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91.
The identification of fossil conifer stomata was conducted from a lake sedimentary sequence on the Liupan Mountains, southwestern Loess Plateau, in order to reconstruct detailed forest history and assess the potential of using stomata as a supplement to pollen analysis to determine the local presence of conifers over the last 3200 years. The pollen has already been analyzed in our previous study. Reference conifer stomata were prepared for the fossil stomata identification and demonstrate that the conifer stomata could be identified to at least genus level. Our stomata and pollen results reveal three clear stages of the forest recession during the late Holocene: mixed conifer-deciduous forest, steppe-forest and steppe. Combined stomata and pollen results confirm the local presence of conifer trees (Abies and Pinus) in the vicinity of the Tianchi Lake. Abundant Abies trees existed during 3200-2200 cal yr BP. Then it decreased and finally demised at ca.210 cal yr BP. Pinus trees have been continuously present with very low abundance throughout the late Holocene. Application of comprehensive studies comprising both stomata and pollen analyses allowed a detailed determination of the stages of conifer trees presence in the vicinity of the Tianchi Lake. Our study suggests that the analysis of fossil stomata is a valuable methodological tool for the provision of unambiguous evidence of the past local presence of the coniferous taxa in this region. 相似文献
92.
93.
分析了楚雄市1965-2010年的降雨特点,采用同时期城区(楚雄站)与郊区(南华站、双柏站)雨量横向对比和城市化发展不同时期同一站雨量纵向对比的方法,研究了城市化对年降雨量、季节雨量和不同类型降雨发生次数等的影响.通过研究发现:城市化发展对城市年雨量、汛期雨量有不同程度的增加作用;受城市化发展的影响,降雨年内分配集中雨季的趋势更加明显;城市化发展使不同类型降雨发生次数有增加或减少,对大雨和暴雨有增加作用,小雨和中雨有所减少.城市化发展使城市大雨和暴雨增多,所以要加强城市洪涝、滑坡和泥石流灾害的防治工作. 相似文献
94.
基于遥感和GIS的黄土高原中阳县土壤侵蚀评价 总被引:4,自引:0,他引:4
黄土高原地区是我国水土流失最严重的地区,定量评价土壤侵蚀量、土壤侵蚀强度及其空间分布特征,可以为水土流失防治措施的制定提供依据.基于遥感和GIS,应用修正的通用土壤流失方程(RUSLE)对山西中阳县土壤侵蚀进行评价.结果表明,中阳县平均土壤侵蚀模数为2 874.25t/(km2·a),属于中度侵蚀地区,年平均土壤侵蚀量408.78×104 t.发生中度以上土壤侵蚀面积占总面积的29.95%,土壤侵蚀量占总侵蚀量的85.61%,这部分土地是土壤侵蚀防治的主要区域.黄土区发生轻度以上土壤侵蚀面积大,所占比例多,侵蚀程度严重.微度土壤侵蚀面积以有林地为主,轻度和中度土壤侵蚀面积主要是未成林造林地和灌木林地,发生强烈以上土壤侵蚀面积主要是疏林地、未利用地、灌木林地和农田,土地类型、植被覆盖与土壤侵蚀有着密切关系,林业生态工程建设对于中阳县控制水土流失和改善生态环境有着重要作用. 相似文献
95.
In the last decade, there has been increasing interest in climate change, pasture degradation and its driving forces, and innovations in nomadic pastoralism on the Tibetan Plateau. However, little is known of indigenous strategies of adaptation to pasture degradation, which limits the effectiveness of adaptation strategies planned by local government. This paper analyzes nomads’ strategies of adaptation to pasture degradation on the basis of a field survey of three townships of Dalag County in the source regions of the Yangtze and Yellow rivers. Pastures there have evidently degraded, with pastures in Wasai mainly in a state of slight or medium degradation and those in Manzhang and Jianshe in a state of medium or severe degradation. With the degradation of pasture, the grazing time is reduced, which affects the livelihoods of nomads. Although the Four-Package Project has commenced in this region, there is still severe fodder shortage in winter and spring. The traditional hay storage strategy does not work because of pasture degradation, and few nomads establish fenced and artificial pastures. Therefore, nomads have employed other strategies, such as renting pasture, providing supplementary feed, and diversifying their livelihoods. Local strategies taken by nomads can provide valuable insights into ecological restoration and livelihood improvement in the region and suggest changes to means promoted by local government. It is necessary to seek new means that combine the best aspects of nomadic pastoralism with modern stockbreeding technologies to help nomads adapt to pasture degeneration and improve their livelihoods. 相似文献
96.
应用激光烧蚀多接收器电感耦合等离子体质谱仪(LA-ICP-MS)方法,对青藏高原北部卡巴纽尔多湖西牙扎康塞一带巴颜喀拉山群碎屑岩系英安质沉凝灰岩夹层中的锆石进行了U-Pb同位素测定。研究结果表明,除1个继承锆石206Pb/238U年龄为419±3Ma外,其余20个岩浆锆石的206Pb/238U年龄变化于242~254Ma之间,其中7个测点的206Pb/238U年龄加权平均值为244±2Ma(MSWD=1.3),代表了火山岩的喷发年龄,说明该地区巴颜喀拉山群形成于中三叠世早期,而非传统认为的晚三叠世。该套地层时代的确定,为进一步研究巴颜喀拉山群的物质组成与形成时代提供了新的证据。 相似文献
97.
98.
青藏高原羌塘中部中—上奥陶统达瓦山组的建立及意义 总被引:3,自引:2,他引:1
首次在藏北羌塘中部达瓦山地区识别出一套复理石沉积,厚度大于3000m,未见顶底,岩石组合以变质砂岩、杂砂岩和千枚岩为主,夹双峰式火山岩,火山岩岩性为流纹岩和玄武岩。用LA-ICP-MS同位素测定技术对流纹岩夹层中的锆石进行了U-Pb同位素测定,获得锆石的年龄为454.0±2.0Ma和458.3±1.8Ma,时代为中晚奥陶世达瑞威尔—桑比期,代表了达瓦山组的主体形成时代。在剖面测制和区域对比的基础上建立了中—上奥陶统达瓦山组,代表冈瓦纳大陆北缘中—晚奥陶世裂谷型深水-半深水复理石沉积。中—上奥陶统达瓦山组的发现和建立完善了该区中晚奥陶世的地层系统,进一步确定该区中—晚奥陶世洋盆的存在,对冈瓦纳大陆北缘中—晚奥陶世岩相古地理研究具有重要意义。 相似文献
99.
将前人对黄土高原黄土地层划分的研究成果进行全面总结、分析,结合洛川、宝鸡、西峰3个经典剖面,利用土壤
地层学、沉积学、磁性地层学、古生物学、年代学、地球化学等研究方法对黄土高原的黄土地层进行详细对比以及进一步
细分。将早、中更新世界线调整为B/M极性倒转开始的对应层位——L9顶部。根据岩性和气候旋回周期的差异性等,将离
石黄土三分,S8/L9作为离石黄土内部划分的一条新界线,S1~L5划分为上离石黄土,S5~S8划分为中离石黄土,L9~L15划分为
下离石黄土。在分析过程中发现关键层位(S0,L1,S1,S5,L9,L15,L33) 的年龄具有差异性。 相似文献
100.
We present three 3D numerical models of deep subduction where buoyant material from an oceanic plateau and a plume interact with the overriding plate to assess the influence on subduction dynamics,trench geometry,and mechanisms for plateau accretion and continental growth.Transient instabilities of the convergent margin are produced,resulting in:contorted trench geometry;trench migration parallel with the plate margin;folding of the subducting slab and orocline development at the convergent margin;and transfer of the plateau to the overriding plate.The presence of plume material beneath the oceanic plateau causes flat subduction above the plume,resulting in a "bowed" shaped subducting slab.In plateau-only models,plateau accretion at the edge of the overriding plate results in trench migration around the edge of the plateau before subduction is re-established directly behind the trailing edge of the plateau.The plateau shortens and some plateau material subducts.The presence of buoyant plume material beneath the oceanic plateau has a profound influence on the behaviour of the convergent margin.In the plateau + plume model,plateau accretion causes rapid trench advance.Plate convergence is accommodated by shearing at the base of the plateau and shortening in the overriding plate.The trench migrates around the edge of the plateau and subduction is re-established well behind the trailing edge of the plateau,effectively embedding the plateau into the overriding plate.A slab window forms beneath the accreted plateau and plume material is transferred from the subducting plate to the overriding plate through the window.In all of the models,the subduction zone maintains a relatively stable configuration away from the buoyancy anomalies within the downgoing plate.The models provide a dynamic context for plateau and plume accretion in Phanerozoic accretionary orogenic systems such as the East China Orogen and the Central Asian Orogen(Altiads),which are characterised by accreted ophiolite complexes with diverse geochemical affinities,and a protracted evolution of accretion of exotic terranes including oceanic plateau and terranes with plume origins. 相似文献