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121.
Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime 总被引:2,自引:0,他引:2
Wildfires are an integral part of Mediterranean ecosystems; humans impact on landscapes imply changes in fuel amount and continuity, and thus in fire regime. We tested the hypothesis that fire regime changed in western Mediterranean Basin during the last century using time series techniques. We first compiled a 130-year fire history for the Valencia province (Spain, Eastern Iberian Peninsula, Western Mediterranean Basin) from contemporary statistics plus old forest administration dossiers and newspapers. We also compiled census on rural population and climatic data for the same period in order to evaluate the role of climate and human-driven fuel changes on the fire regime change. The result suggested that there was a major fire regime shift around the early 1970s in such a way that fires increased in annual frequency (doubled) and area burned (by about an order of magnitude). The main driver of this shift was the increase in fuel amount and continuity due to rural depopulation (vegetation and fuel build-up after farm abandonment) suggesting that fires were fuel-limited during the pre-1970s period. Climatic conditions were poorly related to pre-1970s fires and strongly related to post-1970s fires, suggesting that fire are currently less fuel limited and more drought-driven than before the 1970s. Thus, the fire regime shift implies also a shift in the main driver for fire activity, and this has consequences in the global change agenda. 相似文献
122.
Christopher S. Romanek Concepcin Jimnez-Lpez Alejandro Rodriguez Navarro Monica Snchez-Romn Nita Sahai Max Coleman 《Geochimica et cosmochimica acta》2009,73(18):5361-5376
A set of free-drift experiments was undertaken to synthesize carbonates of mixed cation content (Fe, Ca, Mg) from solution at 25 and 70 °C to better understand the relationship between the mineralogy and composition of these phases and the solutions from which they precipitate. Metastable solid solutions formed at 25 °C which are not predicted from the extrapolation of higher temperature equilibrium assemblages; instead, solids formed that were intermediary in chemical composition to known magnesite–siderite and dolomite solid solutions. A calcite–siderite solid solution precipitated at 25 °C, with the percentage of CaCO3 in the solid being proportional to the aqueous Ca/Fe ratio of the solution, while Mg was excluded from the crystal structure except at relatively high aqueous Mg/Ca and Mg/Fe ratios and a low Ca content. Alternatively, at 70 °C Mg was the predominant cation of the solid solutions. These results are compatible with the hypothesis that the relative dehydration energies of Fe, Ca and Mg play an important role in the formation of mixed cation carbonates in nature. 相似文献
123.
Greig A. Paterson Andrew P. Roberts Conall Mac Niocaill Adrian R. Muxworthy Lucia Gurioli José G. Viramonté Carlos Navarro Shoshana Weider 《Bulletin of Volcanology》2010,72(3):309-330
Paleomagnetic data from lithic clasts collected from Mt. St. Helens, USA, Volcán Láscar, Chile, Volcán de Colima, Mexico and
Vesuvius, Italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to
highlight the usefulness of the paleomagnetic method for determining emplacement temperatures. At Mt. St. Helens, the temperature
of the deposits (T
dep
) at three sites from the June 12, 1980 eruption was found to be ≥532°C, ≥509°C, and 510–570°C, respectively. One site emplaced
on July 22, 1980 was emplaced at ≥577°C. These new paleomagnetic temperatures are in good agreement with previously published
direct temperature measurements and paleomagnetic estimates. Lithic clasts from pyroclastic deposits from the 1993 eruption
of Láscar were fully remagnetized above the respective Curie temperatures, which yielded a minimum T
dep
of 397°C. Samples were also collected from deposits thought to be pyroclastics from the 1913, 2004 and 2005 eruptions of
Colima. At Colima, the sampled clasts were emplaced cold. This is consistent with the sampled clasts being from lahar deposits,
which are common in the area, and illustrates the usefulness of the paleomagnetic method for distinguishing different types
of deposit. T
dep
of the lower section of the lithic rich pyroclastic flow (LRPF) from the 472 A.D. deposits of Vesuvius was ~280–340°C. This
is in agreement with other, recently published paleomagnetic measurements. In contrast, the upper section of the LRPF was
emplaced at higher temperatures, with T
dep
~520°C. This temperature difference is inferred to be the result of different sources of lithic clasts between the upper
and lower sections, with the upper section containing a greater proportion of vent-derived material that was initially hot.
Our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature
estimation. 相似文献
124.
125.
Armand Hernández Santiago Giralt Roberto Bao Alberto Sáez Melanie J. Leng Philip A. Barker 《Journal of Paleolimnology》2010,44(2):413-429
The late glacial-Holocene transition from the Lago Chungará sedimentary record in northern Chilean Altiplano (18°S) is made up of laminated sediments composed of light-white and dark-green pluriannual couplets of diatomaceous ooze. Light-white sediment laminae accumulated during short-term extraordinary diatom blooms whereas dark-green sediment laminae represent the baseline limnological conditions during several years of deposition. Diatom oxygen isotope ratios (δ18Odiatom) from 40 consecutive dark-green laminae, ranging from 11,990 to 11,450 cal year BP, show that a series of decadal-to-centennial dry–wet oscillations occurred. Dry periods are marked by relatively high isotope values whereas wet episodes are indicated by lower values. This interpretation agrees with the reconstructions of terrigenous inputs and regional effective moisture availability carried out in the lake but there is a systematic temporal disagreement between them owing to the non-linear response of the lacustrine ecosystem to environmental forcings. Furthermore, the δ18Odiatom record tracks effective moisture changes at a centennial scale. Three major phases have been established (11,990–11,800, 11,800–11,550, and 11,550–11,450 cal year BP). Each phase is defined by an increasing isotope trend followed by a sudden depletion. In addition, several wet and dry events at a decadal scale are superimposed onto these major trends. Spectral analyses of the δ18Odiatom values suggest that cycles and events could have been triggered by both El Niño-Southern Oscillation (ENSO) and solar activity. Significant ENSO frequencies of 7–9 years and 15–17 years, and periodicities of the solar activity cycles such as 11 years (Schwabe), 23 years (Hale) and 35 years (Brückner) have been recognised in the oxygen isotope time series. Time–frequency analysis shows that although solar and ENSO forcing were present at the onset of the Holocene, they were more intense during the late glacial period. The early Holocene might have been mainly governed by La Niña-like conditions that correspond to wet conditions over the Andean Altiplano. 相似文献
126.
Javier Graciá-Carpio Santiago García-Burillo Pere Planesas 《Astrophysics and Space Science》2008,313(1-3):331-335
The sample of nearby LIRGs and ULIRGs for which dense molecular gas tracers have been measured is building up, allowing for
the study of the physical and chemical properties of the gas in the variety of objects in which the most intense star formation
and/or AGN activity in the local universe is taking place. This characterisation is essential to understand the processes
involved, discard others and help to interpret the powerful starbursts and AGNs at high redshift that are currently being
discovered and that will routinely be mapped by ALMA. We have studied the properties of the dense molecular gas in a sample
of 17 nearby LIRGs and ULIRGs through millimeter observations of several molecules (HCO+, HCN, CN, HNC and CS) that trace different physical and chemical conditions of the dense gas in these extreme objects. In
this paper we present the results of our HCO+ and HCN observations. We conclude that the very large range of measured line luminosity ratios for these two molecules severely
questions the use of a unique molecular tracer to derive the dense gas mass in these galaxies. 相似文献
127.
M. Domínguez V. Jiménez L. Kowalski S. Navarro L. Castañer 《Planetary and Space Science》2008,56(8):1169-1179
A novel design of a wind sensor for the surface of Mars is described. This sensor is to be included in the Rover Environment Monitoring Station (REMS) to be launched as a part of the Mars Science Laboratory in 2009. A 2D hot film anemometer composed of four hot points and a reference point has been conceived and implemented in the preliminary design model. It uses a closed thermal feedback loop based on thermal sigma-delta modulation. In this paper, the first results obtained in a Mars-like environment are shown, and indicate that sensitivities are expected to be in the range of 0.5 m/s and 10° in wind speed and direction. 相似文献
128.
129.
H. M. Tovmassian S. G. Navarro G. H. Tovmassian L. J. Corral 《Astrophysics and Space Science》1995,224(1-2):577-578
TheIRAS colours of some B stars with anomalous UV extinction confirm that they have circumstellar dust. 相似文献
130.
In a total of 181 photographic spectrograms, obtained at the Manuel Foster Observatory of the Catholic University in Santiago (Chile), the equivalent widths of the absorption lines HeI 4471 and MgII 4481 have been determined for 10 selected southern Be stars and 4 normal B-type stars. The mean equivalent widths of each star are compared to published mean values from largers samples of B stars. On the average, Be stars coincide with normal B-type stars in their equivalent width. However, a striking difference between rapidly and slowly rotating stars in their line ratios (HeI/MgII) was found, which is probably an artifact due to a systematic misclassification of spectral types of rapidly rotating B stars (v sini>200 km s–1). Six of the ten Be stars show evidence for variability in HeI and/or MgII with time-scales shorter than a few days and amplitudes up to a factor 2 in equivalent width. Line variability occurs in the entire range of projected rotation velocity (70 km s–1<v sini<350 km s–1), but seems to be restricted to spectral types B2-B4. In addition, variations at longer time-scales (years) were observed in two cases. Three of the variable stars (HR 4074, HR 4537, and Ara) present correlated variations in both lines, one ( Cen) anti-correlated variability. We interpret our results in terms of nonradial pulsations and stress the importance to study the HeI/MgII lines and their variability in a larger sample of Be, Bn, and normal B stars, including archival material in order to follow-up line variability for several decades in the past. 相似文献