全文获取类型
收费全文 | 153篇 |
免费 | 2篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 8篇 |
地球物理 | 34篇 |
地质学 | 44篇 |
海洋学 | 8篇 |
天文学 | 37篇 |
自然地理 | 18篇 |
出版年
2020年 | 4篇 |
2019年 | 1篇 |
2018年 | 4篇 |
2017年 | 3篇 |
2016年 | 3篇 |
2015年 | 2篇 |
2014年 | 3篇 |
2013年 | 4篇 |
2012年 | 3篇 |
2011年 | 4篇 |
2010年 | 5篇 |
2009年 | 7篇 |
2008年 | 10篇 |
2007年 | 13篇 |
2006年 | 9篇 |
2005年 | 4篇 |
2004年 | 8篇 |
2003年 | 6篇 |
2002年 | 5篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1966年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有155条查询结果,搜索用时 15 毫秒
1.
2.
Philip M. Marren Terence S. McCarthy Stephen Tooth Dion Brandt Glenn G. Stacey Anita Leong Beth Spottiswoode 《Sedimentary Geology》2006,190(1-4):213-226
Along a 28 km reach of the Klip River, eastern Free State, South Africa, mud- and sand-dominated meanders have developed in close proximity within a floodplain wetland up to 1.5 km wide, providing an unusual opportunity to compare their characteristics under similar hydrological conditions. Throughout the reach, the channel bed is grounded on sandstone/shale bedrock although the banks are alluvial, and most river activity occurs during summer high flows. The reach can be divided into three geomorphological zones: Zone 1 (0–11 km), a muddy proximal part with a single meandering channel (w/d < 10) and near-permanent standing water in oxbows and backswamps; Zone 2 (11–17.5 km), a transitional mud-to-sand part with one main channel (w/d 20–30), a number of sinuous palaeochannels and oxbows, and only limited standing water; and Zone 3 (17.5–28 km), a sandy distal part with a single meandering channel (w/d 15–30), scroll bars and oxbows, and little standing water. Each zone also has a distinctive sedimentology: Zone 1 is characterised by an 3–4 m thick succession of basal sand and minor granules overlain by dominantly muddy sediment deposited primarily by oblique accretion in meander bends; Zone 2 is characterised by < 4 m of interbedded sand and mud deposited primarily by lateral point-bar accretion, although a history of avulsions also attests to the importance of abandoned-channel accretion; and Zone 3 is characterised by < 3 m of dominantly sand deposited primarily by lateral point-bar accretion. This unusual downstream sediment coarsening trend, and the associated changes in channel and floodplain character, are independent of sediment inputs from tributaries, and result from a downstream increase in bankfull unit stream power from < 3.5 W m− 2 (Zone 1) to 4–10 W m− 2 (Zone 3). Mud is deposited primarily in low-energy Zone 1 but is conveyed in suspension more effectively through higher energy Zones 2 and 3, only forming drapes over sandy lateral accretion deposits during waning flood stages. The downstream increase in unit stream power is controlled in part by a slight downstream increase in floodplain gradient that may be related to a subtle variation in the erosional resistance of the bedrock underlying the channel bed. These findings add to previous work on meandering rivers by demonstrating that mud-dominated meanders can occur in long-term erosional settings where the channel bed is grounded on bedrock, and that downstream fining trends may be reversed locally. 相似文献
3.
4.
James McCarthy 《Geoforum》2004,35(3):327-341
Recent multilateral trade agreements are among the major manifestations of neoliberalism. They are also emerging as some of the most important sites of environmental governance in the 21st century. I argue here that these trade agreements, particularly the sweeping new protections they provide for investors, are redefining property rights and environmental governance in fundamental ways. I suggest that in addition to furthering the centuries-long process of the enclosure of nature under capitalism, the neoliberal agenda of NAFTA and similar trade agreements also involves something new: the privatization, or primitive accumulation, of conditions of production as an accumulation strategy. I explore these dynamics through examination of two cases, one in the United States and one in Mexico. I also explore the roles of social movements in these dynamics. 相似文献
5.
Anabranching is characteristic of a number of rivers in diverse environmental settings worldwide, but has only infrequently been described from bedrock-influenced rivers. A prime example of a mixed bedrock-alluvial anabranching river is provided by a 150-km long reach of the Orange River above Augrabies Falls, Northern Cape Province, South Africa. Here, the perennial Orange flows through arid terrain consisting mainly of Precambrian granites and gneisses, and the river has preferentially eroded bedrock joints, fractures and foliations to form multiple channels which divide around numerous, large (up to 15 km long and 2 km wide), stable islands formed of alluvium and/or bedrock. Significant local variations in channel-bed gradient occur along the river, which strongly control anabranching style through an influence on local sediment budgets. In relatively long (>10 km), lower gradient reaches (<0.0013) within the anabranching reach, sediment supply exceeds local transport capacity, bedrock usually only crops out in channel beds, and channels divide around alluvial islands which are formed by accretion in the lee of bedrock outcrop or at the junction with ephemeral tributaries. Riparian vegetation probably plays a key role in the survival and growth of these islands by increasing flow roughness, inducing deposition, and stabilising the sediments. Less commonly, channels may form by eroding into once-continuous island or floodplain surfaces. In shorter (<10 km), higher gradient reaches (>0.0013) within the anabranching reach, local transport capacity exceeds sediment supply, bedrock crops out extensively, and channels flow over an irregular bedrock pavement or divide around rocky islands. Channel incision into bedrock probably occurs mainly by abrasion, with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting. Mixed bedrock-alluvial anabranching also occurs in a number of other rivers worldwide, and appears to be a stable and often long-lived river pattern adjusted to a number of factors commonly acting in combination: (1) jointed/fractured granitoid rock outcrop; (2) erosion-resistant banks and islands; (3) locally variable channel-bed gradients; (4) variable flow regimes. 相似文献
6.
7.
8.
Francine M. G. McCarthy John H. McAndrews Steve M. Blasco Sarah H. Tiffin 《Journal of Paleolimnology》2007,37(3):453-470
High-resolution seismic reflection profile data show that the modern sediment cover (over the last 150 years) in Georgian
Bay is thin and spatially discontinuous. Sediments rich in ragweed pollen, largely derived from siltation linked to land clearing
and European settlement, form a thin, discontinuous veneer on the lakebed. Much of the lakebed consists of exposed sediments
deposited during the late glacial or early postglacial. Accumulation rates of modern sediments range from < 0 mm/year (net
erosion) to ∼3.2 mm/year, often within a few hundred metres spatially. These rates are much lower than those reported for
the main basin of Lake Huron and the other Great Lakes, and are attributed to the low sediment supply. Only a few small rivers
flow into Georgian Bay, and most of the basin is surrounded by bedrock of Precambrian gneiss and granite to the east, and
Silurian dolostone, limestone and shale to the west. Thick deposits of Pleistocene drift, found on the Georgian Bay shoreline
only between Meaford and Port Severn, are the main sediment source for the entire basin at present. Holocene to modern sediments
are even absent from some deep basins of Georgian Bay. These findings have implications for the ultimate fate of anthropogenic
contaminants in Georgian Bay. While microfossil assemblages in the ragweed-rich sediments record increased eutrophication
over the last 150 years, most pollutants generated in the Georgian Bay catchment are not accumulating on the lakebed and are
probably exported from the Bay. 相似文献
9.
The polar motion prediction is computed as a least-squares extrapolation of the polar motion data. The least-squares model consists of a Chandler circle with constant or variable amplitude, annual and semiannual ellipses, and a bias. The model with constant amplitude of the Chandler oscillation is fit to the last three years of polar motion data and the model with variable amplitude of the Chandler oscillation is fit to the whole time series ranging from 1973.0 to 2001.1. The variable amplitude of the Chandler oscillation is modeled from the envelope of the Chandler oscillation filtered by the Fourier transform band pass filter from the long-term IERS EOPC01 polar motion series. The accuracy of the polar motion prediction depends mostly on the phase variation of the annual oscillation, which is treated as a constant in the least-squares adjustment. There were two significant changes of the annual oscillation phase of the order of 30° before the two El Niño events in 1982/83 and 1997/98. 相似文献
10.
Dorothy L. Parker Takami Morita Rebecca Verity James K. McCarthy 《Geochimica et cosmochimica acta》2007,71(23):5672-5683
To examine the pathways that form Mn(III) and Mn(IV) in the Mn(II)-oxidizing bacterial strains Pseudomonas putida GB-1 and MnB1, and to test whether the siderophore pyoverdine (PVD) inhibits Mn(IV)O2 formation, cultures were subjected to various protocols at known concentrations of iron and PVD. Depending on growth conditions, P. putida produced one of two oxidized Mn species - either soluble PVD-Mn(III) complex or insoluble Mn(IV)O2 minerals - but not both simultaneously. PVD-Mn(III) was present, and MnO2 precipitation was inhibited, both in iron-limited cultures that had synthesized 26-50 μM PVD and in iron-replete (non-PVD-producing) cultures that were supplemented with 10-550 μM purified PVD. PVD-Mn(III) arose by predominantly ligand-mediated air oxidation of Mn(II) in the presence of PVD, based on the following evidence: (a) yields and rates of this reaction were similar in sterile media and in cultures, and (b) GB-1 mutants deficient in enzymatic Mn oxidation produced PVD-Mn(III) as efficiently as wild type. Only wild type, however, could degrade PVD-Mn(III), a process linked to the production of both MnO2 and an altered PVD with absorbance and fluorescence spectra markedly different from those of either PVD or PVD-Mn(III). Two conditions, the presence of bioavailable iron and the absence of PVD at concentrations exceeding those of Mn, both had to be satisfied for MnO2 to appear. These results suggest that P. putida cultures produce soluble Mn(III) or MnO2 by different and mutually inhibitory pathways: enzymatic catalysis yielding MnO2 under iron sufficiency or PVD-promoted oxidation yielding PVD-Mn(III) under iron limitation. Since PVD-producing Pseudomonas species are environmentally prevalent Mn oxidizers, these data predict influences of iron (via PVD-Mn(III) versus MnO2) on the global oxidation/reduction cycling of various pollutants, recalcitrant organic matter, and elements such as C, S, N, Cr, U, and Mn. 相似文献