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Sean J. Fitzsimons - One of the best experts on this subject based on the ideXlab platform.

  • Debris-Rich Basal Ice as a Microbial Habitat, Taylor Glacier, Antarctica
    Geomicrobiology Journal, 2013
    Co-Authors: Scott N. Montross, Reginald Lorrain, Jean-louis Tison, Shawn M. Doyle, Brent C. Christner, Mark L. Skidmore, Denis Samyn, Sean J. Fitzsimons
    Abstract:

    Two ∼4 m vertical sequences of Basal Ice were collected from tunnels dug into the northern lateral margin of Taylor Glacier, McMurdo Dry Valleys, Antarctica. In both cases the Basal sequences exhibit two contrasting Ice facies groups; clean (debris-free) and banded dispersed (debris-rich). Debris-rich Ices exhibit elevated CO2 and depleted O2 concentrations compared to the clean facies. Bacterial cell numbers, respiration rates, and nutrient concentrations are highest in debris-rich layers. Together, our geochemical and biological data indicate that microbial heterotrophic respiration is likely occurring in situ within the Basal Ice matrix at ambient temperatures near −15°C. This implies that the Basal Ice zone of polar glaciers and larger Ice sheets is a viable subglacial microbial habitat and active biome of significant volume that has not previously been considered.

  • Rotating micro-structures in Antarctic cold Basal Ice: implications for glacier flow and its interpretation
    International Journal of Earth Sciences, 2010
    Co-Authors: Denis Samyn, Sean J. Fitzsimons, Reginald D. Lorrain
    Abstract:

    Structural analyses were conducted in the Basal zone of an Antarctic glacier. The studied Basal Ice sequence was retrieved from a 20-m-long subglacial tunnel dug at the margin of the glacier and is at the temperature of −17°C. For the first time, rotating clast systems embedded within debris-rich Ice were thin-sectioned using specially designed cutting techniques. The observed structures reflect the occurrence of pervasive shearing at the base of the glacier, and can be used as shear sense indicators. In addition, some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the Ice. It is showed here that cautious analysis of deformation structures present in debris-bearing Ice may bring insights not only into the flow dynamics of the embedding matrix, but also into the behaviour of the interstitial fluid network at the base of cold glaciers and Ice sheets.

  • Detecting biogeochemical activity in Basal Ice using fluorescence spectroscopy
    Annals of Glaciology, 2010
    Co-Authors: Joel D. Barker, Jonathan L. Klassen, Martin Sharp, Sean J. Fitzsimons, Raymond J. Turner
    Abstract:

    Dissolved organic matter (DOM) is an important component of aquatic carbon and nutrient budgets and is a metabolic substrate for organisms at the base of aquatic food chains. Active microbial communities in glaciers affect the abundance and characteristics of organic matter (OM) that is exported to downstream ecosystems. However, how OM is biogeochemically altered in glaciers remains unknown and studies documenting active microbial activity by detecting in situ biogeochemical modifications of OM are lacking due to difficulties characterizing OM and the low concentrations of DOM typical of glacier environments. To address this issue, we measure the abundance and fluorescence characteristics of DOM in Basal Ice at Victoria Upper Glacier (VUG), McMurdo Dry Valleys, Antarctica. We compare these observations with the results of microbial incubations from the same Basal Ice samples to determine whether the occurrence of fluorophores indicative of recent microbial activity is linked to the presence of culturable microbial communities containing organisms that could have produced them. Psychrotolerant bacteria were isolated from Basal Ice samples and were associated with marine humic-like fluorescence. This is interpreted as being indicative of in situ microbial degradation of OM within Basal Ice at VUG. Marine humic-like material is a recalcitrant form of OM, and its biogeochemical transformation from a relatively labile form of OM in glacier Ice may function as a carbon sink.

  • Dynamic implications of discontinuous recrystallization in cold Basal Ice: Taylor Glacier, Antarctica
    Journal of Geophysical Research, 2008
    Co-Authors: Denis Samyn, A. Svensson, Sean J. Fitzsimons
    Abstract:

    Crystallographic investigations have been conducted of cold (-17°C) debris-bearing Ice from the base of an Antarctic outlet glacier (Taylor Glacier). The 4-m-thick sequence studied has been retrieved from a 20-m-long tunnel dug from the glacier snout and has been analyzed with an automatic Ice fabric analyzer (AIFA). The top and bottom of the sequence consists of clean meteoric Ice (englacial facies), whereas alternating debris-rich and clean bubbly Ice layers are found in the middle part (stratified facies). Ice from the englacial facies displays a polygonal texture and a strong c-axis clustering toward the vertical, denoting recrystallization through "subgrain rotation" (SGR). In contrast, clean Ice from the stratified facies shows SGR fabrics which are delimited at the contact with debris-rich layers by large, interlocking grains organized in ribbons. These two distinct textures within the stratified facies are associated with looser c-axis patterns at the scale of single thin sections, which is interpreted as resulting from "migration recrystallization" (MR). The change from SGR to MR trends marks a clear increase in grain boundary and nucleation kinetics (hence the term "discontinuous recrystallization") and may be associated with strain localization at rheological interfaces during Basal Ice genesis. Analogies with bottom Ice from deep polar Ice sheets, where temperature is commonly higher than at the studied site, are highlighted. Two recrystallization scenarios are proposed, accounting for the development of both types of fabrics. It is shown that by controlling the repartition of stress and strain energy within Basal Ice, the rheology of debris-bearing Ice layers plays a decisive role in recrystallization dynamics at structural interfaces. We also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate Ice sheets, provided that strain accumulation has been high enough in the former. This challenges the common belief that migration fabrics observed in bottom Ice from deep Ice sheets are exclusive to warm, stagnant, annealed Ice. Copyright 2008 by the American Geophysical Union.

  • Mechanisms of Basal Ice formation in polar glaciers: An evaluation of the apron entrainment model
    Journal of Geophysical Research, 2008
    Co-Authors: Sean J. Fitzsimons, Reginald Lorrain, Nicola Webb, Sarah Mager, Shelley Macdonell, Denis Samyn
    Abstract:

    [1] Previous studies of polar glaciers have argued that Basal Ice can form when these glaciers override and entrain Ice marginal aprons that accumulate adjacent to steep Ice cliffs. To test this idea, we have studied the morphology, structure, composition, and deformation of the apron and Basal Ice at the terminus of Victoria Upper Glacier in the McMurdo dry valleys, which are located on the western coast of the Ross Sea at 77 Si n southern Victoria Land, Antarctica. Our results show that the apron has two structural elements: an inner element that consists of strongly foliated Ice that has a steep up-glacier dip, and an outer element that lacks a consistent foliation and has a down-glacier, slope-parallel dip. Although strain measurements show that the entire apron is deforming, the inner element is characterized by high strain rates, whereas relatively low rates of strain characterize the outer part of the apron. Co-isotopic analyses of the Ice, together with analysis of solute chemistry and sedimentary characteristics, show that the apron is compositionally different from the Basal Ice. Our observations show that aprons may become deformed and partially entrained by advancing glaciers. However, such an Ice marginal process does not provide a satisfactory explanation for the origin of Basal Ice observed at the Ice margin. Our interpretation of the origin of Basal Ice is that it is formed by subglacial processes, which are likely to include deformation and entrainment of subglacial permafrost.

Reginald Lorrain - One of the best experts on this subject based on the ideXlab platform.

  • Debris-Rich Basal Ice as a Microbial Habitat, Taylor Glacier, Antarctica
    Geomicrobiology Journal, 2013
    Co-Authors: Scott N. Montross, Reginald Lorrain, Jean-louis Tison, Shawn M. Doyle, Brent C. Christner, Mark L. Skidmore, Denis Samyn, Sean J. Fitzsimons
    Abstract:

    Two ∼4 m vertical sequences of Basal Ice were collected from tunnels dug into the northern lateral margin of Taylor Glacier, McMurdo Dry Valleys, Antarctica. In both cases the Basal sequences exhibit two contrasting Ice facies groups; clean (debris-free) and banded dispersed (debris-rich). Debris-rich Ices exhibit elevated CO2 and depleted O2 concentrations compared to the clean facies. Bacterial cell numbers, respiration rates, and nutrient concentrations are highest in debris-rich layers. Together, our geochemical and biological data indicate that microbial heterotrophic respiration is likely occurring in situ within the Basal Ice matrix at ambient temperatures near −15°C. This implies that the Basal Ice zone of polar glaciers and larger Ice sheets is a viable subglacial microbial habitat and active biome of significant volume that has not previously been considered.

  • Mechanisms of Basal Ice formation in polar glaciers: An evaluation of the apron entrainment model
    Journal of Geophysical Research, 2008
    Co-Authors: Sean J. Fitzsimons, Reginald Lorrain, Nicola Webb, Sarah Mager, Shelley Macdonell, Denis Samyn
    Abstract:

    [1] Previous studies of polar glaciers have argued that Basal Ice can form when these glaciers override and entrain Ice marginal aprons that accumulate adjacent to steep Ice cliffs. To test this idea, we have studied the morphology, structure, composition, and deformation of the apron and Basal Ice at the terminus of Victoria Upper Glacier in the McMurdo dry valleys, which are located on the western coast of the Ross Sea at 77 Si n southern Victoria Land, Antarctica. Our results show that the apron has two structural elements: an inner element that consists of strongly foliated Ice that has a steep up-glacier dip, and an outer element that lacks a consistent foliation and has a down-glacier, slope-parallel dip. Although strain measurements show that the entire apron is deforming, the inner element is characterized by high strain rates, whereas relatively low rates of strain characterize the outer part of the apron. Co-isotopic analyses of the Ice, together with analysis of solute chemistry and sedimentary characteristics, show that the apron is compositionally different from the Basal Ice. Our observations show that aprons may become deformed and partially entrained by advancing glaciers. However, such an Ice marginal process does not provide a satisfactory explanation for the origin of Basal Ice observed at the Ice margin. Our interpretation of the origin of Basal Ice is that it is formed by subglacial processes, which are likely to include deformation and entrainment of subglacial permafrost.

  • Gas isotopes in Ice reveal a vegetated central Greenland during Ice sheet invasion
    Geophysical Research Letters, 2006
    Co-Authors: Roland Souchez, Reginald Lorrain, Jérôme Chappellaz, Jean Jouzel, Amaelle Landais, Jean-louis Tison
    Abstract:

    Environmental conditions prevailing during build-up of the Greenland Ice Sheet (GIS) are not yet established. Here we use results from gas isotopes from Basal Ice of the GRIP Ice core to show that central Greenland was vegetated during invasion by the Ice sheet. The δ 13CCH4 and δ 18O of O2 values obtained were never encountered before in Ice core studies. Such values are indeed the signature of a significant biological activity.

  • Strain-induced phase changes within cold Basal Ice from Taylor Glacier, Antarctica, indicated by textural and gas analyses
    Journal of Glaciology, 2005
    Co-Authors: Denis Samyn, Sean J. Fitzsimons, Reginald Lorrain
    Abstract:

    This paper reports detailed textural and gas measurements conducted in cold Basal Ice (-178C) from the margin of Taylor Glacier, an outlet glacier of the East Antarctic Ice sheet. The analyzed samples were retrieved from a Basal Ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout. The Basal sequence exhibits two contrasting Ice facies, defined as the englacial and stratified facies. On the one hand, analysis of Ice crystal textures from the Basal Ice sequence provides evidence for localized ductile deformation, especially within the stratified facies where significant dynamic recrystallization was detected. On the other hand, high-resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies. These gas composition changes are typical of melting-refreezing processes but are not associated with any significant loss of gas volume. Given the specific subglacial thermal conditions at the margin of Taylor Glacier, we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre-melt phase. It is argued that such processes may play an important role in the post-genetic geochemical evolution of cold debris-laden Ice and may be enhanced through intense strain conditions.

  • equifinality of Basal Ice facies from an antarctic cold based glacier
    Annals of Glaciology, 2003
    Co-Authors: Suzanne Sleewaegen, Denis Samyn, Sean J. Fitzsimons, Reginald Lorrain
    Abstract:

    Threedebris-bearingIcefacieswererecognizedatthebaseofSuessGlacier, a cold-basedglacierdamminga lakeinTaylorValley, SouthVictoria Land, Antarctica.These faciesaretermed``amber Ice'',``solidfacies''and``Basal stratifiedfacies''.Thispaper uses stable- isotope composition ( dD and d 18 O), gas content and gas composition (CO 2, O2 and N2) to develop an understanding of the processes responsible for the formation of these facies.The Basal Iceischaracterizedbya strikingdifferencein Icepropertiesbetweentheinnermostend of a 25m long tunnel dug 200m upstream fromthe glacier front and the front itself. At the glacier front, co-isotopic data plot along a well-defined freezing slope ( S ˆ 5.6), whereas, insidethetunnel, the isotopic data offset fromthefreezing slope and fromthe local meteoric water-line (which has a slope of 8.2). CO 2 concentrations rise from a minimum of about 1000ppmv inthetunnelto about 220000ppmvatthe front.Takentogether, these character- istics strongly suggest an increasing contribution of liquidwater inthe formation of Basal Ice towards the glacier terminus.We therefore conclude that visually similar Basal Ice facies can have differentorigins.

Bryn Hubbard - One of the best experts on this subject based on the ideXlab platform.

  • debris entrainment and landform genesis during tidewater glacier surges
    Journal of Geophysical Research, 2015
    Co-Authors: Bryn Hubbard, Edward J. Fleming, Harold Lovell, Douglas I. Benn, Sven Lukas
    Abstract:

    The englacial entrainment of Basal debris during surges presents an opportunity to investigate processes acting at the glacier bed. The subsequent melt-out of debris-rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity. We investigate the link between debris entrainment and proglacial geomorphology by analyzing Basal Ice, englacial structures, and ridge networks exposed at the margins of Tunabreen, a tidewater surge-type glacier in Svalbard. The Basal Ice facies display clear evidence for brittle and ductile tectonic deformation, resulting in overall thickening of the Basal Ice sequence. The formation of debris-poor dispersed facies Ice is the result of strain-induced metamorphism of meteoric Ice near the bed. Debris-rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into Basal crevasses and hydrofracture exploitation of thrust faults, reoriented crevasse squeezes, and preexisting fractures. These structures are observed to melt-out and form embryonic geometrical ridge networks at the base of a terrestrially grounded Ice cliff. Ridge networks are also located at the terrestrial margins of Tunabreen, neighboring Von Postbreen, and in a submarine position within Tempelfjorden. Analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris-rich englacial structures. This in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges.

  • Basal Ice facies: a review and unifying approach
    Quaternary Science Reviews, 2009
    Co-Authors: Bryn Hubbard, Simon J Cook, Hayley Coulson
    Abstract:

    Abstract Over the past ∼30 years numerous Basal Ice facies have been identified, named and classified. However, the resulting facies descriptions and names are inconsistent and no single scheme encompasses all of the different Ice types that exist at different Ice masses. In this paper, we review and critique existing Basal Ice facies names, descriptions and classification schemes, and propose a new, non-genetic approach that has the capacity to name and describe all Basal Ice types. We define six fundamental Basal cryofacies and a further 12 composite cryofacies which can all be defined on the basis of a cursory evaluation of debris disposition and concentration. More detailed cryofacies description is based on characterizing three sets of Ice properties that can also be estimated visually in the field: (a) the thickness of the Basal Ice facies and its constituent sub-layers, (b) the concentration and texture of debris included within any or all of those layers, and (c) the concentration and size of bubbles included within any or all of those layers. We also propose a shorthand method for the presentation of this descriptive information. Here, codes for the layer thicknesses are presented as standard text, and codes for the included debris characteristics and included bubble characteristics are presented as superscripts and subscripts respectively. Sub-layers are characterized similarly, but within a nested sequence of brackets. We evaluate the effectiveness of these two schemes by using them to rename and reclassify several existing Basal Ice facies. Results indicate that the schemes are robust and that they provide a coherent, non-genetic framework for the effective naming and description of Basal cryofacies.

  • Basal Ice facies: a review and unifying approach
    Quaternary Science Reviews, 2009
    Co-Authors: Bryn Hubbard, Simon Cook, Hayley Coulson
    Abstract:

    Hubbard, B., Cook, S., Coulson, H. (2009). Basal Ice facies: a review and unifying approach. Quaternary Science Reviews, 28 (19-20), 1956-1969. Bryn Hubbard is corresponding author Sponsorship: NERCOver the past 30 years numerous Basal Ice facies have been identified, named and classified. However, the resulting facies descriptions and names are inconsistent and no single scheme encompasses all of the different Ice types that exist at different Ice masses. In this paper, we review and critique existing Basal Ice facies names, descriptions and classification schemes, and propose a new, non-genetic approach that has the capacity to name and describe all Basal Ice types. We define six fundamental Basal cryofacies and a further 12 composite cryofacies which can all be defined on the basis of a cursory evaluation of debris disposition and concentration. More detailed cryofacies description is based on characterizing three sets of Ice properties that can also be estimated visually in the field: (a) the thickness of the Basal Ice facies and its constituent sub-layers, (b) the concentration and texture of debris included within any or all of those layers, and (c) the concentration and size of bubbles included within any or all of those layers. We also propose a shorthand method for the presentation of this descriptive information. Here, codes for the layer thicknesses are presented as standard text, and codes for the included debris characteristics and included bubble characteristics are presented as superscripts and subscripts respectively. Sub-layers are characterized similarly, but within a nested sequence of brackets. We evaluate the effectiveness of these two schemes by using them to rename and reclassify several existing Basal Ice facies. Results indicate that the schemes are robust and that they provide a coherent, non-genetic framework for the effective naming and description of Basal cryofacies.Peer reviewe

  • Ice core evidence for the thickness and character of clear facies Basal Ice at Glacier de Tsanfleuron, Switzerland
    Journal of Glaciology, 2000
    Co-Authors: Bryn Hubbard, Jean-louis Tison, Laurent Janssens, Baruch Spiro
    Abstract:

    Five Ice cores have been retrieved from a transect close to the terminus of Glacier de Tsanfleuron, Switzerland. The cores extend from the Ice surface to the glacier bed, and are 3.5–44.8 m long. Stratigraphic logging based on bubble size and density reveals the presence of a highly metamorphosed Basal Ice layer, about 10 m thick, from which all traces of bubble-rich Ice have been removed. This bubble-poor Ice, which corresponds closely with clear-facies Ice observed in cavities beneath numerous temperate-based glaciers, contrasts with the overlying bubble-rich or bubble-foliated englacial Ice and the underlying debris-rich and bubble-free dispersed-facies Basal Ice. Down-core patterns in major-ion composition, stable-isotope composition and total gas content and composition are generally consistent with formation of clear-facies Ice by deformation-related metamorphism of bubbly, englacial Ice. In addition, isotopic data suggest that storage of downward-percolating meltwaters occurs close to the upper surface of the clear-facies Ice layer, perhaps reflecting a local variation in Ice permeability across the transition from englacial to clear-facies Ice. Enrichment in crustally derived ionic species is noted in the lowermost decimetres of the debris-free, clear-facies Ice that immediately overlies debris-rich dispersed-facies Basal Ice. This ionic enrichment in debris-free Ice is interpreted in terms of active inter-granular meltwater flow within some decimetres of the glacier bed.

  • On the sedimentological character of Alpine Basal Ice facies
    Annals of Glaciology, 1996
    Co-Authors: Bryn Hubbard, Martin Sharp, Wendy Lawson
    Abstract:

    Seven Basal Ice facies have been defined on the basis of research at eleven glaciers in the western Alps. The concentration and texture of the debris incorporated in these facies are described. Grain-size distributions are characterised in terms of their: (i) mean size and dispersion, (ii) component Gaussian modes, and (iii) self-similarity.Firnified glacier Ice contains low concentrations (≈0.2 g 1−1) of well-sorted and predominantly fine-grained debris that is not self-similar over the range of particle diameters assessed. In contrast, Basal Ice contains relatively high concentrations (≈4–4000 g 1−1 by facies) of polymodal (by size fraction against weight) debris, the texture of which is consistent with incorporation at the glacier bed. Analysis by grain-size against number of particles suggests that these Basal facies debris textures are also self-similar. This apparent contradiction may be explained by the insensitivity of the assessment of self-similarity to variations in mass distribution. Comparison of typical size–weight with size–number distributions indicates that neither visual nor statistical assessment of the latter may be sufficiently rigorous to identify self-similarity.Apparent fractal dimensions may indicate the relative importance of fines in a debris distribution. Subglacially derived Basal facies debris has a mean fractal dimension of 2.74. This value suggests an excess of fines relative to a self-similar distribution of cubes, which has a fractal dimension of 2.58. Subglacial sediments sampled from the forefield of Skalafellsjökull, Iceland, have fractal dimensions of 2.91 (A-horizon) and 2.81 (B-horizon). Debris from the A-horizon, which is interpreted as having been pervasively deformed, both most closely approaches self-similarity and has the highest fractal dimension of any of the sample groups analyzed.

Denis Samyn - One of the best experts on this subject based on the ideXlab platform.

  • Debris-Rich Basal Ice as a Microbial Habitat, Taylor Glacier, Antarctica
    Geomicrobiology Journal, 2013
    Co-Authors: Scott N. Montross, Reginald Lorrain, Jean-louis Tison, Shawn M. Doyle, Brent C. Christner, Mark L. Skidmore, Denis Samyn, Sean J. Fitzsimons
    Abstract:

    Two ∼4 m vertical sequences of Basal Ice were collected from tunnels dug into the northern lateral margin of Taylor Glacier, McMurdo Dry Valleys, Antarctica. In both cases the Basal sequences exhibit two contrasting Ice facies groups; clean (debris-free) and banded dispersed (debris-rich). Debris-rich Ices exhibit elevated CO2 and depleted O2 concentrations compared to the clean facies. Bacterial cell numbers, respiration rates, and nutrient concentrations are highest in debris-rich layers. Together, our geochemical and biological data indicate that microbial heterotrophic respiration is likely occurring in situ within the Basal Ice matrix at ambient temperatures near −15°C. This implies that the Basal Ice zone of polar glaciers and larger Ice sheets is a viable subglacial microbial habitat and active biome of significant volume that has not previously been considered.

  • Rotating micro-structures in Antarctic cold Basal Ice: implications for glacier flow and its interpretation
    International Journal of Earth Sciences, 2010
    Co-Authors: Denis Samyn, Sean J. Fitzsimons, Reginald D. Lorrain
    Abstract:

    Structural analyses were conducted in the Basal zone of an Antarctic glacier. The studied Basal Ice sequence was retrieved from a 20-m-long subglacial tunnel dug at the margin of the glacier and is at the temperature of −17°C. For the first time, rotating clast systems embedded within debris-rich Ice were thin-sectioned using specially designed cutting techniques. The observed structures reflect the occurrence of pervasive shearing at the base of the glacier, and can be used as shear sense indicators. In addition, some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the Ice. It is showed here that cautious analysis of deformation structures present in debris-bearing Ice may bring insights not only into the flow dynamics of the embedding matrix, but also into the behaviour of the interstitial fluid network at the base of cold glaciers and Ice sheets.

  • Dynamic implications of discontinuous recrystallization in cold Basal Ice: Taylor Glacier, Antarctica
    Journal of Geophysical Research, 2008
    Co-Authors: Denis Samyn, A. Svensson, Sean J. Fitzsimons
    Abstract:

    Crystallographic investigations have been conducted of cold (-17°C) debris-bearing Ice from the base of an Antarctic outlet glacier (Taylor Glacier). The 4-m-thick sequence studied has been retrieved from a 20-m-long tunnel dug from the glacier snout and has been analyzed with an automatic Ice fabric analyzer (AIFA). The top and bottom of the sequence consists of clean meteoric Ice (englacial facies), whereas alternating debris-rich and clean bubbly Ice layers are found in the middle part (stratified facies). Ice from the englacial facies displays a polygonal texture and a strong c-axis clustering toward the vertical, denoting recrystallization through "subgrain rotation" (SGR). In contrast, clean Ice from the stratified facies shows SGR fabrics which are delimited at the contact with debris-rich layers by large, interlocking grains organized in ribbons. These two distinct textures within the stratified facies are associated with looser c-axis patterns at the scale of single thin sections, which is interpreted as resulting from "migration recrystallization" (MR). The change from SGR to MR trends marks a clear increase in grain boundary and nucleation kinetics (hence the term "discontinuous recrystallization") and may be associated with strain localization at rheological interfaces during Basal Ice genesis. Analogies with bottom Ice from deep polar Ice sheets, where temperature is commonly higher than at the studied site, are highlighted. Two recrystallization scenarios are proposed, accounting for the development of both types of fabrics. It is shown that by controlling the repartition of stress and strain energy within Basal Ice, the rheology of debris-bearing Ice layers plays a decisive role in recrystallization dynamics at structural interfaces. We also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate Ice sheets, provided that strain accumulation has been high enough in the former. This challenges the common belief that migration fabrics observed in bottom Ice from deep Ice sheets are exclusive to warm, stagnant, annealed Ice. Copyright 2008 by the American Geophysical Union.

  • Mechanisms of Basal Ice formation in polar glaciers: An evaluation of the apron entrainment model
    Journal of Geophysical Research, 2008
    Co-Authors: Sean J. Fitzsimons, Reginald Lorrain, Nicola Webb, Sarah Mager, Shelley Macdonell, Denis Samyn
    Abstract:

    [1] Previous studies of polar glaciers have argued that Basal Ice can form when these glaciers override and entrain Ice marginal aprons that accumulate adjacent to steep Ice cliffs. To test this idea, we have studied the morphology, structure, composition, and deformation of the apron and Basal Ice at the terminus of Victoria Upper Glacier in the McMurdo dry valleys, which are located on the western coast of the Ross Sea at 77 Si n southern Victoria Land, Antarctica. Our results show that the apron has two structural elements: an inner element that consists of strongly foliated Ice that has a steep up-glacier dip, and an outer element that lacks a consistent foliation and has a down-glacier, slope-parallel dip. Although strain measurements show that the entire apron is deforming, the inner element is characterized by high strain rates, whereas relatively low rates of strain characterize the outer part of the apron. Co-isotopic analyses of the Ice, together with analysis of solute chemistry and sedimentary characteristics, show that the apron is compositionally different from the Basal Ice. Our observations show that aprons may become deformed and partially entrained by advancing glaciers. However, such an Ice marginal process does not provide a satisfactory explanation for the origin of Basal Ice observed at the Ice margin. Our interpretation of the origin of Basal Ice is that it is formed by subglacial processes, which are likely to include deformation and entrainment of subglacial permafrost.

  • Stable Isotope Composition of the Basal Ice from Taylor Glacier, Southern Victoria Land, Antarctica
    2007
    Co-Authors: Sarah Mager, Sean J. Fitzsimons, Russell D. Frew, Denis Samyn
    Abstract:

    Summary A tunnel excavated into the margin of Taylor Glacier revealed a Basal sequence containing a thick sequence of layers of clean clear Ice and debris-rich Ice which contained strong deformation features, as well as units of clean bubbly Ice. Analysis of the isotopic composition of the Basal Ice shows a strong linear relationship that plots on a slope of 8, which is usually interpreted as meteoric in origin. However, the physical appearance of the laminated Ice is inconsistent with a meteoric-origin interpretation and has the outward appearance of Ice usually inferred as the product of Basal melt-refreeze processes like regelation. We consider this apparent tension between physical appearance and isotopic composition of the Taylor Glacier Basal Ice to be a limitation of the stable isotope approach, and that the technique employed here is unable to diagnose small-scale processes like regelation.

Peter G. Knight - One of the best experts on this subject based on the ideXlab platform.

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