现代沉积年分辨的137Cs计年──以云南洱海和贵州红枫湖为例

1964,1975及1986年三个¹³⁷CS时标计算出红枫湖和洱海沉积物平均堆积速率完全一致,说明1975年和1986年次级蓄积峰作为计年时标的可靠性。洱海沉积物平均堆积速率为0．047±0．002g／(cm²·a),与²¹⁰Pb方法所获得的结果一致;红枫湖为0．17±0．01g／(cm²·a),与其建湖以来沉积物实际堆积的观察结果吻合。洱海1964年沉积物中¹³⁷CS的比活度(校正到沉积年代)仅46．4Bq／kg,而红枫湖达145Bq／kg。由于红枫湖沉积物堆积速率约为洱海的3．7倍,所以其1964年层节沉积物中¹³⁷CS蓄积量比洱海大11.5倍。红枫湖沉积物中¹³⁷CS累计值的99．4％分配于1985年以前,而洱海的19．4％分配于1986年以后,显示出¹³⁷CS在洱海沉积物中的较大扩散能力。     

Approaches to equilibrium in the distribution of trace elements among the principal minerals in a high-grade metamorphic terrane

X-ray fluorescence, instrumental neutron activation, and particle-induced X-ray emission methods were used to determine the distribution of numerous trace elements among garnet (Grt), Ca-pyroxene (Cpx), hornblende (Hbl), biotite (Bt), plagioclase (Pl) and K-feldspar (Kf) in a high-grade metamorphic terrane within the Grenville Province of the Canadian Shield. Results are presented as distribution formulae, e.g. Sr: Kf 1.1 Pl 16 Hbl 2.2 Cpx 1.0 Bt 1.2 Grt Sc: Hbl 1.1 Cpx 1.0 Grt 7.8 Bt 22 Pl 2.6 Kf V: Hbl 1.15 Bt 2.07 Cpx 6.0 Grt (1.4% CaO)>1 (Pl, Kf) Zn: Bt 1.6 Hbl 1.62 Cpx 2.9 Grt 10 Pl Ga: Bt 1.2 Hbl 1.2 Pl 2.5 Cpx 1.3 Grt where numbers are distribution ratios, e.g. ppm Sr in Hbl/ppm Sr in Cpx=2.2. Examples of inter-element similarities and differences are (a) both Rb and Cs are concentrated in biotite relative to K-feldspar, but for Rb the ratio is 2.3 and for Cs it is 16, (b) the distribution formulae for seven lanthanides are similar except for the position of garnet, e.g. Ce: Hbl 2.7 Cpx 2.8 Pl 1.1 Bt 11 Kf 16 Grt Yb: Grt 2.8 Hbl 2.7 Cpx 9 Pl 1.0 Bt 7 Kf and (c) all of Sr, eight lanthanides, Zr, V and Cr are concentrated in hornblende relative to Ca-pyroxene by a factor that lies in the narrow range of 2.2–3.1. There is a larger variation (departure from the mean) in some distribution ratios than in others. Thus the mean ratios (Hbl/Cpx) for each of six elements and in parentheses the percentage relative standard deviation are Zn 1.62 (8.6), V 2.38 (12), Cr 2.42 (18), Sr 2.7 (28), Ba 2.9 (36) and Ni 1.66 (38). We suggest that variation of this kind is the result of differences from place to place in the magnitude of deformation and recrystallization (which facilitated the rearrangement of atoms), combined with rates of lattice and crystal-boundary diffusion that are unique for the various elements, thus permitting some trace elements to approach equilibrium more closely than others.     

U, Th, K and Cs activity concentrations along the southern coast of the Caspian Sea, Iran

The determination of activity concentrations of the radioactive elements ²³⁸U, ²³²Th, ⁴⁰K and ¹³⁷Cs was performed on grab samples taken from a polluted environment. The samples were sliced into strata from 5 cm depth, dried and ground to sieved through a 170 mesh size prior to the analysis. Activity concentration was quantified using gamma spectroscopy. The results showed that the concentrations of activity in the sediment samples are 177 ± 12.4, 117 ± 11.5, 1085 ± 101.6 and 131 ± 4.8 Bq kg⁻¹ for ²³⁸U, ²³²Th, ⁴⁰K and ¹³⁷Cs, respectively. In general, the distribution of activity concentrations along the southern coast of the Caspian Sea area exceeded international limits. The hazard index of the samples was 0.19-0.88, with an average of 0.49. The mean values of radium equivalent activity and dose rate are 176 Bq kg⁻¹ and 63 nGy h⁻¹, respectively.     

辽宁省海岸带主要河口区近百年沉积速率变化规律

基于¹³⁷Cs法和²¹⁰Pb法的相互校验,利用13孔岩心663个样品,定量分析了辽宁省海岸带主要河口区近百年沉积速率变化规律.研究表明,辽宁海岸带不同地区、不同时间段沉积速率是不同的.辽河河口区沉积最快,为23.66 mm/a,属于中高速沉积区;辽西海岸带沉积最慢,且总的趋势是沉积速率逐渐减小.随深度增加,辽西海岸带沉积速率逐渐加大,辽东湾大辽河、大凌河入海口沉积速率出现多个峰值,辽东半岛黄海西北岸沉积速率则逐渐减少.     

独流减河盐沼210Pbexc和137Cs剖面记录的现代洪水事件沉积

在独流减河入海口附近河道之间的盐沼采用人工探坑侧壁连续取样,获得2个站位的沉积物样品。通过²¹⁰Pb和¹³⁷Cs定年法建立年代框架,结合粒度分析、有孔虫鉴定和水文数据,探讨了该区的现代洪水事件沉积。结果表明,该区沉积物的²¹⁰Pb_exc比活度-深度剖面中存在2处明显的低值沉积层,分别在3~6 cm和10~14 cm深度,这2个沉积层分别对应¹³⁷Cs曲线的次峰和主峰。这两层与上下相邻层位相比呈现粒径较粗、分选较差,有孔虫丰度较低、破损百分比较高的特征。²¹⁰Pb测年结果显示,¹³⁷Cs曲线下部的主峰对应的年龄为1963年,与全球性的最大峰值形成时间相对应,而上部的次峰对应的并非1986年切尔诺贝利核泄漏事故形成的次峰。区内水文数据显示,在1963和1996年分别发生了2次较大的洪水事件。因此,推断S4和S5站位剖面中2个特殊的沉积层是由洪水事件造成的。本研究说明高分辨率沉积特征的多指标研究可以较为准确地揭示洪水等灾害性事件发生的年代和频率,这有利于系统地揭露海陆过渡带地区地质历史时期灾害和气候事件的频率和强度,为灾害预防和未来规划提供科学依据。     

Slope‐area thresholds of road‐induced gully erosion and consequent hillslope–channel interactions

Pikes Peak Highway is a partially paved road between Cascade, Colorado and the summit of Pikes Peak. Significant gully erosion is occurring on the hillslopes due to the concentration of surface runoff, the rearrangement of drainage pathways along the road surface and adjacent drainage ditches, and the high erodibility of weathered Pikes Peak granite that underlies the area. As a result, large quantities of sediment are transported to surrounding valley networks causing significant damage to water quality and aquatic, wetland, and riparian ecosystems. This study establishes the slope/drainage area threshold for gullying along Pikes Peak Highway and a cesium‐137 based sediment budget highlighting rates of gully erosion and subsequent valley deposition for a small headwater basin. The threshold for gullying along the road is S_cr = 0 · 21A^–0·45 and the road surface reduces the critical slope requirement for gullying compared to natural drainages in the area. Total gully volume for the 20 gullies along the road is estimated at 5974 m³, with an erosion rate of 64 m³ yr^–1 to 101 m³ yr^–1. Net valley deposition is estimated at 162 m³ yr^–1 with 120 m³ yr^–1 unaccounted for by gullying. The hillslope–channel interface is decoupled with minimal downstream sediment transport which results in significant local gully‐derived sedimentation. Copyright © 2013 John Wiley & Sons, Ltd.     

Cs2SO4溶液结构的初步研究

用北京同步辐射(BSRF)研究了Cs2SO4溶液298 K的水合结构。研究结果表明,溶液中占优势的物种是六水合铯离子和八水合硫酸根离子,揭示了硫酸铯溶液结构,并讨论了浓度对水合结构的影响。硫酸铯溶液中没有发现离子对的存在。     

太湖THS孔现代沉积物137Cs和210Pb的分布及计年

太湖THS沉积物柱状岩心中存在3个明显的137Cs蓄积峰,高含水率和有机质含量是前苏联切尔诺贝利核电站核泄漏对应的137Cs蓄积峰向表层迁移的一个可能的原因。利用137Cs核素1963年和1975年对应的蓄积峰计算出沉积速率为0.34 cm/a。利用137Cs核素1963年对应的蓄积峰进行校正,采用210Pb计年的CRS模式获得不同时段的沉积速率,发现在80年代末沉积物堆积通量最高,达到0.6 g/(cm2.a)。两种计年方法的结合有助于认识沉积速率的变化情况。     

137Cs测年在海岸盐沼中的应用

海岸盐沼广泛分布在海岸和河口环境,滩面有盐生植被覆盖,并且被潮水沟分割成片。研究海岸盐沼沉积过程和沉积速率,对于深入认识人类活动与海岸系统之间的相互作用具有非常重要的意义1。37Cs测年技术是近20年用来测定现代沉积速率的一个有效方法,人工核试验产生的137Cs被释放到大气中,再经气-水、水-沉积物界面的交换和吸附而沉淀在地层里,通过几次出现的峰值年代来计算不同时段的沉积速率;该方法在计算湖泊沉积速率方面取得了成功,在陆架、海湾、潮滩等区域也取得了较好的成果。但由于该方法对研究区域地层记录的稳定性有较高的要求,沉积物黏土含量、分选程度均对137Cs含量有显著影响;海岸盐沼由于植被生长茂盛,根系伸入地层深处,对沉积地层有较大扰动,因此,很难分辨出特征峰值,需要借助其他手段来综合判断特征沉降峰值。     

Assessment of accumulation rates in small reservoirs by core analysis, 137Cs measurements and bathymetric mapping in Burkina Faso

Reservoir siltation is one of the most important off‐site effects of soil erosion. In Burkina Faso a large number of small dams have been built to store rainfall and runoff water in the rainy season, which is much needed for domestic use, irrigation and stock watering in the dry season. This study quantifies the siltation rates of two small reservoirs in southwestern Burkina Faso by comparing the initial and current reservoir bed morphology and by analyzing variations in accumulated sediment in terms of stratigraphical changes and ¹³⁷Cs concentrations. Results of a bathymetric survey show that both reservoirs have already lost approximately 10% to 15% of their original storage capacity at normal pool level and more than 60% of their initial inactive storage volume at spillway level in the last 15 to 20 years. During this period, inflow sediment had accumulated on the bottom of the reservoir to a thickness of 0.3 m to 0.5 m. Sediment core measurements confirmed this thickness by showing a clear stratigraphical change in core properties with significant variations in the ¹³⁷Cs concentrations at these specific depths. The thickness of accumulated sediment on top of the relict soil indicates that the reservoirs have a ‘half‐life’ of about 40 to 45 years before siltation might affect its water supply and flood control. Compared with other reservoir studies in West Africa these siltation rates can be seen as reasonable but not alarmingly high. Nevertheless, many of the small dams built during the 1980s are likely to reach their ‘half‐life’ in about 10 to 20 years causing hydrological and geomorphological changes upstream and downstream of the dams. Therefore, further research studies should focus on prevention measures to reduce the off‐site effects of erosion and thus to slow down the process of siltation. Copyright © 2015 John Wiley & Sons, Ltd.