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991.
中国第四纪孢粉学研究进展 总被引:29,自引:1,他引:28
随着全球变化研究的不断深入,中国第四纪孢粉学研究已取得的长足的发展。化石样品的时间分辨率不断提高,在黄土高原10万年尺度的孢粉记录分析已达到了小于千年的分辨率,在内蒙古调角海子钻孔的研究已达到百年分辨率。其次,为了准确解译化石孢粉的生态意义,开展了大量表土花粉研究,在中国北方建立了花粉-气候转换函数、气候-花粉响应面模型;系统地开展了表土花粉模拟生物群区的研究,并取得了良好的结果;开展了华北平原不 相似文献
992.
Atmospheric modelling in GPS analysis and its effect on the estimated geodetic parameters 总被引:4,自引:2,他引:2
Permanently operating Global Positioning System (GPS) receivers are used today, for example, in precise positioning and determination
of atmospheric water vapour content. The GPS signals are delayed by various gases when traversing the atmosphere. The delay
due to water vapour, the wet delay, is difficult to model using ground surface data and is thus often estimated from the GPS
data. In order to obtain the most accurate results from the GPS processing, a modelling of the horizontal distribution of
the wet delay may be necessary. Through simulations, three such models are evaluated, one of which is developed in this paper.
In the first model the water vapour is assumed to be horizontally stratified, thus the wet delay can be described by only
one zenith parameter. The second model gives the wet delay with one zenith and two horizontal gradient parameters. The third
model uses the correlation between the wet-delay values in different directions. It is found that for large gradients and
strong turbulence the two latter models yield lower errors in the estimated vertical coordinate and wet-delay parameters.
For large gradients this improvement is up to 7 mm in the zenith wet-delay parameter, from 9 mm down to 2 and 4 mm for the
second and third models, respectively.
Received: 7 May 1998 / Accepted: 1 March 1999 相似文献
993.
Špičák A. Horálek J. Boušková A. Tomek Č. Vaněk J. 《Studia Geophysica et Geodaetica》1999,43(1):87-106
We are proposing a hypothesis that earthquake swarms in the West Bohemia/Vogtland seismoactive region are generated by magmatic activity currently transported to the upper crustal layers. We assume that the injection of magma and/or related fluids and gases causes hydraulic fracturing which is manifested as an earthquake swarm at the surface. Our statements are supported by three spheres of evidence coming from the western part of the Bohemian Massif: characteristic manifestations of recent geodynamic activity, the information from the neighbouring KTB deep drilling project and from the 9HR seismic reflection profile, and the detailed analysis of local seismological data. (1) Recent manifestations of geodynamic activity include Quaternary volcanism, rich CO
2
emissions, anomalies of mantle-derived
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He, mineral springs, moffets, etc. (2) The fluid injection experiment in the neighbouring KTB deep borehole at a depth of 9 km induced hundreds of micro-earthquakes. This indicates that the Earth's crust is near frictional failure in the western part of the Bohemian Massif and an addition of a small amount of energy to the tectonic stress is enough to induce an earthquake. Some pronounced reflections in the closely passing 9HR seismic reflection profile are interpreted as being caused by recent magmatic sills in the crust. (3) The local broadband seismological network WEBNET provides high quality data that enable precise localization of seismic events. The events of the January 1997 earthquake swarm are confined to an extremely narrow volume at depths of about 9 km. Their seismograms display pronounced reflections of P- and S-waves in the upper crust. The analysis of the process of faulting has disclosed a considerable variability of the source mechanism during the swarm.
We conclude that the mechanism of intraplate earthquake swarms generated by magma intrusions is similar to that of induced seismicity. As the recent tectonic processes and manifestations of geodynamic activity are similar in European areas with repeated earthquake swarm occurrence (Bohemian Massif, French Massif Central, Rhine Graben), we assume that magma intrusions and related fluid and gas release at depths of about 10 km are the universal cause of intraplate earthquake swarm generation 相似文献
994.
Postglacial rebound and fault instability in Fennoscandia 总被引:5,自引:0,他引:5
The best available rebound model is used to investigate the role that postglacial rebound plays in triggering seismicity in Fennoscandia. The salient features of the model include tectonic stress due to spreading at the North Atlantic Ridge, overburden pressure, gravitationally self-consistent ocean loading, and the realistic deglaciation history and compressible earth model which best fits the sea-level and ice data in Fennoscandia. The model predicts the spatio-temporal evolution of the state of stress, the magnitude of fault instability, the timing of the onset of this instability, and the mode of failure of lateglacial and postglacial seismicity. The consistency of the predictions with the observations suggests that postglacial rebound is probably the cause of the large postglacial thrust faults observed in Fennoscandia. The model also predicts a uniform stress field and instability in central Fennoscandia for the present, with thrust faulting as the predicted mode of failure. However, the lack of spatial correlation of the present seismicity with the region of uplift, and the existence of strike-slip and normal modes of current seismicity are inconsistent with this model. Further unmodelled factors such as the presence of high-angle faults in the central region of uplift along the Baltic coast would be required in order to explain the pattern of seismicity today in terms of postglacial rebound stress. The sensitivity of the model predictions to the effects of compressibility, tectonic stress, viscosity and ice model is also investigated. For sites outside the ice margin, it is found that the mode of failure is sensitive to the presence of tectonic stress and that the onset timing is also dependent on compressibility. For sites within the ice margin, the effect of Earth rheology is shown to be small. However, ice load history is shown to have larger effects on the onset time of earthquakes and the magnitude of fault instability. 相似文献
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999.
GIS支持的全球气候变化对中国Holdridge生命地带的可能影响研究 总被引:7,自引:0,他引:7
该文以地理信息系统为支持手段 ,根据Holdridge气候—植被生命地带的分类体系 ,首先研究了在目前气候条件下中国Holdridge生命地带空间分布的基本格局 ;然后采用GFDL、GISS、OSU3个GCMs模拟的 2×CO2 情景下的气温和降水作为未来气候条件 ,揭示了 2×CO2 情景下中国Holdridge生命地带在空间分、面积以及海拔高度等方面的变化 相似文献
1000.
The release of dissolved actinium to the ocean: A global comparison of different end-members 总被引:1,自引:1,他引:0
Walter Geibert Matt Charette Guebuem Kim Willard S. Moore Joseph Street Megan Young Adina Paytan 《Marine Chemistry》2008,109(3-4):409
The measurement of short-lived 223Ra often involves a second measurement for supported activities, which represents 227Ac in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of 227Ac, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles 227Ac data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of 227Ac near its potential sources.The average 227Ac activities of nearshore marine end-members range from 0.4 dpm m− 3 at the Gulf of Mexico to 3.0 dpm m− 3 in the coastal waters of the Korean Strait. In analogy to 228Ra, we find the extension of adjacent shelf regions to play a substantial role for 227Ac activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean 227Ac inventory of 1.35 * 1018 dpm 227Acex in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm m−2 y− 1 from the deep-sea floor. For the shelf regions, we obtain a global inventory of 227Ac of 4.5 * 1015 dpm, which cannot be converted directly into a flux value, as the regional loss term of 227Ac to the open ocean would have to be included.Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to 231Paex. Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in 227Ac/228Ra and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including 227Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution. 相似文献