全文获取类型
收费全文 | 900篇 |
免费 | 53篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 23篇 |
大气科学 | 107篇 |
地球物理 | 221篇 |
地质学 | 319篇 |
海洋学 | 56篇 |
天文学 | 128篇 |
自然地理 | 105篇 |
出版年
2021年 | 14篇 |
2020年 | 14篇 |
2019年 | 7篇 |
2018年 | 24篇 |
2017年 | 32篇 |
2016年 | 33篇 |
2015年 | 29篇 |
2014年 | 29篇 |
2013年 | 59篇 |
2012年 | 36篇 |
2011年 | 48篇 |
2010年 | 41篇 |
2009年 | 42篇 |
2008年 | 51篇 |
2007年 | 34篇 |
2006年 | 42篇 |
2005年 | 34篇 |
2004年 | 28篇 |
2003年 | 35篇 |
2002年 | 31篇 |
2001年 | 24篇 |
2000年 | 22篇 |
1999年 | 14篇 |
1998年 | 14篇 |
1997年 | 14篇 |
1996年 | 21篇 |
1995年 | 22篇 |
1994年 | 9篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 13篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 13篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1974年 | 7篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1915年 | 5篇 |
排序方式: 共有959条查询结果,搜索用时 15 毫秒
951.
Joel C. Hoffman Gregory S. Peterson Anne M. Cotter John R. Kelly 《Estuaries and Coasts》2010,33(6):1391-1405
We characterized stable isotope mixing along a river-Great Lake transition zone in the St. Louis River, an important fish
nursery in western Lake Superior, and used it to identify food web linkages supporting young fish production. We observed
a broad, spatial pattern in the carbon stable isotope ratio (δ13C); downriver enrichment in particulate organic carbon and aquatic vegetation δ13C, as well as pelagic, benthic and littoral invertebrate δ13C, reflected isotope mixing along the river-lake transition zone. Fishes with similarly enriched δ13C were used to identify benthopelagic and littoral trophic pathways. River and Lake Superior organic matter (OM) sources contributed
to both pathways. Differences between the δ13C in fishes and invertebrate prey revealed that fish production was supported at multiple spatial scales. The result was that
the food web specific to any location along the transition zone incorporated multiple OM sources from across the watershed. 相似文献
952.
Using multiple logistic regression and GIS technology to predict landslide hazard in northeast Kansas, USA 总被引:60,自引:0,他引:60
Landslides in the hilly terrain along the Kansas and Missouri rivers in northeastern Kansas have caused millions of dollars in property damage during the last decade. To address this problem, a statistical method called multiple logistic regression has been used to create a landslide-hazard map for Atchison, Kansas, and surrounding areas. Data included digitized geology, slopes, and landslides, manipulated using ArcView GIS. Logistic regression relates predictor variables to the occurrence or nonoccurrence of landslides within geographic cells and uses the relationship to produce a map showing the probability of future landslides, given local slopes and geologic units. Results indicated that slope is the most important variable for estimating landslide hazard in the study area. Geologic units consisting mostly of shale, siltstone, and sandstone were most susceptible to landslides. Soil type and aspect ratio were considered but excluded from the final analysis because these variables did not significantly add to the predictive power of the logistic regression. Soil types were highly correlated with the geologic units, and no significant relationships existed between landslides and slope aspect. 相似文献
953.
Kyu-Myong Kim William K.-M. Lau Yogesh C. Sud Gregory K. Walker 《Climate Dynamics》2010,35(1):115-126
Effects of aerosol radiative forcing on the diurnal and seasonal cycles of precipitation over West Africa and eastern Atlantic
Ocean are investigated for the boreal summer season: June–July–August. An eight year (2000–2007) average of GCM simulated
rainfall data is compared with the corresponding TRMM rainfall data. The comparison shows that the amplitude of the diurnal
cycles of rainfall over land and ocean are reasonably well simulated. Over land, the phase of the simulated diurnal cycle
of precipitation peaks several hours earlier than that of the TRMM data. Corresponding differences over the ocean(s) are relatively
smaller. Some of the key features of the aerosol induced model simulated field anomalies are: (a) aerosol direct radiative
forcing which increases the atmospheric stability and reduces the daytime moist convection and convective precipitation; (b)
the aerosol induced changes in the diurnal cycle of precipitation are out of phase with those of the TRMM data over land,
but are in-phase over the ocean; (c) aerosols reduce the amplitude of the diurnal cycle of precipitation over land and enhance
it over ocean. However, the phase of the diurnal cycle is not affected much by the aerosol radiative forcing both over land
and ocean. During the boreal summer, aerosol radiative forcing and induced circulation and precipitation cool the Sahel and
the southern part of Sahara desert more than the adjacent areas to the north and south, thereby shifting the peak meridional
temperature gradient northward. Consequently, an anomalous easterly jet is found north of its climatological location. This
anomalous jet is associated with increased cyclonic circulation to the south of its axis, resulting in an anomalous monsoon
rain belt in the Sahel. 相似文献
954.
Peter Good Jonathan M. Gregory Jason A. Lowe Timothy Andrews 《Climate Dynamics》2013,40(3-4):1041-1053
A fast simple climate modelling approach is developed for predicting and helping to understand general circulation model (GCM) simulations. We show that the simple model reproduces the GCM results accurately, for global mean surface air temperature change and global-mean heat uptake projections from 9 GCMs in the fifth coupled model inter-comparison project (CMIP5). This implies that understanding gained from idealised CO2 step experiments is applicable to policy-relevant scenario projections. Our approach is conceptually simple. It works by using the climate response to a CO2 step change taken directly from a GCM experiment. With radiative forcing from non-CO2 constituents obtained by adapting the Forster and Taylor method, we use our method to estimate results for CMIP5 representative concentration pathway (RCP) experiments for cases not run by the GCMs. We estimate differences between pairs of RCPs rather than RCP anomalies relative to the pre-industrial state. This gives better results because it makes greater use of available GCM projections. The GCMs exhibit differences in radiative forcing, which we incorporate in the simple model. We analyse the thus-completed ensemble of RCP projections. The ensemble mean changes between 1986–2005 and 2080–2099 for global temperature (heat uptake) are, for RCP8.5: 3.8 K (2.3 × 1024 J); for RCP6.0: 2.3 K (1.6 × 1024 J); for RCP4.5: 2.0 K (1.6 × 1024 J); for RCP2.6: 1.1 K (1.3 × 1024 J). The relative spread (standard deviation/ensemble mean) for these scenarios is around 0.2 and 0.15 for temperature and heat uptake respectively. We quantify the relative effect of mitigation action, through reduced emissions, via the time-dependent ratios (change in RCPx)/(change in RCP8.5), using changes with respect to pre-industrial conditions. We find that the effects of mitigation on global-mean temperature change and heat uptake are very similar across these different GCMs. 相似文献
955.
Abandoned channel fill sequences in the tidal estuary of a small mountainous,dry‐summer river 下载免费PDF全文
Andrew B. Gray Gregory B. Pasternack Elizabeth B. Watson Miguel A. Goñi 《Sedimentology》2016,63(1):176-206
This study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub‐centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi‐annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre‐scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ13C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry. 相似文献
956.
辽宁南部万福变质核杂岩的发现及其区域构造意义 总被引:9,自引:1,他引:9
根据宏观与微观构造测量,结合白垩纪沉积盆地组成与结构、区域岩浆活动性及其测年资料等的综合分析,揭示出在辽宁南部辽南变质核杂岩东侧存在另一个变质核杂岩构造,即万福变质核杂岩。该核杂岩具有典型的三层结构:拆离断层带由不同层次构造岩构成,上盘为元古宇岩石,下盘是太古宇岩石和就位于其中的同构造花岗质侵入体。万福变质核杂岩形成于早白垩世,与辽南变质核杂岩构成一个变质核杂岩对,两者在很多方面具有相似性。该变质核杂岩对的厘定可能为阐明华北晚中生代岩石圈的力学和流变学属性以及岩石圈减薄过程提供了依据。 相似文献
957.
黄河三角洲滨海湿地健康条件评价概念模型 总被引:3,自引:0,他引:3
滨海湿地健康与生物学特征主要取决于区域上的水文与盐度体制以及景观尺度上的土地利用现状。然而,由于滨海湿地条件评价的指标和标准并不十分清楚,因此,对滨海湿地系统条件进行评价,目前仍是环境科学的难点。中国地质调查局(CGS)青岛海洋地质研究所与美国地质调查局湿地研究中心合作先后为美国密西西比河下游生态环境及中国黄河三角洲(YRD)滨海湿地评价建立了概念模型。本文将陈述YRD湿地评价的概念模型。此模型的建立在于确定滨海湿地当前的条件和随时间改善或退化的过程,以及确定优先管理的区域。CGS项目之所以选取YRD作为滨海湿地的研究对象主要是因为它具有重要的生态意义。由于上游来水减少或黄河断流,该区湿地生境十分脆弱。本文提出此概念模型可为今后湿地条件评价指标确定、调查研究活动和数据采集提供指导。通过该模型的构建,使环境变化可用具体指标来度量,从而服务于滨海湿地生态系统的保护与管理活动。 相似文献
958.
Brent D. Turrin Fara Lindsay Jeremy S. Delaney Jisun Park Gregory F. Herzog Carl Swisher Jr Cyrena A. Goodrich 《Meteoritics & planetary science》2023,58(3):304-327
The Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC3, including ureilitic (70%–80%) and diverse non-ureilitic materials. We determined the 40Ar/39Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS-D (EL6), as well as a clast of ureilitic trachyandesite MS-MU-011, also known as ALM-A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the 40Ar/39Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published 40Ar/39Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum-rich inclusion and is consistent with the interpretation that 2008 TC3 was a polymict ureilite. With allowance for its 10-Ma uncertainty, the 4549-Ma 40Ar/39Ar age of ALM-A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative 39Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of 40Ar over the last 4.5 Ga. The other five such samples lost some 40Ar at estimated times no earlier than 3800–4500 Ma bp . Clustering of ages in the low-temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic-ray exposure ages based on 38Ar concentrations average 17 ± 5 Ma but may include some early irradiation. 相似文献
959.