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221.
DAVID RHIND 《International journal of geographical information science》2013,27(3):229-241
Abstract This paper summarizes the numerous developments in geographical information systems (GIS) in the United Kingdom over the past few years, covering the development of systems, the creation of data bases, the elucidation of user needs, the education and training of staff and the role adopted by government. The review is organized by the different sectors now actively involved in GIS. 相似文献
222.
223.
ESPERANZA MUÑOZ‐SALINAS MIGUEL CASTILLO‐RODRÍGUEZ VLAD MANEA MARINA MANEA DAVID PALACIOS 《Geografiska Annaler: Series A, Physical Geography》2010,92(3):311-328
Lahars are hazardous events that can cause serious damage to people who live close to volcanic areas; several were registered at different times in the last century, such as at Mt St Helens (USA) in 1980, Nevado del Ruiz (Colombia) in 1985 and Mt Pinatubo (Philippines) in 1990. Risk maps are currently used by decision‐makers to help them plan to mitigate the hazard‐risk of lahars. Risk maps are acquired based on a series of tenets that take into account the distribution and chronology of past lahar deposits, and basically two approaches have been used: (1) The use of Flow Simulation Software (FSS), which simulates flows along channels in a Digital Elevation Model and (2) The Geochronological Method (GM), in which the mapping is based on the evaluation of lahar magnitude and frequency. This study addresses the production of a lahar risk map using the two approaches (FSS and GM) for a study area located at Popocatépetl volcano – Central Mexico. Santiago Xalitzintla, a town located on the northern flank of Popocatépetl volcano, where volcanic activity in recent centuries has triggered numerous lahars that have endangered local inhabitants, has been used for the case study. Results from FSS did not provide satisfactory findings because they were not consistent with lahar sediment observations made during fieldwork. By contrast, the GM produced results consistent with these observations, and therefore we use them to assess the hazard and produce the risk map for the study area. 相似文献
224.
BY GREGORY C. WILES DAVID J. BARCLAY NICOLÁS E. YOUNG 《Geografiska Annaler: Series A, Physical Geography》2010,92(1):101-109
In Alaska, lichenometry continues to be an important technique for dating late Holocene moraines. Research completed during the 1970s through the early 1990s developed lichen dating curves for five regions in the Arctic and subarctic mountain ranges beyond altitudinal and latitudinal treelines. Although these dating curves are still in use across Alaska, little progress has been made in the past decade in updating or extending them or in developing new curves. Comparison of results from recent moraine-dating studies based on these five lichen dating curves with tree-ring based glacier histories from southern Alaska shows generally good agreement, albeit with greater scatter in the lichen-based ages. Cosmogenic surface-exposure dating of Holocene moraines has the potential to test some of the assumptions of the lichenometric technique and to facilitate the development of a new set of improved lichen dating curves for Alaska. 相似文献
225.
Forest vegetation in the southeastern United States extends westward beyond the Ozark and Ouachita plateaus in Arkansas and Missouri into the Central Plains. Along this transect, luxuriant forests give way to mixed forests and grasslands that include smaller trees and progressively fewer tree species and eventually to grassland‐dominated landscapes in central Kansas, Oklahoma, and Texas. This transition is directly related to decreasing precipitation with distance to the west of the Mississippi River valley. Many species, however, have abrupt western range boundaries related to physiography and hydrogeomorphic processes. The western range limits for many trees correspond to Coastal Plain boundaries that at a regional scale impose sharper range boundaries than would be expected on the basis of decreasing precipitation. Also, riparian habitats within stream valleys extending westward from the Coastal Plain provide suitable habitats for trees in the dry regions of the Great Plains. The presence of riparian trees in this region is determined largely by the presence or absence of groundwater conditions necessary for survival. For floodplain trees, then, it is primarily habitat—not climate—that determines the location of range boundaries. 相似文献
226.
WILLIAM R. NORMARK† CHARLES K. PAULL‡ DAVID W. CARESS‡ WILLIAM USSLER III‡ RAY SLITER 《Sedimentology》2009,56(6):1690-1704
Erosional and depositional bedforms have been imaged at outcrop scale in the upper Redondo Fan, in the San Pedro Basin of offshore Southern California in ≥600 m water depths, using an Autonomous Underwater Vehicle developed by the Monterey Bay Aquarium Research Institute. The Autonomous Underwater Vehicle is equipped with multibeam and chirp sub-bottom sonars. Sampling and photographic images using the Monterey Bay Aquarium Research Institute Remotely Operated Vehicle Tiburon provide groundtruth for the Autonomous Underwater Vehicle survey. The 0·3 m vertical and 1·5 m lateral bathymetric resolution and 0·1 m sub-bottom profile resolution provide unprecedented detail of bedform morphology and structure. Multiple channels within the Redondo Fan have been active at different times during the Late Holocene (0 to 3000 yr bp ). The currently active channel extending from Redondo Canyon makes an abrupt 90° turn at the canyon mouth before resuming a south-easterly course along the east side of the Redondo Fan. This channel is floored by sand and characterized by small steps generally <1 m in relief, spaced 10 to 80 m in the down-channel direction. A broader channel complex lies along the western side of the fan valley that was last active more than 850 years ago. Two distinct trains of large scours, with widths ranging from tens to a few hundred metres and depths of 20 m, occur on the floor of the western channel complex, which has a thin mud drape. If observed in cross-section only, these large scours would probably be misidentified as the thalweg of an active channel. 相似文献
227.
WAYNE OSBORN DAVID MATTY MICHAEL VELBEL PETER BROWN JOHN WACKER 《Meteoritics & planetary science》1997,32(6):781-790
Abstract— A 0.5 kg stony meteorite associated with a bright bolide seen over southeastern Michigan on 1994 October 20 has been recovered. The circumstances of the fall and recovery of this chondrite, named Coleman, are presented. The most likely trajectory from the observations of the event implies preatmospheric orbital parameters typical of meteorites. Gamma-ray spectrometry of the cosmogenic radionuclides showed that the recovered mass was an interior fragment of a larger body and revealed abnormally high 22Na and 26Al activity. Electron microprobe analysis yielded compositions of Fa24.1 and Fs20.3, which are consistent with an L-chondrite classification. Petrographically, the presence of chondrules, the observed mineralogy and the degree of chondrule-matrix integration suggests assignment to petrologic type 6. 相似文献
228.
JEAN SOMMÉ RÉ-V. MUNAUT ANNE-FRANCOISE EMONTSPOHL NICOLE LIMONDIN DAVID LEFÈVRE NICOLE CUNAT-BOGÈ JACQUES MOUTHON ÉTIENNE GILOT 《Boreas: An International Journal of Quaternary Research》1994,23(3):231-243
A deep boring (30 m), carried out at Watten (Nord, France) in 1990, has provided an opportunity to undertake palaeoecological studies based on stratigraphy, sedimentology, palynology, malacology and 14 C datings on Early Weichselian and Holocene deposits. The location of the site in the inner zone of the coastal plain, extending in the Aa valley from the southernmost shores of the North Sea upstream to Watten, allows the comparison between interstadials recorded in a pure fluvial environment and the overlying Holocene sediments, in which alternation of marshy, fluvial and marine deposits are observed. The Early Weichselian sequence can be correlated with the Brørup and Odderade interstadials described in northwestern Europe, and belongs within the same palaeogeographical area. At that time the conditions were already continental at Watten. However, the last (Holocene) interglacial has been characterized, at least since the Boreal chronozone, by an oceanic climate and a progressive invasion of the sea, sometimes interrupted by stillstands and withdrawals. 相似文献
229.
Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania 总被引:5,自引:0,他引:5
MASON PAUL R. D.; DOWNES HILARY; THIRLWALL MATTHEW F.; SEGHEDI IOAN; SZAKCS ALEXANDRU; LOWRY DAVID; MATTEY DAVID 《Journal of Petrology》1996,37(4):927-959
Miocene to Pleistocene calc-alkaline volcanism in the East Carpathianarc of Romania was related to the subduction of a small oceanbasin beneath the continental TiszaDacia microlate. Volcanicproducts are predominantly andesitic to dadtic in composition,with rare basalts and rhyodacites (51l71% SiO2; mg-number0.650.26) and have medium- to high-K calcalkaline andshoshonitic affinities. Mg, Cr and Ni are low in all rock-types,indicating the absence of primary erupted compositions. Detailedtrace element and Sr, Nd, Pb and 0 isotope data suggest thatmagmas were strongly crustally contaminated. Assimilation andfractional crystallization (AFC) calculations predict the consumptionof 535% local upper-crustal metasediments or sedimentsfrom the palaeo-accretionary wedge. Variations in the isotopiccomposition of the contaminants and parental magmas caused variationsin the mixing trajectories in different parts of the arc Themost primitive isotopic compositions are found in low-K dacitesof the northern Cdlimani volcanic centre and are interpretedas largely mantle derived. A second possible mantle reservoirof lower 149 Nd/144 Nd and lower 206 Pb/204 Pb is identifiedfrom back-arc basic calc-alkaline rocks in the south of thearc Both magmatic reservoirs have elevated isotopic characteristics,owing either to source bulk mixing (between depleted or enrichedasthenosphere and <1% average subducted local sediment) orlower-crustal contamination. KEY WORDS: Carpathians; assimilation; calc-alkaline; Sr-Nd-Pb-0 isotopes; laser flurination 相似文献
230.
Cross-platform architecture of a sequence boundary in mixed siliciclastic-carbonate lithofacies, Middle Cambrian, southern Great Basin, USA 总被引:1,自引:0,他引:1
Stratigraphic analysis of mixed siliciclastic-carbonate lithofacies within the Middle Cambrian Bonanza King Formation of the southern Great Basin reveals three distinct facies associations that record a range of depositional environments from semi-arid tidal flats to deeper subtidal, restricted lagoons. Stratigraphic trends, cross-platform facies variations and correlation of individual surfaces across 250 km of the study area suggest that these mixed lithofacies were deposited in three temporally distinct phases. (1) Extensive progradation of mixed peritidal environments culminated in a prolonged episode of subaerial exposure marked by an areally extensive intraclast breccia (0·5–1·2 m thick) that we interpret to be a major Type 1 sequence-bounding disconformity. (2) Abrupt flooding of the exposed platform resulted in the deposition of mixed deeper subtidal lithofacies, including a condensed interval of fissile, fossiliferous shale. (3) Progressive shallowing and aggradational accumulation was accompanied by a decrease in siliciclastics and a shift to pure carbonate deposition. Deep-water siliciclastics and megabreccias record deposition along the base-of-slope off the Middle Cambrian shelf-edge, and are interpreted to represent lowstand deposits emplaced during the prolonged episode of subaerial exposure of the shallow shelf. The presence of fine siliciclastics in both peritidal facies and sharply overlying deeper subtidal facies of the study interval within the Bonanza King suggests a variable, but relatively continuous, influx of terrigenous material throughout an extended period of accommodation change, apparently asynchronous with respect to the predictive model of reciprocal sedimentation. We suggest that the primary siliciclastic source changed with relative sea-level position. During lowered sea level, aeolian processes acting upon the unvegetated Cambrian craton transported fine siliciclastics onto peritidal and shallow-subtidal environments. During higher sea level, coastal siliciclastic reservoirs supplied sediment that was transported for long distances by geostrophic currents flowing along the submerged platform. As opposed to many Cambro-Ordovician grand cycles that are commonly interpreted to consist of a transgressive shaly half-cycle grading upward into a regressive carbonate half-cycle, the sequence boundary within this Middle Cambrian succession occurs within siliciclastic-rich, mixed lithofacies rather than in adjoining purer carbonates, implying that some ‘grand cycles’ should not be considered synonymous with ‘sequences’. Interbasinal correlations of the Type 1 sequence boundary within the mixed unit are speculative, primarily because of the inherent imprecision of available trilobite biostratigraphy. However, there is evidence that an extended episode of subaerial exposure may have been continent-wide during the Ehmaniella trilobite biochron. 相似文献