全文获取类型
收费全文 | 144篇 |
免费 | 2篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 10篇 |
地球物理 | 30篇 |
地质学 | 36篇 |
海洋学 | 16篇 |
天文学 | 16篇 |
自然地理 | 33篇 |
出版年
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 1篇 |
2019年 | 1篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2014年 | 3篇 |
2013年 | 9篇 |
2012年 | 5篇 |
2011年 | 2篇 |
2010年 | 8篇 |
2009年 | 6篇 |
2008年 | 3篇 |
2007年 | 5篇 |
2006年 | 1篇 |
2005年 | 2篇 |
2004年 | 8篇 |
2003年 | 3篇 |
2002年 | 6篇 |
2001年 | 5篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 10篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1971年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有147条查询结果,搜索用时 15 毫秒
11.
John C. Patterson 《Aquatic Sciences - Research Across Boundaries》1991,53(2-3):218-238
A model of the time dependent relationship between productivity and light intensity following changes in light intensity is briefly described. The model incorporates two response timescales simulating initial response and photoinhibition, although additional timescales could easily be incorporated. The model is calibrated against one set of time dependent data, and applied to two simple models of motion in the upper mixed layer of a lake. The two models are: organised motion simulating Langmuir cells, and disorganised motion simulating the turbulent velocity field associated with surface wind stirring. The depth and therefore light histories for a number of photosynthesising particles are calculated by these models, and used by the productivity model to calculate mean productivities. The results show that the influence of the time dependent nature of the productivity relationship depends on the ratio of the mixed layer depth to the euphotic depth, and to a less extent, on the rate at which the particles circulate in the mixed layer. 相似文献
12.
The ionization quotients of aqueous carbon dioxide (carbonic acid) have been precisely determined in NaCl media to 5 m and from 50° to 300°C using potentiometric apparatus previously developed at Oak Ridge National Laboratory. The pressure coefficient was also determined to 250°C in the same media. These results have been combined with selected information in the literature and modeled in two ways to arrive at the best fits and to derive the thermodynamic parameters for the ionization reaction, including the equilibrium constant, activity coefficient quotients, and pressure coefficients. The variation with temperature of the two fundamental quantities and were examined along the saturation vapor pressure curve and at constant density. The results demonstrated again that for reactions with minimal electrostriction changes the magnitudes and variations of and with temperature are small and, in addition, and are approximately independent of salt concentration.The results have also been applied to an examination of the solubility of calcite as a function of pH (in a given NaCl medium) for the neutral to acidic region both for systems with fixed CO2 pressure and systems where the calcium ion concentration equals the concentration of carbon. The pH of saturated solutions of calcite with PCO2 of 12 bars increases from 5.1 to 5.5 between 100° and 300°C. 相似文献
13.
Concentrations of lead were measured in a surface transect and at two vertical profile stations (15°N and 20°S) in the Central Pacific. These measurements complement similar measurements made earlier in the North Pacific at 33°N and in the Northwest Atlantic at 34°N [1,2], as well as recent measurements of eolian lead input fluxes near each of these locations [3]. The new transect of surface water concentrations of lead corroborates previous measurements, which decrease from 13 ng/kg at 30°N to 4 ng/kg at 17°S in the Central Pacific [4]. This transect gradient is shown to overlie a similar geographic gradient of subsurface maximum concentrations of lead in the three Pacific vertical profile stations, decreasing from 14 ng/kg at 33°N to 11 ng/kg at 14°N to 2.5 ng/kg at 20°S. Lead concentrations at each of those locations exhibit maxima at 400 m, decreasing concentrations to 2500 m and approximately concentrations of 0.8–1.1 ng/kg below that depth. The subsurface maximum at the northwest Atlantic profile station (36 ng/kg at 34°N) is also congruent with surface water lead concentrations which decrease from 806 ng/kg to 32 ng/kg in an offshore transect from Rhode Island to 34°N, 66°W [5], and the shape of the Atlantic profile is congruent with those in the Pacific. There is a positive correlation between the magnitudes of eolian lead input fluxes and the magnitudes of the upper water maxima in lead concentration profiles at corresponding locations as follows: South Pacific easterlies 3 ng/cm2 yr vs. 2.5 ng/kg; North Pacific easterlies 6 ng/cm2 yr vs. 11 ng/kg; North Pacific westerlies 50 ng/cm2 yr vs. 14 ng/kg; and North Atlantic westerlies 170 ng/cm2 yr vs. 36 ng/kg.This relationship enables one to view the anthropogenic perturbations of the marine lead cycle on a global scale, since the industrial origin of eolian and seawater lead has been established by correlations between geographic patterns of industrial lead emissions to the atmosphere and isotopic ratios of industrial leads [3] and by geographic patterns of Pb/silicate-dust ratios and lead isotopic ratios in ocean surface waters [3–5]. These new data coupled with earlier biogeochemical data indicate that surface water concentrations of lead in the North Pacific and North Atlantic are now conservatively estimated to be 8 to 20-fold greater and those in the South Pacific are 2-fold greater than natural concentrations because of industrial emissions of lead to the atmosphere. 相似文献
14.
A fingernail clam (Sphaerium simile, Sphaeriidae) from Science Lake, a small watershed located in Allegany State Park, New York, USA and a zebra mussel (Dreissena polymorpha, Dreissenidae) from Keuka Lake, New York, the third largest Finger Lake of central New York, were selected to evaluate the applicability of using 18O(CaCO3) and 13O(CaCO3) values for sub-weekly climate records. Seasonal variation in 18O(CaCO3) values was compared with predicted equilibrium values to test the hypothesis that lacustrine molluscs produce shell aragonite according to environmental variables. For the purpose of comparison, aragonite temperature-fractionation equations determined by Grossman& Ku (1986) and Patterson et al. (1993) were used. Sphaerium simile appears to produce 18O(CaCO3) values predicted by Patterson et al. (1993), while Dreissena polymorpha produces 18O(CaCO3) values in agreement with Grossman & Ku (1986). We attribute the difference to family-specific temperature-fractionation relationships. Because both types of mollusc record climate variables with a high degree of integrity, they should each serve as excellent paleoclimate proxies.The fingernail clam collected from a small watershed exhibits higher variation about the seasonal pattern than did the zebra mussel collected from a large watershed. This is attributed to the increased sensitivity of the small watershed to storm perturbation. Analysis of fossil molluscs from such watersheds might be useful in discerning paleo-storminess. 相似文献
15.
Lauren Patterson Marie Urban Aaron Myers Budhendra Bhaduri Eddie Bright Phillip Coleman 《GeoJournal》2007,69(1-2):93-102
Geospatial technologies and digital data have developed and disseminated rapidly in conjunction with increasing computing
efficiency and Internet availability. The ability to store and transmit large datasets has encouraged the development of national
infrastructure datasets in geospatial formats. National datasets are used by numerous agencies for analysis and modeling purposes
because these datasets are standardized and considered to be of acceptable accuracy for national scale applications. At Oak
Ridge National Laboratory a population model has been developed that incorporates national schools data as one of the model
inputs. This paper evaluates spatial and attribute inaccuracies present within two national school datasets, Tele Atlas North
America and National Center of Education Statistics (NCES).
Schools are an important component of the population model, because they are spatially dense clusters of vulnerable populations.
It is therefore essential to validate the quality of school input data. Schools were also chosen since a validated schools
dataset was produced in geospatial format for Philadelphia County; thereby enabling a comparison between a local dataset and
the national datasets.
Analyses found the national datasets are not standardized and incomplete, containing 76 to 90 percent of existing schools.
The temporal accuracy of updating annual enrollment values resulted in 89 percent inaccuracy for 2003. Spatial rectification
was required for 87 percent of NCES points, of which 58 percent of the errors were attributed to the geocoding process. Lastly,
it was found that by combining the two national datasets, the resultant dataset provided a more useful and accurate solution. 相似文献
16.
17.
The geographic variation in the isotopic composition of lead in surface waters of the central Pacific provides new evidence of the global anthropogenic perturbation of the element's cycle. Ratios of 206Pb/207Pb decrease from 1.196 in the northern hemisphere (19°N, 158°W) to 1.176 in the southern hemisphere (15°S, 150°W). This decrease parallels the geographic variation in surface concentrations of soluble lead which decrease from 13 ng kg?1 at the northern station to 4 ng kg?1 at the southern station. Both the 206Pb/207Pb and the 206Pb/208Pb ratios of those waters fit between the isotopic ratios of Australian (Broken Hill) and North American (Mississippi Valley) leads which are the predominant sources of leads in anthropogenic emissions to the Pacific Ocean basin. 相似文献
18.
Alpha thermochronology of carbonates 总被引:1,自引:0,他引:1
Peter Copeland E. Bruce Watson Desmond Patterson 《Geochimica et cosmochimica acta》2007,71(18):4488-4511
Step-heating experiments on 17 calcites from 11 different samples and 6 dolomites from 5 samples suggest a closure temperature of He in carbonates ∼70 ± 10 °C for a cooling rate of 10 °C/m.y. The bulk closure temperature in some samples may tend slightly higher due to the presence of diffusion domains larger (and therefore more retentive) than the sites in which the majority of He resides. The diffusivity of He in calcite is independent of the genesis of the mineral (igneous, sedimentary, metamorphic) or the source of the He (radiogenic, common, or laboratory induced) and in all samples analyzed the effective diffusion dimension for He is smaller than the size of the crystals investigated. Although calcite is a low-U mineral, this shortcoming can be overcome by analyzing large samples (>2 mm diameter) provided samples have a minimum of ∼0.3 ppm U; samples with smaller concentrations of alpha-producers are unlikely to produce enough radiogenic 4He sufficient to overwhelm He present in the crystals at the time they passed through their closure temperature. 相似文献
19.
"This paper investigates the life, migration and retirement experiences of elderly British residents of Tuscany and its adjacent districts.... Many pathways have led the elderly British to Tuscany; only a minority have simply migrated from the UK on retirement.... Key themes explored include class background, reasons for migration to Tuscany, previous connections to Italy, post-retirement behaviour, integration and future plans. The findings challenge some commonly-held ?expectations' about the nature of retirement migration." 相似文献
20.
Matthew S. Lachniet Leah Johnson Yemane Asmerom Stephen J. Burns Victor Polyak William P. Patterson Lindsay Burt April Azouz 《Quaternary Science Reviews》2009,28(27-28):3348-3360
We present a high-resolution terrestrial archive of Central American rainfall over the period 100–24 and 8.1–6.5 ka, based on δ18O time series from U-series dated stalagmites collected from a cave on the Pacific Coast of Costa Rica. Our results indicate substantial δ18O variability on millennial to orbital time scales that is interpreted to reflect rainfall variations over the cave site. Correlations with other paleoclimate proxy records suggest that the rainfall variations are forced by sea surface temperatures (SST) in the Atlantic and Pacific Oceans in a fashion analogous to the modern climate cycle. Higher rainfall is associated with periods of a warm tropical North Atlantic Ocean and large SST gradients between the Atlantic and Pacific Oceans. Rainfall variability is likely linked to the intensity and/or latitudinal position of the intertropical convergence zone (ITCZ). Periods of higher rainfall in Costa Rica are also associated with an enhanced sea surface salinity gradient on either side of the isthmus, suggesting greater freshwater export from the Atlantic Basin when the ITCZ is stronger and/or in a more northerly position. Further, wet periods in Central America coincide with high deuterium excess values in Greenland ice, suggesting a direct link between low latitude SSTs, tropical rainfall, and moisture delivery to Greenland. Our results indicate that a stronger tropical hydrological cycle during warm periods and large inter-ocean SST gradients enhanced the delivery of low latitude moisture to Greenland. 相似文献