Radioisotopes 137Cs Research Articles

Quantifying the threat to archaeological sites from the erosion of cultivated soil

Ploughing is probably the greatest agent of attrition to archaeological sites world-wide. In every country, every year, a bit more is shaved off buried strata and a bit more of the past becomes unreadable. On the other hand, people must eat and crops must be planted. How can the fields be best managed to get the best of both worlds? Perhaps the most pressing need for resource managers is to know how quickly a particular field is eroding: negotiation and protection is then possible. Up to now that has been difficult to measure.The new procedure presented here, which draws on the unexpected benefits of nuclear weapons testing, shows how variation in the concentration of the radioisotope 137Cs can be used to monitor soil movements over the last 40 years. The measurements allow a site's ‘life expectancy’ to be calculated, and there are some promising dividends for tracking site formation processes.

A chronological assessment of Lake Okeechobee (Florida) sediments using multiple dating markers

Reconstructing recent limnological history often relies on lead-210 dating to accurately ascribe a chronology to a sediment profile. In Lake Okeechobee, Florida, a large, shallow subtropical lake that may experience severe mixing, multiple dating methods are required to confirm that conformable sedimentation has been preserved and that the assumptions of the 210Pb method are satisfied. This study uses stratigraphic profiles of heavy metals, 137Cs, PCBs and pollen as independent dating markers to validate the sediment chronology as determined by 210Pb for three cores from the central mud zone of the lake. Unsupported 210Pb and most dating markers show distinct concentration/depth profiles, suggesting that the sediments have not been severely mixed for at least the last 75 years. Onset and maximum activity of the radioisotope 137Cs in the cores coincides with the 210Pb-dated interval of 1945–1970. This agrees well with the known timing of atmospheric deposition of 137Cs that resulted from above-ground nuclear testing during late 1940s until 1963. Sediment core profiles of atmospherically deposited metals such as Zn and Pb, which reflect regional increases during industrialization and decreases after regulation in the 1970s, exhibit expected concentration increases and peaks coinciding within 5–15 years of the predicted 210Pb dates. Uranium, a contaminant in some phosphate fertilizers, shows large concentration increases at core depths dated to be about 1950 by 210Pb, matching the intensification of agriculture after WWII. PCBs, which are expected to peak in the early 1970s, were measured in one core, and the observed peak corresponds to a 210Pb date of about 1960. Pollen makers were unable to verify specific events, but increases in disturbance taxa and declines in native types correspond generally with the expected dates assigned by 210Pb dating. Conformity between the 210Pb defined dates and independent markers of < ±15 years confirm that Lake Okeechobee sediments do preserve a sequential and reliable stratigraphic history of the lake, useful for reconstructing past limnological conditions.

A sediment trap and radioisotope study to determine resuspension of particle reactive substances in the sound between Sweden and Denmark

Sediment traps and the radioisotopes 137Cs, 210Pb, 7Be, 234Th and 228Th were used to study downward fluxes and resuspension of sediments in the shallow sound (Øresund) between Sweden and Denmark connecting the Baltic Sea with the North Sea. The water transport through Øresund covers approximately 30% of the total water transport between the North Sea and the Baltic Sea. The heavy boat traffic and the densely populated surroundings coupled to a shallow sound makes resuspension events an important factor in transporting released pollutants. Over the time period studied mass-fluxes recorded by the sediment traps ranged between 175 g m −2 day −1 near bottom during winter time to less than 1 g m −2 day −1 during late spring when the water column was stratified. A combination of sediment traps at different levels in the water column and radioisotope ratios shows that resuspended seabed sediments accounts for a large portion of the collected trap material, ranging from nearly 100% during the winter period to some 50% during stratified periods in late spring. In general the flux of all radioisotopes studied showed a linear relation with the mass-flux. The correlation of flux with organic carbon was better than with mass-flux. A comparison between atmospheric fluxes of 7Be and 210Pb and the fluxes of these isotope measured in the sediment traps shows that the agreement for 7Be is within a factor of two while for 210Pb the sediment trap fluxes are totally dominated by resuspended matter. The fraction of resuspended sediments in the traps may be quantified partly by using the 7Be/ 210Pb or 228Th/ 232Th ratios.

Levels of 137Cs in muddy sediments on the seabed in the Bay of Cádiz (Spain). Part II. Model of vertical migration of 137Cs

This second part of the study reports the development of a model to describe the vertical migration of the artificial radioisotope 137Cs in the sediment column on the seabed of the Bay of Cádiz. The application of the model provides an overall picture of the process of sedimentation in the Inner Bay of Cádiz. The spatial distribution of the rate of sedimentation enables us to study the sources of sediments and the means by which the sediments have been transported. A method has been derived from the rate of sedimentation to perform the dating of the layers of sediment. The model describes the behaviour of 137Cs in the area under study, taking into account the time of residence in the zones that are the source of accumulation, the origin of the sedimentary material, together with the diffusion of the radionuclide in the sediment of the seabed.

Selecting plants to minimise radiocaesium in the food chain

Caesium (Cs) is an alkali metal with chemical properties similar to potassium (K). It has no known role in plant nutrition and it is not toxic to plants at the micromolar concentrations occurring naturally in soil solutions. However, two radioisotopes of Cs (134Cs and 137Cs) are of environmental concern due to their relatively long half-lives, emissions of β and γ radiation during decay, and rapid incorporation into biological systems. There is considerable interest in remediating sites contaminated by these isotopes using phytoextraction and, since the produce from radiocaesium-contaminated areas may enter the food chain, the introduction of `safe' crops that do not accumulate Cs. This article reviews the molecular mechanisms of Cs uptake by plants, and provides a perspective on strategies to develop: (1) plants that extract Cs efficiently from soils (for the phytoremediation of land), or (2) `safe' crops that minimise the entry of radiocaesium directly into the human food chain.

Tansley Review No. 113

Caesium (Cs) is a Group I alkali metal with chemical properties similar to potassium (K). It is present in solution as the monovalent cation Cs+. Concentrations of the stable caesium isotope 133Cs in soils occur up to 25 μg g−1 dry soil. This corresponds to low micromolar Cs+ concentrations in soil solutions. There is no known role for Cs in plant nutrition, but excessive Cs can be toxic to plants. Studies of the mechanism of Cs+ uptake are important for understanding the implications arising from releases of radioisotopes of Cs, which are produced in nuclear reactors and thermonuclear explosions. Two radioisotopes of Cs (134Cs and 137Cs) are of environmental concern owing to their relatively long half‐lives, emissions of β and γ radiation during decay and rapid incorporation into biological systems. The soil concentrations of these radioisotopes are six orders of magnitude lower than those of 133Cs. Early physiological studies demonstrated that K+ and Cs+ competed for influx to excised roots, suggesting that the influx of these cations to root cells is mediated by the same molecular mechanism(s). The molecular identity and/or electrophysiological signature of many K+ transporters expressed in the plasma membrane of root cells have been described. The inward‐rectifying K+ (KIR), outward‐rectifying K+ (KOR) and voltage‐insensitive cation (VIC) channels are all permeable to Cs+ and, by analogy with their bacterial counterparts, it is likely that ‘high‐affinity’ K+/H+ symporters (tentatively ascribed here to KUP genes) also transport Cs+. By modelling cation fluxes through these transporters into a stereotypical root cell, it can be predicted that VIC channels mediate most (30–90%) of the Cs+ influx under physiological conditions and that the KUP transporters mediate the bulk of the remainder. Cation influx through KIR channels is likely to be blocked by extracellular Cs+ under typical ionic conditions in the soil. Further simulations suggest that the combined Cs+ influxes through VIC channels and KUP transporters can produce the characteristic ‘dual isotherm’ relationship between Cs+ influx to excised roots and external Cs+ concentrations below 200 μM. Thus, molecular targets for modulating Cs+ influx to root cells have been identified. This information can be used to direct future genetic modification of plants, allowing them to accumulate more, or less, Cs and thereby to remediate contaminated sites. contents Summary 241 I. introduction: caesium in the environment 242 II. uptake of caesium by plant roots 243 III. molecular mechanisms catalysing caesium uptake 245 IV. modelling caesium influx to root cells 249 V. perspective 253 Acknowledgements 254

A quantitative model for estimating mean annual soil loss in cultivated land using 137Cs measurements

The radioisotope 137Cs has been widely used to determine rates of cultivated soil loss, Many calibration relationships (including both empirical relationships and theoretical models) have been employed to estimate erosion rates from the amount of 137Cs lost from the cultivated soil profile. However, there are important limitations which restrict the reliability of these models, which consider only the uniform distribution of 137Cs in the plough layer and the depth. As a result, erosion rates they may be overestimated or underestimated. This article presents a quantitative model for the relation the amount of 137Cs lost from the cultivated soil profile and the rate of soil erosion. According to a mass balance model, during the construction of this model we considered the following parameters: the remaining fraction of the surface enrichment layer (F R), the thickness of the surface enrichment layer (H s), the depth of the plough layer (H p), input fraction of the total 137Cs fallout deposition during a given year t (F t ), radioactive decay of 137Cs (k), sampling year (t). The simulation results showed that the amounts of erosion rates estimated using this model were very sensitive to changes in the values of the parameters F R, H s, H p. We also observed that the relationship between the rate of soil loss and 137Cs depletion is neither linear nor logarithmic, is very complex. Although the model is an improvement over existing approaches to derive calibration relationships for cultivated soil, it requires empirical information on local soil properties and the behavior of 137Cs in the soil profile. There is clearly still a need for more precise information on the latter aspect and, in particular, on the retention of 137Cs fallout in the top few millimeters of the soil profile and on the enrichment and depletion effects associated with soil redistribution (i.e. for determining accurate values of F R and H s).

Excitation functions for the production of some radioisotopes of Cs, Xe and I in proton bombardment of natXe up to 100 MeV

Integral excitation functions for the production of 16 radioisotopes of Cs, Xe and I in the bombardment of natXe with protons were measured up to 100 MeV. The results were compared with geometry-dependent hybrid-model calculations performed by means of the computer code ALICE/85/300, employing both the standard methods offered by the code to take nucleon pairing and shell-structure effects into account. Good overall agreement was obtained in both cases.

Cesium-137 redistribution in the sediments of the playa, Lake Tyrrell, northwestern Victoria. II. Patterns of cesium-137 and pollen redistribution

Near-surface sediments of the northern basin of the playa, Lake Tyrrell, were analysed for the fallout radioisotope 137Cs (to 20 cm depth) and pollen grains (to 1 cm) and the salt-crust depth at each sampling site was measured. 137Cs areal concentrations (MBq km −2) were found to have a highly significant ( p < 0.001) negative correlation with salt-crust thickness and a highly significant ( p < 0.001) positive correlation with pollen concentrations (no. cm −3). Areal concentrations of 137Cs and pollen were generally depleted in the southwest of the sampled area where salt crust thickness was greatest (≈10 cm), and enriched in the northeast where salt crust was absent. A number of possible mechanisms affecting the distribution of 137Cs and pollen on the lake floor have been identified. 137Cs may be adsorbed on fine-grained sedimentary particles and/or absorbed by algae or bacteria. The transport of these particles, suspended in brine bodies blown across the lake floor by wind and deposited on the sediment surface as the brine evaporates, results in the northerly redistribution of 137Cs. Regional pollen, which acts as a fine sedimentary particle, is similarly affected. The salt crust, in the southwest of the basin, was seen as a barrier to the penetration of 137Cs-labelled particles and pollen grains to the sediment surface and, with the seasonal dissolution of the halite and redistribution of brine, as a source for these materials. The northeast area of the basin acts as a sink for these materials. Local erosion of top soil on the lake margins, and subsequent reworking, may contribute additional material. These redistributed materials eventually become part of the sedimentary sequence, and consideration should be given to redistribution mechanisms before selection of suitable core sites, and when interpreting palaeoenvironmental data from salt lakes. Recommended strategies are outlined.

Certain peculiarities of the plankton biomass distribution in the surface waters of the Indian Ocean in winter 1959/60. (n Russian; English abstract) : Bogorov V.G. and M.Ye. Vinogradov, 1961. Okeanolog. Issled., Mezhd. Komitet Proved. Mezhd. Geofiz. Goda, Prezidiume, Akad. Nauk, SSSR, (4): 66–71

Geochemical studies of the ecosystems of 184 Siberian lakes in three largest zones of northern Asia (humid, arid, and semiarid) and in mountainous area were carried out. The contents of natural radionuclides, radiocesium, and rare-earth elements in conjugate components of the systems and the types of the main sources of the bottom sediment material have been determined.Dating of the bottom sediments was made by the activity of radioisotopes 137Cs and 210Pb, which permitted estimation of the sedimentation rates in lakes in different regions of Siberia: 0.35 cm/year in the south and 0.25–0.3 cm/year in the north.Six main ions have been determined in the waters of the studied lakes: Ca2+, Mg2+, Na+, HCO3−, SO42−, and Cl−. The distribution of natural radionuclides in the stratified sections of bottom sediments of Siberian lakes evidences the stable sedimentation and characterizes their contents in the soils of water-catchment areas, which can be considered background contents there. Sediments enriched in organic matter have higher concentrations of U and lower ones of Th and K. The Th/K ratio in the studied bottom sediments is the same as in the soils. The Th/U ratios are somewhat lower than those in the soils because U is accumulated by chemogenic and organic components. The overall 137Cs pollution of bottom sediments of Siberian lakes is close to the global background (40 mCi /km2 in 2000), but in the Altai Territory and Buryatia and Altai Republics it is twice higher. The uneven areal and temporal distribution of residual radiocesium is observed not only in the lacustrine sediments but also in the lake water areas. The REE patterns of bottom sediments of different mineral types are similar to those of continental crust and clays of the Russian Platform, though organogenic and carbonate sediments have higher absolute REE contents than terrigenous ones. Pelitic fraction is the main REE concentrator in the bottom sediments.

Calcul de la propagation de la marée dans une zone côtière : Bonnefilie R. and F. Voyer, 1962. Mém.et Trav., Soc. Hydrotechn., France, Supp. au Vol. 1 (1962): 111–122

Geochemical studies of the ecosystems of 184 Siberian lakes in three largest zones of northern Asia (humid, arid, and semiarid) and in mountainous area were carried out. The contents of natural radionuclides, radiocesium, and rare-earth elements in conjugate components of the systems and the types of the main sources of the bottom sediment material have been determined.Dating of the bottom sediments was made by the activity of radioisotopes 137Cs and 210Pb, which permitted estimation of the sedimentation rates in lakes in different regions of Siberia: 0.35 cm/year in the south and 0.25–0.3 cm/year in the north.Six main ions have been determined in the waters of the studied lakes: Ca2+, Mg2+, Na+, HCO3−, SO42−, and Cl−. The distribution of natural radionuclides in the stratified sections of bottom sediments of Siberian lakes evidences the stable sedimentation and characterizes their contents in the soils of water-catchment areas, which can be considered background contents there. Sediments enriched in organic matter have higher concentrations of U and lower ones of Th and K. The Th/K ratio in the studied bottom sediments is the same as in the soils. The Th/U ratios are somewhat lower than those in the soils because U is accumulated by chemogenic and organic components. The overall 137Cs pollution of bottom sediments of Siberian lakes is close to the global background (40 mCi /km2 in 2000), but in the Altai Territory and Buryatia and Altai Republics it is twice higher. The uneven areal and temporal distribution of residual radiocesium is observed not only in the lacustrine sediments but also in the lake water areas. The REE patterns of bottom sediments of different mineral types are similar to those of continental crust and clays of the Russian Platform, though organogenic and carbonate sediments have higher absolute REE contents than terrigenous ones. Pelitic fraction is the main REE concentrator in the bottom sediments.

Radio-Isotopes of Ba and Cs

Radio-Isotopes of Ba and Cs