全文获取类型
收费全文 | 1250篇 |
免费 | 69篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 106篇 |
地球物理 | 256篇 |
地质学 | 529篇 |
海洋学 | 134篇 |
天文学 | 164篇 |
综合类 | 1篇 |
自然地理 | 115篇 |
出版年
2022年 | 3篇 |
2021年 | 21篇 |
2020年 | 23篇 |
2019年 | 26篇 |
2018年 | 29篇 |
2017年 | 38篇 |
2016年 | 47篇 |
2015年 | 44篇 |
2014年 | 38篇 |
2013年 | 72篇 |
2012年 | 48篇 |
2011年 | 75篇 |
2010年 | 54篇 |
2009年 | 59篇 |
2008年 | 66篇 |
2007年 | 60篇 |
2006年 | 63篇 |
2005年 | 60篇 |
2004年 | 40篇 |
2003年 | 42篇 |
2002年 | 38篇 |
2001年 | 26篇 |
2000年 | 31篇 |
1999年 | 28篇 |
1998年 | 15篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 9篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 13篇 |
1991年 | 16篇 |
1990年 | 4篇 |
1989年 | 10篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 6篇 |
1985年 | 21篇 |
1984年 | 14篇 |
1983年 | 11篇 |
1982年 | 10篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1977年 | 3篇 |
1976年 | 7篇 |
1975年 | 11篇 |
1974年 | 6篇 |
1973年 | 8篇 |
排序方式: 共有1324条查询结果,搜索用时 15 毫秒
101.
We use a simple approach to estimate the present-day thermal regime along the northwestern part of the Western Indian Passive Margin, offshore Pakistan. A compilation of bottom borehole temperatures and geothermal gradients derived from new observations of bottom-simulating reflections (BSRs) allows us to constrain the relationship between the thermal regime and the known tectonic and sedimentary framework along this margin. Effects of basin and crustal structure on the estimation of thermal gradients and heat flow are discussed. A hydrate system is located within the sedimentary deep marine setting and compared to other provinces on other continental margins. We calculate the potential radiogenic contribution to the surface heat flow along a profile across the margin. Measurements across the continental shelf show intermediate thermal gradients of 38–44 °C/km. The onshore Indus Basin shows a lower range of values spanning 18–31 °C/km. The Indus Fan slope and continental rise show an increasing gradient from 37 to 55 °C/km, with higher values associated with the thick depocenter. The gradient drops to 33 °C/km along the Somnath Ridge, which is a syn-rift volcanic construct located in a landward position relative to the latest spreading center around the Cretaceous–Paleogene transition. 相似文献
102.
Kenneth Neil Mertens Manuel Bringué Nicolas Van Nieuwenhove Yoshihito Takano Vera Pospelova Andre Rochon Anne De Vernal Taoufik Radi Barrie Dale R. Timothy Patterson Kaarina Weckström Elinor Andrén Stephen Louwye Kazumi Matsuoka 《第四纪科学杂志》2012,27(7):734-744
Process length variation of cysts of the dinoflagellate Protoceratium reticulatum (Claparède et Lachmann) Bütschli in surface sediments from the North Pacific was investigated. The average process length showed a significant inverse relation to annual seawater density: σt annual = ?0.8674 × average process length + 1029.3 (R2 = 0.84), with a standard error of 0.78 kg m?3. A sediment trap study from Effingham Inlet in British Columbia revealed the same relationship between average process length and local seawater density variations. In the Baltic–Skagerrak region, the average process length variation was related significantly to annual seawater density: σt annual = 3.5457 × average process length ? 993.28 (R2 = 0.86), with a standard error of 3.09 kg m?3. These calibrations cannot be reconciled, which accentuates the regional character of the calibrations. This can be related to variations in molecular data (small subunit, long subunit and internal transcribed spacer sequences), which show the presence of several genotypes and the occurrence of pseudo‐cryptic speciation within this species. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
103.
Deposition in the New Jersey Pinelands was very acidic (pH=4.17) and contained high levels of SO2
−2 based on bulk deposition measurementsfrom July 1984–July 1986. Streamwater over the same interval in undisturbed watersheds
was less acidic (pH =4.52) and had proportionately less SO4
−2. A preliminary alkalinity budget for undisturbed watersheds suggested that SO4
−2 retention within Pinelands watersheds accounted for a large portion of the total alkalinity generated and thereby lessened
the impact of acidic deposition on surface waters. The only process capable of explaining the retention of SO4
−2 was microbial sulfate reduction in the extensive wetlands surrounding Pinelands streams which occurred at high rates. 相似文献
104.
Anne F. Skinner 《Quaternary Science Reviews》1988,7(3-4)
Marine aragonite, in the form of corals and/or shells, provides useful markers of geological and archaeological events. It is, therefore, important to have simple and accurate methods of dating these materials. Electron spin resonance (ESR) has previously been shown to be a reliable method for establishing the age of aragonitic coral samples in the time period approximately 100 ka B.P. The primary purpose of the present work is to discuss the problems encountered in extending this method to considerably older samples, up to 600 ka BP in age. In this time period there are questions about the stability of the ESR signal. The samples investigated are aragonitic corals from reef terraces of Barbados, West Indies, all of which have previously been dated by the
methods, and by U-series disequilibrium, for samples below the limit of this method. There is generally good agreement for samples up to about 300 ka in age; older samples, even unrecrystallized, appear younger when dated by ESR than by
. The source of this discrepancy is not clear. The explanation of thermal fading is not adequate. However, it appears likely that in most cases ESR will be able to be used to date materials up to this age. Further investigation is needed to determine tests that will distinguish datable samples from non-datable ones. 相似文献
105.
Thomas E. Young Harry W. Green II Anne M. Hofmeister David Walker 《Physics and Chemistry of Minerals》1993,19(6):409-422
Wadsleyite (β-(Mg,Fe)2SiO4) is a major constituent of the Earth's transition zone and is known to accommodate OH. The portion of the transition zone between 400–550 km could be an important source or sink for hydroxyl in plumes and slabs intersecting this region. Micro-infrared spectroscopy has been carried out on the β-phase and coexisting metastable olivine synthesized in a multianvil apparatus at 14 GPa and 1550–1650 K under hydrous conditions. Single-crystal and polycrystal specimens of both phases were analyzed in the 1800–8500 cm?1 frequency region to determine the speciation, abundances, and partitioning behavior of the hydrous components in coexisting β-phase and olivine. β-phase spectra consistently show three distinct OH bands at 3329, 3580, and 3615 cm?1. OH concentrations range from 10000–65000 H/106 Si. A strong positive correlation of grain size and extent of transformation with OH concentration in the β-phase indicates that grain-growth and transformation rates are enhanced in a hydrous environment. Olivine spectra are variable, but consistently show a prominent broad-band absorbance representing molecular H2O, consistent with the infrared signature of the starting material. OH concentrations in olivine range from <300–1400 H/106 Si. The highest OH concentrations measured for olivine and the β-phase may represent solubility limits, in which case the OH solubility ratio between these two phases is approximately 1∶40. Where both phases coexist and are undersaturated with OH, the partitioning ratio of OH between them is about 1∶100. The large solubility contrast between olivine and the β-phase suggests a mechanism for hydrating the transition zone via olivine carried down in subducting slabs. Plumes impinging on an OH-rich upper transition region could cause H2 or H2O to be released upon transformation of the β-phase to olivine, resulting in initiation of secondary upwellings. If dissolution of OH weakens the β-phase, and if OH is present in the mantle, the region between 400–550 km could be a zone of low viscosity. 相似文献
106.
To determine the genetic structure of the bay anchovy (Anchoa mitchilli) within Chesapeake Bay, 16 isozyme systems encoding 21 loci for 20 population were examined using horizontal starch gel electrophoresis. Contingency Chisquare analysis revealed significant allelic frequency differences at nine loci (AAT-1, AAT-2, ALD-1, CPK-2, GAP-1, GLY-1, LDH-1, MDH-1, and MDH-2). Two loci, ALD-1 and MDH-1, were responsible for nine of 14 tests not conforming to Hardy-Weinberg expectations, with some of these deviations attributed to possible scoring and/or sampling error. Estimates for mean average heterozygosity were relatively high, ranging from 0.40 to 0.096, with 33–57% of the loci polymorphic. A low Fst value (0.041) along with high genetic identity estimates (I=0.997) indicated little substructuring of bay anchovy populations within Chesapeake Bay. 相似文献
107.
108.
John W. Morgan Marie-Josée Janssens H. Takahashi Jan Hertogen Edward Anders 《Geochimica et cosmochimica acta》1985,49(1):247-259
We have analyzed 10 H-chondrites for 20 trace elements, using RNAA. The meteorites included 4 of petrologic type 4 and 2 each of types 3, 5 and 6.The data show that H-chondrites are not isochemical. H3's are depleted by some 10% not only in Fe (Dodd, 1976), but also in the siderophiles Os, Re, Ir, Ni, Pd, Au, and Ge. Moreover, the abundance pattern of siderophiles varies systematically with petrologic type. As similar fractionations of REE have been observed by Nakamura (1974), it appears that both the proportions and compositions of the main nebular condensates varied slightly during accretion of the H-chondrites. Thus the higher petrologic types are independent nebular products, not metamorphosed descendants of lower petrologic types.Abundances of highly volatile elements (Cs, Br, Bi, Tl, In, Cd, Ar36) correlate with petrologic type, declining by ≤ 10?3 from Type 3 to Type 6. The trends differ from those for artificially heated Type 3's (Ikramuddinet al., 1977b; Herzoget al., 1979), but agree passably with theoretical curves for nebular condensation. Apparently the low volatile contents of higher petrologic types are a primary feature, not the result of metamorphic loss.The mineralogy of chondrites suggests that they accreted between 405 K (absence of Fe3O4) and 560 K (presence of FeS), and the abundances of Tl, Bi, and In further restrict this interval to 420–500 K. Accretion at 1070 ± 100 K, as proposed by Hutchisonet al. (1979, 1980), leads to some extraordinary problems. Volatiles must be injected into the parent body after cooling, which requires permeation of the body by 1011 times its volume of nebular gas. This process must also achieve a uniform distribution of the less volatile elements (Rb, Cu, Ag, Zn, Ga, Ge, Sn, Sb, Se, F), without freezeout in the colder outer layers.Factor analysis of our data shows 3 groupings: siderophiles (Os, Re, Ir, Ni, Pd, Au, and Ge), volatiles (Ag, Br, In, Cd, Bi, and Tl) and alkalis (Rb and Cs). The remaining 5 elements (U, Zn, Te, Se, and Sb) remain unassociated. 相似文献
109.
Thermally, the lithosphere may be defined as that outer portion of the earth in which heat is transferred primarily by conduction. It generally includes the crust and part of the mantle. The thermal regime of continental lithosphere is determined by many factors including heat flow from the asthenosphere, the vertical and lateral variation of both thermal conductivity and radiogenic heat production, tectonic history, and such superficial processes as climatic history and the shallow hydrothermal regime. From studies of the global heat flow data set, two generalizations regarding continental lithosphere have arisen, namely that: 1) there is a negative correlation between heat flow and tectonic age of continental lithosphere; and 2) the thermal evolution of continental lithosphere is similar to that of ocean basins with the result that the “stable geotherm” is similar in both environments. When continental heat-flow data are studied from a regional rather than a global point of view, considerable doubt arises as to the general applicability of either statement. R. U. M. Rao and his associates have demonstrated that while Precambrian terranes do have demonstrably lower heat flows than, say, Tertiary terranes, the data are not normally distributed and it is not possible to establish a negative correlation between heat flow and age in any rigorous statistical way. The scatter in the relation may be explained in terms of the variations in the duration, intensity and even the sign of continental thermotectonic events in contrast to the simple situation (creation of new oceanic lithosphere at mid-ocean ridges) which prevails in the oceans. The scatter also is partially attributable to the large and laterally variable radiogenic component of heat flow on continents. For a province for which a heat flow-heat production relation has been established, much of the scatter in surface heat flow due to crustal radiogenic heat production versus age is eliminated by determining reduced heat flow (surface heat flow minus radiogenic component) as a function of tectonic age, but much scatter remains, and it is still not possible to establish a heat flux-age relation in a rigorous way. Primarily because of the spatial variability in radiogenic heat production, no single geotherm can be used to characterize the thermal regime of a stable continental terrane. Thus, while some sites on stable continental blocks may have a geotherm fortuitously similar to that for old ocean basins, there is no reason to expect that this will be true generally, and many stable continental terranes will be characterized by geotherms markedly different from the geotherm for old ocean basins. 相似文献
110.