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.     

Altitude-dependent distribution of ~(137)Cs in the environment:a case study of Aragats massif,Armenia

This paper considers the distribution of technogenic 137Cs and naturally occurring radionuclides:238 U,232 Th and40 R concentrations in soils and 137Cs in atmospheric dry depositions by altitudinal belts of the Aragats mountain massif,Republic of Armenia.Undisturbed soil samples were collected at altitudes from 1000 to 3200 m.For the determination of geochemical variability,two soil sampling campaigns were undertaken.Atmospheric dry depositions were sampled from five stations at1100-3200 m collected onto organic fiber filters between June and December 2016.137Cs activity was measured using a high-purity Germanium detector coupled to a multichannel analyzer(Canberra).Results indicated that specific activity of 137Cs in soils at 1000 m is495-528 Bq m^-2,andat3200 mis10,500-11,470 Bq m^-2.No correlation observed for 137Cs versus naturally occurring radionuclides,which varies in distribution by altitude.Specific activities of 137Cs in dry atmospheric depositions varies from 1.06 at 846 m to2.37 Bq m^-2 per quarter at 3200 m and increases as the altitude increases.Activities of 137Cs in soil and dry atmospheric deposition correlated significantly,and 137Cs activity in soils and atmospheric dry depositions decrease as the absolute altitude decreases.The 50-year effective dose from exposure to 137Cs fallout varies with altitude from 0.007 to 1.42 m Sv.     

青海共和盆地达连海湖积物137Cs示踪的初步结果

通过对青海省共和盆地达连海干湖盆中的＾１３７Ｃｓ和粒度样品的测定,结合区域调查结果及其他相关资料的分析,确定了两个＾１３７Ｃｓ蓄积峰和一个蓄积谷,由此建立了达连海湖相沉积物的＾１３７Ｃｓ时标,即１９６３年的＾１３７Ｃｓ最大蓄积峰值和２０世纪８０年代末至９０年代初大规模土地开垦引起的、与异常剧烈风沙沉积有关的＾１３７Ｃｓ最小蓄积谷,以及１９９４年的湖泊干涸事件形成的次一级＾１３７Ｃｓ蓄积峰,并估算出     

Sediment accumulation rates in subpolar fjords – Impact of post-Little Ice Age glaciers retreat,Billefjorden, Svalbard

The Little Ice Age (∼1600–1900 AD) and 20th century sediment accumulation rates in Billefjorden, a subpolar fjord on Svalbard, were reconstructed by applying ²¹⁰Pb, ¹³⁷Cs and AMS ¹⁴C datings. The modern sediment accumulation rate decreases from more than 0.39 cm y⁻¹ at the fjord head to 0.08 cm y⁻¹ close to the fjord mouth. However, during the Little Ice Age the sediments accumulated at a much lower rate of 0.02 cm y⁻¹ in the central fjord basin. This difference is most likely related to the rapid retreat of glaciers during the 20th century, when most of them withdrew up to 2 km. The post-Little Ice Age increases in temperature and a negative glacier mass balance resulted in a larger meltwater discharge transferring substantial amounts of sediments released from the glaciers, as well as those eroded from previously stored unconsolidated glacial sediments. A comparison of data from the subpolar fjords of Svalbard suggests that the increase in the sediment accumulation rate is a common trend, and further increases might be expected if climate warming continues. The properties of the fjord sediments (grain size, IRD, coarse-fraction composition, clay mineralogy) from the Little Ice Age and the 20th century showed no distinct differences. The change in the accumulation rate may be the most evident sedimentary record of this climatic change.     

海岸带地区~（137）Cs沉积剖面类型划分及其意义

作为一种人工核素,137Cs年代学方法已在海岸地区得到广泛应用,并成为重建现代地质过程的重要方法。搜集了海岸带地区报道的200余组137Cs沉积剖面数据（其中中国海岸带地区121个站位,其它地区100余站位）,通过对137Cs沉积剖面形态的分析,并与理想137Cs沉积剖面对比研究,结果显示,海岸带地区137Cs沉积剖面表现为以下6种主要类型：①不连续137Cs曲线、②无特征峰型的连续137Cs曲线、③特征峰型偏下的连续137Cs曲线、④特征峰型偏上的连续137Cs曲线、⑤比活度向下增大的137Cs曲线和⑥比活度向下减小的137Cs曲线。各种类型的沉积剖面反映的沉积环境是多样的;侵蚀作用、混合作用和沉积物供给变化是控制137Cs沉积剖面的主要因素;同时,137Cs定年的取样要尽量做到无扰动,数据解释要考虑到取样分辨率的影响;在没有大气沉降监测的海岸带地区,其附近沉积环境相对稳定地区的137Cs沉积剖面可作为潮间带、浅海区的背景参考值。     

ASSESSMENT OF SOIL LOSS ON UNCULTIVATED SLOPE LAND BY USING 137CS TECHNIQUE IN THE UPPER YANGTZE RIVER BASIN OF CHINA

Radionuclides are used as tracers of sediments in areas where no plot data or field monitoring data are not available. Caesium-137 can provide important information on both soil loss rates and sediment sources. The 137Cs technique is employed to assess soil erosion rates in forest and grass lands in the Upper Yangtze River Basin, China. The study area includes 6 forestlands and 4 grasslands in 8 counties and cities over the eastern part of the basin. The ^137Cs reference inventories ranged from 620.9 Bq m-2 to 2560.3 Bq m^-2. For the 6 forestlands, the average 137Cs inventory over a field ranges from 960.8 Bq m^-2 to 2560.3 Bq m^-2. The average erosion rate is estimated from the ^137Cs measurements between 0 to 588 t km^-2yr^-1. For the 4 grasslands, the average ^137Cs inventory ranges from 119.0 Bq m2 to 869.7 Bq m^-2. The average rate of erosion from grassland varies from 0 to 4435 t km^-2 yrl. Where the erosion was more severe, the ^137Cs inventory was lower. The results showed that there is no clear relationship between ^137Cs inventory, slope length and slope gradient on uncultivated land. The erosion rates for uncultivated land and cultivated land is compared. The result indicates that the erosion rates on uncultivated land is much lower than those on cultivated slopes and the severity of the soil erosion is closely related to vegetation cover.     

Cs+水合结构的密度泛函理论计算研究

基于密度泛函理论在ωB97XD/gen def2-tzvp基组水平上对[Cs(H_2O)n]~+(n=1～15)水合团簇结构的振动光谱进行了研究。研究表明,随着水分子数的增加,在气相和液相中,八配位的Cs~+水合团簇是最稳定的;随着水分子数的增加,Cs-O键长增大,Cs~+上的电荷密度减小。当水分子数大于11时,Cs-O键长和Cs~+上的电荷密度变化很小。O(H_2O)-O(H_2O)距离、∠O(H_2O)-Cs-O(H_2O)°统计结果表明,15个水分子足以形成Cs~+的近似饱和水合层。此外,红外光谱显示[Cs(H_2O)n]~+水合团簇中O-H伸缩振动随着水分子数的增加而发生变化,由此可以作为鉴别水合层位置的一种参考方法。     

Dating recent colluvial sequences with Pb and Cs along an active fault scarp, the Eliki Fault, Gulf of Corinth, Greece

Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the ²¹⁰Pb and ¹³⁷Cs dating methods. Here, we test the applicability of using ²¹⁰Pb and ¹³⁷Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The ²¹⁰Pb and ¹³⁷Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, ²¹⁰Pb dating, corroborated by ¹³⁷Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by ²¹⁰Pb and ¹³⁷Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the ¹⁴C-based estimated dates for the upper unit. At two other trenches ²¹⁰Pb and ¹³⁷Cs dating only provided minimum ages (based on the presence or absence of ²¹⁰Pb_excess and ¹³⁷Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset.