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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
91

The origin, composition and behaviour of basal ice at Nigardsbreen, Norway

Gemmell, J. Campbell January 1985 (has links)
The basal ice of temperate-type glacier Nigardsbreen is described in terms of a sediment facies model-after Lawson (1979). The ice is characterised using data obtained from sedimentological, chemical and stable isotopic analyses. These indicate that ice nearest the glacier bed is distinguishable from the ice above descriptively and genetically. In particular, the measurement of paired Oxygen and Hydrogen isotope contents, in accordance with theory proposed by Jouzel and Souchez (1982), provides a means of separating the basal facies ice component, formed by subglacial/basal regelation processes, from ice formed by 'normal' non-fractionating firnification processes. In the latter case the basic isotope content is close to that of the original precipitation whilst in the former this precipitation (meteoric) relationship is modified when ice is subjected to more or less predictable levels of fractionation during regelation. Some 1-1.5m of the basal ice, both early and late in the melt season, was found to originate from regelation. Within the basal facies ice, both stratified debris-laden layers comprising fine gravel-dominated debris up to concentrations of 1kg/l (c30% by volume) and clear intercalated ice types, reflect the regelation processes involved in debris entrainment and the freezing on of water. Thick (up to 40cm) layers of clean macrocrystalline 'regelation1 ice are considered to have been recrystallised. Coisotopic analysis may be used to highlight the fact that mixing of waters of differing origins occurs at the ice-bed interface and that the prediction of the extent of refreezing proposed by Jouzel and Souchez appears untenable. Major cation chemical analysis, undertaken to obtain data for use in the Souchez and Tison (1981) model of basal ice formational processes, which purports to discriminate between ice formed from water squeezed through the basal ice and water flowing, or ponded, at the bed, proved inconclusive. It appears that considerable desorption from clay minerals occurs and that squeezing of basal waters through the stratified facies may occur. In the absence of a clear statement of mineralogical composition in the basal debris clay fraction, which was attempted at Nigardsbreen, the Souchez and Tison model is seen to require development before it will reveal new information about basal processes. Measurements of temperature, sliding velocity and strain within subglacial cavities, in conjunction with observations on changes in sedimentological and isotopic conditions throughout the basal facies zone, are used to test and vindicate a descriptive model of the character, origin and evolution of the basal ice. The model is in turn proposed for further test.
92

Evaluation of Greenland Near Surface Air Temperature Datasets

Reeves Eyre, James Edward Jack, Reeves Eyre, James Edward Jack January 2016 (has links)
Near-surface air temperature (SAT) over Greenland has important effects on mass balance of the ice sheet, but it is unclear which SAT datasets are reliable in the region. Here extensive in-situ SAT measurements are used to assess monthly mean SAT from seven global reanalysis datasets, four gridded SAT analyses, one satellite retrieval and two dynamically downscaled reanalyses. Strengths and weaknesses of these products are identified, and their biases are found to vary by season and glaciological regime. MERRA2 reanalysis overall performs best with mean absolute error less than 2 °C in all months. Ice sheet-average annual mean SAT from different datasets are highly correlated in recent decades, but their 1901–2000 trends differ in sign. Compared with the MERRA2 climatology combined with gridded SAT analysis anomalies, thirty-one earth system model historical runs from the CMIP5 archive reach ~5 °C for the 1901–2000 average bias and have opposite trends for a number of sub-periods.
93

A reconstruction of the eastern margin of the late Weichselian Ice Sheet in northern Britain

Stewart, Fiona S. January 1991 (has links)
No description available.
94

Electromagnetic reflections inside ice sheets

Miners, William Dingle January 1999 (has links)
When radio echo sounding polar ice sheets weak stratified reflections are visible deep inside the ice sheets. These reflections are often called internal layers. Previously it has been suggested as a result of glacier flow models that these reflections can be treated as surfaces of equal age. In order for a reflection to be related to a single age feature in an ice sheet a one dimensional wave model must be adequate to model the propagation of a wavelet down to the feature and back to the surface. In this thesis four different one dimensional models are constructed each including different physics. It is shown that for the frequencies of interest to radio echo sounding it is sufficient to use the non-dispersive high frequency values of permittivity and conductivity for the ice in the models. The models are used on data from two drill sites. The first site is Berkner Island where I constructed an instrument to measure the electrical conductivity of the 181 metre long ice core. The second site is the Greenland Ice Core Project (GRIP) site at Summit of length 3028 metres. For both sites permittivity and conductivity profiles inside the ice sheet are calculated and put into the models with an estimate of the transmitted wavelet to produce expected radio echo profiles at the sites. For Berkner despite altering many parameters no match between model result and radar data was obtained. For GRIP a satisfactory match was obtained between model result and radar data. It is concluded that the weak, specular (plane like), st Ratified reflections at depth can be treated as isochrones. The strong reflections at shallow depths are a result of a combination of spherical reflection surfaces and interference between many closely spaced layers and cannot necessarily be treated as isochrones.
95

Nature and dynamics of ice-stream beds : assessing their role in ice-sheet stability

Davies, Damon January 2018 (has links)
Ice streams are fast flowing outlet glaciers through which over 90% of the ice stored within the Antarctic Ice Sheet drains. The dynamic behaviour of ice streams is therefore crucial in controlling the mass balance of the ice sheet. Over the past few decades, Antarctica has been losing mass. Much of this mass loss has been focussed around coastal regions of the Antarctic Ice Sheet. Some of the most dramatic changes such as grounding-line retreat, acceleration and surface elevation change have been observed in Pine Island Glacier (PIG) and its neighbouring ice streams. This is of particular concern because these ice streams account for 10% of the discharge from the west Antarctic Ice Sheet and therefore have the potential to contribute significantly to global sea-level rise. One of the key challenges in accurately forecasting this future sea-level rise is improving understanding of processes occurring at the beds of ice streams. This requires detailed knowledge of the properties and dynamics of the bed. This thesis aims to address this knowledge gap by investigating the spatial and temporal characteristics of the bed of PIG using high-resolution geophysical data acquired in its trunk and tributaries and beneath the ice shelf. The thesis begins by analysing radar-derived high-resolution maps of subglacial topography. These data show a contrasting topography across the ice-bed interface. These diverse subglacial landscapes have an impact on ice flow through form drag, controlled by the size and orientation of bedrock undulations and protuberances. The next chapter provides a quantitative analysis of these landscapes using Fast Fourier analysis of subglacial roughness. This analysis investigates the roughness signature of subglacial bedforms and the how the orientation and wavelength of roughness elements determine their correlation with ice dynamic parameters. The slow-flowing inter-tributary site is found to have a distinct signature comparable to 'ribbed' patterns of modelled basal shear stress and transverse 'mega rib' bedforms. Roughness oriented parallel to ice flow with wavelengths approaching mean ice thickness are found to have the highest correlation with ice dynamic parameters. The temporal stability of PIG is analysed using repeat radar measurements. No significant change is observed over a period of 3-6 years with no evidence of rapid erosion or the evolution of subglacial bedforms as observed in previous repeat measurements of ice-stream beds elsewhere in Antarctica. This suggests that the widespread deforming till layer detected in extensive seismic reflection surveys is in steady state. Lastly, the thesis explores geomorphological evidence of twentieth-century grounding-line retreat beneath PIG Ice Shelf using high-resolution geophysical data acquired from autonomous underwater vehicle surveys. Evidence of erosion, deposition, meltwater flow and post-glacial modification is observed in fine detail. The observed distribution of sediment supported previous surveys indicating a geological transition coinciding with the ridge that acted as a former stable grounding-line location. Metre-scale resolution images of recently deglaciated ice stream beds were found to reveal bedforms that are not detectable with traditional offshore bathymetric surveys. Together these findings reveal the role of short wavelength topography as both an influence on, and product of fast ice stream flow. It also highlights the spatial diversity of subglacial environments and the need to focus future research on tying detailed observations of ice-stream beds with knowledge of basal properties over time.
96

Contribution à l'étude du comportement viscoplastique d'un multicristal de glace : hétérogénéité de la déformation et localisation, expériences et modèles.

Mansuy, Philippe 02 February 2001 (has links) (PDF)
Le monocristal de glace est un mat ériau particuli èrement anisotrope dans le domaine viscoplastique, qui se d éforme principalement par glissement des dislocations parall èlement aux plans de base. Un mod èle de comportement du monocristal, bas e sur les mod èles existants en m étallurgie, est propos é pour reproduire les essais de la litt érature. Dans la glace polycristalline, cette tr es forte anisotropie viscoplastique provoque des incompatibilités de d éformation intergranulaires qui engendrent une tr ès forte h ét érog en éit é de la d éformation intragranulaire. Pour étudier les m écanismes conduisant à cette localisation de la d éformation, des exp ériences de fluage ont et e r éalis ées en laboratoire froid, sur des éprouvettes de glace constitu ées d'un multicristal en inclusion dans une matrice de glace macroscopiquement isotrope. La d éformation a ét é suivie a l'aide de photographies des éprouvettes prises au cours de l'essai, une caract érisation fine étant r ealis ée sous microscope optique apr ès d émontage. Di fférentes con -figurations d'inclusions ont permis d'obtenir une d éformation homog ène dans le grain, ou au contraire de provoquer la localisation de la d éformation sous forme de bandes en genou, de bandes de flexion ou de polygonisation. Une loi constitutive, consid érant le grain de glace comme un mat eriau orthotrope de r évolution poss édant une faible résistance au cisaillement parall èlement aux plans de base, a ét é impl ément ée dans un code aux él éments finis pour simuler les exp eriences. Lorsque la d eéformation est homog ène (inclusion monocristalline), les simulations reproduisent fi d èlement les exp ériences. Dans le cas d'une inclusion multicristalline, les simulations permettent de reproduire relativement bien la forme g én érale des grains et leur orientation, et donnent des informations sur les zones o u la localisation est susceptible de se produire.
97

Development and applications of a full-stress flowband model for ice using the finite volume method /

Price, Stephen F., January 2006 (has links)
Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 149-159).
98

Contribution à l'étude de la fonction de transfert air neige en régions polaires

Silvente, Eric 01 March 1993 (has links) (PDF)
L'atmosphère des régions centrales des calottes polaires constitue un terrain privilégié pour appréhender la réponse de notre environnement atmosphérique à des phénomènes naturels globaux. Cependant, la méconnaissance des mécanismes régissant l'incorporation des impuretés dans la neige limite l'interprétation des paléodonnées chimiques extraites des carottes polaires en terme de composition chimique de l'atmosphère passée. Cette étude s'est focalisée sur le transfert air neige du nitrate, du chlore et de l'ammonium en région centrale du Groenland (Summit). Nous avons montré qu'il était possible de mesurer des espèces telles que l'acide nitrique, l'acide chlorhydrique, présentes en été à l'état de traces gazeuses dans l'atmosphère de Summit. Nous avons montré la viabilité en régions polaires de la méthode de collecte par tubes dénudeurs enduits de fluorure de sodium. L'autre méthode de piégeage employée lors de nos prélèvements (filtres Nylon), montrait qu'elle n'était pas adaptée aux collectes effectuées en régions polaires. Nos mesures de chlore et de nitrate ont démontré clairement que ces deux espèces étaient présentes en été sous forme gazeuse dans la troposphère de Summit. L'étude de suivis de puits et de neiges fraîches ont montré que ces deux espèces chimiques n'étaient pas incorporées avec la même efficacité au sein de la précipitation mais que leurs comportements étaient similaires une fois déposées sur le manteau neigeux. Concernant l'ammonium présent à l'état de particule dans la troposphère de Summit, nous avons mis au point un protocole strict permettant d'effectuer des mesures correctes de cet aérosol. Nous montrons que le signal atmosphérique de l'ammonium n'est pas déformé lors de sa transcription par la précipitation et lors de l'évolution du manteau neigeux. Nous en avons déduit que la majeure partie des études atmosphériques de ces particules qui n'étaient pas cohérentes avec les mesures au sein du manteau neigeux étaient à revoir du fait de la difficulté de la mesure analytique de NI-4+.
99

The analysis of radar altimeter waveform reflections over continental ice sheets /

Nuth, Vannaroth, January 1999 (has links)
Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 101-105). Available also in a digital version from Dissertation Abstracts.
100

Meteorological differences between Rabots glaciär and Storglaciären and its impact on ablation

Eriksson, Pia January 2014 (has links)
In the Kebnekaise Massif, Northern Sweden, the west facing glacier, Rabots glaciär, is loosing volume at a significantly higher rate than east facing, Storglaciären. By analyzing data from automatic weather stations situated on the ablation area on the glaciers we investigated the effect of meteorological differences on ablation. There was a difference in micro-climate between Rabots glaciär and Storglaciären. Generally Storglaciären had slightly warmer and drier air, had less or a thinner cloud layer but more precipitation. On both glaciers a glacier wind is dominant but high wind velocities were common especially on Storglaciären indicating a larger influence from the synoptic system. There was a good correlation for temperature and vapor pressure between the glaciers that indicate that both glaciers are strongly affected by the synoptic system. The meteorological parameters have similar effect on the ablation on the glaciers. Temperature, vapor pressure and the turbulent heat fluxes are the only meteorological parameters that suggest a linear affect on ablation. Net shortwave radiation contribute with the greatest amount of energy for ablation but decreased in relative importance as the temperature increased. Shortwave radiation, sensible and latent heat contributed with a total 184Wm-2 on Rabots glaciär and 222Wm-2 on Storglaciären. Rabots glaciär seem to have a significantly greater relative importance of the turbulent heat fluxes than Storglaciären. Although the differences in micro-climate were not great, using the ablation for Storglaciären to estimate ablation on Rabots glaciär would over estimate the ablation with 0.5m w.e..

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