1999年夏季中国首次北极考察区水团特征   总被引：11，自引：3，他引：11       下载免费PDF全文

高郭平  董兆乾  侍茂崇 《极地研究》2003,15(1):11-20

依据 1 999年 7月至 9月中国首次北极考察队在白令海、楚科奇海和南加拿大海盆的现场调查资料 ,本文分析了三个海区的水团特征 :( 1 )白令海水团主要由季节变化显著的白令海上层水团和中层水团以及深层水团组成 ;( 2 )楚科奇海水文特征受融结冰过程影响较大 ,1 999年7月和 8月差异较大 ,其水团主要为浅海变性水团 ,包括两个次级水团 ,楚科奇海夏季水和来自北太平洋以及北冰洋变性的外海入侵水 ;( 3)南加拿大海盆的水团主要由受融结冰过程影响的表层水团、源于太平洋水的次表层水、源自北大西洋的中层水团和深层水团组成  相似文献   

The Vulnerability of Canada to Volcanic Hazards     

Mark V. Stasiuk  Catherine J. Hickson  Taimi Mulder 《Natural Hazards》2003,28(2-3):563-589

Western Canada lies in a zone of active tectonics and volcanism, but thedispersed population has witnessed few eruptions due to the remoteness of the volcanoes and their low level ofactivity. This has created a false perception that Canada's volcanoes are extinct.There are more than 200 potentially-active volcanoes in Canada, 49of which have erupted in the past 10,000 years. They occur in five belts, with origins related totectonic environment. The minimum annual probability of a Canadian volcanic eruption is approximately 1/200;for an effusive (lava) eruption the probability is about 1/220, and for a significant explosive eruptionit is about 1/3333. In-progress studies show that there have been earthquakes associated with at least 9 ofthe youngest Canadian volcanoes since 1975. A scenario of an eruption of Mt. Cayley (50.1°N,123.3°W) shows how western Canada is vulnerable to an eruption. The scenario is basedon past activity in the Garibaldi volcanic belt and involves both explosive and effusive activity.The scenario impact is largely a result of the concentration of vulnerable infrastructure in valleys.Canadian volcanoes are monitored only by a regional seismic network,that is capable of detecting a M > 2 event in all potentially-active areas.This level of monitoring is probably sufficient to alert scientistsat or near eruption onset, but probably insufficient to allow a timelyforecast of activity. Similarly the level of geological knowledge about the volcanoes is insufficient to createhazard maps. This will improve slightly in 2002 when additional monitoring is implemented in theGaribaldi volcanic belt. The eruption probabilities, possible impacts, monitoring limitations and knowledgegaps suggest that there is a need to increment the volcanic risk mitigation efforts.  相似文献   

Geology and mineral potential of Ethiopia: a note on geology and mineral map of Ethiopia   总被引：1，自引：0，他引：1  

Solomon Tadesse  Jean-Pierre Milesi  Yves Deschamps   《Journal of African Earth Sciences》2003,36(4):273-313

This work presents a geoscientific map and database for geology, mineral and energy resources of Ethiopia in a digital form at a scale of 1:2,000,000, compiled from several sources. The final result of the work has been recorded on CD-ROM in GIS format so that the map and the database could be available to users on a personal computer.Metallic resources (precious, rare, base and ferrous–ferroalloy metals) are widely related to the metamorphic meta-volcano-sedimentary belts and associated intrusives belonging to various terranes of the Arabian–Nubian Shield, accreted during the East and West Gondwana collision (Neoproterozoic, 900–500 Ma).Industrial minerals and rock resources occur in more diversified geological environments, including the Proterozoic basement rocks, the Late Paleozoic to Mesozoic sediments and recent (Cenozoic) volcanics and associated sediments.Energy resources (oil, coal, geothermal resources) are restricted to Phanerozoic basin sediments and Cenozoic volcanism and rifting areas.  相似文献   

The “Taourirt” magmatic province, a marker of the closing stage of the Pan-African orogeny in the Tuareg Shield: review of available data and Sr–Nd isotope evidence     

Abla Azzouni-Sekkal  Jean-Paul Ligeois  Faten Bechiri-Benmerzoug  Safia Belaidi-Zinet  Bernard Bonin 《Journal of African Earth Sciences》2003,37(3-4):331

The Tuareg Shield, located between the Archaean to Palaeoproterozoic Saharan metacraton and the West African craton, is composed of 23 recognized terranes that welded together during the Neoproterozoic Pan-African orogeny (750–520 Ma). Final convergence occurred mainly during the 620–580 Ma period with the emplacement of high-K calc-alkaline batholiths, but continued until 520 Ma with the emplacement of alkali-calcic and alkaline high-level complexes. The last plutons emplaced in central Hoggar at 539–523 Ma are known as the “Taourirt” province. This expression is redefined and three geographical groups are identified: the Silet-, Laouni- and Tamanrasset-Taourirts. The Silet-Taourirts are cross-cutting Pan-African island arc assemblages while the two others intrude the Archaean–Palaeoproterozoic LATEA metacraton. The Taourirts are high-level subcircular often nested alkali-calcic, sometimes alkaline, complexes. They are aligned along mega-shear zones often delimiting terranes. Mainly granitic, they comprise highly differentiated varieties such as alaskite (Silet-Taourirts) and topaz–albite leucogranite (Tamanrasset-Taourirts). Different subgroups were defined on the basis of REE patterns and major and other trace elements. The Taourirt province displays a wide transition from dominant alkali-calcic to minor alkaline granite varieties. Sr isotopes indicate that these complexes were affected by fluid circulation during the Ordovician along shear zones probably contemporaneous to the beginning of the Tassilis sandstone deposition. Nd isotope systematic indicates a major interaction with the upper crust during the emplacement of highly differentiated melts, particularly in samples showing seagull wing-shaped REE patterns. On the other hand, all Taourirt plutons are strongly contaminated by the lower crust: _Nd vary from −2 to −8 and T_DM from 1200 to 1700 Ma. This implies the presence of an old crust at depth, also below the Silet-Taourirts, which are emplaced within Pan-African island arc assemblages. A model is proposed for the genesis of the Taourirt province where reworking of the mega-shear zones, which dissected the LATEA metacraton, provoked a linear delamination of the lithospheric mantle, asthenosphere uprise and partial melting of the lower crust (or strong interaction with), giving rise to a mixed source.  相似文献   

Water table control on rill initiation and implications for erosional response     

R. B. Bryan  D. L. Rockwell 《Geomorphology》1998,23(2-4)

On sloping sandy agricultural soil sites near Toronto, Canada, summer storms adequate to generate runoff occur frequently, but rill development occurs mainly in spring when snowmelt or rainfall-induced runoff occurs above frozen subsoil. This suggests that on low and moderate slopes on these soils rill initiation is controlled primarily by hydraulic impedance close to the surface, rather than critical hydraulic conditions in runoff. Laboratory flume experiments were carried out on 10 m slopes at 1.5°, 5° and 9° with loamy sand/clay composite soil sample to test this hypothesis. Runoff with hydraulic conditions adequate for rill initiation occurred rapidly in most tests, but on 1.5° and 5° slopes little knickpoint scour or sediment transport occurred before water table development. This coincided with reduced surface soil strength, knickpoint scour and marked increase in sediment discharge, particularly on 5° slopes where increase was 20- to 30-fold. Further increase in sediment discharge occurred when water tables reached the surface. On 9° slopes runoff occurred more quickly, with higher hydraulic values. Significant rill incision and sediment discharge occurred well before water table development, and ultimately reached much higher values than on lower slopes. Results show that soil erodibility can change dramatically over short time periods during storms due to soil moisture conditions, and that the presence of a hydraulic impedance close to the surface which causes a perched water table to develop can strongly influence rill incision and sediment transport. The influence is unlikely to be marked on soils which are very erodible regardless of moisture conditions, or on extremely resistant soils. It will also be limited on very gentle or steep sites, but can be a significant factor in rill development on intermediate slopes.  相似文献   

Peatland Initiation During the Holocene in Continental Western Canada   总被引：4，自引：0，他引：4  

Linda A. Halsey  Dale H. Vitt  Ilka E. Bauer 《Climatic change》1998,40(2):315-342

Today, the southern limit of peatlands in continental western Canada is largely limited by thermal seasonal aridity, although physiographic parameters of substrate texture, topography, and salinity also exsert important controls on the presence and absence of peatlands. Factors that control peatland distribution today also operated in the past, thus the initiation of peatlands during the Holocene was mainly limited by aridity and physiography. Calibrated radiocarbon dates of basal peat deposits from 90 locations across continental western Canada indicate that peat formation began approximately 8,000 to 9,000 years BP in nucleation zones along the upper elevations of the Montane region of Alberta and in northern Alberta uplands after an initial deglacial lag. Predictions of maximum early Holocene summer insolation by climate simulations provide a mechanism for limiting peatland establishment during the early Holocene. From 6,000 to 8,000 years ago, peat formation in continental western Canada expanded eastwards into Manitoba responding to decreases in summer insolation. Peatland expansion during the early Holocene was more extensive in Alberta than in Manitoba in response to a southwesterly shift in the Arctic front. The displacement of the Arctic front allowed for more frequent incursions of moist Pacific air into Alberta while limiting it in Manitoba. After 6,000 years BP, the trend of southeasterly peatland expansion continued. Peatlands are youngest in the southern Boreal Forest and Aspen Parkland Region as well as in the lower elevations of the Peace-Wapiti River drainage basin, forming over the last 3,000 to 4,000 years. Peatlands are also young in the lower elevations of the Hudson Bay Lowlands where peat initiation has been limited by timing of emergence from glacial rebound. The spatial and temporal distribution of peatland initiation during the Holocene is verified by existing pollen records and corroborates some simulated climate models.  相似文献   

FRIENDS OR FOE? INVASIVE SPECIES AND PUBLIC GREEN SPACE IN TORONTO     

JENNIFER FOSTER  L. ANDERS SANDBERG 《Geographical review》2004,94(2):178-198

ABSTRACT. Environmentalists and ecologists typically regard invasive species as ecologically detrimental and undesirable. Although the ecological impacts of invasive species are well documented, less attention has been devoted to the sociocultural contexts guiding responses to species invasion. In this study the roles of invasive species are reconsidered through three prominent green spaces in Toronto, Canada: the Don Valley Brick Works, High Park, and the Leslie Street Spit. The case studies challenge popular negative assumptions about invasive species and suggest that they can serve important functions both for local ecosystems and for human communities. The case studies also provide lessons on the tension between and within different environmental imaginaries informing invasive species management. Invasives are often compatible with recreational interests, whereas naturalization efforts are ecologically sensitive and costly. Invasives can help restore human‐made wastelands, and naturalization efforts often benefit wealthy rather than poor neighborhoods.  相似文献   

High-resolution body wave tomography beneath the SVEKALAPKO array – II. Anomalous upper mantle structure beneath the central Baltic Shield     

Senén Sandoval  Edi Kissling  Joerg Ansorge  the SVEKALAPKO Seismic Tomography Working Group 《Geophysical Journal International》2004,157(1):200-214

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Transnational ruptures and sutures: questions of identity and social relations among Guatemalans in Canada     

Catherine L. Nolin 《GeoJournal》2002,56(1):59-67

This article examines the ruptures and sutures of Guatemalan refugee transnationalism in the context of settlement in southern Ontario, Canada in the 1980s and 1990s. Political violence became deeply imbedded in `community' relations and subsequently ruptured the social fabric of Guatemala. Through the optic of transnationalism, this research examines the ways in which Guatemalans in Canada work to transform, rely on, and create new primary social relations that stretch across borders when face-to-face `community' connections are no longer possible. Transnational ruptures rather than transnational flows are evident due to varying legal status, the continuing instability, impunity, and insecurity associated with living conditions in Guatemala. Ruptures are reinforced by low income levels (in both countries) leading to lack of communication, and the physical distance between the two countries that inhibits regular travel. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Social policy at work? Equality and equity in women's paid employment in Canada     

Kim England  Gunter Gad 《GeoJournal》2002,56(4):281-294

Under mounting pressure from women, Canada introduced employment policies to address the gender wage gap and women's access to a wider range of jobs. The policies were generally introduced between the late 1970s and early 1990s. Over time these policies have shifted from focusing on equality to emphasising equity. Two pivotal policies are pay equity and employment equity. In the end, the impact of these policies is difficult to assess. Keeping the European situation in mind we argue that they be considered as a component of broader changes rather than as specific causes of improving the situation of women in the Canadian labour market. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献