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141.
MICHAEL WILLIAMS 《Geographical review》1998,88(2):275-300
ABSTRACT. The first half of the twentieth century marked both an end to confidence in human control of the earth and a prelude to the contemporary era of intense environmental concern after 1950. This conspectus of the environmental history/historical geography of the transitional years between 1900 and 1950 focuses on rising concerns over resource supplies and exploitation, particularly land settlement, timber supplies, soil erosion, and river-basin control. Conservation and the pervasiveness of ecological thinking and analogy form a strong underlying theme. 相似文献
142.
As in other regions colonised from Europe within the last few centuries, Australia's vegetation and soils have been dramatically changed by clearing, cropping and grazing. In southeastern Australia, particularly on the Southern Tablelands, the impacts of European settlement are clearly manifested by channel incision. By using stratigraphic and documentary evidence, in conjunction with aerial photographs, sediment budgets for the post-settlement period have been constructed to define the fluxes and stores of sediment for each of the major geomorphic components of the 136 km2 catchment of Jerrabomberra Creek near Canberra. Using these budgets, and some plausible assumptions, it has been possible to approximate the history of both the sediment delivery ratio and sediment yield for this catchment. While the quantities estimated in this analysis are approximations, the trends through time are credible. Sediment yield increased rapidly to a peak after European settlement, and has returned to a level between the peak and the pre-European value. The delivery ratio has followed a similar trend. The most general conclusion to emerge is that in this landscape both the total sediment flux and the sediment yield of the catchment have been dominated by channel erosion. This result is contrary to the findings in many parts of the world where sheet and rill erosion dominates the fluxes. The soil conservation implication of these results is clear: to control off-site effects of erosion, the focus must be on the channels. 相似文献
143.
Results of lithostratigraphic and mineral magnetic analysis of two surficial sediment cores (21 cm and 45 cm in length) collected from the Southern basin of Lake Baikal at a water depth of 1390 m, are presented. The sediments have been measured for a wide range of mineral magnetic parameters in order to assess their value in the identification of turbidite layers. Particle size and geochemical data are also presented and these explain some of the down core variations in magnetic mineralogy. It is suggested that changes in the particle size frequency distributions down core may be related to fossil diatom shells. One of the cores has been dated using 210Pb. The sediment cores were cross-correlated using low frequency magnetic susceptibility (f) and these cores can also be correlated with a nearby core collected earlier in 1992. Changes in the magnetic parameters of lf, IRMs and HIRM210 suggest that there are significant changes in the concentration of ferrimagnetic minerals in the sediment cores, indicating changing sediment sources and/or increasing concentrations of spheroidal carbonaceous particles and the dissolution of minerals through reduction below the oxidised layer within the sediment core. 相似文献
144.
同时满足多个阻尼反应谱的人造地震动时程合成 总被引:1,自引:0,他引:1
应用M.Hirasawa和M.Watabe(1992)提出的方法,进行了相应的数学推导:即以满足某个阻尼比的反应谱的人造地震动作为初始时程,采用奇异值分解法,通过解线性方程组求出校正时程,并多次叠以逐步辐近 相似文献
145.
The Day Nui Con Voi belt in Vietnam is the southeasternmost part of the Red River shear zone in Asia. It is a narrow high-grade metamorphic core complex consisting of garnet–sillimanite–biotite gneisses, mylonite bands, amphibolite layers and migmatites. Geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite-facies conditions of 690−60+30°C and 0.65±0.15 GPa and the subsequent mylonitization occurred under greenschist-facies conditions of 480°C and under 0.3 GPa. Fifteen synkinematic hornblende and biotite separates from gneisses, amphibolites and mylonites were dated with the K/Ar method. Hornblende separates from the Day Nui Con Voi give K–Ar ages of 26.4–28.5 Ma, and the biotite separates do give 24.5–24.7 Ma. Combination of thermobarometric and geochronological data yields the cooling history of 500°C at 28 Ma and 300°C at 24 Ma with a cooling rate of 70–110°C Ma−1, and 23 km post-metamorphic exhumation of the core complex. The first 16 km exhumation from the peak of metamorphism (at probably 31 Ma) to 28 Ma was triggered by the left-lateral strike-slip displacement of the Red River shear zone. 相似文献
146.
147.
The pattern and style of mantle convection govern the thermal evolution, internal dynamics, and large-scale surface deformation of the terrestrial planets. In order to characterize the nature of heat transport and convective behaviour at Rayleigh numbers, Ra, appropriate for planetary mantles (between 104 and 108), we perform a set of laboratory experiments. Convection is driven by a temperature gradient imposed between two rigid surfaces, and there is no internal heating. As the Rayleigh number is increased, two transitions in convective behaviour occur. First we observe a change from steady to time-dependent convection at Ra≈105. A second transition occurs at higher Rayleigh numbers, Ra≈5×106, with large-scale time-dependent flow being replaced by isolated rising and sinking plumes. Corresponding to the latter transition, the exponent β in the power law relating the Nusselt number Nu to the Rayleigh number (NuRaβ) is reduced. Both rising and sinking plumes always consist of plume heads followed by tails. There is no characteristic frequency for the formation of plumes. 相似文献
148.
Ralph Fyfe 《第四纪科学杂志》2007,22(6):571-578
New and existing pollen data from Wales, UK, are used to assess the level of landscape openness at the regional and local scale. At the regional scale, the existing pollen data support the high‐forest model of vegetation structure by 6000 cal. yr. BC prior to any palynological signal for anthropogenic impact in the region. New data from two sites in southwest Wales follow the general regional pattern of early to middle Holocene vegetation succession, but are striking owing to maintenance of high non‐arboreal pollen percentages (NAPs) throughout the mid‐Holocene. It is argued that these NAPs indicate that a significant degree of openness can be found at the local level (and beyond the confines of the peat‐forming site) within regions characterised by closed woodland. It is possible that woodland development in these areas may have been suppressed by reduced drainage, although the role of grazing animals in maintenance of clearings cannot be assessed. Implications for this local degree of openness are discussed, in particular the significance of vegetation heterogeneity at the local scale for the construction of place or locale for prehistoric communities. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
149.
研究了军曹鱼(Rachycentron canadum)早期阶段的生长和摄食特性。结果表明:1~29日龄间,全长与日龄之间呈指数函数相关:y=3.6922e0.0995x(R=0.9913);体重与日龄间呈指数函数相关:y=0.0002e0.276x(R=0.9830);体重与全长之间为幂函数相关:y=5×10-6x2.786(R=0.9957);口径与日龄的关系为直线函数关系:y=0.1034x-0.0044(R=0.9939)。鱼苗早期阶段的生长速度存在显著性差异。军曹鱼早期鱼苗在3~6日龄时主要摄食原生动物、单轮幼虫和轮虫;7~12日龄时主要摄食桡足类无节幼体;13日龄以后主要摄食桡足类成体。仔、稚鱼阶段,军曹鱼的摄食活动呈现明显的昼夜节律,其饱满度指数均在12时前后出现最高峰,日摄食率分别为57.36%和14.78%。 相似文献
150.