La Formation glaciaire de la Mambéré (République Centrafricaine): Reconstitution paléogéographique et implications à l'échelle du Paléozoique africain

The Mambéré Formation constitutes a horizontal unit located in the western and southwestern part of the Central African Republic. Stratigraphical and sedimentological study provides strong argument to attribute these deposits a glacial origin. A palaeogeographical reconstruction has been outlined in order to precise the age of the formation. Two main categories of glacial deposits have been recognized:

glaciogenic deposits made of basal tills (with facetted pebbles) and flow tills (with flattened blunt pebbles);

reworked glacial deposits formed of sandstones and conglomeratic sandstones, in continuous beds, lenses or isolated blocks, together with siltstones and bedded sandstones.

The southerly provenance of the detrital material is demonstrated by quartz grain surface analysis and heavy-mineral study. This material results essentially from the dismantling of the Precambrian Schistoquartzitic Complex and secondarily from the Granitogneissic Complex. According to the palaeomagnetic polar paths and the migration of the glacial centers over the African continent during the Palaeozoic, the Mambéré Formation may be attributed a Lower Silurian age by reference to similar formations observed in Cameroon or a Lower Carboniferous age by comparison with the glacial formations reported from Niger and Egypt.     

Restructuring of polygalacturonate on alumina upon hydration—Effect on phosphate sorption kinetics

Hydration of organic coatings in soils is expected to affect the sorption of oxyanions onto hydrous Fe and Al oxides. We hypothesized that the hydration of polygalacturonate (PGA) coatings on alumina (Al₂O₃) increases their permeability for phosphate. Pure and PGA-coated alumina were equilibrated in deionized water for 2 and 170 h at pH 5 and 20 °C before studying (i) their porosity with N₂ gas adsorption and ¹H NMR relaxometry, (ii) structural changes of PGA-coatings with differential scanning calorimetry (DSC), and (iii) the kinetics of phosphate sorption and PGA desorption in batch experiments. Scanning electron micrographs revealed that PGA molecules formed three-dimensional networks with pores ranging in size from <10 to several hundred nanometers. Our NMR results showed that the water content of intraparticle alumina pores decreased upon PGA sorption, indicating a displacement of pore water by PGA. The amount of water in interparticle alumina pores increased strongly after PGA addition, however, and was attributed to water in pores of PGA and/or in pores at the PGA-alumina interface. The flexibility of PGA molecules and the fraction of a PGA gel phase increased within one week of hydration, implying restructuring of PGA. Hydration of PGA coatings increased the amount of phosphate defined as instantaneously sorbed by 84%, showing that restructuring of PGA enhanced the accessibility of phosphate to external alumina surfaces. Despite the fact that the efficacy of phosphate to displace PGA was higher after 170 h than after 2 h, a higher phosphate surface loading was required after 170 h to set off PGA desorption. Our findings imply that the number of PGA chain segments directly attached to the alumina surface decreased with time. We conclude that hydration/dehydration of polymeric surface coatings affects the sorption kinetics of oxyanions, and may thus control the sorption and transport of solutes in soils.     

无水氯化镍及芳烃基二膦酸四乙酯的制备

1，4—二溴苯，4，4’—二溴联苯、4，4’—二溴三联和亚磷酸三乙酯在催化剂无氯化镍催化下，干燥氮气保护．反应得到相应的有机二磷酸四乙酯。     

ABUNDANCE AND DISTRIBUTION OF PLATINUM-GROUP ELEMENTS (PGE) IN PERIDOTITE FROM THE DAGZHUKA OPHIOLITE IN TIBET:IMPLICATIONS FOR MANTLE METASOMATISM

ABUNDANCE AND DISTRIBUTION OF PLATINUM-GROUP ELEMENTS (PGE) IN PERIDOTITE FROM THE DAGZHUKA OPHIOLITE IN TIBET:IMPLICATIONS FOR MANTLE METASOMATISM     

快速城市化地区水系结构变化特征——以深圳市为例

为了探讨城市化对水系结构的影响,应用深圳市1982、2002年的二期水系详查图,定量分析了水系长度、密度、分枝比、分枝能力和分维数等参数的变化特征。结果表明: 快速城市化过程中, 深圳市的河道总长度减少、密度减小, 河道数目减少, 特别是低级别的支流减少较多; 河道的分枝比和分枝能力都有不同程度的弱化, 单个水系片区和全市的河流分维数都下降,河流具有简单化的趋势, 河流的多元化特征削弱; 对于不同水系片区来说, 其变化速度不同, 表明城市化过程对水系结构的干扰存在空间差异性。建议强力推进河流的蓝线规划和滨水区的开发管理, 将水网的保护和规划纳入到城市规划的管理范围; 注重在开发利用中的保护, 减少各类新建城市用地、农用地整理过程中对水系的破坏; 加大水土保持力度, 减少因水土流失导致河流淤塞; 加强管理, 严禁向河道中倾倒垃圾。     

Fossil wood

Plants have been making wood (secondary xylem) for more than 370 million years. This familiar material is one of the keys to their massive success. Wood allows plants to attain breathtaking heights and maximize the capture of sunlight for growth. By creating complex, multi‐layered forests, the evolution of wood, has done more to shape life on land than almost any other innovation. Wood is one of the most common terrestrial fossils encountered in the geological record. Whether preserved as huge petrified logs or as minute chunks of charcoal, fossil wood is abundant in rocks of Late Devonian age and younger. It is of enormous scientific importance, shedding light on the identity and stature of ancient trees. As a record of growth over a sustained period, it also tells us much about the climates and environments in which those trees lived. In this article, I explain some aspects of the origin, evolution, preservation, and identification of fossil wood, and emphasise its great significance for geology.     

反向沙丘近地层气流变化及其对沙丘形态的影响

反向沙丘形态-动力学过程是风沙地貌的重要研究内容.选择腾格里沙漠东南缘推平沙地发育的反向沙丘为研究对象,采用二维超声风速仪对沙丘表面5个位置0.25 m和0.5 m高度的气流进行野外观测.结果表明:(1)气流方向影响近地层气流特征.以沙丘脊线垂线为轴,0.25 m高度处沙丘脊线的垂线两侧风向变化不完全对称.当丘顶气流方...     

秦直道旅游开发问题探讨

秦直道是古代连接中原地区和北方少数民族地区的一条重要纽带,它是秦帝国传递中央政令和传播中原文化的重要通讯带,也是秦始皇为抵御匈奴、调动兵力和物资的一条战略要道,在历史上起到了极其重要的作用.通过对秦直道及其现阶段旅游资源开发现状的介绍和在对秦直道旅游资源开发的可行性综合评价与分析的基础上,提出秦直道旅游资源开发的初步构想.     

河西走廊西端灌丛沙丘发育过程及其对沙漠化的指示意义

在风沙活动强烈的荒漠地区开展环境变化研究常缺乏符合要求的地质测年材料。灌丛沙丘是干旱区发育的一种特殊生物地貌类型,具有计年和储存环境信息的功能,可以用来重建沙丘的发育过程和过去的环境变化。以河西走廊西端花海盆地发育的灌丛沙丘沉积剖面为研究对象,基于AMS¹⁴C年代学及沉积物粒度、碳酸盐含量、地球化学元素等代用指标的分析,与文献记载、沙漠化监测数据和近几十年的器测气象资料对比分析,试图揭示区域灌丛沙丘的发育过程及其环境指示意义。结果表明：花海盆地灌丛沙丘的发育时间已超过800 年;20世纪60年代中后期至70年代早期人类活动的干扰曾使灌丛沙丘的堆积过程中断,但丰富的物源随后又促使灌丛沙丘再次快速发育。沙丘物源曾发生过剧烈的变化,使得灌丛沙丘沉积物中的粗颗粒组分和中值粒径并不适于作为环境代用指标重建区域的风沙环境变化,而细颗粒组分（<63 μm和<10 μm）和碳酸盐含量的变化,则揭示了研究区在20世纪70-80年代（沙丘高度300~420 cm）时获得了丰富的物质来源。灌丛沙丘沉积物中的Zr/Rb比值,能够作为良好的环境代用指标指示1970-1988年（沙丘高度300~420 cm）区域处在风沙活动较强烈的环境;近几十年花海盆地土地沙漠化明显,尤其是20世纪70年代和80年代。灌丛沙丘可以作为有较高可信度的地质记录载体,在河西走廊西端重建较长时间尺度的、高分辨率的区域气候环境演变历史。