绿洲荒漠过渡带生态环境变化预警线提取与分析研究以新疆和田绿洲为例

运用MODIS时间序列数据,以干旱区和田绿洲为例,在对2002年、2004年SMVC-NDVI（Seasonal Maximum Value Composite NDVI）图进行分类后,通过对土地覆盖变化类型多度、重要度和评价等级的计算,将干旱区绿洲的生态环境强烈变化区定位于低草和中草的界限,并以此提取了过渡带生态环境变化的预警线。结果表明：从2002—2004年,预警线呈现整体缩小趋势,说明绿洲整体的生态环境在这3 a内有所退化,向恶性发展。通过对以气候因子为主的自然和人类活动因素定量分析后,认为预警线对干旱区荒漠绿洲脆弱生态环境的连续实时监测和可持续性发展研究具有很好的指示作用。     

沙漠地区春季近地层气象要素分布规律的观测研究

利用2005年1月至2006年4月朱日和地区20 m气象塔的风向、风速、气温、相对湿度的观测资料,分析沙漠地区春季近地层气象要素的分布规律。结果表明: 春季温度回升,风速最大,相对湿度最小,利于起沙,故沙尘天气频繁。风速满足幂指数率分布规律,并且幂指数m能够很好的反映出风速梯度的变化情况;在沙尘暴、扬沙、背景、浮尘的天气条件下,春季近地面层风速梯度依次增大,湍流动量、热量交换系数依次减小;风向以西南为主。浮尘、扬沙天气各气层平均增温率分别大于或小于同时段的背景大气;沙尘暴期间温度下降,平均降温率为0.61 ℃\5h-1。春季相对湿度的平均递减率(递增率)与平均增温率(降温率)的大小正相关。浮尘天气相对湿度的平均递减率大于同时段的背景大气;扬沙天气相对湿度的平均递减率小于同时段的背景大气;沙尘暴天气相对湿度增大,平均增大率为2.80%\5h-1。     

Fluid evolution in base‐metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland

The Dalradian and Ordovician–Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base‐metal sulphide‐bearing vein deposits associated with kilometre‐scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H₂O–CO₂–salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base‐metal deposition and occurred at temperatures between 220 and 360°C and pressures of between 1.6 and 1.9 kbar. Caledonian mineralization in Ordovician–Silurian metamorphic rocks occurred at temperatures between 300 and 360°C and pressures between 0.6 and 1.9 kbar. A later, probably Carboniferous, stage of mineralization was associated with base‐metal sulphide deposition and involved a low to moderate temperature (T_h 70 to 240°C), low to moderate salinity (0 to 20 wt% NaCl eq.), H₂O–salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long‐term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids. Copyright © 2004 John Wiley & Sons, Ltd.     

Deterministic sensitivity analysis for a model for flow in porous media

A deterministic method for sensitivity analysis is developed and applied to a mathematical model for the simulation of flow in porous media. The method is based on the singular value decomposition (SVD) of the Jacobian matrix of the model. It is a local approach to sensitivity analysis providing a hierarchical classification of the directions in both the input space and of those in the output space reflecting the degree of sensitiveness of the latter to the former. Its low computational cost, in comparison with that of statistical approaches, allows the study of the variability of the results of the sensitivity analysis due to the variations of the input parameters of the model, and thus it can provide a quality criterion for the validity of more classical probabilistic global approaches. For the example treated here, however, this variability is weak, and deterministic and statistical methods yield similar sensitivity results.     

The volcanic acidification of glacial Lake Caviahue,Province of Neuquen,Argentina

Lake Caviahue (northern Patagonia, Argentina) is a large glacial lake acidified by volcanic fluids from Copahue volcano. The lake and the feeding rivers were sampled annually from 1997 till early 2006, including the eruptive period of 2000. Lake Caviahue waters evolved over time, with the most concentrated waters in 2000 during the eruptive period, followed by gradual dilution that was interrupted by renewed acidification in 2003–2004. Inversion of the lake water data and application of a dynamic non-steady state model for the lake provides our best quantitative estimates for the variation in element fluxes over the 9-year period. The model flux results agree well with most of the measured fluxes. The Copahue hydrothermal system had gently declining element fluxes between 1997 and mid-1999, although the lake was still becoming more concentrated. About 2–3 months before the 2000 eruption, element fluxes increased strongly, but the hydrothermal fluxes almost shutoff directly after the main eruptive events. The fluxes of several elements recovered post-2001, with an increase in element fluxes in 2003–2004; the lake became more dilute between 2004 and 2006. The intrusion of new magma into the hydrothermal system just prior to the 2000 eruption led to enhanced water rock interaction, with higher concentrations of the rock forming elements in the fluids, and the hot spring flow rate increased as a result of the higher pressure in the reservoir. The fluids became saturated in alunite and jarosite, and they were already saturated with anhydrite. Precipitation of these minerals possibly led to a decreased permeability of the hydrothermal reservoir, leading to the strongly reduced element fluxes just after the eruption. In addition, K, Al and S were retained in the newly precipitated minerals as well, further diminishing their export. The acidification in 2003–2004 may have resulted from a new small intrusion of magma or resulted from seismic activity that created new permeability and fresh rock surfaces for water rock interaction. The volcano is a significant source of toxic trace elements such as F, As, B and Li as well as a nutrient (P) for the local watershed. Monitoring of the hydrothermal fluids in the river that drains Copahue, especially the S/Cl, Mg/Cl and Mg/K values as well as the magnitude of the element fluxes would provide the best information for eruption forecasting for this volcano.     

Major and trace element geochemistry of neutral and acidic thermal springs at El Chichón volcano,Mexico: Implications for monitoring of the volcanic activity

Four groups of thermal springs with temperatures from 50 to 80 °C are located on the S–SW–W slopes of El Chichón volcano, a composite dome-tephra edifice, which exploded in 1982 with a 1 km wide, 160 m deep crater left. Very dynamic thermal activity inside the crater (variations in chemistry and migration of pools and fumaroles, drastic changes in the crater lake volume and chemistry) contrasts with the stable behavior of the flank hot springs during the time of observations (1974–2005). All known groups of hot springs are located on the contact of the basement and volcanic edifice, and only on the W–SW–S slopes of the volcano at almost same elevations 600–650 m asl and less than 3 km of direct distance from the crater. Three groups of near-neutral (pH ≈ 6) springs at SW–S slopes have the total thermal water outflow rate higher than 300 l/s and are similar in composition. The fourth and farthest group on the western slope discharges acidic (pH ≈ 2) saline (10 g/kg of Cl) water with a much lower outflow rate (< 10 l/s).     

Migration and enrichment of trace elements of Lower Palaeozoic carbonate rock strata in Beijing

Analyses of trace elements of the Lower Palaeozoic carbonate rock strata in Beijing show that the contents of As, Hg, F increase from primary carbonate rocks to weathered carbonate rocks and from primary carbonate rocks to the soil coexisting with carbonate rocks, but the distribution regularity of S is not obvious. In the whole weathered stages, the sorption of As is mainly affected by Fe2O3. In soil Fe2O3 is also the main affecting factor of Hg enrichment. The main existing forms of Hg in primary carbonate rocks should simply be physical adsorption, coprecipitation and false isomorphous form between surface of carbonate rock and Hg. In soil the enrichment of F has little relationship with sul-fides and Fe2O3. In primary carbonate rocks, F is mainly absorbed by sulfides and clay minerals, etc. Weathered samples have closer genetic relationships with primary carbonate rocks. This also implies that weathered carbonate rocks have the close existing forms to that of primary carbonate rocks. In primary carbonate rocks FeS2 and FeS are the main forms of S, and sulfides have fixation effect on some heavy metals, whereas in weathered carbonate rocks and soil the fixation effect is weakened.     

Crustal thickness and adakite occurrence in the Philippines: Is there a relationship?

Adakites are increasingly being recognized worldwide in a variety of tectonic settings. Models on the formation of this geochemically distinct class of volcanic rocks have evolved from partial melting of subducted young, hot oceanic slabs to magmatism resulting from oblique subduction, low‐angle or flat subduction, or even slab‐tearing. Some workers have also pointed to the partial melting of thickened crust to explain the generation of adakitic melts. Rare earth element ratios from adakites and adakitic rocks in the Philippines were used in this study to obtain approximations of the levels where they were generated. These were tied to available geophysical data that defines the crustal thickness of the areas where the samples were collected. High Sm/Yb and La/Yb ratios denote the involvement of amphiboles, and in some cases garnet, in the generation of adakites and adakitic magmas. The presence of amphibole and garnet as residual phases suggests high pressures corresponding to thicker crust (～30 to 45 km). Adakites and adakitic rocks formed through processes other than melting of subducted young oceanic crust would need ≥30 km to account for the heavy rare earth element signatures. If mantle fractionation is not the process involved, crustal thickness is critical to generate adakites and adakitic rocks.     

土石坝横向裂缝的土工离心机模型试验研究

岸坡突变所产生的不均匀沉降变形常常是导致土石坝心墙发生横向裂缝的主要原因。使用糯扎渡高心墙堆石坝心墙混合土料进行了模拟心墙发生横向裂缝的土工离心机模型试验。试验中再现了由于岸坡坡度变化导致坝顶发生横向裂缝的现象。采用工程中常用的3种裂缝分析方法：倾度法、Leonards法和有限元应力法对试验结果进行了分析。结果表明,前两种方法仅在测点较多时才是适用的,而有限元应力法对土石坝心墙横向裂缝问题更为有效。