The Weichselian Late-glacial in southwestern Europe (Iberian Peninsula,Pyrenees, Massif Central,northern Apennines)     

J. -L. de Beaulieu  V. Andrieu  P. Ponel  M. Reille  J. J. Lowe 《第四纪科学杂志》1994,9(2):101-107

This paper presents a summary of Late-glacial environmental changes in southwestern Europe (lberian Peninsula, Pyrenees, Massif Central and the northern Apennines). The emphasis is on palaeoclimatic interpretations inferred from key sites in the region from which the most detailed records are available and which have been radiocarbon dated. The earliest evidence for climatic improvement following the end of the last glacial stage is dated to ca. 15 ka BP and is found at a few sites only. By 13 ka BP, a more widespread and marked climatic improvement is evident, although it is difficult to be precise in the timing and magnitude of the event. There are significant variations in detail between the Late-glacial records, but evidence for a significant cooling correlated with the Younger Dryas event is widespread throughout the region. Just two sites in the region provide evidence for an earlier, less emphatic phase of climatic cooling, which is tentatively equated with the ‘Older Dryas’ of continental northern Europe. Dry conditions appear to have predominated throughout the region in the later part of the Younger Dryas and the early Holocene.  相似文献   

3-D Non-linear Travel-time Tomography: Imaging High Contrast Velocity Anomalies   总被引：1，自引：0，他引：1  

Chao-ying?BaiEmail author  Stewart?Greenhalgh 《Pure and Applied Geophysics》2005,162(11):2029-2049

In this paper we present an approach for 3-D travel-time tomography, which works well in reconstructing high contrast velocity anomalies in both location and strength. It uses a revised ‘irregular’ approach to the shortest-path method as the ray tracer and a damped minimum norm, and constrained least-squares CG approach as the inversion solver. In ray tracing, the advantages of the revised ‘irregular’ over the ‘regular’ approach are that the secondary nodes introduced on the cell surfaces significantly improve accuracy of computed travel times, without dramatically increasing the total number of cells and nodes; the tri-linear velocity function defined across the cell guarantees accurate ray tracing in a high velocity contrast medium; and the capacity to calculate a relatively large 3-D model, due to the fast run speed (at least one order of magnitude over the ‘regular’ approach) and less number of total nodes. The introduction of ‘soft’ and ‘hard’ bounds into the inversion process changes the conditioning and makes the solution meaningful in a physical sense. Thus the artifacts caused by noise and high velocity contrasts are substantially suppressed and the image quality is considerably improved, making the solution realistic with noisy or inconsistent travel-time data. Several numerical tests indicate that we can obtain good quality images even for high velocity contrast anomalies (say more than 20%) in the target region. This means the inversion algorithm is an efficient and effective procedure. Meanwhile, the inversion procedure is not very sensitive to the quality of the travel-time data, which is promising for practical usage.  相似文献   

‘Natural’ rivers, ‘hydromorphological quality’ and river restoration: a challenging new agenda for applied fluvial geomorphology     

Malcolm. D. Newson  Andrew R. G. Large 《地球表面变化过程与地形》2006,31(13):1606-1624

Fluvial geomorphology is rapidly becoming centrally involved in practical applications to support the agenda of sustainable river basin management. In the UK its principal contributions to date have primarily been in flood risk management and river restoration. There is a new impetus: the European Union's Water Framework and Habitats Directives require all rivers to be considered in terms of their ecological quality, defined partly in terms of ‘hydromorphology’. This paper focuses on the problematic definition of ‘natural’ hydromorphological quality for rivers, the assessment of departures from it, and the ecologically driven strategies for restoration that must be delivered by regulators under the EU Water Framework Directive (WFD). The Habitats Directive contains similar concepts under different labels. Currently available definitions of ‘natural’ or ‘reference’ conditions derive largely from a concept of ‘damage’, principally to channel morphology. Such definitions may, however, be too static to form sustainable strategies for management and regulation, but attract public support. Interdisciplinary knowledge remains scant; yet such knowledge is needed at a range of scales from catchment to microhabitat. The most important contribution of the interdisciplinary R&D effort needed to supply management tools to regulators of the WFD and Habitats regulations is to interpret the physical habitat contribution to biodiversity conservation, in terms of ‘good ecological quality’ in rivers, and the ‘hydromorphological’ component of this quality. Contributions from ‘indigenous knowledge’, through public participation, are important but often understated in this effort to drive the ‘fluvial hydrosystem’ back to spontaneous, affordable, sustainable self‐regulation. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

The role of groundwater in cliff instability: an example at Cape Blanc‐Nez (Pas‐de‐Calais,France)     

Guillaume Pierre  Philippe Lahousse 《地球表面变化过程与地形》2006,31(1):31-45

Cliff retreat in northern Boulonnais is described. The distribution of the amount of retreat is related to structural and topographic factors. Both control the runoff–infiltration balance and therefore the mechanical behaviour of the rocks, which determines the modes of failure. The spatial variability of the retreat rate is explained, but predicting the temporal variability of the retreat rate, which is central to risk management, is much more difficult. Rainfall and piezometric surface data enhanced a ‘piston flow’ mechanism during November 2000. The result of its occurrence on the stability of the cliff and conditions of its recurrence are examined with a view to better understanding the rate of recession of coastal cliffs and wiser management of risk. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Âges KAr des roches magmatiques du fossé de Garoua (Cameroun) : leur place dans le cadre de la « Ligne du Cameroun »     

Raymond Montigny  Ismaïla Ngounouno  Bernard Druelle 《Comptes Rendus Geoscience》2004,336(16):285

KAr analyses on whole rock and minerals are reported for the Kokoumi anorogenic pluton and alkaline volcanics of the Benue valley, west of Garoua (northern Cameroon), which belong to the northern part of the ‘Cameroon Line’. The two formations yield similar ages of 39 and 37 Ma, respectively. These values likely correspond to the time of emplacement. They are in agreement with the interpretation considering the ‘Cameroon Line’ as a huge lithospheric crack tapping a hot deep asthenospheric zone. To cite this article: R. Montigny et al., C. R. Geoscience 336 (2004).  相似文献   

Solution-collapse depressions and suspensates in the limnocrenic lake of banyoles (ne Spain)     

M. Canals  H. Got  R. Julia  J. Serra 《地球表面变化过程与地形》1990,15(3):243-254

A high resolution seismic reflection survey in the Banyoles limnocrenic solution lake allowed penetration of dense suspensates occupying cone-like bottom depressions of different size. The depressions result from the dissolution and collapse of underlying Eocene calcareous and gypsiferous materials over which lacustrine sediments of varying thicknesses have accumulated. The suspensates occupying the depressions present three main types of seismic signatures: stratified, semistratified, and transparent. The densities of the suspensates and the water depths of their tops, which fluctuate continuously, vary from one depression to another. The maximum seismically recorded suspensate thickness is 44 m. Morphological and structural features, seismic characters, and variable degrees of hydraulic activity, point to the existence of different stages of maturity in the lake bottom depressions. This work brings new insight on the dynamics and evolution of limnocrenic solution lakes.  相似文献   

Fluvial chronology and valley floor evolution of the upper bowmont valley,borders region,Scotland     

Richard Tipping 《地球表面变化过程与地形》1994,19(7):641-657

Two major fluvial terrace surfaces, and four less significant surfaces, are identified by aerial photographic interpretation, field mapping and levelling on one c. 2 km long reach of the upper Bowmont Water, an upland stream draining the northern Cheviot Hills, Scotland. The oldest terrace surface remains undated. Cartographic, radiocarbon and palynological dating suggest that later terraces formed very recently, within the last 250 years, with the most prominent terrace fill having aggraded in the 18th century. Incision below this terrace surface is dated to approximately the end of the 18th century, correlated with channel trenching at other sites in the region. Detailed palaeoecological and documentary evidence is used to examine whether climatic or land-use changes might have instigated aggradation, and a link with increased precipitation and flooding during part of the ‘Little Ice Age’ is suggested.  相似文献   

Spatial and temporal variation of diverse inter‐tidal algal assemblages in Southwest O‘ahu          下载免费PDF全文

Traci Erin Cox  Heather L. Spalding  Michael S. Foster 《Marine Ecology》2017,38(3)

Spatial and temporal variation in tropical inter‐tidal communities is poorly known, making predictions about the effects of climate change and other anthropogenic disturbances difficult. Along Southwest O‘ahu, Hawai'i, local residents are concerned about the environmental effects of coastal development and the perceived loss of targeted algal species, which are collected for human consumption. To describe the coastal benthic community and better understand the processes that form and maintain it, the abundance and composition of macroalgae were sampled in the region's inter‐tidal zone from 2006 to 2015. Sixty‐six macroalgal species and two broad algal assemblages were identified that corresponded to substrate topography and sand influence at a similar tidal elevation. Along flat carbonate benches with a sand beach, Phaeophyceae and Rhodophytes occurred in almost equal proportions, while shores with slightly more topographic relief and angular substrate were dominated by Rhodophytes. Foliose or turf algal forms were most common. Surveys captured the local invasion of an alga, Avrainvillea sp. and significant declines in abundant macroalgae in 2015 after a period of unseasonably warm, calm water. Temporal changes in algal assemblages were related to maximum water temperature and wave height but not precipitation. Thus, algal assemblages appear to be structured by local beach morphology as they interact with sand and wave activity and episodically by unusual weather events. However, manipulation and continuous monitoring of the algal assemblages coupled to sensing of the localized environment is necessary to confirm factors related to assembly maintenance and recent species shifts.  相似文献   

2018年阿拉斯加湾M_W7.9地震震源复杂性          下载免费PDF全文

张旭  许力生  杜海林  易磊 《地球物理学报》2019,62(5):1680-1692

2018年1月23日,在美国阿拉斯加湾海域发生了一次M_W7.9地震.震源机制解表明这次地震以走滑为主,可能发生在近东西向或南北向的陡倾角断层上,早期余震并非线型展布.我们利用视震源时间函数分析确定了此次地震的总体破裂方向,并结合余震的空间展布特征构建了相互交叉的双断层模型,进而通过联合反演远场P波和SH波数据获得了此次地震的时空破裂过程.视震源时间函数分析表明总体破裂方向既非东西也非南北,而且反演结果表明,两个断层上都发生了错动,总体破裂时间~50 s,释放标量地震矩~8.11×10²⁰ Nm.震源时间函数表现出多事件特征,且两个断层破裂的时间过程也不相同.破裂首先在南北向断层的南端开始,很快触发了东西向断层,最后终止于南北向断层的北端.每个断层都具有相当的时空复杂性,位错分布很不均匀.东西向断层具有三个凹凸体,一个位于震源附近,其他两个位于断层两端.南北向断层有两个凹凸体,均位于断层北段,最大滑动量~5.0 m就出现在这里.发生最大位错的南北向断层延伸至阿拉斯加海沟,增加了触发阿拉斯加海沟其他断层发生破裂的可能性.  相似文献   

典型厄尔尼诺期间台风降水δ18O变化分析: 以2018年22号台风“山竹”为例     

杨云月  许涛  罗翠榆  刘娟  姜修洋 《热带海洋学报》2020,39(4):34-41

台风因其特殊的物理结构, 带来的降雨有别于一般的暴雨事件, 其降水稳定同位素组成与一般的大气降水事件也有较大的差异。本文根据2018年第22号台风“山竹”登陆前后广州、东莞两地气象资料和每小时间隔的降水样品收集, 分析了此次台风在两地的降水稳定同位素变化特征及其影响因素。台风“山竹”影响期间, 广州降水δ¹⁸O值变化范围为-5.7‰~-19.2‰, 变化幅度达13.5‰, 其平均值为-15.5‰; 东莞降水δ¹⁸O值变化范围为-7.3‰~-20.0‰, 变化幅度达12.7‰, 平均值为-14.8‰; 两地降水δ¹⁸O值均呈现为3个阶段倒U型的变化特征。受到蒸发作用的影响, 两地台风前端和尾端的降水氧同位素值相对偏正, 其中广州为-5.7‰~-9.3‰, 东莞为-7.3‰~-8.1‰。两地台风中端的降水稳定同位素值极端偏负, δ¹⁸O值变化范围分别为-16.0‰~-19.2‰(广州)和-13.0‰~-20.0‰(东莞), 是该地区迄今为止已报道的最为偏负的降水δ¹⁸O值。分析认为, 在厄尔尼诺状态下生成的远源台风“山竹”强度增强, 其内部具有更强的对流和水汽循环过程, 导致降水δ¹⁸O值极端偏负。  相似文献