全文获取类型
收费全文 | 30400篇 |
免费 | 626篇 |
国内免费 | 314篇 |
专业分类
测绘学 | 734篇 |
大气科学 | 2392篇 |
地球物理 | 6601篇 |
地质学 | 10575篇 |
海洋学 | 2543篇 |
天文学 | 6192篇 |
综合类 | 51篇 |
自然地理 | 2252篇 |
出版年
2020年 | 194篇 |
2019年 | 191篇 |
2018年 | 385篇 |
2017年 | 375篇 |
2016年 | 527篇 |
2015年 | 392篇 |
2014年 | 536篇 |
2013年 | 1461篇 |
2012年 | 660篇 |
2011年 | 1008篇 |
2010年 | 850篇 |
2009年 | 1116篇 |
2008年 | 1052篇 |
2007年 | 980篇 |
2006年 | 1026篇 |
2005年 | 876篇 |
2004年 | 913篇 |
2003年 | 854篇 |
2002年 | 850篇 |
2001年 | 690篇 |
2000年 | 693篇 |
1999年 | 665篇 |
1998年 | 629篇 |
1997年 | 636篇 |
1996年 | 539篇 |
1995年 | 532篇 |
1994年 | 506篇 |
1993年 | 472篇 |
1992年 | 451篇 |
1991年 | 388篇 |
1990年 | 468篇 |
1989年 | 374篇 |
1988年 | 410篇 |
1987年 | 464篇 |
1986年 | 390篇 |
1985年 | 563篇 |
1984年 | 644篇 |
1983年 | 627篇 |
1982年 | 509篇 |
1981年 | 504篇 |
1980年 | 510篇 |
1979年 | 469篇 |
1978年 | 456篇 |
1977年 | 427篇 |
1976年 | 443篇 |
1975年 | 396篇 |
1974年 | 429篇 |
1973年 | 422篇 |
1972年 | 262篇 |
1971年 | 207篇 |
排序方式: 共有10000条查询结果,搜索用时 906 毫秒
781.
The effect of CaO and MgO, with or without TiO2 and P2O5, on the two-melt field in the simplified system Fe2SiO4–KAlSi3O8–SiO2 has been experimentally determined at 1,050°–1,240°C, 400 MPa. Despite the suppressing effect of MgO, CaO, and pressure on silicate melt immiscibility, our experiments show that this process is still viable at mid-crustal pressures when small amounts (0.6–2.0 wt%) of P2O5 and TiO2 are present. Our data stress that the major element partition coefficients between the two melts are highly correlated with the degree of polymerisation (nbo/t) of the SiO2-rich melt, whatever temperature, pressure, or exact composition. Experimental immiscible melt compositions in natural systems at 0.1 MPa from the literature (lunar and tholeiitic basalts) plot on similar but distinct curves compared to the simplified system. These relations between melt polymerisation and partition coefficients, which hold for a large range of compositions and fO2, are extended to various volcanic and plutonic rocks. This analysis strengthens the proposal that silicate melt immiscibility can be important in volcanic rocks of various compositions (from tholeiitic basalts to lamprophyres). However, the majority of proposed immiscible compositions in plutonic rocks are at least not coexisting melts, but may have suffered accumulation of early crystallized minerals. 相似文献
782.
Summary Lenses and pods of mafic rocks from the Monotonous Unit near Svetlik are characterized by eclogite facies mineral assemblages;
however some inclusion patterns (oriented quartz rods in clinopyroxene and cuboids of disordered graphite in garnet) that
are usually known from ultra-high pressure rocks were also observed in one sample. Conventional thermobarometry yielded maximum
P–T conditions of 2.0–2.5 GPa and 750 °C. Decompression and heating at amphibolite and granulite facies conditions resulted in
the formation of at least five distinct types of symplectites. These include symplectitic intergrowth of ilmenite and clinopyroxene
after titanite, described here for the first time from the Moldanubian Zone. In addition, symplectites of plagioclase and
biotite with accessory amounts of spinel after tabular pseudomorphs (after phengite?) are also reported here. Mass balance
relations indicate that symplectites of diopside + plagioclase after omphacite and plagioclase + spinel (sapphirine) after
kyanite + garnet, formed by nearly isochemical reactions. All other symplectite-forming reactions were allochemical and were
accelerated by the presence of fluid in the primary phases. Preserved zoning pattern in garnet with high compositional gradient
in some samples suggests that the rocks were affected briefly by granulite facies overprint. 相似文献
783.
Whereas certain linkages between stream channel morphology and stream ecology are fairly well-understood, how geomorphology influences trophic interactions remains largely unknown. As a first step, a simple, heuristic model is developed that couples reach-scale geomorphic morphology with trophic dynamics between vegetation, detritus, herbivores, and predators. Predation is assumed to increase with depth beyond a threshold depth, and herbivory is assumed to decrease with velocity beyond a threshold velocity. Results show that the modeled food chain is sensitive to channel geometry, particularly around the threshold conditions for predators and herbivores. Importantly, geomorphic influences are not isolated to a particular trophic level, but rather are transferred through the food chain via top-down and bottom-up effects. The modeled system is particularly sensitive to changes in the end-members of the food chain: vegetation and predators. Results illustrate that geomorphic disturbances, known to affect a single trophic level (e.g., fish), likely impact multiple trophic levels in the stream ecosystem via trophic interactions. Such impacts at the multiple trophic level are poorly understood. While limited by the lack of empirical long-term data for testing and calibration, this simple model provides a structure for generating hypotheses, collecting targeted data, and assessing the potential impacts of stream disturbance or restoration on entire stream ecosystems. Further, the model illustrates the potential for future coupled stream models to explore spatial and temporal linkages. 相似文献
784.
Flow and form in rehabilitation of large-river ecosystems: An example from the Lower Missouri River 总被引:2,自引:0,他引:2
On large, intensively engineered rivers like the Lower Missouri, the template of the physical habitat is determined by the nearly independent interaction of channel form and flow regime. We evaluated the interaction between flow and form by modeling four combinations of modern and historical channel form and modern and historical flow regimes. The analysis used shallow, slow water (shallow-water habitat, SWH, defined as depths between 0 and 1.5 m, and current velocities between 0 and 0.75 m/s) as an indicator of habitat that has been lost on many intensively engineered rivers and one that is thought to be especially important in rearing of young fishes. Two-dimensional hydrodynamic models for modern and historical channels of the Lower Missouri River at Hermann, Missouri, indicate substantial differences between the two channels in total availability and spatial characteristics of SWH. In the modern channel, SWH is maximized at extremely low flows and in overbank flows, whereas the historical channel had substantially more SWH at all discharges and SWH increased with increasing discharge. The historical channel form produced 3–7 times the SWH area of the modern channel regardless of flow regime. The effect of flow regime is evident in increased within-year SWH variability with the natural flow regime, including significant seasonal peaks of SWH associated with spring flooding. Comparison with other reaches along the Lower Missouri River indicates that a) channel form is the dominant control of the availability of habitat even in reaches where the hydrograph is more intensively altered, and b) rehabilitation projects that move toward the historical condition can be successful in increasing topographic diversity and thereby decreasing sensitivity of the availability of habitat to flow regime. The relative efficacy of managing flow and form in creating SWH is useful information toward achieving socially acceptable rehabilitation of the ecosystem in large river systems. 相似文献
785.
Mongolian gerbils (Meriones unguiculatus) and Daurian pikas (Ochotona dauurica) are two key small mammal species in the Inner Mongolian grasslands, China. Mongolian gerbils displayed density-dependent population growth, but the time lag of density dependence was short and within season. The spring–autumn population growth rate was inversely related to population density in Mongolian gerbils of the Erdos desert grasslands. The autumn–spring population growth rate of Mongolian gerbils was inversely related to winter precipitation. Precipitation had stronger effects on the population growth of gerbils during the non-breeding season. The monthly population growth rate of Daurian pikas was positively related to the monthly precipitation and was inversely related to population density in central Inner Mongolia. Daurian pikas select habitats with tall plants. Increased precipitation enhances plant production and increases the height of plants in arid and semi-arid regions. This enhancement of plant height might increase the population growth of Daurian pikas. Mongolian gerbils live in short grasslands. Increased winter precipitation might result in higher winter mortality of Mongolia gerbils, or taller plants might supress gerbil population growth in wet years. Therefore, responses of small mammal species to changes in precipitation depend on the life history of small mammals and changes in vegetation induced by climatic changes. 相似文献
786.
Robert Gilbert Sarah Crookshanks Kyle R. Hodder John Spagnol Roland B. Stull 《Journal of Paleolimnology》2006,35(4):737-745
Severe rainfall in mid October, 2003 produced the largest floods in almost a century of record on rivers in the Cordillera
of southwestern British Columbia. Sediment deposited in Lillooet Lake as a result of this event is clearly distinguished by
stratigraphy, colour, texture, magnetic properties, and organic content. Each of these physical properties is related to the
lacustrine processes, especially turbid underflow, that distributed the sediment through the lake. The flood, which lasted
less than a week, delivered 8–12 times the amount of sediment that accumulates in most entire years in the deepest, central
parts of the lake. Recognition of events of this type in the stratigraphic record offers a means of assessing the changing
nature of extreme hydroclimatic events, and their relation to more ubiquitous, lower-energy processes. 相似文献
787.
David W. Kelley Stefanie A. Brachfeld Edward A. Nater Herbert E. Wright Jr. 《Journal of Paleolimnology》2006,35(1):193-206
Sediments from Lake Pepin on the Mississippi River, southeastern Minnesota, are used as provenance tracers to assess variations
in hydrology and sediment-transport during the middle Holocene. Three rivers contribute sediment to Lake Pepin, and each catchment
is characterized by a distinctly different geologic terrain. The geochemical fingerprint for each drainage basin was determined
from the elemental composition of heavy minerals in the silt-sized fraction of modern sediment samples. Down-core elemental
abundances were compared with these fingerprints by use of a chemical-mass-balance model that apportions sediment to the source
areas. We observed a decreased contribution from the Minnesota River during the interval ~6700–5500 14C yr BP, which we attribute to decreased discharge of the Minnesota River, likely controlled by a combination of precipitation,
snow melt, and groundwater input to the river. This hydrologic condition coincides with the mid-Holocene prairie period recorded
by fossil pollen data. The occurrence of this feature in a proxy record for hydrologic variations supports the hypothesis
that the mid-Holocene prairie period reflects drier conditions than before or after in midwestern North America. 相似文献
788.
Georg Schettler Jens Mingram Jörg F. W. Negendank Liu Jiaqi 《Journal of Paleolimnology》2006,35(2):271-288
Palaeovariations of the atmospheric 210Pb flux in Northeast China (Long Gang area, Jilin province), quantified by high resolution 210Pb measurements on seasonally laminated sediments of Lake Sihailongwan are presented on decadal scale. The mean flux of unsupported
210Pb between 1790 and 1970 for the centre of the lake basin is 517 Bq m−2 yr−1 with maximum deviations between −23% and +27% of this value. Flux rates above this average were found between 1783 and 1813,
around 1836, and between 1860 and 1901. The mean 210Pbexc flux rates derived were clearly lower around 1821, 1908, 1930, and 1953. The 210Pbexc flux reached its minimum of 344 Bq m−2 yr−1 during the period 1977–1982. The atmospheric flux of unsupported 210Pb is correlated with the precipitation frequency during the summer monsoon and shows coinciding variability with geochemical
proxies that document the groundwater inflow into the lake. Al2O3-rich dust of remote provenance scavenged by wet-deposition in the rainy season is the major carrier of the atmospheric 210Pbexc flux. 相似文献
789.
790.