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   1616篇
  免费   113篇
  国内免费   10篇
测绘学   46篇
大气科学   171篇
地球物理   541篇
地质学   584篇
海洋学   108篇
天文学   206篇
综合类   7篇
自然地理   76篇
  2023年   7篇
  2022年   15篇
  2021年   38篇
  2020年   45篇
  2019年   31篇
  2018年   39篇
  2017年   60篇
  2016年   104篇
  2015年   94篇
  2014年   113篇
  2013年   176篇
  2012年   137篇
  2011年   90篇
  2010年   85篇
  2009年   76篇
  2008年   55篇
  2007年   31篇
  2006年   63篇
  2005年   34篇
  2004年   32篇
  2003年   31篇
  2002年   45篇
  2001年   26篇
  2000年   12篇
  1999年   12篇
  1998年   16篇
  1997年   17篇
  1996年   9篇
  1995年   18篇
  1994年   11篇
  1993年   9篇
  1992年   7篇
  1991年   19篇
  1990年   9篇
  1989年   9篇
  1987年   9篇
  1986年   5篇
  1985年   10篇
  1984年   13篇
  1983年   12篇
  1982年   9篇
  1981年   8篇
  1980年   5篇
  1979年   13篇
  1977年   4篇
  1976年   6篇
  1975年   8篇
  1974年   7篇
  1973年   10篇
  1969年   7篇
排序方式: 共有1739条查询结果,搜索用时 10 毫秒
101.
In 1992 and 2004, heavy metals concentrations were measured in surficial sediments from Todos Santos Bay, located in Ensenada, Baja California, Mexico. The aim was to search for relationships between metal enrichment factors and a biological adverse effects index. Unlike Ni, the elements Cd, Cu and Zn showed significant correlations (p<0.05) between enrichment factors and the biological adverse effects index. Cu showed a 0.74:1 relationship, which means that any enrichment above 0.74 could represent biological adverse effects. On the other hand, Cd and Zn enrichments must be >5.5 and >1.5, respectively, in order for the sediments to be considered toxic. In general, data showed that most of the metal concentrations in Todos Santos Bay sediments could not cause adverse effects to biota. Only Ensenada's harbor and the zone next to a dredging dumping site showed metal enrichments that could be toxic.  相似文献   
102.
Cutting and removal of oil-impacted marsh plants are still used worldwide as a clean-up and recovery technique. To experimentally test the efficacy of cutting and removing marsh plants under subtropical conditions, we simulated an oil spill (Bunker MF-180) in Spartina alterniflora marshes and compared the responses of plant height, biomass, density of culms and number of flowering plants in high and low energy areas in Paranaguá Bay (S Brazil) for about 9 months. Cutting and removal were inefficient in promoting or accelerating the recovery of the impacted areas. Cut or uncut impacted marshes fully recovered within 6 months, both in low and high energy areas. Plant cutting should be practiced only when there is an effective risk of contamination of groundwater near urban areas, when obvious aesthetical issues are involved in areas of touristic interest or when there are real short-term conservation risks to threatened species.  相似文献   
103.
 At the northern part of the Portuguese mainland, the upflow zone of several hot and cold HCO3/Na/CO2-rich mineral waters is mainly associated with important NNE–SSW faults. Several geochemical studies have been carried out on thermal and non-thermal hydromineral manifestations that occur along or near these long tectonic alignments. The slight chemical differences that exist between these meteoric hot and cold HCO3/Na/CO2-rich mineral waters seem to be mainly caused by CO2. δ13C(TIDC) values observed in these groundwaters range between –6.00 and –1.00‰ versus V-PDB (V denotes Vienna, the site of the International Atomic Energy Agency; PDB originates from the CaCO3 of the rostrum of a Cretaceous belemnite, Belemnitella americana, collected in the Peedee formation of South Carolina, USA) indicating a deep-seated (mantle) origin for most of the CO2. Nevertheless, in the case of the heavier δ13C(TIDC) values, the contribution of metamorphic CO2 or the dissolution of carbonate rock levels at depth cannot be excluded. Concerning the hot waters, the lack of a positive 18O-shift should be attributed to water-rock interaction in a low temperature environment, rather than to the isotopic influence of CO2 on the δ18O-value of the waters. Received: 9 August 1999 · Accepted: 8 March 2000  相似文献   
104.
Summer stream water quality was monitored before and following the logging of 50% of the boreal forest within three small watersheds (<50 ha) nested in the ‘Ruisseau des Eaux‐Volées’ Experimental Watershed, Montmorency Forest (Québec, Canada). Logging was conducted in winter, on snow cover according to recommended best management practices (BMPs) to minimize soil disturbance and protect advance growth. A 20‐m forest buffer was maintained along perennial streams. In watershed 7·2, cut‐blocks were located near the stream network and logging was partially allowed within the riparian buffer zone. In watersheds 7·5 and 7·7, logging occurred farther away from the stream network. Observations were also made for watershed 7·3 that collected the runoff from watersheds 7·2 and 7·5, and watershed 7·6, the uproad portion of watershed 7·7. The control watershed 0·2 was contiguous to the impacted watersheds and remained undisturbed. Following clearcutting, changes in summer daily maximum and minimum stream temperatures remained within ± 1 °C while changes in diurnal variation did not decrease by more than 0·5 °C. Concentrations of NO3? greatly increased by up to 6000% and concentrations of K+ increased by up to 300% during the second summer after logging. Smaller increases were observed for Fetotal (up to 71%), specific conductance (up to 26%), and Mg2+ (up to 19%). Post‐logging pH decreased slightly by no more than 7% while PO43? concentration remained relatively constant. Suspended sediment concentrations appeared to increase during post‐logging, but there was not enough pre‐logging data to statistically confirm this result. Logging of moderate intensity and respecting established BMPs may account for the limited changes of water quality parameters and the low exceedances of the criteria for the protection of aquatic life. The proximity of the cutover to the stream network and logging within the riparian zone did not appear to affect water quality. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   
105.
106.
107.
The lunar rock and mineral characterization consortium (LRMCC) has conducted coordinated mineralogy/petrography/spectroscopy analyses of a suite of pristine lunar basalts. Four basalt slabs (two low‐Ti, two high‐Ti) and paired thin sections were analyzed. Thin sections were analyzed for mineralogy/petrography, while the slabs were used to prepare particulate separates of major mineral phases and bulk samples. Mineral separates and particulate bulk samples were crushed to controlled grain sizes and their reflectance spectra measured in the NASA RELAB at Brown University. The resulting data set provides an essential foundation for spectral mixing models, offers valuable endmember constraints for space weathering analyses, and represents critical new ground truth results for lunar science and exploration efforts.  相似文献   
108.
The Middle Berriasian deposits of the Jura platform in Switzerland and France have already been well studied in terms of high-resolution sequence stratigraphy and different orders of depositional sequences (large-, medium-, and small-scale) have been defined. The hierarchical stacking pattern of the sequences and the time span represented by the investigated interval imply that sea-level fluctuations in the Milankovitch frequency band as well as differential subsidence caused the observed changes of accommodation on the Jura platform. The present study focuses on three small-scale sequences within the transgressive interval of a large-scale sequence. The initial flooding of the platform is marked by a facies change from supra- and intertidal (Goldberg Formation) to shallow-marine subtidal deposits (Pierre Chatel Formation). Detailed logging and facies analysis of 11 sections allow recognizing small environmental changes that define elementary sequences within the well-established small-scale sequences and distinguishing between autocyclic and allocyclic processes in sequence formation. It is concluded that the small-scale sequences correspond to the 100-ka orbital eccentricity cycle, while allocyclic elementary sequences formed in tune with the 20-ka precession cycle. Based on the correlation of elementary and small-scale sequences it can be shown that the Jura platform has been flooded stepwise by repeated transgressive pulses. Differential subsidence and pre-existing platform morphology further controlled sediment accumulation and distribution during the transgression. The combination of high-resolution sequence stratigraphy and cyclostratigraphy then enables the reconstruction of hypothetical palaeogeographic maps in time increments of a few ten thousand years.  相似文献   
109.
Wild mushrooms are recognized as important non-wood forest products in mountainous ecosystems, but their real potential for generating rural economies has not been fully evaluated due to the difficulties in obtaining reliable productivity data, minimizing their true potential as contributor to rural economies. Mushroom yield models based on large data series from Pinus forest ecosystems in the region of Catalonia(Spain), combined with data from the Spanish National Forest Inventory allow us to estimate the potential mushroom productivity by forest ecosystems. The results of 24,500 tons/yr of mushrooms of which 16,300 tons are classified as edible and 7,900 tons are commonly marketed demonstrate the importance of mushroom productions in Catalonian pine forests, mostly located in mountainous areas where the development of agricultural activities is limited. Economic mushroom value is estimated at 48 million € for the edible mushroom and 32 million € for those corresponding to marketable yields, confirming the potential of this non-wood forest product. These production results and corresponding economic values provide a basis for the incorporation of wild mushrooms as significant non-wood forest products in the development of forest policies in mountainous areas.  相似文献   
110.
Conventional water treatments when downsized to cover the demands of small and scattered populations are costly and ineffective. In fact, many small towns cannot afford the high average treatment costs nor provide the skill and complex management requirements. Even when these facilities are available, waste water might not be properly treated. Land application system treatments (LAST) have been shown to be preferable on the basis of their technical effectiveness and financial viability. The recently approved Spanish regulations (RDL 1620/2007, pp 50369–50661) represent an opportunity for the implementation of well-designed LASTs. This paper presents an assessment methodology for the design of LASTs by combining technical, financial and location criteria. The method is applied to the design of LASTs to cover the water disposal demand of twelve municipalities located within the protected natural landscape, and surrounding areas of ‘El Rebollar’, Salamanca, Spain.  相似文献   
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