全文获取类型
收费全文 | 859篇 |
免费 | 66篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 57篇 |
地球物理 | 285篇 |
地质学 | 310篇 |
海洋学 | 105篇 |
天文学 | 95篇 |
综合类 | 5篇 |
自然地理 | 66篇 |
出版年
2024年 | 5篇 |
2023年 | 8篇 |
2022年 | 7篇 |
2021年 | 24篇 |
2020年 | 33篇 |
2019年 | 35篇 |
2018年 | 35篇 |
2017年 | 39篇 |
2016年 | 47篇 |
2015年 | 39篇 |
2014年 | 46篇 |
2013年 | 50篇 |
2012年 | 50篇 |
2011年 | 59篇 |
2010年 | 59篇 |
2009年 | 49篇 |
2008年 | 41篇 |
2007年 | 40篇 |
2006年 | 30篇 |
2005年 | 30篇 |
2004年 | 35篇 |
2003年 | 22篇 |
2002年 | 24篇 |
2001年 | 16篇 |
2000年 | 17篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有937条查询结果,搜索用时 31 毫秒
161.
Harris E. Mason Paolo Montagna Laura Kubista Malcolm McCulloch 《Geochimica et cosmochimica acta》2011,75(23):7446-7457
Recent development of paleo-nutrient proxies based on the phosphorus/calcium (P/Ca) ratio in tropical- and deep-water corals (also known as cold-water corals) require an understanding of the processes by which P is incorporated into the coral skeletal aragonite. Here, we apply single- and double-resonance solid-state nuclear magnetic resonance (NMR) spectroscopy to determine the speciation of P in coral aragonite. The results show that the majority of P occurs as phosphate defects in the aragonite structure, but in many samples a significant fraction of the P occurs also in crystalline hydroxylapatite inclusions. Quantification of the amount of hydroxylapatite indicates that its presence is not related simply to external environmental factors and that it can occur at varying abundances in different parts of the same corallite. Since there is currently no model available to describe the relationship between dissolved inorganic phosphate and its incorporation as apatite inclusions into carbonates, careful screening of samples which contain only phosphate in the aragonite structure or selective microsampling could improve proxy development. 相似文献
162.
This article is intended to explain the snow avalanche occurrence, as a natural phenomenon directly influenced by the local natural conditions, for the well-delimited area represented by the Piatra Mica massif, belonging to the Piatra Craiului mountain range (southern Carpathians). In this respect, depending on the factors that may trigger or encourage the avalanches, some vulnerable areas with avalanche occurring conditions have been identified, based on the analysis of the relationships among the factors controlling the avalanche vulnerability in the study area. These factors are mainly represented by the slope aspect, which induces from the very beginning some specific features for each type of slope (north-, east-, south and west-facing slopes), the geological structure, slope gradient and topography. At the same time, the general climatic and biological features have been taken into account, from the point of view of their importance for avalanche occurrence and distribution. Depending on the microrelief exhibited by the avalanche chutes, one can establish distinct dynamical features for each of the four major slopes of the massif. It is worth mentioning that for this study area, this is the first paper dealing with avalanche phenomenon, vulnerable space, control factors and landscape dynamics. In accomplishing this demarche, we used detailed mappings in the field in several stages, the processing of satellite images, analytical (declivities, the exposure of slopes, etc.) and synthetic maps from which the dynamic of sectors with avalanches resulted. The findings of this investigation may further be employed for solving the problems raised by avalanche-prone areas, as well as for devising a better strategy for the effective management of the mountain realm. 相似文献
163.
164.
Laura Navarro Enric Ballesteros Cristina Linares Bernat Hereu 《Estuarine, Coastal and Shelf Science》2011
There are deep-water populations of the endemic and threatened Mediterranean brown alga Cystoseira zosteroides in the Medes Islands Marine Reserve (NW Mediterranean). Here, the distribution, population structure, individual growth, mortality and recruitment rates of this species over two years are described in relation to the effects of an exceptional storm. We found a high spatial variability in the structure and dynamics of C. zosteroides populations at small geographical scales, suggesting that environmental factors acting at the population level display a key role in population size structure, mortality and recruitment. The elevated mortality rates recorded at some locations (almost 80%) is amongst the highest recorded for perennial algae as a consequence of a single storm, emphasizing the importance of episodic catastrophic events in the maintenance of these deep-water, slow-growing populations. These findings are of particular importance not only to fully understand the ecology of C. zosteroides, but also for its conservation. 相似文献
165.
166.
Manuela Lasagna Domenico Antonio De Luca Laura Debernardi Paolo Clemente 《Environmental Earth Sciences》2013,70(6):2767-2784
This paper discusses the effects of the dilution process on levels of aquifer contamination attenuation, developing analytical equations for application in theoretical and practical cases. The key finding is that, in aquifers, the dilution process causes a reduction of the contaminant concentration. Analytical equations for the assessment of the contaminant attenuation capacity, by means of the dilution process in an aquifer, are put forward. In particular, the attenuation of the contaminant concentration in an aquifer, and thus the dilution, is described by the volumetric flow rate per unit width perpendicular to the flow direction (q u). Moreover, a classification of q u was developed, considering six classes of attenuation capacity due to dilution. For the equations validation, nitrate contamination attenuation due to dilution was analysed in two sample areas in Piedmont (North-West Italy). The results confirmed the hypothesis, evidencing high nitrate concentrations in areas with low q u values, connected to low dilution rate of the contaminant in the groundwater; vice versa, low nitrate concentrations were evidenced in areas with high q u values, due to a high nitrate dilution rate. 相似文献
167.
Laura Carmine Elsa Aristodemou Christopher Pain Ann Muggeridge Cassiano de Oliveira 《Geophysical Prospecting》2008,56(1):115-140
The purpose of this work was to investigate a new and fast inversion methodology for the prediction of subsurface formation properties such as porosity, salinity and oil saturation, using time‐dependent nuclear well logging data. Although the ultimate aim is to apply the technique to real‐field data, an initial investigation as described in this paper, was first required; this has been carried out using simulation results from the time‐dependent radiation transport problem within a borehole. Simulated neutron and γ‐ray fluxes at two sodium iodide (NaI) detectors, one near and one far from a pulsed neutron source emitting at ~14 MeV, were used for the investigation. A total of 67 energy groups from the BUGLE96 cross section library together with 567 property combinations were employed for the original flux response generation, achieved by solving numerically the time‐dependent Boltzmann radiation transport equation in its even parity form. Material property combinations (scenarios) and their correspondent teaching outputs (flux response at detectors) are used to train the Artificial Neural Networks (ANNs) and test data is used to assess the accuracy of the ANNs. The trained networks are then used to produce a surrogate model of the expensive, in terms of computational time and resources, forward model with which a simple inversion method is applied to calculate material properties from the time evolution of flux responses at the two detectors. The inversion technique uses a fast surrogate model comprising 8026 artificial neural networks, which consist of an input layer with three input units (neurons) for porosity, salinity and oil saturation; and two hidden layers and one output neuron representing the scalar photon or neutron flux prediction at the detector. This is the first time this technique has been applied to invert pulsed neutron logging tool information and the results produced are very promising. The next step in the procedure is to apply the methodology to real data. 相似文献
168.
Bioremediation of petroleum hydrocarbons in anoxic marine sediments: Consequences on the speciation of heavy metals 总被引:3,自引:0,他引:3
Antonio Dell’Anno Francesca Beolchini Massimo Gabellini Laura Rocchetti Antonio Pusceddu Roberto Danovaro 《Marine pollution bulletin》2009,58(12):1808-1814
We investigated the effects of biostimulation and bioagumentation strategies applied to harbor sediments displaying reducing conditions and high concentrations of petroleum hydrocarbons and heavy metals. We compared the microbial efficiency of hydrocarbon removal from sediments maintained for 60 days in anoxic conditions and inoculated with acetate, sulfate-reducing bacterial strains and acetate and sulfate-reducing bacteria. All treatments determined a significant increase in the microbial growth and significant decreases of hydrocarbon contents and of redox potential values. The addition of sulfate-reducing bacterial strains to the sediment was the most efficient treatment for the hydrocarbon removal. In all experiments, significant changes of the heavy metals’ phase repartition were observed. The results reported here suggest that the biodegradation of petroleum hydrocarbons in anoxic marine sediments may be enhanced by stimulating microbial anaerobic metabolism, but care should be applied to monitor the potential changes in the mobility and bioavailability of heavy metals induced by bio-treatments. 相似文献
169.
Roberto Sulpizio Rosanna Bonasia Pierfrancesco Dellino Mauro A. Di Vito Luigi La Volpe Daniela Mele Giovanni Zanchetta Laura Sadori 《Journal of Volcanology and Geothermal Research》2008
Ash samples from tephra layers correlated with the Pomici di Avellino (Avellino Pumice) eruption of Somma-Vesuvius were collected in distal archives and their composition and particle morphology investigated in order to infer their behaviour of transportation and deposition. Differences in composition and particle morphologies were recognised for ash particles belonging to the magmatic Plinian and final phreatomagmatic phases of the eruption. The ash particles were dispersed in opposite directions during the two different phases of the eruption, and these directions are also different from that of coarse-grained fallout deposits. In particular, ash generated during magmatic phase and injected in the atmosphere to form a sustained column shows a prevailing SE dispersion, while ash particles generated during the final phreatomagmatic phase and carried by pyroclastic density currents show a general NW dispersion. These opposite dispersions indicate an ash dispersal influenced by both high and low atmosphere dynamics. In particular, the magmatic ash dispersal was first driven by stratospheric wind towards NE and then the falling particles encountered a variable wind field during their settling, which produced the observed preferential SE dispersal. The wind field encountered by the rising ash clouds that accompanied the pyroclastic density currents of the final phreatomagmatic phase was different with respect to that encountered by the magmatic ash, and produced a NW dispersal. These data demonstrate how ash transportation and deposition are greatly influenced by both high and low atmosphere dynamics. In particular, fine-grained particles transported in ash clouds of small-scale pyroclastic density currents may be dispersed over distances and cover areas comparable with those injected into the stratosphere by Plinian, sustained columns. This is a point not completely addressed by present day mitigation plans in case of renewal of activity at Somma-Vesuvius, and can yield important information also for other volcanoes potentially characterised by explosive activity. 相似文献
170.
Mark B. Edlund Laura D. Triplett Mark D. Tomasek Kathy Bartilson 《Journal of Paleolimnology》2009,41(4):679-689
Paleolimnological studies show that phosphorus (P) loads to the federally protected St. Croix River, a tributary of the Upper
Mississippi River, have increased about threefold over the last century. Ongoing management efforts to protect and restore
the river hinge on the question of whether the increased nutrient load results from point-source discharges or nonpoint runoff
from agricultural intensification and urban expansion. Here we determine the historical contribution of point source phosphorus
(P) loads to the St. Croix watershed from 1900–2000 A.D. Historical point source loads were estimated based on discharge volumes,
demographics, industrial sources, wastewater technologies, and facility discharge records, where available. Sewering in the
basin began in 1905, and since that time, there have been as many as 169 permitted point source dischargers basinwide, including
municipal, industrial, and agricultural facilities. Early wastewater management typically discharged untreated sewage; technological
advances had secondary treatment in place at most facilities by the 1960s–1970s and much of the municipal population was served
by tertiary treatment by the 1990s. Peak nutrient discharges from point sources occurred in the 1960s–1970s. Detergent phosphorus
bans instituted in the late 1970s for Minnesota and Wisconsin, greater use of land and groundwater effluent disposal, and
improvements in treatment technology brought about decreases in P loads in the 1980s and 1990s. Point-source discharges were
compared to historical total phosphorus loads estimated in a whole-basin phosphorus mass balance to calculate the historical
contribution of point sources, anthropogenic nonpoint sources, and natural or background sources. We estimated 1990s point
source loads at 48 t P yr−1, which represents about 10% of the total phosphorus load (459 t P yr−1, flow-corrected to 412 t P yr−1) to the basin. Without further controls on nutrient inputs to the St. Croix River, annual flow-corrected P loads are projected
to increase to 498 t P yr−1 by the 2020s with point source phosphorus loading contributions at 65 t P yr−1 or 13% of the total load. However, if we exclude background P loads to the St. Croix (166 t P yr−1), recent nutrient loads are primarily from anthropogenic nonpoint sources. Point sources also contribute over 19% of the
current and future phosphorus load that can be attributed to human activities in the watershed. Interstate and federal efforts
to decrease P loading to the St. Croix River by 20% will need to target both point and nonpoint sources.
This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this
special issue of the Journal of Paleolimnology. D.R. Engstrom served as guest editor of the special issue. 相似文献