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排序方式: 共有576条查询结果,搜索用时 15 毫秒
571.
572.
Xiuju Liu Steven M. Colman Erik T. Brown Andrew C. G. Henderson Josef P. Werne Jonathan A. Holmes 《Journal of Paleolimnology》2014,51(2):223-240
We inferred the climate history for Central Asia over the past 20,000 years, using sediments from core QH07, taken in the southeastern basin of Lake Qinghai, which lies at the northeastern margin of the Tibetan Plateau. Results from multiple environmental indicators are internally consistent and yield a clear late Pleistocene and Holocene climate record. Carbonate content and total organic carbon (TOC) in Lake Qinghai sediments are interpreted as indicators of the strength of the Asian summer monsoon. Warm and wet intervals, associated with increased monsoon strength, are indicated by increased carbonate and TOC content. During the glacial period (~20,000 to ~14,600 cal year BP), summer monsoon intensity remained low and relatively constant at Lake Qinghai, suggesting cool, dry, and relatively stable climate conditions. The inferred stable, cold, arid environment of the glacial maximum seems to persist through the Younger Dryas time period, and little or no evidence of a warm interval correlative with the Bølling–Allerød is found in the QH07 record. The transition between the late Pleistocene and the Holocene, about 11,500 cal year BP, was abrupt, more so than indicated by speleothems in eastern China. The Holocene (~11,500 cal year BP to present) was a time of enhanced summer monsoon strength and greater variability, indicating relatively wetter but more unstable climatic conditions than those of the late Pleistocene. The warmest, wettest part of the Holocene, marked by increased organic matter and carbonate contents, occurred from ~11,500 to ~9,000 cal year BP, consistent with maximum summer insolation contrast between 30°N and 15°N. A gradual reduction in precipitation (weakened summer monsoon) is inferred from decreased carbonate content through the course of the Holocene. We propose that changes in the contrast of summer insolation between 30°N and 15°N are the primary control on the Asian monsoon system over glacial/interglacial time scales. Secondary influences may include regional and global albedo changes attributable to ice-cover and vegetation shifts and sea level changes (distance from moisture source in Pacific Ocean). The abruptness of the change at the beginning of the Holocene, combined with an increase in variability, suggest a threshold for the arrival of monsoonal rainfall at the northeastern edge of the Tibetan Plateau. 相似文献
573.
Abstract This paper examines the potential effects of urbanization on streamflow in Maine, USA, from 1950 to 2000. The study contrasts nine watersheds in southern Maine, which has seen steady urban growth over the study period, with nine rural watersheds from northern Maine. Historical population data and current land cover data are used to develop an urbanization score for each watershed. Trends in watershed urbanization over the study period are compared to trends in ecologically relevant streamflow characteristics. The results indicate that trends in northern, rural watersheds are much more consistent than the trends in the southern watersheds. Additionally, trends in the southern watersheds are inconsistent with the hydrological characteristics observed in urban watersheds elsewhere, likely due to the comparatively low level of current urban development in Maine's urban watersheds. Our study suggests that urban areas in Maine have not yet reached an urbanization threshold where streamflow impacts become consistently detectable. Editor Z.W. Kundzewicz Citation Martin, E.H., Kelleher, C., and Wagener, T., 2012. Has urbanization changed ecological streamflow characteristics in Maine (USA)? Hydrological Sciences Journal, 57 (7), 1337–1354. 相似文献
574.
575.
Experimental determination of the coefficient of restitution for meter-scale granite spheres 总被引:4,自引:0,他引:4
We present results of a series of large-scale experiments to measure the coefficient of restitution for 1-m-diameter rocky bodies in impacts with collision speeds up to ∼1.5 m s−1. The experiments were conducted in an outdoor setting, with two 40-ton cranes used to suspend the ∼1300-kg granite spheres pendulum-style in mutual contact at the bottoms of their respective paths of motion. The spheres were displaced up to ∼1 m from their rest positions and allowed to impact each other in normal-incidence collisions at relative speeds up to ∼1.5 m s−1. Video data from 66 normal-incidence impacts suggest a value for the coefficient of restitution of 0.83 ± 0.06 for collisions between ∼1-m-scale spheres at speeds of order 1 m s−1. No clear trend of coefficient of restitution with impact speed is discernable in the data. 相似文献
576.
Mengist Teklay Erik E. Scherer Klaus Mezger Leonid Danyushevsky 《Contributions to Mineralogy and Petrology》2010,159(5):731-751
The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting
to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts
from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic
composition (87Sr/86Sr = 0.70336–0.70356, ε
Nd = +6.6 to +7.0, and ε
Hf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair
islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths
vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, ε
Nd = −8.1 to +2.5, and ε
Hf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield
and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks.
Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or
hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment
of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with
their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that
crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the
host basalts. 相似文献