210Pbex示踪法技术原理及其在土壤侵蚀中的应用

土壤侵蚀是全球性的环境问题之一,严重威胁到资源、环境和社会经济的可持续发展。应用核素示踪技术研究土壤侵蚀已成为当前该领域研究热点,它体现出了很多的优越性,并在今后研究中有着广阔的应用前景。本文介绍了210Pbex的示踪原理,概述了210Pbex在土壤侵蚀示踪研究中的存在的关键性问题及其应用,并对其研究进行了展望。     

内蒙古浑善达克沙地南缘~(137)Cs、~(210)Pb_(ex)复合示踪研究（英文）

Wind-driven soil erosion results in land degradation, desertification, atmospheric dust, and sandstorms. The Hunshandake Sandy Land, an important part of the Two Barriers and Three Belts project, plays important roles in preventing desert and sandy land expansion and in maintaining local sustainability. Hence, assessing soil erosion and soil accumulation moduli and analyzing the dynamic changes are valuable. In this paper, Zhenglan Banner,located on the southern margin of the Hunshandake Sandy Land, was selected as the study area. The soil erosion and accumulation moduli were estimated using the ~(137)Cs and ~(210)Pb_(ex) composite tracing technique, and the dynamics of soil erosion and soil accumulation were analyzed during two periods. The results are as follows:(1) the regional ~(137)Cs reference inventory was 2123.5±163.94 Bq/m~2, and the regional ~(210)Pb_(ex) reference inventory was 8112±1787.62 Bq/m~2.(2) Based on the ~(137)Cs isotope tracing analysis, the erosion moduli ranged from –483.99 to 740.31 t·km~(-2)·a~(-1). Based on the ~(210)Pb_(ex) isotope tracing analysis, the erosion moduli ranged from –441.53 to 797.98 t·km~(-2)·a~(-1).(3) Compared with the earliest 50 years, the subsequent 50 years exhibited lower soil erosion moduli and accumulation moduli.Therefore, the activities of local sand dunes weakened, and the quality of the local ecological environment improved. The multi-isotope composite tracing technique combining the tracers ~(137)Cs and ~(210)Pb_(ex) has potential for similar soil erosion studies in arid or semiarid regions around the world.     

基于 137Cs和 210Pbex方法的高寒草甸坡面土壤侵蚀示踪

高寒草甸是青藏高原主要的草地类型之一。为了解高寒草甸坡面土壤侵蚀特征,以兴海盆地高寒草甸坡面为研究区,分析了高寒草甸坡面上 137Cs和 210Pbex的分布特征;利用 137Cs和 210Pbex示踪方法,通过转换模型估算了高寒草甸坡面土壤侵蚀速率,并对两种方法估算的结果进行了对比。结果表明:(1)选取的高寒草甸背景土壤剖面上 137Cs质量活度最大值分布在 2～4 cm层位, 210Pbex质量活度最大值出现在最表层。(2)研究区坡面 137Cs和 210Pbex质量活度和通量在不同的坡位有明显的差异。(3)根据 210Pbex估算的土壤侵蚀速率高于基于 137Cs估算的土壤侵蚀速率,表明研究区近几十年土壤侵蚀可能呈增加趋势。(4)根据 137Cs和 210Pbex估算的土壤侵蚀速率具有极显著的正相关关系( r = 0.94,P <     

地球化学示踪在现代土壤侵蚀研究中的应用

土壤侵蚀导致土地退化、农产品产量和品质下降、水环境污染等诸多问题 ,引起各方面的广泛关注 ,而且这个问题随着持续增长的人口压力和农业生产方式深刻变革的影响而日益严重。由于目前广泛使用的高差法、遥感研究法、RUSLE、野外调查法、水土流失监测点法等土壤侵蚀研究方法存在着一些不足 ,因此 ,核素地球化学示踪法、稀土元素示踪法、土壤地球化学指标法等地球化学示踪方法应运而生 ,在土壤侵蚀研究中发挥其独特作用。在比较了各种传统的土壤侵蚀研究方法的优势和不足的基础上 ,作者认为 :1 137Cs、 2 10 Pb可用于研究 30、 40年左右平均土壤侵蚀速率 ,7Be可用于示踪季节性土壤侵蚀和一次降雨事件的土粒运移 ,并说明这些核素的示踪原理和侵蚀速率的定量依据 ;2人为施放稀土元素 ( REE)示踪法适合定量一次或多次降雨事件的侵蚀速率 ,可用于土壤侵蚀理论的研究 ,尤其适合于室内模拟实验的研究 ;3土壤磁化率法和土壤理化性质指标法研究土壤侵蚀是一种处于起步阶段的侵蚀研究方法 ,适用于土壤质量退化研究 ,但要用于土壤侵蚀研究特别是侵蚀速率的定量研究还很不成熟。     

内蒙古浑善达克沙地南缘 137Cs、 210Pbex复合示踪研究

土壤风蚀是导致土地退化、荒漠化以及大气扬尘、风沙天气的重要原因。内蒙古浑善达克沙地南缘是国家主体功能区规划确定的“两屏三带”的重要部分,科学估算本地区土壤风蚀强度及其变化,是地区生态安全和生态建设成效评估的重要内容。选择浑善达克沙地南缘的正蓝旗为研究区,运用 ¹³⁷Cs、 ²¹⁰Pb_ex复合示踪技术,对研究区土壤侵蚀速率及其变化进行了分析。结果表明：① 研究区内 ¹³⁷Cs本底值为2123.5±163.94 Bq·m ^-2, ²¹⁰Pb_ex本底值为8112±1787.62 Bq·m ^-2。② 基于 ¹³⁷Cs同位素示踪分析,研究区侵蚀模数为-483.99~740.31 t·km ^-2·a ^-1;基于 ²¹⁰Pb_ex同位素示踪分析,研究区侵蚀模数为-441.53~797.98 t·km ^-2·a ^-1。③ 与20世纪20-70年代相比,20世纪70年代以来土壤侵蚀与堆积速率明显降低,研究区风沙活动明显减弱,区域生态环境质量得到改善。综合 ¹³⁷Cs和 ²¹⁰Pb_ex的多同位素复合示踪技术在干旱、半干旱地区土壤侵蚀研究中具有较大的潜力。     

黑土区土壤侵蚀的REE示踪法研究

应用定位小区段面法实验和稀土元素示踪剂检测,选择吉林省长春市净月开发区和永吉县的两块黑土实验小区进行土壤侵蚀过程研究,探讨了稳定性稀土元素示踪法应用于黑土区土壤侵蚀、沉积、分布和发生过程研究的可能性,并计算出坡面不同部位的侵蚀量与相对侵蚀量,较好地描述降雨对坡面的侵蚀过程。实验表明,坡面的土壤侵蚀率随降雨时间而加大;7°~12°坡面的最大土壤侵蚀量出现在坡面中部和下部(为Nd、Sm和Eu所标记的坡段)。降雨时间和地形坡度大小影响土壤颗粒运动的速度和方向,是土壤侵蚀发生的主要因素。     

海陆相蒸发岩硫同位素值变化和地球化学应用

硫同位素作为一种稳定的同位素,几乎遍布各种自然环境,出现于地质上所能跨越的所有温度范围。自然界中,不同环境下δ~(34)S值的差别可达180‰。研究表明,蒸发沉积硫酸盐的δ~(34)S值可以代表相应水体的δ~(34)S值,通过研究蒸发岩中的δ34S值,结合其它指标,可判断沉积环境,示踪成矿物质来源,研究成矿作用过程,为重建古地理古环境提供依据。在前人研究基础上总结了硫同位素分馏的影响因素及其在地球化学方面的应用;分析了陆相盆地蒸发咸化过程中水体δ~(34)S值的变化特征;对比研究了部分海陆相盆地硫酸盐δ~(34)S值特征,并指出对陆相盆地,不同类型的水体中δ~(34)S值变化趋势仍需进一步研究。     

~(137)Cs示踪法土壤侵蚀量估算的本底值问题

137Cs示踪法因能快速、相对简便地估算土壤侵蚀量而在土壤侵蚀定量研究中得到广泛应用。本底值获取是137Cs示踪法的关键和基础。在具有空间异质性多因素综合作用下,本底值呈现高度的空间异质性。针对本底值空间变异性,从气候气象要素、地形、土壤属性、土地利用/覆被四个方面阐明各因素与本底值空间变异的作用机理。分析了当前137Cs示踪法应用中在本底值获取时参考点存在性及选点的准确性、单个或几个本底值对研究区本底值的代表性和参考点采样设计。提出划分侵蚀测定单元、建立多本底值体系和进行地形校正解决当前137Cs示踪法中本底值存在问题的对策。侵蚀测定计算单元的划分原则和方法、根据已有参考点的137Cs本底值推算各单元137Cs本底值的技术方法、定量化研究各因素对137Cs的作用是今后需要深入的工作。     

~(210)Po法在铀矿勘察中的应用

~(210)Po作为放射性测量方法广泛运用在铀矿勘察中。铀元素在衰变过程中形成能溶于水惰性气体~(222)Rn,当发生构造运动时,含~(222)Rn地下水会沿着构造运动产生的断裂等通道运移至地表,然后经过一系列的长、短期衰变形成稳定的~(210)Po,吸附在土壤中。通过~(210)Po测量可以推断断裂构造的位置,间接的指导铀矿勘察工作。本文通过土壤~(210)Po测量大致查明研究区内,北东向断裂为主干断裂,并发育多条北西向次级断裂。     

放射性核素137Cs在示踪土壤侵蚀研究中的应用

近几十年来,土壤侵蚀日益加剧,伴随着土地质量的退化,已成为影响区域可持续发展的主要环境问题。利用放射性核素137Cs进行土壤侵蚀研究能够简便、快速、准确地获取土壤流失、沉积和空间重新分布等详细信息。从元素来源、空间分布、背景值、示踪原理、活度测定、转换模型等方面对该方法进行了详细描述,在总结其优缺点的基础上,对其未来的发展方向做出了展望。     

Application of210Pb in soils

The spectroscopic measurement of soil samples is described.²¹⁰Pb data from a soil survey in western-Europe are briefly reviewed. The average²¹⁰Pb flux from the atmosphere, as determined from²¹⁰Pb _exc soil inventories, is 90 Bq m^–2a^–1. A simple one-dimensional box-chain model is described. The model simulates the vertical, post depositional transport of natural and fall-out radionuclides in the soil. Simulation of measured²¹⁰Pb _exc ,¹³⁴Cs,¹³⁷Cs, and²⁴¹Am soil profiles shows that mixing (bioturbation) is a very efficient transport mechanism. Lead seems to be strongly fixed to organic and clay particles. It is transported by the displacement of the organic and clay carrier substances. The mean residence time of lead, caesium, plutonium and americium in organic rich forest soils is in the order of 250–1000 years. An applicability study in investigate the use of²¹⁰Pb in erosion problems showed erosion rates from 60–180 g m^–2a^–1 on organic rich forest and meadow sites with 10°–25° slopes.This is the eighth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

Using unsupported lead-210 measurements to investigate soil erosion and sediment delivery in a small Zambian catchment

Traditional techniques used to assemble information on rates of erosion and soil redistribution possess many important limitations. As a result, the use of environmental radionuclides, and more particularly ¹³⁷Cs measurements, has attracted increasing attention in recent years as a means of obtaining spatially distributed information on rates of erosion and deposition. The application of the ¹³⁷Cs approach is, however, hampered in some areas of the world where ¹³⁷Cs inventories are low and the low concentrations of ¹³⁷Cs found in soils and sediments cause problems for laboratory analysis. These problems will increase as time progresses due to the radioactive decay of the existing inventory, most of which was deposited as fallout ca. 40 years ago. This contribution explores the potential for using another fallout radionuclide, namely unsupported ²¹⁰Pb, as an alternative to ¹³⁷Cs, in the small (63 km²) Upper Kaleya catchment in southern Zambia where ¹³⁷Cs inventories are already very low. The approach employed with unsupported ²¹⁰Pb is similar to that used for ¹³⁷Cs, although the essentially constant fallout of unsupported ²¹⁰Pb through time means that the resulting estimates of erosion and soil redistribution rates reflect a longer period of time (ca. 100 years rather than ca. 40 years). The estimates of erosion and deposition rates derived from the unsupported ²¹⁰Pb measurements are used to construct typical sediment budgets for the three main land-use types in the Upper Kaleya catchment, namely, commercial cultivation, communal cultivation and bush grazing. The results obtained from the unsupported ²¹⁰Pb are compared with equivalent results based on ¹³⁷Cs measurements provided by a previous investigation undertaken in the study catchment. The two sets of results are highly consistent. The study reported confirms the viability of using unsupported ²¹⁰Pb as an alternative to ¹³⁷Cs in this environment and demonstrates that conjunctive use of both radionuclides can provide additional information on the erosional history of a study area.     

Distribution coefficients of210Pb and210Po in laboratory and natural aquatic systems

We have measured the distribution coefficient (Kd) of²¹⁰Po and²¹⁰Pb in laboratory systems and in natural freshwater systems. In the laboratory systems, an inverse relationship was observed between the particle concentration of sand or lake sediment, and the distribution coefficients of²¹⁰Po and²¹⁰Pb. The slope of the log-linearK _d vs particle concentration relation is consistent with existingK _d-particle concentration theories. These laboratory observations are consistent with similar measurements in two lakes. TheK _d values of Po and Pb for the bottom sediment-pore water system with a high particle concentration were 10 to 100 times lower than those for dilute concentrations of particles suspended in the lake water. TheK _d of²¹⁰Pb in the sediments was >10⁴ so that the diffusive transport of²¹⁰Pb has only a small influence on the interpretation of²¹⁰Pb concentration-depth profiles and the²¹⁰Pb dating of these sediments.This is the second of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing.     

The transport and mass balance of fallout radionuclides in Blelham Tarn, Cumbria (UK)

Although lake sediment archives are widely used for reconstructing historical records of atmospherically delivered pollutants, the quantitative relationship between fallout levels and their record in the sediments is complex and not well known. The original input signal from the atmosphere can be significantly distorted by mediating transport processes in the catchment, through the water column and within the sediments themselves. Since these processes also affect the fallout radionuclide ²¹⁰Pb commonly used to date sediments, a better understanding of their impact is also important to improving the accuracy and reliability of sediment dating. Blelham Tarn has been the subject of a number of palaeolimnological investigations using radiometric dating techniques since the early 1970s. More recently it was the site of a major study carried out within the EU Transuranics project concerning the long-term fate of fallout radionuclides in catchment/lake systems. This paper reviews the radiometric data from this study and uses the results to determine mass balances for fallout ²¹⁰Pb, ¹³⁷Cs and ^239+240Pu in Blelham Tarn, and their spatial distribution over the bed of the lake. Atmospheric fluxes were determined by measuring concentrations in rainwater and cumulative inventories in soil cores from non-eroding sites. Sediment records in a grid of 16 cores were used to determine the spatial distribution over the bed of the lake, and net inputs from the catchment. Mass balance calculations indicate that c. 47% of ²¹⁰Pb in the sediments derives from erosive inputs from the catchment. For ^239+240Pu the figure rises to 61%. Reduced amounts of ¹³⁷Cs in the sediments are attributed to greater losses of this radionuclide from the water column via the outflow due to its greater solubility. Inputs of radionuclides from the catchment are concentrated near one of the major input streams. Away from this part of the lake the sediment record is dominated by direct atmospheric fallout, though the detailed pattern is influenced by sediment focussing. A one parameter catchment/lake transport model is developed that incorporates the assumption that transport rates will decline with time as fallout on the catchment diffuses into the soil and becomes less available for removal. Values of the transport parameter were calculated for ²¹⁰Pb and ^239+240Pu and found to be comparable. The results suggest that it will take c. 11000 years to remove 50% of ^239+240Pu from the catchment to the lake.     

210Pb and 137Cs dating methods in lakes: A retrospective study

²¹⁰Pb has been used for more than two decades to provide the geochronology of annually deposited sediments and to construct pollution histories. Evidence from some lakes suggests that this radionuclide may be adequately mobile to compromise dating reliability. This study provides one test of that possibility by comparing recent measurements of ²¹⁰Pb and trace metals to ones carried out more than 20 yrs in the past. ¹³⁷Cs dating is used to confirm sediment accumulation rates in the recent cores. In the three Connecticut, USA, lakes studied, sediment accumulation rates changed abruptly to higher values between 40-50 yrs ago (increasing by factors of 2.2, 2.9, and 3.0). In all three lakes, rates calculated from ²¹⁰Pb distributions both above and below this horizon agreed, within measurement uncertainty, in recent and older cores. Furthermore, when the older data were corrected for 20 yrs of burial, the changes in slope in ²¹⁰Pb distributions occurred at the same depth in each pair of cores. The depth of sharp peaks in concentrations of trace metals also matched. In general, this evidence supports the idea that sediments in these lakes have simply been buried, without significant diagenetic remobilization of ²¹⁰Pb and trace metals . Nevertheless, some important differences were also observed. For two of the three lakes, there was a significant difference in average sediment accumulation rate during the past 33 yrs as calculated from ¹³⁷Cs and ²¹⁰Pb in the recent cores. Most potential causes for this difference can be ruled out, and it appears that one of the two nuclides is remobilized compared to the other. There were also significant differences in the total inventories of both ²¹⁰Pb and trace metals (both up to 2 ×) between recent and older cores in some cases. This may be due to dissimilar sediment focusing, since it is not known for certain whether the new cores were collected at exactly the same sites as in the past.     

Low-background gamma counting: applications for210Pb dating of sediments

Sediment cores from three lakes were dated with²¹⁰Pb using a constant rate of supply (CRS) model. We used low-background gamma counting to measure naturally occurring levels of²¹⁰Pb,²²⁶Ra, and¹³⁷Cs in sediment samples because sample preparation is simple and non-destructive,²²⁶Ra activity provides a direct measure of supported²¹⁰Pb activity for each sample analyzed, and¹³⁷Cs activity may provide an independent age marker for the 1962–1963 peak in atmospheric fallout of this radionuclide. In one core supported²¹⁰Pb activity was estimated equally well from²²⁶Ra activity of each sampling interval or from the mean total²¹⁰Pb activity of constant activity samples at depth. Supported²¹⁰Pb activity was constant with depth in this core. In a short freeze core, determining²²⁶Ra activity of every sample proved advantageous in estimating supported²¹⁰Pb activity because supported²¹⁰Pb activity could be estimated from²¹⁰Pb measurements only at the deepest sampling interval. Supported²¹⁰Pb activity estimated from²²⁶Ra activity also yielded more precise estimates of highly variable sedimentation rates. In the third core²²⁶Ra activity exceeded²¹⁰Pb activity at the top of the core and varied 20 fold with depth. This high input of²²⁶Ra in disequilibrium with²¹⁰Pb is attributed to recent erosion of radium-bearing materials in the drainage basin. These data invalidate the assumption that supported²¹⁰Pb activity is constant in sediment cores and can be estimated from the mean total²¹⁰Pb activity at depths where²¹⁰Pb activity is constant. We recommend using gamma counting or another independent assay of²²⁶Ra to validate the assumption of constant supported²¹⁰Pb activity in sediment cores if there is reason to expect that²²⁶Ra activity varies with depth.This is the fourth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr. P.G. Appleby is guest editing this series.     

Post-depositional 137Cs Mobility in the Sediments of Three Shallow Coastal Lagoons, SW England

We present ¹³⁷Cs profiles for three low lying coastal lagoons in Southwest England that show a decline in activity with sediment depth. ¹³⁷Cs inventories are lower than expected by comparison with local reference inventories despite the fact that sampling was undertaken in the deep-water zone of each lake where sediment and ¹³⁷Cs focusing would be expected. At all three locations, lake sediment ⁷Be and unsupported ²¹⁰Pb (²¹⁰Pb_un) inventories are not significantly lower than the local reference inventory. ¹³⁷Cs inventories in the study cores range from 38 to 95% of local reference inventories. The standing water level and mud: water interface at two sites are below maximum tide level and, at all three sites, salinity increases significantly in the water columns between low and high tide and in the pore waters of the underlying sediments. We suggest that the difference in hydrostatic pressure between sea level and standing water levels in the lagoons forces salt water up through the sediment column and that monovalent cations (especially Na⁺ and K⁺) replace ¹³⁷Cs on exchange sites leading to the upward migration and loss of ¹³⁷Cs. Rising sea levels may therefore contribute to remobilisation and release of ¹³⁷Cs to the aquatic environment from the sediments of coastal lagoons.     

A sedimentary record of human disturbance from Lake Miragoane,Haiti

Lake Miragoane, Haiti is one of the largest, natural freshwater lakes in the Caribbean (A=7.06 km², z_max=41.0 m, conductivity = 350 S cm^–1). Lake waters are dominated by calcium and bicarbonate ions. The lake was thermally stratified, and oxygen profiles were clinograde during summer visits in 1983 and 1985. A 72-cm mud-water interface core was taken near the center of the lake and dated with ²¹⁰Pb. The local ²¹⁰Pb fallout rate is low (0.09 pCi cm^–2 yr^–1), about 20% of the global average. Bulk sedimentation rates ranged from 0.008 to 0.030 g cm^–2 yr^–1 during the past 130 years (0–8 cm depth). Sediment geochemistry and pollen have been analyzed in the topmost 58 cm of the section. Tentative ages were assigned to the core by extrapolation of ²¹⁰Pb dates. According to this preliminary chronology, the bottom part of the core (58–30 cm) records pre-Columbian sedimentation (1000–500 B.P.) and contains pollen evidence of intact, dry and mesic forest. Pre-Columbian deposits are rich in organic matter (x = 30%) and relatively poor in carbonates (x = 15% as CO₂). The top 30 cm of the core preserve the record since European contact (500 B.P. to present). Pollen data reveal two episodes of deforestation following European arrival. Consequent soil erosion is documented by a decrease in organic matter content (x = 15%) and an increase in carbonates (x = 27% as CO₂). Surficial sediments reflect the widespread deforestation and soil loss that characterize the watershed today.