全文获取类型
收费全文 | 1850篇 |
免费 | 156篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 54篇 |
大气科学 | 199篇 |
地球物理 | 604篇 |
地质学 | 688篇 |
海洋学 | 130篇 |
天文学 | 233篇 |
综合类 | 12篇 |
自然地理 | 99篇 |
出版年
2023年 | 6篇 |
2022年 | 15篇 |
2021年 | 47篇 |
2020年 | 51篇 |
2019年 | 35篇 |
2018年 | 58篇 |
2017年 | 81篇 |
2016年 | 120篇 |
2015年 | 104篇 |
2014年 | 126篇 |
2013年 | 195篇 |
2012年 | 155篇 |
2011年 | 107篇 |
2010年 | 108篇 |
2009年 | 79篇 |
2008年 | 65篇 |
2007年 | 38篇 |
2006年 | 68篇 |
2005年 | 43篇 |
2004年 | 38篇 |
2003年 | 31篇 |
2002年 | 48篇 |
2001年 | 32篇 |
2000年 | 13篇 |
1999年 | 13篇 |
1998年 | 18篇 |
1997年 | 16篇 |
1996年 | 11篇 |
1995年 | 22篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1992年 | 7篇 |
1991年 | 22篇 |
1990年 | 11篇 |
1989年 | 9篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 13篇 |
1978年 | 5篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1974年 | 7篇 |
1973年 | 10篇 |
1969年 | 7篇 |
排序方式: 共有2019条查询结果,搜索用时 31 毫秒
61.
André C. Colonese Giovanni Zanchetta Elisabeth Dotsika Russell N. Drysdale Anthony E. Fallick Renata Grifoni Cremonesi Giuseppe Manganelli 《第四纪科学杂志》2010,25(8):1347-1359
This paper compares stable isotope (δ18O and δ13C) records of early–middle Holocene land snail shells from the archaeological deposits of Grotta di Latronico 3 (LTR3; southern Italy) with modern shell isotopic data. No substantial interspecific variability was observed in shell δ18O (δ18Os) of modern specimens (Pomatias elegans, Cornu aspersum, Eobania vermiculata, Helix ligata and Marmorana fuscolabiata). In contrast, interspecific shell δ13C (δ13Cs) variability was significant, probably due to different feeding behaviour among species. The δ18Os values of living land snails suggest that species hibernate for a long period during colder months, so that the signal of 18O‐depleted winter rainfall in their δ18Os is lost. This suggests that δ18Os and δ13Cs values of Pomatias elegans from this archaeological succession provide valuable clues for seasonal (spring–autumn) climatic conditions during the early–middle Holocene. The δ18Os values of fossil specimens are significantly lower than in modern shells and in agreement with other palaeoclimatic records, suggesting a substantial increase of precipitation and/or persistent changes in air mass source trajectories over this region between ca. 8.8 cal ka BP and 6.2–6.7 ka ago. The δ13Cs trend suggests a transition from a slightly 13C‐enriched to a 13C‐depleted diet between early and middle Holocene compared to present conditions. We postulate that this δ13Cs trend might reflect changes in the C3 vegetation community, potentially combined with other environmental factors such as regional moisture increase and the progressive decrease of atmospheric CO2 concentration. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
62.
Andrés Gutiérrez-Rodríguez Mikel Latasa Marta Estrada Montserrat Vidal Celia Marrasé 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(4):486-500
The carbon flux through major phytoplankton groups, defined by their pigment markers, was estimated in two contrasting conditions of the Northwestern Mediterranean open ocean ecosystem: the spring bloom and post-bloom situations (hereafter Bloom and Post-bloom, respectively). During Bloom, surface chlorophyll a (Chl a) concentration was higher and dominated by diatoms (53% of Chl a), while during Post-bloom Synechococcus (42%) and Prymnesiophyceae (29%) became dominant. The seawater dilution technique, coupled to high pressure liquid chromatography (HPLC) analysis of pigments and flow cytometry (FCM), was used to estimate growth and grazing rates of major phytoplankton groups in surface waters. Estimated growth rates were corrected for photoacclimation based on FCM-detected changes in red fluorescence per cell. Given the 30% average decrease in the pigment content per cell between the beginning and the end of the incubations, overlooking photoacclimation would have resulted in a 0.40 d?1 underestimation of phytoplankton growth rates. Corrected average growth rates (μo) were 0.90±0.20 (SD) and 0.40±0.14 d?1 for Bloom and Post-bloom phytoplankton, respectively. Diatoms, Cryptophyceae and Synechococcus were identified as fast-growing groups and Prymnesiophyceae and Prasinophyceae as slow-growing groups across Bloom and Post-bloom conditions. The higher growth rate during Bloom was due to dominance of phytoplankton groups with higher growth rates than those dominating in Post-bloom. Average grazing rates (m) were 0.58±0.20 d?1 (SD) and 0.31±0.07 d?1. The proportion of phytoplankton growth consumed by microzooplankton grazing (m/μo) tended to be lower in Bloom (0.69±0.34) than in Post-bloom (0.80±0.08). The intensity of nutrient limitation experienced by phytoplankton indicated by μo/μn (where μn is the nutrient-amended growth rate), was similar during Bloom (0.78) and Post-bloom (0.73). Primary production from surface water (PP) was estimated with 14C incubations. A combination of PP and Chl a synthesis rate yielded C/Chl a ratios of 34±21 and 168±75 (g:g) for Bloom and Post-bloom, respectively. Transformation of group-specific Chl a fluxes into carbon equivalents confirmed the dominant role of diatoms during Bloom and Synechococcus and Prymnesiophyceae during Post-bloom. 相似文献
63.
Mostafa Oukassou Omar Saddiqi Jocelyn Barbarand Samira Sebti Lahssen Baidder André Michard 《地学学报》2013,25(2):151-159
The origin of the Anti‐Atlas relief is one of the currently debated issues of Moroccan geology. To constrain the post‐Variscan evolution of the Central Anti‐Atlas, we collected nine samples from the Precambrian basement of the Bou Azzer‐El Graara inlier for zircon and apatite fission‐track thermochronology. Zircon ages cluster between 340 ± 20 and 306 ± 20 Ma, whereas apatite ages range from 171 ± 7 Ma to 133 ± 5 Ma. Zircon ages reflect the thermal effect of the Variscan orogeny (tectonic thickening of the ca. 7 km‐thick Paleozoic series), likely enhanced by fluid advection. Apatite ages record a complex Mesozoic–Cenozoic exhumation history. Track length modelling yields evidence that, (i) the Precambrian basement was still buried at ca. 5 km depth by Permian times, (ii) the Central Anti‐Atlas was subjected to (erosional) exhumation during the Triassic‐Early Cretaceous, then buried beneath ca. 1.5 km‐thick Cretaceous‐Paleogene deposits, (iii) final exhumation took place during the Neogene, contemporaneously with that of the High Atlas. 相似文献
64.
Reconstructing fluvial bar surfaces from compound cross‐strata and the interpretation of bar accretion direction in large river deposits 下载免费PDF全文
Renato P. Almeida Bernardo T. Freitas Bruno B. Turra Felipe T. Figueiredo André Marconato Liliane Janikian 《Sedimentology》2016,63(3):609-628
The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood‐related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood‐increment boundary surfaces to be widely spaced. Cross‐strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar‐scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross‐strata and cross‐strata set boundaries. The method can be used in the absence of abundant flood‐increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces. 相似文献
65.
66.
67.
68.
[Translated by the editorial staff] Simulating the precipitation regime of Northern Africa is challenging for regional climate models, particularly because of the strong spatial and temporal variability of rain events in the region. In this study we evaluate simulations conducted with two recent versions of regional climate models (RCM) developed in Canada: the CRCM5 and CanRCM4. Both are also used in the COordinated Regional Climate Downscaling EXperiment (CORDEX)-Africa. The assessment is based on the occurrence, duration, and intensity indices of daily precipitation in Maghreb during the fall and spring seasons from 1998 to 2008. We also examine the links between the North-Atlantic Oscillation (NAO) index, weather systems, and the precipitation regime over the region. During the rainy season (September to February), the CRCM5 reproduces the frequency and intensity of extreme precipitation adequately, as well as the occurrence of days with rain, while the CanRCM4 underestimates precipitation extremes. The study of links between weather systems and the precipitation regime shows that, along the Atlantic coast, precipitation (occurrence, intensity, and wet sequences) increases significantly with storm frequency in the fall. In winter, these links grow stronger going east, from the Atlantic coast to the Mediterranean coast. The negative phases of the NAO index are statistically associated with the increase in rain intensity, extremes, and accumulation along the Atlantic coast in the fall. However, the link weakens in winter over these regions and strengthens along the Mediterranean coast as the precipitation frequency rises during negative phases of the NAO. Both RCMs generally reproduce the links between the NAO and the precipitation regime well, regardless of location. 相似文献
69.
70.
Anabele Lindner Cira Souza Pitombo Samille Santos Rocha José Alberto Quintanilha 《地球空间信息科学学报》2016,19(4):245-254
Studies in transportation planning routinely use data in which location attributes are an important source of information. Thus, using spatial attributes in urban travel forecasting models seems reasonable. The main objective of this paper is to estimate transit trip production using Factorial Kriging with External Drift (FKED) through an aggregated data case study of Traffic Analysis Zones in São Paulo city, Brazil. The method consists of a sequential application of Principal Components Analysis (PCA) and Kriging with External Drift (KED). The traditional Linear Regression (LR) model was adopted with the aim of validating the proposed method. The results show that PCA summarizes and combines 23 socioeconomic variables using 4 components. The first component is introduced in KED, as secondary information, to estimate transit trip production by public transport in geographic coordinates where there is no prior knowledge of the values. Cross-validation for the FKED model presented high values of the correlation coefficient between estimated and observed values. Moreover, low error values were observed. The accuracy of the LR model was similar to FKED. However, the proposed method is able to map the transit trip production in several geographical coordinates of non-sampled values. 相似文献