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361.
A scenario for the future development of the Dutch Wadden Sea is derived from an evolutionary model for tidal basins during a rise in sea level. The model is based on the evolution of the Atlantic/Subboreal Holland tidal basin, between 7000 BP and 3500 BP. It emphasizes the balance between the storage capacity created by a sea-level rise and the amount of sediment available.
If the rate of relative sea-level rise exceeds the rate of sediment supply, the innermost (central) portions of the basin will not receive sufficient sediment for an intertidal morphology to be preserved. Eventually, sand will be deposited only in tidal channels and in the flood-tidal delta through which the sediment is supplied, mud deposition will occur in the interchannel areas and salt marshes will disappear. 相似文献
362.
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364.
Dirk Verschuren 《Journal of Paleolimnology》1994,10(3):253-263
Aquatic invertebrates are intrinsically capable of rapid and sensitive response to changes in their lacustrine habitat. Fossil invertebrate assemblages preserved in the sediments of a climate-sensitive lake can thus produce high-resolution proxy records of past climate. In shallow lakes, however, a potential conflict exists between the sensitivity of biota to frequent habitat change in their fluctuating environment and the increased probability of disturbance of selected proxy records by bioturbation and physical mixing of sediments. I investigated this problem with tropical-African aquatic invertebrate faunas in a paleolimnological sensitivity study that incorporates both the response of biota to short-term habitat change and the taphonomic integrity of fossil assemblages in a small, shallow, and hydrologically closed lacustrine basin. Analysis of chironomid, cladoceran, and ostracode remains in a210Pb-dated short core from Lake Oloidien (Kenya) indicates that habitat changes accompanying the late 19th- and 20th-century fluctuations in lake level (Z
max
range: 3 to 18 m) and salinity (conductivity range:c. 400 toc. 2000–4000 µS cm–1) were sufficient to drastically alter the composition of local benthic and planktonic invertebrate faunas. This response remained relatively unaffected by taphonomic phenomena during its incorporation into the sediment record. Results indicate that tropical-African aquatic invertebrate faunas in suitable climate-sensitive lakes are a valuable tool to resolve paleoclimatic fluctuations on a timescale of decades.This paper was presented at the VI Palaeolimnology Symposium, held at Canberra in April 1993. Dr. Mark Brenner guest edited this contribution. 相似文献
365.
Prof. Dr. Dirk Anthony Ballendorf 《GeoJournal》1988,16(2):137-142
The former United Nations Trust Territory of the Pacific Islands (TTPI) which has been administered by the United States since
the end of World War II, has divided itself up into four separate political entities: (1) the Federated States of Micronesia,
(2) the Republic of Belau, (3) the Marshall Islands Republic, and (4) the Commonwealth of the Northern Mariana Islands. The
first three of these aforementioned entities are freely-associated states with the United States. This is a new form of political
arrangement for these areas and it is also new for the United States. All of these areas have a colonial history of domination
by Spain, Germany, Japan, and the US, and these colonial powers have all left legacies which the new states have to deal with.
There are also a number of natural environmental factors — such as vast distance and the limited store of natural resources
- which are constraints that the new states have to face and overcome with the help of the former colonial powers and the
rest of the world. Whether or not these new states can be successful in their continued economic and social development at
a pace rapid enough to bring sufficient foreign capital, depends on their own energy and also on the nature and quality of
the outside assistance. 相似文献
366.
Marek Pękala Jan Dirk Kramers Hans Niklaus Waber Thomas Gimmi Peter Alt-Epping 《Applied Geochemistry》2009
The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of 238U, 234U and 230Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of 234U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both 238U and 230Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of 234U from the rock matrix to the pore water by the decay of 238U will be established. Diffusive redistribution separates 234U from its immobile parent 238U resulting in bulk rock 234U/238U activity disequilibria. These may provide a means of estimating the mobility of 234U in the rock if the diffusion rate of 234U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements. 相似文献
367.
368.
Contrasting sediment flux in Val Lumnezia (Graubünden, Eastern Swiss Alps), and implications for landscape development 总被引:1,自引:0,他引:1
Marco Schwab Fritz Schlunegger Heinz Schneider Gregor Stöckli Dirk Rieke-Zapp 《Swiss Journal of Geoscience》2009,102(2):211-222
This paper presents qualitative estimates of sediment discharge from opposite valley flanks in the S–N-oriented Val Lumnezia,
eastern Swiss Alps, and relates inferred differences in sediment flux to the litho-tectonic architecture of bedrock. The valley
flank on the western side hosts the deep-seated Lumnezia landslide where an area of ca. 30 km2 has experienced slip rates of several centimetres per year, potentially resulting in high sediment discharge to the trunk
stream (i.e. the Glogn River). High slip rates have resulted in topographic changes that are detectable on aerial photographs
and measurable with geodetic tools. In contrast, a network of tributary channels dissects the valley flank on the eastern
side. There, an area of approximately 18 km2 corresponding to < 30% of the surface has experienced a change in the landscape mainly by rock avalanche and rock fall, and
the magnitudes of changes are below the calibration limit of digital photogrammetry. We thus infer lower magnitudes of sediment
discharge on the eastern tributaries than on the western valley side, where landsliding has been the predominant erosional
process. These differences are interpreted to be controlled by the dip-slope situation of bedrock on the western side that
favours down-slope slip of material.
Morphometric investigations reveal that the western valley side is characterized by a low topographic roughness because this
valley flank has not been dissected by a channel network. It appears that high sediment discharge of the Lumnezia landslide
has inhibited the establishment of a stable channel network and has largely controlled the overall evolution of the landscape.
This contrasts to the general notion that channelized processes exert the first-order control on landscape evolution and formation
of relief and needs to be considered in future studies about landscape architecture, drainage network and sediment discharge. 相似文献
369.
Dirk Fuchs 《Proceedings of the Geologists' Association. Geologists' Association》2019,130(3-4):339-344
A concretion from the lower Tithonian Kimmeridge Clay Formation (Pectinatus Zone) found by Steve Etches yielded a gladius of a coleoid cephalopod. It is peculiar in shape and has an unusual ornamentation of radiating ribs and tubercles. The new form is named Etchesia martilli n. gen. n. sp. and preliminarily placed within the octobrachian family Muensterellidae based on its limpet-like gladius. Through the presence of radiating ribs as well as the absence of a narrow anterior rachis E. martilli n. gen. n. sp. is similar to Pearceiteuthis buyi from the Oxford Clay Formation (Callovian). The new muensterellid is unique in having an enrolled patella apex, which is located close to the posterior gladius rim. E. martilli n. gen. n. sp. represents the first muensterellid coleoid from the Kimmeridge Clay Formation. A phylogenetic relationship of E. martilli n. gen. n. sp. (and Pearceiteuthis) with cirrate and incirrate octopods is discussed, although further information on soft parts such as the muscular mantle is necessary. 相似文献
370.
Much of the knowledge about ocean circulation stems from rather simple analytical models. The behavior of the meridional overturning and, more specifically, the thermohaline-induced part of the global ocean circulation, under changing surface conditions, is often judged by the bifurcation structure of box models with very low (low-order) resolution. The present study proposes a new low-order model of the thermohaline-driven circulation, which is constructed by severe truncation of a spectral decomposition of the two-dimensional equations of motion (vorticity and heat/salt balances). The physical ingredients of the new model are superior to box models because it has a continuous lateral and vertical representation of the fields and finite diffusion coefficients for heat and salt. The building of the spectral model involves much mathematical labor because the structure functions must be constructed in accordance with the boundary conditions for conservation of momentum, mass, heat, and salt. Furthermore, a number of complicated coupling coefficients must be evaluated. Like the box models, the spectral model is a dynamical system with mathematical complexity, but in most of the versions that we analyze, it still can be handled by standard analytical procedures. These versions are the spectral counterparts of the classical box models of Stommel, Rooth, and Welander, adjusted to the Atlantic overturning. A detailed comparison of the model types reveals a similar bifurcation pattern of box and spectral low-order configurations under symmetric and asymmetric forcing conditions and slight perturbations thereof (we use mixed boundary conditions for heat and salt and the surface freshwater flux as a continuation parameter). Comparison of the spectral low-order models with models towards a higher resolved range, namely, the two-dimensional overturning models for the meridional plane, reveals a close resemblance as well. A major difference of box and spectral models is the appearance of parameter windows in the latter, where only unstable steady states exist. The spectral models then show limit cycles, as well as chaotic trajectories with time scales of thousands of years. 相似文献