首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   678篇
  免费   51篇
  国内免费   46篇
测绘学   26篇
大气科学   136篇
地球物理   183篇
地质学   180篇
海洋学   66篇
天文学   89篇
综合类   1篇
自然地理   94篇
  2024年   4篇
  2023年   3篇
  2022年   6篇
  2021年   27篇
  2020年   32篇
  2019年   21篇
  2018年   21篇
  2017年   26篇
  2016年   34篇
  2015年   33篇
  2014年   34篇
  2013年   48篇
  2012年   27篇
  2011年   41篇
  2010年   43篇
  2009年   46篇
  2008年   42篇
  2007年   39篇
  2006年   42篇
  2005年   31篇
  2004年   32篇
  2003年   20篇
  2002年   21篇
  2001年   15篇
  2000年   15篇
  1999年   5篇
  1998年   8篇
  1997年   7篇
  1995年   7篇
  1994年   7篇
  1993年   5篇
  1992年   9篇
  1991年   3篇
  1990年   5篇
  1989年   2篇
  1987年   1篇
  1985年   4篇
  1984年   2篇
  1983年   2篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
  1975年   1篇
  1974年   1篇
排序方式: 共有775条查询结果,搜索用时 15 毫秒
11.
This study investigates whether there has been a decline in fish stocks of a coral reef in Thailand, and if so, whether that decline is due to small-scale fishing. The research methods used included fish sampling by UVC, interviews and questionnaires with key informants and artisanal fishers. The results confirmed that there has been a decline in piscivores and carnivores (though not herbivores), and the main perceived threat came not from artisanal but from commercial fishing. Recommendations are to replace the largely unrestricted fishery with a more regulated regime backed by effective enforcement.  相似文献   
12.
The above article published online on 16 June 2011 in Wiley Online Library ( http://www.onlinelibrary.wiley.com ), and in Volume 39, pp. 697‐704, has been retracted by agreement between the Editor‐in‐Chief, Prisca Henheik, and Wiley‐VCH Verlag GmbH & Co. KgaA. The retraction has been agreed following an investigation carried out by Washington State University (WSU) which determined that research misconduct had occurred with respect to fabricating experimental data and falsifying data that formed the basis of a section of this article on the part of the corresponding author. Furthermore, WSU determined that the corresponding author did not declare an existing commercial conflict of interest when submitting the paper to the peer‐‐reviewed journal CLEAN ‐ Soil, Air, Water.  相似文献   
13.
The 2002 Reform of the Common Fisheries Policy (CFP) was claimed to be a radical overhaul of a failing system. Several EU fish stocks—particularly North Sea cod—had reached dangerously low levels, and there was widespread dissatisfaction with the way in which the CFP was operating. The European Commission took the opportunity of the legal requirement to review some features of the CFP (principally access provisions) in 2002, to undertake a broader reappraisal of the CFP. One of the features of this reappraisal was an attempt to improve the CFP's system of governance by increasing the amount of stakeholder participation, decentralisation, transparency, accountability, effectiveness and coherence. In this paper, the conclusion is reached that this attempt to improve the quality of governance in the 2002 CFP reform package has been more rhetorical than real.  相似文献   
14.
15.
http://www.sciencedirect.com/science/article/pii/S1674987114001601   总被引:1,自引:2,他引:1  
Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time[P-T-t) evolution of the Nagercoil Block at the southernmost tip of peninsular India.An inferred peak metamorphic assemblage of garnet,K-feldspar.sillimanite,plagioclase,magnetite,ilmenite,spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900℃.Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 C.SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages.New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation,and the minimum age of the metamorphic peak,to around560 Ma.The data suggest two stages of monazite growth.The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite.Relatively REE-depleted rims,which have a pronounced negative europium anomaly,grew during melt crystallisation along the retrograde path at around 535 Ma.Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer,supporting a long-lived high-grade metamorphic history.The metamorphic conditions,timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.  相似文献   
16.
The morphological evolution of embayed beaches on a microtidal coast is assumed to largely respond to the degree of exposure to wave conditions, decreasing the mobility with increasing beach indentation (and vice versa). However, the number of sediment arrivals at the beach or the impact of extreme storms can modify this relationship. Here, we present an analysis of 10 embayed beaches along the Catalan coast with different morphometric and sedimentary characteristics to identify the most relevant parameters controlling the morphological evolution of these embayed beaches at the inter-annual and decadal scales. The study was mostly based on LiDAR topographic data collected from 2012 to 2017, aerial photographs from 1945 to 2021, sediment sampling and a long-term series analysis of the forcing parameters (waves, sea level, precipitation and land-use changes). The results show a net loss of volume on all the studied beaches at an inter-annual scale and a general shoreline retreat during the last few decades, suggesting the influence of common processes on the evolution of the studied beaches. Smaller pocket beaches with medium-to-high indentations are more sensitive to changes induced by local factors and show higher variability in the volume of the emerged beach and shoreline position than larger beaches. The most relevant factors influencing the evolution of the studied beaches on a decadal scale were identified as changes in sea level and the reduction in sediment inputs provided by streams due to land-use changes in the drainage basin. At the inter-annual scale, the impact of extreme events is the main factor controlling beach behaviour. These general trends can be opposite locally for beaches that receive large amounts of sediment via longshore transport from adjacent beaches.  相似文献   
17.
Catchments in the Loess Plateau have been under the influence of human activities for centuries. In the last four decades, soil conservation measures have accelerated and intensified. These measures were designed to reduce soil erosion, improve agricultural productivity, and enhance environmental quality. It is important to evaluate the effects of these measures on hydrology in order to develop sustainable catchment management plans in the region. This study evaluated changes in stream flow data for four selected catchments in the Loess Plateau following large‐scale soil conservation measures. The non‐parametric Mann–Kendall test was used to identify trends in annual stream flow and the results showed significant downward trends in three of the four catchments. The Pettitt test indicated that a change point occurred in 1978 in these three catchments. Annual precipitation in all the catchments showed no significant trend during the period of record. Comparison of daily flow duration curves for two 20‐year periods (1957–1978) and (1979–2003) showed significant changes in stream flow regime. Reduction in most percentile flows varied between 20 and 45%, and the reduction in low flows was greatest. Overall, the reductions in daily flow were increasing with time, with significant changes occurring in the 1990s. However, it is not clear whether these catchments have seen the full effects of the soil conservation measures, so the results of this study might underestimate the final impact of soil conservation on stream flow regime. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
18.
19.
Abstract The Shimanto accretionary complex on the Muroto Peninsula of Shikoku comprises two major units of Tertiary strata: the Murotohanto Sub-belt (Eocene-Oligocene) and the Nabae Sub-belt (Oligocene-Miocene). Both sub-belts have been affected by thermal overprints following the peak of accretion-related deformation. Palaeotemperatures for the entire Tertiary section range from ~ 140 to 315°C, based upon mean vitrinite reflectance values of 0.9–5.0%Rm. Values of illite crystallinity index are consistent with conditions of advanced diagenesis and anchimetamorphism. Illite/mica b0 lattice dimensions indicate that burial pressures were probably no greater than 2.5kbar. In general, levels of thermal maturity are higher for the Murotohanto Sub-belt than for the Nabae Sub-belt. The Eocene-Oligocene strata also display a spatial decrease in thermal maturity from south to north and this pattern probably was caused by regional-scale differential uplift following peak heating. Conversely, the palaeothermal structure within the Nabae Sub-belt is fairly uniform, except for the local effects of mafic intrusions at the tip of Cape Muroto. There is a paleotemperature difference of ~ 90°C across the boundary between the Murotohanto and Nabae Sub-belts (Shiina-Narashi fault), and this contrast is consistent with approximately 1200 m of post-metamorphic vertical offset. Subduction prior to Middle Miocene probably involved the Kula or fused Kula-Pacific plate and the background geothermal gradient during the Eocene-Oligocene phase of accretion was ~ 30–35°C/km. Rapid heating of the Shimanto Belt evidently occurred immediately after a Middle Miocene reorganization of the subduction boundary. Hot oceanic lithosphere from the Shikoku Basin first entered the subduction zone at ~ 15 Ma; this event also coincided with the opening of the Sea of Japan and the rapid clockwise rotation of southwest Japan. The background geothermal gradient at that time was ~ 70°C/km. Whether or not all portions of the inherited (Eocene-Oligocene) palaeothermal structure were overprinted during the Middle Miocene remains controversial.  相似文献   
20.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号