Pu and137Cs in coastal sediments

Analyses are presented of¹³⁷Cs,²³⁸Pu, and^239,240Pu, 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 of^239,240Pu 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 the^239,240Pu/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, the^239,240Pu/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.     

Radionuclide ratios in wet and dry deposition samples from June 1976 through December 1977

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.     

Fallout radionuclides in the Pacific Ocean: Vertical and horizontal distributions,largely from GEOSECS stations

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 alongσ_t 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.     

Pu, Cs and excess Pb in Russian Arctic sediments

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.     

Anthropogenic Radionuclides in the Caspian Sea

Analysis and interpretation of the distribution of anthropogenic radionuclides ⁹⁰Sr, ¹³⁷Cs, and ^239,240Pu in the Caspian Sea water are presented. These radionuclides are shown to be of environmental importance and to be useful for studying water mass dynamics.     

Depositional history of artificial radionuclides in the Ross Ice Shelf,Antarctica

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.     

Geochronological reconstruction of Cs transport from the Coruh river to the SE Black Sea: comparative assessment of radionuclide retention in the mountainous catchment area

The deposition record of ¹³⁷Cs was traced in the SE Black Sea sediments adjacent to the Coruh river mouth in comparison with the earlier studied chronology of ¹³⁷Cs deposition in front of the Danube delta (NW Black Sea). In both cases, the ¹³⁷Cs profiles showed two subsurface peaks attributable to maximum fallout of ‘bomb’ and Chernobyl radionuclides. The Coruh profile revealed a larger contribution of ‘bomb’ ¹³⁷Cs in comparison with the Chernobyl input, suggesting different coverage of NW and SE Black Sea regions with the Chernobyl fallout. The ¹³⁷Cs-derived dating showed that maximum deposition of particulate bound ¹³⁷Cs in sediments adjacent to the Coruh river mouth was delayed for 14 yr relative to date of Chernobyl accident, reflecting a buffer effect of the watershed soils. This transit time is 3 times longer than in the Danube catchment area, indicating a difference in retention processes in these mountainous (Coruh) and lowland (Danube) river basins. The ¹³⁷Cs profile in Coruh sediments showed penetration of ¹³⁷Cs to much greater depth than would be expected from ¹³⁷Cs fallout chronology, suggesting the sediment mixing rate of 1.3 cm² yr⁻¹. This value was used to evaluate deposition chronology of ¹³⁷Cs, applying the model developed for pulse fallout case. Comparing the measured and modelled data has allowed differentiation of the flood-induced discharge of the ¹³⁷Cs-containing suspended matter and the slower transit of eroded soil particles from the contaminated catchment areas. The obtained results may be used for the prediction of period when the pollutants, deposited over the river basins, can reach the Black Sea.     

241Pu and241Am in sediments from coastal basins off California and Mexico

In sediments deposited in the coastal basins off the western North American continent since the early 1960's, there is more²⁴¹Am activity than one would predict if its presence was solely due to the decay of²⁴¹Pu that was produced during the testing of weapons in 1961–1962 (taken as July 1, 1962 for calculations). This excess can be accounted for by the decay of²⁴¹Pu, if pre-1962 fallout (fallout from the pre-moratorium testing) has been continuously introduced into the recent sediments along with fallout from post-moratorium testing. This hypothesis is supported by the plutonium depth profiles which do not reflect direct fallout from the atmosphere, as characterized by two peaks corresponding to the two major weapons testing periods, but continuously increasing plutonium and americium concentrations from 1954 to 1974. Thus, the nuclides may be coming from a large reservoir that has accumulated and mixed fallout since 1952. Such a concentration gradient for all of the nuclides requires their immobilization in the sediment after deposition.     

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

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 and^239,240Pu 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 (K_d) 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, theK_d 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.     

Variability of natural and bomb-produced radionuclide distributions in abyssal red clay sediments

The nature of sedimentation and mixing are examined in abyssal red clay sediments from the North Central Pacific using three types of indicators: ²³⁰Th/²³²Th, organic¹⁴C, and¹³⁷Cs and ^239.240Pu.²³⁰Th/²³²Th analysed revealed that the clay sedimentation rate in three box cores collected within a 50 km radius was less than 1.0 mm/10³ yr. However, analyses of the organic carbon in thin layers of sediment revealed that radiocarbon was present much deeper in the cores (down to 20 cm) than was expected from the ²³⁰Th/²³²Th distribution. In addition, both the stratigraphy and inventory of radiocarbon was significantly different between box cores. The distributions and inventories of¹³⁷Cs and^239.240Pu were similar to that found for radiocarbon, further illustrating the spatial variability of radionuclides in oligotrophic North Pacific red clays. These data suggest that bioturbational processes are important for transporting organic carbon down into the sediment column.     

Concentrations of 137Cs, 90Sr, 108m Ag, 239+240 Pu and atom ratio of 240Pu/239Pu in tanner crabs, Chionoecetes japonicus and Chionoecetes opilio collected around Japan

The anthropogenic radionuclides, ¹³⁷Cs, ⁹⁰Sr, ^108mAg, ^239+240Pu, were measured in two Chionoecetes species, red queen crab (Chionoecetes japonicus) and snow crab (Chionoecetes opilio) collected around Japan during 1996–2007. There was no increase in the concentrations of these radionuclides and no large variation of the atom ratio of ²⁴⁰Pu/²³⁹Pu during this research period. These results indicated that the source of the radionuclides was not the radioactive wastes dumped by the former USSR and Russia and originated from past nuclear weapon tests. The higher atom ratio in the crab species than that from global fallout would be contributed by the Pacific Proving Grounds close-in fallout. The variability of the concentration of radionuclides in the crab species would result from the variability of the composition and quantity in the diet. However, the decrease in the concentration of radionuclides with sampling depth would depend on the concentration in the seawater and diet.     

Transport of the environmental radionuclides in an alpine watershed

The radionuclides¹³⁷Cs,²¹⁰Pb and⁷Be have been examined in the alpine Rhoˆne watershed (Switzerland) during a period of two years in order to evaluate their usefulness as tracers of the removal and transport rate of top-soil particles and particle-reactive contaminants of atmospheric origin. The specific activities of the radionuclides in fluvial suspension show a distinct seasonal pattern which depends on the hydrologic regime of the stream and the sources of the suspended matter. Input-output budgets based on the atmospheric deposition and fluvial removal of three radionuclides in the alpine Rhoˆne watershed are used to estimate their erosional residence times. The simplest one box model yields mean residence times of about 800 and 1400 years for¹³⁷Cs and²¹⁰Pb, respectively. The removal rate of short-lived⁷Be suggests that a part of the watershed (0.6–2.3% of the total surface) is exposed to a rapid erosion, in which the mean residence time of the radionuclides is in the range of 1–220 days. This has little influence on the calculated residence time of¹³⁷Cs but increases the estimated residence time of²¹⁰Pb in soil to over 1800 years. The use of²¹⁰Pb-⁷Be pair in fluvial output may be very helpful in the assessment of the impact of atmospheric pollutants on the water quality in rivers and lakes.     

Interpretation procedures for the determination of sediment parameters from time-dependent flux inputs

A formulation for the effects of a time-dependent input flux to a sedimentation system consisting of a diffusive layer underlain by a non-diffusive medium has been developed. The intent is to provide interpretation procedures for the determination of sedimentation rate, diffusion coefficient and mixed layer thickness from observed tracer concentrations. They have been applied, for illustrative purposes, to published¹³⁷Cs and^239,240Pu measurements.     

Distribution of 137Cs in the Lena River estuary-Laptev Sea system.

Surface sediment samples in the Laptev Sea have average 137Cs content of 7.1 Bq kg(-1), a value intermediate between that of the western Kara Sea (23 Bq kg(-1) and the East Siberian Sea (4.2 Bq kg-'). Both surface sediment content and sediment inventory of 137Cs in the Laptev Sea sediments show significant variability, and the influence of a variety of environmental factors.137Cs concentrations in the Laptev Sea surface sediments range from 0.8 to 16 Bq kg(-1). There is a marked increase in 137Cs content of surface sediment samples collected near the Lena River delta, and a local enrichment in the 137Cs inventories at these sites is also evident. Fine-grained mixed-layer illite/ smectite rich sediments in the estuary provide effective adsorption sites to fix 137Cs, in spite of desorption processes associated with low salinities in estuarine mixing. The Lena River-Laptev Sea mixing zone is a major site of sea-ice production. River and shelf sediments are incorporated into sea-ice formed in this region (Holmes and Creager, 1974). The irregular 137Cs activity profiles of the Lena River estuary cores indicate disturbance or removal of 137Cs-laden sediments via sea-ice related processes. Lena River and Estuary sediments may have served as a secondary source (i.e. other than direct fallout) of 137Cs in sea-ice. North-east of the Lena River estuary, sediment contains a thin layer of 137Cs-bearing material over an erosion surface. The 137Cs-laden surface layer may be the result of transient deposition of estuarine sediments being delivered by sea-ice or spring floods.     

Rating Water Quality in Sevastopol Bay by the Fluxes of Pollutant Deposition in Bottom Sediments

Sevastopol Bay is used as an example for the development of criteria for rating anthropogenic impact by elimination fluxes from the water area of post-Chernobyl (⁹⁰Sr, ¹³⁷Cs, ^239,240Pu) and natural (²¹⁰Pо) radionuclides, as well as mercury and organochlorine compounds. The differentiation of the bay water area into zones with different biogeochemical conditions and the balance approach to interpreting field observation data were used to assess the conditioning capacity of Sevastopol Bay ecosystem for conservative radioactive and chemical substances by elimination fluxes of pollutants into aqueous depot, which is the open part of the Black Sea and into the geological depot, i.e., its bottom sediment stratum.     

Atmospherically-derived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake sediments: Evidence from7Be,210Pb,and239,240Pu

Cosmogenic⁷Be(t_1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess²¹⁰Pb and/or^239,240Pu 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 and^239,240Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.     

The state of Lake Ladoga pollution by global 90Sr, 137Cs, and 239,240Pu

The state of radioactive pollution of Lake Ladoga has been studied. The purification half-life of lake water from ⁹⁰Sr was found to be 21.7 year. The ratio of ⁹⁰Sr storage in lake water volume to its storage in silts (0?C10 cm layer) was 2: 1. Lake water purification from global ¹³⁷Cs was slower than that from ¹³⁷Cs originating from Chernobyl emergency emission because of the chronic supplement of global ¹³⁷Cs reserve by atmospheric fallouts. The behavior of ⁹⁰Sr, ¹³⁷Cs, and ^239,240Pu in bed soils was in agreement with the geochemical nature of the elements. By the distribution of ^{239, 240}Pu in bed soils, the sedimentation of substances of non-radiation nature was estimated at 0.3 and 0.5 mm/year at denting and flat relief, respectively.     

Organic carbon flow in the Ob, Yenisey Rivers and Kara Sea of the Arctic region

Stable carbon isotope and elemental C/N ratios of the organic fraction of a set of samples along a transect in the Ob and Yenisey Rivers into the Kara Sea in the Arctic were measured. Previously, the concentrations of 239,240Pu and 137Cs in these same samples had been determined. The coupled measurements were carried out to assess possible connectivity between organic carbon flow into the Kara Sea and transport of radioactive nuclides in this marine environment. Organic carbon flow into the Kara Sea is influenced significantly by terrigenous sources carried by the Ob and Yenisey Rivers. The carbon isotope-organic carbon relationship provides evidence that a rich source of terrigenous carbon exists in the riverine system. A weak, but significant relationship between stable carbon isotope ratio and 137Cs suggests that most of the 137Cs is derived from riverine particles, as compared to Pu which is also derived from in situ scavenging within the water column.     

137Cs湖泊沉积年代学方法应用的局限——以Crawford湖为例

＾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     

Transport and deposition of plutonium in the ocean: evidence from Gulf of Mexico sediments

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.