Peter G. Knight

The basal ice layer of glaciers and ice sheets

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. Analysis of stable isotopes, gas and cation composition, ice crystallography, debris composition and structural glaciology has elucidated many mechanisms for BIL formation. Common elements can be identified from a range of locations. Key issues are entrainment of ice and debris from the bed, formation of new ice, metamorphism of existing ice in the BIL, and thickening of the BIL by deformation. A distinction can be made between basal ice formed by accretion of material from the bed (‘stratified facies’) and basal ice formed by metamorphic processes within the ice close to the bed (‘dispersed facies’).

Glacier science and environmental change.

Glacier Science and Environmental Change is an authoritative and comprehensive reference work on contemporary issues in glaciology. It explores the interface between glacier science and environmental change, in the past, present, and future. Written by the worlds foremost authorities in the subject and researchers at the scientific frontier where conventional wisdom of approach comes face to face with unsolved problems, this book provides: state-of-the-art reviews of the key topics in glaciology and related disciplines in environmental change cutting-edge case studies of the latest research an interdisciplinary synthesis of the issues that draw together the research efforts of glaciologists and scientists from other areas such as geologists, hydrologists, and climatologists color-plate section (with selected extra figures provided in color at www.blackwellpublishing.com/knight ). The topics in this book have been carefully chosen to reflect current priorities in research, the interdisciplinary nature of the subject, and the developing relationship between glaciology and studies of environmental change. Glacier Science and Environmental Change is essential reading for advanced undergraduates, postgraduate research students, and professional researchers in glaciology, geology, geography, geophysics, climatology, and related disciplines.

Two-facies interpretation of the basal layer of the Greenland ice sheet contributes to a unified model of basal ice formation

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.

Stacking of basal debris layers without bulk freezing-on: isotopic evidence from West Greenland

This paper tests and falsifies the theory that the development of thick sequences of vertically stacked clean and debris-laden ice 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 50 and 5180 of ice from the ice-sheet margin near S0ndre Stmmfjord 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 debrisbearing 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.

Ice Flow around large obstacles as indicated by basal ice exposed at the margin of the Greenland Ice Sheet

Spatial variations in the debris-bearing basal ice layer exposed at the ice-sheet margin in West Greenland reflect the geography of basal melting and ice flow around large obstacles close to the margin. This paper demonstrates the character of the basal ice layer, which comprises fine material incorporated in an interior, subglacial environment and coarser material entrained in an ice-marginal environment. We develop an empirical model of ice flow close to a lobate margin of the ice sheet in which ice convergence and divergence, and limited subglacial melting affect the character and distribution of the basal ice at the margin. There is a tendency for the convergence and divergence to thicken the basal layer in lobate areas and to thin it in inter-lobate areas. Under certain circumstances, basal melting may remove much of the layer from beneath the snouts of larger lobes, thus causing the basal layer to be thickest in an intermediate location.

Discharge of debris from ice at the margin of the Greenland ice sheet

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 m 3 m −1 a −1 . This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in 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 localized, 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.

Glaciohydraulic supercooling: the process and its significance

Glaciohydraulic supercooling is a process that allows water at the base of a glacier to remain liquid at a temperature below its freezing point in response to the geometry of water flow and subglacial pressure. Supercooling, and subsequent freezing, of subglacial water has implications for glacier dynamics, sediment transfer and landform evolution, and an understanding of the process is important both for understanding modern glacial environments and for reconstructing glacial environments of the past. However, recent research on glaciohydraulic supercooling has raised controversy both about the significance of the process and about the way in which it has been applied within the discipline. In this paper, we review recent work on supercooling in glaciers, assess its significance to glaciology, geomorphology and Quaternary science, and identify key issues requiring further research in order to resolve some of the controversy surrounding the topic. We suggest that, while glaciohydraulic supercooling is a very significant process, its adoption as an explanation of some phenomena has been premature, and that further research is required to test its true significance both in modern settings and in the glacial geologic record.

Preservation of basal-ice sediment texture in ice-sheet moraines

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 faciesa basal ice at the ice-sheet margin is clearly re#ected in the sedimentology of the ice-sheet moraine. Geographical variations in the texture or grain size of moraine sediments may thus re#ect geographical variations in basal ice. This o!ers a new approach to reconstructing the basal-ice characteristics, and hence the thermal and dynamic properties, of former ice sheets. ( 2000 Elsevier Science Ltd. All rights reserved.

Identification of basal layer debris in ice-marginal moraines, Russell Glacier, West Greenland

Different processes of ice formation and deformation at the glacier bed create distinctive basal ice facies. The geographical distribution of these facies at the glacier margin can indicate the distribution of subglacial environments. Previous research has shown that debris in dispersed facies basal ice at the margin of the Greenland ice sheet has a distinctive particle-size distribution that can also be recognised in moraines deposited where the dispersed facies is present. This permits reconstruction of the distribution of basal ice characteristics and subglacial conditions from moraine sediments. Here, we study stratified and debris-band facies of the basal ice layer at the same site. We find that, contrary to previous suggestions, these two facies can also be distinguished sedimentologically. Co-variant plots of the C40 (aggregate shape) and RA (aggregate roundness) indices show that pebble and cobble sized clasts (−2φ to −7φ) in the debris bands are statistically more angular than those in the stratified facies. Sedimentological characteristics of the parent facies are retained in ice-marginal moraines. Combined with previous work, these new observations suggest that the geographical distribution of basal ice facies at a glacier margin can be reconstructed from sedimentological characteristics preserved in moraines.

The concept of transport capacity in geomorphology

The notion of sediment-transport capacity has been engrained in geomorphological and related literature for over 50 years, although its earliest roots date back explicitly to Gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering. Despite cross fertilization between different process domains, there seem to have been independent inventions of the idea in aeolian geomorphology by Bagnold in the 1930s and in hillslope studies by Ellison in the 1940s. Here we review the invention and development of the idea of transport capacity in the fluvial, aeolian, coastal, hillslope, debris flow, and glacial process domains. As these various developments have occurred, different definitions have been used, which makes it both a difficult concept to test, and one that may lead to poor communications between those working in different domains of geomorphology. We argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons. First, as sediment becomes entrained in a flow, the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment. Secondly, environmental sediment transport is complicated, and the range of processes involved in most movements means that simple relationships are unlikely to hold, not least because the movement of sediment often changes the substrate, which in turn affects the flow conditions. Thirdly, the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space. Consequently, new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems.