全文获取类型
收费全文 | 1047篇 |
免费 | 41篇 |
国内免费 | 16篇 |
专业分类
测绘学 | 27篇 |
大气科学 | 107篇 |
地球物理 | 252篇 |
地质学 | 343篇 |
海洋学 | 82篇 |
天文学 | 162篇 |
综合类 | 4篇 |
自然地理 | 127篇 |
出版年
2022年 | 6篇 |
2021年 | 19篇 |
2020年 | 14篇 |
2019年 | 19篇 |
2018年 | 25篇 |
2017年 | 26篇 |
2016年 | 38篇 |
2015年 | 26篇 |
2014年 | 30篇 |
2013年 | 62篇 |
2012年 | 32篇 |
2011年 | 51篇 |
2010年 | 46篇 |
2009年 | 61篇 |
2008年 | 49篇 |
2007年 | 57篇 |
2006年 | 41篇 |
2005年 | 40篇 |
2004年 | 34篇 |
2003年 | 34篇 |
2002年 | 20篇 |
2001年 | 22篇 |
2000年 | 30篇 |
1999年 | 28篇 |
1998年 | 20篇 |
1997年 | 14篇 |
1996年 | 22篇 |
1995年 | 10篇 |
1994年 | 9篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 16篇 |
1990年 | 10篇 |
1989年 | 17篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 5篇 |
1983年 | 18篇 |
1982年 | 20篇 |
1981年 | 5篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 8篇 |
1971年 | 3篇 |
1930年 | 3篇 |
排序方式: 共有1104条查询结果,搜索用时 15 毫秒
991.
Extent and Chronology of the Ross Sea Ice Sheet and the Wilson Piedmont Glacier along the Scott Coast at and Since the Last Glacial Maximum 总被引:1,自引:0,他引:1
Brenda L. Hall & George H. Denton 《Geografiska Annaler: Series A, Physical Geography》2000,82(2-3):337-363
During the last glacial maximum, a coalescent ice mass consisting of the grounded Ross Sea ice sheet and an expanded Wilson Piedmont Glacier covered the southern Scott Coast. This coalescent ice mass was part of a larger grounded ice sheet that occupied the Ross Sea Embayment during the last glacial maximum. Deglaciation of the western Ross Sea Embayment adjacent to the southern Scott Coast was delayed until shortly before 6500 14 C yr bp , aconclusion based on ages of marine shells from McMurdo Sound, a relative sea-level curve, and algae that lived in ice-dammed lakes. Therefore, most recession of grounded ice in the Ross Sea Embayment occurred in mid to late Holocene time, after deglacial sea-level rise due to melting of Northern Hemisphere ice sheets essentially was accomplished. Rising sea level alone could not have driven grounding-line retreat back to the present-day Siple Coast. 相似文献
992.
Peter Roseacute;n Roland Hall Tom Korsman Ingemar Renberg 《Journal of Paleolimnology》2000,24(2):109-123
The relationships between diatoms (Bacillariophyceae) in surface sediments of lakes and summer air temperature, pH and total organic carbon concentration (TOC) were explored along a steep climatic gradient in northern Sweden to provide a tool to infer past climate conditions from sediment cores. The study sites are in an area with low human impact and range from boreal forest to alpine tundra. Canonical correspondence analysis (CCA) constrained to mean July air temperature and pH clearly showed that diatom community composition was different between lakes situated in conifer-, mountain birch- and alpine-vegetation zones. As a consequence, diatoms and multivariate ordination methods can be used to infer past changes in treeline position and dominant forest type. Quantitative inference models were developed to estimate mean July air temperature, pH and TOC from sedimentary diatom assemblages using weighted averaging (WA) and weighted averaging partial least squares (WA-PLS) regression. Relationships between diatoms and mean July air temperature were independent of lake-water pH, TOC, alkalinity and maximum depth. The results demonstrated that diatoms in lake sediments can provide useful and independent quantitative information for estimating past changes in mean July air temperature (R2
jack = 0.62, RMSEP = 0.86 °C; R2 and root mean squared error of prediction (RMSEP) based on jack-knifing), pH (R2
jack = 0.61, RMSEP = 0.30) and TOC (R2
jack = 0.49, RMSEP = 1.33 mg l-1). The paper focuses mainly on the relationship between diatom community composition and mean July air temperature, but the relationships to pH and TOC are also discussed. 相似文献
993.
Eight meadows of the seagrassThalassia testudinum Banks ex König representing a gradient of freshwater influence in Charlotte Harbor, Florida (United States), were sampled on a bimonthly basis from April 1995 to August 1996. Spatial and temporal variation in the density, biomass, productivity, and epiphyte loads of short shoots were determined. Physical factors such as water temperature, salinity, and light extinction coefficients were also measured. Areal blade production (g dw m?2 d?1) ofT. testudinum was not strongly associated with water temperature, salinity, or the amount of subsurface irradiance reaching the bottom at each station. Variation in production could be described by a linear combination of the independent variables water temperature and salinity. Water clarity (expressed as the percent of subsurface irradiance reaching the bottom) was positively related to salinity. The lack of a clear relationship between water clarity and areal production was probably due to water clarity being highest during times of the year when water temperatures were too cold to support growth ofT. testudinum. Our results suggest that seagrass light requirements determined by averaging irradiance levels measured during the growing season might be more relevant than those established by averaging light measurements collected throughout the year. The use of field studies for estimating lower salinity tolerances of seagrasses might be inappropriate for those systems where water clarity is positively associated with salinity. 相似文献
994.
995.
Margaret O. Hall Michael J. Durako James W. Fourqurean Joseph C. Zieman 《Estuaries and Coasts》1999,22(2):445-459
The Florida Bay ecosystem has changed substantially in the past decade, and alterations in the seagrass communities have been particularly conspicuous. In 1987 large areas ofThalassia testudinum (turtlegrass) began dying rapidly in western Florida Bay. Although the rate has slowed considerably, die-off continues in many parts of the bay. Since 1991, seagrasses in Florida Bay have been subjected to decreased light availability due to widespread, persistent microalgal blooms and resuspended sediments. In light of these recent impacts, we determined the current status of Florida Bay seagrass communities. During the summer of 1994, seagrass species composition, shoot density, shoot morphometrics, and standing crop were measured at 107 stations. Seagrasses had been quantified at these same stations 10 yr earlier by Zieman et al. (1989).T. testudinum was the most widespread and abundant seagrass species in Florida Bay in both 1984 and 1994, and turtlegrass distribution changed little over the decade. On a baywide basis,T. testudinum density and biomass declined significantly between surveys; mean short-shoot density ofT. testudinum dropped by 22% and standing crop by 28% over the decade.T. testudinum decline was not homogeneous throughout Florida Bay; largest reductions in shoot density and biomass were located principally in the central and western bay. Percent loss ofT. testudinum standing crop in western Florida Bay in 1994 was considerably greater at the stations with the highest levels of standing crop in 1984 (126–215 g dry wt m−2) than at the stations with lower levels of biomass. While turtlegrass distribution remained consistent over time, both the distribution and abundance of two other seagrasses,Halodule wrightii andSyringodium filiforme, declined substantially between 1984 and 1994. Baywide,H. wrightii shoot density and standing crop declined by 92%, andS. filiforme density and standing crop declined by 93% and 88%, respectively, between surveys. Patterns of seagrass loss in Florida Bay between 1984 and 1994 suggest die-off and chronic light reductions were the most likely causes for decline. If die-off and persistent water-column turbidity continue in Florida Bay, the long-term future of seagrasses in the bay is uncertain. 相似文献
996.
Tj. C. E. van Weering I. R. Hall H. C. de Stigter I. N. McCave L. Thomsen 《Progress in Oceanography》1998,42(1-4)
To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur results in along-slope and periodic off-slope directed transport of particles, possibly by entrainment of particles in a detached bottom or intermediate nepheloid layer. From the shelf to the abyssal plain the surface sediments on the Goban Spur change from terrigenous sandy shelf sediments into clayey silts. 210Pb activity decreases exponentially down core, reaching a stable background value at 10 cm (shallower stations) to 5 cm (deeper stations) sediment depth. 210Pb profiles of repeatedly sampled stations indicate negligible annual variability of mixing and flux. The 210Pbxs flux to the sediment shows a decreasing trend with increasing water depth. Below about 2000 m the average 210Pbxs flux is about 0.3 dpm cm−2 y−1, a third of the fluxes measured on the shelf and upper slope stations. Sediment mixing rates (Db) correlate with macro- and meiofaunal density changes and are within the normal oceanic ranges. Lower mixing rates on the lower slope likely reflect lower organic carbon fluxes there. Mass accumulation rates on Meriadzek Terrace are at maximum 80 g m−2 y−1, almost twice as high as at Goban Spur stations of comparable depth. A minimum accumulation rate of 16.6 g m−2 y−1 is found at the Goban Spur upper slope break. Organic carbon burial rates are low compared to other margins and range from a lowest value of 0.05 g m−2 y−1 at the upper slope break to 0.11 g m−2 y−1 downslope. A maximum organic carbon burial rate of 0.41 g m−2 y−1 is found on Meriadzek Terrace. Carbonate burial rates increase along the northern transect from the shelf (13 g m−2 y−1) via a low (9.3 g m−2 y−1) on the upper slope break to the deep sea (30.7 g m−2 y−1). Carbonate burial is highest on Meriadzek Terrace (44.5 g m−2 y−1). The N.W. European Margin at Goban Spur and Meriadzek Terrace cannot be considered a major carbon depocenter. 相似文献
997.
We examine 800-year time series of internally generated variability in both a coupled ocean-atmosphere model where water
vapor anomalies are not allowed to interact with longwave radiation and one where they are. The ENSO-like phenomenon in the
experiment without water vapor feedback is drastically suppressed both in amplitude and geographic extent relative to the
experiment with water vapor feedback. Surprisingly, the reduced amplitude of ENSO-related sea surface temperature anomalies
in the model without water vapor feedback cannot be attributed to greater longwave damping of sea surface temperature. (Differences
between the two experiments in radiative feedback due to clouds counterbalance almost perfectly the differences in radiative
feedback due to water vapor.) Rather, the interaction between water vapor anomalies and longwave radiation affects the ENSO-like
phenomenon through its influence on the vertical structure of radiative heating: Because of the changes in water vapor associated
with it, a given warm equatorial Pacific sea surface temperature anomaly is associated with a radiative heating profile that
is much more gravitationally unstable when water vapor feedback is present. The warm sea surface temperature anomaly therefore
results in more convection in the experiment with water vapor feedback. The increased convection, in turn, is related to a
larger westerly wind-stress anomaly, which creates a larger decrease in upwelling of cold water, thereby enhancing the magnitude
of the original warm sea surface temperature anomaly. In this manner, the interaction between water vapor anomalies and longwave
radiation magnifies the air-sea interactions at the heart of the ENSO phenomenon; without this interaction, the coupling between
sea surface temperature and wind stress is effectively reduced, resulting in smaller amplitude ENSO episodes with a more limited
geographical extent.
Received: 26 March 1999 / Accepted: 25 October 1999 相似文献
998.
Sub-ophiolite metamorphic rocks from NW Anatolia, Turkey 总被引:4,自引:0,他引:4
The metamorphic rocks from near Kütahya in north-west Anatolia record different stages in the history of closure of the Neo-Tethyan İzmir–Ankara–Erzincan ocean. Sub-ophiolite metamorphic rocks within the Tavşanlı zone are a tectonically composite sequence of quartz–mica schists, amphibole schists, amphibolites and garnet amphibolites. They show increasing metamorphic grade towards the base of the ophiolite. A first metamorphic event, typical of sub-ophiolite metamorphic sole rocks, was characterized by high-grade assemblages, and followed by retrograde metamorphism. A second event was marked by a medium-to high-pressure overprint of the first-stage metamorphic assemblages with assemblages indicating a transition between the blueschist and greenschist facies. The chemistry of the sub-ophiolite metamorphic rocks indicates an ocean island basalt origin, and Ar–Ar dating indicates a high temperature metamorphic event at 93±2 Ma. Counter-clockwise P–T–t paths recorded by the sub-ophiolite metamorphic rocks are interpreted to result from intra-oceanic thrusting during the closure of the İzmir– Ankara–Erzincan ocean, initiating subduction, which formed the high-temperature assemblages. Further subduction then produced the widespread blueschists of the Tavşanlı zone during the Late Cretaceous. Later cold thrusting obducted the ophiolite (with the metamorphic sole welded to its base), ophiolitic melanges and blueschists onto the Anatolide passive margin in the latest Cretaceous. All these events pre-date the final Anatolide–Pontide continent–continent collision. 相似文献
999.
Derek Hall 《GeoJournal》2000,50(2-3):249-253
This paper attempts to provide an evaluation of cross-border movement - short-term, long-term and permanent - in southeastern
Europe, identifying the functions of such movement in relation to ongoing restructuring processes. It also suggests a range
of theoretical frameworks appropriate for further research in this area. While refugee flight and permanent displacement have
resulted from some of the more pernicious and explicit processes of reinvigorated nationalism in the region, patterns of petty
and larger scale trading, labour and education migration, leisure mobility and tourism, better reflect the differential impact
of local, regional and global push and pull factors and the new configurations of national boundaries and neighbourly relations.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
1000.
Jeremy B. Platt 《Natural Resources Research》2007,16(3):263-275
Fuel prices in 2006 continued at record levels, with uranium continuing upward unabated and coal, SO2 emission allowances, and natural gas all softening. This softening did not continue for natural gas, however, whose prices
rose, fell and rose again, first following weather influences and, by the second quarter of 2007, continuing at high levels
without any support from fundamentals. This article reviews these trends and describes the remarkable increases in fuel expenses
for power generation. By the end of 2005, natural gas claimed 55% of annual power sector fuel expenses, even though it was
used for only 19% of electric generation. Although natural gas is enormously important to the power sector, the sector also
is an important driver of the natural gas market—growing to over 28% of the market even as total use has declined. The article
proceeds to discuss globalization, natural gas price risk, and technology developments. Forces of globalization are poised
to affect the energy markets in new ways—new in not being only about oil. Of particular interest in the growth of intermodal
traffic and its a little-understood impacts on rail traffic patterns and transportation costs, and expected rapidly expanding
LNG imports toward the end of the decade. Two aspects of natural gas price risk are discussed: how understanding the use of
gas in the power sector helps define price ceilings and floors for natural gas, and how the recent increase in the natural
gas production after years of record drilling could alter the supply–demand balance for the better. The article cautions,
however, that escalation in natural gas finding and development costs is countering the more positive developments that emerged
during 2006. Regarding technology, the exploitation of unconventional natural gas was one highlight. So too was the queuing
up of coal-fired power plants for the post-2010 period, a phenomenon that has come under great pressure with many consequences
including increased pressures in the natural gas market. The most significant illustration of these forces was the early 2007
suspension of development plans by a large power company, well before the Supreme Court’s ruling on CO2 as a tailpipe pollutant and President Bush’s call for global goals on CO2 emissions.
相似文献
Jeremy B. PlattEmail: |