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1.
The analyses of plutonium isotopes in dated strata of polar ice sheets indicate that the241Pu/239+240Pu activity ratio produced in the U.S.-dominated nuclear tests during the 1950s was about 26, while that in the U.S.S.R.-dominated weapons tests in the early 1960s was between 12 and 14. This difference provides time horizons for sedimentary deposits. Further, the239Pu/240Pu ratio may show a similar difference in fallout values from these two periods of weapons testing and may provide an additional, and perhaps more sensitive, chronology for sediments.  相似文献   

2.
Mass spectrometric analyses of low levels of global fallout plutonium separated from Atlantic marine samples have differentiated fallout239Pu and240Pu in aquatic samples for the first time. The results show no single characteristic240Pu/239Pu ratio in marine samples; the observed range is from 0.11 to 0.24 on an atom basis. There are indications that differences exist in the chemical or physical form of plutonium from atmospheric fallout in Atlantic surface water and that selective concentration in surface organisms is occurring. No single240Pu/239Pu value is found in pelagic sediments collected from different depths and locations. Discounting sources other than fallout, our results show that the plutonium deposited at any given time since atmospheric testing began may have carried a unique240Pu/239Pu tag. This label may be extremely useful to trace fallout plutonium through biogeochemical cycles.  相似文献   

3.
Anthropogenic radionuclides have reached the Hudson estuary as global fallout from nuclear weapons testing and through local releases from commercial nuclear reactors. Significant activities of238Pu and239,240Pu (fallout-derived),134Cs and60Co (reactor-released), and137Cs (derived from both sources), have accumulated in the sediments throughout the estuary, with the primary zone of accumulation near the downstream end of the system in New York harbor. The estuary appears to have trapped nearly all of the239,240Pu delivered as fallout, and consequently, ocean dumping of dredged harbor sediment is currently the primary means for the net transport of these nuclides to coastal waters. In contrast, only 10–30% of the137Cs,134Cs and60Co delivered to the estuary have been retained on the fine particles which accumulate at a rapid rate in the harbor.The primary factors which have governed the distribution of anthropogenic radionuclides in Hudson sediments are: (1) spread of fine particles labeled with both fallout and reactor nuclides throughout the axis of the estuary, (2) differences in timing of the peak fallout years (1962–1964) and years of maximum reactor releases (1971–1972), (3) large variations in sediment accumulation rates, ranging from a few millimeters per year or less to many tens of centimeters per year, (4) appreciable desorption of137Cs and134Cs from particles at higher salinities, and (5) possible enhanced desorption of60Co at higher salinities (relative to134Cs and137Cs) which may be associated with the release of reduced manganese from the harbor sediments.  相似文献   

4.
Analyses are presented of137Cs,238Pu, and239,240Pu, in relation to depth in sediment, in 21 gravity cores. These cores span the ranges of times 1964–1975, and of water depths 12–2000 m; they come from three distinct sedimentation areas off the northeast coast of the United States. Although the ranges of total sediment inventories of239,240Pu and of137Cs from the various areas hardly overlap, the range of ratios of the inventories of these two nuclides is probably the same in all the areas. In the shallow-water cores the239,240Pu/137Cs ratio regularly diminishes with depth in the core, and a tendency is seen for curves of this function to have similar slopes in each area; ratios of238Pu/239,240Pu show no change with depth in these shallow-water cores. In the deeper-water cores, the239,240Pu/137Cs ratio shows no systematic change with depth, but sometimes the238Pu/239,240Pu ratio shows a minimum at the sediment surface, and is much higher deeper in the cores. We believe that these phenomena can be explained in terms of a complicated bioturbational process moving the nuclides, together, down into the sediments, of chemical resolubilization, at depth, of plutonium only, and of its subsequent upward translocation in the interstitial solution. Some re-immobilization of plutonium near the sediment surface is implied, and a mechanism is suggested for this, based on displacement of plutonium from organic complexes by the increasing concentrations, in upper layers of the sediment, of re-oxidized dissolved iron.  相似文献   

5.
137Cs和241Am在滇池、剑湖沉积孔柱中的蓄积分布及时标意义   总被引：16，自引：2，他引：14  
项亮  吴瑞金  吉磊 《湖泊科学》1996,8(1):27-34
自70年代以来,对湖泊沉积物中核试验散落核素的研究已成为研究现代湖泊沉积过程和沉积年代及沉积速率的一个重要方面,本文通过对分别采自云南省滇池、剑湖的两个沉积孔柱中~(137)Cs和~(241)Am蓄积分布的研究,测定了它们60、70年代以来的沉积速率分别是42.1和28.7mg/(cm~2·a)。研究发现,虽然~(241)Am的含量远低于~(137)Cs,但~(241)Am具有沉积后迁移能力较小,半衰期长等特点,因而随着时间的推移,在未来湖泊现代沉积作用的研究中将会显示出越来越重要的作用。  相似文献   

6.
A study of sediments in the Gulf of Mexico shows dramatic gradients in Pu content and isotope ratios from the continental shelf to the Sigsbee Abyssal Plain. In terms of predicted direct fallout inventory of Pu, one shelf core contains 745% of the predicted inventory, while abyssal plain sediments contain only 15–20% of the predicted value. Absolute Pu concentrations of shelf sediments are also conspicuously high, up to 110 dpm/kg, compared to 13.5 dpm/kg in Mississippi River suspended sediment. There is no evidence of Pu remobilization in Gulf of Mexico shelf sediments, based on comparison of Pu profiles with Mn/Al and Fe/Al profiles. Horizontal transport of fallout nuclides from the open ocean to removal sites in ocean margin sediments is concluded to be the source of both the high concentrations and high inventories of Pu reported here.The shelf sediments show240Pu/239Pu ratios close to 0.179, the average stratospheric fallout value, but the ratios decrease progressively across the Gulf to low values of 0.06 in abyssal plain sediments. The source of low-ratio Pu in deep-water sediments may be debris from low yield tests transported in the troposphere. Alternatively, it may represent a fraction of the Pu from global stratospheric fallout which has been separated in the water column from the remainder of the Pu in the ocean. In either case, the low-ratio material must have been removed rapidly to the sea floor where it composes a major fraction of the Pu in abyssal plain sediments. Pu delivered by global atmospheric fallout from the stratosphere has apparently remained for the most part in the water or has been transported horizontally and removed into shallow-water sediments.  相似文献   

7.
137Cs湖泊沉积年代学方法应用的局限——以Crawford湖为例   总被引：17，自引：5，他引：12  
项亮 《湖泊科学》1995,7(4):307-313
＾１３７Ｃｓ湖泊沉积年代学方法是测定现代湖泊沉积物沉积年代和沉积速率的重要同位素年代学方法之一。对加拿大Ｃｒａｗｆｏｒｄ湖采集的沉积孔柱内＾１３７Ｃｓ垂直分布的研究发现,该方法给出时的标有明显偏差。比较＾２１０Ｐｂ和纹层等年代方法的结果,其１９６３年时标蓄积峰值所在位置明显移向表层,而作为１９５４年时标的该核素出现蓄积的层位则远早于该年沉积物蓄积层位。研究还表明,孔柱中较高的间隙水含量、缺少足以吸  相似文献   

8.
The annual fluxes of artificial radionuclides (238Pu,239+240Pu,241Am,137Cs,90Sr and3H) from the atmosphere to the Ross Ice Shelf in Antarctica were determined from measurements in strata dated by210Pb. Recognizable sources include the U.S. tests (Mike-Ivy and Castle Hill) in the early 1950s, the U.S.S.R. tests of the early 1960s, the SNAP-9A burnup of 1964 and the French and Chinese tests in the late 1960s and 1970s. There are several problems still awaiting resolution: the differences in atmospheric chemistries of fission products and of transuranics produced in weapons tests and the anomalous fluxes of238Pu to the ice shelf which do not appear to reflect a one-year stratospheric residence. There is no evidence for a smearing of the fallout record as a consequence of diffusion of these radionuclides in the glacial column.  相似文献   

9.
Cosmogenic7Be(t1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess210Pb and/or239,240Pu have provided limits on rates of sediment accumulation in these environments and indices of the efficiency of the sediments as collectors of reactive nuclides over longer time scale.In sediment cores from Long Island Sound (marine) and Lake Whitney (fresh-water)7Be was measurable in the top 2–3 cm. Diffusion-analog particle-mixing coefficients calculated from these data are in the range of 10?7 cm2/s. For Long Island Sound the coefficients are lower by factors of 3–6 than those estimated from the depth distributions of excess234Th at the same stations [14]. For Lake Whitney the calculated mixing coefficient is an upper limit because of the possibility of a sampling artifact.Measurements of total (wet + dry) atmospheric deposition of7Be in New Haven give an average flux of 0.07 dpm/cm2 day during March-November, 1977; this is equivalent to a steady-state inventory of 5.4 dpm/cm2 in a perfect collector. Sediment cores from Long Island Sound contain about half this7Be inventory, consistent with either a mean residence time for7Be in the water column of about one half-life or with post-depositional loss of7Be from Long Island Sound sediments. The Lake Whitney cores contain about 5 dpm/cm2, much nearer the atmospheric delivery. A higher inventory of7Be in fresh-water, as compared to marine, sediments could be due either to a shorter mean residence time for7Be in fresh water or to lateral transport processes in the lake or its catchment. High inventories of excess210Pb and239,240Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.  相似文献   

10.
From GEOSECS stations, largely, the 1974 distributions of Pu and of137Cs are described in the Pacific Ocean north of about 20°S latitude. Changes in some of these distributions are described from 1978 cruises by the authors.The Pacific exhibited, everywhere, a shallow subsurface layer of Pu-rich water with its concentration maximum at about 465 m in 1974; over a large portion of the central North Pacific a second layer of Pu-labelled water, less concentrated than the shallow layer, lay just above the bottom. Similar features were not observed in the case of137Cs.The inventories of both Pu and137Cs in the water column at most 1974 stations are substantially greater than those to be expected from world-wide fallout alone; these inventory excesses appear to be attributable to close-in fallout, but only if the ratio Pu/137Cs in this source was much higher than in world-wide fallout. The North Pacific mean ratio of the inventories is 2.2 times that observed in world-wide fallout.Resolubilization of Pu both from sinking particles and from sediments explains peculiarities of its depth distributions.There is little evidence for tracer movement by sliding downward along density surfaces;137Cs appears to have moved to depth by downmixing at the edge of the Kuroshio, and then moved horizontally and upward alongσt contours. The shallow Pu-rich layer shows no coordination with density, salinity or O2 isopleths. The deep Pu-rich layer is restricted to a narrow range of O2 concentrations that confirm its origin in the Aleutian Trench and rapid spread southward and laterally. Near-bottom circulation processes have been much more active than here-to-fore described.  相似文献   

11.
238Pu,239Pu and137Cs in rain and dry fallout and90Sr in rain samples were measured at Woods Hole, Massachusetts, from June 1976 through December 1977. The dry fallout was estimated to be about 7.8% of the total deposition of239Pu and137Cs.239Pu/137Cs ratios, almost constant at about 0.011 in rain or dry fallout, February through December 1977, suggested that fractionation between the refractory and volatile radionuclides is insignificant in stratospheric fallout. This supports the idea of regional homogeneity of radionuclide ratios in fallout.  相似文献   

12.
Possible relation between periodic glaciation and the flexure of the Galaxy   总被引：1，自引：0，他引：1  
A purely radiochemical method is described for analysis of241Pu in environmental samples. Data are given showing that fallout-contaminated marine samples, of several kinds and provenances, exhibit241Pu/239,240Pu ratios slightly higher than, but not certainly distinguishable from, those reported for fallout-contaminated soils. Such ratios, however, in samples from several planned or accidental Pu releases, are clearly different, suggesting241Pu will be a useful tracer in following the spread of released Pu. The data have also clear implications for the source, and growth, of environmental241Am.  相似文献   

13.
The concentrations of214Pb (half-life=26.4minutes) and22Rn (half-life=3.84days) have been measured in deep groundwaters of Gujarat, India. The results show that the abundance of214Pb in the water is only ～25% of that expected from its production through the radioactive decay of dissolved222Rn. This deficiency if modelled in terms of a first-order removal, yields a residence time of ～10 minutes for214Pb in these waters. The estimated residence time for214Pb is the shortest observed for any nuclide in natural water systems and suggests that reactive nuclides lead like could be removed from aqueous phases to adjoining solid surfaces on extremely short time scales. Results of laboratory experiments using the212Pb-224Ra pair are compatible with the observed fast removal of214Pb from groundwaters.Re-evaluation of234Th residence times in these waters using a model with a recoil flux of234Th into aqueous phase, the same as that of222Rn, yields values in the range of 23 to<176 minutes, very similar to that of214Pb. This “concordancy” in the residence times seems to suggest that the geochemical behaviour of234Th and214Pb in these waters is quite similar.  相似文献   

14.
We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO2-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides137Cs,239,240Pu and241Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and241Am are supplied to the deep water by advection.The depth distributions of the naturally occurring radionuclides232Th,228Th and230Th reflect their sources to the oceans.232Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of232Th decrease with depth to values 0.01 dpm/1000 l.228Th shows high activities in near surface and near bottom water, due to the distribution of its parent,228Ra. Dissolved230Th, produced throughout the water column from234U decay, increases with depth to 3000 m. Values in the deep water (> 3000 m) are nearly constant ( 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and241Am data.The ratio of particulate to dissolved activity for both230Th and228Th is 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths228Th tends to have a higher particulate/dissolved ratio than230Th, suggesting uptake and release of230Th and228Th by different processes. This could occur if228Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.230Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.230Th,241Am and239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with 15% of the230Th on particles compared with 7% for Am and 1% for Pu. Distribution coefficients (Kd) are 1.3–1.6 × 107 for Th, 5–6 × 106 for Am and 7–10 × 105 for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show 85% oxidized (V + VI) Pu. However, theKd value for Pu may be an upper limit because Pu, like228Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.  相似文献   

15.
The activity ratios of Pu and radiocesium isotopes have been used to delineate the major sources (such as global and close-in (debris) fallout, nuclear fuel reprocessing and fabrication plant effluents) in the environment. We have measured 238Pu, 239,240Pu, 137Cs, and excess 210Pb concentrations in 107 surficial sediments as well as in 5 sediment cores collected in the summer months of 1993 and 1994 from the Ob and Yenisey Rivers (Russia) and the Kara sea. A comparison of the sediment core inventories of 239,240Pu and 137Cs, along with the 238Pu/239,240Pu activity ratios, with those expected from global fallout allows us to estimate the relative amounts, if any, of reactor-derived 238Pu and 239,240Pu from the dumped reactor sites in the study area.

In surficial sediment samples collected in 1993 and 1994, the 239,240Pu concentrations varied between 4.2 and 856 mBq kg−1, with a mean of 239 mBq kg−1. In samples with a measurable 238Pu, the 238Pu/239,240Pu activity ratios varied between 0.010 and 0.069, with an average value of 0.035 ± 0.014. This range can be compared to the average 238Pu/239,240Pu activity ratio of 0.030 for the year 1993 from nuclear weapons testing and SNAP fallout obtained from soil studies, indicating very little (≤ 5%) additional sources of 238Pu to the sediments in the study area. The inventories of Pu in the 5 sediment cores from the study area varied between 2.67 ± 0.67 and 24.5 ± 2.2 Bq m−2 with a mean value of 8.83 Bq m−2. The 137Cs concentrations in the upper 3 cm of the sediments varied between below detection limit to 71.4 Bq kg−1, with a mean of 14.9 Bq kg−1. The 137Cs inventories in the 5 sediment cores varied between 156.7 ± 28.3 and 1600 ± 153.3 Bq m−2, with a mean value of 583.3 Bq m−2. The mean ratio of inventories of Pu to that of 137Cs, 0.015, is comparable to the values in other places in the Arctic region. There is a significant correlation between total organic carbon and concentrations of 137Cs, 239,240Pu and 210Pb, suggesting that organic matter-enriched fine particles are a likely carrier phase for these nuclides. The ratio of 137Cs/239,240Pu also suggests that most of these nuclides are derived from global fallout.  相似文献   


16.
An experiment was designed to assess the relative importance of sediment accumulation and bioturbation in determining the vertical distribution of nuclides in estuarine sediments. A diver-collected core, 120 cm long, was raised from central Long Island Sound and analyzed down its length for:210Pb and226Ra;239, 240Pu; and Mn, Zn, Cu, and Pb. Sampling for chemical analysis was guided by X-radiography of the core. Excess210Pb (relative to226Ra) is roughly homogeneous in the top 2–4 cm of the core, then decreases quasi-exponentially to zero at (or above) 15 cm.239, 240Pu and excess Zn, Cu, and Pb, relative to background values at greater depths in the core, are distributed like excess210Pb in the top 10–15 cm. The absence of Mn enrichment at the top of the core, in contrast to other cores raised from this station, suggests that 1–3 cm of sediment was lost by erosion at the site of this core sometime prior to sampling. Below 15 cm excess210Pb and excess Zn, Cu, and Pb are found only in the bulk sample from 25 to 30 cm and in clearly identifiable burrow fillings dissected from 70 cm and 115 cm depth. Infilling of large burrows, excavated and then abandoned by crustaceans, is therefore a mechanism for transfer of surficial material to depth in these sediments.The bioturbation rate in the top several centimeters at this station has been determined previously using234Th (24-day half-life). The distribution of239, 240Pu can be used to estimate a bioturbation rate for the underlying layer (to ～10 cm depth); this rate is found to be 1–3% of the maximum mixing rate for the top 2–3 cm. Using these two mixing rates in a composite-layer, mixing + sedimentation model, the distribution of excess210Pb in the top 15 cm was used to constrain the sediment accumulation rate, ω. While the apparent rate of sediment accumulation (assuming no mixing below 2–4 cm) is 0.11 cm/yr, the model requires ω < 0.05 cm/yr. Thus in an area of slow sediment accumulation, a low rate of bioturbation below the surficial zone of rapid mixing causes an increase of at least a factor of two in apparent accumulation rate.  相似文献   

17.
Unsupported226Ra (t12 = 1620years) in marine sediments can provide a basis for measuring rates of accumulation of the order of centimeters per thousand years. The excess radium apparently enters the sediments incorporated in phytoplankton. The sensitivity of the method depends upon the initial value of the unsupported226Ra and of the value of230Th, a parent of226Ra, in the sedimentary components.226Ra dating was applied to a sediment taken from the slope of the San Clemente Basin in the Southern California coastal region. Rates of sedimentation over two half-lives of the nuclide were found to be either 5.2 or 5.3 cm/1000 years depending upon which of two models for the geochronology is used. One model assumes that the230Th brings to the deposit an amount of226Ra in equilibrium with it. The other is based upon the growth of the226Ra from the230Th in the sedimentary components.238+239Pu and210Pb levels in the upper strata indicated sedimentation rates of the order of 100–500 cm/1000 years, i.e. much faster accumulations. We suggest these derived rates are spurious and reflect bioturbative activities of surface-living organisms.  相似文献   

18.
The occurrence of plutonium isotopes derived from the Windscale reprocessing facility discharges and weapons test fallout in intertidal sediments of the Wyre estuary, northwest England, is reported. Windscale plutonium appears to be slowly transported to the estuary in association with sedimentary material, and its distribution in surface and buried sediments is controlled by the variations in organic matter content of the sediments and/or their grain size characteristics. Results from chemical leaching experiments suggest that very little plutonium is ion exchangeable or in humic substances, but it is in other organic and Fe/Mn hydrous oxide fractions of the sediments. Plutonium shows a similar depositional behaviour to mercury which enters the Wyre directly in local industrial discharges.  相似文献   

19.
Particulates amounting to 0.1–2.0 g efficiently collected from large volumes of Atlantic and Pacific surface waters have been analyzed for carbonate, opal, quartz and several natural and man-made radioisotopes.The concentrations of particles range between 10 and 600 μg/kg. In the equatorial regions particle concentrations are low and similar in both the oceans. At higher latitudes (>30°N or S), the Atlantic waters, however, have higher concentrations of particles compared to those in the Pacific. The latitudinal distribution exhibits a north-south symmetry with higher concentrations in the 30°–60° belt. Based on the particulate abundance for CaCO3 and opal and their sedimentation, we have estimated their production and in-situ integrated dissolution rates for a few regions.Radioisotopes having different source functions, namely14C and239Pu injected due to nuclear weapon tests,234Th,230Th and228Th produced in-situ in seawater,232Th which derives primarily from land,210Pb introduced via wet precipitations and226Ra introduced through diffusion from deep-sea sediments have been measured in the particulates. The relative enrichment factors for these nuclides in particles vary as Th ? Pu > Pb > Ra. The atmospheric bomb fallout pattern is discernible in the surface particulates; the239Pu concentration increases with latitude in both the hemispheres; however, the values are about a factor of two lower in the southern hemisphere.The distribution pattern of radioisotopes is found to be complex, even for234Th whose source function in the oceans is uniform. In view of the differences in the source functions it becomes possible to delineate the principal geochemical/geophysical processes which determine the concentrations of these nuclides in surface waters.  相似文献   

20.
Xenon isotopic analyses by stepwise heating are presented for two neutron-irradiated chondrites, Arapahoe (L5) and Bjurböle (L4). The iodine-xenon formation age of Arapahoe is the oldest yet observed, 9.9 ± 0.8 m.y. before that of Bjurböle. It is thus unlikely that younger ages found in carbonaceous chondrite magnetite record the condensation of the solar nebula. The composition of trapped xenon in Arapahoe is normal except for a deficiency of129Xe, where we infer 129/Xe132Xe= 0.56 ? 0.04, well below the apparent primordial solar system value. This need not conflict with higher values in other metamorphosed meteorites since growth of129Xe from decay of129I in xenon-depleted environments can be substantial. The contrast with apparent average solar system composition cannot be easily explained, however, since there is no way to generate one composition from the other. The simplest way to achieve low129Xe seems to be to suppose that before decay to129Xe r-process production at mass 129 condensed into dust as129I, and that Arapahoe's parent body formed in a region of the solar system substantially depleted of this dust before any isotopic homogenization by vaporization of the remaining dust. Arapahoe is not unique in having trapped129Xe-deficient xenon, nor in any other respect yet observed, so some such history evidently characterizes major groups of meteorites.  相似文献   

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