全文获取类型
收费全文 | 5691篇 |
免费 | 228篇 |
国内免费 | 88篇 |
专业分类
测绘学 | 169篇 |
大气科学 | 546篇 |
地球物理 | 1420篇 |
地质学 | 1950篇 |
海洋学 | 551篇 |
天文学 | 750篇 |
综合类 | 22篇 |
自然地理 | 599篇 |
出版年
2021年 | 59篇 |
2020年 | 64篇 |
2019年 | 86篇 |
2018年 | 124篇 |
2017年 | 116篇 |
2016年 | 156篇 |
2015年 | 144篇 |
2014年 | 202篇 |
2013年 | 324篇 |
2012年 | 242篇 |
2011年 | 264篇 |
2010年 | 192篇 |
2009年 | 304篇 |
2008年 | 285篇 |
2007年 | 263篇 |
2006年 | 221篇 |
2005年 | 191篇 |
2004年 | 196篇 |
2003年 | 188篇 |
2002年 | 179篇 |
2001年 | 130篇 |
2000年 | 127篇 |
1999年 | 104篇 |
1998年 | 97篇 |
1997年 | 87篇 |
1996年 | 84篇 |
1995年 | 83篇 |
1994年 | 74篇 |
1993年 | 63篇 |
1992年 | 60篇 |
1991年 | 67篇 |
1990年 | 64篇 |
1989年 | 58篇 |
1988年 | 55篇 |
1987年 | 62篇 |
1986年 | 41篇 |
1985年 | 72篇 |
1984年 | 80篇 |
1983年 | 70篇 |
1982年 | 65篇 |
1981年 | 75篇 |
1980年 | 62篇 |
1979年 | 55篇 |
1978年 | 38篇 |
1977年 | 54篇 |
1976年 | 63篇 |
1975年 | 46篇 |
1974年 | 56篇 |
1973年 | 47篇 |
1972年 | 25篇 |
排序方式: 共有6007条查询结果,搜索用时 15 毫秒
991.
Gary M. McMurtry David R. Tappin Peter N. Sedwick Ian Wilkinson Jan Fietzke Bruce Sellwood 《Sedimentary Geology》2007,200(3-4):155-165
Deposits of coral-bearing, marine shell conglomerate exposed at elevations higher than 20 m above present-day mean sea level (MSL) in Bermuda and the Bahamas have previously been interpreted as relict intertidal deposits formed during marine isotope stage (MIS) 11, ca. 360–420 ka before present. On the strength of this evidence, a sea level highstand more than 20 m higher than present-day MSL was inferred for the MIS 11 interglacial, despite a lack of clear supporting evidence in the oxygen-isotope records of deep-sea sediment cores. We have critically re-examined the elevated marine deposits in Bermuda, and find their geological setting, sedimentary relations, and microfaunal assemblages to be inconsistent with intertidal deposition over an extended period. Rather, these deposits, which comprise a poorly sorted mixture of reef, lagoon and shoreline sediments, appear to have been carried tens of meters inside karst caves, presumably by large waves, at some time earlier than ca. 310–360 ka before present (MIS 9–11). We hypothesize that these deposits are the result of a large tsunami during the mid-Pleistocene, in which Bermuda was impacted by a wave set that carried sediments from the surrounding reef platform and nearshore waters over the eolianite atoll. Likely causes for such a megatsunami are the flank collapse of an Atlantic island volcano, such as the roughly synchronous Julan or Orotava submarine landslides in the Canary Islands, or a giant submarine landslide on the Atlantic continental margin. 相似文献
992.
A New Specimen of Liaoceratops yanzigouensis (Dinosauria: Neoceratopsia) from the Early Cretaceous of Liaoning Province, China 下载免费PDF全文
A new specimen of basal neoceratopsian dinosaur Liaoceratops yanzigouensis is described. The specimen comes from the Lujiatun Bed of the Lower Cretaceous Yixian Formation in Beipiao City of western Liaoning Province, and is represented by a very well preserved three-dimensional partial skull and mandible. It is also the smallest among the three specimens of L. yanzigouensis, and several features in the new specimen, such as the short preorbital length and the round rostroventral orbital rim, can be ontogenetically-related. The superb exposure of the palatal complex may be caused by the removing of its brain by a small predator in the contemporary Jehol Biota. 相似文献
993.
Daniel Bernoulli Lukas Hottinger Silvia Spezzaferri Peter Stille 《Swiss Journal of Geoscience》2007,100(2):215-225
Shallow-water limestones of presumed Late Cretaceous and Eocene age, interbedded with basaltic lavas, were described by earlier authors from São Nicolau in the northwestern part of the Cabo Verde archipelago. If confirmed, these ages would imply late Mesozoic shallow-marine and subaerial volcanic activity in the Cabo Verde archipelago, and document a geological history very different from that known so far from other Cabo Verde Islands, from which no subaerial volcanic activity before the mid-Cenozoic is known. Our re-investigation of the foraminiferal fauna indicates a Late Miocene age for the presumed Late Cretaceous and Eocene limestones. The hypothesis of a long-lived hot spot, active by the Early Cretaceous, and of a major island-building stage in the Cabo Verde Islands during this period, is therefore not supported by the present bio- or chronostratigraphic data. 相似文献
994.
Peter Malik 《Environmental Geology》2007,51(5):707-711
Hydrograph analyses of groundwater depletion process in the spring were used for estimation of karstification degree and groundwater
sensitivity to pollution in the whole catchment area. Differences of individual depletion hydrographs enable assessment of
the anticipated extent of absorption, attenuation and self-purification processes during the groundwater penetration through
the rock environment between its infiltration and its outflow in the spring. The method was applied in the SW part of the
Velka Fatra Mountains (Slovakia) “Tlsta” hydrogeological structure. In total, 209 individual recession curves from 20 gauged
springs were analyzed. Depending on characteristic groundwater depletion hydrographs with independent sub-regimes, categories
of groundwater sensitivity to pollution were defined. Finally, resulting sensitivities to pollution were linked to lithostratigraphical
units in the area. 相似文献
995.
Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ~1,000 m of picritic lavas and discrete 10- to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaîgat and Maligât Formations of which the former includes the Naujánguit member, which consists of picrites with 7–29 wt% MgO, 80–1,400 ppm Ni, 5.7–9.4 ppb Pt and 4.2–12.9 ppb Pd. The Naujánguit member contains two horizons of contaminated basalts, the Asûk and Kûgánguaq, which have elevated SiO2 (52–58 wt%) and low to moderate MgO (7.5–12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16–0.63 ppb) and Pd (0.13–0.68 ppb) abundances, and in the case of the Asûk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril’sk region. These basalts belong to the 0- to 500-m thick, ~5,000- to 10,000-km3 Nadezhdinsky Formation, which is centered in the Noril’sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metal-depleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10–20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N–S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril’sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril’sk system. 相似文献
996.
997.
Control of wind strength and frequency in the Aral Sea basin during the late Holocene 总被引:1,自引:0,他引:1
Philippe Sorrel Hedi Oberhnsli Nikolaus Boroffka Danis Nourgaliev Peter Dulski Ursula Rhl 《Quaternary Research》2007,67(3):371-382
Changing content of detrital input in laminated sediments traced by XRF scanning and microfacies analyses reflect prominent variations in sedimentation processes in the Aral Sea. A high-resolution record of titanium from a core retrieved in the northwestern Large Aral Sea allows a continuous reconstruction of wind strength and frequency in western Central Asia for the past 1500 yr. During AD 450–700, AD 1210–1265, AD 1350–1750 and AD 1800–1975, detrital inputs (bearing titanium) are high, documenting an enhanced early spring atmospheric circulation associated with an increase in intensity of the Siberian High pressure system over Central Asia. In contrast, lower titanium content during AD 1750–1800 and AD 1980–1985 reflects a diminished influence of the Siberian High during early spring with a reduced atmospheric circulation. A moderate circulation characterizes the time period AD 700–1150. Unprecedented weakened atmospheric circulation over western Central Asia are inferred during ca. AD 1180–1210 and AD 1265–1310 with a considerable decrease in dust storm frequency, sedimentation rates, lamination thickness and detrital inputs (screened at 40-μm resolution). Our results are concurrent with changes in the intensity of the Siberian High during the past 1400 yr as reported in the GISP2 Ice Core from Greenland. 相似文献
998.
Methods reported in the literature for rock fracture simulations include approaches based on stochastic geometry, multiple-point
statistics and a combination of geostatistics for fracture density and object-based modelling for fracture geometries. The
advantages and disadvantages of each of these approaches are discussed with examples. By way of review, the authors begin
with the geostatistical indicator simulation method, based on the truncated–Gaussian algorithm; this is followed by multiple-point
statistical simulation and then the stochastic geometry approach, which is based on marked point process simulation. A new
approach, based on pluriGaussian structural simulation, is then introduced. The new approach incorporates in the simulation
the spatial correlation between different sets of fractures, which in general, is very difficult, if not impossible, to accomplish
in the three methods reviewed. Each simulation method is summarised together with detailed simulation procedures for each.
A published two-dimensional fracture dataset is used as a means of assessing the performance of each simulation method and
of demonstrating the concepts discussed in the text. 相似文献
999.
Lucinid bivalves dominate the infauna of tropical seagrass sediments. While the effect of seagrass on lucinids has been studied,
the reverse effect has largely been ignored. Lucinids can alter porewater chemistry (i.e., increase porewater nutrients by
suspension feeding and decrease porewater sulfides by oxygen introduction and bacterial oxidation), which can potentially
change seagrass productivity and growth morphology. To observe correlations between porewater chemistry and lucinid presence,
a field survey and laboratory microcosm experiment were conducted. Survey sampling sites with lucinids had significantly lower
sulfide and higher ammonium concentrations than sampling sites without lucinids. There was no difference in phosphate concentration
among sampling sites. Both lucinid species used in the microcosm experiment (Ctena orbiculata andLucinesca nassula) significantly lowered sulfide concentrations in the sediment porewater. Microcosm and field survey results were incorporated
into a sulfide budget. In seagrass sediments, lucinids remove 2–16% of the total sulfide produced. Sulfide is a major stressor
to both plants and animals in Florida Bay sediments; this removal may be important to maintaining seagrass productivity and
health. Oxygen introduction into sediments byC. orbiculata was estimated in a dye experiment.C. orbiculata were added to small tubes containing sieved mud and incubated in a bath of seawater with a Rhodamine WT. Rhodamine WT accumulation
in the sediment was measured. A first order estimate showed that oxygen introduction can account for less than 5% ofC. orbiculata sulfide removal. 相似文献
1000.
Peter Brack Hans Rieber Roland Mundil Wolfgang Blendinger Florian Maurer 《Swiss Journal of Geoscience》2007,100(3):327-348
The depositional architecture and the geometric relationships between platform-slope deposits and basinal sediments along
with paleontological evidence indicate the time interval of the younger Anisian Reitziites reitzi ammonoid zone to largely represent the main stage of platform aggradation at the Cernera and Bivera/Clapsavon carbonate platforms.
Published and new U-Pb age data of zircons from volcaniclastic layers bracketing the stratigraphic interval of platform growth
constrain the duration of platform evolution to a time span shorter than 1.8±0.7m.y., probably in the order of 0.5-1m.y.,
reflecting fast rates of vertical platform aggradation exceeding 500 m/m.y. In the range of growth potentials for shallow-water
carbonate systems estimated in relation to the time span of observation, this high rate is in agreement with values for short
intervals of 105-106yrs (e.g., Schlager 1999).
After drowning, the platforms at Cernera and Bivera/Clapsavon were blanketed by thin pelagic carbonates. On the former platform
flanks the draping sediments in places comprise red nodular pelagic limestones (Clapsavon Limestone) similar in facies to
the Han Bulog Limestones occurring elsewhere in Middle Triassic successions of the Mediterranean Tethys. The drowning of vast
areas of former carbonate platforms possibly triggered the onset of bottom-water circulation in adjacent basins as suggested
by the abrupt transition from laminated to bioturbated pelagic nodular limestones in the Buchenstein Formation which occurred
close to the time of initial platform submergence. During the Late Ladinian the topographic features of the drowned platforms
were onlapped by rapidly deposited, predominantly clastic successions including coarse breccias and volcanic rocks sealing
and preserving the peculiar stratigraphic setting. 相似文献