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171.
本文用多维谱分析的方法研究了1985年1月-1987年12月南极长城站地面气象要素的谱特征。该地区的气象要素与北半球一样,也普遍存在3-5天、准一周、准二周和3-4周振荡周期。在长城站所在的乔治王岛地区,从地面到平流层中下部,气压系统都存在着显著的准二周振荡,这种振荡是由上往下传播的。无论冬、夏季,准二周和准一周振荡都是由西往东传播的,而扰动的南北传播方向则与季节及振荡周期长度有关。南极地区的中期振荡特征与500hPa极地涡旋的活动和强度变化有密切关系。 相似文献
172.
A climatic-limnologic model of diatom succession for paleolimnological interpretation of varved sediments at Elk Lake,Minnesota 总被引:1,自引:0,他引:1
J. Platt Bradbury 《Journal of Paleolimnology》1988,1(2):115-131
Sediment traps placed in the profundal region of Elk Lake, north central Minnesota during the 1979 spring and 1983–84 fall and spring seasons monitored seasonal diatom production for two climatically distinctive periods. The spring of 1979 was one of the coldest and wettest on record. Ice out at Elk Lake was 10 days later than average, and spring circulation was short. Fragilaria crotonensis dominated the late spring and early summer diatom production in association with Synedra and Cyclotella species, perhaps because rates of phosphorus supply were low compared to silica. The winter of 1983–84 was drier than usual, and the early but cold spring of 1984 caused ice out at Elk Lake to be about 1 week earlier than normal. Spring storms promoted a long and full circulation that allowed Stephanodiscus minulutus to bloom, presumably in response to increased phosphorus loading related to deep and vigorous circulation. The two dominant diatoms in Elk Lake, F. crotonensis and S. minutulus may reflect climatic patterns that control lake circulation. The climatically regulated occurrence of these diatoms is generally, but not specifically, comparable to their distribution in lake surface sediments throughout Minnesota. A combination of regional and lake-specific studies on the controls of diatom succession and distribution provides climatic insights for interpreting paleolimnogical records of diatoms. 相似文献
173.
《The Professional geographer》1988,40(1):116-131
Books reviewed in this article: Boundary Control and Legal Priniciples . Curtis M. Brown , Walter G. Robillard and Donald A. Wilson . Lost Initiatives: Canada's Forest Industries, Forest Policy, and Forest Conservation . R. Peter Gillis and Thomas R. Roach . The Navajo Atlas: Resources, People, and History of the Dine Bikeyah . James M. Goodman . The City and the Sign . M. Gottdiener AND Alexandros Ph . Lagopoulos , EDS. The Comfortable House: North American Suburban Architecture 1890–1930 . Alan Gowans . Arabic-Islamic Cities. Building and Planning Principles . Besim Salim Hakim . Contemporary Climatology . Ann Henderson -Sellers and Peter J. Robinson . A Trace of Desert Waters . Samuel G. Houghton . Geography, Resources and Environment. Vol. 1: Selected Writings of Gilbert F. White. Vol. 2: Themes from the Work of Gilbert F. White . Robert W. Kates AND Ian Burton , ED. Spatial Dimensions of Unemployment and Underemployment (A Case Study of Rural Punjab) . Gopal Krishan . Maps of the Holy Land: Cartobibliography of Printed Maps, 1475–1900 . Compiled by Eran Laor , assisted by Shoshana Klein . The Geography of Third World Cities . Stella Lowder . A Perspective on U.S. Farm Problems and Agricultural Policy . Lance Mc Kinzie , Timothy G. Baker AND Wallace E. Tyner . Soil Erosion and Its Control. R.P.C . Morgan , ED Geopolitics . Patrick O'Sullivan . Landscape, Meanings and Values . Edmund C. Penning -Rowsell AND David Lowenthal , EDS. 相似文献
174.
The proximity of the Great Barrier Reef (GBR) Marine Park to areas of intensive agriculture and increasing urbanisation places the park under potential threat of contamination by land-based pollutants. Passive samplers were deployed at inshore reef and river mouth sites in the Wet Tropics region of the GBR during a dry and a wet season to measure levels of land-based organic pollutants in this environment. Two types of passive sampling devices were deployed: (i) a polar sampler, which can be used to monitor polar herbicides and (ii) semipermeable membrane devices (SPMDs) which sequester more hydrophobic compounds (e.g. PAHs, chlorpyrifos). Herbicides (diuron, simazine, atrazine, hexazinone and/or flumeturon) were detected at low concentrations (ng L−1) at all sites sampled and in both seasons. Chlorpyrifos was not detected while PAHs were present in SPMDs at levels below limits of detection. The results show that the GBR environment does contain low levels of organic pollutants and that passive sampling provides a sensitive monitoring tool for measuring waterborne organic pollutants. 相似文献
175.
Joseph A. Shacat William J. Green Eric H. Decarlo Silvia Newell 《Aquatic Geochemistry》2004,10(3-4):325-352
Lake Joyce is one of the least studied lakes of the McMurdo Dry Valleys. Similar to other lakes in this region, Lake Joyce is a closed-basin, permanently ice-covered, meromictic lake. We present here a detailed investigation of major ions, nutrients, and dissolved trace elements for Lake Joyce. Specifically, we investigate the role of iron and manganese oxides and hydrous oxides in trace metal cycling.Lake Joyce is characterized by fresh, oxic waters overlying an anoxic brine, primarily Na–Cl. Surface waters have a maximum nitrate concentration of 26M with a molar dissolved inorganic nitrogen to phosphorus ratio of 477. The supply of nitrogen is attributed to atmospheric deposition, possibly from polar stratospheric clouds. Dissolved phosphorus is scavenged by hydrous iron oxides. The pH is highest (10.15) just beneath the 7-m thick ice cover and decreases to a minimum of 7.29 in the redox transition zone. Dissolved Al exceeds 8M in surface waters, and appears to be controlled by equilibrium with gibbsite. In contrast, concentrations of other trace elements in surface waters are quite low (e.g., 5.4nM Cu, 0.19nM Co, <20pM La). Dissolved Fe, Mn, Ni and Cd were below our detection limits of 13 nM, 1. 8 nM, 4.7 nM and 15pM (respectively) in surface waters. There was a 6-m vertical separation in the onset of Mn and Fe reduction, with dissolved Mn appearing higher in the water column than Fe. Based on thermodynamic calculations, dissolved Mn appears to be controlled by equilibrium with hausmannite (Mn3 O4 ). Co tracks the Mn profile closely, suggesting Co(III) is bound in the lattice of Mn oxides, whereas the Ce profile is similar, yet the Ce anomaly suggests oxidative scavenging of Ce. Release of Cu, Ni, Cd and trivalent REE appears to be controlled by pH-induced desorption from Fe and Mn oxides, although Cu (and perhaps Ni) may be scavenged by organic matter in surface waters. 相似文献
176.
Heightened sensitivity of a poorly buffered high arctic lake to late-Holocene climatic change 总被引:4,自引:0,他引:4
Neal Michelutti Marianne S.V. Douglas Alexander P. Wolfe John P. Smol 《Quaternary Research》2006,65(3):421-430
A diatom-based paleolimnological investigation was conducted on late Holocene sediments from a poorly buffered lake, informally named “Rock Basin Lake”, on Ellesmere Island, Arctic Canada. The fossil diatom record is unlike any other obtained thus far from high arctic regions, exhibiting dynamic assemblage shifts over the entire 3300 yr sedimentary record. Multiple proxies (i.e., diatoms, pH reconstructions, biogenic silica, C/N ratios, total organic carbon) appear to sensitively track rapid limnological changes, which are associated with distinct climate intervals as inferred from other regional proxy records. The highly responsive nature of the diatom assemblages in Rock Basin Lake, relative to those recorded from nearby alkaline sites, appears to be related to this lake's limited ability to buffer changes in pH. The dynamic species responses suggest that the diatoms in Rock Basin Lake are faithfully tracking climatic changes, and that low-alkalinity lakes may provide the most sensitive diatom-based paleolimnological records from high arctic regions. 相似文献
177.
178.
长城是中华民族的象征,是世界上规模最大的文化遗产。但近年来长城却面临着严重的人为破坏和自然破坏的威胁,特别是人为破坏日益加剧。为进一步加强长城保护管理工作,实施宁夏境内明长城资源调查项目,开展明长城基础地理信息数据生产、明长城专题要素数据采集、测量明长城长度、制作明长城专题影像图等工作。 相似文献
179.
Sediment provenance in a tropical fluvial and marine context by magnetic 'fingerprinting' of transportable sand fractions 总被引:1,自引:0,他引:1
B. A. MAHER S. J. WATKINS G. BRUNSKILL† J. ALEXANDER‡ C. R. FIELDING§ 《Sedimentology》2009,56(3):841-861
The sources and fluxes of sediment to the Great Barrier Reef lagoon from north‐eastern Australian rivers have been the subject of much concern and study, with the large catchments of the Burdekin and Fitzroy Rivers thought to be the key sources at present. Here, the utility of newly developed magnetic ‘fingerprinting’ methods for identifying sediment provenance, both onshore and offshore, and in association with individual large flood events, is investigated. Within the Burdekin catchment, sediments are mobilized from different subcatchments by runoff generated by intense, localized rainfall events. Magnetic measurements were made on untreated and acid‐treated samples of river channel sediments within the Burdekin River subcatchments and from the estuarine and inner shelf depocentres of Burdekin River sediments. The acid treatment removes all discrete magnetic particles and coatings, and leaves magnetic inclusions (protected within host silicate grains) as the basis of the measured magnetic signature of a sample. The magnetic properties of the acid‐treated samples display statistically distinct sediment provenance groupings. Sand samples from the Upper Burdekin River appear magnetically distinct from samples from tributaries of the Burdekin (e.g. Hann Creek, Fanning River) and also from nearby coastal rivers, including the Haughton. Suspended sand samples from a Burdekin flood event in 2000 appear to have a different source compared with those from floods in 1998 and 1999. Comparisons of the terrestrial, acid‐treated sand fractions with the same, acid‐treated, sand‐size fractions from transects taken offshore suggest that the surface sediments in Upstart Bay and Bowling Green Bay have different sources. Some of these sources are as yet unidentified but may represent the unsampled, lower‐discharge south‐western Burdekin subcatchments, and/or along‐shore drift of sand from the south, perhaps even from the Fitzroy River, over millennial timescales of cyclone pumping. The magnetic inclusion method precludes any obfuscation or confounding of sediment source, which might arise from hydraulic sorting and/or post‐depositional magnetic diagenesis or authigenesis. 相似文献
180.
James A. Smith Claus-Dieter Hillenbrand Alastair G.C. Graham 《Quaternary Research》2009,71(2):190-200
Subglacial meltwater plays a significant yet poorly understood role in the dynamics of the Antarctic ice sheets. Here we present new swath bathymetry from the western Amundsen Sea Embayment, West Antarctica, showing meltwater channels eroded into acoustic basement. Their morphological characteristics and size are consistent with incision by subglacial meltwater. To understand how and when these channels formed we have investigated the infill of three channels. Diamictons deposited beneath or proximal to an expanded grounded West Antarctic Ice Sheet are present in two of the channels and these are overlain by glaciomarine sediments deposited after deglaciation. The sediment core from the third channel recovered a turbidite sequence also deposited after the last deglaciation. The presence of deformation till at one core site and the absence of typical meltwater deposits (e.g., sorted sands and gravels) in all three cores suggest that channel incision pre-dates overriding by fast flowing grounded ice during the last glacial period. Given the overall scale of the channels and their incision into bedrock, it is likely that the channels formed over multiple glaciations, possibly since the Miocene, and have been reoccupied on several occasions. This also implies that the channels have survived numerous advances and retreats of grounded ice. 相似文献