Age and extent of the Scandinavian ice sheet in northwest Russia     

EILIV LARSEN  ASTRID LYSÅ  IGOR DEMIDOV  SVEND FUNDER  MICHAEL HOUMARK-NIELSEN  KURT H. KJÆR  REW S. MURRAY 《Boreas: An International Journal of Quaternary Research》1999,28(1):115-132

The last glacial maximum (LGM) of the Scandinavian ice sheet in the Arkhangelsk region has been identified morphologically as ridges and hummocks in an otherwise flat topography. Stratigraphically the limit is marked by the presence of till above Mikhulinian (last interglacial) sediments inside the ridges and by the absence of till outside the ridges. During the LGM, ice flowed into the region from the north and northwest forming a lobe in the Dvina-Vaga depression. The continuation northward, northeast of Arkhangelsk, is still somewhat uncertain, but evidence suggests that the outer margin of the Scandinavian ice sheet was situated in the Mezen drainage basin. Luminescence and radiocarbon dates suggest that the maximum position was attained after some 17 ka ago, and that deglaciation started close to 15 ka ago. This age for the maximum position is younger than the maximum position in the western peripheral areas of the Scandinavian ice sheet. This may be accounted for by initial ice build-up in the west followed by a successive migration of the ice divide(s) to the east as ice growth continued. Deglaciation was either by lateral retreat or isolation of dead ice masses causing areal downwasting.  相似文献   

AVIFAUNAL RESEARCH AND GEOGRAPHICAL PERSPECTIVES     

MICHAEL K. STEINBERG 《Geographical review》2010,100(2):iii-iv

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The sensitivity of supercritical atmospheric boundary-layer flow along a coastal mountain barrier     

MICHAEL TJERNSTRÖm 《地球，A辑:动力气象学与海洋学》1999,51(5):880-901

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Sedimentary facies and geomorphic evolution of a blocked‐valley lake: Lake Futululu,northern Kwazulu‐Natal,South Africa     

SUZANNE E. GRENFELL  WILLIAM N. ELLERY  MICHAEL C. GRENFELL  LISA F. RAMSAY  TYREL J. FLÜGEL 《Sedimentology》2010,57(5):1159-1174

Blocked‐valley lakes are formed when tributaries are impounded by the relatively rapid aggradation of a large river and its floodplain. These features are common in the landscape, and have been identified in the floodplains of the Solimões‐Amazon (Brazil) and Fly‐Strickland Rivers (Papua New Guinea), for example, but their inaccessibility has resulted in studies being limited to remotely sensed image analysis. This paper documents the sedimentology and geomorphic evolution of a blocked‐valley lake, Lake Futululu on the Mfolozi River floodplain margin, in South Africa, while also offering a context for the formation of lakes and wetlands at tributary junctions. The study combines aerial photography, elevation data from orthophotographs and field survey, and longitudinal sedimentology determined from a series of cores, which were sub‐sampled for organic content and particle size analysis. Radiocarbon dating was used to gauge the rate and timing of peat accumulation. Results indicate that following the last glacial maximum, rising sea‐levels caused aggradation of the Mfolozi River floodplain. By 3980 years bp , aggradation on the floodplain had impounded the Futululu drainage line, creating conditions suitable for peat formation, which has since occurred at a constant average rate of 0·13 cm year^?1. Continued aggradation on the Mfolozi River floodplain has raised the base level of the Futululu drainage line, resulting in a series of back‐stepping sedimentary facies with fluvially derived sand and silt episodically prograding over lacustrine peat deposits. Blocked‐valley lakes form where the trunk river has a much larger sediment load and catchment than the tributary stream. Similarly, when the relative difference in sediment loads is less, palustrine wetlands, rather than lakes, may be the result. In contrast, where tributaries drain a steep, well‐connected catchment, they may impound much larger trunk rivers, creating lakes or wetlands upstream.  相似文献   

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging     

By RALPH KAHN  REAS PETZOLD  MANFRED WENDISCH  EIKE BIERWIRTH  TILMAN DINTER  MICHAEL ESSELBORN  MARCUS FIEBIG  BIRGIT HEESE  PETER KNIPPERTZ  DETLEF MÜLLER  ALEXANDER SCHLADITZ  WOLFGANG VON HOYNINGEN-HUENE 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):239-251

Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006 field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer (MISR) observations, and to place the suborbital aerosol measurements into the satellite's larger regional context. On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical thickness (AOT) agrees with field measurements point-by-point to within 0.05–0.1. This is about as well as can be expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an extended region. The field data also validate MISR's ability to distinguish and to map aerosol air masses, from the combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand kilometres away.  相似文献   

Regional Saharan dust modelling during the SAMUM 2006 campaign     

By BERND HEINOLD  INA TEGEN  MICHAEL ESSELBORN  KONRAD KANDLER  PETER KNIPPERTZ  DETLEF MÜLLER  ALEXANDER SCHLADITZ  MATTHIAS TESCHE  BERNADETT WEINZIERL  ALBERT ANSMANN  DIETRICH ALTHAUSEN  BENOIT LAURENT  REAS MASSLING  THOMAS MÜLLER  REAS PETZOLD  KERSTIN SCHEPANSKI  ALFRED WIEDENSOHLER 《Tellus. Series B, Chemical and physical meteorology》2009,61(1):307-324

The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the unique comprehensive data set of near-source dust properties during the 2006 SAMUM field campaign, the performance of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size distributions and the position of the maximum dust extinction in the vertical profiles agree well with the observations. However, the spatio-temporal evolution of the dust plumes is not always reproduced due to inaccuracies in the dust source placement by the model. While simulated winds and dust distributions are well matched for dust events caused by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or influenced by small-scale topography that is not resolved by the model. In contrast to long-range dust transport, in the vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and likely explains the absence of the observed sharply defined dust layers.  相似文献   

Microstructure and thermal history of metal particles in CH chondrites     

J. I. GOLDSTEIN  R. H. JONES  P. G. KOTULA  J. R. MICHAEL 《Meteoritics & planetary science》2007,42(6):913-933

Abstract We have studied metal microstructures in four CH chondrites, Patuxent Range (PAT) 91546, Allan Hills (ALH) 85085, Acfer 214, and Northwest Africa (NWA) 739, to examine details of the thermal histories of individual particles. Four types of metal particles are common in all of these chondrites. Zoned and unzoned particles probably formed as condensates from a gas of chondritic composition in a monotonic cooling regime, as has been shown previously. We have demonstrated that these particles were cooled rapidly to temperatures below 500 K after they formed, and that condensation effectively closed around 700 K. Zoned and unzoned particles with exsolution precipitates, predominantly high‐Ni taenite, have considerably more complex thermal histories. Precipitates grew in reheating episodes, but the details of the heating events vary among individual grains. Reheating temperatures are typically in the range 800–1000 K. Reheating could have been the result of impact events on the CH parent body. Some particles with precipitates may have been incorporated into chondrules, with further brief heating episodes taking place during chondrule formation. In addition to the four dominant types of metal particles, rare Ni‐rich metal particles and Si‐rich metal particles indicate that the metal assemblage in CH chondrites was a mixture of material that formed at different redox conditions. Metal in CH chondrites consists of a mechanical mixture of particles that underwent a variety of thermal histories prior to being assembled into the existing brecciated meteorites.  相似文献   

Stratigraphy of a Neoglacial end moraine in Norway     

MICHAEL J. ALEXANDER  PETER WORSLEY 《Boreas: An International Journal of Quaternary Research》1973,2(3):117-142

There exposures in an outer end moraine ridge at Autre Okstindbredal, north Norway, are described and interpreted. The presence of perennially frozen ground is attributed to the present or very recent climate of the area and suggest the occurence of true sporadic permafrost. Several buried soil horizons including podzols are recognised within the moraine ridge sections. These are considered to be derived. It is thought that their occurance as imbricate stress is best explained by a basal freeze-in mechanism associated with marginal thrusting when the local glacier possessed a sub-polar thermal régime. This freeze-in phase is of late Neoglacial age but not necessarily associated with the widespread eighteenth century advance in Scandinavia. The soils in part date from the postglacial climatic optimum.  相似文献   

The planform of epeirogeny: vertical motions of Australia during the Cretaceous   总被引：1，自引：0，他引：1  

MARK Russell  MICHAEL Gurnis 《Basin Research》1994,6(2-3):63-76

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The Recreational Value of In-Stream Flows     

CHRIS COCKLIN  IAIN FRASER  MICHAEL HARTE 《New Zealand geographer》1994,50(1):20-29

The decision in 1988 by the Rangitikei-Wanganui Catchment Board to allocate flows away from use for electricity generation into recreation use brought into focus the conflict between market and non-market valuation of river uses. The travel cost method (TCM) of non-market valuation is used to estimate the in-stream recreation values of the Upper Wanganui and Whakapapa rivers.  相似文献