全文获取类型
收费全文 | 41603篇 |
免费 | 629篇 |
国内免费 | 254篇 |
专业分类
测绘学 | 844篇 |
大气科学 | 2861篇 |
地球物理 | 8203篇 |
地质学 | 15388篇 |
海洋学 | 3688篇 |
天文学 | 8989篇 |
综合类 | 109篇 |
自然地理 | 2404篇 |
出版年
2022年 | 257篇 |
2021年 | 440篇 |
2020年 | 499篇 |
2019年 | 576篇 |
2018年 | 1308篇 |
2017年 | 1314篇 |
2016年 | 1200篇 |
2015年 | 657篇 |
2014年 | 1112篇 |
2013年 | 1873篇 |
2012年 | 1349篇 |
2011年 | 1720篇 |
2010年 | 1448篇 |
2009年 | 1805篇 |
2008年 | 1651篇 |
2007年 | 1700篇 |
2006年 | 1568篇 |
2005年 | 1541篇 |
2004年 | 1714篇 |
2003年 | 1520篇 |
2002年 | 1031篇 |
2001年 | 870篇 |
2000年 | 797篇 |
1999年 | 723篇 |
1998年 | 715篇 |
1997年 | 723篇 |
1996年 | 588篇 |
1995年 | 572篇 |
1994年 | 501篇 |
1993年 | 453篇 |
1992年 | 409篇 |
1991年 | 424篇 |
1990年 | 437篇 |
1989年 | 391篇 |
1988年 | 369篇 |
1987年 | 400篇 |
1986年 | 415篇 |
1985年 | 508篇 |
1984年 | 545篇 |
1983年 | 542篇 |
1982年 | 496篇 |
1981年 | 454篇 |
1980年 | 429篇 |
1979年 | 408篇 |
1978年 | 375篇 |
1977年 | 384篇 |
1976年 | 344篇 |
1975年 | 353篇 |
1974年 | 340篇 |
1973年 | 370篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Oliver E. Jagoutz 《Contributions to Mineralogy and Petrology》2010,160(3):359-381
Results of simple model calculations that integrate cumulate compositions from the Kohistan arc terrain are presented in order
to develop a consistent petrogenetic model to explain the Kohistan island arc granitoids. The model allows a quantitative
approximation of the possible relative roles of fractional crystallization and assimilation to explain the silica-rich upper
crust composition of oceanic arcs. Depending in detail on the parental magma composition hydrous moderate-to-high pressure
fractional crystallization in the lower crust/upper mantle is an adequate upper continental crust forming mechanism in terms
of volume and compositions. Accordingly, assimilation and partial melting in the lower crust is not per se a necessary process
to explain island arc granitoids. However, deriving few percent of melts using low degree of dehydration melting is a crucial
process to produce volumetrically important amounts of upper continental crust from silica-poorer parental magmas. Even though
the model can explain the silica-rich upper crustal composition of the Kohistan, the fractionation model does not predict
the accepted composition of the bulk continental crust. This finding supports the idea that additional crustal refining mechanism
(e.g., delamination of lower crustal rocks) and/or non-cogenetic magmatic process were critical to create the bulk continental
crust composition. 相似文献
972.
The Mississippi Valley-type (MVT) Pb–Zn ore district at Mežica is hosted by Middle to Upper Triassic platform carbonate rocks
in the Northern Karavanke/Drau Range geotectonic units of the Eastern Alps, northeastern Slovenia. The mineralization at Mežica
covers an area of 64 km2 with more than 350 orebodies and numerous galena and sphalerite occurrences, which formed epigenetically, both conformable
and discordant to bedding. While knowledge on the style of mineralization has grown considerably, the origin of discordant
mineralization is still debated. Sulfur stable isotope analyses of 149 sulfide samples from the different types of orebodies
provide new insights on the genesis of these mineralizations and their relationship. Over the whole mining district, sphalerite
and galena have δ
34
S values in the range of –24.7 to –1.5‰ VCDT (–13.5 ± 5.0‰) and –24.7 to –1.4‰ (–10.7 ± 5.9‰), respectively. These values are
in the range of the main MVT deposits of the Drau Range. All sulfide δ
34
S values are negative within a broad range, with δ
34
S
pyrite <δ
34
S
sphalerite <δ
34
S
galena for both conformable and discordant orebodies, indicating isotopically heterogeneous H2S in the ore-forming fluids and precipitation of the sulfides at thermodynamic disequilibrium. This clearly supports that
the main sulfide sulfur originates from bacterially mediated reduction (BSR) of Middle to Upper Triassic seawater sulfate
or evaporite sulfate. Thermochemical sulfate reduction (TSR) by organic compounds contributed a minor amount of 34S-enriched H2S to the ore fluid. The variations of δ
34
S values of galena and coarse-grained sphalerite at orefield scale are generally larger than the differences observed in single
hand specimens. The progressively more negative δ
34
S values with time along the different sphalerite generations are consistent with mixing of different H2S sources, with a decreasing contribution of H2S from regional TSR, and an increase from a local H2S reservoir produced by BSR (i.e., sedimentary biogenic pyrite, organo-sulfur compounds). Galena in discordant ore (–11.9
to –1.7‰; –7.0 ± 2.7‰, n = 12) tends to be depleted in 34
S compared with conformable ore (–24.7 to –2.8‰, –11.7 ± 6.2‰, n = 39). A similar trend is observed from fine-crystalline sphalerite I to coarse open-space filling sphalerite II. Some variation
of the sulfide δ
34
S values is attributed to the inherent variability of bacterial sulfate reduction, including metabolic recycling in a locally
partially closed system and contribution of H2S from hydrolysis of biogenic pyrite and thermal cracking of organo-sulfur compounds. The results suggest that the conformable
orebodies originated by mixing of hydrothermal saline metal-rich fluid with H2S-rich pore waters during late burial diagenesis, while the discordant orebodies formed by mobilization of the earlier conformable
mineralization. 相似文献
973.
Effects of natural and calcined oyster shells on Cd and Pb immobilization in contaminated soils 总被引:5,自引:0,他引:5
Yong Sik Ok Sang-Eun Oh Mahtab Ahmad Seunghun Hyun Kwon-Rae Kim Deok Hyun Moon Sang Soo Lee Kyoung Jae Lim Weon-Tai Jeon Jae E. Yang 《Environmental Earth Sciences》2010,61(6):1301-1308
In Korea, soils adjacent to abandoned mines are commonly contaminated by heavy metals present in mine tailings. Further, the
disposal of oyster shell waste by oyster farm industries has been associated with serious environmental problems. In this
study, we attempted to remediate cadmium (Cd)- and lead (Pb)-contaminated soils typical of those commonly found adjacent to
abandoned mines using oyster shell waste as a soil stabilizer. Natural oyster shell powder (NOSP) and calcined oyster shell
powder (COSP) were applied as soil amendments to immobilize Cd and Pb. The primary components of NOSP and COSP are calcium
carbonate (CaCO3) and calcium oxide (CaO), respectively. X-ray diffraction, X-ray fluorescence and scanning electron microscope analyses conducted
in this study revealed that the calcination of NOSP at 770°C converted the less reactive CaCO3 to the more reactive CaO. The calcination process also decreased the sodium content in COSP, indicating that it was advantageous
to use COSP as a liming material in agricultural soil. After 30 days of incubation, we found that the 0.1 N HCl-extractable
Cd and Pb contents in soil decreased significantly as a result of an increase in the soil pH and the formation of metal hydroxides.
COSP was more effective in immobilizing Cd and Pb in the contaminated soil than NOSP. Overall, the results of this study suggest
that oyster shell waste can be recycled into an effective soil ameliorant. 相似文献
974.
Volcanogenic massive sulfide deposits in ophiolite complexes are usually attributed to the Cyprus type. They associate with
basaltic volcanics that are formed in mid-ocean or back-arc spreading centers and much less frequently in intra-plate settings.
The deposits are characterized by copper or copper-zinc ores that are enriched in Ni, Co, and in places Mn and As, but are
very poor in Pb and demonstrate a low to moderate content of Ag and Au. Typically, the deposits are low to very low in ore
and metal reserves. Cyprus-type deposits were irregularly distributed during geological history. The most ancient of them
were formed in the Neoproterozoic, while the bulk of the deposits are Ordovician or Cretaceous in age. Their possible Paleoproterozoic
analogues can be found in the Svecofennian belt (Outokumpu ore district), while modern ones are confined to the Explorer and
Endeavour Ridges and southern segment of the Juan de Fuca Ridge. 相似文献
975.
E. A. Avvakumova 《Astronomy Reports》2010,54(1):17-27
We searched for signs of the presence of circumstellar gaseous matter in photometric data for massive contact early-type binaries
by analyzing residual curves (the dependence of the difference between the observed and theoretical brightness variations
on the orbital-period phase) for three such stars. The residual curves make it possible to estimate the influence of gas in
the common envelope on the observed light curves for different phase intervals and to qualitatively describe the character
of the distortion of the light from the system’s components. Changes of the residual curves from filter to filter indicate
varying conditions in the circumstellar matter. Changes of the residual curves from one observation epoch to another indicate
varying conditions in the circumstellar matter. We compared the residual curves obtained for different photometric bands and
epochs via a correlation analysis. The distortion of light from the components of LY Aurigae in the ultraviolet differs from
that in the visual. The distortion of light from the components of SV Centauri is appreciable, but not selective, and does
not vary in time, while the distortion of light from BH Centauri possesses a strong selective component. A comparison of the
radii computed for the components of BH Centauri and SV Centauri shows that the gas distribution near these binaries varies
in time. 相似文献
976.
977.
This paper reports the results of petrographic and geochemical studies of the Miocene-Pleistocene volcanic rocks that accompanied
the formation of the deep-water basins of the Sea of Japan and Sea of Okhotsk. The geochemical types of these rocks, their
geodynamic settings, and their derivation from different magmatic sources were determined. The marginal-sea basaltoids of
the Sea of Japan are derivatives of fluid-enriched mantle (EMI), while the volcanics of the Kurile basin were generated from
mantle enriched in continental crust (EMII). In spite of the different conditions of their genesis, they share some common
geochemical features, in particular, their calc-alkaline signatures. These traces of the influence of sialic crust on magma
generation confirm the development of the basins of both these seas on continental basement. 相似文献
978.
Yong Wang Qi-Long Miao Chong-Yi E Jian-Kang Han Yuan-Yuan Ding 《Environmental Earth Sciences》2010,60(6):1257-1266
The δ18O data obtained from an 18.7 m ice core drilled in Chongce Ice Cap at an elevation of 6,530 m a.s.l. in the West Kunlun Mountains
on the northern Tibetan Plateau show a strong correlation with the summer temperature of the middle to upper troposphere over
the mid-high latitudes of Eurasia. Based on this, the δ18O record can be used as a proxy of the June–September mean temperature of the mid-upper troposphere (MUT) from 1903 to 1992.
The time span of the ice core record is much longer than the meteorological data available only after 1948. Using the empirical
mode decomposition method (EMD), the δ18O record is decomposed into various frequency components and compared with the solar irradiance variations of the same period.
The results show that (1) The June–September mean temperature of the MUT over Eurasian mid-high latitudes is completely decomposed
into four IMF (intrinsic mode function) components and an increasing trend. (2) Solar irradiance is decomposed into the Schwabe
cycle, the Hale cycle, the Gleissberg cycle, and an increasing trend. (3) The correlation coefficients between the June and
September mean temperatures of the MUT over Eurasian mid-high latitudes and solar irradiance on the longer timescales (at
least more than 11-year) show the significant correlations; their phase changes are basically identical in general, and (4)
the 11-year Schwabe cycle exists in the June–September mean temperature of the MUT over Eurasian mid-high latitudes during
most of the time from 1903 to 1992, and only in the two high-temperature phases (1929–1944 and from 1975 to the present) may
global warming disturb this relation. A full understanding of this phenomenon would shed insight into the potential consequence
of global warming on the MUT. 相似文献
979.
S. V. Kalenskii S. Kurtz V. I. Slysh P. Hofner C. M. Walmsley L. E. B. Johansson P. Bergman 《Astronomy Reports》2010,54(10):932-939
We present the results of VLA observations of a maser candidate in the low-mass star formation region L1157 in the 70-61
A
+ transition at 44 GHz. The line is emitted by a compact, undoubtedly maser source associated with clump B0a, which is seen
in maps of L1157 in thermal lines of methanol and other molecules. A much weaker compact source is associated with clump B1a,
which is brighter than B0a in thermal methanol lines. The newly detected masers may form in thin layers of turbulent post-shock
gas. In this case, the maser emission may be beamed, so that only an observer located in or near the planes of the layers
can observe strong masers. On the other hand, the maser lines are double with a “red” asymmetry, indicating that the masers
may form in collapsing clumps. A detailed analysis of collapsing-cloud maser models and their applicability to the masers
in L1157 will be developed in subsequent papers. 相似文献
980.
I. Yu. Melekestseva V. A. Kotlyarov P. V. Khvorov V. N. Ivanov V. E. Beltenev I. G. Dobretsova 《Geology of Ore Deposits》2010,52(8):800-810
The porous fine-grained to microcrystalline copper-zinc ore of the Semenov-2 hydrothermal field, a site in the Semenov hydrothermal
cluster discovered in 2007 (13°31′N, MAR), is anomalously enriched in Au (22–188 ppm) and Ag (127–1787 ppm). Chalcopyrite,
isocubanite, würtzite, and opal are major minerals; sphalerite, marcasite, pyrite, and covellite are auxiliary; and galena,
pyrrhotite, native gold, silver telluride, barite, and aragonite are sporadic. Gold containing 0.31 to 23.07 wt % Ag occurs
as up to 9-μm-sized subhedral, dendritelike, and elongated grains mostly hosted in opal and less frequently in sphalerite
and in pores within isocubanite-chalcopyrite aggregate. An elongated grain (2 × 4 μm in size) of the Ag-Te phase was found
in a pore. So far only basalts have been dredged from the Semenov-2 field, but anomalous gold and silver concentrations suggest
the influence of ultramafic rocks; the latter were found 1.5 km westward, in the Semenov-1 hydrothermal field. Mineral assemblage
and morphology of gold particles indicate its primary origin in contrast to the hydrothermal fields hosted in basalts, where
gold is a product of remobilization. Zonal gold grains, found on oceanic floor for the first time, are characterized by low
Ag content in the cores and high Ag content in the outer rims, reflecting variation in formation conditions. 相似文献