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171.
After an unusually long quiet period of nearly 6 years, in 1998the Piton de la Fournaise volcano started a new cycle of intensevolcanic activity. We report geochemical data on the first nineevents (53 samples), from the long-lived initial eruption (sixand a half months) of 1998 to the high-flux picritic eruptionof January 2002. Pb isotopes and trace elements display systematic,coupled variations, which are mostly confined to the beginningand the end of the period. Two well-defined binary mixing trendsare shown by PbPb and Pbtrace element relationships.These trends indicate a change of end-member components betweenMarch and June 2001 that coincides with the transition fromsteady-state basalts to picrites. A three-component mixing modelinvolving a homogeneous plume and two contaminants successfullyexplains the data. The PbPb relationship requires thattwo mixing processes occur successively: plume-derived magmainteracts first with altered oceanic crust, and the resultinghybrid product then interacts at shallower level with the oldlavas constituting the base of the volcanic edifice. Assimilationof edifice material decreased continuously from 1998 to 2002,whereas assimilation of oceanic crust drastically increasedduring the late-stage picritic eruption. These results suggestthat picrites may have resided for an unusually long time atan oceanic crustal level before ascending rapidly through thevolcanic edifice with little interaction with channel walls. KEY WORDS: assimilation; lead isotopes; picrites; Piton de la Fournaise; trace elements 相似文献
172.
THOMAS J BASSETT 《The Geographical journal》2005,171(1):24-35
This paper examines wildlife decline in northern Côte d'Ivoire with emphasis on the political-economic and cultural dimensions of market hunting. Hunting and the trade in wild animal meat are situated within the economic diversification strategies of impoverished farmers and the flourishing of hunter associations. The role of hunter associations is critical in the rise in the number of hunters and firearms in the case study area of Kakoli. Initiation into the national hunters' association, Binkadi, allows card-carrying members to bear arms without legal repercussions. Higher quality arms and rural poverty motivate many of these recruits to engage in market hunting. Game market surveys for 1981–82 and 1997–99 in Kakoli show tremendous pressure on both vulnerable and robust species alike. Game depletion is conceptualized as the outcome of interacting social and biophysical processes that produce hunting pressure and habitat change. By drawing attention to the political-economic, ecological, and cultural dimensions of wildlife decline, this study implicates a diversity of actors and institutions in the dynamics of game depletion. 相似文献
173.
PETER JANSSON GUNHILD ROSQVIST THOMAS SCHNEIDER 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):37-50
ABSTRACT. In order to perform inverse modelling of climate variability based on palaeoclimate proxy records, the complexity of intermediate steps in the chain of processes from the climate forcing to the responding proxy has to be considered. In reconstructing climate-forced glacier fluctuations from proglacial lacustrine sediments it is important to understand how climate affects glacier dynamics. A glacier system is complex with many factors influencing sediment production, transport and deposition. Fluvial and mass movement processes in the proglacial environment may affect lake sedimentation substantially. We argue that it is easy to over-interpret glaciolacustrine sediment variability by ignoring these complications. The sediment records may contain individual layers resulting from single precipitation or melt events, as well as persistent changes in climate-forced glacier dynamics. We conclude that it is necessary to consider all possible influencing factors in order to derive reliable palaeoclimate data from lacustrine sediment sequences. 相似文献
174.
Sedimentological outcrop analysis and sub‐surface ground‐penetrating radar (GPR) surveys are combined to characterize the three‐dimensional sedimentary architecture of Quaternary coarse‐grained fluvial deposits in the Neckar Valley (SW Germany). Two units characterized by different architectural styles are distinguished within the upper part of the gravel body, separated by an erosional unconformity: (i) a lower unit dominated by trough‐shaped depositional elements with erosional, concave‐up bounding surfaces that are filled by cross‐bedded sets of mainly openwork and filled framework gravel; and (ii) an upper unit characterized by gently inclined sheets of massive and openwork gravels with thin, sandy interlayers that show lateral accretion on a lower erosional unconformity. The former is interpreted as confluence scour pool elements formed in a multi‐channel, possibly braided river system, the latter as extensive point bar deposits formed by the lateral migration of a meandering river channel. The lateral accretion elements are locally cut by chute channels mainly filled by gravels rich in fines, and by fine‐grained abandoned channel fills. The lateral accretion elements are associated with gravel dune deposits characterized by steeply inclined cross‐beds of alternating open and filled framework gravel. Floodplain fines with a cutbank and point bar morphology cover the gravel deposits. The GPR images, revealing the three‐dimensional geometries of the depositional elements and their stacking patterns, confirm a change in sedimentary style between the two stratigraphic units. The change occurred at the onset of the Holocene, as indicated by 14C‐dating of wood fragments, and is related to a re‐organization of the fluvial system that probably was driven by climatic changes. The integration of sedimentological and GPR results highlights the heterogeneity of the fluvial deposits, a factor that is important for modelling groundwater flow in valley‐fill aquifers. 相似文献
175.
The vertical component of the turbulent flow acceleration term, , is used to determine the net positive vertical force that may support a suspended sediment load. A dimensionless criterion, Λ, is proposed for the maintenance of suspension, defined as the ratio of the maximum vertical turbulent stress to immersed weight of the suspended load above a unit bed area. In order that a suspension be maintained: where v ′ is instantaneous vertical turbulent velocity, σ and ρ are solid and fluid densities, respectively and m is the suspended load dry mass. The Λ criterion is dynamic, being a ratio of stresses and is analogous in this respect to Shields dimensionless stress criterion, θ, for the initiation of bedload motion. The new criterion is successful in predicting the maintenance of steady-state suspended sediment transport in open channel shear flow and deposition from non-uniform particulate density flows of wall jet type. 相似文献
176.
MICHAEL P. EKBERG THOMAS V. LOWELL ROBERT STUCKENRATH 《Boreas: An International Journal of Quaternary Research》1993,22(3):189-204
Despite the application of radiocarbon dating for more than three decades along the southern margin of the Laurentide Ice Sheet, fundamental questions about the timing of glacial advances remain. For one of its sublobes, the Miami, we undertook areal mapping, detailed lithostratigraphic analysis, and radiocarbon dating to interpret four pulses of ice advance. On top of the undated sediments deposited during the first advance is a major unconformity. The second advance occurred about 20,000 BP and marks the beginning of the late Wisconsin glaciation. A minor recession (more than 30 km) ensued, but plants did not reoccupy the landscape. A third advance of the ice margin produced a stone-rich lodgement till to within 20 km of the late Wisconsin maximum. The final ice motion only occurred in the northern part of the study area and may be of local extent. Large accumulations of supraglacial gravity flowtills and outwash mark the final ice-margin retreat. Of these. only the second advance is well dated. This study implies that the number of advances of the ice margin is fewer than previously suggested. Consequently we argue that several of the sublobes across the southern margin of the Laurentide Ice Sheet acted in unison for the interval of 22.000 to 18.000 BP implying ice-sheet external forcing. 相似文献
177.
178.
A. ALEXIS D. MAROT P. THOMAS Doctor-Engineer Laboratoire Genie Vivil Nantes Saint-Nazaire BP Saint-Nazaire France. Doctor Laboratoire Genie Vivil Nantes Saint-Nazaire BP Saint-Nazaire France. Professor Laboratoire Genie Vivil 《国际泥沙研究》2001,16(3)
l INTRODUCTIONTo ensure the long-term futhe of dams, the emPtying of storage reservoirs is often realised with tWoobjectives:'-periodic insPection of the sttucthe,-flushing oftalrials Which sedimatd within the reservoir.Both aims are lthely to cause the instability of sedhoent, and these displacements can induceenvirOInnenal distheances that have to be reduced to the Ininimum. This imPlies a good knowledge ofthese maerials and of the processes Which originated the disPlacements.The se… 相似文献
179.
Partial Melting and Assimilation of Dolomitic Xenoliths by Mafic Magma: the Ioko-Dovyren Intrusion (North Baikal Region, Russia) 总被引:5,自引:0,他引:5
WENZEL THOMAS; BAUMGARTNER LUKAS P.; BRUGMANN GERHARD E.; KONNIKOV EDUARD G.; KISLOV EVGENIY V. 《Journal of Petrology》2002,43(11):2049-2074
A petrological study was carried out on Mg-skarn-bearing dunitecumulates that are part of the Neo-Proterozoic Ioko-Dovyrenintrusion (North Baikal region, Russia). Skarn xenoliths containbrucite pseudomorphs after periclase, forsterite and Cr-poorspinel. Fine-grained forsteritespinel skarns occur withthe brucite skarns or as isolated schlieren. Field relationshipsreveal that the Mg-skarns formed from silica-poor dolomiticxenoliths by interaction with the mafic magma of the Ioko-Dovyrenintrusion. Rapid heating of dolomitic xenoliths by the maficmagma caused the decomposition of dolomite into calcite + periclase,releasing much CO2. Further heating quantitatively melted thecalcite. A periclase-rich restite was left behind after extractionof the low-density, low-viscosity calcite melt. The extractedcalcite melt mixed with the surrounding mafic melt. This resultedin crystallization of olivine with CaO contents up to 1·67wt %. A local decrease in the silica concentration stabilizedCaAl2SiO6-rich clinopyroxene. Brucite/periclase-free forsteritespinelskarns probably originated by crystallization from the maficmelt close to the xenoliths at elevated fO2. The high fO2 wascaused by CO2-rich fluids released during the decompositionof the xenoliths. The above case study provides the first evidencefor partial melting of dolomite xenoliths during incorporationby a mafic magma. KEY WORDS: dunite; dolomite assimilation; partial melting 相似文献
180.
A polarizable sphere embedded in a conducting half-space can give rise to negative voltage transients in a coincident-loop time-domain electromagnetic system. Such transients have been observed in field situations. Our results are obtained from a model in which the contributions of the host rock, the currents in the sphere, and the interaction between the sphere and the host rock are separated and superposed. This model uses approximations to the integral equation solutions rather than finite-element or finite-difference approximations, and so allows very rapid computation. The theoretical demonstration suggests that interpretation of the negative voltage transients as a polarization response is valid, but more detailed interpretation of polarization properties may not be possible, because the superposition of the polarization response on the normal response depends strongly on the position of the target. 相似文献