中国湖泊沉积物137 Cs和210 Pbex断代的一些问题

137 Cs和 210 Pbex是湖泊沉积物断代常用的两种核素,沉积物中核素的深度分布,不但随核素的大气沉降量变化,也与流域环境变化和沉积后核素的再分布有关。我国环境复杂,人类活动强烈,137 Cs和 210 Pbex深度分布的非理想曲线常见于我国一些湖泊的沉积物。本文简要介绍了湖泊沉积物 137 Cs和 210 Pbex深度分布理想曲线和断代基本原理,在参阅大量有关中国湖泊沉积物 137 Cs和 210 Pbex断代文献的基础上,讨论了 137 Cs和 210 Pbex非理想深度分布曲线的解译和沉积物断代的几个常见问题。主要观点如下: 1)我国湖泊沉积物不存在所谓的1974年和1986年蓄积峰;2)一些湖泊沉积出现的非单峰型 137 Cs深度分布曲线,可能与人类活动或湖流扰动湖泊底泥、流域侵蚀产沙环境变化和 137 Cs主要来源于冰川融水补给等因素有关;3)沉积物中的核素垂向运移,不改变剖面中1963年 137 Cs蓄积峰的位置,不影响其断代标志意义;4)流域环境突发事件引起的沉积剖面中的 137 Cs和 210 Pbex明显异常,是断代的可靠标志等。最后介绍了根据沉积剖面 137 Cs和 210 Pbex总量与本底值对比,求算沉积速率的核素质量平衡法。     

210Pb和137Cs定年技术在湖泊沉积物中的应用与问题

湖泊沉积记录研究对了解流域侵蚀、揭示区域气候环境变化及人类活动影响、建立外生地球化学循环模式等均具有重要意义.放射性同位素~(210)Pb和~(137)Cs组合定年法是百年尺度湖泊沉积记录研究的首选方法,在过去几十年中得到了广泛应用,大力推动了湖泊沉积记录的高分辨率气候环境变化研究的开展.然而,如何理解和选择适用的计算模式来尽可能地提高定年准确度目前仍存在争议,这在一定程度上限制了该方法的应用.鉴于上述问题,本文基于本实验室近年来从青藏高原获取的若干支代表性湖芯的定年结果,从样品前处理、仪器及测试、数据分析及计算等全过程对~(210)Pb、~(137)Cs定年原理和方法进行了较为系统的阐述;在此基础上,提出了~(210)Pb定年样品封装原则,探讨了高纯锗γ谱仪在放射性活度测量中的影响因素,并建立了准确有效的比活度校正方法;明确指出了恒定初始浓度模式(constant initial concentration,CIC)可用于计算沉积物每层的年龄,而不是部分研究中认为的平均沉积速率,这对于深刻理解该定年模式并提供准确的定年结果具有重要的参考意义.同时,通过对比两种放射性核素的分布特征发现,相对于~(210)Pb而言,~(137)Cs更容易发生垂向迁移进而影响其时标准确性,这需引起研究者的充分重视,因此,当二者定年结果出现明显差异时,建议以~(210)Pb定年结果为准.     

内蒙古乌梁素海~(210)Pb和~(137)Cs测年与现代沉积速率

对乌梁素海沉积物柱心样品进行210Pb、137Cs测年分析表明,柱心剖面上有明显的1963年蓄积峰,这个蓄积峰对乌梁素海的现代沉积有明显的时标意义。根据210Pb的CRS模式,可以计算出每个样品深度所对应的年代,在该沉积柱心中与137Cs时标吻合较好。乌梁素海的沉积速率并不稳定,变化比较大,表明乌梁素海近113 a来沉积环境不稳定。210Pb、137Cs两种计年方法的结合有助于认识沉积速率变化较大的沼泽的沉积状况,也有助于对核素计年方法的理解。     

长江河口水下三角洲137Cs地球化学分布特征

文章通过对长江口水下三角洲采集的10个柱状样放射性核素¹³⁷Cs的分析可以得知,长江口水下三角洲¹³⁷Cs剖面中均存在清晰的最大蓄积峰,其峰值比活度介于5.68±1.03～21.74±1.39Bq/kg之间,平均值为14.11±1.10Bq/kg,最大蓄积峰所处的深度为55～117cm。剖面中¹³⁷Cs最大蓄积峰应该与1963年的¹³⁷Cs散落沉降相对应。长江口水下三角洲的表层沉积物中的¹³⁷Cs比活度范围介于0～9.19±1.12Bq/kg之间,并且与长江流域其他地区的表层¹³⁷Cs比活度相一致。长江口水下三角洲可探测到的¹³⁷Cs比活度的最大深度范围在88～160cm的范围内变化,¹³⁷Cs蓄积总量为2361.30±174.38～17714.94±262.14Bq/m²,平均值为9664.97±100.05Bq/m²,¹³⁷Cs比活度的最大深度及¹³⁷Cs蓄积总量均表现出从岸向海逐渐增加的趋势。实测的¹³⁷Cs总量均大于长江流域的¹³⁷Cs背景值,说明了长江口水下三角洲的¹³⁷Cs蓄积既有大气散落直接沉降的来源,又有流域侵蚀带来的¹³⁷Cs输入,并且主要以后者为主。通过放射性核素示踪模型分析长江口水下三角洲¹³⁷Cs散落蓄积特征可以得知,长江口水下三角洲¹³⁷Cs的蓄积以长江流域来源为主,说明了放射性核素¹³⁷Cs在长江口水下三角洲沉积物中的蓄积主要受流域侵蚀因素的影响。     

江苏北部潮滩沉积物中~(137)Cs和~(210)pb的分布特征(英文)

Seven cores were collected from different sediment zones of tidal flats at Xin-yanggang in north Jiangsu province in August 2007. Sediment grain-size distribution and radioisotopes of ~(137)Cs and ~(210)Pb analysis were carried out for these cores. Sediment rates of the cores and radioisotopes distribution in surface sediment in different zones of the tidal flat were calculated from the ~(137)Cs and ~(210)Pb activities in sediments cores. The results indicated that each tidal zone had experienced different evolution phases, hydrological dynamics in the tidal flats made the grain-size of the surface sediment change gradually. ~(137)Cs and ~(210)Pb activities on the superficial layer of the cores varied spatially and the reason was discussed. On tidal flats, the fluctuation of ~(137)Cs and ~(210)Pb activities in the cores reflected the special sedimentary characteristics. Vegetation affects the grain-size distribution and the vertical profiles of ~(137)Cs and ~(210)Pb in the upper depths. ~(137)Cs and ~(210)Pb chronology got the comparable average sediment rates on the tidal flat. The characteristics of ~(137)Cs and ~(210)Pb in the cores reflected various depositional dynamical environments in different tidal zones and gave information on the different evolvement phases of the tidal zones. Based on the information of grain-size distribution, texture of the cores, sediment rates and topography, the evolution lines of the tidal flat were reconstructed.     

Preliminary results of usingCs to study wind erosion in the Qinghai-Tibet Plateau

The world fallout of caesium-137 (¹³⁷Cs) associated with nuclear weapons testing during the 1950s and 1960s has provided a valuable man-made tracer for studies of soil erosion and sediment delivery. But relatively few researchers have used it to estimate wind erosion. In this paper, the¹³⁷Cs technique is introduced into the study of wind erosion and its modern processes in the Qinghai-Tibet Plateau. Two¹³⁷Cs reference inventories of 982·11 and 2376·04 Bq m⁻²was established preliminarily, which distribute in the south and mid-north parts of the study area respectively. By analysing the patterns of¹³⁷Cs depth profiles from sampling sites, the aeolian processes of erosion and deposition along nearly 40 years has been revealed, i.e. the shrub coppice dunes(S1) and semi-fixed dunefields (S3) had experienced the alternation of erosion and deposition, while the grasslands (S4, S6 and S7) and dry farmlands (S5) suffered erosion only. By using the¹³⁷Cs model, the average wind erosion rates for shrub coppice dune (S1), semi-fixed dunefields (S3), dry farmlands (S5) and grasslands (S4, S6 and S7) were estimated to be 84·14, 69·43, 30·68 and 21·84 tha⁻¹a⁻¹respectively, and for the whole Plateau, averaging 47·59 ha⁻¹a⁻¹which can be regarded as the medium erosion standard. These results derived from¹³⁷Cs for the first time have significant implications for the further research of wind erosion and desertification control in the Qinghai-Tibet Plateau.     

滇西和黔中表土中7Be与137Cs分布特征对比研究

滇西和黔中28个表土剖面分层土样^7Be、^137Cs比活度对比分析可见：滇西地区表土层顶部^7Be/^137Cs为100－1000，而黔中地区仅10－100；两地区表土层中^7Be的表观比活度和最大渗透深度总体相近；^137Cs在黔中表土层中的比活度总体较滇西地区略高。而黔中表土中^7Be累计值的偏低现象，映射了该地区强烈的水土流失。为解释滇西和黔中地区之间表土层顶部^7Be/^137Cs的地区差异，需进一步讨论两核素的湖泊沉积物蓄积特征。     

纹层计年与137Cs、210Pb法对比研究东北二龙湾玛珥湖现代沉积速率

通过对东北二龙湾玛珥湖顶部无扰动岩心显微研究,证明此岩心发育有良好的年纹层。据精确统计,0~5.5 cm间为36个年纹层,纹层计年的误差为4％~7％,年纹层厚度为1.1~2 mm不等,平均沉积速率为1.53 mm/a,5.5 cm处对应于1965年。而二龙湾沉积物（从6.5 cm到5.5 cm)岩心的¹³⁷Cs比活度值急剧增加,从（4.67±0.29） dpm/g增加到（12.06±0.47） dpm/g,在5.5 cm处有一个较为明显的蓄积峰值,这一峰值被确认对应为1963年全球¹³⁷Cs散落的高峰期。另外据²¹⁰Pb_ex比活度CIC模式,5.5 cm处约为1962年,这和¹³⁷Cs比活度曲线的1963年时标相一致。印证了纹层计年的精确性,完全可以建立高分辨率时间序列。     

137 Cs示踪农耕地土壤侵蚀速率模型的比较研究

137 Cs示踪技术广泛地应用于农耕地土壤侵蚀研究，目前已建立了许多运用 137 Cs估算土壤侵蚀速率的模型。这些模型主要分为两类：经验模型与理论模型。其中理论模型中的质量平衡模型应用较多。质量平衡模型主要有：Wal l ing模型、张信宝模型、杨浩模型和周维芝模型。重点讨论这 4种质量平衡模型的异同。详细阐述了这四种模型的建立过程，并用图形模拟的方法给出各个模型所刻画的土壤侵蚀速率与土壤剖面中 137 Cs相对损失率的关系。分析表明，尽管各个模型在建立的假设和方法上以及对 137 Cs沉降过程的处理上存在一定程度的差异，但是各个模型所 刻画的土壤侵蚀速率与土壤剖面中 137 Cs相对损失率的关系实质上都是幂函数的形式，而且这 4条曲线的走势基本一致，各自计算的土壤侵蚀速率差异也较小。因此，在利用 137 Cs技术示踪农耕地土壤侵蚀速率时，这 4个模型都可以应用。     

Quantifying sheet wash erosion rates in a mountainous semi‐arid basin using environmental radionuclides and a stream power model

Erosion rates and processes define how mountainous landscapes evolve. This study determines the range of erosion rates in a semi‐arid landscape over decadal time spans and defines the dominant processes controlling variability in erosion rates. The varying topography and climatic regimes of the Xiying Basin (Qilian Shan Mountains, China) enables us to examine the relative roles of sheet wash versus rainsplash and the influence of vegetation on soil erosion and deposition. Soil erosion rates since 1954 were determined using ¹³⁷Cs along 21 transects at four sites with varying gradient, rainfall, and vegetation cover. The mean ¹³⁷Cs derived soil erosion rate ~0.42 mm/a was consistent with the catchment level erosion rate derived from total sediment yield for a 44 year record. However, there is considerable variability in ¹³⁷Cs erosion rates both between transects and along transects, perhaps reflecting variation not only in the effectiveness of individual processes but also in their relative roles. We compare the ¹³⁷Cs‐derived erosion rates with 1‐D models for sediment flux that incorporate sheet wash and rainsplash processes, testing them over a previously untested 60 year timescale. The variability in ¹³⁷Cs erosion rates along transects is best replicated by sheet wash dominated simulations, suggesting that this is the dominant erosion process in this semi‐arid landscape. The functional form of the sheetwash model can also explain our observations that ¹³⁷Cs erosion rates decrease with upslope length (i.e. distance down slope) while its variability increases. However, sparsely vegetated sites, located in slightly drier locations, have higher erosion rates, and are not as accurately modeled as densely vegetated sites, suggesting that patchiness of vegetation introduces fine scale variability in erosion rates on these slopes. Copyright © 2015 John Wiley & Sons, Ltd.