全文获取类型
收费全文 | 103篇 |
免费 | 1篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 1篇 |
大气科学 | 3篇 |
地球物理 | 18篇 |
地质学 | 46篇 |
海洋学 | 19篇 |
天文学 | 8篇 |
自然地理 | 10篇 |
出版年
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 5篇 |
2019年 | 1篇 |
2016年 | 1篇 |
2015年 | 2篇 |
2014年 | 4篇 |
2013年 | 7篇 |
2012年 | 2篇 |
2011年 | 1篇 |
2010年 | 4篇 |
2009年 | 6篇 |
2008年 | 6篇 |
2007年 | 3篇 |
2006年 | 4篇 |
2005年 | 1篇 |
2003年 | 2篇 |
2002年 | 1篇 |
2001年 | 1篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1997年 | 1篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 8篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有105条查询结果,搜索用时 48 毫秒
41.
42.
Evaluation of the flushing rates of Apalachicola Bay, Florida via natural geochemical tracers 总被引:4,自引:0,他引:4
We used naturally occurring radium isotopes as tracers of water exchange in Apalachicola Bay, a shallow coastal-plain estuary in northwestern Florida. The bay receives fresh water and radium from the Apalachicola River, and mixes with Gulf of Mexico waters through four inlets. We deployed moored buoys with attached Mn-fibers at several stations throughout the estuary during two summer and two winter periods. After deployment for at least one tidal cycle we measured the ratio of the two short-lived radium isotopes 223Ra (half-life = 11 d) and 224Ra (3.6 d) to estimate “radium ages” of the water in the bay.During our four seasonal deployments the river discharge ranged from 338 to 1016 m3 s− 1. According to our calculations the water turnover time in the bay during these samplings ranged from 6 to 12 days. Age contours in the bay showed that winds and tides as well as river discharge influence the water movement and the residence time of freshwater in the bay. We also calculated the mean age of river water in the bay which was between 5 to 9 days during the studied periods. We suggest that this approach can be used to quantify transport processes of dissolved substances in the bay. For example, soluble nutrient or pollutant transport rates from a point source could be examined. We conclude that the radium age technique is well suited for flushing rate calculations in river dominated shallow estuaries. 相似文献
43.
A.S. Gale P. Montgomery W.J. Kennedy J.M. Hancock J.A. Burnett J.M. McArthur 《地学学报》1995,7(6):611-622
Review of biostratigraphical evidence from different regions shows that criteria used by workers on various marine fossil groups to define the Santonian-Campanian boundary differ considerably in relative age and position. Probably the most widely recognizable of these criteria is the extinction of the distinctive crinoid Marsupites testudinarius (North America, Europe, Asia, north Africa, Australia), which, coincides exactly with two separate definitions of the boundary - appearances of the ammonite Placenticeras bidorsatum and the belemnite Gonioteuthis granulataquadrata - and may also coincide with a third - entry of the planktic foraminiferan Globotruncana elevata. A comparison of evidence from upper Santonian and lower Campanian successions in widely separated regions allows us to place a series of important biostratigraphical markers in correct order. Defining the boundary at the extinction of M. testudinarius corresponds to a 87 Sr/86 Sr of 0.707479, and a small positive excursion in δ13 C. The base of magnetochron 33R, generally considered to coincide with, or fall just above the base of the Campanian, is shown to lie within the upper Santonian Uintacrinus socialis Zone. 相似文献
44.
We investigated submarine ground water discharge and salt water-fresh water interactions at two locations along the shoreline of the Upper Gulf of Thailand to evaluate mechanisms of water and material transport into the coastal zone. Our data set illustrates the value of using a combined approach consisting of automatic seepage meters to monitor flow rates while assessing the conductivity (salinity) of the subterranean fluids via remote resistivity measurements. Negative correlations between electric conductivities of fluids measured directly inside seepage meter chambers and the remotely assessed resistivities of subsurface pore water show that such measurements may evaluate the spatial distribution of flow rates as well as the subterranean water quality in the coastal zone. Combined seepage and resistivity measurements may thus provide a more complete understanding of coastal ground water dynamics. 相似文献
45.
J.M. McArthur W.J. Kennedy A.S. Gale M.F. Thirlwall M. Chen J. Burnett J.M. Hancock 《地学学报》1992,4(3):385-393
In the Boreal Chalk of northwestern Europe the base of the Maastrichtian Stage is defined by entry of the belemnite Belemnella . In the Kronsmoor section, in northwestern Germany, which is a standard section for the European Chalk, 87 Sr/86 Sr at the Campanian/ Maastrichtian (C/M) boundary is 0.707723±9 (10). An isotopic correlation of this boundary to the US Western Interior, for which a highly refined cephalopod biostratigraphy exists, suggests that this boundary, as defined at Kronsmoor, occurs within the Baculites jenseni zone. This correlationagrees with the latest placement based on biostratigraphic criteria. On87 Sr/86 Srcriteria the boundary at Kronsmoor correlates to the English Chalk at Trunch, Norfolk, UK, at a level 3.5 m lower than its position based on biostratigraphic criteria.
At Kronsmoor,87 Sr/86 Sr increases in a quasi-linear manner through much of the section, suggesting that, averaged over intervals of 1 Myr, the gross sedimentation rate and temporal rate of change of 87 Sr/86 Sr were approximately constant through the section. For US samples, zonally-plotted values of 87 Sr/86 Sr define a quasi-linear trend, which suggests a moderate uniformity of zonal duration from the Baculites compressus zone (73.2±0.7 Ma, 10) to the Baculites grandis zone (70.1±0.7 Ma, 10). 相似文献
At Kronsmoor,
46.
D. S. Burnett 《Meteoritics & planetary science》2013,48(12):2351-2370
The Genesis Discovery mission returned solar matter in the form of the solar wind with the goal of obtaining precise solar isotopic abundances (for the first time) and greatly improved elemental abundances. Measurements of the light noble gases in regime samples demonstrate that isotopes are fractionated in the solar wind relative to the solar photosphere. Theory is required for correction. Measurement of the solar wind O and N isotopes shows that these are very different from any inner solar system materials. The solar O isotopic composition is consistent with photochemical self‐shielding. For unknown reasons, the solar N isotopic composition is much lighter than essentially all other known solar system materials, except the atmosphere of Jupiter. Ne depth profiling on Genesis materials has demonstrated that Ne isotopic variations in lunar samples are due to isotopic fractionation during implantation without appealing to higher energy solar particles. Genesis provides a precise measurement of the isotopic differences of Ar between the solar wind and the terrestrial atmosphere. The Genesis isotopic compositions of Kr and Xe agree with data from lunar ilmenite separates, showing that lunar processes have not affected the ilmenite data and that solar wind composition has not changed on 100 Ma time scales. Relative to Genesis solar wind, ArKrXe in Q (the chondrite noble gas carrier) and the terrestrial atmosphere show relatively large light isotope depletions. 相似文献
47.
Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary 总被引:1,自引:0,他引:1
Isaac R. Santos William C. Burnett Thorsten Dittmar I G.N.A. Suryaputra Jeffrey Chanton 《Geochimica et cosmochimica acta》2009,73(5):1325-9900
We hypothesize that nutrient cycling in a Gulf of Mexico subterranean estuary (STE) is fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. We estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions. The results indicate a large variability in nutrient concentrations over tidal and seasonal time scales. At high tide, nutrient concentrations in shallow beach groundwater were low as a result of dilution caused by seawater recirculation. During ebb tide, the concentrations increased until they reached a maximum just before the next high tide. The dominant form of nitrogen was dissolved organic nitrogen (DON) in freshwater, nitrate in brackish waters, and ammonium in saline waters. Dissolved organic carbon (DOC) production was two-fold higher in the summer than in the winter, while nitrate and DON production were one order of magnitude higher. Oxic remineralization and denitrification most likely explain these patterns. Even though fresh SGD accounted for only ∼5% of total volumetric additions, it was an important pathway of nutrients as a result of biogeochemical inputs in the mixing zone. Fresh SGD transported ∼25% of DOC and ∼50% of total dissolved nitrogen inputs into the coastal ocean, with the remainder associated with a one-dimensional vertical seawater exchange process. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime SGD nutrient fluxes (40% higher for DOC and 60% higher for nitrogen in the summer compared to the winter). We suggest that coastal primary production and nutrient dynamics in the STE are linked. 相似文献
48.
Dorothy S. Woolum D. S. Burnett Marian Furst J. R. Weiss 《Earth, Moon, and Planets》1975,12(2):231-250
Anin situ measurement of the lunar neutron density from 20 to 400 g cm?2 depth below the lunar surface was made by the Apollo 17 Lunar Neutron Probe Experiment (LNPE) using particle tracks produced by the10B (n,α)7Li reaction. Both the absolute magnitude and the depth profile of the neutron density are in good agreement with theoretical calculations by Lingenfelter, Canfield, and Hampel. However, relatively small deviations between experiment and theory in the effect of Cd absorption on the neutron density and in the relative149Sm to157Gd capture rates reported previously (Russet al., 1972) imply that the true lunar157Gd capture rate is about one half of that calculated theoretically. 相似文献
49.
Jeroen E. Sonke William C. Burnett Jurian A. Hoogewerff Sieger R. van der Laan Jaco Vangronsveld D. Reide Corbett 《Journal of Paleolimnology》2003,29(1):95-107
Pb-210 dating of two metal-polluted organic sedimentcores obtained near a former pyrometallurgical zinc smelter in Lommel, Belgiumhave been used to reconstruct atmospheric lead deposition rates during the20th century. Independent knowledge concerning historical pollutionevents and 137Cs fall-out profiles has allowed a criticalevaluation of the CRS, CIC and CF-CS models for the 210Pb ageinterpretation. Resulting ages for the three models suggest that, in this case,the CIC model gives the most accurate interpretation of historical pollutionevents and atmospheric lead fall-out. The 210Pbwater-sediment flux was estimated at 141–1158Bq·m–2·yr–1 for one site and62–106 Bq·m–2·yr–1 at theother site, during the last century. The large difference illustrates thatsediment focusing was important on a small spatial scale (10 m).The direction of focusing correlates with the predominant wind direction.Maximum atmospheric lead deposition rates were found to be 1.63 ± 0.59g·m–2·yr–1 around 1968 AD,which is 2 orders of magnitude larger than the Belgian average in 1980 AD, and5 orders larger than Holocene atmospheric lead deposition. 相似文献
50.
Sedimentary phosphorites occurring on the sea floor off Peru and Chile have been analyzed for U and Th isotopes, to establish their ages and hence obtain clues for their mode of formation. Fission-track distribution studies indicate that the U is primarily associated with the apatite fraction. Uranium-series disequilibrium methods, therefore, should be applicable, if the U incorporation is syngenetic with the apatite.The fractionation of U isotopes between oxidation states in the relatively young phosphorites from South America is low compared to that in older deposits. This supports the contention of Kolodny and Kaplan (1970) that the major mechanism of fractionation is displacement of 234U atoms into sites where they are more ‘oxidizable’ than the 238U parent. Age estimates based on 234U(IV) and 230Th contents are internally consistent and range from late Pleistocene to Recent.The results indicate that marine phosphorites are currently forming in this area of intense oceanic upwelling. The age pattern during the last 150,000 yr suggests a correlation with eustatic high sea level stands and implies that conditions were more favorable for apatite genesis in this area during interglacials rather than during glacial times. 相似文献