全文获取类型
收费全文 | 254篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 12篇 |
大气科学 | 37篇 |
地球物理 | 57篇 |
地质学 | 72篇 |
海洋学 | 23篇 |
天文学 | 48篇 |
综合类 | 2篇 |
自然地理 | 17篇 |
出版年
2024年 | 5篇 |
2023年 | 1篇 |
2022年 | 3篇 |
2021年 | 11篇 |
2020年 | 7篇 |
2019年 | 7篇 |
2018年 | 18篇 |
2017年 | 12篇 |
2016年 | 11篇 |
2015年 | 7篇 |
2014年 | 11篇 |
2013年 | 17篇 |
2012年 | 15篇 |
2011年 | 20篇 |
2010年 | 11篇 |
2009年 | 20篇 |
2008年 | 20篇 |
2007年 | 16篇 |
2006年 | 7篇 |
2005年 | 5篇 |
2004年 | 7篇 |
2003年 | 6篇 |
2002年 | 4篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1988年 | 1篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有268条查询结果,搜索用时 31 毫秒
171.
Marcelo C. L. Cohen Paloma Maria Pinto Camargo Luiz C. R. Pessenda Flávio Lima Lorente Adriana V. De Souza José Augusto Martins Corrêa José Bendassolli Marianne Dietz 《第四纪科学杂志》2021,36(6):1013-1027
The high sea-level stand during the mid-Holocene is a benchmark in mangrove dynamics along the north-east/south-east coast of Brazil and provides a reference point for landward and seaward mangrove migrations corresponding to changes in relative sea level (RSL). However, evidence of the impacts associated with RSL fall on the northern Brazilian coast is scarce. Multi-proxy data from the highest tidal flats of the Bragança Peninsula in northern Brazil revealed modern herbaceous areas were occupied by mangroves Rhizophora and Avicennia from ~6250 to ~5850 cal a bp , and only Avicennia between ~5850 and ~5000 cal a bp . The same tidal flats were vegetation-free between ~5000 and ~4300 cal a bp . A combination of a high sea-level stand (0.6 ± 0.1 m) at ~5000 cal a bp and a dry early–middle Holocene in the Amazon probably caused an increase in porewater salinity of tidal flats, which resulted in a mangrove succession from Rhizophora to Avicennia dominance. RSL fall accentuated this process, contributing to mangrove degradation between ~5000 and ~4300 cal a bp . RSL fall, and a wetter period over the past ~4300 cal a bp caused a mangrove migration from highest to lowest flats, followed by expansion of herbaceous vegetation on the highest flats. 相似文献
172.
Mariano?PoissonEmail author Pascal?Démoulin Marcelo?López Fuentes Cristina?H.?Mandrini 《Solar physics》2016,291(6):1625-1646
The photospheric spatial distribution of the main magnetic polarities of bipolar active regions (ARs) present during their emergence deformations are known as magnetic tongues. They are attributed to the presence of twist in the toroidal magnetic-flux tubes that form the ARs. The aim of this article is to study the twist of newly emerged ARs from the evolution of magnetic tongues observed in photospheric line-of-sight magnetograms. We apply the procedure described by Poisson et al. (Solar Phys. 290, 727, 2015a) to ARs observed over the full Solar Cycle 23 and the beginning of Cycle 24. Our results show that the hemispherical rule obtained using the tongues as a proxy of the twist has a weak sign dominance (53 % in the southern hemisphere and 58 % in the northern hemisphere). By defining the variation of the tongue angle, we characterize the strength of the magnetic tongues during different phases of the AR emergence. We find that there is a tendency of the tongues to be stronger during the beginning of the emergence and to become weaker as the AR reaches its maximum magnetic flux. We compare this evolution with the emergence of a toroidal flux-rope model with non-uniform twist. The variety of evolution of the tongues in the analyzed ARs can only be reproduced when using a broad range of twist profiles, in particular having a large variety of twist gradients in the direction vertical to the photosphere. Although the analytical model used is a special case, selected to minimize the complexity of the problem, the results obtained set new observational constraints to theoretical models of flux-rope emergence that form bipolar ARs. 相似文献
173.
Multidecadal changes in the relationship between extreme temperature events in Uruguay and the general atmospheric circulation 总被引:1,自引:0,他引:1
We analyze changes in the relationship between extreme temperature events and the large scale atmospheric circulation before and after the 1976 climate shift. To do so we first constructed a set of two temperature indices that describe the occurrence of warm nights (TN90) and cold nights (TN10) based on a long daily observed minimum temperature database that spans the period 1946?C2005, and then divided the period into two subperiods of 30?years each (1946?C1975 and 1976?C2005). We focus on summer (TN10) and winter (TN90) seasons. During austral summer before 1976 the interannual variability of cold nights was characterized by a negative phase of the Southern Annular Mode (SAM) with a cyclonic anomaly centered off Uruguay that favoured the entrance of cold air from the south. After 1976 cold nights are associated not with the SAM, but with an isolated vortex at upper levels over South Eastern South America. During austral winter before 1976, the El Ni?o phenomenon dominated the interannual variability of warm nights through an increase in the northerly warm flow into Uruguay. However, after 1976 the El Ni?o connection weakened and the variability of warm nights is dominated by a barotropic anticyclonic anomaly located in the South Atlantic and a low pressure center over South America. This configuration also strengthens the northward flow of warm air into Uruguay. Our results suggest that changes in El Ni?o evolution after 1976 may have played a role in altering the relationship between temperature extreme events in Uruguay and the atmospheric circulation. 相似文献
174.
The proposed techniques in recent years for the analysis of superficial waves with multiple transducers have been focused in the identification of different types of waves in the ground. The multistation tests avoid the ambiguous phase unwrapping procedure required to obtain experimental dispersion curves for the SASW technique. However, the soil profiles with stiffness inversion in depth involve the contribution of higher modes, and the inversion process through “apparent” or “effective” dispersion curves presents difficulties since these contributions depend of the transducers layout. The technique proposed herein is based on the test simulation through an updating model with low computational cost and good accuracy that include all propagation modes. Two actual test cases performed with only six transducers shows the advantages of a new objective function called “spatial phase dispersion” whose experimental determination does not require subjective interventions by the analyst. 相似文献
175.
Succession and seasonal onset of colonization in subtidal hard‐bottom communities off northern Chile
Aldo S. Pacheco Jürgen Laudien Martin Thiel Marcelo Oliva Olaf Heilmayer 《Marine Ecology》2011,32(1):75-87
Although there is extensive information concerning the colonization sequences of benthic communities, little is known about the successional development of subtidal hard‐bottom habitats in highly productive coastal upwelling areas. In these systems, succession is predicted to be fast due to high growth rate of the later dominant colonizers. Using artificial hard substrata a field experiment was conducted in a rocky subtidal area off Northern Chile (Humboldt Current System) and monitored at 3‐month intervals to test the following hypotheses: (i) epibenthic succession may proceed through consecutive replacement of species, (ii) there is a fast convergence rate towards natural communities, and (iii) different seasonal starting points for the colonization will produce different community structure over a 1‐year period of exposure. Panels were installed on a vertical wall at 17 m water depth. Three replicate panels were sampled every 3 months over a period of 27 months. As a reference, six haphazardly selected plots from the surrounding natural community were surveyed at each sampling date. To evaluate how seasonally varying substratum availability affects community development, further panels were exposed for a 12‐month period, starting in four different seasons (n = 3 replicates per season). Community succession was slow and occurred through progressive changes, between early encrusting red corallines, middle Balanus flosculus and late Lagenicella variabilis. After 27 months, the community composition, but not its structure, was similar between experimental and reference communities on surrounding rocky bottoms. Seasonality had no effects and after 1 year of exposure the experimental communities converged towards a common structure. This study indicates that succession of subtidal epibenthic communities follows a slow and predictable pattern with a dominant late colonial species. In addition, aseasonal variability might be more relevant during colonization and succession in this upwelling ecosystem. 相似文献
176.
Nicolai Mirlean Paulo Baisch Marcelo P. Travassos Cristina Nassar 《Geo-Marine Letters》2011,31(1):65-73
Arsenic levels (up to 130 mg kg−1) substantially exceeding the official threshold have recently been documented in beach and nearshore sediments along more
than 50 km of coastline in the Brazilian state of Espírito Santo between 19°50′ and 20°12′S. In an attempt to assess the sources
of this enrichment, we performed a study on arsenic distribution in the main mineral substances and living organisms in the
beach environment. Laboratory tests on arsenic retention by beach carbonate debris have also been carried out. The data suggest
that sedimentary arsenic occurs largely bound to particles of the calcareous red alga Corallina panizzoi, whereby live specimens contained much smaller amounts of this metalloid than was the case for nonliving material (2.4 and
20.3 mg kg−1, respectively). Experimental tests confirmed the ability of C. panizzoi detritus to retain arsenic at pH intervals and ionic strength characteristic of seawater. There are two potential sources
of that metalloid for calcareous debris in sediments: brown macroalgae, which were found to contain high levels of As (up
to 66.3 mg kg−1), and ferruginized sandstones (up to 23.0 mg kg−1). We argue that any contribution of brown algae to beach sediment enrichment by As would be minor, and consider the ferrous
sandstones from coastal sedimentary rocks of the Barreiras Group as the principal large-scale source of arsenic in the marine
environment of Espírito Santo. The experimental data, together with field studies, corroborate the interpretation that arsenic
anomalies in sediments with calcareous debris can form when weathered continental rocks even only slightly enriched in As
are leached by marine waters, and the As is at least partially retained by biogenic calcareous detritus in nearshore sediments.
Considering that rocks of the Barreiras Group are exposed to marine erosion far to the north of Espírito Santo, we estimate
that marine sediments containing calcareous material are “anomalously” enriched in As along approximately 2,000 km of the
Brazilian tropical coastline. 相似文献
177.
Marcelo Marchesin Deborah da Paixão Vasconcellos 《Astrophysics and Space Science》2014,352(2):443-460
We study a highly symmetric nine-body problem in which eight positive masses, called the primaries, move four by four, in two concentric circular motions such that their configuration is always a square for each group of four masses. The ninth body being of negligible mass and not influencing the motion of the eight primaries. We assume all the nine masses are in the same plane and that the masses of the primaries are \(m_{1}=m_{2}=m_{3}=m_{4}=\tilde{m}\) and m 5=m 6=m 7=m 8=m and the radii associated to the circular motion of the bodies with mass \(\tilde{m}\) is λ∈[λ 0,1] and for the bodies with mass m is 1. We prove the existence of central configurations which characterize such arrangement of the primaries and we study the influence of the parameter λ, the ratio of the radii of the two circles, on the masses m and \(\tilde{m}\) . We use a synodical system of coordinates to eliminate the time dependence on the equations of motion. We show the existence of equilibria solutions symmetrically distributed on the four quadrants and their dependence on the parameter λ. Finally, we show that there can be 13, 17 or 25 equilibria solutions depending on the size of λ and we investigate their linear stability. 相似文献
178.
Marcelo Armengot Nestor Sánchez Javier López-Santiago Ana Inés Gómez de Castro 《Astrophysics and Space Science》2014,354(1):113-119
Large sky surveys are providing a huge amount of information for studies of the interstellar medium, the galactic structure or the cosmic web. Setting into a common frame information coming from different wavelengths, over large fields of view, is needed for this kind of research. GALEX is the only nearly all-sky survey at ultraviolet wavelengths and contains fundamental information for all types of studies. GALEX field of view is circular embedded in a squared matrix of 3840×3840 pixels. This fact makes it hard to get GALEX images properly overlapped with the existing astronomical tools such as Aladin or Montage. We developed our own software for this purpose. In this article, we describe this software and makes it available to the community. 相似文献
179.
Mauricio M. Perillo James L. Best Miwa Yokokawa Tomohiro Sekiguchi Tomohiro Takagawa Marcelo H. Garcia 《Sedimentology》2014,61(7):2063-2085
The development of bedforms under unidirectional, oscillatory and combined‐flows results from temporal changes in sediment transport, flow and morphological response. In such flows, the bedform characteristics (for example, height, wavelength and shape) change over time, from their initiation to equilibrium with the imposed conditions, even if the flow conditions remain unchanged. These variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record. Hence, understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record. This article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional, purely oscillatory and combined‐flow conditions, which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples. The present results yield a unified model for bedform development and equilibrium under unidirectional, oscillatory and combined‐flows. The experimental results show that the processes of bedform genesis and growth are common to all types of flows, and can be characterized into four stages: (i) incipient bedforms; (ii) growing bedforms; (iii) stabilizing bedforms; and (iv) fully developed bedforms. Furthermore, the development path of bedform; growth exhibits the same general trend for different flow types (for example, unidirectional, oscillatory and combined‐flows), bedform size (for example, small versus large ripples), bedform shape (for example, symmetrical or rounded), bedform planform geometry (for example, two‐dimensional versus three‐dimensional), flow velocities and sediment grain sizes. The equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition. 相似文献
180.
Guillermo Bosch †‡ Rodolfo Barbá ‡§ Nidia Morrell †‡§ Virpi Niemela †‡¶ Pablo Ostrov ‡ Marcelo Arnal § Cristina Cappa § Ricardo Morras § Guillermo Giménez de Castro 《Monthly notices of the Royal Astronomical Society》2003,341(1):169-178
We present new spectroscopy in the optical range and 21-cm H i data covering the Ruprecht 55 (Ru 55) field in the Puppis window where several authors have proposed the existence of one (or two) clusters.
We have determined new MK spectral types for about 50 stars in the region, finding 43 OB-type stars among them. LS 985 was found to be an O9 V + O9.5 III binary and it is the earliest type of star in our observed sample.
We have identified a stellar OB association (Ru 55), which is most likely related to a depletion detected in our H i data, as: (i) they are located at the same distance (6 kpc), within observational errors; (ii) both have similar radial velocities (∼67 km s−1 ); (iii) current OB stars could have provided the energy needed to blow the cavity; (iv) the dynamical time-scale for the hole buildup matches the age estimated for the earliest OB stars; and (v) LS 985 might be responsible for ionizing the H i cavity inner walls close to it. 相似文献
We have determined new MK spectral types for about 50 stars in the region, finding 43 OB-type stars among them. LS 985 was found to be an O9 V + O9.5 III binary and it is the earliest type of star in our observed sample.
We have identified a stellar OB association (Ru 55), which is most likely related to a depletion detected in our H i data, as: (i) they are located at the same distance (6 kpc), within observational errors; (ii) both have similar radial velocities (∼67 km s