全文获取类型
收费全文 | 1431篇 |
免费 | 62篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 58篇 |
大气科学 | 114篇 |
地球物理 | 351篇 |
地质学 | 399篇 |
海洋学 | 184篇 |
天文学 | 264篇 |
综合类 | 7篇 |
自然地理 | 119篇 |
出版年
2021年 | 14篇 |
2020年 | 19篇 |
2019年 | 9篇 |
2018年 | 26篇 |
2017年 | 29篇 |
2016年 | 40篇 |
2015年 | 23篇 |
2014年 | 38篇 |
2013年 | 71篇 |
2012年 | 45篇 |
2011年 | 54篇 |
2010年 | 58篇 |
2009年 | 65篇 |
2008年 | 55篇 |
2007年 | 51篇 |
2006年 | 58篇 |
2005年 | 36篇 |
2004年 | 39篇 |
2003年 | 40篇 |
2002年 | 39篇 |
2001年 | 30篇 |
2000年 | 39篇 |
1999年 | 25篇 |
1998年 | 27篇 |
1997年 | 24篇 |
1996年 | 20篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1993年 | 20篇 |
1992年 | 24篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 11篇 |
1988年 | 14篇 |
1987年 | 22篇 |
1986年 | 15篇 |
1985年 | 34篇 |
1984年 | 42篇 |
1983年 | 28篇 |
1982年 | 28篇 |
1981年 | 33篇 |
1980年 | 29篇 |
1979年 | 16篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 20篇 |
1975年 | 18篇 |
1974年 | 9篇 |
1973年 | 15篇 |
1971年 | 5篇 |
排序方式: 共有1496条查询结果,搜索用时 93 毫秒
61.
Samuel Ebert Addi Bischoff Dennis Harries Sarah Lentfort Jean‐Alix Barrat Andreas Pack Jrme Gattacceca Robbin Visser Peter Schmid‐Beurmann Stephan Kimpel 《Meteoritics & planetary science》2019,54(2):328-356
Based on the high abundance of fine‐grained material and its dark appearance, NWA 11024 was recognized as a CM chondrite, which is also confirmed by oxygen isotope measurements. But contrary to known CM chondrites, the typical phases indicating aqueous alteration (e.g., phyllosilicates, carbonates) are missing. Using multiple analytical techniques, this study reveals the differences and similarities to known CM chondrites and will discuss the possibility that NWA 11024 is the first type 3 CM chondrite. During the investigation, two texturally apparent tochilinite–cronstedtite intergrowths were identified within two thin sections. However, the former phyllosilicates were recrystallized to Fe‐rich olivine during a heating event without changing the textural appearance. A peak temperature of 400–600 °C is estimated, which is not high enough to destroy or recrystallize calcite grains. Thus, calcites were never constituents of the mineral paragenesis. Another remarkable feature of NWA 11024 is the occurrence of unknown clot‐like inclusions (UCLIs) within fine‐grained rims, which are unique in this clarity. Their density and S concentration are significantly higher than of the surrounding fine‐grained rim and UCLIs can be seen as primary objects that were not formed by secondary alteration processes inside the rims. Similarities to chondritic and cometary interplanetary dust particles suggest an ice‐rich first‐generation planetesimal for their origin. In the earliest evolution, NWA 11024 experienced the lowest degree of aqueous alteration of all known CM chondrites and subsequently, a heating event dehydrated the sample. We suggest to classify the meteorite NWA 11024 as the first type 3 CM chondrite similar to the classification of CV3 chondrites (like Allende) that could also have lost their matrix phyllosilicates by thermal dehydration. 相似文献
62.
Jan Leitner Knut Metzler Christian Vollmer Christine Floss Pierre Haenecour Jnos Kodolnyi Dennis Harries Peter Hoppe 《Meteoritics & planetary science》2020,55(6):1176-1206
We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine‐grained chondrule rims in six Mighei‐type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM‐related carbonaceous chondrite Sutter's Mill. Sixteen O‐anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix‐normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O‐rich grains are enriched in 17O, and could originate from low‐mass asymptotic giant branch stars. One grain is enriched in 17O and significantly depleted in 18O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core‐collapse supernova origin showing enhanced 18O/16O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O‐anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O‐rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe‐rich silicates, one forsterite‐like Mg‐rich silicate, two Al‐oxides with spinel‐like compositions, and one Fe‐(Mg‐)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm × 735 nm) revealed that it was crystalline åkermanite (Ca2Mg[Si2O7]) or a an åkermanite‐diopside (MgCaSi2O6) intergrowth. 相似文献
63.
The results of a 2007 survey of fishers, managers, scientists, and enforcement officials indicate that noncompliance is a significant problem in the Northeast multispecies groundfish (NEGF) fishery, as it has been for at least 20 years. The percent of total harvest taken illegally is estimated to be 12–24%, which is significantly higher than estimates of 6–14% in the 1980s. Thirty-seven percent of fishers, 61% of fishery managers and 80% of fishery enforcement staff believe that “the combined adverse impact of all violations on the health and manageability of fish resources” is significant, highly significant, or extremely significant. Many fishers believe that illegal fishing will prevent them from ever benefiting from stock rebuilding programs. 相似文献
64.
Diana Tingley Jóhann Ásmundsson Edward Borodzicz Alexis Conides Ben Drakeford Ingi Rúnar Eðvarðsson Dennis Holm Kostas Kapiris Sakari Kuikka Bogi Mortensen 《Marine Policy》2010
This paper presents the findings of risk identification and risk perception research conducted in relation to the fisheries systems of four distinct and diverse European countries: Faroes, Iceland, Greece and the UK. Risk research traditionally attempts to quantify the potential threat or consequences from a range of risk events or hazards. This research, however, adopted a social sciences perspective and so assumed that a risk event or hazard can mean different things to different people and that these perceptions are also context and culturally dependent. Risk perceptions were examined and risk registers developed in each country for a range of stakeholder groups. A ‘mental modelling’ approach was adopted in a series of qualitative interviews. Findings were examined in terms of a wide range of psychological, social and cultural risk theories. Differences in risk perceptions were noted between stakeholder groups and countries and contextual influences were examined such as the widely differing fisheries management systems used in each country. This research provides one of the first attempts to systematically evaluate risks and perceptions across a range of fisheries-systems. The findings support social science theories which argue that risk is a subjective, as opposed to objective, concept and that this subjectivity will therefore affect our attempts to assess and manage those hazards we think we can potentially influence or control. 相似文献
65.
The sea pen Funiculina quadrangularis (Pallas, 1766) is a species of conservation concern in Scottish coastal waters, due to its restricted geographical distribution and high sensitivity to demersal fishing activities. Reproduction in F. quadrangularis was investigated in a population located in southern Loch Linnhe, west Scotland. This was accomplished through the analysis of trends in oocyte size-frequency distribution and relative fecundity over a 12-month period. Funiculina quadrangularis is dioecious and the study population exhibited a sex ratio of 1:1. Oogenesis in female F. quadrangularis is characterised by the maintenance of a large pool of asynchronously developing oocytes throughout the year, of which a small proportion (<10%) mature with increasing sychronicity and are spawned in midwinter. The reasons for this distinct pattern of oogenesis and winter spawning remain unclear, although the potential influence of environmental cues and the role of endogenous factors in relation to this sea pen's deep-sea habit are discussed. Whilst the duration of oogenesis is prolonged (>12 months), it is proposed that spawning is a brief and synchronous annual event. Relative fecundity is high and is independent of colony size, varying between approximately 500–2000 oocytes per 1 cm rachial midsection. This measure of fecundity exhibited pronounced seasonality and was significantly lower during the post-spawning winter months. Total fecundity in F. quadrangularis is considered to be high; although a small proportion of the total number of oocytes is spawned annually, this is compensated for by large colony size. Funiculina quadrangularis produces large oocytes (>800 μm), indicative of the production of lecithotrophic larvae. 相似文献
66.
Jeffrey E. Moore Bryan P. Wallace Rebecca L. Lewison Ramúnas Žydelis Tara M. Cox Larry B. Crowder 《Marine Policy》2009
This paper reviews the available information (observer programs, estimates, statutes, regulations) for bycatch of marine mammals, sea turtles, and seabirds in fisheries of the United States. Goals of the review were to evaluate the state of knowledge of bycatch and the role of existing protective legislation in shaping bycatch management for different taxa. Pressing issues are identified, as well as knowledge gaps and policy limitations that hinder multi-species bycatch reduction. The USA has made important progress toward reducing bycatch in its fisheries, but the efficacy of its management has been limited somewhat by a focus on taxon- and fishery-specific regulation and the lack of consistent mandate across taxa for taking a cumulative perspective on bycatch. Applying consistent criteria across taxa for setting bycatch limits (e.g., extending the approach used for marine mammals to sea turtles and seabirds) would be the first step in a multi-species approach to bycatch reduction. A population-based multi-species multi-gear approach to bycatch would help identify priority areas where resources are needed most and can be used most effectively. 相似文献
67.
68.
69.
Stream temperature and riparian microclimate were characterized for a 1·5 km wildfire‐disturbed reach of Fishtrap Creek, located north of Kamloops, British Columbia. A deterministic net radiation model was developed using hemispherical canopy images coupled with on‐site microclimate measurements. Modelled net radiation agreed reasonably with measured net radiation. Air temperature and humidity measured at two locations above the stream, separated by 900 m, were generally similar, whereas wind speed was poorly correlated between the two sites. Modelled net radiation varied considerably along the reach, and measurements at a single location did not provide a reliable estimate of the modelled reach average. During summer, net radiation dominated the surface heat exchanges, particularly because the sensible and latent heat fluxes were normally of opposite sign and thus tended to cancel each other. All surface heat fluxes shifted to negative values in autumn and were of similar magnitude through winter. In March, net radiation became positive, but heat gains were cancelled by sensible and latent heat fluxes, which remained negative. A modelling exercise using three canopy cover scenarios (current, simulated pre‐wildfire and simulated complete vegetation removal) showed that net radiation under the standing dead trees was double that modelled for the pre‐fire canopy cover. However, post‐disturbance standing dead trees reduce daytime net radiation reaching the stream surface by one‐third compared with complete vegetation removal. The results of this study have highlighted the need to account for reach‐scale spatial variability of energy exchange processes, especially net radiation, when modelling stream energy budgets. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
70.
Dennis J. Geist James D. Myers Carol D. Frost 《Contributions to Mineralogy and Petrology》1988,99(1):105-112
Sr and Nd isotopic ratios are reported from mineral separates and bulk rock samples from the Edgecumbe Volcanic Field in southeast Alaska, a suite of lavas that has developed by assimilation. Megacrysts in the basaltic unit and the rhyodacites are in isotopic equilibrium with their matrices. Because the rhyodacites developed as the result of assimilation, the assimilate must have been completely dissolved. In contrast, megacrysts in the two andesitic units are in isotopic disequilibrium with their matrix. These megacrysts must be a mixture of those inherited from the contaminant, crystals in equilibrium with the groundmass, and possibly crystals inherited from a mafic liquid. 相似文献