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971.
Martin Thöni 《Chemical Geology》2003,194(4):353-379
Interpretation of Sm-Nd garnet ages is frequently impaired by one of the following restrictions: (a) high-LREE inclusions, (b) isotopic disequilibrium, and (c) the uncertainty about the closure temperature. These issues are addressed by way of an evaluation of garnet Sm-Nd data from different rock types of the Austroalpine basement units, Eastern Alps, including metabasic eclogites, mica schist and paragneiss, metapegmatite and metagranite.Nd concentration in handpicked garnet varies between 0.021 and 23.1 ppm in metabasites, 0.49 and 17.4 ppm in metapelites and between 0.024 and 4.6 ppm in metapegmatites and metagranites. The overall range of 147Sm/144Nd is 0.15-2.5 in garnet from metabasites, 0.12-3.03 in metapelite garnet and 0.66-7.21 in Mn-rich garnet from metapegmatites and metagranites. A clear negative correlation between Nd concentration and Sm/Nd is observed in garnets from all these lithologies. Therefrom, it is concluded that even optically “clean” garnet separates may contain high-LREE microinclusions, such as epidote-allanite, zoisite, apatite, sphene, monazite or zircon. However, very low Nd concentrations correlated with low Sm/Nd as well as high Nd concentrations (>5 ppm) correlated with fairly high Sm/Nd ratios (0.8) have also been observed. Apart from replicate analyses within as well as between samples with a common PT-history, leaching experiments are a useful technique to elucidate any distorting influence of unequilibrated inclusions on the garnet age, especially if the observed Sm/Nd ratio is low (<0.5). Leaching of garnet separates with HCl (2.5, 5.8 M) produces no obvious element fractionation, but may improve Sm/Nd, and hence age precision, considerably. Isotopic disequilibrium between garnet and other matrix minerals is observed preferentially in basic eclogites, derived from gabbroic precursors.Sm-Nd garnet analysis allows the recognition of several distinct garnet-forming events in the Eastern Alps.(a) A Variscan high-P event is documented in metabasites from the northern-central Ötztal basement around 360-350 Ma, whereas garnet from sillimanite-bearing gneisses dates the Variscan thermal peak in the western part of the same subunit around 345-330 Ma.(b) A long-lived, Permian to Triassic event (285-225 Ma), correlated with crustal extension and low-P metamorphism, is documented by spessartine-rich garnet from metapegmatites as well as almandine-rich garnet cores from mica schist.(c) Age data of garnet from eo-Alpine (Cretaceous) deeply subducted rocks of the southern/eastern Austroalpine units are related to near-peak PT, eclogite- to amphibolite-facies metamorphic conditions (peak: 2 GPa/685 °C), and/or incipient isothermal decompression, due to fast, tectonically driven exhumation (110/100-85 Ma). At cooling rates of 20-30 °C/Ma (exhumation rates: 3-5 km/Ma), the Sm-Nd closure temperature (Tc) for mm-sized garnet in these rocks is estimated at 650-680 °C. 相似文献
972.
1999年6月下旬长江中下游梅雨锋低涡扰动的结构研究 总被引:28,自引:12,他引:28
1999年6月下旬在长江中下游发生长达10天之久的连续性暴雨,作者对其进行了全程中尺度四维同化数值模拟,利用时空高分辨的模式输出,对梅雨锋上与强降水密切相关的5个低涡的活动特点和四维结构进行了综合分析:识别和追踪每个低涡的活动路径,分析其主要特征的垂直结构、水平分布和时间演变,并通过轨迹和气块物理属性个别变化的计算,揭示构成低涡的三股气流,分析低涡区水汽及冷空气来源和湿斜压锋区的形成. 相似文献
973.
选择西峰和段家坡两个黄土剖面,对磁化率变化显著的L6-S5-L5层段,通过对样品中的趋磁细菌(MB)用相同的培养基富集及生长培养,初步定量模拟了MB和磁小体(MSs)对黄土-古土壤序列磁化率的贡献。富集培养MB后,段家坡剖面样品磁化率表现出不同程度的增高,最大达116.4%(DS5-1)。以古土壤样品XS5-1、DS5-1为介质生长培养MB,DS5-1磁化率的增长与接种的菌液量呈近似线性关系。此外,生长培养后古土壤样品频率磁化率与FeO/Fe2O3比值呈现出相似的变化趋势,证实了磁小体的形成。由此表明:1)MB和MSs是影响黄土剖面磁化率的因素之一,受气候和地质环境等因素的影响,不同地区黄土剖面MB的生长和MSs的形成存在差异,因此对磁化率的贡献也不尽相同;2)MSs是黄土-古土壤序列中超细粒磁性物质的组成部分。当气候环境比较适宜MSs形成及保存时,MSs在黄土剖面中超细粒(≤50nm)磁性组分中比重会相应增高,对磁化率和频率磁化率的影响也更为突出。 相似文献
974.
Larry W. Dennis Gary E. Maciel Patrick G. Hatcher Bernd R.T. Simoneit 《Geochimica et cosmochimica acta》1982,46(6):901-907
Cretaceous black shales from DSDP Leg 41, Site 368 in the Eastern Atlantic Ocean were thermally altered during the Miocene by an intrusive basalt. The sediments overlying and underlying the intrusive body were subjected to high temperatures (up to ~ 500°C) and, as a result, their kerogen was significantly altered. The extent of this alteration has been determined by examination by means of 13C nuclear magnetic resonance, using cross polarization/magic-angle spinning (CP/MAS). Results indicate that the kerogen becomes progressively more aromatic in the vicinity of the intrusive body. Laboratory heating experiments, simulating the thermal effects of the basaltic intrusion, produced similar results on unaltered shale from the drill core. The 13C CP/MAS results appear to provide a good measure of thermal alteration. 相似文献
975.
Maria Clara Fava Maurizio Mazzoleni Narumi Abe Eduardo Mario Mendiondo Dimitri P. Solomatine 《水文科学杂志》2020,65(7):1096-1111
ABSTRACTPoorly monitored catchments could pose a challenge in the provision of accurate flood predictions by hydrological models, especially in urbanized areas subject to heavy rainfall events. Data assimilation techniques have been widely used in hydraulic and hydrological models for model updating (typically updating model states) to provide a more reliable prediction. However, in the case of nonlinear systems, such procedures are quite complex and time-consuming, making them unsuitable for real-time forecasting. In this study, we present a data assimilation procedure, which corrects the uncertain inputs (rainfall), rather than states, of an urban catchment model by assimilating water-level data. Five rainfall correction methods are proposed and their effectiveness is explored under different scenarios for assimilating data from one or multiple sensors. The methodology is adopted in the city of São Carlos, Brazil. The results show a significant improvement in the simulation accuracy. 相似文献
976.
Mid-latitude ionospheric perturbation associated with the Spacelab-2 plasma depletion experiment at Millstone Hill 总被引:1,自引:0,他引:1
Elevation scans across geomagnetic mid latitudes by the incoherent scatter radar at Millstone Hill captured the ionospheric response to the firing of the Space Shuttle Challenger OMS thrusters near the peak of the F layer on July 30, 1985. Details of the excitation of airglow and the formation of an ionospheric hole during this event have been reported in an earlier paper by Mendillo et al.. The depletion (factor 2) near the 320 km Shuttle orbital altitude persisted for 35 min and then recovered to near normal levels, while at 265 km the density was reduced by a factor of 6; this significant reduction in the bottomside F-region density persisted for more than 3 hours. Total electron content in the vicinity of the hole was reduced by more than a factor of 2, and an oscillation of the F-region densities with 40-min period ensued and persisted for several hours. Plasma vertical Doppler velocity varied quasi-periodically with a 80-min period, while magnetic field variations observed on the field line through the Shuttle-burn position exhibited a similar 80-min periodicity. An interval of magnetic field variations at hydromagnetic frequencies (95 s period) accompanied the ionospheric perturbations on this field line. Radar observations revealed a downward phase progression of the 40-min period density enhancements of -1.12° km–1, corresponding to a 320-km vertical wavelength. An auroral-latitude geomagnetic disturbance began near the time of the Spacelab-2 experiment and was associated with the imposition of a strong southward IMF Bz across the magnetosphere. This created an additional complication in the interpretation of the active ionospheric experiment. It cannot be determined uniquely whether the ionospheric oscillations, which followed the Spacelab-2 experiment, were related to the active experiment or were the result of a propagating ionospheric disturbance (TID) launched by the enhanced auroral activity. The most reasonable conclusion is that the ionospheric oscillations were a result of the coincident geomagnetic disturbance. The pronounced depletion of the bottomside ionosphere, however, accentuated the oscillatory behavior during the interval following the Shuttle OMS burn. 相似文献
977.
978.
979.
Mapping of Land-Atmosphere Heat Fluxes and Surface Parameters with Remote Sensing Data 总被引:3,自引:0,他引:3
A variational data assimilation scheme is used to infer two key parameters ofthe surface energy balance that control the partitioning of available energy intolatent, sensible, and ground heat fluxes (LE, H, and G). Remotely sensedland surface temperature (LST) is the principal data source. Maps ofdiurnal energy balance components are presented for a basin with varied landcover (Arno Basin, Italy) for a 18-day period in July 1996.Given available energy, the major unknown (dimensionless) parameters requiredfor partitioning among fluxes are: (1) Landscape effects on near-surfaceturbulence as captured by the bulk heat transfer coefficient CBN underneutral conditions and (2) surface control of the relative magnitudes of LEand H as represented by the evaporative fraction EF. The data assimilationscheme merges 1.1-km resolution remotely sensed LST images (based onoptical, thermal and microwave measurements from two different satelliteplatforms) into a parsimonious model of heat diffusion. Both the measurementsand the model predictions are considered uncertain. Posterior error statisticsthat represent uncertainty of the estimated parameters are also derived.Maps of CBN show spatial patterns consistent with the dominant land useand basin physiography. Daily maps of EF exhibit spatial variationscorresponding to land cover and land use – the day-to-day variations inEF show fluctuations consistent with rain events and drydowns experiencedduring the period. Based on these parameters and available environmentalvariables, maps of diurnal LE and H may be produced (in this paper daytimeLE maps are reported).The application demonstrates that remotely sensed land surface temperaturesequences contain significant amount of information of the partitioning ofavailable energy among the fluxes. The variational data assimilation frameworkis shown to be an efficient and parsimonious approach without reliance onempirical relationships such as those based on vegetation indices. 相似文献
980.
FAN Qi WANG Anyu FAN Shaoii WU Dui DENG Xuejiao LIU Yingwei 《Acta Meteorologica Sinica》2003,17(3):337-349
Now more comprehensive cloud microphysical processes have been included in advanced three-dimensional mesoscale meteorological model such as PSU/NCAR MM5 model,so the model can be used in the prediction of fog.In this paper,MM5 was utilized to simulate an advection fog occurring in Nanling Mountain area.The simulated results were compared with the facts obtained by detailed observation experiment.The results showed that the simulation was successful in the following aspects:(1)the formation and development of the fog;(2)the temporal variation of the maximum liquid water content;(3)the diffusion of the cold air,especially the temporal variation of the ground temperature;and (4)the uplift of the air and the formation and development of the low-lever inversion.Besides,we did some sensitivity numerical experiments and discussed the effects of the radiation,the release of condensation latent heat and the change of soil moisture and temperature on the formation and development of fog.The success of numerical simulation experiment of fog has proved that the numerical forecasting of fog is promising. 相似文献