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21.
Data on the major and trace element chemistry of a suite oftwenty pantelleritic pitchstone lavas from the Quaternary Ethiopianvolcano Fantale is presented. This reveals a contrast betweenthe composition of the pre-caldera flows and the more siliceous,less peralkaline post-caldera lavas. Comparison with experimental and theoretical studies suggeststhat nearly all of the major and trace element variation withinthe two suites can be explained by assuming fractional crystallizationof alkali-feldspar, the most abundant phenocryst phase. Fractionationof the mafic phases appears to have been less significant. The trace element data strongly indicate that the lavas allbelong to a single suite. However, it is suggested that themajor element chemistry of the post-caldera flows was modifiedby the loss of volatiles at the time of the formation of thecaldera, an event which coincided with the eruption of a 2 km3welded ash-flow tuff. 相似文献
22.
Environmental impacts of the Norse settlement: palaeoenvironmental data from Myvatnssveit, northern Iceland 总被引:2,自引:0,他引:2
IAN T. LAWSON FREDERICK J. GATHORNE-HARDY MIKE J. CHURCH ANTHONY J. NEWTON KEVIN J. EDWARDS REW J. DUGMORE AARNI EINARSSON 《Boreas: An International Journal of Quaternary Research》2007,36(1):1-19
Lawson, I. T., Gathorne‐Hardy, F. J., Church, M. J., Newton, A. J., Edwards, K. J., Dugmore, A. J. & Einarsson, Á. 2007 (January): Environmental impacts of the Norse settlement: palaeoenvironmental data from Mývatnssveit, northern Iceland. Boreas, Vol. 36, pp. 1–19. Oslo. ISSN 0300–9483. The first stratigraphically continuous pollen profile spanning the Norse and Medieval periods from the archaeologically‐rich Mvatnssveit region of northern Iceland is presented. Detailed analyses were made of the tephra, sediment characteristics, pollen and chironomids of a 3 kyr sediment sequence from Helluvastjörn, a small, shallow lake. The pollen data show a steady decline in the percentage abundance of tree birch (Betula pubescens) pollen between the Norse settlement (landnám, c. AD 870) and c. AD 1300, a pattern that contrasts with the abrupt fall in birch pollen percentages immediately following the Norse colonization at almost all previously studied sites in Iceland. Some lines of evidence suggest that the gradual birch decline could be a result of reworking of soil pollen, but independent evidence suggests that this may not necessarily be the case. The pollen record indicates that birch woodland was replaced by acidophilic taxa (notably Empetrum nigrum and Sphagnum), again contrasting with the more usual pattern of Poaceae expansion seen in post‐landnám pollen diagrams from mires close to farm sites. Chironomid and Pediastrum accumulation data show that the limnic environment became more productive immediately after landnám, probably because of anthropogenic disturbance. An increase in sedimentation rate after landnám appears initially to have been caused by increased lake productivity, while reworked inorganic soil materials became a significant contributor to the sediments after c. AD 1200. The data suggest that the impact of settlement on terrestrial vegetation may have been more variable than previously thought, while freshwater ecosystems experienced significant and rapid change. 相似文献
23.
The Klokken Gabbro--Syenite Complex, South Greenland: Cryptic Variation and Origin of Inversely Graded Layering 总被引:2,自引:0,他引:2
The Klokken stock has an outer sheath of vertically banded alkaligabbro which passes through syenogabbros into unlaminated syenitessurrounding a central layered syenite core. The layered seriescomprises sheets of granular syenite interleaved with drusylaminated syenite which shows repeated inversely graded minerallayering in which normal leucosyenite becomes progressivelymore hedenbergite-rich upwards. Tops to layers are nearly feldspar-free,fayalitemagnetite horizons. Model cumulate textures areexhibited, and cross-bedding and normally graded channel structuresare encountered. Granular layers often rest on ultramafic layersand load structures occur at the junctions. The series is cutby a sheet of biotite-syenodiorite and finally by quartzsyeniteaplites. Plagioclase (An57 in gabbros) is progressively mantled by ternaryalkali feldspar and is absent only in the layered series inwhich alkali feldspars cluster around Ab65 Or35. Pyroxene showsunbroken evolution with respect to Fe/Fe + Mg, from augite toferrohedenbergite and sodie hedenbergite, with acmite in aplites,but Al and Ti decrease suddenly between syenogabbro and unlaminatedsyenite stages. Olivines show parallel evolution but there isa jump in Fe: Mg between gabbros and unlaminated syenites fromFo52 to Fo34, with subsequent steady evolution to Fo3Te5Fa92in laminated syenites. Biotites show steady increase in Fe/Fe+ Mg throughout the syenites, approaching pure annite, but thegabbro-syenogabbro series show a reversal, interpreted as indicatingcrystallization under conditions of increasing fo2. Ilmenomagnetitesshow systematically varying minor element ratios, with a suddendrop in MgAl2O4 content between gabbros and syenites. Amphiboles(ferroedenites and sodic hastingsites) appear only in the mostfractionated syenites in the layered series. The Klokken chamber was lined by gabbro with only limited insitu fractionation to syenogabbro. The termination of gabbrocrystallization was marked by ingress of water into the conduit,perhaps causing the liquid to embark on its fractionation trendtowards oversaturated residua. The mineralogical breaks areconsistent with a drop in temperature, and increase in aSIO2at this point. The chamber was subsequently filled by a drytrachytic liquid which fractionated in situ, initially by congelationagainst the walls as unlaminated syenite. The layered seriesdeveloped within this chamber. The granular syenites show invertedcryptic and phase-layering (downward appearance of amphibole),and downward increase in grain size. They are sheets stopedoff the roof of the intrusion, representing chilled equivsalentsof the fractionating trachytic liquid of which the laminatedsyenites are the bottom accumulated products. In the inverselygraded layers the cumulus phases have slightly more advancedcompositions than the same phases in either normal rock or whenpresent as an intercumulus phase in adjacent parts of layers.The layering is attributed to crystallization at varying degreesof undercooling in a magma in which all phases exhibit a narrowcrystallization interval and which was subject to rhythmic P-build-upand sudden release. Crystal accumulation took place under nearstagnant to rhythmie P-build-up and sudden release. Crystalaccumulation took place under near stagnant conditions in athin chamber immediately beneath the roof of the intrusion. 相似文献
24.
DARREL A. SWIFT DAVID C. W. SANDERSON PETER W. NIENOW ROBERT G. BINGHAM IAN C. COCHRANE 《Boreas: An International Journal of Quaternary Research》2011,40(3):446-458
Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446–458. 10.1111/j.1502‐3885.2010.00196.x. ISSN 0300‐9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample‐group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence‐based techniques. 相似文献
25.
KROON NORRIS KLAUS IAN T. ALEXANDER LEON PAUL BARDOT CHARLES E. BARKER JEAN-PIERRE BELLIER CHARLES D. BLOME LEON J. CLARKE JOCHEN ERBACHER KRISTINA L. FAUL MARY ANNE HOLMES BRIAN T. HUBER MIRIAM E. KATZ KENNETH G. MACLEOD SANDRA MARCA FRANCISA C. MARTINEZ-RUIZ ISAO MITA MUTSUMI NAKAI JAMES G. OGG DOROTHY K. PAK THOMAS K. PLETSCH JEAN M. SELF-TRIAL NICHOLAS J. SHACKLETON JAN SMIT WILLIAM USSLERIII DAVID K. WATKINS JOEN WIDMARK & PAUL A. WILSON 《Geology Today》1998,14(6):222-226
26.
PER SANDGREN IAN SNOWBALL 《Boreas: An International Journal of Quaternary Research》2001,30(2):115-130
The Kullen Peninsula in northwest Skåne, at the time of the Weichselian deglaciation an island surrounded by the Kattegat Sea, is the earliest known deglaciated area in Sweden. Sediment stratigraphic and mineral magnetic properties, combined with radiocarbon dating, were used to determine and date the isolation of present-day lake basins from the sea. Significant environmental changes, which reflect the isolations, are supported by previously published palaeoecological data and cannot be related to climate changes. Basins situated above the marine limit (ML) have short (in the order of centimetres) minerogenic sequences that are magnetically characterized by low concentrations of detrital magnetite. In contrast, the pre-isolation sediments in basins below the ML, especially those deposited in sheltered positions in the landscape, have thick sequences (in the order of metres) of authigenic greigite-bearing sediments. Age determinations of the isolation level are based on the AMS radiocarbon dating of terrestrial plant macrofossil remains and previously published pollen stratigraphical investigations. Supported by the upper level of a sandy beach deposit preserved on the generally steep till covered slopes, the marine limit can be determined to 88–89 m a.s.l., which developed at the regional deglaciation c. 17000 calendar years ago. The results indicate that the deglaciation shoreline level remained fairly constant, relative to the sea level, for c. 1000 years and was followed by a gentle regression. The presented shoreline displacement curve from the Kullen Peninsula extends c. 1000 calendar years further back in time than any previously published records from the Swedish west coast. 相似文献
27.
In this study, the movement of polar lows is addressed from a novel and perspicuous viewpoint. The usual assumption has been that these mesoscale systems are, to a first approximation, advected by the larger scale flow in which they are embedded. However, when there are two or more polar lows in the same vicinity, binary interactions between pairs of polar lows can cause a cyclonic co-rotation of the pair. In geographic coordinates these interactions can appear as unusual loops, twists and turns in the low tracks, due to the interplay between the cyclonic co-rotation and the translation of the binary pair by the ambient flow. However when the tracks are replotted in centroid relative coordinates, a mutual rotation is unmistakable. Satellite imagery from several case studies has been examined to accurately determine the polar low tracks, and thus the co-rotations. Using surface wind observations, a theoretical rotation rate can be estimated, based on barotropic vortex dynamics. There is a good correspondence between the observed and calculated rotation rates. Recognizing the existence of binary interactions between polar lows emphasises the connection between polar lows and tropical cyclones, where binary interactions have been described in several studies. Indeed, in the cases studied here, binary interactions are strongest during the secondary (convective) stage of polar low development where the lows are somewhat analogous to arctic hurricanes. 相似文献
28.
Analytical and numerical models of the neutral and stably-stratifiedatmospheric boundary layer are reviewed. Theoretical arguments andcomputational models suggest that a quasi-steady state is attainable in aboundary layer cooled from below and it is shown how this may be incorporatedwithin a time-steady, one-dimensional model. A new length-scale-limitedk- model is proposed for flows where a global maximum mixing length isimposed by the finite boundary-layer depth or, in stably-stratifiedconditions, by the Obukhov length, whilst still reducing to a form consistentwith the logarithmic law in the surface layer. Simulations compare favourablywith data from the Leipzig experiment and from Cardington airfield inEngland. 相似文献
29.
30.
PHILIP J. ASHWORTH GREGORY H. SAMBROOK SMITH JAMES L. BEST JOHN S. BRIDGE STUART N. LANE IAN. A. LUNT ARNOLD J. H. REESINK CHRISTOPHER J. SIMPSON ROBERT E. THOMAS 《Sedimentology》2011,58(7):1860-1883
The depositional stratigraphy of within‐channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within‐channel heterogeneity, knowledge of which is important for reservoir characterization. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1·3 km long, 80 m wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (< 1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of ground‐penetrating radar surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m long, 400 m wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that, in terms of grain‐size trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits which, therefore, renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object‐based models of sandy braided river alluvial architecture. 相似文献