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1.
Oxygen isotope ratios in streamflow of the Missouri River basin vary geographically due to differences in source precipitation and the integration of waters from upstream regions. Average '18O values in the Missouri River main stem systematically increase from less than -17 in the headwaters to about -9 in the lower basin. Seasonal variations at a given location result from fluctuations in meteoric precipitation, residence time in reservoirs and groundwater systems, evaporation, and snowmelt. Average water chemistry values are successfully predicted for the upstream stations of two reaches on the lower Missouri River based on changes in discharge along each reach and water quality measurements collected at the downstream stations. Source regions for some dissolved ions found in the lower Missouri River are also identified. Sodium and sulfate originate predominantly from the basin above Sioux City, Iowa, while nitrate is largely derived from agricultural regions below Sioux City. 相似文献
2.
To explore planetary evolution, we provide conductive cooling profiles that account for planet size, phonon diffusivity and various internal heating scenarios. Our new analytical solution for simple cooling of spheres reveals that heat is removed from only Earth's outermost ~1000 km over geological time. Numerical models with decaying heat production show that any upward concentration of radionuclides causes high temperatures at shallow depths, forcing interior temperatures to increase with time while producing a thermal gradient that forbids lower mantle convection. Hence, differentiation drives upper mantle magmatism and tectonics, leaving a quiescent but hot deep interior, while slowly melting the core. 相似文献
3.
Late Weichselian glaciation history of the northern North Sea 总被引:8,自引:1,他引:8
HANS PETTER SEJRUP HAFLIDI HAFLIDASON INGE AARSETH EDWARD KING CARL FREDRIK FORSBERG DAVID LONG KÅRE ROKOENGEN 《Boreas: An International Journal of Quaternary Research》1994,23(1):1-13
Based on new data from the Fladen, Sleipner and Troll areas, combined with earlier published results, a glaciation curve for the Late Weichselian in the northern North Sea is constructed. The youngest date on marine sedimentation prior to the late Weichselian maximum ice extent is 29.4 ka BP. At this time the North Sea and probably large parts of southern Norway were deglaciated (corresponding to the Alesund interstadial in western Norway). In a period between 29.4 and c. 22 ka BP, the northern North Sea experienced its maximum Weichselian glaciation with a coalescing British and Scandinavian ice sheet. The first recorded marine inundation is found in the Fladen area where marine sedimentation started close to 22 ka BP. After this the ice fronts receded both to the east and west. The North Sea Plateau, and possibly parts of the Norwegian Trench, were ice-free close to 19.0 ka, and after this a short readvance occurred in this area. This event is correlated with the advance recorded at Dimlington, Yorkshire, and the corresponding climatostratigraphic unit is denoted the Dimlington Stadial (18.5 ka to 15.1 ka). The Norwegian Trench was deglaciated at 15.1 ka in the Troll area. The data from the North Sea, together with the results from Andwa, northern Norway (Vorren et al . 1988; Møller et al . 1992), suggest that the maximum extent of the last glaciation along the NW-European seaboard from the British Isles to northern Norway was prior to c . 22 ka BP. 相似文献
4.
Strontium and oxygen isotopic variations in Mesozoic and Tertiary plutons of central Idaho 总被引:2,自引:0,他引:2
Regional variations in initial 87Sr/86Sr ratios (r
i) of Mesozoic plutons in central Idaho locate the edge of Precambrian continental crust at the boundary between the late Paleozoic-Mesozoic
accreted terranes and Precambrian sialic crust in western Idaho. The r
i values increase abruptly but continuously from less than 0.704 in the accreted terranes to greater than 0.708 across a narrow,
5 to 15 km zone, characterized by elongate, lens-shaped, highly deformed plutons and schistose metasedimentary and metavolcanic
units. The chemical and petrologic character of the plutons changes concomitantly from ocean-arc-type, diorite-tonalite-trondhjemite
units to a weakly peraluminous, calcic to calcalkalic tonalite-granodiorite-granite suite (the Idaho batholith). Plutons in
both suites yield Late Cretaceous ages, but Permian through Early Cretaceous bodies are confined to the accreted terranes
and early Tertiary intrusions are restricted to areas underlain by Precambrian crust. The two major terranes were juxtaposed
between 75 and 130 m.y. ago, probably between 80 and 95 m.y.
Oxygen and strontium isotopic ratios and Rb and Sr concentrations of the plutonic rocks document a significant upper-crustal
contribution to the magmas that intrude Precambrian crust. Magmas intruding the arc terranes were derived from the upper mantle/subducted
oceanic lithosphere and may have been modified by anatexis of earlier island-arc volcanic and sedimentary units.
Plutons near the edge of Precambrian sialic crust represent simple mixtures of the Precambrian wall-rocks with melts derived
from the upper mantle or subducted oceanic lithosphere with r
i of 0.7035. Rb/Sr varies linearly with r
i, producing “pseudoisochrons” with apparent “ages” close to the age of the wall rocks. Measured δ
18O values of the wall rocks are less than those required for the assimilated end-member by Sr-O covariation in the plutons,
however, indicating that wall-rock δ
18O was reduced significantly by exchange with circulating fluids. Metasedimentary rocks of the Belt Supergroup are similarly
affected near the batholith, documenting a systematic depletion in 18O as much as 50 km from the margin of the batholith.
Plutons of the Bitterroot lobe of the Idaho batholith are remote from the accreted terranes and represent mixtures of Precambrian
wall-rocks with melts dominated by continental lower crust (r
i>0.708) rather than mantle. “Pseudoisochrons” resulting from these data are actually mixing lines that yield apparent “ages”
less than the true age of the wall rocks and meaningless “ri”. Assimilation/ fractional-crystallization models permit only insignificant amounts of crystal fractionation during anatexis
and mixing for the majority of plutons of the region. 相似文献
5.
RJ Andres WI Rose RE Stoiber SN Williams O Matías R Morales 《Bulletin of Volcanology》1993,55(5):379-388
Measurements of the sulfur dioxide (SO2) emission rate from three Guatemalan volcanoes provide data which are consistent with theoretical and laboratory studies of eruptive and shallow magma chamber processes. In particular, unerupted magma makes a major contribution to the measured SO2 emission rates at Santiaguito, a continuously erupting dacitic volcanic dome. Varying shallow magma convection rates can explain the variations in SO2 emission rates at Santiaguito. At Fuego, a basaltic volcano currently in repose, SO2 emission rate measurements are consistent with a high level magma body that is crystallizing and releasing volatiles. At Pacaya, a continuously erupting basaltic volcano, recent SO2 emission rate measurements support laboratory simulation studies of strombolian eruptions; these studies indicate that the majority of gas escapes during eruptions and little gas escapes between eruptions.Average SO2 emission rates over the last 20 years for Santiaguito, Fuego and Pacaya are 80, 160 and 260 Mg/d, respectively. On a global scale, these three volcanoes account for 1% of the annual global volcanic output of SO2. Santiaguito and Pacaya, together, emit 6% of the total annual SO2 emitted by continuously erupting volcanoes.Even though SO2 measurements at these volcanoes have been made infrequently and by different investigators, the collective data help to establish a useful baseline by which to judge future changes. A more complete record of SO2 emission rates from these volcanoes could lead to a better understanding of their eruption mechanisms and reduce the impact of their future eruptions on Guatemalan society. 相似文献
6.
Quaternary geology and deglaciation of the continental shelf off Troms, north Norway 总被引:1,自引:0,他引:1
The study is based mainly on 4700 km shallow seismic profiling, soil mechanical, and micropaleontological analyses from forty localities, and seven radiocarbon datings. Six foraminiferal assemblages are recognized. The thickness of Quaternary deposits ranges from sparse to more than 200 m. They consist of top sand, soft sensitive clay, and glacial drift divided into four seismic stratigraphic units. The oldest unit, which is the thickest and most extensive, is built up of various sediments and is partly eroded, especially in the southern part. The remaining units occur as three complex linear belts running broadly parallel to the present coast and were deposited during deglaciation. Dating of the outer unit, the Nordvestsnaget Drift, suggests a maximum age of 13,300 years B.P., and datings from the top of the middle unit, the Mulegga Drift, have given a minimum age of about 12,200 years B.P. Seismic stratigraphy shows that the inner unit, the Havbrobakken Drift, is younger than the Mulegga Drift, but no datings have been obtained so far. 相似文献
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10.
Method for calibrating a theoretical model in karst springs: an example for a hydropower station in South China 总被引:1,自引:0,他引:1
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Mingming Luo Zhihua Chen Robert E. Criss Hong Zhou Hamza Jakada Tingting Shi 《水文研究》2016,30(25):4815-4825
A theoretical, dimensionless rainfall–runoff model was used to simulate the discharge of Wulongdong spring in western Hubei province, South China. The single parameter (time constant τ) in the model is easy to obtain by fitting the recession rate of the observed hydrographs. The model was scaled by simply matching the total annual flow volume of the model to the observed value. Annual distribution of actual evapotranspiration was embedded in the model input to calculate the accumulated deficit of soil moisture before each rain event. Hourly precipitation input data performed better than daily data, defining τ of 0.85 days and returning a Nash–Sutcliffe efficiency of 0.89 and the root mean square error of 0.07. This model offers an effective way to simulate the discharge of karst springs that respond sensitively to rainfall events. The model parameters of a successful simulation can be used to estimate the recharge area and indicate the intrinsic response time of the basin. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献