全文获取类型
收费全文 | 916篇 |
免费 | 21篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 21篇 |
大气科学 | 76篇 |
地球物理 | 193篇 |
地质学 | 342篇 |
海洋学 | 70篇 |
天文学 | 150篇 |
综合类 | 2篇 |
自然地理 | 95篇 |
出版年
2021年 | 5篇 |
2020年 | 13篇 |
2019年 | 12篇 |
2018年 | 7篇 |
2017年 | 22篇 |
2016年 | 22篇 |
2015年 | 16篇 |
2014年 | 19篇 |
2013年 | 43篇 |
2012年 | 23篇 |
2011年 | 39篇 |
2010年 | 37篇 |
2009年 | 42篇 |
2008年 | 38篇 |
2007年 | 38篇 |
2006年 | 27篇 |
2005年 | 23篇 |
2004年 | 25篇 |
2003年 | 27篇 |
2002年 | 30篇 |
2001年 | 17篇 |
2000年 | 13篇 |
1999年 | 8篇 |
1998年 | 15篇 |
1997年 | 15篇 |
1996年 | 15篇 |
1995年 | 18篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 13篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 18篇 |
1986年 | 11篇 |
1985年 | 16篇 |
1984年 | 14篇 |
1983年 | 18篇 |
1982年 | 27篇 |
1981年 | 17篇 |
1980年 | 14篇 |
1979年 | 20篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 10篇 |
1973年 | 9篇 |
1972年 | 7篇 |
排序方式: 共有949条查询结果,搜索用时 31 毫秒
91.
The zooplankton community of Mission Bay, San Diego, California, was monitored over two years, to study spatial and temporal
patterns and the response of zooplankton species composition to environmental variation. Data were collected every two weeks
from six stations and included hydrographic parameters, dissolved nutrient concentrations, and phytoplankton and zooplankton
species composition. Hydrography varied seasonally, along a spatial gradient from the mouth to the back of the bay, and between
the two years around the influence of rainfall. Spatially, Mission Bay could be divided into three regions during this study
based on hydrography and zooplankton species composition. Zooplankton species composition followed a predictable seasonal
progression, with different groups of species being characteristic to particular times of the year. Variability in zooplankton
species composition was also evident between years, as certain species were more common in one or the other year of the study.
Spatial patterns were more consistent than temporal ones, and related to distance from the mouth of the bay during much of
the year and distance from freshwater inlets during the relatively short rainy season. Multivariate analysis revealed that
variation in zooplankton species composition was best related to measured abiotic factors (temperature, salinity, rainfall,
and tidal velocity). 相似文献
92.
Pleistocene tectonics inferred from fluvial terraces of the northern Upper Rhine Graben, Germany 总被引:1,自引:0,他引:1
This study of fluvial terraces of the River Rhine and tributaries aims to search for indications of Pleistocene tectonic activity. The study area includes the northern Upper Rhine Graben (URG), the Mainz Basin and the adjacent Rhenish Massif with the Middle Rhine Valley. High rates of Quaternary surface processes, large amount of human modifications, relatively slow tectonic deformation and presently low intra-plate seismic activity characterize this area. Therefore, the records of relatively slow tectonic deformation are less well preserved and thus difficult to detect. This study uses the relative position of fluvial terraces to determine the more local effects of fault movements on the terraces and to evaluate their displacement rates and patterns. The research is based on a review of previous terrace studies and new terrace mapping from the eastern Mainz Basin and the bordering URG using topographic map interpretations and field observations. This newly mapped sequence of terrace surfaces can be correlated to other terraces in the vicinity on the basis of relative height levels. Terrace correlation between the western Mainz Basin and Middle Rhine Valley relies on a single chronostratigraphic unit (Mosbach sands) and additional relative height correlations. This is the first study to present a continuous correlation of terraces from the western margin of the URG to the Rhenish Massif and enables the study of the transition from the subsiding graben to the uplifted Rhenish Massif. By means of a longitudinal profile, which ranges from the URG to the Rhenish Massif, the influence of individual fault movements on the terrace levels and the large-scale regional uplift is demonstrated. It is evident from the profile that the uplift of Early to Middle Pleistocene terraces increases northwards, towards the Rhenish Massif. The uplift was diachronic, with a significant pulse occurring first in the northern URG (Lower Pleistocene) and later in the Rhenish Massif (Middle Pleistocene). The largest vertical displacements are recorded for the boundary fault separating the Mainz Basin and the Rhenish Massif (Hunsrück–Taunus Boundary Fault) and for faults bounding the northeastern Mainz Basin. The motions and displacement rates calculated for individual faults indicate deformation rates in the order of 0.01–0.08 mm/year. At this stage, the calculation of displacement rates depends mostly on a single dated stratigraphic unit. Additional dating of terrace deposits is urgently needed to better constrain the temporal development of the terrace sequence and the impact of tectonic movements. 相似文献
93.
Yongping YUAN Ronald BINGNER Randall WILLIAMS Richard LOWRANCE David BOSCH Joe SHERIDAN 《国际泥沙研究》2007,22(1):60-69
The United States Department of Agriculture (USDA) Annualized Agricultural Non-Point Source Pollution model (AnnAGNPS) is used to help evaluate a watershed response to agricultural management practices to control water quality. However, AnnAGNPS version 3.5 does not contain features to estimate the effect of a riparian buffer (RB) system on water quality. The Riparian Ecosystem Management Model (REMM) is used to simulate the impact of riparian buffer systems on water quality. However, frequently the lack of measured upland loadings that are required by REMM simulation limits the application of REMM. To address this data gap, a study was conducted to integrate AnnAGNPS with REMM for RB system simulation. AnnAGNPS was used to simulate water and sediment loadings from an upland field into a three-zone RB system at the Gibbs Farm located in the Georgia coastal plain. These AnnAGNPS outputs were used as the inputs to REMM. REMM was used to simulate water and sediment movement along the riparian buffers. The AnnAGNPS simulated amount of annual runoff at the edge of the field was close to observed amounts (Nash-Sutcliffe efficiency of 0.92). It is believed that a substantial portion of sand was removed from the runoff one meter into the grass buffer where the samplers were located; therefore, sand was excluded from the AnnAGNPS simulation for comparison with observed sediment. Excluding sand, the AnnAGNPS predicted amount of annual sediment matches the observed amount fairly well (Nash-Sutcliffe efficiency of 0.46). In addition, based on evaluating the percent reduction of sediment at each zonal interface, the AnnAGNPS/REMM model well simulated the function of the RB system to reduce sediment. 相似文献
94.
An intensive approach to Barrow, Alaska’s adaptations to climate change and variability during recent decades suggests reconsideration
of the interconnected roles of science, policy, and decision-making structures. First, profound uncertainties are inherent
in unique interactions among the many natural and human factors affecting Barrow’s vulnerability. Science cannot significantly
reduce these uncertainties through extensive approaches, but intensive approaches can reconstruct and update local trends,
clarify the underlying dynamics, and harvest experience for policy purposes. Second, sound policies to reduce Barrow’s vulnerability
to coastal erosion and flooding must incorporate these profound uncertainties and the multiple values of the community. Minimizing
vulnerability to climate change is only one of the community’s interests, and must compete with other interests for limited
time, attention, funds and other resources. Third, the community itself is in the best position to understand its own context,
to decide on sound policies, and to take responsibility for those decisions. In short, local context matters in science, policy,
and decision-making structures for adaptation to climate change and variability. Overall, cognitive constraints may be the
most important human dimension of climate change. Factoring the global problem into more tractable local problems would make
the most of our cognitive capacity. 相似文献
95.
Teh-Lung Ku Shangde Luo Tim K. Lowenstein Jianren Li Ronald J. Spencer 《Quaternary Research》1998,50(3):261-275
Uranium-series dating on a 186-m core (DV93-1) drilled from Badwater Basin in Death Valley, California, and on calcareous tufas from nearby strandlines shows that Lake Manly, the lake that periodically flooded Death Valley during the late Pleistocene, experienced large fluctuations in depth and chemistry over the last 200,000 yr. Death Valley has been occupied by a long-standing deep lake, perennial shallow saline lakes, and a desiccated salt pan similar to the modern valley floor. The average sedimentation rate of about 1 mm/yr for core DV93-1 was punctuated by episodes of more-rapid accumulation of halite. Arid conditions similar to the modern conditions prevailed during the entire Holocene and between 120,000 and 60,000 yr B.P. From 35,000 yr B.P. to the beginning of the Holocene, a perennial saline lake existed, over 70 m at its deepest. A much deeper and longer lasting perennial Lake Manly existed from about 185,000 to 128,000 yr B.P., with water depths reaching about 175 m, if not 330 m. This lake had two significant “dry” excursions of 102–103yr duration about 166,000 and 146,000 yr B.P., and it began to shrink to the point of halite precipitation between 128,000 and 120,000 yr B.P. The two perennial lake periods correspond to marine oxygen isotopic stages (OIS) 2 and 6. Based on the shoreline tufa ages, we do not rule out the possible existence 200,000 yr ago of yet a third perennial lake comparable in size to the OIS 6 lake. The234U/238U data suggest that U in tufa owes its origin mainly to Ca-rich springs fed by groundwater that emanated along lake shorelines in southern Death Valley, and that an increase of this spring-water input relative to the river-water input apparently occurred during OIS 6. 相似文献
96.
A. D. Beard H. Downes E. Hegner S. M. Sablukov V. R. Vetrin K. Balogh 《Contributions to Mineralogy and Petrology》1998,130(3-4):288-303
Minor magmatic intrusions of kimberlite, melilitite and cpx-melilitite occur in the southern part of the Kola Peninsula,
Russia, on the Terskii Coast and near the town of Kandalaksha. They yield K-Ar ages of 382 ± 14 Ma and 365 ± 16 Ma, similar
to the magmatic rocks from the Kola Alkaline Province. The Terskii Coast kimberlites have mineralogical and geochemical affinities
with group 1 kimberlites, whereas the Kandalaksha monticellite kimberlite more closely resembles calcite kimberlites. The
lower Al2O3 content in the Kola kimberlites indicates a strongly depleted harzburgitic source, while higher Al2O3 in the melilitites suggests a lherzolitic source. The Terskii Coast kimberlites are anomalously potassic and significantly
enriched in P and Ba compared to other group 1 kimberlites. In contrast, the melilitites are sodic and are anomalously depleted
in P compared to worldwide melilitites. Trace element patterns of the Kola kimberlites and melilitites indicate the presence
of K- and P-rich phases in the mantle source. To account for the K-troughs shown by both magma types, a K-rich phase such
as phlogopite is thought to be residual in their sources; however, the anomalous K-enrichment in the Terskii Coast kimberlites
may indicate that an additional metasomatic K-rich phase (e.g. K-richterite and/or a complex K-Ba-phosphate) existed in the
kimberlite source. The P-depletion in the melilitites may suggest that a phosphate phase such as apatite remained residual
in the melilititic source. However, anomalous P-enrichment in the kimberlites cannot be explained by complete melting of the
same phase because the kimberlites are a smaller degree melt; thus, it is most likely that another metasomatic phosphate mineral
existed in the source of the kimberlites. The Kola kimberlites and melilitites are all strongly LREE-enriched but the kimberlites
have a steeper REE pattern and are significantly more depleted in HREE, indicating a higher proportion of garnet in their
source. Higher Nb/Y ratios and lower SiO2 values in the kimberlites indicate that they were a smaller degree partial melt than the melilitites. The presence of diamonds
in the Terskii Coast kimberlites indicates a relatively deep origin, while the melilitites originated from shallower depth.
The non-diamondiferous Kandalaksha monticellite kimberlite has lower abundances of all incompatible trace elements, suggesting
a higher degree of partial melting and/or a less enriched and shallower source than the Terskii Coast kimberlites. The 87Sr/86Sri, 143Nd/144Ndi and Pb isotope compositions confirm that the Terskii Coast kimberlites have close affinities with group 1 kimberlites and
were derived from an asthenospheric mantle source, while the Kandalaksha monticellite kimberlite and Terskii Coast melilitites
were derived from lithospheric mantle. Impact of a Devonian asthenospheric mantle plume on the base of the Archaean-Proterozoic
lithosphere of the Kola Peninsula caused widespread emplacement of kimberlites, melilitites, ultramafic lamprophyres and other
more fractionated alkaline magmas. The nature of the mantle affected by metasomatism associated with the plume and, in particular,
the depth of melting and the stability of the metasomatic phases, gave rise to the observed differences between kimberlites
and the related melilitites and other magmas.
Received: 3 March 1997 / Accepted: 7 October 1997 相似文献
97.
Simon J. Woodings rew J. Williams Ralph Martin Ronald R. Burman & David G. Blair 《Monthly notices of the Royal Astronomical Society》1998,301(1):L5-L9
Following the proposal by Damineli that the central object of Eta Carinae may be an early-type binary, we perform numerical simulations of the X-ray emission from colliding stellar winds. A synthetic light curve has been generated which qualitatively agrees with the recent X-ray variability, and provides further support for the binary model. In particular, the model predicts a rise in the observed X-ray emission towards periastron, followed by a sharp drop and subsequent recovery. This is indeed what is seen in the RXTE light curve, although some problems concerning the X-ray spectrum at periastron still need to be explained. The simulations suggest that the width of the periastron dip will provide strong constraints on the binary and stellar wind properties of the components of Eta Car. 相似文献
98.
The emission of gas from the earth's crust is a complex process influenced by meteorological and seasonal processes which must be understood for effective application of gas emission to geochemical exploration. Free mercury vapor emission and radon emanation are being measured in a shallow instrument vault at a single nonmineralized site in order to evaluate these influences on gas emission.Mercury concentrations in the instrument vault average 9.5 ng/m3 and range from < 1 ng/m3 to 53 ng/m3 with a strong seasonal effect. Mercury has a direct relationship to vault temperature, air temperature, soil temperature, barometric pressure, water table, and the frozen or thawed state of the soil. Air and soil temperature, barometric pressure, and relative humidity are most important in influencing mercury emission while soil moisture is also important in radon emanation. Diurnal cycles are common but do not occur on all days. A heavy precipitation event on a dry soil seals the soil resulting in a rise in mercury concentration. Precipitation on a soil that is already wet does not increase mercury emission because of the compensation caused by lowering of the soil temperature by the precipitation event. Freezing of the soil changes the physical state of the vault-soil-soil gas-atmosphere system and emits the lowest concentrations of mercury. Phase lag effects are likely important. Stepwise multiple regression of mercury as dependent variable with meteorological and seasonal parameters as independent variables gives a cumulative R value of 0.563 and R2 of 0.317. The short-term noise coupled with phase lags are an important factor.The radon measurements integrated over weekly intervals smooth out much of the short-term noise. Stepwise multiple regression of radon as dependent variable with meteorological and seasonal parameters as independent variables gives a cumulative R value of 0.967 and R2 of 0.934. In this portion of the study the variation in the radon emanation is adequately predicted by meteorological and seasonal parameters. 相似文献
99.
Ronald T. Merrill 《Surveys in Geophysics》1975,2(3):277-311
Experimental evidence and theory indicate that chemical changes occur in many igneous rocks at sufficiently low temperatures to significantly affect the remanent magnetization. Some chemical changes lead to self-reversals of magnetization that are not reproducible in laboratory experiments. Such self-reversals appear to be very rare in subaerially-erupted basalts, but they probably are much more common in some other rock types, such as granites and diorites. The stability of the natural remanent magnetization in igneous rocks can be decreased, left unaltered, or increased by chemical changes. In addition, chemical changes will usually affect the intensity of magnetization in a rock; the intensity can increase, decrease, or (rarely) be left unaltered by a chemical change. Such changes are important to consider in the development of improved techniques for obtaining reliable estimates of the intensity of the Earth's magnetic field in the past and in correctly interpreting marine magnetic anomalies. Finally, experiments and theory are presented which suggest that many of the chemical changes in igneous rocks will only occasionally produce significant changes in the direction of the magnetization. 相似文献
100.