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Broken ice, Kinder Downfall, Derbyshire, UK

Investigating the effects of glacial shearing of sediment on luminescence


 

Posted by Darrel A. Swift on 4 January 2012

Bateman, M.D., Swift, D.A., Piotrowski, J.A. and Sanderson, D.C.W. in press: Investigating the effects of glacial shearing of sediment on luminescence. Quaternary Geochronology.

Previously reported low luminescence of basal glacial sediment has raised the possibility that processes operating at the ice-bedrock interface have the potential to reset (or ‘bleach’) natural luminescence signals (Swift et al., 2011). This finding indicates that certain types of glacial sediment (for example, subglacial diamicts) might be amenable to dating using luminescence-based techniques. Using a purpose-built ring-shear apparatus situated in a light-controlled environment, we have investigated the potential for mineral grains to be reset when subjected to conditions similar to those experienced by sediment that has undergone transport at the ice-bedrock interface.  Reported here are the preliminary results of an initial experiment that used medium quartz sand with a naturally-acquired palaeodose of ~4.3 Gy that had been obtained from a relict dune system. Incremental sampling during the shearing experiment and measurements were made to track changes in the luminescence properties of the sand as strain/shearing increased. The results indicate that increased strain/shearing resulted in an increase in the number of zero-dose grains and evolution of the De distribution from unimodal to multimodal. In light of the very much longer shearing distances that subglacial sediment would endure in nature, these results would appear to suggest that geomechanical processes at the ice-bed interface of glaciers and ice sheets may be a viable mechanism for ‘bleaching’ sediment.

Reference: Swift, D.A., Sanderson, D.C.W., Nienow, P.W., Bingham, R.B., Cochrane, I.C., 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d’Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas 40: 446–458.

DOI: 10.1016/j.quageo.2011.11.012

Origin and significance of ‘dispersed facies’ basal ice: Svinafellsjokull, Iceland


 

Posted by Darrel A. Swift on 27 July 2011

Cook, S.J., Swift, D.A., Graham, D.J. & Midgley, N.G. 2011: Origin and significance of ‘dispersed facies’ basal ice: Svinafellsjokull, Iceland. Journal of Glaciology 57: 710-720.

Dispersed facies basal ice – massive (i.e. structureless) ice with dispersed debris aggregates – is present at the margins of many glaciers and, as a product of internal glacial processes, has the potential to provide important information about the mechanisms of glacier flow and the nature of the subglacial environment. The origin of dispersed facies is poorly understood, with several hypotheses having been advanced for its formation, and there is disagreement as to whether it is largely a sedimentary or a tectonic feature. We test these established hypotheses at the temperate glacier Svinafellsjokull, Iceland, and find that none fully account for dispersed facies characteristics at this location. Instead, dispersed facies physical, sedimentological and stable-isotope (δ18O, δD) character- istics favour a predominantly tectonic origin that we suggest comprises the regelation and strain- induced metamorphism of debris-rich basal ice that has been entrained into an englacial position by tectonic processes operating at the base of an icefall. Further thickening of the resultant dispersed facies may also occur tectonically as a result of ice flow against the reverse bed slope of a terminal overdeepening. Lack of efficient subglacial drainage in the region of the overdeepening may limit basal melting and thus favour basal ice preservation, including the preservation of dispersed facies. Despite the relatively low sediment content of dispersed facies (~1.6% by volume), its thickness (up to 25 m) and ubiquity at Svınafellsjokull results in a significant contribution to annual sediment discharge (1635–3270 m3 a–1) that is ~6.5 times that contributed by debris-rich stratified facies basal ice.

DOI: 10.3189/002214311797409703

Anomalous luminescence of subglacial sediment at Haut Glacier d’Arolla, Switzerland: a consequence of resetting at the glacier bed?


 

Posted by Darrel A. Swift on 14 July 2011

Swift, D.A., Sanderson, D.C.W., Nienow, P.W., Bingham, R.G. & Cochrane, I.C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d’Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas 40: 446–458.

Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d’Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample-group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence-based techniques.

DOI: 10.1111/j.1502-3885.2010.00196.x

Sedimentary signatures of basal ice formation and their preservation in ice-marginal sediments


 

Posted by Darrel A. Swift on 1 February 2011

Cook, S.J., Graham, D.J., Swift, D.A., Midgley, N.G. & Adam, W.G. 2010: Sedimentary signatures of basal ice formation and their preservation in ice-marginal sediments. Geomorphology 125: 122-131.

Reconstruction of subglacial processes from sedimentological evidence is central to our understanding of glaciological conditions beneath former ice masses. At Svínafellsjökull, southeast Iceland, we assess the extent to which two different processes of basal ice formation (regelation and glaciohydraulic supercooling) can be identified from ice-marginal sediments. Our results indicate that the sedimentary characteristics of deposits produced by these two processes can be distinguished from one another and that it may be possible to recognise evidence of these processes in Quaternary sediments and to reconstruct their spatial pervasiveness. Sediments derived from the melting of regelation basal ice have (i) a massive structure; (ii) a sediment matrix (0 to 10Φ) dominated by coarse sand; and (iii) a higher proportion of angular clasts than supercool basal ice and associated sediments. Sediments derived from “supercool” basal ice (i) can be either massive or layered; (ii) tend to have a silt-dominated matrix; and (iii) contain a slightly higher proportion of rounded and well-rounded clasts than regelation basal ice and sediments. Previous studies indicate that the dominance of silt within supercool basal ice may be unique to this process, and hence, supercooling should leave a readily recognisable signature in the sedimentary record. Our results from Svínafellsjökull lend support to that idea, although we suggest that further work is required to assess whether silt dominance is a process signature diagnostic of supercooling, and in particular, the extent to which subglacial sediment supply determines the sedimentary character of basal ice facies and associated sediments.

DOI: 10.1016/j.geomorph.2010.08.018

INQUA 2011 session on glacial erosion announced:
Glacial overdeepening: processes, forms and significance


 

Posted by Darrel A. Swift on 18 October 2010

XVIII INQUA-Congress
Quaternary sciences – the view from the mountains
21-27 July 2011 in Bern, Switzerland

Session: Glacial overdeepening: processes, forms, and significance

Glacially-overdeepened valleys and basins are common features beneath contemporary ice masses and in formerly glaciated mountain belts, their forelands and other formerly glaciated lowland areas. These and their sediments are of large practical and scientific interest, influencing the dynamics and hydrology of ice masses, presenting opportunities for glacial and climate reconstruction, and raising management issues related to aggregate, groundwater and hydrocarbon resources and radioactive waste disposal in deep geological repositories. We solicit a broad range of submissions that include theoretical, numerical, experimental and field studies that advance our understanding of the age, formation, morphology and infilling of overdeepened valleys and basins, as well as their significance for glacial systems and the management and economical and societal importance of formerly glaciated landscapes.

Conveners: Urs H. Fischer, Mads Huuse, Kelly MacGregor, Darrel A. Swift

URL: http://www.inqua2011.ch/?a=programme&subnavi=sessions&id=66

A reassessment of the role of ice sheet glaciation in the long-term evolution of the East Greenland fjord region


 

Posted by Darrel A. Swift on 1 May 2008

Swift, D.A., Persano, C., Stuart, F.M., Gallagher, K. & Whitham, A. 2008: A reassessment of the role of ice sheet glaciation in the long-term evolution of the East Greenland fjord region. Geomorphology 97 – Glacial Landscape Evolution – Implications for Glacial Processes, Patterns and Reconstructions: 109-125.

The long-term evolution of the East Greenland fjord region is investigated using geomorphological and stratigraphical evidence to: (1) assess the nature of landscape modification caused by late Cenozoic ice sheet glaciation; and (2) relate patterns of glacial landscape modification to first-order (i.e. continent-margin scale) topography and geology. Geomorphological and stratigraphical evidence demonstrates evolution of the first-order topography and incision of at least part of the present first-order fjord system by ~55 Ma. This hypothesis is tested using apatite (U–Th)/He ages for samples from two bedrock profiles near Kong Oscar Fjord. The thermochronology supports landscape evolution before 55 Ma, followed by relative tectonic stability, because it indicates rapid denudation around the time of rifting that occurred prior to continental breakup (i.e. between ~75 and 55 Ma). The nature of landscape modification caused by late Cenozoic glacial erosion appears to have been controlled by first-order topography and geology, with selective ice sheet erosion in areas of high-elevation Caledonian basement and apparently little glacial erosion of low-elevation Mesozoic sedimentary strata. Nevertheless, fjord morphometry demonstrates systematic evolution of the first-order fjord system from confined and overdeepened fjords in Caledonian basement to wider and disproportionately larger fjords in Mesozoic strata. The latter indicates that changes in lithological strength enabled the development of more efficient fjord morphology under full glacial conditions that may have promoted fast ice flow.

DOI: 10.1016/j.geomorph.2007.02.048

Transverse englacial debris-rich ice bands at Kviarjokull, southeast Iceland


 

Posted by Darrel A. Swift on 15 July 2006

Swift, D.A., Evans, D.J.A. & Fallick, A.E. 2006: Transverse englacial debris-rich ice bands at Kviarjokull, southeast Iceland. Quaternary Science Reviews 25: 1708-1718.

Thick exposures of debris-rich ice at various Icelandic glaciers are central to the debate over the prevalence of glacial sediment transfer by glaciohydraulic supercooling. We present physical analyses of ice and debris at Kvíárjökull, a temperate glacier in southeast Iceland with a terminal glacier-bed overdeepening, where stratified debris-rich ice forms up to metre-thick transverse englacial bands. Our results are not consistent with debris-rich ice formation predominantly by supercooling because: (1) 137-Cs was absent from sediment filtered from debris-rich ice; (2) isotopic analysis (δD and δ18O) demonstrated no clear pattern of isotopic enrichment of debris-rich ice with respect to englacial ice; and (3) melt-out debris from debris-rich ice included large striated clasts from both fluvial and basal sources. We support transverse englacial debris-rich ice band formation by the thickening and elevation of basal materials in a region of longitudinally compressive ice flow situated between the reverse slope of the overdeepening and the base of an ice fall. Debris band form and distribution are likely to be controlled by thrusting along transverse englacial foliae associated with the formation of band ogives on the glacier surface.

DOI: doi:10.1016/j.quascirev.2006.01.003

Basal sediment evacuation by subglacial meltwater: suspended sediment transport from Haut Glacier d’Arolla, Switzerland


 

Posted by Darrel A. Swift on 27 July 2005

Swift, D.A., Nienow, P.W. & Hoey, T.B. 2005: Basal sediment evacuation by subglacial meltwater: suspended sediment transport from Haut Glacier d’Arolla, Switzerland. Earth Surface Processes and Landforms 30: 867–883.

Proglacial suspended sediment transport was monitored at Haut Glacier d’Arolla, Switzerland, during the 1998 melt season to investigate the mechanisms of basal sediment evacuation by subglacial meltwater. Sub-seasonal changes in relationships between suspended sediment transport and discharge demonstrate that the structure and hydraulics of the subglacial drainage system critically influenced how basal sediment was accessed and entrained. Under hydraulically inefficient subglacial drainage at the start of the melt season, sediment availability was generally high but sediment transport increased relatively slowly with discharge. Later in the melt season, sediment transport increased more rapidly with discharge as subglacial meltwater became confined to a spatially limited network of channels following removal of the seasonal snowpack from the ablation area. Flow capacity is inferred to have increased more rapidly with discharge within subglacial channels because rapid changes in discharge during highly peaked diurnal runoff cycles are likely to have been accommodated largely by changes in flow velocity. Basal sediment availability declined during channelization but increased throughout the remainder of the monitored period, resulting in very efficient basal sediment evacuation over the peak of the melt season. Increased basal sediment availability during the summer appears to have been linked to high diurnal water pressure variation within subglacial channels inferred from the strong increase in flow velocity with discharge. Basal sediment availability therefore appears likely to have been increased by (1) enhanced local ice-bed separation leading to extra-channel flow excursions and[sol ]or (2) the deformation of basal sediment towards low-pressure channels due to a strong diurnally reversing hydraulic gradient between channels and areas of hydraulically less-efficient drainage.

DOI: 10.1002/esp.1197

Seasonal evolution of runoff from Haut Glacier d’Arolla, Switzerland and implications for glacial geomorphic processes


 

Posted by Darrel A. Swift on 8 July 2005

Swift, D.A., Nienow, P.W., Hoey, T.B. & Mair, D.W.F. 2005: Seasonal evolution of runoff from Haut Glacier d’Arolla, Switzerland and implications for glacial geomorphic processes. Journal of Hydrology 309: 133-148.

Statistical classification of hydrograph form is used to elucidate the controls on diurnal runoff cycle evolution at Haut Glacier d’Arolla, Switzerland during the 1998 and 1999 melt seasons. Hydrographs are objectively grouped using statistical techniques into four principal types that are qualitatively interpreted as rising, falling, peaked-falling and peaked hydrographs. Peaked hydrographs are further grouped on the basis of the magnitude of their bulk flow, baseflow and diurnal flow components. Comparison with the evolution of meltwater sources and pathways demonstrates that runoff cycles evolve systematically during the melt season in response to removal of the seasonal snowpack from the ablation area. Peaked hydrographs predominate following the onset of snowpack removal and demonstrate an increasing and progressively earlier diurnal peak, but also an unusually low baseflow component that is probably due to surface melt mainly contributing direct to subglacial channels. Runoff cycle evolution has potentially significant geomorphic implications because peaked surface runoff cycles result in the formation of hydraulically efficient, channelised subglacial drainage and a significant increase in the gradient of the relationship between suspended sediment transport and discharge. Increasingly peaked diurnal cycles also result in increased basal sediment availability, most likely related to high diurnal water pressure variation within subglacial channels that may also have enhanced rates of basal sliding and hence subglacial erosion. Differences in runoff cycle form and evolution therefore have the potential to significantly influence glacial erosion rates and sediment yields.

DOI: doi:10.1016/j.jhydrol.2004.11.016

Geomorphic implications of subglacial drainage configuration: rates of basal sediment evacuation controlled by seasonal drainage system evolution


 

Posted by Darrel A. Swift on 15 May 2002

Swift, D.A., Nienow, P.W., Spedding, N. & Hoey, T.B. 2002: Geomorphic implications of subglacial drainage configuration: rates of basal sediment evacuation controlled by seasonal drainage system evolution. Sedimentary Geology 149: 5-19.

Although basal sediment evacuation by subglacial meltwater is widely assumed to dominate the sediment budget of most temperate glaciers and ice caps, its potential influence for glacial geomorphic processes and landforms has largely been overlooked in the glacial literature. Data collected at Haut Glacier d’Arolla, Switzerland during the 1998 ablation season are used to test theoretical predictions that subglacial drainage configuration should exert a primary control on rates of basal sediment evacuation by subglacial meltwater. Seasonal evolution of the subglacial drainage system enables observation of sediment evacuation from both distributed and channelised drainage systems. Daily suspended sediment load exhibits an extreme, nonlinear increase with respect to discharge between winter and summer periods, which correspond to periods of predominantly distributed and channelised subglacial drainage, respectively. These results suggest that subglacial drainage configuration should significantly impact upon: (a) rates of subglacial erosion and debris production; (b) the nature of debris transport and the proportion of sediment in glacial versus fluvioglacial transport pathways; and (c) rates and styles of ice-marginal sedimentation. In contrast to their distributed counterparts, channelised drainage configurations are predicted to encourage subglacial erosion through the removal of basal sediment from the ice–bedrock interface and, with the exception of supraglacial transport, will discourage sediment transport by alternative glacial mechanisms. Direct sedimentation at the glacier margin may therefore be reduced, resulting in smaller moraines despite an overall increase in sediment production. An appreciation of the coupling between glacier hydrology and geomorphic processes suggests new frameworks for the understanding of rates and styles of glacial erosion, sediment transfer and deposition, and the complex geomorphology of ice-marginal environments.

DOI: doi:10.1016/S0037-0738(01)00241-X