral distribution are attributable to the supply of239,240Pu by the Yangtze River discharge. On the contrary,137Cs sediment inventory shows a decrease to the south, and the fact can be accounted for by the southward dispersion of fine silt particles discharged from the Yellow River. Total137Cs inventory is smaller than the estimated fallout input, and the fact seems to indicate dispersion of137Cs out of the shelf region. Vertical profiles of239,240Pu and137Cs contents in sediments differ from that of natural210Pb, implying the effect of varied accumulation rates of the artificial radionuclides over the sediment particle mixing by benthic organisms. Apparent maximum sediment particle mixing coefficient (D B ) calculated from the excess210Pb profiles in stations located between the inner and outer shelves ranged from 1.4 to 8.3 cm2y–1. ThisD B value is higher than that in the Okinawa Trough (1.0 cm2y–1), but lower than previously estimatedD B value (26 cm2y–1) in the outer shelf mud.  相似文献   

3.
239, 240Pu,137Cs and90Sr concentrations were determined in sea waters from the central and western North Pacific in 1980 and 1982. The results are consistent with those reported earlier for North Pacific waters. The profiles of90Sr and137Cs show a monotonic decrease with depth, whilst239, 240Pu shows a distinct subsurface maximum at a depth between 400 and 1,000 m. The calculated inventories of these nuclides significantly exceed the global mean fallout inputs for these latitudes. This may be due to local fallout input to the ocean at times of large-scale nuclear weapon tests in the equatorial North Pacific. The existence of measurable amounts of137Cs and239, 240Pu in deep waters suggests that these nuclides are transported by sinking particulate matter from the surface to the deep ocean.  相似文献   

4.
Excess 210Pb was determined in six abyssal cores from the Eastern Atlantic, Hatteras Abyssal Plain, and Puerto Rico Trench. A bioturbation rate for each core was evaluated using the mixing model of Guinasso and Schink. The rates were compared to mixing rates evaluated for the same cores using 239,240Pu. In all cases, the excess 210Pb-derived mixing rates were lower than the plutonium-derived mixing rates. 239,240Pu was also present at greater depths than excess 210Pb. One possible explanation for these observations is the association of plutonium with particles of greater food value which are bioturbated at faster rates.  相似文献   

5.
Profiles of 210Pb and 239 + 240Pu from sediment cores collected throughout Massachusetts Bay (water depths of 36–192 m) are interpreted with the aid of a numerical sediment-mixing model to infer bioturbation depths, rates and processes. The nuclide data suggest extensive bioturbation to depths of 25–35 cm. Roughly half the cores have 210Pb and 239 + 240Pu profiles that decrease monotonically from the surface and are consistent with biodiffusive mixing. Bioturbation rates are reasonably well constrained by these profiles and vary from 0.7 to 40 cm2 yr−1. As a result of this extensive reworking, however, sediment ages cannot be accurately determined from these radionuclides and only upper limits on sedimentation rates (of 0.3 cm yr−1) can be inferred. The other half of the radionuclide profiles are characterized by subsurface maxima in each nuclide, which cannot be reproduced by biodiffusive mixing models. A numerical model is used to demonstrate that mixing caused by organisms that feed at the sediment surface and defecate below the surface can cause the subsurface maxima, as suggested by previous work. The deep penetration depths of excess 210Pb and 239 + 240Pu suggest either that the organisms release material over a range of >15 cm depth or that biodiffusive mixing mediated by other organisms is occurring at depth. Additional constraints from surficial sediment 234Th data suggest that in this half of the cores, the vast majority of the present-day flux of recent, nuclide-bearing material to these core sites is transported over a timescale of a month or more to a depth of a few centimeters below the sediment surface. As a consequence of the complex mixing processes, surface sediments include material spanning a range of ages and will not accurately record recent changes in contaminant deposition.  相似文献   

6.
Concentrations and inventories of137Cs and239,240Pu were determined in sea waters and sediments columns from the western North Pacific from 1980 to 1986.The239,240Pu/137Cs activity ratio in the water column shows a tendency to increase from the surface (10–3) to bottom waters (10–1), but the ratio in sediment is within a rather narrow range (10–2 to 10–1), indicating more effective removal of plutonium from the water column than137Cs. In regions south of 40N, the radionuclide inventories in the water column significantly exceed the estimated global fallout (stratospheric fallout due to the atmospheric nuclear explosion), especially in the case of239,240Pu. These excess inventories imply that local or close-in fallout derived from nuclear explosions in the equatorial North Pacific are well-preserved and retained in the regions, despite about 20 years since the atmospheric nuclear explosion moratorium. Data suggesting lateral transport of137Cs in surface water from north of 40N to southern regions is shown. Some data on90Sr contents are also shown.  相似文献   

7.
We report here thermal ionization mass spectrometry measurements of 239Pu, 240Pu, 241Pu, 242Pu, and 237Np isolated from oceanic, estuarine, and riverine sediments from the Arctic Ocean Basin. 238Pu/239+240Pu 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 241Pu/239Pu 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.  相似文献   

8.
Radionuclide measurements have been conducted on sediment cores collected in 1992 in the south-eastern region of the Barents Sea, known as the Pechora Sea. Cesium-137 and 239,24OPu activities in surface sediments are generally less than 30 Bq/kg, with the highest levels being measured in sediments off the southwestern coastline of the island of Novaya Zemlya. High correlations between both 137Cs and 239,24OPu and the concentration of fine (< 63 μm) particles in surface sediments indicate that much of the variance in radionuclide concentrations throughout the Pechora Sea can be explained by particle size fractionation. However, elevated activities of 137Cs (138 Bq/kg), 60Co (92 Bq/kg), 241Am (433 Bq/kg), and especially 239,24OPu (8.47 × 103 Bq/kg) were measured in one surface sediment sample from the fjord of Chernaya Bay on the southern coast of Novaya Zemlya. The source of radioactive contamination is two underwater nuclear tests conducted in Chernaya Bay in 1955 and 1957.The 238Pu/239,240Vu activity ratio of 0.0245 in Chernaya Bay is equivalent to values measured in global fallout. The 240Pu/239Pu atom ratio (0.0304), measured by mass spectrometry, is much lower than values (0.18) typical of global fallout, but is consistent with ratios measured for fallout from the early (1951–1955) series of weapons tests at the Nevada Test Site. The timing of the Chernaya Bay source term, estimated from the 241Am/241Pu ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear tests. Relatively low initial yields of 241Pu (241Pu/239Pu atom RATIO = 0.00 123) in these tests have resulted in relatively low 241Am/239,240Pu activity ratios (0.05) in recent sediments in Chernaya Bay.Radionuclide tracer profiles in cores from the Pechora Sea can be simulated using a two-layer biodiffusion model with rapid, near-homogeneous mixing in the surface mixed layer and reduced mixing in the deep layer. Lead-210 profiles are consistent with a wide range of sedimentation and mixing rates in the deep sediment layer. However, the 137Cs and 239,240Pu results further constrain the model parameters and indicate that the downward transport of radionuclides in the sediments is governed primarily by sediment mixing, with sediment burial playing a secondary role.  相似文献   

9.
The contents of plutonium isotopes (239Pu and238Pu), thorium isotopes (232Th,230Th and228Th) and protactinium-231 in sea water collected in the North Pacific, the East China Sea and the Japan Sea were determined. These nuclides were sequentially analyzed byα-ray spectrometry after separating them mainly with solvent extraction technique. The contents of239Pu in surface sea water ranged from 0.6 to 1.6 pCi/10001,238Pu/239Pu activity ratios being 0.2～0.7. The228Th/232Th activity ratios for the North Pacific waters varied between 7.6 and 30, whereas the sample from the East China Sea showed the very high value, 65. The contents of231Pa are less than 6 percent of that in equilibrium with its parent235U. Furthermore, the analysis of plutonium isotopes in recent coral from Yoron Island was carried out and it was confirmed that plutonium isotopes have concentrated in recent coral with the concentration factor of about 1～2×103.  相似文献   

10.
Surface intertidal sediments from 35 sites in the Irish Sea have been analysed for their 238Pu and 239,240Pu activities, together with an intensive study of plutonium in sediments of the Esk Estuary (NW England). The range of plutonium activities for the whole survey were 0·14–4118 and 1·3–16 026 Bq kg−1 for 238Pu and 239,240Pu, respectively. The levels of Pu activity, derived from the Sellafield nuclear fuel reprocessing effluents, in sediments are controlled by lithological factors and the influence of transport and post-depositional processes. Grain size distribution is particularly important, the major part of plutonium activity being in the mud fraction of all sediments.The data suggest that over the Irish Sea coastline, dynamic mixing of sediment grains by reworking and resuspension and/or by dispersion in tidal currents are important in determining plutonium distributions. The exponential decrease in sediment plutonium activities away from the Sellafield source is attributed to the progressive mixing with older contaminated and uncontaminated sediments.  相似文献   

11.
Anthropogenic 90Sr, 239,240Pu and 241Am were used as tracers of water mass circulation in the Crozet Basin of the South Indian Ocean, represented by three main water fronts—Agulhas (AF), Subtropical (STF) and Subantarctic (SAF). Higher 90Sr concentrations observed north of 43°S were due to the influence of AF and STF, which are associated with the south branch of the Subtropical gyre, which acts as a reservoir of radionuclides transported from the North to the South Indian Ocean. On the other hand, the region south of 43°S has been influenced by SAF, bringing to the Crozet Basin Antarctic waters with lower radionuclide concentrations. The 238Pu/239,240Pu activity ratios observed in water and zooplankton samples indicated that, even 35 years after the injection of 238Pu to the Indian Ocean from the burn-up of the SNAP-9A satellite, the increased levels of 238Pu in surface water and zooplankton are still well visible. The radionuclide concentrations in seawater and their availability to zooplankton are responsible for the observed 210Po, 239,240Pu and 241Am levels in zooplankton.  相似文献   

12.
Bivalve shells offer several advantages over tissues for the monitoring of heavy metal pollutants in the marine environment. They are easier to handle and to store. The problem of whether to depurate the animals before analyses is avoided. The shells appear to be more sensitive to environmental heavy metals levels over the long term than do the soft parts. Of the substances examined (Cd, Cu, Zn, Pb, Ag, Ni, 238Pu and 239 + 240Pu) only Pb and Pu displayed a strong covariance between soft tissue and shell concentrations. There were strong correlations between metals in the shell but not in the soft tissues in general. The byssal threads, because of their enrichment of transuranic elements and of their ease in handling, may be useful in monitoring these metals. A very weak discharge of 238Pu to marine waters adjacent to a nuclear reactor was detected in the byssal threads of mussels.  相似文献   

13.
《Marine Chemistry》2001,73(2):125-152
We report here bioturbation and sediment accumulation rates determined from replicate sediment cores at four different sampling sites on the Palos Verdes shelf, Southern California, using bomb fallout and natural radionuclides (137Cs, 239,240Pu, 210Pb, 234Th, and 14C), along with supporting measurements of organic carbon (OC), porosity and granulometry. Present-day particle reworking, on time scales of several months, is restricted to the upper 3 cm, with rates ranging from 13 to 200 cm2/year, as deduced from 234Thxs profiles. There is little evidence that particle reworking reached depths significantly greater than 5 cm. Post-1963 (or post-1971) sediment accumulation rates ranged from 0.7 to 1.4 g/cm2/year (equivalent to 1.1–1.8 cm/year for surficial sediments), as calculated from Pu and Cs isotope profiles, with little change over time or distance from the outfall. Lateral transport of older sediment and multiple sediment sources on the Palos Verdes shelf is suggested from radiocarbon measurements on foraminifera and bulk sedimentary organic matter at two sampling sites, which showed variable, old and refractory sources of OC. Pre-1953 sediments accumulated at rates that were at least 0.4 g/cm2/year (≥0.3 cm/year), based on 210Pbxs dating. Given the abundance of sediment sources to the Palos Verdes shelf, the high sedimentation rates, and shallow particle mixed layers, contaminant-enriched layers should continue to move deeper into the sediments.  相似文献   

14.
Arctic sea ice can incorporate sediment and associated chemical species during its formation in shallow shelf environments and can also intercept atmospherically transported material during transit. Release of this material in ice ablation areas (e.g. the Fram Strait) enhances fluxes of both sediments and associated species in such areas. We have used a suite of natural (7Be, 210Pb) and anthropogenic (137Cs, 239Pu, 240Pu) radionuclides in sea ice, sea-ice sediments (SIS), sediment trap material and bottom sediments from the Fram Strait to estimate transit times of sea ice from source to ablation areas, calculate radionuclide fluxes to the Fram Strait and investigate the role of sea-ice entrained sediments in sedimentation processes. Sea ice intercepts and transports the atmospherically supplied radionuclides 7Be and 210Pb, which are carried in the ice and are scavenged by any entrained SIS. All of the 7Be and most of the excess 210Pb measured in SIS collected in the Fram Strait are added to the ice during transit through the Arctic Ocean, and we use these radionuclides as chronometers to calculate ice transit times for individual ice floes. Transit times estimated from the 210Pb inventories in two ice cores are 1–3 years. Values estimated from the 7Be/210Pbexcess activity ratio of SIS are about 3–5 years. Finally, equilibrium values of the activity ratio of 210Pb to its granddaughter 210Po in the ice cores indicate transit times of at least 2 years. These transit times are consistent with back-trajectory analyses of the ice floes. The latter, as well as the clay-mineral assemblage of the SIS (low smectite and high illite content), suggest that the sampled sea-ice floes originated from the eastern Siberian Arctic shelf seas such as the eastern Laptev Sea and the East Siberian Sea. This result is in agreement with the relatively low activities of 239,240Pu and 137Cs and the 240Pu/239Pu atom ratios (∼0.18, equivalent to that in global fallout) in SIS, indicating that prior global atmospheric fallout, rather than nuclear fuel reprocessing facilities, forms the main source of these anthropogenic radionuclides reaching the western Fram Strait at the time of sampling (1999). Transport of radionuclides by sea ice through the Arctic Ocean, either associated with entrained SIS or dissolved in the ice, accounts for a significant flux in ablation areas such as the Fram Strait, up to several times larger than the current atmospheric flux in the area. Calculated fluxes derived from sea-ice melting compare well to fluxes obtained from sediment traps deployed in the Fram Strait and are consistent with inventories in bottom sediments. 240Pu/239Pu atomic ratios lower than 0.18 in bottom sediments from the Fram Strait provide evidence that plutonium from a source other than atmospheric fallout has reached the area. Most likely sources of this Pu include tropospheric fallout from atomic weapons testing of the former Soviet Union prior to 1963 and Pu released from nuclear reprocessing facilities, intercepted and transported by sea ice to the ablation areas. Future work is envisaged to more thoroughly understand the actual mechanisms by which radionuclides are incorporated in sea ice, focusing on the quantification of the efficiency of scavenging by SIS and the effect of melting and refreezing processes over the course of several years during transit.  相似文献   

15.
Thermodynamic considerations based on existing data from various laboratory studies of plutonium species in aqueous solution are used to predict the speciation of this radioactive pollutant in seawater. Oxidation-reduction data for plutonium suggest that Pu(VI) should be very dominant in seawater solution compared to Pu(IV), and that Pu(III) and Pu(V) should be absent. The disproportionation reactions and the alpha reduction mechanism are probably of no consequence to the oxidation state in seawater. However, the irreversible hydrolysis of Pu4+ and the associated formation of polymeric Pu(OH)4 colloids are important mechanisms of speciation control and plutonium removal to sediments, by adsorption onto suspended matter. Stability constants for plutonium complexation with inorganic ligands in seawater suggest that Pu(VI) dissolved in seawater will be dominantly PuO2CO3OH−.The theoretical predictions of plutonium speciation and behaviour in seawater are compared to the only available data on plutonium speciation in seawater (Nelson and Lovett, 1978). Good agreement between the predictions and field observations was obtained, within the limitations imposed by the scarcity of data on this subject.  相似文献   

16.
Laboratory experiments were carried out to assess the role of photochemical reactions upon the dissolution of 239 + 240pu and 241Am from marine sediment in sea water. Supplementary information was obtained by comparing their behaviour with that of 54Mn under similar experimental conditions. Irradiation from natural sunlight resulted in more than a ten-fold increase in the extent of 239 + 240Pu desorption relative to that observed in the dark. Remobilisation of 54Mn from sediment was also enhanced by natural sunlight, albeit to a lesser extent than 239 + 240Pu, whilst the behaviour of 241Am was largely unaffected. Data for concentrations of dissolved 239 + 240Pu(IV) and 239 + 240Pu(V) species showed that only the oxidised form was significantly affected by irradiation, indicating remobilisation occurs as a result of photooxidation reactions. Further experiments were carried out using artificial light sources to establish the influence of wavelength. Data from these investigations indicated 239 + 240Pu photooxidation (hence desorption) was a function of both light intensity and wavelength. Remobilisation decreased concomitant with light intensity but increased as the wavelength decreased. Similar trends were observed for photoreduction of 54Mn, although differences were less pronounced than those observed for 239 + 240Pu.  相似文献   

17.
The results of the U.S. Mussel Watch Monitoring Program for the period 1976–1978 for trace metals and artificial radionuclides in bivalves are presented. The substances analysed included Ag, Cu, Zn, Cd, Ni, Pb, 238Pu, 239+240Pu and 241Am. The analyses of organic substances will be presented elsewhere. The concentrations of these substances in the bivalves may reflect upwelling processes, anthropogenic inputs or natural levels. Off the California coast, mussels show markedly elevated Pu and Cd concentrations in coastal areas adjacent to the most intensive upwelling zones. Elevated levels of Pb, for example, are found in organisms living adjacent to highly urbanized places. The general patterns of distribution repeat themselves year after year at a given site. Thus, it is concluded that annual monitoring activities may not be necessary and that a frequency of sampling of several or so years may be more appropriate to identify pollution problems. Finally, national or regional baselines for metal concentrations in bivalves from unpolluted waters are proposed. National baselines for Pb in the west coast mussels of 1·0 parts 10?6 and for Ag in east coast mussels of 0·05 parts 10?6 are suggested.  相似文献   

18.
In order to estimate the deposition rate of extraterrestrial material onto a manganese crust in a search for supernova debris, we analyzed the contents of 10Be, 230Th, 231Pa, and 239,240Pu in a sample of manganese crust collected from the North Pacific Ocean. On the basis of the depth profile of 10Be, the growth rate of the manganese crust was determined to be 2.3 mm Myr−1. The uptake rates of 10Be, 230Th, and 231Pa onto the manganese crust were estimated to be 0.22–0.44%, 0.11–0.73%, and 1.4–4.5%, respectively, as compared to the deposition rates onto the deep-sea sediments near the sampling station, while that for 239,240Pu was 0.14% as compared to the total inventory of seawater and sediment column. Assuming that sinking particles represent 0.11–4.5% of the uptake rates, the deposition rate of extraterrestrial material onto the manganese crust was estimated to be 2–800 μg cm−2Myr−1 according to the uptake of 10Be onto the manganese crust. Further, our estimate is similar to the value of 9–90 μg cm− 2Myr−1 obtained using the integrated global production rate of 10Be and the deposition rate of 10Be onto the manganese crust.  相似文献   

19.
1958年,Thomas等人首次测定了海洋生物中的鈈(Pu),从而开始了海洋环境中超铀元素的研究。七十年代,超铀元素的研究已成了海洋同位素化学的重要內容之一,甚至超过了对~(90)Sr,~(187)Cs等核素的研究。因为超铀元  相似文献   

20.
The extent and kinetics of Np(V)O2+ adsorption from dilute aqueous solutions and seawater onto a variety of synthetic and natural solids were determined at 25°C and 1 atm total pressure. Extensive and complex adsorption reactions were found, contrary to speculations in the literature that NpO2+ should behave as a simple monovalent ion with a low affinity for surfaces. When normalized to adsorption per unit solid surface area, the ranking for the synthetic solids was aragonite ? calcite > goethite ? MnO2 ≈ clays. Natural materials generally followed the same behavior patterns as their synthetic counterparts. The dissolved/adsorbed ratio was found to be constant over a wide range (10?13–10?7M) of NpO2+ concentrations. At higher concentrations the extent of adsorption decreased until a solubility limit was reached at approximately 10?5 M.Solution composition had the most significant influence for NpO2+ adsorption on goethite, where much more extensive adsorption occurs in dilute solutions than in seawater. When seawater is added to a dilute solution, extensive desorption of NpO2+ from goethite occurs. Tests conducted on NpO2+ adsorbed on carbonates indicated that it remained in the V oxidation state.There is a growing consensus that Pu dissolved in natural waters also occurs dominantly in the V oxidation state as PuO2+ ion. Consequently, these results for NpO2+ may serve as a guide for Pu behavior when also in the V oxidation state. The fact that most adsorbed Pu is found in the III or IV oxidation states indicates that reduction of Pu may occur subsequent to adsorption in the V oxidation state.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号