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81.
82.
Solomon Tafesse Joanne M. Robison Fernlund Wenjuan Sun Fredrik Bergholm 《Sedimentology》2013,60(4):1100-1110
Angularity is an important parameter in the characterization of particle morphology that is used to interpret the transport history of particles in sedimentary deposits. In the past, visual classification using silhouette charts was widely used to determine particle angularity, but this approach is subjective and time‐consuming. With advances in modern image analysis techniques and low‐cost software packages, it is possible to rapidly quantify particle angularity more objectively than using visual classification methods. This study re‐examines the performance of three existing image analysis methods and one new image analysis procedure, applied to six rock and sediment samples that were visually different in angularity. To facilitate comparison between the angularity results, measurements were reduced to rankings for each aggregate sample. These results show that the four image analysis methods rank the angularity of the samples differently, and that none rank the mean angularity index in the same order as the angularity ranking using visual classification. Therefore, further research is needed to develop an image analysis method that can quantify the angularity of sedimentary particles more precisely. 相似文献
83.
Joanne S. Johnson Jeremy D. Everest Philip T. Leat Nicholas R. Golledge Dylan H. Rood Finlay M. Stuart 《Quaternary Research》2012,77(2):273-280
Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic 10Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the 10Be ages are in the range 10.2–21.7 ka, showing a general trend of progressive ice-sheet thinning since at least 22 ka until 10 ka. The data also provide a minimum estimate (490 m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic 3He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46 ka, probably reflecting exhumation by periglacial processes. 相似文献
84.
In 1981 a magnetic and bathymetric survey was carried out over Explorer Seamount located approximately between 48°50′ and
49°30′N and 130°40′ and 131°30′W. During the cruise four pillow fragments were dredged from peaks in this area. Based on the
observed magnetic data, it was identified that the main peak of Explorer Seamount should be of the Bruhnes age while the magnetic
high in the NW of study area would be Gauss. This would yield a spreading rate of about 2 cm per year for the Matuyama epoch.
The results of intensities of remanence measured from the pillow fragments indicate that the ages are less than 106 years. Similarly the Curie temperature is consistent with ages of around 106 years. An attempt was made to model the observed magnetic field using a series of uniformly magnetized NE-SW trending blocks
of alternating polarity. Agreement between the model and observed results is satisfied. Oxidation ratios calculated from measured
samples suggest that the main peak of Explorer Seamount is not far from the present day ridge axis, which agrees with the
younger interpretation from the magnetic profiles.
The results mentioned above strongly, but not conclusively, favour an interpretation in which the southeastern peak of Explorer
Seamount is of the Bruhnes age and the recent spreading is at Explorer trench, then the Sovanco Fracture Zone should be thought
of as a complex triangular area between the northern end of Juan de Fuca Ridge and Explorer Seamount rather than as a simple
transform fault as defined conventionally. 相似文献
85.
Joanne?C.?Bintz Scott?W.?NixonEmail author Betty?A.?Buckley Stephen?L.?Granger 《Estuaries and Coasts》2003,26(3):765-776
We investigated the independent and interactive effects of nutrient loading and summer water temperature on phytoplankton,
drift macroalgae, and eelgrass (Zostera marina) in a coastal lagoon mesocosm experiment conducted from May through August 1999. Temperature treatments consisted of controls
that approximated the 9-yr mean daily temperatures for Ninigret and Point Judith Lagoons in Rhode Island (United States) and
treatments approximately 4°C above and 4°C below the controls. Nutrient treatments consisted of the addition of 6 mmol N m−2d−1 and 0.5 mmol P m−2 d−1 to mesocosms 4°C above and 4°C below the 9-yr daily mean. Nutrient enrichment produced marked phytoplankton blooms in both
cool and warm treatments during early summer. These were replaced after midsummer by dramatic growths of macroalgal mats ofEnteromorpha flexuosa and, to a lesser degree,Cladophora sericea. No phytoplankton blooms were observed in the cool unenriched treatments, but blooms did develop in the mean temperature
and warm mesocosms during the second half of the summer that were similar in intensity, though of shorter duration, than those
observed earlier in the enriched systems. Macroalgal blooms did not occur in the unenriched mesocosms. Sustained warm water
temperatures markedly decreased eelgrass density and belowground production and increased the time interval between the initiation
of new leaves, particuarly when the biomass of macroalgae was high. The negative effect of elevated water temperature on eelgrass
was significantly increased under conditions of elevated inorganic nutrient input. By the end of summer, virtually all of
the measures of eelgrass health declined in rank order from cool, to mean, to cool enriched, to warm, to warm enriched treatments.
It is likely that the marked declines in eelgrass abundance observed during recent decades in the Northeast have resulted
from an interaction of increasing nutrient enrichment combined with increasing summer water temperatures. 相似文献
86.
87.
A Weichselian Late Pleniglacial fluvio-aeolian deposit has been investigated in the southern Netherlands. Three main structural lineaments have been distinguished: (1) very small, vertical platy structures (microjoints), in a parallel and a columnar configuration; (2) large joints and normal faults with minor displacement (‘Grubbenvorst type’), arranged in a conjugate fault-system; (3) large joints and normal faults (‘wedge-type’), located adjacent to ice-wedge casts. Since clay is absent, the occurrence of the vertical platy structures cannot be attributed to desiccation cracking. The vertical platy structures are interpreted as the result of thermal contraction cracking of a relatively thin layer, due to a sudden temperature drop. The large joints and normal faults of the Grubbenvorst type are the result of failure of the sediment due to the melting of the permafrost in the Late Pleniglacial, just before the formation of the Beuningen Gravel Bed. In other areas large periglacial convolutions have been formed during the same period. The normal faults and joints of the wedge type are more generally known. They are the result of failure of the sediment adjacent to a melting ice wedge. 相似文献
88.
Importance of inherited rift margin structures in the early North Alpine Foreland Basin, Switzerland 总被引:1,自引:0,他引:1
The earliest evolution of the North Alpine Foreland Basin in Switzerland was characterized by deposition in small, structurally partitioned sub-basins during the Late Cretaceous and Early Tertiary, rather than in a single, large foredeep. These sub-basins, which were probably located between old rift margin fault-blocks reactivated during Alpine compression, were incorporated into the thrust wedge during thin-skinned deformation. In eastern Switzerland, the most external sub-basins with respect to the orogenic wedge (North Helvetic Flysch and Blattengrat units) have at their base an unconformity attributed to flexural forebulge erosion. More internal sub-basins (Sardona and Prättigau units) contain a conformable succession from the underlying passive margin stage and are dominated by deep-water sedimentation. In western Switzerland, both external sub-basins, now found in the Helvetic Diablerets and Wildhorn nappes, and deep-water internal sub-basins (Höchst-Meilleret Flysch, Neisen Flysch, Tarentaise Flysch) preserve a well-developed basal unconformity. Comparison of the eastern and western Swiss transects shows important intrabasinal lateral variations to be present. The western Swiss area was a topographic high for much of the Late Cretaceous and Early Tertiary; this is demonstrated by the increased chronostratigraphic gap at the karstified basal unconformity surface in western Switzerland. The strata onlapping this unconformity young to the west, suggesting that drowning of the emergent area was delayed compared with the east. In addition, reactivation and uplift of the rift margin structures occurred earlier in western Switzerland compared with eastern Switzerland. There is therefore strong evidence for lateral topographic gradients in the early foreland basin caused by differential amounts of tectonic reactivation of rift margin structures. In the early foreland basin-fill, these lateral variations are as important in determining depositional patterns as strike-normal changes across the basin. 相似文献
89.