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51.
Rebecca M. Briant Martin R. Bates Steve Boreham Nigel G. Cameron G. Russell Coope Michael H. Field B. Marcus Hatch Jonathan A. Holmes David H. Keen Aiobhean A. Kilfeather Kirsty E.H. Penkman Rianne M.J. Simons Jean-Luc Schwenninger Francis F. Wenban-Smith Nicola J. Whitehouse John E. Whittaker 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(1):1-26
Constraining the speed of sea level rise at the start of an interglacial is important to understanding the size of the ‘window of opportunity’ available for hominin migration. This is particularly important during the last interglacial when there is no evidence for significant hominin occupation anywhere in Britain. There are very few finer grained fossiliferous sequences in the Channel region that can be used to constrain sea level rise and they are preserved only to the north of the Channel, in England. Of these, the sequence at Stone Point SSSI is by far the most complete. Data from this sequence has been previously reported, and discussed at a Quaternary Research Association Field Meeting, where a number of further questions were raised that necessitated further data generation. In this paper, we report new data from this sequence – thin section analysis, isotopic determinations on ostracod shells, new Optical Stimulated Luminescence ages and Amino Acid Recem analyses. These show early sea level rise in this sequence, starting during the pre-temperate vegetation zone IpI, but no early warming. The implications of this almost certainly last interglacial sequence for the human colonisation of Britain and our understanding of the stratigraphic relationship of interglacial estuarine deposits with their related fluvial terrace sequences is explored. 相似文献
52.
Claude Robin Pablo Samaniego Jean-Luc Le Pennec Michel Fornari Patricia Mothes Johannes van der Plicht 《Bulletin of Volcanology》2010,72(9):1109-1129
Fieldwork, radiometric (40Ar/39Ar and 14C) ages and whole-rock geochemistry allow a reconstruction of eruptive stages at the active, mainly dacitic, Pichincha Volcanic
Complex (PVC), whose eruptions have repeatedly threatened Quito, most recently from 1999 to 2001. After the emplacement of
basal lavas dated at ∼1100 to 900 ka, the eruptive activity of the old Rucu Pichincha volcano lasted from ∼850 ka to ∼150 ka
before present (BP) and resulted in a 15 × 20 km-wide edifice, which comprises three main building stages: (1) A lower stratocone
(Lower Rucu, ∼160 km3 in volume) developed from ∼850 to 600 ka; (2) This edifice was capped by a steeper-sided and less voluminous cone (the Upper
Rucu, 40–50 km3), the history of which started 450–430 ka ago and ended around 250 ka with a sector collapse; (3) A smaller (8–10 km3) but more explosive edifice grew in the avalanche amphitheatre and ended Rucu Pichincha's history about 150 ka ago. The Guagua
Pichincha volcano (GGP) was developed from 60 ka on the western flank of Rucu with four growth stages separated by major catastrophic
events. (1) From ∼60 to 47 ka, a basal effusive stratocone developed, terminating with a large ash-and-pumice flow event.
(2) This basal volcano was followed by a long-lasting dome building stage and related explosive episodes, the latter occurring
between 28–30 and 22–23 ka. These first two stages formed the main GGP (∼30 km3), a large part of which was removed by a major collapse 11 ka BP. (3) Sustained explosive activity and viscous lava extrusions
gave rise to a new edifice, Toaza (4–5 km3 in volume), which in turn collapsed around 4 ka BP. (4) The ensuing amphitheatre was partly filled by the ∼1-km3 Cristal dome, which is the historically active centre of the Pichincha complex. The average output rate for the whole PVC
is 0.29 km3/ka. Nevertheless, the chronostratigraphic resolution we obtained for Lower Rucu Pichincha and for the two main edifices of
Guagua Pichincha (main GGP and Toaza), leads to eruptive rates of 0.60–0.65 km3/ka during these construction stages. These output rates are compared to those of other mainly dacitic volcanoes from continental
arcs. Our study also supports an overall SiO2 and large-ion lithophile elements enrichment as the PVC develops. In particular, distinctive geochemical signatures indicate
the involvement of a new magma batch at the transition between Rucu and Guagua. At the GGP, the same phenomenon occurs at
each major collapse event marking the onset of the ensuing magmatic stage. Since the 11-ka-BP collapse event, this magmatic
behaviour has led to increasingly explosive activity. Four explosive cycles of between 100 and 200 years long have taken place
at the Cristal dome in the past 3.7 ka, and repose intervals between these cycles have tended to decrease with time. As a
consequence, we suggest that the 1999–2001 eruptive period may have initiated a new eruptive cycle that might pose a future
hazard to Quito (∼2 million inhabitants). 相似文献
53.
Lance A.M. Benner Steven J. Ostro Michael C. Nolan Jon D. Giorgini Jean-Luc Margot Michael W. Busch 《Icarus》2008,198(2):294-304
Radar observations of 214 near-Earth asteroids (NEAs) reveal a very strong correlation of circular polarization ratio with visible-infrared taxonomic class, establishing distinct differences in the centimeter-to-several-decimeter structural complexity of objects in different spectral classes. The correlation may be due to the intrinsic mechanical properties of different mineralogical assemblages but also may reflect very different formation ages and collisional histories. The highest ratios are measured for groups associated with achondritic igneous rocky meteorites: the E class, whose parent body may be 3103 Eger, and the V class, derived from the mainbelt asteroid (and Dawn mission target) 4 Vesta. 相似文献
54.
An algorithm for determining if any given point,P, on the surface of a sphere is located inside, outside, or along the border of an arbitrary spherical polygon,S, is described. The polygon is described by specifying coordinates of its vertices, and coordinates of some pointX which is known to lie withinS. The algorithm is based on the principle that an arc joiningX andP will cross the border ofS an odd number of times ifP lies outsideS, and an even number of times ifP lies withinS. The algorithm has been implemented as a set of FORTRAN subroutines, and a listing is provided. The algorithm and subroutine package can be used with spherical polygons containing holes, or with composited spherical polygons. 相似文献
55.
Controlling boundary conditions with a four-dimensional variational data-assimilation method in a non-stratified open coastal model 总被引:1,自引:1,他引:0
Vincent?Taillandier Vincent?Echevin Laurent?Mortier Jean-Luc?DevenonEmail author 《Ocean Dynamics》2004,54(2):284-298
An innovative way to take the large-scale circulation influence into account in coastal primitive-equation models is explored by an inverse modelling approach. Restricted to barotropic external forcing, this work is a first step in the development of a four-dimensional variational (4DVAR) data-assimilation approach to estimate the best initial and open-boundary conditions that force a coastal model according to interior observations. This development is founded on the OPA modelling system which representation of barotropic coastal dynamics is restricted to motions of long time scales ( a day) due to its rigid lid approximation. Twin experiments are performed in an academic configuration of the Gulf of Lions (located in the northwestern Mediterranean Sea) to study the sensitivity of a remote barotropic forcing to different observational networks measuring surface currents deployed in this area. Three monitoring designs are tested for a large-scale barotropic perturbation in the hindcast mode. It is shown that the space and time distribution of observations acts on the efficiency of the 4DVAR method and then allows coarser datasets.Responsible Editor: Phil Dyke 相似文献
56.
57.
Sedimentation processes occurring in an active convergent setting are well illustrated in the Lesser Antilles island arc. The margin is related to westward subduction of the North and/or the South America plates beneath the Caribbean plate. From east to west, the arc can be subdivided into several tectono-sedimentary depositional domains: the accretionary prism, the fore-arc basin, the arc platform and inter-arc basin, and the Grenada back-arc basin. The Grenada back-arc basin, the fore-arc basin (Tobago Trough) and the accretionary prism on the east side of the volcanic arc constitute traps for particles derived from the arc platform and the South American continent. The arc is volcanically active, and provides large volumes of volcaniclastic sediments which accumulate mainly in the Grenada basin by volcaniclastic gravity flows (volcanic debris avalanches, debris flows, turbiditic flows) and minor amounts by fallout. By contrast, the eastern side of the margin is fed by ash fallout and minor volcaniclastic turbidites. In this area, the dominant component of the sediments is pelagic in origin, or derived from South America (siliciclastic turbidites). Insular shelves are the locations of carbonate sedimentation, such as large platforms which develop in the Limestone Caribbees in the northern part of the margin. Reworking of carbonate material by turbidity currents also delivers lesser amounts to eastern basins of the margin. This contrasting sedimentation on both sides of the arc platform along the margin is controlled by several interacting factors including basin morphology, volcanic productivity, wind and deep-sea current patterns, and sea-level changes. Basin morphology appears to be the most dominant factor. The western slopes of the arc platform are steeper than the eastern ones, thus favouring gravity flow processes. 相似文献
58.
59.
Florian Genier Jean-Luc Epard François Bussy Tomas Magna 《Swiss Journal of Geoscience》2008,101(2):431-452
Detailed field work and zircon analysis have improved the knowledge of the lithostratigraphy at the base of the Siviez-Mischabel nappe in the Mattertal (St-Niklaus-Törbel area). They confirm the existence of an overturned limb and clarify the structure of the St-Niklaus syncline. The following formations can be observed:
- Polymetamorphic gneisses; composed of paragneisses, amphibolites and micaschists (Bielen Unit, pre-Ordovician).
- Fine-grained, greyish quartzite and graywacke with kerogen-rich horizons (Törbel Formation, presumed Carboniferous).
- Green or white micaschists characterized by brown carbonate spots associated with white conglomeratic quartzites (Moosalp Formation, Early Permian).
- Massive, green or white, fine grained, microconglomeratic or conglomeratic quartzites with characteristic pink quartz pebbles (Bruneggjoch Formation, Late Permian-Early Triassic).
60.
Pierre?CondamineEmail author Etienne?Médard Jean-Luc?Devidal 《Contributions to Mineralogy and Petrology》2016,171(11):95
Melting experiments have been performed at 3 GPa, between 1150 and 1450 °C, on a phlogopite-peridotite source in the garnet stability field. We succeeded to extract and determine the melt compositions of both phlogopite-bearing lherzolite and harzburgite from low to high degrees of melting (? = 0.008–0.256). Accounting for the presence of small amounts of F in the mantle, we determined that phlogopite coexists with melt >150 °C above the solidus position (1150–1200 °C). Fluorine content of phlogopite continuously increases during partial melting from 0.2 to 0.9 wt% between 1000 and 1150 °C and 0.5 to 0.6 wt% between 1150 and 1300 °C at 1 and 3 GPa, respectively. The phlogopite continuous breakdown in the lherzolite follows the reaction: 0.59 phlogopite + 0.52 clinopyroxene + 0.18 garnet = 0.06 olivine + 0.23 orthopyroxene + 1.00 melt. In the phlogopite-harzburgite, the reaction is: 0.93 phlogopite + 0.46 garnet = 0.25 olivine + 0.14 orthopyroxene + 1.00 melt. Melts from phlogopite-peridotite sources at 3 GPa are silica-undersaturated and are foiditic to trachybasaltic in composition from very low (0.8 wt%) to high (25.6 wt%) degrees of melting. As observed at 1 GPa, the potassium content of primary mantle melts is buffered by the presence of phlogopite, but the buffering values are higher, from 6.0 to 8.0 wt% depending on the source fertility. We finally show that phlogopite garnet-peridotite melts are very close to the composition of the most primitive post-collisional lavas described worldwide. 相似文献