Freezing-rate effects on the physical characteristics of basal ice formed by net adfreezing

Hubbard, Bryn

doi:10.3189/s0022143000005773

Cited by 17 publications

(15 citation statements)

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“…Solid Stratified Subfacies The debris characteristics of the solid stratified subfacies are consistent with frozen subglacial till [Sugden et al, 1987;Sharp et al, 1994;Evans et al, 2006]. Regelation is unlikely to have been responsible for its formation as it rarely produces layers >0.1 m thick [Nye, 1970;Hubbard and Sharp, 1993] and is typically associated with low debris concentrations [Kamb and LaChapelle, 1964;Hubbard, 1991], which contrasts with the thickness and high debris concentrations of the solid stratified subfacies. Vertical regelation into the bed is most effective at incorporating coarse-grained, clast-supported debris [Alley et al, 1997], which contrasts with the silt-rich matrix of the solid stratified subfacies (Figure 5c).…”

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Debris entrainment and landform genesis during tidewater glacier surges

et al. 2015

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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.

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confidence: 78%

Debris entrainment and landform genesis during tidewater glacier surges

et al. 2015

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“…For example, at CI‐0 at a depth of 4.6 m (Figures a and g) a quasi‐vertical bright streaking feature is observed. This is interpreted as being formed by an upper and lower freezing‐front rejecting dissolved gases and forming two coalescing vertical bubble trains, characteristic of the linear progression of a freezing front through standing water [ Hubbard , ]. The bright speckles that are present throughout U3 are interpreted as bubble clouds.…”

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confidence: 99%

Ice and firn heterogeneity within Larsen C Ice Shelf from borehole optical televiewing

et al. 2017

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We use borehole optical televiewing (OPTV) to explore the internal structure of Larsen C Ice Shelf (LCIS). We report a suite of five ~90 m long OPTV logs, recording a light‐emitting diode‐illuminated, geometrically correct image of the borehole wall, from the northern and central sectors of LCIS collected during austral spring 2014 and 2015. We use a thresholding‐based technique to estimate the refrozen ice content of the ice column and exploit a recently calibrated density‐luminosity relationship to reveal its structure. All sites are dense and strongly influenced by surface melt, with frequent refrozen ice layers and mean densities, between the depths of 1.87 and 90 m, ranging from 862 to 894 kg m−3. We define four distinct units that comprise LCIS and relate these to ice provenance, dynamic history, and past melt events. These units are in situ meteoric ice with infiltration ice (U1), meteoric ice which has undergone enhanced densification (U2), thick refrozen ice (U3), and advected continental ice (U4). We show that the OPTV‐derived pattern of firn air content is consistent with previous estimates, but that a significant proportion of firn air is contained within U4, which we interpret to have been deposited inland of the grounding line. The structure of LCIS is strongly influenced by the E‐W gradient in föhn‐driven melting, with sites close to the Antarctic Peninsula being predominantly composed of refrozen ice. Melting is also substantial toward the ice shelf center with >40% of the overall imaged ice column being composed of refrozen ice.

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“…This is particularly effective when meltwater flows into areas of the bed that are cold-based (Weertman, 1961;Hubbard & Sharp, 1989;Hubbard, 1991), and thus explains why polythermal glaciers are important transporters of large volumes of sediment debris . Freeze-on by conductive cooling occurs as a result of changing basal thermal conditions (Alley et al, 1997).…”

Section: Characteristicsmentioning

confidence: 99%

Neoproterozoic Glaciated Basins: A Critical Review of the Snowball Earth Hypothesis by Comparison with Phanerozoic Glaciations

Etienne¹,

Allen

Rieu

et al. 2007

Glacial Sedimentary Processes and Products

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The Neoproterozoic is widely considered to have experienced some of the most severe climatic perturbations recorded in Earth history, with extensive glaciations often referred to as 'Snowball Earth' events. The Snowball Earth and competing hypotheses seek to explain a wide range of geological data on Neoproterozoic pre-, syn-and post-glacial successions including glacial sedimentology, chemostratigraphy, palaeoceanography, geochronology, palaeomagnetism and palaeogeography, geodynamics, tectonics, palaeontology and palaeobiogeochemistry. However, our understanding of the Phanerozoic and particularly the Cenozoic and contemporary glacial geological record is often relatively neglected when evaluating the evidence for apparent severe and prolonged periods of globally synchronous glaciation. This paper presents a review of the available geological data for Neoproterozoic glacial successions in the light of what we know about the Cenozoic and recent glacial record. Most Neoproterozoic successions are shown to exhibit spatial and temporal variability, with sediment stacking patterns and facies associations indicative of dynamic ice masses. These characteristics are typical of sedimentary sequences deposited along glaciated continental margins throughout Earth history, without the need for global synchroneity or necessarily severe climatic excursions. Although recurrent very widespread glaciation is envisaged in the Neoproterozoic, the presence of analogous glacigenic and interglacial successions in the Neoproterozoic and Cenozoic suggest the operation of a similar set of processes across a similar range of depositional environments. Consequently, an unambiguous sedimentary record of hydrological shutdown during a prolonged global glaciation appears to be lacking. This indicates either a preservational bias in Neoproterozoic successions of the advance and recessional stages of glacial epochs, or the occurrence of dynamic, non-global glaciations during the Neoproterozoic.

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Freezing-rate effects on the physical characteristics of basal ice formed by net adfreezing

Cited by 17 publications

References 37 publications

Debris entrainment and landform genesis during tidewater glacier surges

Debris entrainment and landform genesis during tidewater glacier surges

Ice and firn heterogeneity within Larsen C Ice Shelf from borehole optical televiewing

Neoproterozoic Glaciated Basins: A Critical Review of the Snowball Earth Hypothesis by Comparison with Phanerozoic Glaciations

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