全文获取类型
收费全文 | 1569篇 |
免费 | 57篇 |
国内免费 | 11篇 |
专业分类
测绘学 | 72篇 |
大气科学 | 132篇 |
地球物理 | 355篇 |
地质学 | 504篇 |
海洋学 | 129篇 |
天文学 | 277篇 |
综合类 | 6篇 |
自然地理 | 162篇 |
出版年
2021年 | 11篇 |
2020年 | 16篇 |
2019年 | 21篇 |
2018年 | 37篇 |
2017年 | 39篇 |
2016年 | 42篇 |
2015年 | 31篇 |
2014年 | 44篇 |
2013年 | 86篇 |
2012年 | 59篇 |
2011年 | 65篇 |
2010年 | 68篇 |
2009年 | 81篇 |
2008年 | 72篇 |
2007年 | 69篇 |
2006年 | 55篇 |
2005年 | 56篇 |
2004年 | 44篇 |
2003年 | 44篇 |
2002年 | 45篇 |
2001年 | 27篇 |
2000年 | 46篇 |
1999年 | 26篇 |
1998年 | 27篇 |
1997年 | 21篇 |
1996年 | 25篇 |
1995年 | 21篇 |
1994年 | 13篇 |
1993年 | 23篇 |
1992年 | 25篇 |
1991年 | 14篇 |
1990年 | 24篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 23篇 |
1983年 | 27篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 13篇 |
1974年 | 10篇 |
1973年 | 10篇 |
1972年 | 20篇 |
1971年 | 13篇 |
1969年 | 9篇 |
排序方式: 共有1637条查询结果,搜索用时 15 毫秒
611.
Provenance,tectonic setting and source of Archean metasedimentary rocks of the Browns Range Metamorphics,Tanami Region,Western Australia 总被引:1,自引:0,他引:1
T. Nazari-Dehkordi C. Spandler N. H. S. Oliver J. Chapman R. Wilson 《Australian Journal of Earth Sciences》2017,64(6):723-741
This study combines U–Pb age and Lu–Hf isotope data for magmatic and detrital zircons, with whole-rock geochemistry of the Browns Range Metamorphics (BRM), Western Australia. The BRM are medium- to coarse-grained metasandstones that consist of angular to sub-rounded detrital quartz and feldspars with minor granitic lithic fragments. The sequence has undergone partial to extensive quartz–muscovite alteration and rare-earth-element mineralisation and has been intruded by mafic/ultramafic, syenitic and pegmatitic intrusive rock units. Uranium–Pb and Lu–Hf isotopic data on detrital zircons from the metasandstones and intruding granitic rocks yield a well-defined age of ca 3.2 to ca 3.0 Ga for all samples, with relatively radiogenic ?Hf values (?Hf = –1.7 to 5.1) indicating derivation from Mesoarchean granite basement of juvenile origin. This is consistent with geochemical and petrological data that support deposition from a granitic source in a continental rift basin setting. The timing of sediment deposition is constrained between the ca 3.0 Ga age of the source rocks and ca 2.5 Ga age of the granitic intrusive bodies that cross-cut the metasedimentary rocks. The ca 2.5 Ga zircons from the intrusive rocks have ?Hf model ages of ca 3.4 to ca 3.1 Ga, which is consistent with formation via partial melting of the BRM, or the Mesoarchean granite basement. Zircons of the Gardiner Sandstone that unconformably overlies the BRM return detrital ages of ca 2.6 to ca 1.8 Ga with no trace of ca 3.1 Ga zircons, which discounts a significant contribution from the underlying BRM. The Mesoarchean age and isotopic signatures of the BRM zircons are shared by some zircon records from the Pine Creek Orogen, and the Pilbara, Yilgarn and Gawler cratons. Collectively, these records indicate that juvenile Mesoarchean crust is a more significant component of Australian cratons than is currently recognised. This work also further demonstrates that detrital minerals in Paleoproterozoic/Archean sedimentary rocks are archives to study the early crustal record of Earth. 相似文献
612.
613.
Mengnjo Jude Wirmvem Takeshi Ohba Linus Anye Nche Brice Tchakam Kamtchueng Wiylahnyuy Edith Kongnso Mumbfu Ernestine Mimba Tasin Godlove Bafon Muga Yaguchi Gloria Eneke Takem Wilson Yetoh Fantong Ako Andrew Ako 《Environmental Earth Sciences》2017,76(9):354
High water demand for domestic use in Douala with over 3 million inhabitants is met mainly by shallow groundwater. Field measurements and water sampling in January 2015 were carried out to examine the major controls on the groundwater composition and spatial view of ions in the water, timing of recharge and link between the recharge process and quality of the water. Fifty-two water samples were analysed for major ions and stable hydrogen and oxygen isotopes. Low pH values (3.61–6.92) in the groundwater indicated an acidic aquifer; thus, prone to acidification. The dominant water type was Na–Cl. Nitrate, which exceeded the WHO guide value of 50 mg/l in 22% of the groundwater, poses a health problem. Mass ratios of Cl?/Br? in the water ranged from 54 to 3249 and scattered mostly along the mixing lines between dilute waters, septic-tank effluent and domestic sewage. A majority of the samples, especially the high NO3 ? shallow wells, clustered around the septic-tank effluent-end-member indicating high contamination by seepage from pit latrines; hence, vulnerable to pollution. Stable isotopes in the groundwater indicated its meteoric origin and rapid infiltration after rainfall. The δ18O values showed narrow ranges and overlaps in rivers, springs, open wells and boreholes. These observations depict hydraulic connectivity, good water mixing and a homogeneous aquifer system mainly receiving local direct uniform areal recharge from rainfall. The rapid and diffused recharge favours the leaching of effluent from the pit toilets into the aquifer; hence, the high NO3 ? and Cl? in shallow wells. Silicate weathering, ion exchange and leaching of waste from pit toilets are the dominant controls on the groundwater chemistry. Drilling of deep boreholes is highly recommended for good-quality water supply. However, due the hydraulic connection to the shallow aquifer, geochemical modelling of future effects of such an exploitation of the deeper aquifer should support groundwater management and be ahead of the field actions. 相似文献
614.
615.
616.
Marine biodiversity and ethnobiodiversity of Bellona (Mungiki) Island, Solomon Islands 总被引:2,自引:2,他引:0
R.R. Thaman Tepaikea Puia Wilson Tongabaea Ashley Namona Teddy Fong 《Singapore journal of tropical geography》2010,31(1):70-84
This paper reports on the marine biodiversity and ethnobiodiversity of Bellona, a small island in Solomon Islands inhabited by Polynesians who have, for centuries, depended on biodiversity for their own sustainability. The Bellonese have names for at least 8 whales and dolphins, 7 reptiles, over 500 finfish, 191 molluscs, 48 crustaceans, 29 echinoderms and a range of corals, other invertebrates and marine plants, most of which have commercial, subsistence or cultural value. If conserved, this inheritance will continue to provide a foundation for continuing sustainability in a rapidly globalizing world. The paper highlights the importance of the preservation, and application to development, of ethnobiodiversity for food and subsistence security, and contingent issues of conservation and sustainability in small island developing states. As the current extinction crisis escalates globally, the rate of attrition of the intangible indigenous knowledge that has coevolved with this threatened biodiversity is probably far greater. In itself, the recording of information on biodiversity and related sustainable livelihoods is not sufficient to ensure sustainability. There is a vital need to integrate this into the formal education system and applied scientific activities at all levels to achieve the right balance between agriculture, wild harvest and trade, upon which sustainability and subsistence affluence still depend. 相似文献
617.
Carlos J. Archanjo Patrick Launeau Maria Helena B. M. Hollanda José Wilson P. Macedo Dunyi Liu 《International Journal of Earth Sciences》2009,98(8):1793-1807
Anisotropy of magnetic susceptibility (AMS) applied to an alkaline granite from Meruoca (NE Brazil) recorded weak anisotropies,
typically below 4%, and a considerable dispersion of the AMS axes. Red-clouded feldspars and clots of metasomatic minerals
enclosed in magmatic crystals indicate that hydrothermal fluids altered the granite. U–Pb isotopic data show high-common Pb
on zircons but allowed the calculation of a mean SHRIMP age of 523 ± 9 Ma attributed to the magmatic crystallization. Growth
of fine oxides by late fluid–rock interactions was responsible for the scattering of AMS. Rock magnetic data indicate they
consist mainly of an oxidized magnetite and (titano)hematite. Shape preferred orientation of mafic aggregates measured in
granite quarries shows that the pluton preserves a gently dipping magmatic foliation. AMS in some quarries with a well-defined
magmatic fabric, however, remains highly dispersed. When AMS mimics the mafic shape fabric, only magnetic foliations share
a common orientation. Locally, AMS grounded in coarse Ti-poor magnetite associated with titanite develops a consistent subhorizontal
oblate fabric that agrees with tectonic models suggesting that the cupola of the pluton has been exposed by erosion. 相似文献
618.
Rachel Marie Wilson Jeffrey Chanton Graham Lewis Douglas Nowacek 《Estuaries and Coasts》2009,32(5):999-1010
Stable isotope ratios of carbon and sulfur were used to assess organic matter utilization of numerically abundant consumers
present in Apalachicola Bay, FL, USA. These results were used to infer nitrogen isotopic enrichment of organic matter sources
in an effort to establish baseline δ
15N enrichment for trophic evaluations. We compared results from concentration-independent and concentration-dependent mixing
models and found that the two methods resulted in widely different conclusions about the importance of organic matter sources
that varied ninefold in sulfur concentrations. Nitrogen isotopic enrichment was used to determine relative trophic positions
of consumer organisms. Source elemental concentrations of nitrogen were also considered in the calculation of relative trophic
levels in a concentration-dependent approach. Concentration-independent and concentration-dependent methods of calculating
trophic results were compared. While relative trophic levels of individual species varied continuously from approximately
1.7 to 3.5, comparisons of trophic level among consumers indicated four possible trophic groupings. Filter feeders (mussels
and oysters) made up the lowest trophic tier while teleost fishes made up the highest trophic tier. Invertebrates sampled
were assigned intermediate nondiscrete trophic levels. Because δ
15N values of important organic matter sources in the system were similar, the concentration-independent and concentration-dependent
methods did not result in significantly different conclusions about trophic level for any of the consumers examined. However,
a comparison of the two methods applied to a hypothetical case found that differences in base δ
15N values ranging approximately 4‰ resulted in significantly different trophic-level assignments when comparing the concentration-dependent
and concentration-independent methods of trophic-level calculations. Our results confirm that consideration of the elemental
concentrations of the base organisms is an important factor in determining source contributions and may affect trophic-level
calculations in systems with a sufficient range of base nitrogen enrichment. However, this result depended on the relative
isotopic signatures of the chosen sources and their elemental concentrations and should be considered individually for each
system. 相似文献
619.
Accuracy of Laser Ablation U-Pb Zircon Dating: Results from a Test Using Five Different Reference Zircons 总被引:2,自引:0,他引:2
Urs Klötzli Eva Klötzli Zekeriya Günes Jan Kosler 《Geostandards and Geoanalytical Research》2009,33(1):5-15
Using a state‐of‐the‐art 193 nm‐LA‐MC‐ICP‐MS system and with careful control of analytical procedures, the long term external reproducibility and accuracy of the ages Phanerozoic zircons measured over a period of months using calibrator bracketing for the 206Pb/238U and 207Pb/206Pb ages were ca. 1% (2 RSD) if a single reference zircon was used for the matrix‐matched calibration. When different reference zircons were used for the calibration, suspicious systematic shifts in the obtained ages were observed and thus a reduction in the overall accuracy of the dating method became obvious. Such shifts were within a few percent range of the U‐Pb and Pb/Pb ages and seemed to vary independently of zircon age and composition. A “test of accuracy” experiment was conducted reducing instrumental effects as far as possible by analysing five different reference zircons mounted on a single mount eight times during the same session. An identical protocol was used for all analyses, with unchanged instrument parameters and with ion beam intensities kept as identical as possible. For data reduction, every zircon served consecutively as the reference zircon for calibration, with the others in the batch treated as unknowns. The known reference age and the four calculated ages obtained using the four other RMs for calibration were then compared. Even using such a strict analytical protocol, shifts in 206Pb/238U, 207Pb/235U and 207Pb/206Pb ratios were still present. They varied non‐systematically and ranged from ?4.35% to 3.08% for the investigated age range (1065 Ma to 226 Ma). Assuming the absence of instrumental effects (i.e., memory, dead‐time correction, non‐linearity of ion counters and interdetector calibration, crystallographic orientation, ablation cell geometry and setup, gas flows), the observed shifts were attributed to matrix and/or ablation related effects. It is proposed that non‐spectral matrix effects in the Ar plasma torch resulted in non‐uniform signal enhancement (or depression?) leading to shifts both in elemental and Pb isotopic ratios. Additionally, the ablated particle size distribution could be an important factor controlling plasma conditions and thus mass bias and fractionation. Until such effects are well understood and controlled, it would seem that any LA‐ICP‐MS zircon U‐Pb and 207Pb/206Pb age determination cannot be meaningfully interpreted at below a ca. 3% to 4% (2 RSD) confidence level. 相似文献
620.
The aim of this study is to improve our knowledge of the attenuation structure in the Southern Apennines using a new amplitude
ratio tomography method (Phillips et al., Geophys Res Lett 32(21):L21301, 2005) applied on both direct and coda envelope measurements
derived from 150 events recorded by 47 stations of the Istituto Nazionale di Geofisica e Vulcanologia National Seismic Network
(Rete Sismica Nazionale Centralizzata). The two-dimensional (2-D) analysis allows us to take into account lateral crustal
variations and heterogeneities of this region. Using the same event and station distribution, we also applied a simple 1-D
methodology, and the performance of the 1-D and 2-D path assumptions is tested by comparing the average interstation variance
for the path-corrected amplitudes using coda and direct waves. In general, coda measurement results are more stable than using
direct waves when the same methodology is applied. Using the 2-D approach, we observe more stable results for both waves.
However, the improvement is quite small, probably because the crustal heterogeneity is weak. This means that, for this region,
the 1-D path assumption is a good approximation of the attenuation characteristics of the region. A comparison between Q tomography images obtained using direct and coda amplitudes shows similar results, consistent with the geology of the region.
In fact, we observe low Q along the Apennine chain toward the Tyrrhenian Sea and higher values to the east, in correspondence with the Gargano zone
that is related to the Apulia Carbonate Platform. Finally, we compared our results with the coda Q values proposed by Bianco et al. (Geophys J Int 150:10–22, 2002) for the same region. The good agreement validates our results
as the authors used a completely independent methodology. 相似文献