Peter Knight's Research |
Selected Extracts from Publications
THE BASAL ICE LAYER OF GLACIERS AND ICE SHEETS.
Knight, P.G. (1997) Quaternary Science Reviews, Vol. 16, 975-993. (Elsevier Science Ltd.)
Abstract In many glaciers and ice sheets there is a basal ice layer (BIL)
in which the ice is conditioned primarily by processes operating at the
bed. The BIL is chemically and physically distinctive, and is characterised
by a component of basally derived sediment. The BIL is: a rheological control
on ice-sheet dynamics; an indicator of subglacial conditions and
processes; an agent of subglacial geologic processes; the source of a substantial
proportion of glacial sediments; a limit to the downward extension of the
climate record from deep ice cores. If debris characteristics of the BIL
are preserved in glacial sediments, former glacier conditions can be inferred.
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EXPERIMENTAL OBSERVATIONS OF SUBGLACIAL DEBRIS ENTRAINMENT
INTO THE VEIN NETWORK OF POLYCRYSTALLINE ICE
Knight, P.G. and Knight, D.A. (1999) GLACIAL GEOLOGY AND GEOMORPHOLOGY http://boris.qub.ac.uk/ggg/papers/full/1999/rp051999/rp05.html Abstract Theoretical models have suggested that subglacial water can enter
basal ice along crystal boundaries. This has previously been proposed as
a mechanism for entraining debris into the base of ice sheets without entraining
layers of frozen meltwater. We describe two sets of laboratory experiments
designed to establish whether the hypothesis that sediment entrainment
into polycrystalline ice could occur along crystal boundaries is mechanically
realistic. One set of experiments explores the response of basal sediment
to the vertical passage of thermal fronts. A second set explores the response
of basal sediment to the imposition of different pressure gradients through
the ice.
KEYWORDS: subglacial debris entrainment; polycrystalline ice; intercrystal
vein network; glacier basal ice.
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GLACIER SLIDING, REGELATION WATER FLOW, AND DEVELOPMENT
OF BASAL ICE
Knight, P.G. and Knight D.A. (1994) Journal of Glaciology 40 (136), 600-601. Extracts Recent theoretical modelling by Lliboutry (1993) has suggested that
standard models of glacier sliding by regelation are flawed. Lliboutry's
analysis predicts that melting of ice in response to flow obstruction by
bed protruberances will occur in a layer of thickness h(w), through which
water will be mobile in the vein capillary network....
Lliboutry has called for field observation of h(w), to constrain his model,
but has recognised problems in identifying h(w) in the field...
We suggest that observations of the dispersed facies of the basal layer,
which is accessible at some glacier margins, could provide the modelling
constaint that Lliboutry has called for and a test of his theory....
We have carried out a series of experiments in the low-temperature laboratory
that demonstrate both debris transport through the vein network of polycrystalline
ice and entrainment of debris into the vein network from beneath the ice
in response to a pressure gradient.... If Lliboutry's zone h(w) does
exist, then the dispersed facies may be a visible consequence of it.
....estimates of h(w) should be possible on the basis of field observations
at ice-margin sites and from deep cores.
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DEBRIS STRUCTURES IN THE BASAL ICE EXPOSED AT THE
MARGIN OF THE GREENLAND ICE SHEET.
Knight, P.G. (1995) Boreas 24, 11-12.
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TWO-FACIES INTERPRETATION OF THE BASAL ICE LAYER
OF THE GREENLAND ICE SHEET CONTRIBUTES TO A UNIFIED MODEL OF BASAL ICE
FORMATION
Knight, P.G. (1994) Geology, Vol. 22, pp.971-974 Abstract Two exposures of basal ice in Alaska and Greenland, which have previously provided the basis for contrasting models of basal-ice development, are in fact directly comparable. Reinterpretation of previous field descriptions, combined with new structural and sedimentological data from West Greenland, facilitates a simple unification of previous models of basal-ice development and a common terminology for the field description of basal-ice sequences. The stratigraphy of the basal-ice sequence at any site indicates the subglacial conditions and processes operating up-glacier of that site. Presented here are interpretations of the major sequence types.
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STACKING OF BASAL DEBRIS LAYERS WITHOUT BULK FREEZING-ON:
ISOTOPIC EVIDENCE FROM WEST GREENLAND.
Knight, P.G. (1989) Journal of Glaciology Vol.35 No.120, pp. 214-216. Abstract This paper tests and falsifies the theory that the development of thick sequences of vertically stacked clean and debris-laden layers at the margin of the Greenland ice sheet can be attributed solely to simple freezing-on of material at the bed. Isotopic analysis in oxygen-18 and deuterium of ice from the ice-sheet margin near Sondre Stromfjord, indicates that the debris-rich and debris-poor elements of the basal sequence have different origins. While the debris bands display isotopic fractionation consistent with a freezing origin, the intercalated clean ice layers do not. The clean ice layers have isotopic values indistinguishable from debris-bearing ice immediately above the debris-band sequence and from unaltered glacier ice, and are entrained by a different process from the debris bands. Debris may be entrained by freezing at the bed, but the development of a vertically stacked sequence of debris bands must be attributed to some other mechanism.
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OBSERVATIONS AT THE EDGE OF THE GREENLAND ICE SHEET:
BOUNDARY CONDITION IMPLICATIONS FOR MODELLERS.
Knight, P.G. (1987) In: Waddington, E.D. and Walder, J.S. "The Physical basis of Ice Sheet Modelling." International Association of Hydrological Sciences Publication 170, pp.359-366. Abstract Basal ice sequences at the margin of the Greenland ice sheet reflect both marginal and interior basal processes, and have important boundary condition implications for modelling. Analysis of stable isotopes (deuterium, oxygen-18) and debris from the margin indicates two separate zones of debris entrainment beneath the ice; a zone of bulk adfreezing near to the margin, and a zone of warm-bed regelation in the interior. Surface strain measurements indicate two predominant marginal flow regimes. Widespread compressive flow is accentuated during the winter by basal adfreezing, with major folding extending up to 1km from the margin, and intense deformation in the extreme marginal zone giving rise to thick stacking of both marginally and interior derived basal layers. By contrast, where rapid streaming flow reaches the margin, extending flow predominates, and bottom melting removes the lower layers of the interior derived regelation ice. |
PROGRESS REPORT: GLACIERS
Knight, P.G. (1998) Progress in Physical Geography 22 (3), 407-411 Section Headings
The Quaternary and geomorphology Extra-terrestrial glaciers Modelling Siege glaciology The future Although the study of glaciers draws expertise from a wide range of disciplines, the number of researchers who would explicitly call themselves glaciologists is relatively small. The International Glaciological Society, which is the principal professional organisation for glaciologists, has a membership of well under 1000. Hughes (1985, p.39) suggested that “glaciology is a small profession”, and in numerical terms he is right. In practical terms, however, the boundaries of glaciology are more difficult to delimit. It is a discipline with tentacles that intertwine with those of many others, and the cultivation of these links has been a prominent feature of recent developments in the broad field of glacier-related studies. A number of new initiatives and publications have seen collaboration between glaciologists and colleagues from neighbouring disciplines, and some of these have involved the continuing expansion of the realm of glaciology beyond terrestrial confines. This review will focus on some areas of recent and future cross-discipline co-operation and dialogue. A major part of modern research effort is dedicated to modelling glacier behaviour. Models are quintessentially collaborative constructions, derived from the integration of diverse elements. A great ice-sheet model would be like a symphony, drawing together into a unified picture the disparate themes of the glacier story. Thermal regime, hydrology, substrate rheology, ice deformation, mass balance, geomorphology, and glacier dynamics combine transparently into a single system. Creating an ice-sheet model is like translating an epic natural poem into something understandable, and usable, at a human scale. Modelling either of whole ice sheets or of components of the glacier system aims to facilitate predictions and reconstructions of glacier behaviour. Adequate modelling for these purposes requires some understanding of the processes and parameters that control glacier behaviour. Developments in modelling procedures need to progress hand in hand with observational glaciological data. Modelling, like remote sensing, needs ground-truth data. Models, like engines, require fuel, and the fuel of a good model is good base-line data. A striking feature of modern glaciology is the tendency for groups of collaborators to focus their attentions over prolonged periods at individual sites. A part of this trend in modern glaciological research programmes has been the ascendancy of what can be called “Siege Glaciology”. This style of research involves comprehensive, long-term, data acquisition on a range of topics at a specific site. Base camps are established, teams of researchers are co-ordinated over periods of several years to handle different aspects of the multi-analytical scientific and technical programmes, and long-term funding and management strategies are employed. In modern glaciology it is a style of research that can be attributed in part to the logistical requirements of collaborative, technology-based field research. Once a logistical and administrative infrastructure for research at a particular site is established, the use of the site becomes self-reinforcing. Once a glacier has been well studied, the data that has been collected there serves as a framework to encourage further study. Furthermore, there is a preference on the part of many glaciologists for multi-parameter research. Because so many aspects of the glacier system are closely interlinked, efforts to understand one part of the system are increasingly deemed to require data from other parts of the system. No self-respecting program of study into the mechanisms of basal motion could proceed without an array of boreholes to monitor basal water pressure, or ploughmeters to test the substrate, or equipment to monitor glaci-seismic activity. Hence major research programs grow up to involve large groups of researchers around a relatively small number of glaciers. There are many examples of this. Reading through the literature of recent years one quickly becomes familiar with a mere handful of glaciers: Storglaciären, Haut Glacier d’Arolla, Variegated Glacier, Jakobshavn Isbrae. In the ice-sheet literature one quickly gets to know the Siple Coast ice streams, the GRIP and GISP2 cores, Vostok, and a few other sites. Each of these, and a small number of others, have huge lists of publications attaching to them, while a huge number of other glaciers or ice-sheet locations are more or less unmentioned in the literature. Research at Haut Glacier d’Arolla, Switzerland, for example, has been generating research publications at an average rate of about one per month throughout the present decade. Siege programmes now constitute a substantial proportion of the research being carried out on glaciers. Glaciology has yet to take full advantage of many possible avenues of
interdisciplinary collaboration. There have been substantial contributions
to glaciology from physicists, chemists and mathematicians, and there has
been some interdisciplinary dialogue in fields such as fracture mechanics
and creep (e.g. Lliboutry, 1987), but much potentially valuable interdisciplinary
collaboration in the fields of material sciences has yet to be explored.
Most of the glaciologists who study glaciers in the field have little contact
with ceramicists, metallurgists or others who study equivalent phenomena
in other materials. Cross-boundary collaborations on physical and chemical
phenomena and in mathematical or technical procedures have not widely
translated to the scale of whole glaciers or whole physical systems. The
sort of collaboration that is possible has been demonstrated recently by
collaboration between glaciologists, hydrologists and chemists in glacier
hydrology and in ice-core analysis through the application of chemical
principals to the study of glaciers. There are further clues to glacier
behaviour hidden in the physical characteristics of the ice that are not
yet being adequately interpreted. Crystallographers and structural glaciologists
will in future throw as much new light on glacier behaviour as chemists
have done in recent years.
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Glacier advance, ice-marginal lakes and routing of
meltwater and sediment: Russell Glacier, Greenland.
Knight, P.G., Waller, R.I., Patterson, C.J., Jones, A.P. and Robinson, Z.P. (2000) JOURNAL OF GLACIOLOGY 46 (154), 423-426. Abstract The ice-sheet margin at Russell Glacier, West Greenland, advanced ~7
m/a between 1968 and 1999. As the ice advanced over moraine ridges, small
changes in position caused major changes in the routing of proglacial water
and sediemnt. These included changes in distribution of ice-marginal lakes,
in the periodic drainage of ice-marginal lakes, in the routing and sediment
content of water draining into the proglacial zone, and in the release
of sediment from the moraines by erosion and mass movements. Proglacial
hydrology and sediment flux appear to be controlled not simply by glacier
mass balance, but by evolving ice-marginal geomorphology, which must be
accounted for in palaeoenvironmental interpretation of proglacial sediments.
Preservation of basal-ice sediment texture in ice-sheet moraines Peter G. Knight, Carrie J. Patterson, Richard I. Waller, Alison P. Jones,
QUATERNARY SCIENCE REVIEWS 19 (2000) 1255-1258 Abstract Ice-sheet moraines near Kangerlussuaq in west Greenland inherit distinctive
particle-size distributions from basal ice, although debris structures
from the basal ice are commonly destroyed by deposition and resedimentation
processes. The abundance of clay and silt in the dispersed facies
basal ice at the ice-sheet margin is clearly reflected in the sedimentology
of the ice-sheet moraine. Geographical variations in the texture or grain
size of moraine sediments may thus reflect geographical variations in basal
ice. This offers a new approach to reconstructing the basal-ice characteristics,
and hence the thermal and dynamic properties, of former ice sheets.
Changes in sediment routing as a consequence of ice-sheet advance, Russell Glacier, Greenland Knight, P. G., Patterson, C. J., and Waller, R. I. Eos Trans. AGU, 82 (47), Fall Meet. Suppl., Abstract (2001)
An advancing ice margin is an extremely complex sedimentary environment.
As the position of the ice margin changes through time, debris sources,
sediment transfer routes and depositional environments can vary rapidly.
The position of an advancing ice-sheet margin with respect to its ancient
marginal moraines controls the way sediment is released from the basal
ice as well as where and how long it is stored. On the basis of our observations
at the edge of the Greenland ice sheet we have identified six stages in
the advance of the glacier that have distinctive sedimentary processes.
For each stage we describe: (1) debris release, routing, and deposition
close to the margin; and (2) processes of erosion, mass movement and deposition
that affect the length of time that the sediment is stored in the ice-proximal
environment. The progress of the ice across the moraine causes changes
in ice-proximal processes that are recorded in the sedimentary record,
and we find that the overtopping of moraine ridges has significant consequences
for both proximal and distal proglacial environments. Sediment production
at this land-based section of the Greenland ice-sheet margin is dominated
by debris released through the basal ice layer, which is up to 30 m thick.
The debris flux through the basal ice is 21.6 m3m-1yr-1
or approximately 75.6 x 103 kg m-1yr-1.
Only negligible amounts are released through englacial, supraglacial or
subglacial transfer. Glaciofluvial sediment production is highly localized
and long sections of the ice margin receive no sediment from glaciofluvial
sources.
Knight, P.G., Waller, R.I., Patterson, C.J., Jones, A.P. and Robinson,
Z.P. (2002)
ABSTRACT. Sediment production at a terrestrial section of the
ice-sheet margin in West Greenland is dominated by debris released through
the basal ice layer. The debris flux through the basal ice at the margin
is estimated to be 12-45 m3 m-1 a-1. This is three orders of magnitude
higher than that previously reported for East Antarctica, an order of magnitude
hugher than sites reported from Norway, Iceland and Switzerland, but an
order of magnitude lower than values previously reported from tidewater
glaciers in Alaska and other high-rate environments such as surging glaciers.
At our site, only negligible amounts of debris are released through englacial,
supraglacial or subglacial sediment transfer. Glaciofluvial sediment production
is highly localised, and long sections of the ice-sheet margin receive
no sediment from glaciofluvial sources. These findings differ from those
of studies at more temperate glacial settings where glaciofluvial routes
are dominant and basal ice contributes only a minor percentage of the debris
released at the margin. These data on debris flux through the terrestrial
margin of an outlet glacier contribute to our limited knowledge of debris
production from the Greenland ice sheet.
Knight, P.G. and
Knight, D.A. (2004)
Invited contribution to AMICS (Antarctic ice-sheet dynamics and climatic change: Modelling and Ice Composition Studies) workshop Dynamic Interaction between the Antarctic Ice Sheet and the Subglacial Environment, Vrije Universiteit Brussel, Brussels, April 2004. Sponsored by the Belgian Federal Science Policy Office (BELSPO). See Powerpoint Slides from presentation here (2MB .pdf file) ABSTRACT:
Interdisciplinary Science Reviews, December 2004, vol. 29, no. 4, pp. 385-393(9) Abstract: Glaciers play a central role in the global environmental
system, and their behaviour is intimately linked to changing patterns of
the ocean–atmosphere circulation, climate, sea level and landscape. Deep
cores retrieved from ice sheets have helped us recognise how climate has
changed over hundreds of thousands of years, and, for many parts of the
Earth's surface, an understanding of landforms, drainage patterns and surface
geology would be impossible without an understanding of glacial processes.
Today, the role of glaciers in the media and in the popular imagination
is dominated by their role in science as indicators of environmental change.
However, it is barely a hundred and fifty years since the significance
of glaciers in this context was first appreciated. Before the middle of
the nineteenth century glaciers occupied a different niche in popular perception,
ruled by their place in the artistic and cultural domain rather than the
scientific. Changing technologies for observing and analysing glacial phenomena
have impacted on both scientific and cultural perceptions of glaciers,
and our understanding is still constrained by technological limitations.
Despite progress in remote sensing and analytical techniques, our reconstructions
of past glaciations remain tentative, our understanding of modern glacial
processes incomplete and our modelling of their future unreliable. In nineteenth
century art, glaciers represented romance, mystery and unassailable majesty.
In twenty-first century science their position is perhaps similar, but
what art calls 'mystery', science calls 'uncertainty'.
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