全文获取类型
收费全文 | 295篇 |
免费 | 10篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 26篇 |
地球物理 | 74篇 |
地质学 | 104篇 |
海洋学 | 51篇 |
天文学 | 21篇 |
综合类 | 1篇 |
自然地理 | 22篇 |
出版年
2021年 | 2篇 |
2020年 | 7篇 |
2019年 | 3篇 |
2018年 | 6篇 |
2017年 | 9篇 |
2016年 | 5篇 |
2015年 | 10篇 |
2014年 | 13篇 |
2013年 | 16篇 |
2012年 | 9篇 |
2011年 | 11篇 |
2010年 | 18篇 |
2009年 | 26篇 |
2008年 | 19篇 |
2007年 | 19篇 |
2006年 | 15篇 |
2005年 | 8篇 |
2004年 | 9篇 |
2003年 | 10篇 |
2002年 | 9篇 |
2001年 | 5篇 |
2000年 | 3篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1968年 | 1篇 |
1953年 | 1篇 |
排序方式: 共有305条查询结果,搜索用时 15 毫秒
191.
Jane Xinxin Zhang Kang‐Tsung Chang Joan Qiong Wu 《International journal of geographical information science》2013,27(8):925-942
Digital elevation models (DEMs) vary in resolution and accuracy by the production method. DEMs with different resolutions and accuracies can generate varied topographic and hydrological features, which can in turn affect predictions by soil erosion models, such as the WEPP (Water Erosion Prediction Project) model. This study investigates the effects of DEMs on deriving topographic and hydrological attributes, and on predicting watershed erosion using WEPP v2006.5. Six DEMs at three resolutions from three sources were prepared for two small forested watersheds located in northern Idaho, USA. These DEMs were used to calculate topographic and hydrological parameters that served as inputs to WEPP. The model results of sediment yields and runoffs were compared with field observations. For both watersheds, DEMs with different resolutions and sources generated varied watershed shapes and structures, which in turn led to different extracted hill slope and channel lengths and gradients, and produced substantially different erosion predictions by WEPP. 相似文献
192.
Lake sediments from four sites in the southwest Yukon Territory, Canada, provided paleotemperature records for the past 2000 yr. An alpine and a forest site from the southeastern portion of the study area, near Kluane Lake, and another alpine-forest pair of lakes from the Donjek River area located to the northwest yielded chironomid records that were used to provide quantitative estimates of mean July air temperature. Prior to AD 800, the southwest Yukon was relatively cool whereas after AD 800 temperatures were more variable, with warmer conditions between ~ AD 1100 and 1400, cooler conditions during the Little Ice Age (~ AD 1400 to 1850), and warming thereafter. These records compare well with other paleoclimate evidence from the region. 相似文献
193.
Joan Feynman 《Solar physics》1975,43(1):249-252
Because of theoretical difficulties in predicting the behavior of multi-ionic plasmas, observation must serve as a guide to expectations concerning the results of further measurements of other parameters. Here we examine the support for the common belief that observations of the helium to hydrogen temperature ratio indicate that the thermal velocities of ionic species in the solar wind tend to be equal. 相似文献
194.
Christopher J. Hintz Timothy J. Shaw G. Thomas Chandler Joan M. Bernhard Daniel C. McCorkle 《Geochimica et cosmochimica acta》2006,70(8):1952-1963
Trace/minor element signatures (DCd, DBa, DMg, and DSr) were measured in the tests (shells) of benthic foraminifera cultured in a trace-metal-concentration-controlled system. The culture system was constructed of inert materials and designed to limit microhabitat effects. This system ensured that variation observed in cultured foraminiferal element:calcium (TE/Ca) signatures was due to biologically mediated (vital) effects only. Two species, Bulimina aculeata and Rosalina vilardeboana, reproduced prolifically during two 4-to-8-month culture periods. In every case (i.e., for both species and each element), the inter-individual variability was larger than the analytical precision. Mean (±1 standard deviation) DE signatures for B. aculeata were: DCd: 1.5 ± 0.4, DBa × 10: 2.1 ± 0.7, DMg × 1000: 0.62 ± 0.15, and DSr × 10: 1.5 ± 0.1. Cultured B. aculeata DMg, calibrated from culture and core-top (live) field specimens, predicted temperatures within ±2.0 °C. The observed inter-individual variability from culture specimens was as large or larger than comparable results from core-top investigations. R. vilardeboana DCd signatures were significantly lower, while DBa, DMg, and DSr signatures were significantly higher than B. aculeata values. Since our culture system minimizes microhabitat variability, the variation in measured TE/Ca ratios suggests that biological processes are a significant factor in inter-individual and inter-species variability. Comparison of cultured and field-collected foraminiferal DBa signatures supports previous findings that pore-water chemistry is a major environmental influence on foraminiferal test chemistry. 相似文献
195.
Isabel Cacho Nick Shackleton Harry Elderfield Francisco J. Sierro Joan O. Grimalt 《Quaternary Science Reviews》2006,25(23-24):3294
Deep-water temperatures (DWTs) from the Western Mediterranean Sea are reconstructed for the last 50 ka based on the analysis of Mg/Ca ratios in benthic foraminifera from core MD95-2043 collected in the Alboran Sea. The exceptionally high sedimentation rates of this core and the robust chronology available allow discussion of the results in the context of the Dansgaard–Oeschger (D–O) rapid climatic variability. The applicability of Mg/Ca thermometry in the Western Mediterranean Deep-Water mass (WMDW) is first tested by the analysis of different benthic species in a collection of Mediterranean core tops. The results indicate the need of a readjustment of the existing Cibicidoides spp. calibrations in order to reconstruct present Western Mediterranean DWT values (12.7 °C). Different physiological effects in the Mg uptake between the C. pachydermus living in different regions could account for this offset in the Mediterranean samples. Consequently, the obtained DWT record still has many uncertainties in absolute terms but trends provide valuable information on past changes in WMDW conditions. The DWT record shows significant oscillations in relation to the D–O cycles, colder values occurred during the time of D–O stadials and warmer ones during D–O interstadials. Surprisingly, the coldest DWTs occurred during the time of Heinrich Event 4 (HE4) and not during the Last Glacial Maximum (LGM) when DWTs were mostly warm. These and other particular features of the DWT reconstruction mimic changes in the vegetation from the Eastern Mediterranean indicating the control of the Mediterranean climate on the DWT record. Paired analyses of Mg/Ca and δ18Occ (calcite δ18O) provide the opportunity to reconstruct deep-water δ18O (δ18Odw) and past salinities and hence changes in past WMDW density. Due to the large error associated with these calculations, they can only be discussed in terms of relative changes between different intervals. The results suggest the dominance of a heavier water end member during glacial times and a lighter one during the early Holocene in relation to the δ18Odw conditions present today. Densest WMDW were formed during most of Marine Isotopic Stage (MIS) 2 and during the D–O Stadials not associated with HEs, while lightest WMDW dominated during D–O Interstadials. The δ18Odw record shows a D–O variability pattern likely controlled by changes in the composition and intensity of the local run-off and also to changes in the δ18Ow signal of the Atlantic inflow. Changes in the residence time of the Mediterranean waters, governed by the global sea level, are also considered to exert an important role governing Mediterranean δ18Ow and salinity, particularly during MIS 2. Overall, our results are consistent with the formation of dense WMDW during D–O stadials and even denser during most of MIS 2. 相似文献
196.
Traditional fecal coliform bacterial indicators have been found to be severely limited in determining the significance and
sources of fecal contamination in ambient waters of tropical and subtropical regions. The bacteriophages that infectBacteroides fragilis have been suggested as better fecal indicators and at least one type may be human specific. In this study, the phages that
infectB. fragilis host RYC2056 (RYC), including phage B56-3, and host ATCC 51477-HSP40 (HSP), including the human specific phage B40-8, were
evaluated in the drainage basins of Tampa Bay, 7 samples (n=62), or 11%, tested positive for the presence of phages infecting
the host HSP, whereas 28 samples, or 45%, tested positive using the host RYC. A survival study was also done to compare the
persistence of phages B56-3 and B40-8 to MS2 coliphage in seawater at various temperatures. The decay rates for MS2 were 0.239
log10 d−1 at 10°C, but increased to 0.896 at 20°C and 2.62 log10 d−1 at 30°C. The twoB. fragilis phages persisted much longer in the seawater compared to the coliphage and showed little variation between the temperatures.
All sewage influents sampled from area wastewater treatment plants contained phages that infected the twoB. fragilis hosts at levels from 1.2×104 to 1.11×105 pfu 100 ml−1 for host RYC and 67 to 350 pfu 100 ml−1 for host HSP. Of the 7 chlorinated effluent samples tested, 3 were positive for the presence of the phage using the host
RYC and the phage enrichment method, with levels estimated to be <10 pfu 100 ml−1. No phages were detected using the host HSP in the treated sewage effluent. Coliphages were found in 3 of the 7 effluent
samples at a range of 30 to 1.2×103 pfu 100 ml−1. 相似文献
197.
Jordi Corominas Ramon Copons José Moya Joan M. Vilaplana Joan Altimir Jordi Amigó 《Landslides》2005,2(4):343-357
Quantitative Risk Assessment (QRA) has become an indispensable tool for the management of landslide hazard and for planning
risk mitigation measures. In this paper we present the evaluation of the rockfall risk at the Solà d’Andorra slope (Andorra
Principality) before and after the implementation of risk mitigation works, in particular, the construction of protective
fences. To calculate the risk level we have (i) identified the potential rockfall release areas, (ii) obtained the volume
distribution of the falling rocks, (iii) determined the frequency of the rockfall events, and (iv) performed trajectographic
analysis with a 3D numerical model (Eurobloc) that has provided both the expected travel distances and the kinetic energy
of the blocks. The risk level at the developed area located at the foot of the rock cliff has been calculated taking into
account the nature of the exposed elements and their vulnerability. In the Forat Negre basin, the most dangerous basin of
the Solà d’Andorra, the construction of two lines of rockfall protection fences has reduced the annual probability of loss
of life for the most exposed person inside the buildings, from 3.8×10−4 to 9.1×10−7 and the societal risk from 1.5×10−2 of annual probability of loss of life to 1.2×10−5. 相似文献
198.
Erin K. Lipp Raymond Kurz Robert Vincent Cesar Rodriguez-Palacios Samuel R. Farrah Joan B. Rose 《Estuaries and Coasts》2001,24(2):266-276
The Charlotte Harbor estuary in southwest Florida was sampled monthly for one year at twelve stations, in the lower reaches of the Myakka and Peace Rivers. The objectives of the study were to address the distribution and seasonal changes in microbial indicators and human pathogen levels in Charlotte Harbor shellfish and recreational waters, and to determine those factors that may be important in the transport and survival of pathogens. Monthly water samples and quarterly sediment samples were analyzed for fecal coliform bacteria, enterococci,Clostridium perfringens, and coliphage. Quarterly samples also were analyzed for the enteric human pathogens,Cryptosporidium spp.,Giardia spp., and enteroviruses. Fecal indicator organisms were generally concentrated in areas of low salinity and high densities of septic systems; however, pollution became widespread during wet weather in, the late fall and winter of 1997–1998, coincident with a strong El Nino event. Between, December 1997 and February 1998, enteroviruses were detected at 75% of the sampling stations; none were detected in other months. Enteric protozoa were detected infrequently and were not related to seasonal influences. Fecal indicators and enteroviruses were each significantly associated with rainfall, streamflow, and temperature. Regression models suggest that temperature and rainfall can predict the occurrence of enteroviruses in 93.7% of the cases. Based on findings in this watershed, factors such as variability in precipitation, streamflow, and temperature show promise in modeling and forecasting periods of poor coastal water quality. 相似文献
199.
We compare the different geomorphological processes which occurred in Pliocene–Quaternary times on two very similar slopes in Mallorca, one located at the Tramuntana Range and the other in the Llevant Ranges. Both slopes have the same geological structure, the same stratigraphic and lithological levels, and the same altitude and orientation. The different slope processes are due to the relationship between the accommodation space and the sedimentation rate in the adjacent basins: In the Valencia Trough (located N of the Tramuntana Range), the sedimentation rate has not been sufficient to fill the accommodation space, whereas in the Alcudia Basin (adjacent to the Llevant Ranges), the sedimentation rate has been sufficient to fill the accommodation space. This difference has resulted in major landslides on the Tramuntana slope, whereas the Llevant slope is characterized mainly by alluvial fans and debris screes. 相似文献
200.
Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic 总被引:4,自引:0,他引:4
Edward L. Habgood Neil H. Kenyon Douglas G. Masson rey Akhmetzhanov Philip P. E. Weaver Joan Gardner† Thierry Mulder‡ 《Sedimentology》2003,50(3):483-510
Abstract A study of the seafloor of the Gulf of Cadiz west of the Strait of Gibraltar, using an integrated geophysical and sedimentological data set, gives new insights into sediment deposition from downslope thermohaline bottom currents. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components. Changes in bedform morphology across the study area indicate a decrease in the peak velocity of the MO from >1 m s?1 to <0·5 m s?1. The associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. A series of downslope‐oriented channels, formed by the MO, are found where the MO starts to descend the continental slope at a water depth of ≈700 m. These channels are up to 40 km long, have gradients of <0·5°, a fairly constant width of ≈2 km and a depth of ≈75 m. Sand waves move down the channels that have mud wave‐covered levees similar to those seen in turbidite channel–levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. Cores from the lobes recovered up to 8·5 m of massive, well‐sorted, fine sand, with occasional mud clasts. This work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel–levee systems. The distribution of sand within these systems is of particular interest, with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings. 相似文献