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871.
The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen 总被引:1,自引:0,他引:1
Daniel M. Sigman Peter J. DiFiore Mathis P. Hain Curtis Deutsch Yi Wang David M. Karl Angela N. Knapp Moritz F. Lehmann Silvio Pantoja 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(9):1419-1439
We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate δ15N and δ18O in the ocean interior. The δ18O of nitrate is our focus because it has been explored less in previous work. Denitrification raises the δ15N and δ18O of mean ocean nitrate by equal amounts above their input values for N2 fixation (for δ15N) and nitrification (for δ18O), generating parallel gradients in the δ15N and δ18O of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the δ15N and δ18O of the residual nitrate that can be transported into the interior. However, the regeneration and nitrification of sinking N can be said to decouple the N and O isotopes of deep ocean nitrate, especially when the sinking N is produced in a low latitude region, where nitrate consumption is effectively complete. The δ15N of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in δ18O due to denitrification or nitrate assimilation with nitrate having the δ18O of nitrification. This lowers the δ18O of mean ocean nitrate and weakens nitrate δ18O gradients in the interior relative to those in δ15N. This decoupling is characterized and quantified in the box model, and agreement with data shows its clear importance in the real ocean. At the same time, the model appears to generate overly strong gradients in both δ18O and δ15N within the ocean interior and a mean ocean nitrate δ18O that is higher than measured. This may be due to, in the model, too strong an impact of partial nitrate assimilation in the Southern Ocean on the δ15N and δ18O of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone. 相似文献
872.
Jeomshik Hwang Steven J. Manganini Daniel B. Montluon Timothy I. Eglinton 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(10):1792-1803
The Northwest Atlantic margin is characterized by high biological productivity in shelf and slope surface waters. In addition to carbon supply to underlying sediments, the persistent, intermediate depth nepheloid layers emanating from the continental shelves, and bottom nepheloid layers maintained by strong bottom currents associated with the southward flowing Deep Western Boundary Current (DWBC), provide conduits for export of organic carbon over the margin and/or to the interior ocean. As a part of a project to understand dynamics of particulate organic carbon (POC) cycling in this region, we examined the bulk and molecular properties of time-series sediment trap samples obtained at 968 m, 1976 m, and 2938 m depths from a bottom-tethered mooring on the New England slope (water depth, 2988 m). Frequent occurrences of higher fluxes in deep relative to shallower sediment traps and low Δ14C values of sinking POC together provide strong evidence for significant lateral transport of aged organic matter over the margin. Comparison of biogeochemical properties such as aluminum concentration and flux, and iron concentration between samples intercepted at different depths shows that particles collected by the deepest trap had more complex sources than the shallower ones. These data also suggest that at least two modes of lateral transport exist over the New England margin. Based on radiocarbon mass balance, about 30% (±10%) of sinking POC in all sediment traps is estimated to be derived from lateral transport of resuspended sediment. A strong correlation between Δ14C values and aluminum concentrations suggests that the aged organic matter is associated with lithogenic particles. Our results suggest that lateral transport of organic matter, particularly that resulting from sediment resuspension, should be considered in addition to vertical supply of organic matter derived from primary production, in order to understand carbon cycling and export over continental margins. 相似文献
873.
In recent years, offshore reservoirs have been developed in deeper and deeper water environments. Steel catenary risers (SCRs) are being considered in deepwater development such as Northern North Sea. SCRs used in conjunction with a semi-submersible or floating production, storage and offloading (FPSO) in deepwater harsh environments present significant design challenges. The large vertical motions at the semi or FPSO induce severe riser response, which results in difficulty meeting strength and fatigue criteria at the hang off and touch down point locations. To improve the understanding of SCR behaviour and increase the confidence in the design of such systems in deepwater harsh environments, a parametric study on a SCR connected to a semi-submersible was carried out in this paper to deal with the factors that mainly influence the loading condition and fatigue life of the riser. Weight-optimized configurations were applied during the course of riser design. Riflex combined with DeepC was the primary analysis tool used for the long-term response of the nonlinear SCR structure simulations, which requires a large amount of computer time. Hence, the parameters affecting the efficiency and accuracy of the simulations have also been studied during the analysis process. 相似文献
874.
Marcondes Lima da Costa Daniel José Lima Sousa Rômulo Simões Angélica 《Journal of South American Earth Sciences》2009,27(2-3):219-234
The eastern region of the Amazon is home to the most important kaolin bauxite producing district in Brazil, referred to as the Paragominas-Capim kaolin bauxite district, which has a reserve of at least 1.0 billion tons of high-quality kaolin used in the paper coating industry. The kaolin deposits are closely related to sedimentary rocks of the Parnaíba basin and their lateritic cover. Two large deposits are already being mined: IRCC (Ipixuna) and PPSA (Paragominas). The geology of the IRCC mine is comprised of the kaolin-bearing lower unit (truncated mature laterite succession derived from the Ipixuna/Itapecuru formation) and the upper unit (immature lateritized Barreiras formation). The lower kaolin unit is characterized by a sandy facies at the bottom and a soft (ore) with flint facies at the top. It is formed by kaolinite, quartz, some iron oxi-hydroxides, mica and several accessories and heavy minerals. The <2 μm kaolinite crystallites only correspond to 41.3–58.3% of the soft kaolin, and large booklets of 15–300 μm are common. The degree of structure order of kaolinite decreases towards the flint kaolin. The chemical composition of the soft kaolin is similar to the theoretical chemical composition of kaolinite, with low iron content, and can be well correlated to most kaolin deposits in the region. The distribution pattern of chemical elements from sandy to flint kaolin (lower unit) suggests a lateritic evolution and erosive truncation. This is quite distinct from the upper unit, which has a mineralogical and chemical pathway relating it to a complete immature lateritic profile. The geological evolution of the IRCC kaolin is similar to that of other deposits in the eastern Amazon region, being comprised of: parent rocks formed in an estuarine marine and fluvio-laccustrine environment during the early Cretaceous; establishment of mature lateritization with the formation of kaolin in the Eocene; marine transgression and regression – (Pirabas and Barreiras formation) with kaolin profile erosion and forward movement of deferruginization and flintization during the Miocene after partial mangrove covering; and immature lateritization – partial kaolin ferruginization during the Pleistocene. 相似文献
875.
Fluid flows in consolidated porous media of volcanic origin are being investigated to support such diverse efforts as the
modeling of thermal/outgassing phenomena at Mount St. Helens and the hydrological modeling of tuffaceous rocks in support
of the Department of Energy’s (DOE) Nevada Nuclear Waste Storage Investigations Project An experimental apparatus was designed
and built to allow water-saturated permeabilities as low as 10−18 m2 to be measured on cores of diameter 5 cm and length 10 cm under steady-state flow conditions. This same apparatus can also
be utilized in a transient (pressure-decay) mode in order to measure permeabilities several orders of magnitude lower than
the steady-state limit.
Tests were conducted on samples of pumice, fractured welded tuff, and welded tuff, representing a permeability range of seven
orders of magnitude Pumice was found to have a permeability of ∼3×10−12 m2, sufficiently high to allow the complete Darcy-to-Ergun regime to be investigated Welded (unfractured) tuff was tested in
the transient mode, yielding a permeability of ∼5×10−19 m2. Two, long-time-scale, steady-flow experiments were conducted on a core of welded tuff containing a single, through-going
fracture. For the first experiment, the core was an integral cylinder containing a naturally occurring fracture. For the second
experiment, the core was separated into two pieces along the existing fracture plane, then rejoined. Effects of essentially
constant, as well as rapidly varied, circumferential stress were studied in both tests. Results showed core permeability to
decay to 2×10−18 m2 in both cases, independent of the initial fracture state (closed versus open). With a naturally occurring fracture, core
permeability decreased by a factor of 2 over a 200-h test period. With an initially open fracture, core permeability decreased
by a factor of 4 under the influence of a comparable 200-h load-time history, after 700 h of testing, core permeability was
reduced by an order of magnitude from its initial level. Final effective hydraulic fracture aperture was calculated to be
10−6 m, corresponding to a calculated effective fracture permeability of 10−13 m2 Fracture flow was thus estimated to account for 80% of the total flow rate through this core under final test conditions. 相似文献
876.
Daniel J. Charman 《第四纪科学杂志》1995,10(4):327-342
Patterned fens are part of the diverse Holocene peatland landscape of the Flow Country, northern Scotland. Their present vegetation and morphology allies them with boreal systems in Scandinavia and North America. However, previous work has shown that their mode of formation is rather different and has suggested a developmental model in which the poor fens have formed only recently and have arisen as a result of local hydrological change. These ideas are tested here by morphological, stratigraphical and macrofossil studies on three additional patterned fens covering a wider geographical and ecological range. Two sites corroborate the hypotheses of the earlier study, and one site which refutes the earlier findings can be shown to be dependent on different topographic controls. A comparison of two patterned fens with an ombrotrophic mire site in the same area shows that hydroseral processes dominate the early development of individual components of the mire complex. The time of initiation of patterned fens coincides with increasing surface wetness on the ombrotrophic mire. This may be due to climatic change and/or the coalescence of the constituent parts of the present integrated blanket mire system. The hydrological changes arising from this led to the emergence of the springs that currently supply the patterned fens with their water and elevated nutrient levels. The study supports the idea that landscape-scale research and conservation are essential for the understanding and hydrological integrity of blanket mire landscapes such as the Flow Country. 相似文献
877.
878.
Nikhil Padmanabhan David J. Schlegel Uro Seljak Alexey Makarov Neta A. Bahcall Michael R. Blanton Jonathan Brinkmann Daniel J. Eisenstein Douglas P. Finkbeiner James E. Gunn David W. Hogg eljko Ivezi Gillian R. Knapp Jon Loveday Robert H. Lupton Robert C. Nichol Donald P. Schneider Michael A. Strauss Max Tegmark Donald G. York 《Monthly notices of the Royal Astronomical Society》2007,378(3):852-872
879.
Age constraints on the evolution of the Austroalpine basement to the south of the Tauern Window 总被引:1,自引:0,他引:1
Siegfried Siegesmund Till Heinrichs Rolf L. Romer Daniel Doman 《International Journal of Earth Sciences》2007,96(3):415-432
The Austroalpine basement to the south of the Tauern Window once was part of the northern margin of Gondwana. It includes
the “Altkristallin” and the phyllitic Thurntaler Complex. In the Altkristallin (AMU, MPU), suites of arc-related metamafic
sequences occur together with calc-alkaline metagranite. SHRIMP U–Pb dating of zircon from calc-alkaline metagranite associated
with an eclogitic amphibolite give an age of 470 ± 3 Ma interpreted as the age of protolith emplacement. In the Thurntaler
Complex, metaporphyroids occur together with tholeiitic as well as alkaline within-plate basalt-type metabasite. The metaryholites
of this association give a crystallization SHRIMP age of 477 ± 4 Ma, which suggests contemporaneity of arc-related and extensional
settings in the Austroalpine basement units. The age data demonstrate widespread magmatic activity associated with the Early-Ordovician
amalgamation at the end of the 550–470 Ma subduction–accretion–collision cycle. The Pb–Pb and U–Pb systematics of step-wise
leached staurolite and kyanite from the peak-metamorphic assemblage of the Altkristallin indicate that (1) step-wise leaching
of staurolite and kyanite yields the age of inclusions rather than the host; (2) zircon inclusions in staurolite suggest an
Ordovician or older age for the precursor of the staurolite-schists; (3) the weighted average of the 206Pb/238U data of the various leaching steps yields a Variscan age for the inclusions (ilmenite, biotite, and andesine). Since these
inclusions are part of the metamorphic mineral assemblage, this age provides a minimum estimate for staurolite growth, i.e.,
metamorphism. Thus, the Pb–Pb and U–Pb systematics of staurolite provide evidence for a Variscan metamorphism of the Austroalpine
basement, e.g., MPU, AMU and Thurntaler Complex, to the south of the Tauern Window. 相似文献
880.
Richard J. Squire Walter Herrmann Daniel Pape D. Ian Chalmers 《Mineralium Deposita》2007,42(5):489-503
The early Palaeozoic Macquarie Arc, southeastern Australia, hosts a variety of major late Ordovician to earliest Silurian
subduction-related deposits (e.g., Cadia East, Ridgeway, Cadia Hill, Cowal and Northparkes). However, there is uncertainty
about whether coeval high-sulfidation epithermal deposits, which occur in intra-oceanic metallogenic belts elsewhere in the
West Pacific, (e.g., Lepanto and Chinkuashih), are also present in the Macquarie Arc. This has led to suggestions that their
absence may be due to the poor preservation potential of deposits that form at relatively shallow crustal levels in ancient
rocks. We present here an interpretation for evolution of the Peak Hill Au–Cu deposit based on the distribution of alteration
facies, sulfur isotope data from several textural forms of pyrite and barite, and an assessment of the regional volcanic and
sedimentary facies architecture. These data show that the Peak Hill deposit displays a distinct sub-vertical zoning with a
pyrophyllite and vuggy-quartz core, that today extends about 350 m east–west and at least 550 m north–south, which grades
out through paragonite+muscovite, kaolinite to a chlorite+epidote alteration zone at the margin. The alteration zoning reflects
both lower temperatures and neutralisation of acid fluids with increasing distance from the core, which represents the conduit
along which hot acidic hydrothermal fluids were channelled. Several temporally overlapping events of silicification, bladed-quartz-pyrite
veining, brecciation and pyrite veining occurred during the last stages of hydrothermal alteration, although most appear to
predate mineralisation. Au–Cu mineralisation was associated with late quartz-pyrite-barite veins, and the highest gold grades
occur mainly in microcrystalline-quartz-altered rocks in the paragonite+muscovite alteration zone, generally within 50 m outward
from the boundary of the pyrophyllite and vuggy-quartz core. Sulfur- and lead-isotope data, and the characteristic zoning
of ore minerals and alteration assemblages support a magmatic source for the hydrothermal fluids. Similarities in many of
the isotopic signatures between Peak Hill and deposits such as Northparkes support generation of the high-sulfidation mineralisation
during the Late Ordovician to earliest Silurian (possibly ca. 440 Ma) metallogenic event. The Late Ordovician to Early Silurian
volcanic and sedimentary facies associations at Peak Hill are consistent with alteration and mineralisation occurring in rocks
deposited in a submarine setting. 相似文献