大气散落核素复合示踪在土壤侵蚀科学中的应用

介绍了大气散落核素 7Be、2 1 0 Pbex、1 37Cs复合在土壤侵蚀和泥沙来源领域中的研究进展 ;阐述了核素比率应用的原理、特点及其能更为敏感地反映土壤侵蚀速率和过程等优点 ;说明多元素复合示踪在推断土壤不同侵蚀过程及估算土壤侵蚀速率方面的作用 ;论述了建立两核素相关深度曲线确定泥沙起源深度的应用原理和特点及其应用范围 ,并对各种方法进行了简单的评价。

中国北方风成堆积的风化作用与环境变迁—U-Th、~(10)Be及元素地球化学的研究

The surface of the Earth is continuously undergoing changes as a result of weathering-erosion, plate tectonics and volcanic processes. Continental weathering-erosion with its complex rock-water interactions is the central process of global biochemical cycling of elements, and affects the long-term ocean atmosphere budget of carbon dioxide both through the consumption of carbonic acid during silicate weathering and through changes in the weathering and burial rates of organic carbon. Rates of the weathering-erosion depend on a variety of factors, in particular rock properties and chemical composition, climate (especially rainfall), structure, and elevation. They are quite variable on a regional scale. Thus, environmental changes in a region could be indicated by the history of weathering-erosion in the region. Recent attention has focused on increased silicate weathering of tectonically uplifted areas in the India-Asia collision zone as a possible cause for falling atmospheric CO_2 levels in the Cenozoic era. The wind blown dust deposits in the Loess Plateau is derived from the arid and semiarid regions in northwestern China, in turn, where the deposits have been derived from the Qinghai-Xizang Plateau and the high mountains around. Therefore, geochemistry of the wind blown loess-paleosol and red clay sequences may provide insight both to paleoenvironmental changes on the Loess Plateau, and to the uplift and weathering-erosion histories of the Qinghai-Xizang Plateau. In this paper, uranium-thorium series nuclides and cosmogenic ~(10)Be have been employed as tracers of weathering intensities and histories of the dust sediments in the Loess Plateau. Major elements, such as Na, Al, Fe etc., are also used to estimate degree of chemical alteration of the dust sediments and to rebuild the history of weathering on the Loess Plateau. First of all, using a low-level HPGe γ-ray detector, we measured U and Th series nuclides in 170 loess and paleosol samples from five sites in the Loess Plateau, going back 2.6 Ma. The results show that ~(238)U activities are disequilibrium with its daughter nuclide ~(230)Th in young loess-paleosol sequence, indicating that weathering was happened both in dust deposition site and in dust source regions. Using concentrations of ~(238)U and ~(232)Th in the samples, we estimated the amounts of ~(238)U leached out of from paleosols due to weathering. Further, based on analyses of ~(230)Th in paleosols deposited in the past ca. 140 ka, we determined when the paleosols weathered in the source regions. We conclude that most of the weathering in the dust-source regions may have occurred during the interglacials before dust deposition.

The Preboreal climate reversal and a subsequent solar-forced climate shift

Accurate chronologies are essential for linking palaeoclimate archives. Carbon-14 wiggle-match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas-Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric C-14; (ii) a temporary decline of atmospheric CO2; and (iii) an increase in the GRIP Be-10 flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright (C) 2004 John Wiley Sons, Ltd.

Evidence from northwest European bogs shows 'Little Ice Age' climatic changes driven by variations in solar activity

Fluctuations in Holocene atmospheric radiocarbon concentrations have been shown to be due to variations in solar activity. Analyses of both Be-10 and C-14 nuclides confirm that production-rate changes during the Holocene were largely modulated by solar activity. Analyses of peat samples from two intact European ombrotrophic bogs show that climatic deteriorations during the 'Little Ice Age' are associated with transitions to increasing atmospheric C-14 content due to greater C-14 production. Both ombrotrophic mires, which are positioned c. 800 km apart, register reactions to globally recorded C-14 fluctuations between AD 1449 and 1464 and an almost identical reaction between AD 1601 and 1604.

Biomineralization of Weathered Rock Rinds:Examples from the Lower Afroalpine Zone on Mount Kenya

Rock rinds have been used for half a century to date glacial deposits and recently inroads have been developed to use nuclides to provide absolute ages of weathering rinds in pebble clasts. Although maximum and minimum rind thicknesses have helped to elucidate time since deposition and allowed stratigraphic division of deposits at glacial rank, little has been done to investigate the wealth of mineral degradation, growth of alteration products and biomineralization that occur in these weathered crusts. In some cases the mass of microbe-mineral intergrowth is nearly present on a 50%/50% basis, with the biotic mass intergrown with mineral matter to such an extent that it probably controls pH and redox phenomena that act as accelerators in the weathering process. Assuming weathering time spans of 2 × 10⁶ years or more for a complete cycle, eventual clast decomposition is the end product. Here we present evidence of microbe-clast intergrowth from selected sites of Pleistocene age (~70 ka to 2.0 Ma) in the lower Afroalpine of Mt. Kenya and hypothesize about its role in rock decomposition and fossilization of biotic end-members. © 2013 Copyright Taylor and Francis Group, LLC.

Radiological impact associated to Technologically Enhanced Naturally Occurring Radioactive Materials (TENORM) from coal-fired power plants emissions

Certain materials used and produced in a wide range of non-nuclear industries contain enhanced activity concentrations of natural radionuclides. In particular, electricity production from coal is one of the major sources of increased human exposure to naturally occurring radioactive materials. A methodology was developed to assess the radiological impact due to natural radiation background. The developed research was applied to a specific case study, the Sines coal-fired power plant, located in the southwest coastline of Portugal. Gamma radiation measurements were carried out with two different instruments: a sodium iodide scintillation detector counter (SPP2 NF, Saphymo) and a gamma ray spectrometer with energy discrimination (Falcon 5000, Canberra). Two circular survey areas were defined within 20 km of the power plant. Forty relevant measurements points were established within the sampling area: 15 urban and 25 suburban locations. Additionally, ten more measurements points were defined, mostly at the 20-km area. The registered gamma radiation varies from 20 to 98.33 counts per seconds (c.p.s.) corresponding to an external gamma exposure rate variable between 87.70 and 431.19 nGy/h. The highest values were measured at locations near the power plant and those located in an area within the 6 and 20 km from the stacks. In situ gamma radiation measurements with energy discrimination identified natural emitting nuclides as well as their decay products (Pb-212, Pb-2142, Ra-226, Th-232, Ac-228, Th-234, Pa-234, U- 235, etc.). According to the results, an influence from the stacks emissions has been identified both qualitatively and quantitatively. The developed methodology accomplished the lack of data in what concerns to radiation rate in the vicinity of Sines coal-fired power plant and consequently the resulting exposure to the nearby population.

Measurements of environmental background radiation in the surrounding area of a coal-fired power plant

Certain materials used and produced in a wide range of non-nuclear industries contain enhanced activity concentrations of natural radionuclides. In particular, electricity production from coal is one of the major sources of increased exposure to man from enhanced naturally occurring materials. Over the past decades there has been some discussion about the elevated natural background radiation in the area near coal-fired power plants due to high uranium and thorium content present in coal. This work describes the methodology developed to assess the radiological impact due to natural radiation background increasing levels, potentially originated by a coal-fired power plant’s operation. Gamma radiation measurements have been done with two different instruments: a scintillometer (SPP2 NF, Saphymo) and a gamma ray spectrometer with energy discrimination (Falcon 5000, Canberra). A total of 40 relevant sampling points were established at locations within 20 km from the power plant: 15 urban and 25 suburban measured stations. The highest values were measured at the sampling points near to the power plant and those located in the area within the 6 and 20 km from the stacks. This may be explained by the presence of a huge coal pile (1.3 million tons) located near the stacks contributing to the dispersion of unburned coal and, on the other hand, the height of the stacks (225 m) which may influence ash’s dispersion up to a distance of 20 km. In situ gamma radiation measurements with energy discrimination identified natural emitting nuclides as well as their decay products (212Pb, 214Pb, 226Ra 232Th, 228Ac, 234Th 234Pa, 235U, etc.). This work has been primarily done to in order to assess the impact of a coal-fired power plant operation on the background radiation level in the surrounding area. According to the results, an increase or at least an influence has been identified both qualitatively and quantitatively.

Radioactivity levels of 238U and 232Th decay series and related dose rates in the surroundings of a coal power plant using high resolution g-spectrometry

Gamma radiations measurements were carried out in the vicinity of a coal-fired power plant located in the southwest coastline of Portugal. Two different gamma detectors were used to assess the environmental radiation within a circular area of 20 km centred in the coal plant: a scintillometer (SPP2 NF, Saphymo) and a high purity germanium detector (HPGe, Canberra). Fifty urban and suburban measurements locations were established within the defined area and two measurements campaigns were carried out. The results of the total gamma radiation ranged from 20.83 to 98.33 counts per second (c.p.s.) for both measurement campaigns and outdoor doses rates ranged from 77.65 to 366.51 Gy/h. Natural emitting nuclides from the U-238 and Th-232 decay series were identified as well as the natural emitting nuclide K-40. The radionuclide concentration from the uranium and thorium series determined by gamma spectrometry ranged from 0.93 to 73.68 Bq/kg, while for K-40 the concentration ranged from 84.14 to 904.38 Bq/kg. The obtained results were used primarily to define the variability in measured environmental radiation and to determine the coal plant’s influence in the measured radiation levels. The highest values were measured at two locations near the power plant and at locations between the distance of 6 and 20 km away from the stacks, mainly in the prevailing wind direction. The results showed an increase or at least an influence from the coal-fired plant operations, both qualitatively and quantitatively.

Tracing pre-eruptive magma degassing using (Pb-210/Ra-226) disequilibria in the volcanic deposits of the 1980-1986 eruption of Mount St. Helens

Disequilibria between Pb-210 and Ra-226 can be used to trace magma degassing, because the intermediate nuclides, particularly Rn-222, are volatile. Products of the 1980-1986 eruptions of Mount St. Helens have been analysed for (Pb-210/Ra-226). Both excesses and deficits of Pb-210 are encountered suggesting rapid gas transfer. The time scale of diffuse, non-eruptive gas escape prior to 1980 as documented by Pb-210 deficits is on the order of a decade using the model developed by Gauthier and Condomines (Earth Planet. Sci. Lett. 172 (1999) 111-126) for a non-renewed magma chamber and efficient Rn removal. The time required to build-up Pb-210 excess is much shorter (months) as can be observed from steady increases of (Pb-210/Ra-226) with time during 1980-1982. The formation of Pb-210 excess requires both rapid gas transport through the magma and periodic blocking of gas escape routes. Superposed on this time trend is the natural variability of (Pb-210/Ra-226) in a single eruption caused by tapping magma from various depths. The two time scales of gas transport, to create both Pb-210 deficits and Pb-210 excesses, cannot be reconciled in a single event. Rather Pb-210 deficits are associated with pre-eruptive diffuse degassing, while Pb-210 excesses document the more vigorous degassing associated with eruption and recharge of the system. (c) 2006 Elsevier B.V. All rights reserved.

Discovery of 157W and 161Os

The nuclides 157W and 161Os have been discovered in reactions of 58Ni ion beams with a 106Cd target. The 161Os α -decay energy and half-life were 6890±12 keV and 640±60 μs. The daughter 157W nuclei β -decayed with a half-life of 275±40 ms, populating both low-lying α-decaying states in 157Ta, which is consistent with a 7/2− ground state in 157W. Fine structure observed in the α decay of 161Os places the lowest excited state in 157W with Iπ=9/2− at 318±30 keV. The branching ratio of View the MathML source indicates that 161Os also has a 7/2− ground state. Shell-model calculations analysing the effects of monopole shifts and a tensor force on the relative energies of 2f7/2 and 1h9/2 neutron states in N=83 isotones are presented.

A combined method for evaluating radon and progeny in waters and its use at Guarani aquifer, São Paulo State, Brazil

A combined method for evaluating radon (Rn-222) and progeny (Pb-214 and Bi-214) in water was developed by using inexpensive alpha scintillation Counting and gamma ray spectrometry through NaI(Tl) scintillation detectors. A groundwater sample collected at the Pocos de Caldas alkaline massif in Brazil was submitted to the technique in order to assure its applicability by comparing the volumetric activities by different methods. Similar volumetric activity was determined for Pb-214 and Bi-214 in the sample analyzed that is compatible with the expected condition of radioactive equilibrium between these nuclides. The combined method was successfully used to analyze groundwater samples from Guarani aquifer in S (a) over tildeo Paulo State, Brazil, and the results of the measurements indicated that Pb-214 and Bi-214 provide useful information concerning the evaluation of the drinking water quality in terms of radiological aspects. This is because they are directly identified in the water samples, without the need of requiring the assumption of the establishment of the transient equilibrium condition with its parent Rn-222. (c) 2005 Elsevier Ltd. All rights reserved.

Hydro(radio)chemical relationships in the giant Guarani aquifer, Brazil

This investigation was carried out within the Parana sedimentary basin and involved the sampling of 78 pumped tubular wells for evaluating the hydrochemistry and radioactivity due to the nuclides (238)U, (234)U, (222)Ra, (226)Ra, and (228)Ra in the Brazilian part of Guarani aquifer. Several significant correlations were found involving the geostatic pressure, for instance, specific flow rate, CO(3)(2-), SO(4)(2-) temperature, dissolved O(2), free CO(2), pH, redox potential Eh, conductivity, Na, HCO(3)-, CO(3)(2-) , SI(calcite), Cl(-), F(-), SO(4)(2-), and B. Carbonates precipitation was evidenced by inverse correlation between CO(3)(2-) and Ca, Mg, Sr, and Ba, whereas Na exhibited an opposite trend, dissolving rather than precipitating with increasing CO(3)(2-) concentration. An inverse correlation between 3 and K was found, possibly related to the increasing tendency of K to recombine with the thickness of the clayey layers. HCO(3)-played an important role on Na, Ca, Mg, and Sr dissolution. The dissolved U content and (234)U/(238)U activity ratio data were plotted on a two-dimensional diagram that was successfully utilized on identifying an unreported zone of U accumulation, though not necessarily of economic size and grade. The variability in chemical and radionuclides data indicated an important influence of the underlying Paleozoic sediments in the composition of waters from Guarani aquifer. The available data allowed estimate the groundwater residence time by two U-isotopes disequilibrium methods. Values of 45-61 ka were initially calculated, depending on the adopted porosity (15-20%), but a longer residence time (- 640 ka) was also estimated, which is more compatible with the hydraulic conductivity data in Guarani aquifer and groundwater flow velocity occurring at Milk River aquifer, Alberta, Canada. Such time range agrees with previously reported (14)C ages exceeding 30 ka BP at the more central parts of the Parana sedimentary basin. (c) 2005 Elsevier B.V. All rights reserved.

ENHANCEMENT OF U-234 IN SPRINGWATERS OF AGUAS-DA-PRATA, SAO-PAULO, BRAZIL

Springwater samples were collected at Aguas da Prata, an important spa in São Paulo State, Brazil. The U-238 and U-234 isotopes were evaluated for Platina, Paiol, Villela, Prata-Radioativa, Prata-Nova, Boi, Vitoria, and Prata-Antiga springs. An inverse logarithmic correlation was found between U-234/U-238 activity ratio and Rn-222 content of the studied waters, which was related to different solution mechanisms of Rn-222 and U-234 nuclides at the rock-water interface. A linear correlation also exists between the U-234/U-238 activity ratios of the studied waters and total dissolved solids, ionic strength, CO2 partial pressure, and index of base exchange, which indicates that the enhancement of U-234 in solution increases with the degree of weathering in the aquifers, A higher U-234/U-238 activity ratio was found to be directly related to a higher value of dissolution rate.

Geo(anthropo)genic factors affecting the radioactivity in groundwaters from Guarani aquifer, Brazil

This investigation was carried out within the Paraná sedimentary basin, involved the sampling of 77 pumped tubular wells, and was realized with the puipose of evaluating the radioactivity in the Brazilian part of Guarani (Botucatu-Pirambóia, Tacuarembó, Misiones) aquifer. The radioactivity due to nuclides belonging to uranium and thorium series decay was investigated in terms of the uranium isotopes 234U and 238U, radon (222Rn), and the radium isotopes 226Ra and 228Ra. The obtained results were compared with the maximum permissible concentration limits in drinking water defined by the Brazilian national standard, as well with the guidelines for drinking water quality established by the World Health Organization. The importance of water-rock/soil interactions was considered in order to explain most of the obtained data.

Radon emissions related to the granitic Precambrian shield in southern Brazil

The equivalent uranium (eU) activity concentration was analysed in selected granite samples at several sites in Porto Alegre, Southern Brazil, to obtain information on the radon (222Rn) generation by the aquifer rock matrices. Radon analyses of ground water and soil samples were also performed. Several samples exhibited a dissolved 222Rn activity concentration exceeding the World Health Organization maximum limit of 100 Bq l-1. The dissolved radon content in ground waters from the Fractured Precambrian Aquifer System exhibited a direct significant correlation with the eU in the rock matrices, which is a typical result of water-rock interactions. Variation in the soil's porosity was confirmed as an important factor for 222Rn release, as expected, due to its gaseous nature. Thus, although the calcic-alkaline to alkaline Precambrian granitoid rocks of the study area are important reservoirs for underground resources, they can release high amounts of radon gas into the liquid phase. © 2013 Copyright Taylor and Francis Group, LLC.