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841.
Larry Syu-Heng Lai Joshua J. Roering Noah J. Finnegan Rebecca J. Dorsey Jiun-Yee Yen 《地球表面变化过程与地形》2021,46(13):2671-2689
The incision rate and steepness of bedrock channels depend on water discharge, uplift rate, substrate lithology, sediment flux, and bedload size. However, the relative role of these factors and the sensitivity of channel steepness to rapid (>1 mm yr−1) uplift rates remain unclear. We conducted field and topographic analyses of fluvial bedrock channels with varying channel bed lithology and sediment source rock along the Coastal Range in eastern Taiwan, where uplift rates vary from 1.8 to 11.8 mm yr−1 and precipitation is relatively consistent (1.5–2.7 m yr−1), to evaluate the controls on bedrock channel steepness. We find that channel steepness is independent of rock uplift rate and annual precipitation but increases monotonically with sediment size and substrate strength. Furthermore, in reaches with uniform substrate lithology (mudstone and flysch), channel steepness systematically varies with sediment source rock but not with channel width. When applied to our data, a mechanistic incision model (saltation-abrasion model) suggests that the steepness of Coastal Range channels is set primarily by coarse-sediment supply. We also observe that larger particles are mainly composed of resistant lithologies derived from volcanic rocks and conglomerates. This result implies that hillslope bedrock properties in the source area exert a dominant control on the steepness of proximal channels through coarse-sediment production in this setting. We propose that channel steepness may be insensitive to uplift rate and flow discharge in fast-uplifting landscapes where incision processes are set by coarse sediment size and supply. Models assuming a proportionality between incision rate and basal shear stress (stream power) may not fully capture controls on fluvial channel profiles in landslide-dominated landscapes. Processes other than channel steepening, such as enhanced bedload impacts and debris-flow scour, may be required to balance rock uplift and incision in these transport-limited systems. 相似文献
842.
OSL chronology and paleoclimatic implications of paleodunes in
the middle and southwestern Qaidam Basin,
Qinghai-Tibetan Plateau 总被引:2,自引:0,他引:2
Aeolian sediments are widely distributed in the Qaidam Basin, northeastern Qinghai-Tibetan Plateau (QTP). The paleodunes are mainly located at the southeastern, middle and southwestern Qaidam Basin. However, ages of paleodunes in the middle and southwestern Qaidam Basin have not been well studied, although they are close to and might connect to the evolution of salt lakes in the central basin. In this study, we use single aliquot regeneration (SAR) protocol of Optically Stimulated Luminescence (OSL) to date the dune sand in these two regions. The results show that: (1) Sand accumulation in these regions started at ca. 4-3 ka and lasted to ca. 0.5 ka when they were stabilized, due to the arid climate in the late Holocene. (2) The underlying fluvial sand was formed during deglaciation at 12.6±0.8 ka. (3) The stabilization periods of the paleodunes correspond to stages of glacier advance in the northeastern QTP, during which lower temperatures caused the decrease of evaporation and increase of the effective moisture, leading to an increase of vegetation cover and stabilization of the dunes. 相似文献
843.
中全新世黄河流域耕地格局重建及对原生植被的影响(英文) 总被引:1,自引:0,他引:1
Yangshao culture is the most important mid-Holocene Neolithic culture in the Yellow River catchment,and thus,a study on the impact of human activities on the environment is important.In the current study,the distribution pattern of the cultivated land in late Yangshao culture is reconstructed using GIS tool and site domain analysis(SDA).The results show that the cultivated land during 5.5-5.0 ka BP was mainly distributed in the Weihe River valley,Luohe River valley,northwestern Henan Plain,Fenhe River valley and eastern Gansu region,especially concentrated in the Xi’an-Baoji line of the Weihe River valley.At that time,at least 37,000 km 2 of lands were reclaimed in the middle and lower reaches of the Yellow River,and 132,000 km 2 of lands were affected by agricultural activities.Human activities had become the driving force of land use/land coverage.Charcoal records indicate that the ancestors of Yangshao culture burnt forests for reclamation,leading to the decrease of arbor pollen at 5 ka BP in core areas of the Yangshao culture.The areas that were significantly affected by human activities accounted for 3.2% of the Yangshao culture influenced area,while the moderately affected areas accounted for 20.1% of Yangshao culture influenced area.Meanwhile,92% of the land areas on the edge and outside of the Yangshao culture influenced area were not affected by human activities.The arbor pollen in these areas did not decrease until 4.0 ka BP. 相似文献
844.
Xiang-Jun Lai Yan Li National Astronomical Observatories /Yunnan Observatory Chinese Academy of Sciences Kunming China Key Laboratory for the Structure Evolution of Celestial Objects China Graduate University of Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2011,(11)
Based on the turbulent convection model(TCM),we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediatemass stars,focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell(HR) diagram.A diffusive mixing model is adopted during the Red Giant Branch(RGB) phase.The properties of the blue loop are changed by modification of the element profiles above the H-burning shell,which results from the incomplete mixing in the bottom... 相似文献
845.
846.
847.
岩浆熔体的氧逸度是岩石学研究中的一个重要物理化学参数。本文从氧逸度的概念、估算方法、影响因素等方面综述了近年来基性及酸性岩浆氧逸度的研究成果。 相似文献
848.
Monthly investigations were mae on the population of Chinese freshwater crab,Sinopotamon yangtsekiense Bott, 1967 from April, 1984 to March, 1985. The data on 4413 specimens show that the growth was affected mainly by temperature.
During the April to November growth period, the crabs' major development occurred from June through October. One year was
required for a fine white oocyte to develop into a mature egg. The reproduction period was June–October. Females bearing eggs
were taken from June–August, and crabs with young were found from July–October. The females reproduced once a year but could
for more than one year. The number of eggs carried by a female varied greatly according to the size of the crab, ranging from
30 to 100 eggs. New-born crabs become mature after 1–2 years. The sex ratio was approximately 1∶1 in the overall population.
However, the larger crabs are predominantly male. The age distribution ofS. yangtsekinese was estimated from size frequency histograms. There were more adult crabs (over 70%) from June to October and more immature
crabs (over 50%) from November to May. 相似文献
849.
Detailed studies indicate that the main rock type of the Neogene high-potassium calc-alkaline volcanic rock association from north Qiangtang is andesite, dacite and rhyolite. They belong to typical crust-generation magmatic system and originate from the special thickened crust of the Tibetan Plateau by dehydration melting. This group of rocks exhibits LREE enrichment but no remarkable Eu anomaly that shows their source region should be a thickened deep crust consisting of eclogitic mass group, implying that the crust had been thickened and an eclogitic deep crust had been formed during the Neogene period in Qiangtang area. This understanding is important and significant to making further discussion on the uplift mechanism and continental dynamics of the Tibetan Plateau. 相似文献
850.
结构地震反应时程分析中的阻尼问题评述 总被引:22,自引:5,他引:22
本文对一般建筑结构地震反应时程分析,主要是弹塑性时程分析中阻尼问题的研究现状进行了分析介绍,指出了存在的问题,并就解决这些问题的关键提出了作者的看法。 相似文献