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271.
本简要介绍我国学1987年以来在国内外冰流动理论和试验研究中取得的进展,包括晶粒长大规律、异向性冰的第三蠕变、压剪联合作用下冰的流动以及全新世冰和冰期冰流动性质的差别等,并加以评论。 相似文献
272.
273.
Above and belowground phenology are critical aspects for plant life in areas of seasonal climate like Mediterranean regions. However, fine root growth phenology is rarely considered in most phenological studies. In this article we describe the above and belowground phenology of four species of Mediterranean sub-shrubs growing along a gradient of temperature and water availability, with special attention at the relationship between both processes. Observations were conducted monthly over a minimum of 12 months per species. Fine root growth varied significantly throughout the year, being higher in autumn than in spring and minimum in summer. In the species growing in cold areas, root growth was also reduced during winter. Shoot growth was maximum in spring for the four study species, buts its beginning was related to the site temperature, being earlier in those species growing in warmer sites. The species displaying a short vegetative period tended to separate root and shoot growth processes throughout the year. These results emphasize the importance of water availability and winter cold on fine root growth. A trade-off appears to exist between the duration of the vegetative period and the overlap between root and shoot growth processes. 相似文献
274.
The Chilean lake district includes diverse lentic ecosystems along ca. 700 km of the country (36°–43°S), including the “Nahuelbutan lakes”, “Araucanian lakes” and “Chiloe lakes”. This area is recognized as an important “hot spot” of benthic freshwater biodiversity in Southern South America. In Chilean temperate lakes, increased nutrient loads of P and N caused eutrophication, particularly in the Nahuelbutan Lakes. The freshwater Hyriidae mussel Diplodon chilensis (Gray, 1828) which is one of the most abundant species in Chilean temperate lakes, is known to be very susceptible to eutrophication. This species presents a clear reduction in its geographic ranges and is considered to be a threatened species in many Chilean lakes. In this study, we used a correlative approach to determine how eutrophication-driven changes in the food supply and in geographical parameters of different Chilean lakes affected the shell growth rates of D. chilensis. The results obtained from sclerochronological analyses of the mussel shells suggest an association with a group of environmental variables, including geographical types (negative), such as latitude and altitude, and limnological types (positive), especially phosphorous and turbidity. However, the D. chilensis populations under extreme conditions of turbidity in eutrophic and hypertrophic lakes are extinct or nearly so. The high positive correlation of the mean D. chilensis growth rates with orthophosphate (R=0.76; P<0.05), in relation to dissolved inorganic nitrogen, suggests that P is the major limiting factor of the primary productivity in Chilean temperate lakes. We discuss some implications of our results in terms of the conservation of biodiversity in temperate lake ecosystems at different taxonomic levels. 相似文献
275.
Molly C. McCanta Malcolm J. Rutherford Julia E. Hammer 《Journal of Volcanology and Geothermal Research》2007,160(3-4):263-284
A series of experiments and petrographic analyses have been run to determine the pre-eruption phase equilibria and ascent dynamics of dacitic lavas composing Black Butte, a dome complex on the flank of Mount Shasta, California. Major and trace element analyses indicate that the Black Butte magma shared a common parent with contemporaneously erupted magmas at the Shasta summit. The Black Butte lava phenocryst phase assemblage (20 v.%) consists of amphibole, plagioclase (core An77.5), and Fe–Ti oxides in a fine-grained (< 0.5 mm) groundmass of plagioclase, pyroxene, Fe–Ti oxides, amphibole, and cristobalite. The phenocryst assemblage and crystal compositions are reproduced experimentally between 890 °C and 910 °C, ≥ 300 MPa, XH2O = 1, and oxygen fugacity = NNO + 1. This study has quantified the extent of three crystallization processes occurring in the Black Butte dacite that can be used to discern ascent processes. Magma ascent rate was quantified using the widths of amphibole breakdown rims in natural samples, using an experimental calibration of rim development in a similar magma at relevant conditions. The majority of rims are 34 ± 10 μm thick, suggesting a time-integrated magma ascent rate of 0.004–0.006 m/s among all four dome lobes. This is comparable to values for effusive samples from the 1980 Mount St. Helens eruption and slightly faster than those estimated at Montserrat. A gap between the compositions of plagioclase phenocryst cores and microlites suggests that while phenocryst growth was continuous throughout ascent, microlite formation did not occur until significantly into ascent. The duration of crystallization is estimated using the magma reservoir depth and ascent rate, as determined from phase equilibria and amphibole rim widths, respectively. Textural analysis of the natural plagioclase crystals yields maximum growth rates of plagioclase phenocryst rims and groundmass microlites of 8.7 × 10− 8 and 2.5 × 10− 8 mm/s, respectively. These rates are comparable to values determined from time-sequenced samples of dacite erupted effusively from Mount St. Helens during 1980 and indicate that syneruptive crystallization processes were important during the Black Butte eruptive cycle. 相似文献
276.
Modeling doline populations with logistic growth functions 总被引:1,自引:0,他引:1
Phillip R. Kemmerly 《地球表面变化过程与地形》2007,32(4):587-601
Initiation and evolution of a large karst depression population consisting of parent and daughter dolines were spatially and temporally modeled using logistic growth functions. Logistic growth models are well suited for analyzing doline population initiation and evolution because they reflect the density‐dependent growth mechanisms present in the evolution of karst depressions. Seven assumptions based upon previous studies were refined into mathematical statements and tested using more than 2000 dolines from a subpopulation of both parent and daughter karst depressions on the Western Highland Rim and Pennyroyal Plain of Tennessee and Kentucky. Logistic growth models quantify the initiation and evolution of doline populations and interface well with recent models describing the evolution of three‐dimensional conduit systems. Logistic growth models should apply to modeling other doline populations in karst terranes with hydraulically efficient, three‐dimensional conduit systems. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
277.
A global two-dimensional chemistry model is developed to study long-term trends of CH4 since industrial revolution.The sources of CH4,CO and NOx are parameterized as functions of latitude and time.With two long-term emission scenarios,long-term trends of CH4 are simulated.The results have a good agreement with observation from ice cores.The modeled CH4 increased from 760 ppbv in 1840 to 1611.9 ppbv in 1991, while the modeled number concentration of tropospheric OH decreased from 7.17×105 cm-3 in 1840 to 5.79×105 cm-3 in 1991.The increase of atmospheric CH4 can be explained by the increase of emission of CH4 and build-up because of decrease of OH radicals that remove CH4 from the atmosphere.The model is also used to simulate the distribution of CH4.Comparisons between the model results and observations show that the model can simulate both latitudinal distribution and seasonal variation of CH4 well. 相似文献
278.
279.
广州市社会空间景观形态演化初探 总被引:5,自引:0,他引:5
林耿 《地理与地理信息科学》2001,17(1):31-36
相对于形体空间而言 ,社会空间是一个更深层面的城市空间领域 ,在城市形体空间日新月异的嬗变中 ,社会空间内涵的演化显得更为缓慢和深刻。以广州市为例 ,探讨这种景观形态演化的特性及其对现代城市形象建设和个性塑造的意义。 相似文献
280.