全文获取类型
收费全文 | 278篇 |
免费 | 8篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 20篇 |
地球物理 | 54篇 |
地质学 | 98篇 |
海洋学 | 37篇 |
天文学 | 37篇 |
自然地理 | 38篇 |
出版年
2023年 | 1篇 |
2021年 | 3篇 |
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 8篇 |
2015年 | 4篇 |
2014年 | 7篇 |
2013年 | 18篇 |
2012年 | 6篇 |
2011年 | 17篇 |
2010年 | 7篇 |
2009年 | 18篇 |
2008年 | 20篇 |
2007年 | 12篇 |
2006年 | 9篇 |
2005年 | 7篇 |
2004年 | 19篇 |
2003年 | 13篇 |
2002年 | 6篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 8篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 8篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 4篇 |
1974年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有290条查询结果,搜索用时 15 毫秒
1.
Concepts derived from previous studies of offshore winds on natural dunes are evaluated on a dune maintained for shore protection during three offshore wind events. The potential for offshore winds to form a lee‐side eddy on the backshore or transfer sediment from the dune and berm crest to the water are evaluated, as are differences in wind speed and sediment transport on the dune crest, berm crest and a pedestrian access gap. The dune is 18–20 m wide near the base and has a crest 4.5 m above backshore elevation. Two sand‐trapping fences facilitate accretion. Data were obtained from wind vanes on the crest and lee of the dune and anemometers and sand traps placed across the dune, on the beach berm crest and in the access gap. Mean wind direction above the dune crest varied from 11 to 3 deg from shore normal. No persistent recirculation eddy occurred on the 12 deg seaward slope. Wind speed on the berm crest was 85–89% of speed at the dune crest, but rates of sediment transport were 2.27 times greater during the strongest winds, indicating that a wide beach overcomes the transport limitation of a dune barrier. Limited transport on the seaward dune ramp indicates that losses to the water are mostly from the backshore, not the dune. The seaward slope gains sand from the landward slope and dune crest. Sand fences causing accretion on the dune ramp during onshore winds lower the seaward slope and reduce the likelihood of detached flows during offshore winds. Transport rates are higher in access gaps than on the dune crest despite lower wind speeds because of flatter slopes and absence of vegetation. Transport rates across dunes and through gaps can be reduced using vegetation and raised walkover structures. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
2.
Nancy L. Chabot Rachel H. Cueva Andrew W. Beck Richard D. Ash 《Meteoritics & planetary science》2020,55(4):726-743
Some of the defining characteristics of the IIG iron meteorite group are their high bulk P contents and massive, coarse schreibersite, which have been calculated to make up roughly 11–14 wt% of each specimen. In this study, we produced two data sets to investigate the formation of schreibersites in IIG irons: measurements of trace elements in the IIG iron meteorite Twannberg and experimental determinations of trace element partitioning into schreibersite. The schreibersite‐bearing experiments were conducted with schreibersite in equilibrium with a P‐rich melt and with bulk Ni contents ranging from 0 to 40 wt%. The partitioning behavior for the 20 elements measured in this study did not vary with Ni content. Comparison of the Twannberg measurements with the experimental results required a correction factor to account for the fact that the experiments were conducted in a simplified system that did not contain a solid metal phase. Previously determined solid metal/P‐rich melt partition coefficients were applied to infer schreibersite/solid metal partitioning behavior from the experiments, and once this correction was applied, the two data sets showed broad similarities between the schreibersite/solid metal distribution of elements. However, there were also differences noted, in particular between the Ni and P contents of the solid metal relative to the schreibersite inferred from the experiments compared to that measured in the Twannberg sample. These differences support previous interpretations that subsolidus schreibersite evolution has strongly influenced the Ni and P content now present in the solid metal phase of IIG irons. Quantitative attempts to match the IIG solid metal composition to that of late‐stage IIAB irons through subsolidus schreibersite growth were not successful, but qualitatively, this study corroborates the striking similarities between the IIAB and IIG groups, which are highly suggestive of a possible genetic link between the groups as has been previously proposed. 相似文献
3.
Nancy England April L. James Krystopher J. Chutko Richard S. Pyrce Huaxia Yao 《水文研究》2019,33(6):905-919
Stable water isotope surveys have increasingly been integrated into river basins studies, but fewer have used them to evaluate impact of hydropower regulation. This study applies hydrologic and water isotope survey approaches to a Canadian Shield river basin with both regulated and natural flows. Historical streamflow records were used to evaluate the influence of three hydroelectric reservoirs and unregulated portions of the basin on downstream flows and changes in water level management implemented after an extreme flood year (1979). In 2013, water isotope surveys of surface and source waters (e.g., rainfall, groundwater, snowmelt) were conducted to examine spatial and temporal variation in contributions to river flow. Seasonal changes in relative groundwater contribution were assessed using a water‐isotope mass balance approach. Within the basin, two regulated reservoirs exhibited inverted hydrographs with augmented winter flows, whereas a third exhibited a hydrograph dominated by spring snowmelt. In 2013, spatial variation in rain‐on‐snow and air temperatures resulted in a critical lag in snowmelt initiation in the southern and northern portions of the basin resulting in a dispersed, double peak spring hydrograph, contrasting with 1979 when a combination of rain‐on‐snow and coincident snowmelt led to the highest flood on record. Although eastern basin reservoirs become seasonally enriched in δ18O and δ2H values, unregulated western basin flows remain less variable due to groundwater driven baseflow with increasing influence downstream. Combined analysis of historical streamflow (e.g., flood of 1979, drought of 2010) and the 2013 water isotope surveys illustrate extreme meteorological conditions that current management activities are unable to prevent. In this study, the influence of evaporative fractionation on large surface water reservoirs provides important evidence of streamflow partitioning, illustrating the value of stable water isotope tracers for study of larger catchments. 相似文献
4.
Describing the spatial variability of heterogeneous snowpacks at a watershed or mountain‐front scale is important for improvements in large‐scale snowmelt modelling. Snowmelt depletion curves, which relate fractional decreases in snow‐covered area (SCA) against normalized decreases in snow water equivalent (SWE), are a common approach to scale‐up snowmelt models. Unfortunately, the kinds of ground‐based observations that are used to develop depletion curves are expensive to gather and impractical for large areas. We describe an approach incorporating remotely sensed fractional SCA (FSCA) data with coinciding daily snowmelt SWE outputs during ablation to quantify the shape of a depletion curve. We joined melt estimates from the Utah Energy Balance Snow Accumulation and Melt Model (UEB) with FSCA data calculated from a normalized difference snow index snow algorithm using NASA's moderate resolution imaging spectroradiometer (MODIS) visible (0·545–0·565 µm) and shortwave infrared (1·628–1·652 µm) reflectance data. We tested the approach at three 500 m2 study sites, one in central Idaho and the other two on the North Slope in the Alaskan arctic. The UEB‐MODIS‐derived depletion curves were evaluated against depletion curves derived from ground‐based snow surveys. Comparisons showed strong agreement between the independent estimates. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
5.
Derek G. Turner Brent C. Ward Duane G. Froese Michel Lamothe Jeffrey D. Bond Nancy H. Bigelow 《Boreas: An International Journal of Quaternary Research》2016,45(3):521-536
At least five Middle to Late Pleistocene advances of the northern Cordilleran Ice Sheet are preserved at Silver Creek, on the northeastern edge of the St Elias Mountains in southwest Yukon, Canada. Silver Creek is located 100 km up‐ice of the Marine Isotope Stage (MIS) 2 McConnell glacial limit of the St Elias lobe. This site contains ~3 km of nearly continuous lateral exposure of glacial and non‐glacial sediments, including multiple tills separated by thick gravel, loess and tilted lake beds. Infrared‐stimulated luminescence (IRSL) and AMS radiocarbon dating constrain the glacial deposits to MIS 2, 4, either MIS 6 or mid‐MIS 7, and two older Middle Pleistocene advances. This chronology and the tilt of the lake beds suggest Pleistocene uplift rates of up to 1.9 mm a?1 along the Denali Fault since MIS 7. The non‐glacial sediment consists of sand, gravel, loess and organic beds from MIS 7, MIS 3 and the early Holocene. The MIS 3 deposits date to between 30–36 14C ka BP, making Silver Creek one of the few well‐constrained MIS 3‐aged sites in Yukon. This confirms that ice receded close to modern limits in MIS 3. Pollen and macrofossil analyses show that a meadow‐tundra to steppe‐tundra mosaic with abundant herbs and forbs and few shrubs or trees, dominated the environment at this time. The stratigraphy at Silver Creek provides a palaeoclimatic record since at least MIS 8 and comprises the oldest direct record of Pleistocene glaciation in southwest Yukon. 相似文献
6.
We have calculated the compressional, vibrational, and thermodynamic properties of Ni3S2 heazlewoodite and the high-pressure orthorhombic phase (with Cmcm symmetry) using the generalized gradient approximation
to the density functional theory in conjunction with the quasi-harmonic approximation. The predicted Raman frequencies of
heazlewoodite are in good agreement with room-temperature measurements. The calculated thermodynamic properties of heazlewoodite
at room conditions agree very well with experiments, but at high temperatures (especially above 500 K) the heat capacity data
from experiments are significantly larger than the quasi-harmonic results, indicating that heazlewoodite is anharmonic. On
the other hand, the obtained vibrational density of states of the orthorhombic phase at 20 GPa reveals a group of low-frequency
vibrational modes which are absent in heazlewoodite. These low-frequency modes contribute substantially to thermal expansivity,
heat capacity, entropy, and Grüneisen parameter of the orthorhombic phase. The calculated phase boundary between heazlewoodite
and the orthorhombic phase is consistent with high-pressure experiments; the predicted transition pressure is 17.9 GPa at
300 K with a negative Clapeyron slope of −8.5 MPa/K. 相似文献
7.
The impacts of climate change on water and nitrogen cycles in arid central Arizona (USA) were investigated by integrating the Second Generation Coupled Global Climate Model (CGCM2) and a widely used, physical process-based model, Soil and Water Assessment Tool (SWAT). With statistically downscaled daily climate data from the CGCM2 as model input, SWAT predicted increased potential evapotranspiration and decreased surface runoff, lateral flow, soil water, and groundwater recharge, which suggests serious consequences for the water cycle in this desert catchment in the future. Specifically, stream discharge is projected to decrease by 31 % in the 2020s, 47 % in the 2050s, and 56 % in the 2080s compared to the mean discharge for the base period (0.73 m3/s). A flow-duration analysis reveals that the projected reduction of stream discharge in the future is attributable to significant decreases in mid-range and low-flow conditions; however, flood peaks would show a slight increase in the future. The drier and hotter future also will decrease the rate of nitrogen mineralization in the catchment and ultimately, nitrate export from the stream. Since mean mineralization rate would decrease by 15 % in the 2020s, 28 % in the 2050s, and 35 % in the 2080s compared to the based period (9.3 g N ha?1 d?1), the combined impact of reduced catchment mineralization and reduced streamflow would predict declining nitrate export: from today’s mean value of 30 kg N/d, to 20, 15 and 12 kg N/d by the 2020s, 2050s, and 2080s, respectively. 相似文献
8.
It is often argued that the growth of major emerging powers, such as China, India, Russia, and Brazil, will have a transformative effect on the world economy and politics, but the implications of their rise on sustainable development diplomacy have remained understudied. Do these new powers have the potential to exercise leadership and address the stalled performance on the sustainable development agenda? Drawing on theories of leadership in multiparty negotiations and the empirical study of two major emerging powers’ coalitions, BRICS (Brazil, Russia, India, China, South Africa) and BASIC (Brazil, South Africa, India, China), this article assesses their leadership potential in sustainable development diplomacy. It finds that emerging powers are already engaging in policy coordination in the sustainable development realm, have begun to use their influence to create incentives for other actors in the system and have been conducting skillful diplomacy to develop their coalitions and forge consensus in global policy-making. However, exerting actual global leadership would require building a stronger policy agenda. One area with particular potential for emerging powers to exert leadership is the clean energy arena of sustainable development diplomacy. 相似文献
9.
Brian W. Zimmer Nancy R. Riggs Gerardo Carrasco-Núñez 《Bulletin of Volcanology》2010,72(10):1223-1240
Cerro Pinto is a Pleistocene rhyolite tuff ring-dome complex located in the eastern Trans-Mexican Volcanic Belt. The complex
is composed of four tuff rings and four domes that were emplaced in three eruptive stages marked by changes in vent location
and eruptive character. During Stage I, vent clearing produced a 1.5-km-diameter tuff ring that was then followed by emplacement
of two domes of approximately 0.2 km3 each. With no apparent hiatus in activity, Stage II began with the explosive formation of a tuff ring ~2 km in diameter adjacent
to and north of the earlier ring. Subsequent Stage II eruptions produced two smaller tuff rings within the northern tuff ring
as well as a small dome that was mostly destroyed by explosions during its growth. Stage III involved the emplacement of a
0.04 km3 dome within the southern tuff ring. Cerro Pinto’s eruptive history includes sequences that follow simple rhyolite-dome models,
in which a pyroclastic phase is followed immediately by effusive dome emplacement. Some aspects of the eruption, however,
such as the explosive reactivation of the system and explosive dome destruction, are more complex. These events are commonly
associated with polygenetic structures, such as stratovolcanoes or calderas, in which multiple pulses of magma initiate reactivation.
A comparison of major and trace element geochemistry with nearby Pleistocene silicic centers does not show indication of any
co-genetic relationship, suggesting that Cerro Pinto was produced by a small, isolated magma chamber. The compositional variation
of the erupted material at Cerro Pinto is minimal, suggesting that there were not multiple pulses of magma responsible for
the complex behavior of the volcano and that the volcanic system was formed in a short time period. The variety of eruptive
style observed at Cerro Pinto reflects the influence of quickly exhaustible water sources on a short-lived eruption. The rising
magma encountered small amounts of groundwater that initiated eruption phases. Once a critical magma:water ratio was exceeded,
the eruptions became dry and sub-plinian to plinian. The primary characteristic of Cerro Pinto is the predominance of fall
deposits, suggesting that the level at which rising magma encountered water was deep enough to allow substantial fragmentation
after the water source was exhausted. Isolated rhyolite domes are rare and are not currently viewed as prominent volcanic
hazards, but the evolution of Cerro Pinto demonstrates that individual domes may have complex cycles, and such complexity
must be taken into account when making hazard risk assessments. 相似文献
10.