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Cesar Casquet Robert J. Pankhurst C. Mark Fanning Edgardo Baldo Carmen Galindo Carlos W. Rapela Jose M. Gonzlez-Casado Juan A. Dahlquist 《Gondwana Research》2006,9(4):524-529
Metamorphism of Grenvillian age (ca. 1.2 Ga; U–Pb zircon dating) is recognized for the first time in the Western Sierras Pampeanas (Sierra de Maz). Conditions reached granulite facies (ca. 780 °C and ca. 780 MPa). Comparing geochronological and petrological characteristics with other outcrops of Mesoproterozoic basement, particularly in the northern and central Arequipa-Antofalla craton, we suggest that these regions were part of a single continental crustal block from Mesoproterozoic times, and thus autochthonous or parautochthonous to Gondwana. 相似文献
854.
High-field cantilever magnetometry as a tool for the determination of the magnetocrystalline anisotropy of single crystals 总被引:1,自引:0,他引:1
Ftima Martín-Hernndez Iris M. Bominaar-Silkens Mark J. Dekkers Jan Kees Maan 《Tectonophysics》2006,418(1-2):21
Cantilever torque magnetometry is utilized widely in physics and material science for the determination of magnetic properties of thin films and semiconductors. Here, we report on its first application in rock magnetism, namely the determination of K1 and K2 of single crystal octahedra of natural magnetite. The design of cantilever magnetometers allows optimization for the specific research question at hand. For the present study, a cantilever magnetometer was used that enables measurement of samples with a volume up to 64 mm3. It can be inserted into an electromagnet with a maximum field of 2 T. The cantilever spring is suitable for torque values ranging from 7.5 × 10− 7 N·m to 5 × 10− 6 N·m. The torque is detected capacitively; the measured capacitance is converted into torque by using a calibrated feedback coil. The magnetometer allows in-situ rotation of the sample in both directions and is, therefore, also suitable to analyze rotational hysteresis effects.The evaluation of the magnetite anisotropy constants involved Fourier analysis of the torque signal on the magnetite crystals' (001) and (110) planes. The absolute anisotropy constant has been computed using the extrapolation-to-infinite-field method. The value of K1 at room temperature is determined at − 1.28 × 104 [J m− 3] (± 0.13, i.e. 10%) and that of K2 at − 2.8 × 103 [J m− 3] (± 0.1, i.e. 2%). These values concur with earlier determinations that could not provide an instrumental error, in contrast with this work.The cantilever magnetometer performs four times faster than other torque magnetometers used for rock magnetic studies. This makes the instrument also suitable for magnetic fabric analysis. 相似文献
855.
Anthony J. Parsons Richard E. Brazier John Wainwright D. Mark Powell 《地球表面变化过程与地形》2006,31(11):1384-1393
Eight runoff plots, located within a small catchment within the Walnut Gulch Experimental Watershed, southern Arizona, were constructed to test the argument that sediment yield (kg m?2) decreases as plot length increases. The plots ranged in length from 2 m to 27·78 m. Runoff and sediment loss from these plots were obtained for ten natural storm events. The pattern of sediment yield from these plots conforms to the case in which sediment yield first increases as plot length increases, but then subsequently decreases. Data from the present experiment indicate that maximum sediment yield would occur from a plot 7 m long. Analysis of both runoff and sediment yield from the plots indicates that the relationship of sediment yield to plot length derives both from the limited travel distance of individual entrained particles and from a decline in runoff coefficient as plot length increases. Particle‐size analysis of eroded sediment confirms the role of travel distance in controlling sediment yield. Whether in response to the finite travel distance of entrained particles or the relationship of runoff coefficient to plot length, the experiment clearly demonstrates that the erosion rates for hillslopes and catchments cannot be simply extrapolated from plot measurements, and that alternative methods for estimating large‐area erosion rates are required. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
856.
The signatories to United Nations Framework Convention on Climate Change are charged with stabilizing the concentrations of greenhouse gases in the atmosphere at a level that prevents dangerous interference with the climate system. A number of nations, organizations and scientists have suggested that global mean temperature should not rise over 2 °C above preindustrial levels. However, even a relatively moderate target of 2 °C has serious implications for the Arctic, where temperatures are predicted to increase at least 1.5 to 2 times as fast as global temperatures. High latitude vegetation plays a significant role in the lives of humans and animals, and in the global energy balance and carbon budget. These ecosystems are expected to be among the most strongly impacted by climate change over the next century. To investigate the potential impact of stabilization of global temperature at 2 °C, we performed a study using data from six Global Climate Models (GCMs) forced by four greenhouse gas emissions scenarios, the BIOME4 biogeochemistry-biogeography model, and remote sensing data. GCM data were used to predict the timing and patterns of Arctic climate change under a global mean warming of 2 °C. A unified circumpolar classification recognizing five types of tundra and six forest biomes was used to develop a map of observed Arctic vegetation. BIOME4 was used to simulate the vegetation distributions over the Arctic at the present and for a range of 2 °C global warming scenarios. The GCMs simulations indicate that the earth will have warmed by 2 °C relative to preindustrial temperatures by between 2026 and 2060, by which stage the area-mean annual temperature over the Arctic (60–90°N) will have increased by between 3.2 and 6.6 °C. Forest extent is predicted by BIOME4 to increase in the Arctic on the order of 3 × 106 km2 or 55% with a corresponding 42% reduction in tundra area. Tundra types generally also shift north with the largest reductions in the prostrate dwarf-shrub tundra, where nearly 60% of habitat is lost. Modeled shifts in the potential northern limit of trees reach up to 400 km from the present tree line, which may be limited by dispersion rates. Simulated physiological effects of the CO2 increase (to ca. 475 ppm) at high latitudes were small compared with the effects of the change in climate. The increase in forest area of the Arctic could sequester 600 Pg of additional carbon, though this effect is unlikely to be realized over next century. 相似文献
857.
Brigitte Leblon Eric Kasischke Marty Alexander Mark Doyle Melissa Abbott 《Natural Hazards》2002,27(3):231-255
Research was carried out to assess the potential of imaging radar systems formonitoring forest fire danger. In Canada, daily forest fire danger ratings are generated by the Canadian ForestFire Danger Rating System (CFFDRS), based on estimates of fire weather indices (FWI) and measured foliar moisturecontent (FMC). To evaluate the potential of imaging radar, an experiment was conducted using test sitesconsisting of jack pine, black spruce and white spruce stands located in the MacKenzie river basin, NorthwestTerritories, Canada. Radar image intensity values from ERS-1 SAR imagery collected over these stands in 1994 werecompared to FWI indices and FMC data. FWI indices were calculated using data from local weather stations.Seasonal trends in radar backscatter (0) were shown to correlate with temperature and precipitation. Significant relationships were found between 0 and FWI codes and indices, particularly in thecase of the black spruce stands, with slow-drying fuels, like duff moisture code (DMC), drought code (DC), and build-upindex (BUI). Rates of changes in 0were related to rates of changes in FMC, particularly in the case ofthe jack pine stands for old FMC and in the case of white spruce stands for composite FMC. 相似文献
858.
Mark A. Walker 《Monthly notices of the Royal Astronomical Society》1998,294(2):307-311
The recent discovery of radio variability of a quasar on short time-scales (hours) prompts us to examine what is expected in respect of the interstellar scintillation of very compact, extragalactic radio sources. We find that large-amplitude, rapid, variability is predicted at commonly observed radio frequencies (1–20 GHz) over the vast majority of the extragalactic sky. As a guide to assist observers in understanding their data, we demonstrate simple techniques for predicting the effects of interstellar scintillation on any extragalactic source. 相似文献
859.
Mark Cropper Gavin Ramsay & Kinwah Wu 《Monthly notices of the Royal Astronomical Society》1998,293(3):222-232
One method of obtaining the mass of the white dwarf in magnetic cataclysmic variables (mCVs) is through their hard X-ray spectra. However, previous mass estimates using this method give lower limits because the temperature of the plasma in the post-shock region (where the hard X-rays are emitted) is lower than the temperature of the shock itself. In AM Her systems, the additional cooling of the post-shock plasma by cyclotron emission will further lower the derived mass. Here we present estimates of the masses of the white dwarf in 13 mCVs derived using Ginga data and a model in which X-rays are emitted from a multi-temperature emission region with the appropriate temperature and density profile. We include in the model reflection from the surface of the white dwarf and a partially ionized absorber. We are able to achieve good fits to the data. We compare the derived masses with previous estimates and the masses for larger samples of isolated white dwarfs and those in CVs. 相似文献
860.