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81.
82.
M. Iole Spalla Davide Zanoni Guido Gosso Michele Zucali 《International Journal of Earth Sciences》2009,98(1):203-226
The volcano–clastic sequence of Trompia Valley, which caps the Tre Valli Bresciane Variscan basement (TVB), comprises the
Dosso dei Galli Conglomerate (DGC), the oldest deposit containing up to metre-sized metamorphic pebbles. This Lower Permian
formation of the Trompia Basin was fed by the erosion products of the Variscan chain. We used microstructural and mineral
chemical data on metamorphic pebbles of the DGC to infer a quantitative tectono-thermal evolution of the eroded pre-Permian
basement and to compare them with those of TVB and the surrounding Southalpine basement units (tectono-metamorphic units = TMUs).
Metapelitic and metaintrusive pebbles record a polyphase metamorphism with two metamorphic re-equilibrations: the first under
epidote amphibolite facies (M1, ) and the second under greenschist facies (M2) conditions. Rock types and metamorphic data largely match those of TVB basement
unit. The structural and metamorphic records in the pebbles are pre-Permian, and the conglomerate matrix is non-metamorphic.
The DGC deposition age (283 ± 1–280.5 ± 2 Ma) constrains the minimal exhumation age of its basement source. The lack of staurolite
bearing assemblages in metamorphic pebbles suggests that the DGC basement source was already exhumed to shallow structural
levels (greenschist facies conditions) before the thermal equilibration consequent upon continental crust thickening induced
by the Variscan collision. 相似文献
83.
Guido Giordano 《Bulletin of Volcanology》1998,60(1):10-26
The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic,
pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions
from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT
Galluccio. The WTT Galluccio eruption was the largest and emplaced ∼ 4 km3 of magma. The internal stratigraphy of all four WTT eruptive units is a complex association of fallout, surge, and pyroclastic
flow deposits. Each eruptive unit is organized into two facies associations, Facies Association A below Facies Association
B. The emplacement of the two facies associations may have been separated by short time breaks allowing for limited reworking
and erosion. Facies Association A consists of interbedded fallout deposits, surge deposits, and subordinate ignimbrites. This
facies association involved the eruption of the most evolved trachytic magma, and pumice clasts are white and well vesiculated.
The grain size coarsens upward in Facies Association A, with upward increases of dune bedform wavelengths and a decrease in
the proportion of fine ash. These trends could reflect an increase in eruption column height from the onset of the eruption
and possibly also in mass eruption rate. Facies Association B comprises massive ignimbrites that are progressively richer
in lithic clast content. This association involved the eruption of more mafic magma, and pumice clasts are gray and poorly
vesiculated. Facies Association B is interpreted to record the climax of the eruption. Phreatomagmatic deposits occur at different
stratigraphic levels in the four WTT and have different facies characteristics. The deposits reflect the style and degree
of magma–water interaction and the local hydrogeology. Very fine-grained, lithic-poor phreatomagmatic surge deposits found
at the base of WTT Cupa and WTT Galluccio could record the interaction of the erupting magma with a lake that occupied the
Roccamonfina summit depression. Renewed magma–water interaction later in the WTT Galluccio eruption is indicated by fine grained,
lithic-bearing phreatomagmatic fall and surge deposits occurring at the top of Facies Association A. They could be interpreted
to reflect shifts of the magma fragmentation level to highly transmissive, regional aquifers located beneath the Roccamonfina
edifice, possibly heralding a caldera collapse event.
Received: 26 August 1996 / Accepted: 27 February 1998 相似文献
84.
Dirk?Sachse Jens?Radke Reinhard?Gaupp Lorenz?Schwark Guido?Lüniger Gerd?GleixnerEmail author 《International Journal of Earth Sciences》2004,93(4):554-564
For the first time 18O and 13C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for 18O and 13C values of dolomitized carbonates and D values of individual n-alkanes. 18Ocarb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The 13Ccarb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since 13Ccarb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, 18Ocarb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high 18Ocarb and low 13Ccarb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C16 and n-C18 which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C27, n-C28, n-C29 and n-C30, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. Dwater values, reconstructed using the n-C16 alkane and 18O water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction. 相似文献
85.
The reported data present the stratigraphy of several sections across a Middle-Late Jurassic Radiolaritic Unit, well exposed
in different thrust sheets pertaining to the Maghrebian chain of Southwestern Sicily. The aim was to define the chronostratigraphical
distribution of the Jurassic biosiliceous sedimentation in the Sicanian palaeogeographical zone, a deep water basin belonging
to the Southern Tethys continental margin.
The radiolarian biostratigraphy indicates that the switching from carbonate to siliceous sedimentation in the Sicanian Basin
is referable to the Bajocian, as shown by the section of Campofiorito, near Corleone. The biostratigraphical dataset allows
the correlation between the onset of biosiliceous sedimentation and the fall of biodiversity in the Sicanian basin with the
carbonate productivity crisis, indicated by the highest eutrophication that affected Western Tethys during Middle Jurassic
times.
Editorial handling: J.-P. Billon-Bruyat & M. Chiari (Guest) 相似文献
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87.
Characterization of sub‐daily thermal regime in alpine rivers: quantification of alterations induced by hydropeaking 下载免费PDF全文
The thermal regime of rivers is threatened by anthropogenic stresses at a large variety of timescales. We focus on sub‐daily thermal alterations induced by the release of hypolimnetic water for hydropower production (thermopeaking). We analyse the thermal signal focusing on the following characteristics that are potentially affected by hypolimnetic releases: (i) sub‐daily thermal rate of change and (ii) oscillation frequencies contained in the thermal signal. Through a proper scaling, we derive two dimensionless at‐a‐station indicators to compare alterations among stations with different locations and physiographic characteristics of the basins. Then we analyse the data from two different thermal datasets (Italy/Switzerland) for a total of 48 stations with 10 min time resolution of temperature data. The stations are grouped according to the absence of upstream hydropeaking releases (29 stations, reference group) and the existence of upstream hydropeaking, hence potentially impacted by thermopeaking (19 stations, altered group). Using a simple statistical approach, based on a non‐parametric definition of outliers, we identify the range of variability of the two indicators for the reference, unaltered group. This range measures the ‘natural’ sub‐daily thermal variability of the proposed indicators. Finally, we investigate the seasonality effects on the two proposed indicators and it results, that sub‐daily alterations mostly occur during summer. The two indicators represent a novel tool for the assessment of river thermal regime alterations and can be easily included in existing methodologies to assess river quality. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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