全文获取类型
收费全文 | 6443篇 |
免费 | 282篇 |
国内免费 | 81篇 |
专业分类
测绘学 | 235篇 |
大气科学 | 512篇 |
地球物理 | 1522篇 |
地质学 | 2161篇 |
海洋学 | 525篇 |
天文学 | 1177篇 |
综合类 | 28篇 |
自然地理 | 646篇 |
出版年
2022年 | 30篇 |
2021年 | 76篇 |
2020年 | 87篇 |
2019年 | 122篇 |
2018年 | 178篇 |
2017年 | 161篇 |
2016年 | 231篇 |
2015年 | 173篇 |
2014年 | 196篇 |
2013年 | 407篇 |
2012年 | 257篇 |
2011年 | 342篇 |
2010年 | 291篇 |
2009年 | 381篇 |
2008年 | 336篇 |
2007年 | 291篇 |
2006年 | 269篇 |
2005年 | 260篇 |
2004年 | 261篇 |
2003年 | 205篇 |
2002年 | 214篇 |
2001年 | 115篇 |
2000年 | 144篇 |
1999年 | 102篇 |
1998年 | 114篇 |
1997年 | 81篇 |
1996年 | 85篇 |
1995年 | 83篇 |
1994年 | 90篇 |
1993年 | 74篇 |
1992年 | 88篇 |
1991年 | 67篇 |
1990年 | 55篇 |
1989年 | 50篇 |
1988年 | 58篇 |
1987年 | 52篇 |
1986年 | 56篇 |
1985年 | 67篇 |
1984年 | 64篇 |
1983年 | 76篇 |
1982年 | 57篇 |
1981年 | 65篇 |
1980年 | 55篇 |
1979年 | 65篇 |
1978年 | 51篇 |
1977年 | 34篇 |
1976年 | 28篇 |
1975年 | 26篇 |
1974年 | 26篇 |
1973年 | 30篇 |
排序方式: 共有6806条查询结果,搜索用时 15 毫秒
131.
Rural Hong Kong contains culturally protected enclaves of remnant woodlands that provide sanctuary for vegetation, wildlife and soil. This study focuses on the influence of natural and cultural factors on soil properties in woodlands with different human activities. At each of the nine selected sites, two pits were dug to collect soil samples for analysis of 24 physical and chemical properties. Analyses of variance between soil attributes and site factors suggest the prominence of woodland size, and important contribution of topography, aspect and parent material. Soil indicators of human impacts and the tenacity of woodland soil towards anthropogenic disturbance were demonstrated. 相似文献
132.
133.
134.
Richard L. Reynolds Joseph G. Rosenbaum Josh Rapp Michael W. Kerwin J. Platt Bradbury Steven Colman David Adam 《Journal of Paleolimnology》2004,31(2):217-233
Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter. 相似文献
135.
136.
137.
Toxicity of tributyltin in the marine mollusc Mytilus edulis 总被引:1,自引:0,他引:1
Josephine A. Hagger Michael H. Depledge Tamara S. Galloway 《Marine pollution bulletin》2005,51(8-12):811
Our previous studies have demonstrated that tributyltin (TBT) is genotoxic to the early life stages of marine mussels and worms. Here, the toxicity of TBT to adult organisms was determined using a suite of biomarkers designed to detect cytotoxic, immunotoxic and genotoxic effects. Exposure of adult mussels, Mytilus edulis, to environmentally realistic concentrations of TBTO for 7 days resulted in a statistically significant decrease in cell viability at concentrations of 0.5 μg/l and above. TBT had no effect on phagocytic activity or antioxidant capacity (FRAP assay). There was a statistically significant increase in DNA damage detected using the comet and micronucleus assays between the controls and 0.5, 1 and 5 μg/l of TBTO (P > 0.0005). Furthermore there was a strong correlation between DNA strand breaks (comet assay) and formation of micronuclei (P = 0.0005; R2 = 61.5%). Possible mechanisms by which TBT could damage DNA either directly or indirectly are discussed including the possibility that TBT is genotoxic due to its ability to disrupt calcium homeostasis. 相似文献
138.
Michael Pearlman Carey Noll Peter Dunn Julie Horvath Van Husson Paul Stevens Mark Torrence Hoai Vo Scott Wetzel 《Journal of Geodynamics》2005,40(4-5):470
The International Laser Ranging Service (ILRS) was established in September 1998 as a service within the IAG to support programs in geodetic, geophysical, and lunar research activities and to provide data products to the International Earth Rotation Service (IERS) in support of its prime objectives. Now in operation for 5 years, the ILRS develops: (1) the standards and specifications necessary for product consistency and (2) the priorities and tracking strategies required to maximize network efficiency. The service collects, merges, analyzes, archives and distributes satellite and lunar laser ranging data to satisfy a variety of scientific, engineering, and operational needs and encourages the application of new technologies to enhance the quality, quantity, and cost effectiveness of its data products. The ILRS works with: (1) the global network to improve station performance; (2) new satellite missions in the design and building of retroreflector targets to maximize data quality and quantity and (3) science programs to optimize scientific data yield. The ILRS Central Bureau maintains a comprehensive web site as the primary vehicle for the distribution of information within the ILRS community. The site, which can be accessed at: http://ilrs.gsfc.nasa.gov is also available at mirrored sites at the Communications Research Laboratory (CRL) in Tokyo and the European Data Center (EDC) in Munich.During the last 2 years, the ILRS has addressed very important challenges: (1) data from the field stations are now submitted hourly and made available immediately through the data centers for access by the user community; (2) tracking on low satellites has been significantly improved through the sub-daily issue of predictions, drag functions, and the real-time exchange of time biases; (3) analysis products are now submitted in SINEX format for compatibility with the other space geodesy techniques; (4) the Analysis Working Group is heavily engaged in Pilot Projects as it works toward an ILRS “standard” global solution and (5) SLR has significantly increased its participation in the International Terrestrial Reference Frame (ITRF) activity, which is important to the success of IGGOS. 相似文献
139.
140.
Postglacial topographic evolution of glaciated valleys: a stochastic landscape evolution model 总被引:1,自引:0,他引:1
The retreat of valley glaciers has a dramatic effect on the stability of glaciated valleys and exerts a prolonged influence on the subsequent fluvial sediment transport regime. We have studied the evolution of an idealized glaciated valley during the period following retreat of ice using a numerical model. The model incorporates a stochastic process to represent deep‐seated landsliding, non‐linear diffusion to represent shallow landsliding and an approximation of the Bagnold relation to represent fluvial sediment transport. It was calibrated using field data from several recent surveys within British Columbia, Canada. We present ensemble model results and compare them with results from a deterministic linear‐diffusion model to show that explicit representation of large landslides is necessary to reproduce the morphology and channel network structure of a typical postglacial valley. Our model predicts a rapid rate of fluvial sediment transport following deglaciation with a subsequent gradual decline, similar to that inferred for Holocene time. We also describe how changes in the model parameters affect the estimated magnitude and duration of the paraglacial sediment pulse. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献