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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.

Densification and refreezing in the percolation zone of the Greenland Ice Sheet : implications for mass balance measurements

Parry, Victoria January 2009 (has links)
In order to increase coverage, mass balance changes of the world’s ice sheets are increasingly derived from surface elevation changes measured via satellite. Across the percolation zone of the Greenland Ice Sheet, meltwater, percolation and refreezing cause a re-distribution of mass through densification which may result in elevation change with no associated mass loss. Therefore, densification processes need to be quantified, spatially and temporally, and accounted for in mass balance measurements. This thesis investigates the relationships between patterns of elevation change and temporally and spatially variable accumulation and densification processes. In doing so, it provides an important contribution to the validation of the European Space Agency’s CryoSat-2 mission by placing error bars on the accuracy to which changes in satellite-measured ice-mass surface elevation represent real changes in ice mass. Temporal variability in near-surface (<10 m) snowpack and firn density and structure was measured in snowpits, shallow cores and using a neutron probe in the spring and autumn of 2004 at ~1945 m elevation (T05, 69o 51N, 47o 15W) in the percolation zone of the Greenland Ice Sheet. Results show that average snowpack density increased by 26% from spring to autumn, with a 5% (7.6 cm) increase in elevation, and a corresponding 32% increase in mass. Spatial variability was investigated at 11 sites along two transects at spatial scales of 1 m – 10 km. Whilst there was little variability in small scale (1 - 100 m) density changes, ‘seasonal densification’ increased at lower elevations, rising to 47% 10 km closer to the ice sheet margin at 1860 m a.s.l. The spatial variability in seasonal densification was further investigated in spring 2006 at seven sites located at ~10 km intervals along a 57 km transect spanning a 350 m elevation range. Snowpits and shallow cores reveal no significant variation in spring (prior to melt) snowpack density but following summer melt and refreezing cycles, seasonal densification decreased with increasing elevation at 32 kg m-3 per 100 m. Measurements at three sites ranging in elevation from 1860 – 2015 m and spanning three melt-seasons show inter-annual variation in the seasonal densification gradient. In order to obtain a longer time series of mass balance, a 17 m core retrieved in spring 2004 was analysed for stratigraphy, density and ionic and isotopic concentrations to identify annual layers. Unfortunately, the seasonal melt cycle (whereby on average 10% of the snowpack undergoes melt), results in a complex stratigraphy and density and ionic concentrations that cannot be resolved into a seasonal signature. However, the δ18O and δ D isotopes show clear sinusoidal fluctuations, which have been used to derive annual mass balance from 1986 to 2003. These show a mean annual accumulation of 53.7 cm w.e. (s.d. 12.9 cm w.e.) although the accuracy of these measurements is compromised by the percolation of meltwater through more than more year’s snowpack. These findings confirm that estimates of mass balance cannot be calculated solely from observed changes in surface elevation. However, predicting spatial and temporal variations in densification is not straightforward because of the complex inter-annual variations in the processes of accumulation, melt, percolation and refreezing.

A Lateglacial plateau icefield in the Monadhliath Mountains, Scotland : reconstruction, dynamics and palaeoclimatic implications

Boston, Clare Mary January 2012 (has links)
The complex record of glaciogenic landforms and sediments in Britain relating to the last British-Irish Ice Sheet provides the opportunity to reconstruct former ice extents, ice dynamics, retreat patterns and examine their links to climate change. Yet in Scotland, as in the rest of Britain, a previously fragmentary approach to palaeoglaciological research has limited our understanding of glacier dynamics and their relationship to climate, particularly during the Last Glacial-Interglacial Transition. The Monadhliath Mountains in the Central Scottish Highlands are dominated by an extensive plateau area that has received little research attention in the past. The few examples of research include work by British Geological Survey officers in the early 1900s and J.R. Young in the 1970s. These studies focussed primarily on the geomorphology and sedimentology of isolated valleys and therefore this PhD research provides the first systematic mapping of the region as a whole. Results of remote and field mapping demonstrate that two coalescent plateau icefields, together covering an area of c. 280 km2, occurred over the southwest and central sector of the Monadhliath Mountains during the Younger Dryas. Equilibrium line altitudes calculated for the icefield are of comparable magnitude to those reconstructed for nearby Younger Dryas ice masses, such as in Drumochter and Creag Meagaidh, but indicate slightly lower precipitation in the Monadhliath Mountains. ELAs of individual outlet glaciers rise steeply from west to east across the plateau, indicating a strong local precipitation gradient. Significant variations in the geomorphology on the plateau and within outlet valleys allowed an examination of former thermal regime and differences in ice dynamics during retreat. In-depth analysis of moraine retreat patterns enabled a detailed insight into palaeoglaciological controls on deglaciation for the first time, concluding that valley morphology and gradient were the most influential factors on the retreat dynamics of the plateau icefield.

Multiscale analysis of the landforms and sediments of palaeo-ice streams

Channon, Heather January 2013 (has links)
Ice streams play a fundamental role in the stability and dynamics of ice sheets. They are defined by their rapid flow and this is enabled by conditions and processes at the icebed interface. A significant limitation to our understanding of this environment is that most studies, of both contemporary and palaeo-ice streams, have focussed on only one or two, discrete spatial scales of analysis and so integration between scales is restricted. This thesis investigates palaeo-ice streams at multiple scales in order to examine their subglacial processes and characteristics, and to assess the links between and the application of different spatial scales of analysis. Seven palaeo-ice streams from the British and Laurentide ice sheets were investigated at the macroscale, which involved geomorphological mapping, spatial analysis of subglacial lineations and examination of bed characteristics. Two ice streams were also investigated at smaller scales, which included sedimentological analysis (mesoscale) and micromorphological analysis (microscale). Macroscale results showed that subglacial lineations display certain spatial characteristics, including: clustering according to elongation ratio; distribution of low elongation ratios throughout the ice streams; and a decrease in maximum elongation ratio towards the ice stream lateral margins. The latter of which is considered to reflect the transverse distribution of ice velocity. In some cases, a decline in subglacial lineation concentration and elongation ratio coincided with topographic obstacles at the ice stream bed. The most common bed characteristics identified were: widespread till, fine grained sedimentary bedrock with a moderate permeability, low relief and a flat topographic curvature. Key subglacial processes identified included deformation, which was observed at all three scales, and high pore water pressures, for which multiple lines of evidence were found at the meso and micro scales. Spatial variability in both strain and pore water pressure was also common. The multiscale approach allowed robust interpretations of fast flow mechanisms, which furthers knowledge of the sediment and landform characteristics that may result from these flow mechanisms. A summary of the processes that can be identified at each of the spatial scales is given.

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