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61.
Jessica C. Errico Jaime D. Barnes Ariel Strickland John W. Valley 《Contributions to Mineralogy and Petrology》2013,166(4):1161-1176
The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ18O value by ~1–3 ‰ from core to rim at a distance of ~120 ± 50 μm from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ18O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ18O in eclogite garnet cores range from 5.7 to 11.6 ‰, preserving the composition of the eclogite protolith. The abrupt decrease in the δ18O values of the garnet rims to values ranging from 3.2 to 11.2 ‰ suggests interaction with a lower δ18O fluid during the final stages of growth during eclogite facies metamorphism (450–600 °C). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185–240 °C at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ18O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ18O garnet rims, and later as temperatures decrease, with rind formation. 相似文献
62.
The biological processes have been proposed as climate variability contributors. Dimethylsulfide (DMS) is the main biogenic sulfur compound in the atmosphere; it is mainly produced by the marine biosphere and plays an important role in the atmospheric sulfur cycle. Currently it is accepted that terrestrial biota not only adapts to environmental conditions but also influences them through regulations of the chemical composition of the atmosphere. In the present study we used a wavelet method to investigate the relationship between DMS, Low cloud cover (LCC), Ultraviolet Radiation A (UVA), Total Solar Irradiance (TSI) and Sea Surface Temperature (SST) in the so called pristine zone of the Southern Hemisphere. We found that the series analyzed have different periodicities which can be associated with large scale climatic phenomena such as El Niño (ENSO) or the Quasi-Biennial Oscillation (QBO), and/or to solar activity. Our results show an intermittent but sustained DMS-SST correlation and a DMSUVA anti correlation; but DMS-TSI and DMS-LCC show nonlinear relationships. The time-span of the series allow us to study only periodicities shorter than 11 years, then we limit our analysis to the possibility that solar radiation influences the Earth climate in periods shorter than the 11-year solar cycle. Our results also suggest a positive feedback interaction between DMS and solar radiation. 相似文献
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Maria I. Velez Jason H. Curtis Mark Brenner Jaime Escobar Barbara W. Leyden Marion Popenoe de Hatch 《Geoarchaeology》2011,26(3):346-364
We inferred the late Holocene environmental history of the Guatemala highlands from multiple lines of evidence in a sediment core from Lake Amatitlán. Inferred environmental changes are generally synchronous with archaeologically documented highland Maya cultural shifts. Population increases in the Middle Preclassic, Early Classic, and Late Postclassic are associated with deforestation and soil erosion. Land abandonment in the Late Preclassic, Late Classic, and Early Postclassic is associated with evidence for reforestation and soil stabilization. Diatoms indicate relatively lower lake level and greater trophic status at times of reduced human impact, from ca. 250 B.C. to A.D. 125 and from ca. A.D. 875 to 1375. Decreased water levels were probably due to drier climate, to reforestation, or both. Lake eutrophication was caused by reduced water volume combined with a legacy of long‐term agricultural activity. Our data contribute to the understanding of relations among ancient Maya culture, climate, and environment. © 2011 Wiley Periodicals, Inc. 相似文献
65.
Jaime Burgos-García Joaquín Delgado 《Celestial Mechanics and Dynamical Astronomy》2013,117(2):113-136
The restricted three-body problem (R3BP) possesses the property that some classes of doubly asymptotic (i.e., homoclinic or heteroclinic) orbits are limit members of families of periodic orbits, this phenomenon has been known as the “blue sky catastrophe” termination principle. A similar case occurs in the restricted four body problem for the collinear equilibrium point $L_{2}$ L 2 . In the restricted four body problem with primaries in a triangle relative equilibrium, we show that the same phenomenon observed in the R3BP occurs. We prove that there exists a critical value of the mass parameter $\mu _{b}$ μ b such that for $\mu =\mu _{b}$ μ = μ b a Hamiltonian Hopf bifurcation takes place. Moreover we show that for $\mu >\mu _{b}$ μ > μ b the stable and unstable manifolds of $L_{2}$ L 2 intersect transversally and the spectrum corresponds to a complex saddle. This proves that Henrard’s theorem applies at least for $\mu $ μ close to $\mu _{b}$ μ b . In particular there exists a family of periodic orbits having the homoclinic orbit as a limit. 相似文献
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68.
Sylvana Melo dos Santos Jaime Joaquim da Silva Pereira Cabral Ivaldo Dario da Silva Pontes Filho 《Natural Hazards》2012,64(1):421-439
The global positioning system (GPS) is widely used for determining the three-dimensional position of points on earth surface. In the last few years, its use has also been increasingly employed for obtaining vertical geometric coordinates, which associated with the orthometrical altitude of the point can be employed in various civil engineering applications, as has been seen in diverse localities throughout the world. The satisfactory performance of this technology for obtaining excellent precision in vertical coordinates demands some conditions, related to the satellites, such as transmission of signals and, principally, in the case of urbanized areas, the presence of obstructions in the proximity of the equipment, which interferes with the quality and quantity of the information collected. GPS is an efficient technique for the detection of small movements, including the monitoring of soil subsidence. The present article describes adequate forms of the use of GPS in such a way as to guarantee good precision of the resulting vertical coordinates, even under unfavorable situations, such as in coastal areas, where the reference networks for monitoring are implanted in only one of the sides of the point?Cobject region, which makes the situation more difficult and requires careful planning so that the use of GPS reaches the desired precision. 相似文献
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Jaime Escobar Mark Brenner Thomas J. Whitmore William F. Kenney Jason H. Curtis 《Journal of Paleolimnology》2008,40(2):715-731
Fifty-seven surface sediment samples from 35 Florida lakes were collected to study testate amoebae. Seven genera, 17 species,
and 28 strains were identified in the 46 sediment samples from 31 lakes that contained testate rhizopods. Seven species accounted
for ≥90% of the individuals in all samples. Sediment total phosphorus (TPsed), organic matter (OM), and total carbon:total
nitrogen ratio (TC:TN) were measured to assess the effect of these variables on thecamoebian assemblages. OM content was the
only sediment variable that influenced presence/absence of thecamoebians. Samples with <5% OM contained no thecamoebians.
Lakes with multiple surface sediment samples showed high Morisita–Horn similarity values (0.74–0.99), indicating that all
sites at which samples were collected in a lake provided representative thecamoebian assemblages. No relationship was observed
between thecamoebian diversity indices and sediment variables. Lake trophic state and pH were examined to explore potential
water column influences on thecamoebian communities. Highest thecamoebian diversity indices were found in mesotrophic to eutrophic
lakes with pH near 8.0. These results suggest that water column conditions have a greater influence on thecamoebian assemblages
than do sediment variables. We used multivariate analysis to evaluate the relations between water quality variables and testate
rhizopod assemblages. Canonical correspondence analysis (CCA) showed that alkalinity and pH are the water column variables
that most influence the relative abundance of species. Thecamoebians thus hold promise as bioindicators of acidification in
Florida lakes. Thecamoebian remains in lake sediment cores should be useful to infer past anthropogenic shifts in lake pH. 相似文献