全文获取类型
收费全文 | 3876篇 |
免费 | 160篇 |
国内免费 | 39篇 |
专业分类
测绘学 | 125篇 |
大气科学 | 340篇 |
地球物理 | 846篇 |
地质学 | 1381篇 |
海洋学 | 317篇 |
天文学 | 669篇 |
综合类 | 17篇 |
自然地理 | 380篇 |
出版年
2023年 | 17篇 |
2022年 | 25篇 |
2021年 | 72篇 |
2020年 | 98篇 |
2019年 | 93篇 |
2018年 | 111篇 |
2017年 | 139篇 |
2016年 | 152篇 |
2015年 | 112篇 |
2014年 | 137篇 |
2013年 | 219篇 |
2012年 | 143篇 |
2011年 | 200篇 |
2010年 | 170篇 |
2009年 | 205篇 |
2008年 | 181篇 |
2007年 | 136篇 |
2006年 | 136篇 |
2005年 | 135篇 |
2004年 | 130篇 |
2003年 | 123篇 |
2002年 | 115篇 |
2001年 | 89篇 |
2000年 | 75篇 |
1999年 | 50篇 |
1998年 | 55篇 |
1997年 | 48篇 |
1996年 | 48篇 |
1995年 | 48篇 |
1994年 | 34篇 |
1993年 | 33篇 |
1992年 | 34篇 |
1991年 | 31篇 |
1990年 | 38篇 |
1989年 | 26篇 |
1988年 | 30篇 |
1987年 | 37篇 |
1986年 | 32篇 |
1985年 | 43篇 |
1984年 | 41篇 |
1983年 | 46篇 |
1982年 | 45篇 |
1981年 | 47篇 |
1980年 | 40篇 |
1979年 | 26篇 |
1978年 | 23篇 |
1977年 | 25篇 |
1976年 | 27篇 |
1975年 | 27篇 |
1974年 | 28篇 |
排序方式: 共有4075条查询结果,搜索用时 0 毫秒
11.
Jon M. Friedrich Hannah L. McLain Jason P. Dworkin Daniel P. Glavin W. Henry Towbin Morgan Hill Denton S. Ebel 《Meteoritics & planetary science》2019,54(1):220-228
X‐ray microcomputed tomography (μCT) is a useful means of characterizing cosmochemical samples such as meteorites or robotically returned samples. However, there are occasional concerns that the use of μCT may be detrimental to the organic components of a chondrite. Small organic compounds such as amino acids comprise up to ~10% of the total solvent extractable carbon in CM carbonaceous chondrites. We irradiated three samples of the Murchison CM carbonaceous chondrite under conditions akin to and harsher than those typically used during typical benchtop X‐ray μCT imaging experiments to determine if detectable changes in the amino acid abundance and distribution relative to a nonexposed Murchison control sample occurred. After subjecting three meteorite samples to ionizing radiation dosages between ~300 Gray (Gy) and 3 kGy with bremstrahlung X‐rays, we analyzed the amino acid content of each sample. Within sampling and analytical errors, we cannot discern differences in the amino acid abundances and amino acid enantiomeric ratios when comparing the control samples (nonexposed Murchison) and the irradiated samples. We conclude that a polychromatic X‐ray μCT experiment does not alter the abundances of amino acids to a degree greater than how well those abundances are measured with our techniques and therefore any damage to amino acids is minimal. 相似文献
12.
13.
Aaron T. Fricke Charles A. Nittrouer Andrea S. Ogston Daniel J. Nowacki Nils E. Asp Pedro W. M. Souza Filho 《地球表面变化过程与地形》2019,44(1):204-218
Depositional environments along the tidal river downstream of Óbidos have been proposed as important sinks for up to one third of the sediment discharge from the Amazon River. However, the morphology of the intertidal floodplain and the dynamics of sediment exchange along this reach have yet to be described. River-bank surveys in five regions along the Amazon tidal river reveal a distinct transition in bank morphology between the upper, central and lower reaches of the tidal river. The upper tidal-river floodplain is defined by prominent natural levees that control the transfer of water and sediment between the mainstem Amazon River and its floodplain. Greater tidal influence in the central tidal river suppresses levee development, and tidal currents increase sediment transport into the distal parts of the floodplain. In the lower tidal river, the floodplain morphology closely resembles marine intertidal environments (e.g. mud flats, salt marshes), with dendritic tidal channels incising elevated vegetated flats. Theory, morphology and geochronology suggest that the dynamics of sediment delivery to the intertidal floodplain of the Amazon tidal river vary along its length due to the relative influence and coupling of fluvial and tidal dynamics. © 2018 John Wiley & Sons, Ltd. 相似文献
14.
15.
Daniel Leduc Ashley A. Rowden Conrad A. Pilditch Elizabeth W. Maas P. Keith Probert 《Marine Ecology》2013,34(3):334-344
Studying the diversity‐ecosystem function relationship in the deep sea is of primary importance in the face of biodiversity loss and for our understanding of how the deep sea functions. Results from the first study of diversity‐ecosystem function relationships in the deep sea (Danovaro et al. 2008; Current Biology, 18, 1–8) are unexpected and show an exponential relationship between deep‐sea nematode diversity and ecosystem function and efficiency, although this relationship appears largely restricted to relatively low diversities [ES(51) <25]. Here, we investigate the relationship between nematode diversity and several independent measures/proxies of ecosystem function (sediment community oxygen consumption, bacterial biomass, bacterial extracellular enzyme activity) and efficiency (ratio of bacterial/nematode carbon to organic C content of the sediment) on the New Zealand continental slope. Nematode diversity at our study sites was relatively high [ES(51) = 30–42], and there was no relationship between species/functional diversity and ecosystem function/efficiency after accounting for the effects of water depth and food availability. Our results are consistent with a breakdown of the exponential diversity‐function relationship at high levels of diversity, which may be due to increased competition or greater functional redundancy. Future studies need to take into account as many environmental factors and as wide a range of diversities as possible to provide further insights into the diversity‐ecosystem function relationship in the largest ecosystem on Earth. 相似文献
16.
Julien Berger Renaud Caby Jean-Paul Liégeois Jean-Claude C. Mercier Daniel Demaiffe 《Contributions to Mineralogy and Petrology》2011,162(4):773-796
We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower
and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90–130-Ma-long evolution of magmatic inputs
and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost
mantle depths (25–30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source
slightly enriched by slab-derived sedimentary components ((La/Sm)N < 1; εNd: +5.4–6.2; 87Sr/86Sr: 0.7027–0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850–1,000°C;
10–12 kbar) and subject to local dehydration–melting reactions, forming trondhjemititic leucosomes with garnet–clinopyroxene–rutile
residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment
faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg
andesitic parental magma. Quartz and hornblende-gabbros (700–660 Ma) with composition typical of hydrous volcanic rocks from
mature arcs ((La/Sm)N: 0.9–1.8; εNd: +4.6 to +5.2; 87Sr/86Sr: 0.7028–0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that
magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred
either in two stages (700–660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T
path (550°C, 6–9 kbar; epidote–amphibolite and greenschist facies conditions) during the main Pan-African collision event
(620–580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation
are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and
Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong
to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent,
a similar story having occurred on the other side of the Sahara with the Mozambique Ocean. 相似文献
17.
Humans constitute one of the main geomorphological agents in modern times. As an example, post-mining regions represent a typical landscape of the Anthropocene. Strong relief modifications are particularly obvious with open pit mining. However, many existing mining areas are lacking detailed pre-mining information for the quantification of anthropogenic relief changes, which is a considerable challenge in regions with historic mining activities. Here, the Ville (Rhenish lignite district, Germany) is used to quantify surface mining induced relief changes in one of the oldest and currently largest lignite districts in Europe. Historical maps from first geodetic mapping in 1893 enabled construction of a historic digital elevation model to quantify the relief changes in comparison to elevation data from 2000 and 2015. The vertical accuracy of the historic data is remarkably high, with relief differences < 2 m in areas not affected by mining. In total, 49.2% of the investigated area (184 km2) shows a relief deficit and 14.5% has positive relief differences. Absolute changes account for more than 80 m heightening (dumpsites of overburden) and lowering of the natural relief (pits). Besides these altitudinal changes, overall steeper slopes are significant for the new topography, but levelling exists likewise. The spatial variabilities are discussed in the context of the regional geology and the mining techniques. Undoubtedly, such large-scale anthropogenic relief changes persist for a very long time and will last as a human legacy far into the future. Only the detailed reconstruction of the pre-mining relief offers the ability to clarify the dimension of humans as geomorphological agents and to understand landscape perception. Due to the fact that the impact of open pit mining has such a large vertical and horizontal extension, their consideration as part of anthropogeomorphology can significantly contribute to support future Critical Zone research in the Anthropocene. 相似文献
18.
A three-dimensional nonlinear numerical model, that has been extensively used previously to predict environmental water flows, was applied to predict the flow over an isolated hill, Askervein. Predictions are reported for winds approaching the hill from 210 ° and 180 ° clockwise from north, both under almost neutral atmospheric conditions.The model predictions were compared with data collected during a major field study in 1983. From the comparisons it was concluded that the model predicts the mean flow variables with good accuracy. Larger discrepancies were found for quantities related to the turbulence, pointing to deficiencies in the turbulence model, and perhaps in some of the measurements. 相似文献
19.
An area in the Fenlands of Eastern England was used to assess the spectral, spatial, and seasonal requirements of airborne multispectral data for identifying wetland archaeological features by detecting crop and soil marks. Ordination of data from a scanner with 11 spectral channels was achieved using the Sheffield method, which calculates the wavebands that produce a 3 band composite image with optimum contrast using band variances and interband correlations. the spatial detail requirements for multispectral data were investigated by applying an edge enhancing filter to single waveband images. the loss of feature clarity with increased levels of spatial smoothing is visually apparent. Seasonal variation in crop mark visibility was quantified by cross tabulating feature visibility in fields known to contain features with time period and sensor type. If remote sensing is to be used for repeated operational archaeological survey then the quality and quantity of information which can be gained from the data must be evaluated. This study attempts to define some of the necessary operational requirements for the use of multispectral data for archaeology. 相似文献
20.