全文获取类型
收费全文 | 255542篇 |
免费 | 5291篇 |
国内免费 | 3262篇 |
专业分类
测绘学 | 7049篇 |
大气科学 | 19207篇 |
地球物理 | 53026篇 |
地质学 | 89075篇 |
海洋学 | 21623篇 |
天文学 | 55162篇 |
综合类 | 1022篇 |
自然地理 | 17931篇 |
出版年
2021年 | 2167篇 |
2020年 | 2597篇 |
2019年 | 2828篇 |
2018年 | 3859篇 |
2017年 | 3612篇 |
2016年 | 5964篇 |
2015年 | 4243篇 |
2014年 | 6966篇 |
2013年 | 14274篇 |
2012年 | 6642篇 |
2011年 | 8129篇 |
2010年 | 7190篇 |
2009年 | 9766篇 |
2008年 | 8540篇 |
2007年 | 7982篇 |
2006年 | 9632篇 |
2005年 | 7769篇 |
2004年 | 7649篇 |
2003年 | 7151篇 |
2002年 | 6666篇 |
2001年 | 5902篇 |
2000年 | 5588篇 |
1999年 | 4808篇 |
1998年 | 4849篇 |
1997年 | 4638篇 |
1996年 | 4226篇 |
1995年 | 4304篇 |
1994年 | 4000篇 |
1993年 | 3761篇 |
1992年 | 3513篇 |
1991年 | 3530篇 |
1990年 | 3626篇 |
1989年 | 3328篇 |
1988年 | 3161篇 |
1987年 | 3713篇 |
1986年 | 3251篇 |
1985年 | 4118篇 |
1984年 | 4629篇 |
1983年 | 4308篇 |
1982年 | 4224篇 |
1981年 | 3842篇 |
1980年 | 3592篇 |
1979年 | 3435篇 |
1978年 | 3442篇 |
1977年 | 3217篇 |
1976年 | 2964篇 |
1975年 | 2899篇 |
1974年 | 2862篇 |
1973年 | 3060篇 |
1972年 | 1989篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A sloping travertine mound, approximately 85 m across and a few metres thick is actively forming from cool temperature waters issuing out of Crystal Geyser, east‐central Utah, USA. Older travertine deposits exist at the site, the waters having used the Little Grand Wash Fault system as conduits. In contrast, the present Crystal Geyser travertine mound forms from 18°C waters which have been erupting for the last 80 years from an abandoned oil well. The present Crystal Geyser travertine accumulation forms from a ‘man‐made’ cool temperature geyser system; nevertheless, the constituents are an analogue for ancient geyser‐fed carbonate deposits. The travertine primary fabric is composed of couplets of highly porous, thin micritic laminae intercalated with thicker iron oxide rich laminae. Low Mg‐calcite is the dominant mineralogy; however, aragonite is a major constituent in deposits proximal to the vent and decreases in abundance distally. Cements exhibit a variety of fabrics, isopachous being common. Constituents include micro‐stromatolites, clasts, pisoids and the common occurrence of Frutexites‐like iron oxide precipitates. Leptothrix, a common iron‐oxidizing bacterium, is believed to be responsible for the production of the dense iron‐rich laminae. Pisoids litter the ground around the vent and rapidly decrease distally in abundance and size. 相似文献
992.
993.
Tectonic control of accommodation space and sediment supply within the Mata Amarilla Formation (lower Upper Cretaceous) Patagonia,Argentina 下载免费PDF全文
Augusto N. Varela 《Sedimentology》2015,62(3):867-896
The Mata Amarilla Formation dates from the early Upper Cretaceous and was deposited during a transition in tectonic regime from the extensional Rocas Verdes Basin to the Austral Foreland Basin. Detailed sedimentological logs and architectural parameters were used to define 13 facies associations. The distribution of facies associations and associated variations in fluvial architecture have enabled large‐scale changes in accommodation space/sediment supply ratios (A/S ratio) to be defined for the three component sections of the Mata Amarilla Formation. The lower and upper sections are characterized by a high A/S ratio, whereas the middle section corresponds to a low A/S ratio. In the western part of the study area, small‐scale variations in the A/S ratio were recognized in the middle section. The strong west to east trend in evolution of the fluvial systems coincides with the direction of propagation of the Patagonian fold and thrust belt, which is located to the west of the study area. Intervals of high A/S ratio (i.e. lower and upper sections) are interpreted to have developed during periods of increased loading by the fold and thrust belt caused by tectonic uplift. In contrast, intervals of low A/S ratio (i.e. middle section) were developed during periods of tectonic quiescence. This article suggests that the large‐scale variations in A/S ratios are related to different rates of migration and growth of the Patagonian fold and thrust belt, whereas the small‐scale variation occurred in response to specific periods of thrusting and folding in the Patagonian fold and thrust belt (i.e. local loads). This field example of the effects of different scales of variation in A/S ratios across the Austral Foreland Basin could be used to recognize similar tectonically forced variations in stratigraphic architecture in other foreland basins throughout the world, as well as to understand the response of fluvial systems to such changes. 相似文献
994.
High‐resolution reconstruction of a coastal barrier system: impact of Holocene sea‐level change 下载免费PDF全文
Mikkel Fruergaard Thorbjørn J. Andersen Lars H. Nielsen Peter N. Johannessen Troels Aagaard Morten Pejrup 《Sedimentology》2015,62(3):928-969
This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply. 相似文献
995.
996.
Journal of Earth System Science - Based on the daily maximum air temperature data from 176 stations in India from 1969 to 2013, the climatological distribution of the number of days with high... 相似文献
997.
Journal of Earth System Science - A severe thunderstorm produced a tornado (F2 on the enhanced Fujita–Pearson scale), which affected the Brahmanbaria district of Bangladesh during... 相似文献
998.
Discussed is the hydrodynamic model of iceberg drift. Presented are the examples ofits use in operational practice for predicting iceberg drift in the Kara Sea and for enhancing ice monitoring in the Barents Sea. 相似文献
999.
1000.