全文获取类型
收费全文 | 669篇 |
免费 | 34篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 46篇 |
地球物理 | 171篇 |
地质学 | 229篇 |
海洋学 | 78篇 |
天文学 | 52篇 |
综合类 | 2篇 |
自然地理 | 110篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 5篇 |
2021年 | 14篇 |
2020年 | 18篇 |
2019年 | 19篇 |
2018年 | 28篇 |
2017年 | 34篇 |
2016年 | 31篇 |
2015年 | 26篇 |
2014年 | 38篇 |
2013年 | 35篇 |
2012年 | 39篇 |
2011年 | 68篇 |
2010年 | 36篇 |
2009年 | 37篇 |
2008年 | 37篇 |
2007年 | 37篇 |
2006年 | 25篇 |
2005年 | 26篇 |
2004年 | 32篇 |
2003年 | 28篇 |
2002年 | 15篇 |
2001年 | 6篇 |
2000年 | 9篇 |
1999年 | 8篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 7篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有707条查询结果,搜索用时 0 毫秒
651.
Trends in the Atlantic tropical cyclones and the cyclones that had tracks through North Carolina were analyzed for more than 100 years. From about 1970, there appears to be an increase in the mean number of storms developing. The number of storms affecting North Carolina each decade has been increasing since the 1960s. In the 1980s, 1990s, and into the 2000s, there was an increase in the number of landfalling storms in North Carolina. Although August and September are the most active months of the Atlantic hurricane season, the hurricane season for North Carolina peaks in September. Wind distribution and frictional convergence associated with landfalling hurricanes in North Carolina are discussed. Convection and precipitation patterns of landfalling hurricanes are presented. Two examples of the effect of spatial surface moisture distribution on intensification of tropical cyclones over land after landfall are discussed. 相似文献
652.
Jennifer L. Irish Ashley E. Frey Julie D. Rosati Francisco Olivera Lauren M. Dunkin James M. Kaihatu Celso M. Ferreira Billy L. Edge 《Ocean & Coastal Management》2010,53(10):645-657
Hurricane flooding is a leading natural threat to coastal communities. Recent evidence of sea level rise coupled with potential future global warming indicate that sea level rise will accelerate and hurricanes may intensify over the coming decades. In regions fronted by barrier islands, the protective capacity of these islands may diminish as they are degraded by rising sea level. Here we present a hydrodynamic and geospatial analysis of the relative role of barrier island degradation on potential future hurricane flooding. For the City of Corpus Christi, Texas, USA, hurricane flooding is projected to rise between 20% and 70% by the 2030s, resulting in an increase in property damages and impacted population. These findings indicate that adaptive management strategies should be developed and adopted for mitigating loss of natural barrier islands when these islands act as protective features for populated bayside communities. Finally, this study illustrates a method for applying models to forecast future storm protection benefits of barrier island restoration projects. 相似文献
653.
Jennifer L. A. Hood William D. Taylor Sherry L. Schiff 《Aquatic Sciences - Research Across Boundaries》2014,76(2):243-258
The use of submersed macrophyte tissue δ 15N to quantify the level of WWTP effluent use in a highly urbanized and agricultural river was evaluated using several methods. Macrophytes, NH4 + and NO3 ? were collected by canoe along two 10 km reaches of river, upstream and downstream of two major municipal WWTPs over 3 years. NH4 + decreased in concentration while δ 15N–NH4 + increased as a function of distance downstream of both WWTPs, changing in one survey from 13 to 31 ‰ over 1 km. This increase is attributed to the combined effects of volatilization, nitrification and uptake. While NO 3 ? concentrations increased downstream of the WWTP over one of the survey reaches, δ 15N–NO 3 ? showed no prominent trend with distance at either. Macrophyte tissue δ 15N increased with distance downstream of both WWTPs, with a slope not significantly different from that of δ 15N–NH4 + suggesting that macrophytes incorporate effluent NH4 + as their main N source in those areas. However, mixing models suggest that towards the end of the reach, where source separation is distinct, macrophytes may utilize background NO 3 ? . Our study indicates the difficulty of deriving precise estimates of effluent use by macrophytes in a system where the δ 15N of the effluent changes rapidly. It also illustrates the utility of macrophytes in describing those changes where the effluent is too attenuated to allow for direct isotopic analysis. 相似文献
654.
Lake Nicaragua, the largest lake in Central America, is a promising site for paleolimnological study of past climate change, tectonic and volcanic activity, and pre-Columbian agriculture in the region. It is near the northern limit of the Intertropical Convergence Zone (ITCZ), which brings the rainy season to the tropics, so effects of decreasing precipitation due to southern migration of the ITCZ through the Holocene should be observable. Because fault zones and an active volcano lie within the lake, the long-term impact of tectonic and volcanic activity can also be examined. Finally, the fertile volcanic soils near the lake may have encouraged early agriculture. We analyzed diatoms, biogenic silica (BSi), total organic carbon (TOC), water content, volcanic glass, and magnetic susceptibility in a sediment core from Lake Nicaragua with eleven accelerator mass spectroscopy radiocarbon dates, spanning ~5,700 years. Sediment accumulation rates decreased from the bottom to the top of the core, indicating a general drying trend through the Holocene. An increase in eutrophic diatom abundance suggests that pre-Columbian agriculture impacted the lake as early as ~5,400 cal yr BP. Above a horizon of coarser grains deposited sometime between ~5,200 and 1,600 cal yr BP, planktonic diatoms increased and remained dominant to the top of the core, indicating that water depth permanently increased. Although magnetic susceptibility peaked and water content dipped at the coarse horizon, volcanic glass fragments did not increase, suggesting that the coarse horizon and subsequent increase in water depth were caused by tectonic rather than by volcanic activity. Decreased accumulation rates of BSi and TOC indicate that water became clearer when depth increased. 相似文献
655.
656.
High-resolution bathymetric mapping has shown that submarine flat-topped volcanic cones, morphologically similar to ones
on the deep sea floor and near mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea),
Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constructed of tholeiitic basalt erupted
during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparently formed
of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana
Ridge, and offshore Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield stage flat-topped cones have steep
(∼25°) sides, remarkably flat horizontal tops, basal diameters of 1–3 km, and heights <300 m. The flat tops commonly have
either a low mound or a deep crater in the center. The rejuvenated-stage flat-topped cones have the same shape with steep
sides and flat horizontal tops, but are much larger with basal diameters up to 5.5 km and heights commonly greater than 200 m.
The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most
of the rejuvenated-stage flat-topped cones formed on slopes <10° and formed adjacent semicircular steps down the flank of
Ni'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive
eruptions lasting years to decades. These, and other submarine volcanic cones of similar size and shape, apparently form as
continuously overflowing submarine lava ponds. A lava pond surrounded by a levee forms above a sea-floor vent. As lava continues
to flow into the pond, the lava flow surface rises and overflows the lowest point on the levee, forming elongate pillow lava
flows that simultaneously build the rim outward and upward, but also dam and fill in the low point on the rim. The process
repeats at the new lowest point, forming a circular structure with a flat horizontal top and steep pillowed margins. There
is a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that
facilitate construction of such landforms include effusive eruption of lava with low volatile contents, moderate to high confining
pressure at moderate to great ocean depth, long-lived steady eruption (years to decades), moderate effusion rates (probably
ca. 0.1 km3/year), and low, but not necessarily flat, slopes. With higher effusion rates, sheet flows flood the slope. With lower effusion
rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure eruptions. If the eruption is too brief, it will
not consolidate activity at a point, and fissure-fed flows will form a pond with irregular levees. The pond will solidify
between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water
will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones, as occurs during the post-shield
stage. The steady effusion of lava on land constructs lava shields, which are probably the subaerial analogs to submarine
flat-topped cones but formed under different cooling conditions.
Received: 30 September 1999 / Accepted: 9 March 2000 相似文献
657.
Dmitry L. Lakshtanov Carine B. Vanpeteghem Jennifer M. Jackson Jay D. Bass Guoyin Shen Vitali B. Prakapenka Konstantin Litasov Eiji Ohtani 《Physics and Chemistry of Minerals》2005,32(7):466-470
We have determined the P-V equation of state of Al-rich H-bearing SiO2 stishovite by X-ray powder diffraction at pressures up to 58 GPa using synchrotron radiation. The sample contained 1.8 wt%
Al2O3 and up to 500 ppm H2O, and had a composition that would coexist with Mg-silicate perovskite in a subducted slab. By fitting a third-order Birch-Murnaghan
equation of state to our compression data, we obtained a bulk modulus K
T0=298(7) GPa with K′=4.3(5). With K′ fixed to a value of 4, the bulk modulus K
T0=304(3) GPa. Our results indicate that Al3+ and H+ have a small effect on the elastic properties of stishovite. Compared with data obtained up to 43.8 GPa, peak intensities
changed and we observed a decreased quality of fit to a tetragonal unit cell at pressures of 49 GPa and higher. These changes
may be an indication that the rutile↔CaCl2 transition occurs between these pressures. After laser annealing of the sample at 58.3(10) GPa and subsequent decompression
to room conditions, the cell volume is the same as before compression, giving strong evidence that the composition of the
recovered sample is also unchanged. This suggests that Al and H are retained in the sample under extreme P-T conditions and
that stishovite can be an agent for transporting water to the deepest lower mantle. 相似文献
658.
659.
Fred Jourdan Jennifer P. Matzel Paul R. Renne 《Geochimica et cosmochimica acta》2007,71(11):2791-2808
The 40Ar/39Ar dating technique requires the activation of 39Ar via neutron irradiation. The energy produced by the reaction is transferred to the daughter atom as kinetic energy and triggers its displacement, known as the recoil effect. Significant amounts of 39Ar and 37Ar can be lost from minerals leading to spurious ages and biased age spectra. Through two experiments, we present direct measurement of the recoil-induced 39Ar and 37Ar losses on Fish Canyon sanidine and plagioclase. We use multi-grain populations with discrete sizes ranging from 210 to <5 μm. One population consists of a mixture between sanidine and plagioclase, and the other includes pure sanidine.We show that 39Ar loss (depletion factor) for sanidine is ∼3% for the smallest fraction. Age spectra of fractions smaller than ∼50 μm show slight departure from flat plateau-age spectrum usually observed for large sanidine. This departure is roughly proportional to the size of the grain but does not show typical 39Ar loss age spectra. The calculated thickness of the total depletion layer d0(sanidine) is 0.035 ± 0.012 (2σ). This is equivalent to a mean depth of the partial depletion layer (x0) of 0.070 ± 0.024 μm. The latter value is indistinguishable from previous values of ∼0.07-0.09 μm obtained by argon implantation experiments and simulation results.We show that it is possible to adequately correct ages from 39Ar ejection loss provided that the d0-value and the size range of the minerals are sufficiently constrained. As exemplified by similar calculations performed on results obtained in a similar study of GA1550 biotite [Paine J. H., Nomade S., and Renne P. R. (2006) Quantification of 39Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions. Geochim. Cosmochim. Acta70, 1507-1517.], the d0(biotite) is 0.46 ± 0.06 μm. The significant difference between empirical results on biotite and sanidine, along with different simulation results, suggests that for biotite, crystal structures and lattice defects of the stopping medium and possibly subsequent thermal degassing (due to ∼150-200 °C temperature in the reactor or extraction line bake out) must play an important role in 39Ar loss.The second experiment suggests that 37Ar recoil can substantially affect the age via the interference corrections with results that suggest up to ∼98% of 37Ar can be ejected from the ∼5 μm grain dimension.Further investigation of silicates of various compositions and structures are required to better understand (and correct) the recoil and recoil-induced effects on both 39Ar and 37Ar and their influences on 40Ar/39Ar dating. 相似文献
660.
Evacuations represent an integral aspect of protecting public safety in locations where intense, fast-spreading forest fires
co-occur with human populations. Most Canadian fire management agencies have as their primary objective the protection of
people and property, and all fire management agencies in Canada recommend evacuations when public safety is in question. This
study provides the first national assessment of wildfire-related evacuations in Canada and documents the loss of homes that
coincided with evacuation events. The most striking finding is that despite the intensity and abundance of wildfire in Canada,
wildfires have displaced a relatively small number of people. Between 1980 and 2007, the median number of evacuees and home
losses per year in Canada were 3,590 and 2, respectively. Evacuees’ homes survived in 99.3% of cases. Patterns of evacuations
and home losses reflected the distributions of forests, wildfire, and people across the Canadian landscape. Most evacuations
occurred in boreal areas, which have relatively low population densities but among the highest percent annual area burned
in Canada. Evacuations were less common in southern parts of the country, where most Canadians reside, but individual wildfires
in these areas had significant impacts. Interactions between wildfire and people in Canada exhibited a unique regional pattern,
and within the most densely populated regions of the country they can be considered ‘low-probability, high-consequence’ events.
This Canadian context is fundamentally different from places such as California, where concentrations of fires and people
overlap across large areas and therefore calls for a fundamentally different fire management response. 相似文献