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Glacier dynamics and subsurface classification of Austfonna, Svalbard: : Inferences from observations and modelling
No full textIce loss from glaciers and ice caps in the Arctic constitute a major contribution to eustatic sea-level rise. Climate change is more pronounced in the Arctic than in other regions, because strong feedback mechanisms such as the albedo feedback lead to enhancement of the initial warming trend. Glaciers and ice caps serve as valuable indicators of past and present climate. However, extraction of climate signals from glaciers is not straightforward. The history, current state and future evolution of glaciers result from external factors, e.g. air temperature or precipitation, and intrinsic glacier dynamics, such as changes in thermal structure or subglacial drainage. Climatically-driven and dynamic processes may interact with each other and either moderate or amplify the glacier’s response to climate change.
This thesis is a contribution to the project GLACIODYN: ’The dynamic response of Arctic glaciers to global warming’, under the framework of the International Polar Year (IPY) 2007–10. It addresses the dynamic behaviour and mass balance regime of Austfonna by combining geophysical techniques for glacier observation and numerical modelling. Austfonna is one of the largest ice caps in the Arctic and by far the largest land-ice mass on the highly glacierized Svalbard archipelago. Observed interior thickening and marginal thinning of Austfonna during the last decade is presumably related to slow glacier dynamics of several basins that are known to have surged in the past and are currently in the quiescent phase. The observed changes may also partly be imposed by changes in the surface mass balance (SMB). Austfonna’s total mass balance during the last decade was negative. Ice-mass loss was primarily attributed to calving from marine terminating outlets and retreat of the marine ice margin. The calving flux depends on the rate at which ice is transported towards the calving front. Previous estimates rely on ice-surface velocity maps that represent snapshots during mid-1990s winter months.
The spatial and temporal variability in snow accumulation is inferred by groundpenetrating radar (GPR). The previously suggested snow-accumulation pattern of Austfonna with about twice as much snow accumulation in the southeast compared to the northwest and a large interannual variability in the total amount of snow is confirmed for the observational period. Rigorous GPR data processing reveals distinct radar zones and allow to distinguish between firn and superimposed ice underneath the last-summer surface, as well as pure glacier ice, representing multi-year-mean accumulation and ablation, respectively. Repeated GPR surveys along coincident transects in spring 2004–07 are utilized to derive time series of radar zones and detect temporal and spatial changes in the SMB regime. Following an initial decrease in 2003–04 the firn area grew in subsequent years of observations, both in terms of the areal extent and thickness, creating favorable conditions for the retention of surface melt in form of internal accumulation and superimposed ice, which constitute a large contribution to the accumulation of Austfonna.
Continuous GPS measurements over the period 2008–10 are utilized to investigate seasonal velocity changes along the central flowlines of two marine-terminating fast-flow units, Basin-3 and Duvebreen. Results from low-frequency GPR and a nearby AutomaticWeather Station (AWS) provide supplementary information with respect to glacier geometry and timing of surface melt. The analysis gives insights into mechanisms that may influence basal motion, characteristic bedrock topography above/below sea level, timing and volume of meltwater input into the subglacial drainage system and drainage characteristics. At Basin-3, the observed annual mean ice-surface velocities exceed those measured by InSAR in the mid 1990s by a factor 3, indicting that ice loss due to calving from this outlet may be significantly larger then previously anticipated. A marked speed-up following the onset of the summer melt period is recorded at both Basin-3 and Duvebreen, implying enhanced basal motion through input of meltwater into the subglacial drainage system. Air temperatures during summer 2009 were significantly higher than in 2008 and the relation between observed ice-surface velocity and melt periods is more complex. This suggests a transition to a hydraulically more efficient drainage system that can accommodate repeated meltwater input without increased basal water pressure.
To gain a better knowledge of the present-day dynamics of Austfonna and surge-type behaviour of individual basins in particular, the existing ice-sheet model SICOPOLIS is employed. Implementation of the new model domain involves compilation of a comprehensive model-input dataset, comprising surface and bedrock topography, and the climatic forcing in terms of precipitation and air temperature. Steady-state model experiments are performed with the present-day geometry as initial conditions and the present-day climate is applied as external forcing. Mechanisms that may govern the dynamic characteristics are investigated by systematically changing the description of basal motion. Depending on the chosen parameter combinations, simulated fast-flow units operate in a mode of steady fast-flow or cyclic surge behaviour. The model results suggest that a change in the basal thermal regime, ultimately controlled by the long-term evolution of the glacier geometry, is decisive for surge-type behaviour, but not sufficient. The model successfully generates surge-type behaviour if the transition from no-slip to full basal sliding occurs within a narrow temperature range near the pressure-melting point (PMP), and if combined with enhanced sliding of ice regions grounded below sea level, e.g. representing plastic deformation of water-saturated sediments. This scenario is in agreement with previous observations of unconsolidated sediments beneath the marine grounded ice and statistical model results, suggesting a thermally controlled soft-bed surge mechanism for Svalbard glaciers. Fast flow is accomplished by basal motion over a temperate bed. Irrespective of the dynamic regime, considerable volumes of temperate ice are, however absent, both in the observed and simulated ice cap. The simulated steady states of Austfonna provide idealized initial conditions for prognostic model runs that allow to assess the uncertainties in the dynamic response of the ice cap to climate change.
Finally, prognostic model runs are performed under the framework of the model initiative GlacMod2010 that aims at investigating the response of an appreciable number of simulated glaciers and ice caps to a set of simple climate-change scenarios. Due to the short simulation period from 2010–2100 (90 years), compared to the principle period of simulated surge-type behaviour (200–500 years), the model was set-up with flow units in steady fast-flow. The prescribed increase in precipitation is found insufficient to compensate for the non-linear increase in mass loss associated with the warming scenario
"Basal conditions of Kongsvegen at the onset of surge - revealed using seismic vibroseis surveys" in the IASC Workshop on the dynamics and mass budget of Arctic glaciers - Abstracts and program booklet.
Get PDFKongsvegen is a well-studied surge-type glacier in the Kongsfjord area of northwest Svalbard. Long-term monitoring has shown that the ice surface velocity has been increasing for the past 4 years; presenting a unique opportunity to study the internal ice structure, basal conditions and thermal regime that play a crucial role in initiating glacier surges. In April 2019, three-component seismic vibroseis surveys were conducted at two sites on the glacier, using a small Electrodynamic Vibrator source (ElViS). Site 1 is in the ablation area and site 2 near the equilibrium line, where the greatest increase in ice surface velocity has been observed.
Initial analysis indicates the conditions at the two sites are significantly different. At
site 1 the ice is around 220 m thick, sitting on a relatively flat and uniform bed, with
no clear change in the bed reflection along the profile. There is a horizontally layered
sediment package ∼60 m thick underlaying the bed. The ice column shows no internal
layering. By contrast at site 2 (Fig. 1), where the ice is around 390 m thick, there is much
more complex internal ice structure. Clear internal ice reflections are visible between
150-250 m depth – where we expect a transition from cold to temperate ice. Further
reflections in the 100 m above the bed indicate there could be shearing or sediment
entrainment in this area. Below the bed, cross-cutting layers are clearly visible and the
bed reflection itself shows changing reflection polarity – suggesting water or very wet
sediment is present in some areas. This suggests ice movement by basal sliding and
shearing is likely
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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