Spatio-temporal analysis of recent groundwater-level trends in the Red River Delta, Vietnam

A groundwater-monitoring network has been in operation in the Red River Delta, Vietnam, since 1995. Trends in groundwater level (1995?C2009) in 57 wells in the Holocene unconfined aquifer and 63 wells in the Pleistocene confined aquifer were determined by applying the non-parametric Mann-Kendall trend test and Sen??s slope estimator. At each well, 17 time series (e.g. annual, seasonal, monthly), computed from the original data, were analyzed. Analysis of the annual groundwater-level means revealed that 35?% of the wells in the unconfined aquifer showed downward trends, while about 21?% showed upward trends. On the other hand, confined-aquifer groundwater levels experienced downward trends in almost all locations. Spatial distributions of trends indicated that the strongly declining trends (>0.3?m/year) were mainly found in urban areas around Hanoi where there is intensive abstraction of groundwater. Although the trend results for most of the 17 time series at a given well were quite similar, different trend patterns were detected in several. The findings reflect unsustainable groundwater development and the importance of maintaining groundwater monitoring and a database in the Delta, particularly in urban areas.     

城市雨水地下回灌过程中悬浮物表面堵塞规律

雨洪资源地下回灌是缓解城市供水压力、解决地下水过量开采引起环境负效应的有效途径之一,但堵塞问题却是制约回灌技术推广的关键问题。基于前人理论,研究了入渗介质与悬浮物粒径中值之比(D₅₀/d₅₀)对堵塞层空间分布的影响。以建立表面堵塞预测模型为目的,利用城市雨水回灌细粒石英砂介质开展室内实验,观测雨水回灌的表面堵塞过程并获取渗透性变化的观测数据。根据物理、化学与生物堵塞的产生条件及堵塞特点,判断雨水回灌产生的堵塞性质为物理堵塞。引入过滤模型描述堵塞的发展过程,确定了模型用于模拟细砂表面堵塞的参数值：孔隙堵塞系数（α）为0.15 m²/kg,阻力增长系数（f′R′）为20 000 m/kg。在不改变以上2个参数值的条件下,用该模型模拟不同悬浮物粒径悬浊水回灌细粒石英砂的堵塞过程,取得了较好的拟合效果。此过滤模型可以用于模拟入渗介质与悬浮物粒径D₅₀/d₅₀值小于5的表面堵塞的发展过程,可定量预测堵塞对回灌速率的影响。     

人工神经网络模型在地学研究中的应用进展

近年来,随着人工神经网络（ANNs）自身技术的不断完善,应用ANNs模型成功解决各类地学问题的案例大量出现。通过对其发展历程进行分析发现,20世纪80年代末国际地学分析中已开始融入ANNs技术,国内则滞后 1～2年。在地学分析中使用的各类人工神经网络类型中,BP模型应用最广,占到85％以上。在10余年的应用过程中,虽然地学的各个分支学科都移植了一种或数种ANNs模型作为其分析工具,但水文、地质、大气、遥感等领域应用较为广泛。传统地学定量分析中的单变量或多变量预测成为人工神经网络地学模型的主要应用客体。同时,诸如模式识别和过程模拟等也是ANNs模型求解的对象。目前,随着建模经验和知识的积累,地学ANNs模型的发展呈现出多种技术综合集成的态势,遗传算法、小波转换、模拟退火算法以及模糊逻辑等方法与ANNs模型融合,成为解决地学分析中非线性问题的利器。     

东秦岭东段新发现的沙坡岭细脉浸染型钼矿地质特征、Re-Os同位素年龄及其地质意义

豫西洛宁沙坡岭钼矿床位于华北克拉通南缘东秦岭钼矿带东段,是新近发现的赋存于太古宙太华群变质岩中的细脉浸染型钼矿床。本文对其地质特征进行了研究,并初步测定了1个辉钼矿样品的Re－Os同位素年龄,获得模式年龄为126.8±1.7Ma,表明沙坡岭钼矿形成于燕山期,接近金堆城、雷门沟钼矿的形成时代,Re同位素含量显示其地幔来源的特征。沙坡岭钼矿形成机制错综复杂,有待进一步研究,特别是深部钻孔工程验证工作,对于验证深部是否隐伏着与成矿相关的斑岩体、储量更大的斑岩型钼矿有着重要作用,找矿潜力巨大。     

Major element geochemistry of purple soils/rocks in the red Sichuan Basin, China: implications of their diagenesis and pedogenesis

Major element compositions and chemical weathering features of the purple soils and their corresponding rocks in the Red Sichuan Basin, China were studied in this paper to infer the diagenetic features of the purple rocks and the pedogenetic features of the purple soils. The results showed that all of the rock and soil samples, except those from Yibin, have strongly similar major element compositions and the distribution patterns of their major element compositions are similar to those of the upper continental crust. Chemical index of alteration, chemical index of weathering and Na₂O/K₂O indicate that the purple soils and rocks have similar chemical weathering intensities, whereas the Al₂O₃–CaO* + Na₂O–K₂O (A–CN–K) relationship, suggests that chemical weathering was strong during diagenesis of the purple rocks, but weak during pedogenesis of the purple soils. Variations in major element compositions of the purplish rock samples, except that from Yibin, do not primarily reflect differences in compositions of the sediment source, sediment recycling and potassium metasomatism, and therefore the effects of chemical weathering on the major element compositions were mainly dependent on climate condition and the residence time of material exposed to chemical weathering. Chemical weathering of the purple rocks reached only moderate degrees under the general influence of warm and humid climate during Jurassic and Cretaceous. Warmer and more humid climate conditions partly resulted in stronger chemical weathering of rocks in the southern region of the basin than in the central and western region, whereas shorter residence time of material in upslope position partly resulted in weaker chemical weathering of rocks in the eastern region than in the central and western region. The same climate and stable tectonic setting led to comparable chemical weathering intensities of rocks in the Ziliujing and Jiaguan Formations. Gradually colder and dryer climate from Early Jurassic to Middle Jurassic then to Late Jurassic and shorter residence time in tectonically active setting of Middle Jurassic and late Late Jurassic resulted in the following order of chemical weathering intensity of the purple Jurassic rocks: Ziliujing Formation ? Shaximiao Formation ≈ Suining Formation > Penglaizhen Formation.     

Geochronological framework and Pb, Sr isotope geochemistry of the Qingchengzi Pb–Zn–Ag–Au orefield, Northeastern China

The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (²⁰⁶Pb/²⁰⁴Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores (²⁰⁶Pb/²⁰⁴Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 17.12 to 17.41, ²⁰⁷Pb/²⁰⁴Pb = 15.47 to 15.54, ²⁰⁸Pb/²⁰⁴Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (²⁰⁶Pb/²⁰⁴Pb = 18.20 to 24.28 and 18.32 to 20.06, ²⁰⁷Pb/²⁰⁴Pb = 15.69 to 16.44 and 15.66 to 15.98, ²⁰⁸Pb/²⁰⁴Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively).Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.     

Silurian collisional suturing onto the southern margin of the North China craton: Detrital zircon geochronology constraints from the Qilian Orogen

The Qilian Orogen of north western China records mid-Paleozoic collisional suturing of arc and continental blocks onto the south western margin of the North China craton. Silurian strata from the retroarc foreland basin mark the transition from ocean closure and northward subduction to the initiation of collision suturing. Detrital zircons were analysed from the western and eastern parts of the basin and show a spectrum of ages from Archean to Paleozoic with major age concentrations at around 2.5 Ga, 1.6 Ga, 1.2 Ga, 0.98 Ga, 0.7 Ga and 0.45 Ga. Archean age grains are derived from the North China craton, whereas the Central Qilian Bloc, which lies to the south provides the likely source for the bulk of the Proterozoic detritus. Paleozoic grains are restricted to Early Silurian samples from the western part of the basin and are considered to have been derived from the magmatic arc related to ocean closure and ultimate collision of the Central Qilian Belt with the North China craton.     

A multi-scale study on land use and land cover quality change: The case of the Yellow River Delta in China

This paper presents a case study of the Yellow River Delta in China, to trace land use and land cover changes during the past 20 years, with an emphasis on land quality changes. Three sets of data are used in this case study: remote sensing data derived from satellite images; crop yield data from statistics; and soil data collected by the researchers in the field. Our study reveals that at the regional scale, LUCC has taken place in a positive direction: vegetation cover has been expanding and crop yields per hectare have been on rise. However, while the overall eco-environment has improved, the improvement is uneven across the Delta region. At local levels, some areas show signs of increased salinization and declining organic content. Both natural forces and human activities are responsible for the LUCC, but human activities play a more important role. While some impacts of human activities are positive, the damages are often long-lasting and irreversible. We also conclude that it is necessary to use both macro data (such as remote sensing data) and micro data (data collected in the field) to study land quality change. The former are efficient in examining land quality changes at the regional scale, the latter can serve to verify ground patterns revealed from macro data and help to identify local variations, so as to get a comprehensive understanding of LUCC and promote sustainable land use and land management. This revised version was published online in July 2006 with corrections to the Cover Date.     

Re–Os age of non-mineralized black shale from the Kupferschiefer, Poland, and implications for metal enrichment

Six samples of non-mineralized black shale from a Kupferschiefer section in the northern part of the Polish Zechstein Basin (Zdrada IG-8 drill hole: 1026.16–1026.90 m) were studied for Re–Os isotopes and selected major and trace elements. These black shales, averaging 5.82 wt.% C_org and 1.69 wt.% S_tot, display very low base metal values (up to 106 ppb Cu, 792 ppb Pb, and 144 ppb Zn) and have abundances of 64–1376 ppb Re and 0.37–1.25 ppb Os, with a ¹⁸⁷Os/¹⁸⁸Os ratio of 6.95–22.5. The regression of all Re–Os data yields an age of 247 ± 20 Ma, which is within the range of uncertainties of previous Rb–Sr and K–Ar geochronological studies. The scatter in the Re–Os data can be explained in terms of fluctuations in sedimentary conditions, i.e., restricted basin with terrestrial influence.     

青藏高原羌塘盆地东部鄂尔陇巴组火山岩的锆石SHRIMP U-Pb年龄及其地质意义

藏北羌塘盆地东部地区三叠系巴贡组之上沉积超覆了一套火山岩-火山碎屑岩. 该套地层下部玄武岩的锆石SHRIMP U-Pb年龄明显可以分为2组,一组年龄相对年轻 （220.4Ma±2.3Ma）,代表了羌塘东部地区鄂尔陇巴组火山岩的喷发年龄;另外一组 年龄相对较老（241.4Ma±3.6Ma）,可能与中三叠世羌塘地区普遍存在的碰撞隆升事 件有关。羌塘中生代（晚三叠世—早白垩世）盆地演化早期的沉积作用经历了由陆相 至海相的超覆过程,沉积超覆作用从冲洪积相开始,伴随着岩浆侵入、火山爆发及火 山碎屑沉积作用,总体上表现为一个向上变深的海侵序列,显示裂谷盆地的特征。