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271.
Despite vigorous tidal and wind mixing, observations in an estuarine tidal inlet in the Wadden Sea show that during part of
the tidal cycle, vertical stratification and internal waves may still develop. Acoustic Doppler current profiler (ADCP) and
conductivity, temperature, depth observations, collected over the past 6 years at 13 h anchor stations (ASs), reveal that
these occur especially during slack tide, when there is little wind and large freshwater discharge from nearby Lake IJssel.
Measurements with a moored ADCP show that in the same tidal phase, strong cross-channel circulation develops, which may suddenly
reverse circulation sense due to passing density fronts. In the vertically stratified phase that follows after the front passage,
propagating mode-one solitary internal waves are observed. These are resonantly generated during decelerating tidal ebb currents
when the (shear) flow passes a transcritical regime (Froude number equal to 1). A combination of photographs (including one
from the International Space Station), bathymetric data, and ASs data leads to the discovery of yet another source of internal
waves in this area, produced during slackening tide by propagating lee waves that develop over a deep trench. We suggest that
both the cross-channel circulation as well as the (solitary) internal waves may locally be of importance for the (re)distribution
and transport of sediments and nutrients and may influence tidally averaged transports. 相似文献
272.
本研究基于10′×10′空间分辨率运用GIS软件对中国植被分布地理配准数字化,提取获得196个优势植物的地理分布数据以及内插相关的气候因子,运用分位数函数计算获得各个植物地理分布的气候阈值; 在此基础上采用生物气候群组(BAGs)的分类方法,对196个优势植物气候因子数据进行聚类分析,同时结合中国植被的地理分布特征,最后划分出29个BAG组,包括13个乔木组(6个常绿阔叶和2个落叶阔叶群组,4个常绿针叶和1个落叶针叶群组)、10个灌木组(5个常绿灌木和5个落叶灌木群组)和6个草本组。本研究的BAGs划分是建立在较高空间分辨率和定量气候因子阈值的基础上,因此,通过计算获得BAGs气候阈值(包括每日温度变化幅度、地表霜冻频率、月均温度、月降雨量、相对湿度、日照时数和>5℃年积温等),运用生物圈碳循环CARAIB模型,模拟出中国19个生物群区的现代潜在分布。模拟结果较好地表达了中国植被类型的地理分布,尤其较详细地展示了热带、亚热带、温带和高原地区的不同植被类型地理分布格局,但由于东北地区、西部草原以及热带原生植被受人类活动严重干扰,使得模拟结果比该地区的原生植被地理分布的实际面积偏大。总体而言,本研究获得的29个生物气候群组对模拟现代生物群区分布提供了理想的分类依据,其气候阈值也是进行过去和未来生物群区模拟的重要参照数据。 相似文献
273.
Yousif K. Kharaka James J. Thordsen Susan D. Hovorka H. Seay Nance David R. Cole Tommy J. Phelps Kevin G. Knauss 《Applied Geochemistry》2009
Sedimentary basins in general, and deep saline aquifers in particular, are being investigated as possible repositories for large volumes of anthropogenic CO2 that must be sequestered to mitigate global warming and related climate changes. To investigate the potential for the long-term storage of CO2 in such aquifers, 1600 t of CO2 were injected at 1500 m depth into a 24-m-thick “C” sandstone unit of the Frio Formation, a regional aquifer in the US Gulf Coast. Fluid samples obtained before CO2 injection from the injection well and an observation well 30 m updip showed a Na–Ca–Cl type brine with ∼93,000 mg/L TDS at saturation with CH4 at reservoir conditions; gas analyses showed that CH4 comprised ∼95% of dissolved gas, but CO2 was low at 0.3%. Following CO2 breakthrough, 51 h after injection, samples showed sharp drops in pH (6.5–5.7), pronounced increases in alkalinity (100–3000 mg/L as HCO3) and in Fe (30–1100 mg/L), a slug of very high DOC values, and significant shifts in the isotopic compositions of H2O, DIC, and CH4. These data, coupled with geochemical modeling, indicate corrosion of pipe and well casing as well as rapid dissolution of minerals, especially calcite and iron oxyhydroxides, both caused by lowered pH (initially ∼3.0 at subsurface conditions) of the brine in contact with supercritical CO2. 相似文献
274.
J. R. Wayland 《Astrophysics and Space Science》1969,3(3):357-364
We have investigated the origin of cosmic radiation in terms of a sudden injection of particles in time, momentum and space. The appropriate boundary conditions for the various regions through which the particles pass were used. With all of the acceleration within a turbulent region, we find that the observed spectrum is explained by a continuous deceleration in which statistical fluctuations dominate. This is contradictory to the usual assumptions in which fluctuations do not play an important part. We find that the time dependent solution has an exponent of the power law spectrum that varies weakly with the momentum and time. The steady state solution shows the usual constant exponent.This work was supported by the National Aeronautics and Space Administration under Grant NsG-695. 相似文献
275.
The Middle to Late Cambrian loop in the North American apparent polar wander path (APWP) has been variously attributed to tectonic rotations, remagnetizations and primary magnetizations. Although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated, the temporally self-consistent nature of the loop has been used as an argument for primary magnetizations. We have studied535 ± 5Ma nepheline syenites and syenites of the McClure Mountain alkalic complex, as well as495 ± 10Ma red trachyte dikes which intruded the complex, in an effort to find a primary TRM. Because Zijderveld analysis yielded consistent results for only one trachyte dike, remagnetization great-circle analysis was employed, giving a pole for the trachyte dikes at the tip of the loop (43°N, 114°E), while the syenites and nepheline synenites gave a pole at the base of the loop (18°N, 142°E). The magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration. KAr whole rock dating of the trachytes suggests a Pennsylvanian age for the alteration, and thus a late Paleozoic remagnetization of the trachytes. Thus, the low-latitude Cambrian pole is confirmed, but we find no evidence in this study to support the primary nature of the Cambrian APWP. 相似文献
276.
Globular clusters rotate significantly, and with the increasing amount of detailed morphological and kinematical data obtained in recent years on galactic globular clusters many interesting features show up. We show how our theoretical evolutionary models of rotating clusters can be used to obtain fits, which at least properly model the overall rotation and its implied kinematics in full 2D detail (dispersions, rotation velocities). Our simplified equal mass axisymmetric rotating model provides detailed two-dimensional kinematical and morphological data for star clusters. The degree of rotation is not dominant in energy, but also non-negligible for the phase-space distribution function, shape and kinematics of clusters. Therefore, the models are well applicable for galactic globular clusters. Since previously published papers on that matter by us made it difficult to do detailed comparisons with observations, we provide a much more comprehensive and easy-to-use set of data here, which uses as entries dynamical age and flattening of observed cluster and then offers a limited range of applicable models in full detail. The method, data structure and some exemplary comparison with observations are presented. Future work will improve modelling and data base to take a central black hole, a mass spectrum and stellar evolution into account. 相似文献
277.
Y. Mizumura J. Kushida K. Nishijima G.V. BicknellR.W. Clay P.G. EdwardsS. Gunji S. HaraS. Hayashi S. KabukiF. Kajino A. KawachiT. Kifune R. KiuchiK. Kodani Y. MatsubaraT. Mizukami Y. MizumotoM. Mori H. MuraishiT. Naito M. OhishiV. Stamatescu D.L. SwabyT. Tanimori G. ThorntonF. Tokanai T. ToyamaS. Yanagita T. YoshidaT. Yoshikoshi 《Astroparticle Physics》2012,35(9):563-572
We have searched for very high energy (VHE) gamma rays from four blazars using the CANGAROO-III imaging atmospheric Cherenkov telescope. We report the results of the observations of H 2356-309, PKS 2155-304, PKS 0537-441, and 3C 279, performed from 2005 to 2009, applying a new analysis to suppress the effects of the position dependence of Cherenkov images in the field of view. No significant VHE gamma ray emission was detected from any of the four blazars. The GeV gamma-ray spectra of these objects were obtained by analyzing Fermi/LAT archival data. Wide range (radio to VHE gamma-ray bands) spectral energy distributions (SEDs) including CANGAROO-III upper limits, GeV gamma-ray spectra, and archival data, even though they are non-simultaneous, are discussed using a one-zone synchrotron self-Compton (SSC) model in combination with a external Compton (EC) radiation. The HBLs (H 2356-309 and PKS 2155-304) can be explained by a simple SSC model, and PKS 0537-441 and 3C 279 are well modeled by a combination of SSC and EC model. We find a consistency with the blazar sequence in terms of strength of magnetic field and component size. 相似文献
278.
P. B. Tissera R. Domínguez-Tenreiro C. Scannapieco A. Sáiz 《Monthly notices of the Royal Astronomical Society》2002,333(2):327-338
We use cosmological smooth particle hydrodynamical (SPH) simulations to study the effects of mergers in the star formation history of galactic objects in hierarchical clustering scenarios. We find that during some merger events, gaseous discs can experience two starbursts: the first one during the orbital decay phase, owing to gas inflows driven as the satellite approaches, and the second one when the two baryonic clumps collide. A trend for these first induced starbursts to be more efficient at transforming the gas into stars is also found. We detect that systems that do not experience early gas inflows have well-formed stellar bulges and more concentrated potential wells, which seem to be responsible for preventing further gas inward transport triggered by tidal forces. The potential wells concentrate owing to the accumulation of baryons in the central regions and of dark matter as the result of the pulling in by baryons. The coupled evolution of the dark matter and baryons would lead to an evolutionary sequence during which systems with shallower total potential wells suffer early gas inflows during the orbital decay phase that help to feed their central mass concentration, pulling in dark matter and contributing to build up more stable systems. Within this scenario, starbursts triggered by early gas inflows are more likely to occur at early stages of evolution of the systems and to be an important contributor to the formation of stellar bulges. Our results constitute the first proof that bulges can form as the product of collapse, collisions and secular evolution in a cosmological framework, and they are consistent with a rejuvenation of the stellar population in bulges at intermediate z with, at least, 50 per cent of the stars (in SCDM) being formed at high z . 相似文献
279.
R. A.Street KeithHorne T. A.Lister A.Penny Y.Tsapras A.Quirrenbach N.Safizadeh J.Cooke D.Mitchell A.Collier Cameron 《Monthly notices of the Royal Astronomical Society》2002,330(3):737-754
We report on the discovery of 25 variable stars plus 13 suspected variables found in the field of the open cluster NGC 6819. The stars were identified from time-series photometric data obtained on the Isaac Newton Telescope, La Palma, during two observing runs covering the 19 nights between 1999 June 22–30 and 1999 July 22–31 . The variables found include 12 eclipsing binaries with an additional three suspected, nine BY Draconis systems, plus four variables of other types, including one star believed to be a Cepheid. Three of the 15 eclipsing binaries are believed to be cluster members. Details of a further 10 suspected variable stars are also included. 相似文献
280.
Grain-scale variations in trace element composition of fluid-altered zircon,Acasta Gneiss Complex,northwestern Canada 总被引:10,自引:0,他引:10
N.?RaynerEmail author R.?A.?Stern S.?D.?Carr 《Contributions to Mineralogy and Petrology》2005,148(6):721-734
SEM back-scattered electron images of zircon separates from a 2.9 Ga granitic dyke from the Acasta Gneiss Complex, Slave Province, reveal primary igneous oscillatory zoning which is transgressively overprinted by irregular alteration domains. Electron microprobe analyses show that altered zircon domains are depleted in Zr and Si relative to unaltered zircon while retaining a constant Zr/Si ratio. SIMS trace element analyses indicate that LREE are preferentially enriched in the altered domains. Altered zircon contains elevated concentrations of Ba (up to 580 ppm), Ca (up to 2% weight of the element) and common Pb (>50 ppb). The presence of LREE, Ca, Ba and common Pb in the zircons is interpreted to be the result of secondary alteration by an aqueous fluid, and not by primary incorporation during crystallization. We propose that the alteration-related elements are located in amorphous zircon domains. 相似文献