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   2267篇
  免费   48篇
  国内免费   26篇
测绘学   48篇
大气科学   107篇
地球物理   579篇
地质学   771篇
海洋学   238篇
天文学   373篇
综合类   10篇
自然地理   215篇
  2021年   21篇
  2020年   31篇
  2019年   28篇
  2018年   42篇
  2017年   32篇
  2016年   63篇
  2015年   44篇
  2014年   55篇
  2013年   120篇
  2012年   64篇
  2011年   114篇
  2010年   78篇
  2009年   95篇
  2008年   82篇
  2007年   93篇
  2006年   86篇
  2005年   90篇
  2004年   76篇
  2003年   55篇
  2002年   74篇
  2001年   40篇
  2000年   39篇
  1999年   43篇
  1998年   28篇
  1997年   30篇
  1996年   32篇
  1995年   41篇
  1994年   33篇
  1993年   26篇
  1992年   25篇
  1991年   32篇
  1990年   23篇
  1989年   24篇
  1988年   34篇
  1987年   27篇
  1986年   25篇
  1985年   35篇
  1984年   38篇
  1983年   41篇
  1982年   32篇
  1981年   30篇
  1980年   27篇
  1979年   32篇
  1978年   24篇
  1977年   34篇
  1976年   31篇
  1975年   25篇
  1973年   19篇
  1972年   22篇
  1971年   18篇
排序方式: 共有2341条查询结果,搜索用时 31 毫秒
961.
Polarographic techniques have been used to determine reduced inorganic sulphur speciation in recent anoxic marine sediments from two hypersaline basins, the Tyro and the Bannock Basins, in the Eastern Mediterranean. The following phases were determined: acid volatile sulphur (AVS), pyritic sulphur and zerovalent sulphur. The determination of AVS and pyrite was based respectively on the acidification and Cr(II) reduction of these sulphur components to H2S. H2S was collected in base and the sulphide concentration was measured by polarography. Standard Na2S and pyrite gave recoveries of 99.6% ± 3.9% and 97% ± 12% respectively. Total zerovalent sulphur in a sediment sample was measured by the reaction of sulphite with thiosulphate. Thiosulphate was measured directly by polarography.

Pyrite is the main phase of inorganic reduced sulphur in the sediments from the Tyro and the Bannock Basins, and it has about the same average level (125 υmoles per gramme dry weight) in the cores recovered from the two areas. However, the distribution of pyrite in the top 100 cm of the two cores differs significantly. In the Bannock Basin a sharp increase is observed with depth, whereas in the Tyro Basin there is a small decrease with depth.

The total amount of reduced inorganic sulphur is less than the total amount of sulphur in the sediments. This indicates that there must be additional sulphur-bearing phases. One of these phases may be gypsum, and indeed, gypsum crystals have been observed in the Bannock Basin.

In neither basin is there a significant correlation between reduced sulphur and organic carbon. The pyrite that occurs in these basins may have been formed syngenetically at the interface of the anoxic brine and oxic seawater. Diagenetic pyrite may have been formed within the sediments of the basins. AVS and total zerovalent sulphur are still observed at depth. We therefore suggest that this may be due to the incomplete transformation of AVS and zerovalent sulphur into pyrite.  相似文献   

962.
George Anastasakis   《Marine Geology》2007,240(1-4):113-135
Santorini volcano has been the largest source of volcaniclastic sediment in the Eastern Mediterranean during the late Quaternary. A dozen cores from the Cretan Basin, south of Santorini, have sampled two megabeds that consist of gravity emplaced volcaniclastic sequences. The uppermost megabed U consists of a succession of five (U5–U1) base cut out turbiditic units. Lower megabed A is a single turbiditic event. Only the uppermost U2 and U1 turbidites are separated from the underlying beds by hemipelagic marls. The texture and composition of the U and A megabeds closely match the texture and composition of the fine, vitric ash of the “Minoan” deposits on Santorini islands, dating from about 3500 yr BP. These megabeds are therefore attributed to rapid accumulation of separate gravity flows fed by the “Minoan” eruption, except for the upper U2 and U1 turbidites deposited from subsequent gravity flows transporting eroded volcaniclastic sediments. With the exception of the margin south of Santorini, dozens of cores retrieved around the margins of the Cretan Basin have a continuous late Quaternary succession that shows no evidence for massive sediment remobilization into the deeper basin, including the passage of the “Minoan” tsunami.

Extensive high-resolution 3.5 kHz records revealed the acoustic character, architecture and distribution of the U and A megabeds and four underlying late Quaternary volcanogenic megabeds in the Cretan Basin. The acoustic facies of megabeds are typical of megaturbidites and consist of an upper, transparent, lower velocity layer that corresponds to the fine-grained upper turbiditic silt and clay section and a lower, strongly reflective higher velocity section that corresponds to the lowest, coarser-grained base of the turbidite that is developed over a sharp erosional surface. Penetration of the high-resolution records reveals the existence of at least six megabeds. Correlation with core lithology and the physical properties of the various lithofacies, including down-core velocity profiles, has allowed us to determine the thickness and volumes of the upper four megabeds which are: U ≤ 9 m thick, volume 3.7 km3; A ≤ 25 m thick, volume 12.2 km3; B ≤ 22 m thick, volume 10.3 km3; C ≤ 15 m thick, volume 8 km3. These thick megabeds are the uppermost products of repeated explosive eruption of Santorini in the late Quaternary. Calculated sedimentation rates from and after the “Minoan” eruption are 9.4 m/1000 yr that rise to over 15.7 m/1000 yr if megabed B was also deposited during this eruption.  相似文献   

963.
The Porcupine Abyssal Plain (NE Atlantic) time-series has shown large, wide-scale, changes in the composition of the benthic community at 4800 m depth (48°50′N, 16°30′W). The abundance of holothurians has increased significantly since 1996 and one species in particular, Amperima rosea, has increased in abundance by three orders of magnitude. Environmental forcing in the form of phytodetrital food supply to the benthos is believed to be driving these changes. Chlorophyll and carotenoid pigments were determined from the gut sediments of seven species of abyssal holothurian, sampled from the Porcupine Abyssal Plain during Autumn 2000 and Spring 2002. These two samples fell either side of the main phytoplankton bloom in the NE Atlantic, providing an opportunity for seasonal comparisons. Significant inter-species differences in pigment profiles were observed among the seven species. Seasonal differences were noted among four species sampled in both time periods. All seven species were collected from the same geographical area and depth. As algal pigments cannot be synthesised by the holothurians, they provide good biomarkers for the composition of the phytodetritus. Differences in pigments from gut sediment profiles are indicative of selective feeding among the holothurians. A. rosea had a gut profile dominated by the pigments zeaxanthin, chlorophyll a/echineone and β-carotene; these pigments were all present in significantly smaller quantities in the other species. The high quantities of these pigments are indicative of a diet rich in cyanobacteria. The gut sediments of A. rosea also lacked many chloropigments characteristic of other phytoplankton groups, which were observed in the guts of other holothurian species. Ovarian tissue for the five species taken in the pre-spring bloom 2002 sample were examined. All species showed similar carotenoid profiles, dominated by zeaxanthin, echinenone and β-carotene, all of which are important compounds for reproductive success in echinoderms. The differences in gut pigment profiles highlight the potential for several species of deposit-feeding holothurians to partition the same phytodetrital food source, possibly providing a mechanism for maintaining the high diversity of deposit feeders at abyssal depths. The dominance of reproductively important carotenoids in the guts and gonads of A. rosea may highlight the ability of this species to rapidly utilise any change in the composition of the phytodetrital flux and translate that advantage into a successful reproductive and recruitment event. The results are discussed in relation to work on bathyal holothurians and the potential for food-driven regime shifts in both the abyssal and bathyal Northeast Atlantic.  相似文献   
964.
A comparison between clones of two species of diatoms, Asterionella japonica and Ditylum brightwellii, isolated from Sandy Hook, New Jersey (polluted site) and Montauk, New York (unpolluted site) indicated that some of the clones from Sandy Hook were more resistant to polychlorinated biphenyls (PCB). A japonica was less sensitive to PCB than D. brightwellii since seven clones of A. japonica from both sites were tolerant of 25 μg litre?1 PCB whereas no clones from either site of D. brightwellii showed such resistance. Growth under high light intensity increased the sensitivity to PCB in all clones except one super-resistant clone from Sandy Hook which was not affected even by additions of 50 μg litre?1 to the growth medium. This interactive effect of light on PCB sensitivity may relate to the photo-adaptive characteristics of each clone.  相似文献   
965.
诺尔博  边效曾 《地质论评》1949,14(Z1):73-74
1940年M.N.Short在其Microscopic D termination of theOre Minerals一书(第二版)中会首先指出Van der veen andFarnham所创之矿物旋转色(rotation color)或偏光色(Polarizationcolor)表,并不尽合实用,因偏光色可因Nicols之装置,光源(弧  相似文献   
966.

In this work, a dynamic GIS modeling approach is presented that incorporates: a) geoinformatic techniques, b) 55-year historical meteorological data, and c) field measurements, in order to estimate soil erosion risk in intensively cultivated regions. The proposed GIS-based modeling approach includes the estimation of soil erosion rates due to surface water flow under current and future climate change scenarios A2 and B1 for the years 2030 and 2050. The soil erosion was estimated using the Universal Soil Loss Equation (USLE). The proposed soil erosion model was validated using field measurements at different sites of the study area. The results show that an extended part of the study area is under intense erosion with the mean annual loss to be 4.85 t/ha year−1. Moreover, an increase in rainfall intensity, especially for scenario B1, can generate a significant increase (32.44 %) in soil loss for the year 2030 and a much more (50.77 %) for the year 2050 in comparison with the current conditions. Regarding the scenario A2, a slight decrease (1.85 %) in soil loss was observed for the year 2030, while for 2050 the results show an adequate increase (7.31 %) in comparison with the present. All these approaches were implemented at one of the most productive agricultural areas of Crete in Greece dominated by olive and citrus crops.

  相似文献   
967.
Liquefaction Potential Index (LPI) is used as an assessing tool of liquefaction susceptibility. Based on the existing correlation of the LPI values with the liquefaction potential of a site, liquefaction hazard maps have been compiled. The evaluation of liquefaction susceptibility of the soil layers that had been taken into account for the calculation of LPI value, on the existing classifications, was based on the “Chinese criteria”. However, Seed et al. [Seed, R.B., Cetin, K.O., Moss, R.E.S., Kammerer, A.M., Wu, J., Pestana, J.M., Riemer, M.F., Sancio, R.B., Bray, J.D., Kayen, R.E., Faris, A., 2003. Recent advances in soil liquefaction engineering: a unified and consistent framework. 26th annual ASCE L.A. Geotechnical Spring Seminar, Long Beach, California. 71 pp.] concluded that those criteria are conservative and proposed a modification of them.The basic aim of this paper is to examine the impact of these modifications to the existing LPI-based calibrations of severity scale of liquefaction-induced failures and to propose a new one if it is needed, using box–whisker plot method. The data set that was analyzed is compiled by borings with SPT from liquefied and non-liquefied sites in Taiwan, in Turkey and in Greece.Moreover, a LPI-based formula is proposed, as an outcome from the application of logistic regression analysis to the data set, which can be used for predicting the liquefaction surface occurrence. In addition, a discriminant function is suggested for classifying a site as liquefiable or not. In this approach, the LPI and the thickness of the non-liquefiable cap layer are taken into account as the classification parameters.  相似文献   
968.
The chemistry of Earth's atmosphere during its first 2–2.5 Ga bears on several branches of geoscience including the origin of prebiotic molecules and life itself, early surface processes, the “faint young sun” problem, carbon isotope systematics, and the transition to an oxidized surface. The geologic record, as sparse as it is for this era, presents several difficulties for attempts to model the atmosphere and its changes through time. The prevailing view for the past 50 years has centered around a moderately oxidized atmosphere of CO2 and N2, and most modeling efforts have been directed at reconciling geologic data, and atmospheric and chemical constraints, with such a composition. Improvements in modeling of early Earth processes and increased knowledge of Archean geology, including new geochemical methods and data, have largely helped support this view of the early atmosphere over the last 25 years, but have also left several nagging questions unanswered. How was a sufficient reservoir (and concentration) of prebiotic molecules produced? What were the major reservoirs for carbon, and how did they develop their isotopic signatures? Is there a solution to the problem of the “faint young sun”? Why was surface oxidation delayed following the advent of oxygenic photosynthesis? Lately, some attempts at answering these questions have suggested the importance of more reducing capacity at the early Earth's surface, but without abandoning the idea of a mainly CO2–N2 atmosphere. It may be that returning to ideas of the early atmosphere current during the 1940s and earliest 1950s could help resolve some of these problems. Such an approach may not only be consistent with the atmospheres of the other terrestrial planets, but may help answer significant questions about the surface history of Mars.  相似文献   
969.
Performance of four mesoscale models namely, the MM5, ETA, RSM and WRF, run at NCMRWF for short range weather forecasting has been examined during monsoon-2006. Evaluation is carried out based upon comparisons between observations and day-1 and day-3 forecasts of wind, temperature, specific humidity, geopotential height, rainfall, systematic errors, root mean square errors and specific events like the monsoon depressions.It is very difficult to address the question of which model performs best over the Indian region? An honest answer is ‘none’. Perhaps an ensemble approach would be the best. However, if we must make a final verdict, it can be stated that in general, (i) the WRF is able to produce best All India rainfall prediction compared to observations in the day-1 forecast and, the MM5 is able to produce best All India rainfall forecasts in day-3, but ETA and RSM are able to depict the best distribution of rainfall maxima along the west coast of India, (ii) the MM5 is able to produce least RMSE of wind and geopotential fields at most of the time, and (iii) the RSM is able to produce least errors in the day-1 forecasts of the tracks, while the ETA model produces least errors in the day-3 forecasts.  相似文献   
970.
In the Witwatersrand approximately 40% of the gold is intimately associated with so-called “carbon” in “carbon seam reefs”, which occur in over a dozen paleoplacers, many of them concentrated at two stratigraphic levels in the 7000-m-thick succession of Archean siliciclastic sedimentary rocks. This is reduced carbon, present as kerogen admixed in various proportions with derivative (now solid) bitumen(s). Oil generation and migration were active geological processes during Early Earth history. Numerous possible source rocks for oil generation, including the carbon seams themselves, occur within the Witwatersrand basin. In the Witwatersrand ore, oil-bearing fluid inclusions are also present, derived like the bitumen, by thermal maturation of the kerogen. The presence of kerogen and bitumen in the Witwatersrand sedimentary rocks, together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover, as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested. Paleochannels, which truncated auriferous carbon seams early in the depositional history, are of widespread occurrence, and micro-synsedimentary faults offset carbon seams. The carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development. The various features highlighting the nature and spatial distribution of Witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes.  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