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81.
In order to establish the magnetic carriers and assess the reliability of previous paleomagnetic results obtained for Eocene marine marls from the south Pyrenean basin, we carried out a combined paleo- and rock-magnetic study of the Pamplona-Arguis Formation, which crops out in the western sector of the southern Pyrenees (N Spain). The unblocking temperatures suggest that the characteristic remanent magnetization (ChRM) is carried by magnetite and iron sulphides. The ChRM has both normal and reversed polarities regardless of whether it resides in magnetite or iron sulphides, and represents a primary Eocene magnetization acquired before folding. Rock magnetic results confirm the presence of magnetite and smaller amounts of magnetic iron sulphides, most likely pyrrhotite, in all the studied samples. Framboidal pyrite is ubiquitous in the marls and suggests that iron sulphides formed during early diagenesis under sulphate-reducing conditions. ChRM directions carried by magnetic iron sulphides are consistent with those recorded by magnetite. These observations suggest that magnetic iron sulphides carry a chemical remanent magnetization that coexists with a remanence residing in detrital magnetite. We suggest that the south Pyrenean Eocene marls are suitable for magnetostratigraphic and tectonic purposes but not for studies of polarity transitions, secular variations and geomagnetic excursions, because it is difficult to test for short time differences in remanence lock-in time for the two minerals. The presence of iron sulphide minerals contributing to the primary magnetization in Eocene marine marls reinforces the idea that these minerals can persist over long periods of time in the geological record. 相似文献
82.
Manuel R. Palacios-Fest Ana Luisa Carreño José R. Ortega-Ramírez Guillermo Alvarado-Valdéz 《Journal of Paleolimnology》2002,27(2):185-206
Paleoecology of Laguna Babícora, Chihuahua, Mexico was reconstructed using ostracode faunal assemblages and shell chemistry. The paleolimnological record is used to show the magnitude of paleoclimatic changes in the area from 25,000 years to the present.Faunal assemblages consist of four species of the genus Limnocythere: L. sappaensis, L. ceriotuberosa, L. bradburyi and L. platyforma, all associated with Candona caudata, Candona patzcuaro and Cypridopsis vidua. A paleosalinity index developed from these assemblages indicates that the lake's salinity fluctuated frequently from oligo- to meso-haline conditions during the last 25,000 years. This pattern and low salinity range are in good agreement with modern TDS (here used as an indicator of salinity) values recorded from 26 wells and one spring from the area (258–975 mg l–1). To estimate paleotemperature we examined the trace element content (Mg/Ca ratios) from individual valves of L. ceriotuberosa and L. platyforma, the two species most commonly recorded in Laguna Babícora.Shell Mg/Ca ratios of 204 specimens of these two species were used to estimate water temperature (Mg/Ca) by means of experimental standard coefficients. Our data show that paleowater temperature ranged from 5.6–21.3 °C (with 2 values ranging from 0.2–4.8 °C), which suggest a close correlation with atmospheric temperatures around the lake. These results are in good agreement with a modern mean winter temperature (3.5 °C) and mean summer temperature (20 °C) recorded in the area between 1970 and 1980. 相似文献
83.
High-resolution, high signal-to-noise ratio, blue-violet spectra of three red giant branch tip stars in M15 have been obtained with the Keck I High-Resolution Echelle Spectrograph. These spectra have been analyzed to determine the abundances of several neutron-capture elements, including the radioactive chronometer element thorium. There are two principal results of this study. First, the abundances of the heavier (Z>/=56) elements for each of the three stars is well matched by a scaled solar system r-process abundance distribution. Second, a weighted mean-observed Th/Eu ratio for the stars implies an age for the neutron-capture material in M15 stars of 14+/-3 Gyr, in reasonable agreement with other recent age estimates for Galactic globular clusters. 相似文献
84.
R. Nieto L. Gimeno J. A. Añel L. De la Torre D. Gallego D. Barriopedro M. Gallego A. Gordillo A. Redaño G. Delgado 《Meteorology and Atmospheric Physics》2007,96(1-2):103-119
Summary The Iberian Peninsula is one of the regions in the world with higher occurrence of cut-off low systems (COL). The aim of this
paper is to analyse the weather events (rainfall and cloudiness layer) associated to COLs in the Iberian Peninsula with tools
not previously used: (a) the use of the new multidecadal COLs database developed by Nieto et al (2005) that permit us to study
a 41 years period (1958–1998), (b) the checking of the expected weather events (rainfall and cloudiness layer) associated
with COLs in a conceptual model (Winkler et al, 2005) and (c) the extensive use of radiosoundings to analyse convective instability
in areas inside and close to the COL. Two points of view are used to make the analysis: (1) a source oriented method, when
a particular COL is followed and its associated precipitation and cloudiness is analysed over four quadrants in which Iberia
was divided and (2) a receptor oriented method, when the precipitation associated to COLs is analysed in given areas, defined
by patterns of precipitation. Results reveal that the precipitation and cloudiness patterns associated to COLs in the conceptual
model reproduce quite well the main characteristics found over the Iberian Peninsula. The generalized idea that most of the
COLs produce intense convective rainfall is show to be misleading. Convective phenomena are important usually when the centre
of the COL is located on the Mediterranean region. Most of the rainfall associated with COLs comes from the baroclinic shield;
specially in cases located over the west half of the Iberian Peninsula. It is shown that nearly 30% of COLs do not induce
any rainfall; most of them located in the southern half of the Peninsula, and mainly during autumn. Only 30% of COLs produce
generalized rainfall over the whole analysed territory, being most of them (about 90%) located over the western half of the
Iberian Peninsula. 相似文献
85.
M. Hoyos V. Soler J. C. Cañaveras S. Sánchez-Moral E. Sanz-Rubio 《Environmental Geology》1998,33(4):231-242
The Candamo Cave contains an important group of paleolithic paintings which have been seriously deteriorated due to mass tourism.
In this work, an analysis was carried out of different climatic parameters (CO2, temperature, humidity, 222Rn) during annual cycles with the cave closed to the public and during an experimental period of controlled visits. The effect
of visits on the geochemical characteristics of karstic water was also analyzed together with the cave ventilation.
The natural variations in the cave air CO2 were above 3000 ppm, the increase produced through visits was only 100–110 ppm and since the humidity is almost permanently
at saturation point, the critical parameter which limits the visitor capacity becomes air temperature. The temperature changes
during the annual cycle are of the order of 1 °C in the external part and less than 0.5 °C in the internal part of the cave
and a maximum increase of 0.13 °C was observed during the period of the visits. The 222Rn and CO2 concentration minimums in the summer period (July–October) show that this is the most propitious time for visits, since the
greatest ventilation is produced in the cave at this time and, therefore, the greatest capacity for recovery. The geochemistry
of the water, on the other hand, indicated that this is the period of the year in which processes of wall corrosion can be
most easily introduced, although this would be of limited magnitude. The visitor capacity calculated was 29 visitors/day.
Received: 29 August 1996 · Accepted: 23 June 1997 相似文献
86.
B. John Makario Londoño A. John Jairo Sanchez E. Luis Eduardo Toro Fernando Gil Cruz Olga Patricia Bohorquez 《Bulletin of Volcanology》1998,59(8):556-561
Coda Q–1 was calculated at Nevado del Ruiz Volcano (NRV) before and after two phreatomagmatic eruptions (November 1985, September
1989) and for a period of stability (May 1987) using a functional form for coda derived from a single scattering model (Sato
1977). Substantial changes were found before and after the eruptions. The highest value of Q–1 was found during the November 1985 eruption, an intermediate value for the September 1989 eruption, and the lowest value
for May 1987. It seems that the changes in coda Q–1 at NRV have a still-unknown relationship with the volume or magnitude of the eruption. A relatively strong frequency dependence
was found for all periods. Also Q–1 clearly changed with time, suggesting that the scattering was strong for the eruption of November 1985 and decreased for
the eruption of September 1989, and that the intrinsic absorption probably increased. This suggests the possibility that crystallization
is taking place in the NRV magma. The clear change of coda Q–1 before and after the eruptions at NRV also suggests the possibility that coda Q–1 is a premonitory tool of activity at this volcano.
Received: 25 October 1996 / Accepted: 21 January 1998 相似文献
87.
It is known that a sharp decrease in the angular velocity of the accretion disk around a black hole could in principle produce a kinematic axisymmetric dynamo, in contrast to the classical situation described by Cowling's antidynamo theorem. Here the effect of a nontrivial poloidal velocity of the disk is studied, showing that a strong gradient of this velocity enhances the possibilities of a working dynamo. 相似文献
88.
89.
An analysis was carried out of the secular period variation of four WU Ma type stars (AW UMa, AP Leo, AG Vir and AH Vir) using
some newly determined times of minima as well as others reported in the literature. It was found that two stars show period
increases, one a period decrease and the remaining shows no variation in period.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
90.
Taal Volcano is peculiar in its violent explosivity in spite of its low altitude. The surrounding topographies suggest that the origin of Taal Volcano is either a caldera or a graben structure. To confirm the caldera origin one must search for a vast quantity of caldera ejecta balancing with the depression. As a first step, a gravity survey was carried out on and around Taal Volcano, and high gravity anomalies were lound on Volcano Island. The distribution of the gravities may suggest a graben structure overlying a denser layer of igneous material. 相似文献