全文获取类型
收费全文 | 30550篇 |
免费 | 536篇 |
国内免费 | 344篇 |
专业分类
测绘学 | 659篇 |
大气科学 | 1904篇 |
地球物理 | 5596篇 |
地质学 | 11809篇 |
海洋学 | 3106篇 |
天文学 | 7022篇 |
综合类 | 76篇 |
自然地理 | 1258篇 |
出版年
2022年 | 318篇 |
2021年 | 491篇 |
2020年 | 540篇 |
2019年 | 590篇 |
2018年 | 1146篇 |
2017年 | 1097篇 |
2016年 | 1218篇 |
2015年 | 587篇 |
2014年 | 1098篇 |
2013年 | 1767篇 |
2012年 | 1223篇 |
2011年 | 1543篇 |
2010年 | 1354篇 |
2009年 | 1653篇 |
2008年 | 1410篇 |
2007年 | 1459篇 |
2006年 | 1369篇 |
2005年 | 799篇 |
2004年 | 760篇 |
2003年 | 698篇 |
2002年 | 718篇 |
2001年 | 641篇 |
2000年 | 581篇 |
1999年 | 480篇 |
1998年 | 492篇 |
1997年 | 459篇 |
1996年 | 397篇 |
1995年 | 359篇 |
1994年 | 386篇 |
1993年 | 302篇 |
1992年 | 297篇 |
1991年 | 296篇 |
1990年 | 327篇 |
1989年 | 223篇 |
1988年 | 228篇 |
1987年 | 272篇 |
1986年 | 216篇 |
1985年 | 316篇 |
1984年 | 282篇 |
1983年 | 260篇 |
1982年 | 276篇 |
1981年 | 207篇 |
1980年 | 245篇 |
1979年 | 197篇 |
1978年 | 211篇 |
1977年 | 167篇 |
1976年 | 163篇 |
1975年 | 174篇 |
1974年 | 169篇 |
1973年 | 167篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We estimate the absolute intensity of the Hα emission line in M17 based on spectroscopic observations with the 70 cm telescope of the Fesenkov Astrophysical Institute and the 1.25 m telescope of the Sternberg Astronomical Institute’s Crimean Laboratory. The visual extinction, A v , is derived for about 250 regions in the nebula via a comparison of the optical (Hα) and radio (21 cm continuum) emissions. The A v values for the eastern, optically bright, part of the nebula are 2–6m. For the western part, which is covered by a dark cloud, the extinction is A v = 7?14.5m. We derived I(Hα)/I([NII] λ6584) ratios and estimated the degree of ionization of nitrogen in the nebula; we find that most nitrogen is in the [NIII] state. 相似文献
992.
993.
In the underground waste isolation projects such as the ANDRA'one in the site of Bure, the transport properties of the surrounding rock mass is of fundamental importance. To measure very low permeability, we use the modified version of the pulse test proposed by Hsieh et al. [Hsieh, P.A., Tracy, J.V., Neuzil, C.E., Bredehoeft, J.D., Silliman, S.E., 1981. A transient laboratory method for determining the hydraulic properties of ‘tight’ rocks — I. Theory. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. Vol. 18, pp. 245-252] which enables the intrinsic permeability, k, and the specific storage coefficient, Ss, of rocks such as mudstone to be characterized. In this paper, the special effort performed on the laboratory apparatus design, to ensure a good sensitivity of the rock response with respect to both parameters, k and Ss, is presented. In addition, two parameters identification procedures are proposed: the graphical method given by Hsieh et al. [Hsieh, P.A., Tracy, J.V., Neuzil, C.E., Bredehoeft, J.D., Silliman, S.E., 1981. A transient laboratory method for determining the hydraulic properties of ‘tight’ rocks — I. Theory. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. Vol. 18, pp. 245-252] and a parameter identification based on the solution of an inverse problem. The efficiency of the apparatus design and the parameters identification procedures is then demonstrated though some pulse tests performed on the Meuse/Haute-Marne mudstone. 相似文献
994.
Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions
Alexander N. Krot Timothy J. Fagan Kazuhide Nagashima Hisayoshi Yurimoto 《Geochimica et cosmochimica acta》2005,69(7):1873-1881
Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa<2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk<15). ∼10% of AOAs contain low-Ca pyroxene (Fs1-3Wo1-5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: (i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5-10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3-5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%-20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally 16O-rich isotopic compositions (Δ17O < −20‰). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are 16O-enriched (Δ17O < −20‰), whereas those of (iii) are 16O-depleted (Δ17O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is 16O-rich (Δ17O ∼ −22‰), whereas the neighboring plagioclase mesostasis is 16O-depleted (Δ17O ∼ −11‰).We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an 16O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted 16O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with 16O-poor nebular gas and may have been transformed into chondrules. The original 16O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains. 相似文献
995.
Complex substrates are degraded in anoxic sediments by the concerted activities of diverse microbial communities. To explore the effects of substrate complexity on carbon transformations in permanently cold anoxic sediments, four substrates—Spirulina cells, Isochrysis cells, and soluble high molecular weight carbohydrate-rich extracts of these cells (Spir-Ex and Iso-Ex)—were added to sediments collected from Svalbard. The sediments were homogenized, incubated anaerobically in gas-tight bags at 0°C, and enzyme activities, fermentation, and terminal respiration were monitored over a 1134 h time course. All substrate additions yielded a fraction (8%-13%) of carbon that was metabolized to CO2 over the first 384 h of incubation. The timecourse of VFA (volatile fatty acid) production and consumption, as well as the suite of VFAs produced, was similar for all substrates. After this phase, pathways of carbon degradation diverged, with an additional 43%, 32%, 33%, and 8% of Isochrysis, Iso-Ex, Spirulina, and Spir-Ex carbon respired to CO2 over the next 750 h of incubation. Somewhat surprisingly, the soluble, carbohydrate-rich extracts did not prove to be more labile substrates than the whole cells from which they were derived. Although Spirulina and Iso-Ex differed in physical and chemical characteristics (solid/soluble, C/N ratio, lipid and carbohydrate content), nearly identical quantities of carbon were respired to CO2. In contrast, only 15% of Spir-Ex carbon was respired, despite the initial burst of activity that it fueled, its soluble nature, and its relatively high (50%) carbohydrate content. The microbial community in these cold anoxic sediments clearly has the capacity to react rapidly to carbon input; extent and timecourse of remineralization of added carbon is similar to observations made at much higher temperatures in temperate sediments. The extent of carbon remineralization from these specific substrates, however, would not likely have been predicted on the basis of general substrate characteristics. 相似文献
996.
Maria Schönbächler Mark Rehkämper Manuela A. Fehr Alex N. Halliday Bodo Hattendorf 《Geochimica et cosmochimica acta》2005,69(21):5113-5122
Stepwise dissolutions of the carbonaceous chondrites Orgueil (CI), Murchison (CM) and Allende (CV) reveal large nucleosynthetic anomalies for Zr isotopes that contrast with the uniform compositions found in bulk meteorites. Two complementary nucleosynthetic components are observed: one enriched and one depleted in s-process nuclides. The latter component, characterized by excess 96Zr, is most distinctive in the acetic acid leachate (up to ε96Zr ≈ 50). The excess decreases with increasing acid strength and the final leaching steps of the experiment are depleted in 96Zr and thus enriched in s-process nuclides. Presolar silicon carbide grains are likely host phases for part of the anomalous Zr released during these later stages. However, by mass balance they cannot account for the 96Zr excesses observed in the early leaching steps and this therefore hints at the presence of at least one additional carrier phase with significant amounts of anomalous Zr. The data provide evidence that average solar system material consists of a homogenized mixture of different nucleosynthetic components, which can be partly resolved by leaching experiments of carbonaceous chondrites. 相似文献
997.
998.
Lina Fernandes G.N. Nayak D. Ilangovan D.V. Borole 《Estuarine, Coastal and Shelf Science》2011,91(3):388-399
Two core sediment samples; one from inner part (ManI) and the other closer to the mouth (ManII); were collected from the intertidal regions of Manori, a tidally influenced creek near Mumbai, India. Both the cores were subjected to various geochemical analyses to determine parameters such as pH, sediment components, total organic carbon, total nitrogen, total phosphorus and selected metals viz., Fe, Mn, Cu, Pb, Co, Ni, Zn, Cr, Al, Ca and V. Analysis of 210Pb activity was employed to assess the sediment deposition trend of the area. The data was further processed using factor and cluster analyses. The results indicate that the sediments from site ManI, had finer sediment composition, higher porosity, organic matter and metal contents but exhibited an erratic decline in 210Pb activity downcore. Also ManI showed higher C:N ratio and enrichment factor values as compared to site ManII. The inner area (ManI) probably received a greater input of organic matter from the erosion of terrestrial matter as well as domestic and industrial discharge. Sediments from site ManII had typical marine organic matter composition (lower C:N ratio). The concentration of metals at this site was also low indicating the contents were getting diluted by freshwater and seawater mixing. 相似文献
999.
Groundwater – the disregarded component in lake water and nutrient budgets. Part 2: effects of groundwater on nutrients 下载免费PDF全文
Lacustrine groundwater discharge (LGD) transports nutrients from a catchment to a lake, which may fuel eutrophication, one of the major threats to our fresh waters. Unfortunately, LGD has often been disregarded in lake nutrient studies. Most measurement techniques are based on separate determinations of volume and nutrient concentration of LGD: Loads are calculated by multiplying seepage volumes by concentrations of exfiltrating water. Typically low phosphorus (P) concentrations of pristine groundwater often are increased due to anthropogenic sources such as fertilizer, manure or sewage. Mineralization of naturally present organic matter might also increase groundwater P. Reducing redox conditions favour P transport through the aquifer to the reactive aquifer‐lake interface. In some cases, large decreases of P concentrations may occur at the interface, for example, due to increased oxygen availability, while in other cases, there is nearly no decrease in P. The high reactivity of the interface complicates quantification of groundwater‐borne P loads to the lake, making difficult clear differentiation of internal and external P loads to surface water. Anthropogenic sources of nitrogen (N) in groundwater are similar to those of phosphate. However, the environmental fate of N differs fundamentally from P because N occurs in several different redox states, each with different mobility. While nitrate behaves essentially conservatively in most oxic aquifers, ammonium's mobility is similar to that of phosphate. Nitrate may be transformed to gaseous N2 in reducing conditions and permanently removed from the system. Biogeochemical turnover of N is common at the reactive aquifer‐lake interface. Nutrient loads from LGD were compiled from the literature. Groundwater‐borne P loads vary from 0.74 to 2900 mg PO4‐P m?2 year?1; for N, these loads vary from 0.001 to 640 g m?2 year?1. Even small amounts of seepage can carry large nutrient loads due to often high nutrient concentrations in groundwater. Large spatial heterogeneity, uncertain areal extent of the interface and difficult accessibility make every determination of LGD a challenge. However, determinations of LGD are essential to effective lake management. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
1000.