A pronounced dry event recorded around 4.2 ka in brine sediments from the northern Red Sea     

Helge W. Arz  Frank Lamy  Jürgen Ptzold 《Quaternary Research》2006,66(3):432-441

Partly laminated sediments were sampled from the brine-filled, anoxic Shaban Deep basin in the northern Red Sea. At about 4200 cal yr BP more than two millennia of anoxic sedimentation is replaced by a sub-oxic facies strongly suggesting the episodic absence of the brine. At the same time stable oxygen isotopes from surface dwelling foraminifera show a sharp increase (within less than 100 yr) pointing to a strong positive salinity anomaly at the sea surface. This major evaporation event significantly enhanced the renewal of deep water and the subsequent ventilation of the small Shaban Deep basin. The timing and strength of the reconstructed environmental changes around 4200 cal yr BP suggest that this event is the regional expression of a major drought event, which is widely observed in the neighboring regions, and which strongly affected Middle East agricultural civilizations.  相似文献   

Two-stage metamorphic evolution of the Bemarivo Belt of northern Madagascar: constraints from reaction textures and in situ monazite dating     

N. JÖNS  V. SCHENK  P. APPEL  T. RAZAKAMANANA 《Journal of Metamorphic Geology》2006,24(4):329-347

New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ～900–950 °C and pressures of ～9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al₂O₃) pointing to even higher temperatures of ～970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.  相似文献   

Heavy metals in freshly deposited sediments of the river Subernarekha, India: an example of lithogenic and anthropogenic effects   总被引：1，自引：0，他引：1  

A. K. Upadhyay  K. K. Gupta  J. K. Sircar  M. K. Deb  G. L. Mundhara 《Environmental Geology》2006,50(3):397-403

Heavy metal distribution patterns in river sediments aid in understanding the exogenic cycling of elements as well as in assessing the effect of anthropogenic influences. In India, the Subernarekha river flows over the Precambrian terrain of the Singhbhum craton in eastern India. The rocks are of an iron ore series and the primary rock types are schist and quartzite. One main tributary, the Kharkhai, flows through granite rocks and subsequently flows through the schist and quartzite layers. The Subernarekha flows through the East Singhbhum district, which is one of India’s industrialised areas known for ore mining, steel production, power generation, cement production and other related activities. Freshly deposited river sediments were collected upstream and downstream the industrial zone. Samples were collected from four locations and analysed in <63-μm sediment fraction for heavy metals including Zn, Pb, Cd and Cu by anodic stripping voltammetry. Enrichment of these elements over and above the local natural concentration level has been calculated and reported. Sediments of the present study are classified by Muller’s geo-accumulation index (I _geo) and vary from element to element and with climatic seasons. During pre-monsoon period the maximum I _geo value for Zn is moderately to highly polluted and for Cu and Pb is moderately polluted, respectively, based on the Muller’s standard. Anthropogenic, lithogenic or cumulative effects of both components are the main reasons for such variations in I _geo values. The basic igneous rock layer through which the river flows or a seasonal rivulet that joins with the main river may be the primary source for lithogenic components.  相似文献   

Early Holocene water budget of the Nakuru-Elmenteita basin, Central Kenya Rift     

Miriam Dühnforth  Andreas G. N. Bergner  Martin H. Trauth 《Journal of Paleolimnology》2006,36(3):281-294

The Nakuru-Elmenteita basin in the Central Kenya Rift, contains two shallow, alkaline lakes, Lake Nakuru (1770 m above sea level) and Lake Elmenteita (1786 m). Ancient shorelines and lake sediments at 1940 m suggest that these two lakes formed a single large and deep lake as a result of a wetter climate during the early Holocene. Here, we used a hydrological model to compare the precipitation–evaporation balance during the early Holocene to today. Assuming that the Nakuru-Elmenteita basin was hydrologically closed, as it is today, the most likely climate scenario includes a 45% increase in mean-annual precipitation, a 0.5°C decrease in air temperature, and an increase of 9% in cloud coverage from the modern values. Compared to the modeling results from other East African lake basins, this dramatic increase in precipitation seems to be unrealistic. Therefore, we propose a significant flow of water from the early Holocene Lake Naivasha in the south towards the Nakuru-Elmenteita basin to compensate the extremely negative hydrological budget of this basin. Since we did not find any field evidence for a surface connection, as often proposed during the last 70 years, the hydrological deficit of the Nakuru-Elmenteita basin could have also been compensated by a subsurface water exchange.  相似文献   

Transparency: aid or obstacle to effective defence of vulnerable environments from reservoir construction? Dam decisions and democracy in North East England     

Christine S McCulloch 《Area》2006,38(1):24-33

Public participation in transparent decisionmaking has been proposed as a panacea for environmental protection. Bargaining theory suggests drawbacks to transparency such as hardening of attitudes and intransigence. Yet achievement of consensus on environmental values for integrated water resource management demands public involvement. Historical study of attempts to prevent reservoir construction in upland valleys designated for protection of nature suggests that 'closed-door' bargaining proved more effective for environmentalists than open, public debate. Records now open for analysis of disputes over Teesdale and Farndale, 1950–1970, allow comparisons to be made between behind-the-scenes deliberations and 'open-door' public discussion. The ambition of progressing from public participation to devolution of environmental decisionmaking and responsibility to the local scale demands more conceptualization of the process of decisionmaking on water resource development.  相似文献   

东亚冬季风异常与广东前汛期旱涝关系的初步分析   总被引：6，自引：3，他引：6       下载免费PDF全文

曾琮胡斯团  梁建茵林良勋谢炯光  纪忠萍李晓娟 易爱民 《应用气象学报》2005,16(5):645-654

利用500hPa北半球格点资料,参照历史平均东亚大槽位置,定义了东亚冬季风强度指数(I₅₀₀)。分析发现,近50年东亚冬季风强度有减弱趋势,特别是1987年以来冬季风明显减弱。根据广东48个站1954～2004年前汛期(4～6月)降水资料,用区域旱涝指数W对广东前汛期旱涝等级进行划分。51年内广东有9年前汛期出现大涝(或特涝),11年出现大旱(或特旱),旱涝发生频繁。冬季风异常对广东前汛期降水的影响比较大,强冬季风年后广东前汛期出现大旱的可能性较大,而出现大涝的可能性很小。  相似文献   

亚洲赤道地区大气动能的纬向传播   总被引：7，自引：1，他引：7  

高辉  陈隆勋  何金海  陶诗言  金祖辉 《气象学报》2005,63(1):22-29

基于 1980～ 1997年 85 0hPa逐日NCEP/NCAR再分析资料讨论了亚洲赤道地区 (0°～ 5°N)大气动能的纬向传播特征。结果表明 ,在亚洲季风区内 ,赤道地区大气动能 (K)的最强中心位于 75°～ 90°E ,次强中心在索马里急流区 (5 0°E附近 )。在 0°～ 5°N ,90°E以东 ,平均的大气动能扰动和赤道上经向风扰动主要起源于西太平洋 ,并向西经南海传播到孟加拉湾。而在孟加拉湾动能中心与索马里急流区之间 ,动能传播方向比较复杂。以上事实说明赤道地区东亚季风系统确实是存在的 ,与印度季风系统中扰动的传播方向不同 ,东亚季风系统中动能和经向风扰动在东西方向上主要受西太平洋的影响。在亚洲赤道季风区 ,这两个系统的交界处约在 95°～ 10 0°E附近 ,比过去界定的偏西 5～ 10个经度。  相似文献   

西北区东部一次暴雨的数值模拟试验   总被引：19，自引：23，他引：19  

王劲松  李耀辉  康凤琴  张华 《高原气象》2002,21(3):258-266

运用双向嵌套的中尺度数值预报模式MM5，对1998年7月上旬西北区东部一次暴雨过程进行了高分辨率数值模拟和敏感性试验。结果表明，该模式能较好地模拟这次暴雨过程，对这次暴雨过程相关的中尺度系统的发生发展也作出了较成功的模拟；大尺度及积云对流尺度的凝结潜热在降水过程中是一个主要因子，潜热释放将加热中高层大气，促使高层大气辐散，低层辐合，垂直运动加强，导致较大的降水；初始时刻不同地区低层大气水汽含量的多寡直接对本次暴雨产生影响，并为这次暴雨提供了水汽源；地面水汽和感热的垂直输送为暴雨的发生发展补充了能量。  相似文献   

武当地块基性岩席群及其地质意义   总被引：6，自引：0，他引：6  

胡健民  孟庆任等 《地质论评》2002,48(4):353-360

侵位于武当地块内部的大量的板状为基性侵入体近年来一直被认为是800Ma左右发生的扬子古大陆裂解的重要依据。然而作者在武当地块北缘与西南部所做的1：5万地质填图以及相应的构造学和同位素年代学研究表明，它们与武当地块顺层伸展滑脱构造主滑脱面之顺层韧性一韧性变形带有密切的空间关系，是被褶皱了的岩席群。最新获得的岩体单颗粒锆石U－Pb年龄为401－407Ma，结合已有的地球化学研究成果，并考虑到邻区同时代碱性岩浆的侵位，作者认为，这套基性岩群代表了泥盆纪时南秦岭地区曾发生过大规模的上地幔岩浆的底侵作用，并因此导致南秦岭上部地壳的伸展减薄。  相似文献   

东天山晚古生代内生金属矿床成矿系列和成矿规律   总被引：12，自引：8，他引：12  

韩春明  毛景文等 《地质与勘探》2002,38(5):5-10

东天山造山带是我国重要的金铜矿产富集区之一，通过初步研究，按照成矿系列的学术思想将区内金属矿床划分为6个矿床成矿亚系列，并对每个成矿亚系列形成的构造环境和主要地质作用进行了简要论述，进而探讨了该地区区域成矿规律与构造演化的关系，总结了该地区构造活动的时－空演化导致该地区独具特色的内生金属矿床侧向分带现象。  相似文献