Quaternary Science Reviews 26 (2007) 585–597 The subglacial thermal organisation (STO) of ice sheets Johan Kleman a,Ã , Neil F. Glasser b a Department of Physical Geography and Quaternary Geology, Stockholm University, S-10691, Stockholm, Sweden b Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, Wales, UK Received 27 April 2006; accepted 19 December 2006 Abstract This paper examines ice-sheet wide variations in subglacial thermal regime and ice dynamics using the landform record exposed on the beds of former mid-latitude ice sheets (the Laurentide, Cordilleran, Fennoscandian and British-Irish Ice Sheets). We compare the landform patterns beneath these former ice sheets to the flow organisation beneath parts of the contemporary Antarctic Ice Sheet inferred from RADARSAT-1 Antarctic Mapping Project (RAMP) data. The evidence preserved in the landform record and observed on contemporary ice masses can be grouped into four major ice-dynamical components that collectively define the subglacial thermal organisation (STO) of ice sheets. These ice-dynamical components are frozen-bed patches, ice streams, ice-stream tributaries and lateral shear zones. Frozen-bed patches appear at a wide range of spatial scales, spanning four orders of magnitude. In some areas, frozen-bed zones comprise large proportions of the bed (e.g. near the ice divide in continental areas), whilst in other areas they constitute isolated ‘‘islands’’ in areas dominated by thawed-bed conditions. Ice streams, narrow zones of fast flow in ice sheets that are otherwise dominated by slow sheet flow, are also common features of Quaternary ice sheets. Tributaries to ice streams flow at velocities intermediate between full ice-stream and sheet flow, and may divert ice drainage from one primary ice-stream corridor to an adjacent one. Sharp lateral boundaries between landforms indicate sliding and non-sliding conditions, respectively. These lateral boundaries represent important discontinuities in the glacial landscape and mark the location of shear zones between thawed-bed ice streams and intervening frozen-bed areas. We use the landform evidence in the area around Great Bear Lake, Canada to trace the evolution of an ice-stream web through time, demonstrating that frozen-bed patches are integral components of this complex system. We conclude that frozen-bed patches are important for the stability of ice sheets because they laterally constrain and isolate peripheral drainage basins and their ice streams. r 2007 Elsevier Ltd. All rights reserved. 1. Introduction The aim of this paper is to examine ice-sheet wide variations in subglacial thermal regime and ice dynamics in order to produce a conceptual model of the subglacial thermal organisation (STO) of ice sheets. Ice sheets are the most dynamic components of the Earth’s topography, growing and shrinking in response to global climatic changes. During Quaternary full-glacial conditions, the Greenland and Antarctic Ice Sheets expanded, but the largest changes occurred in the mid-latitudes, where major ice sheets grew and decayed (e.g. Boulton and Clark, 1990; Kleman et al., 1997; Boulton et al., 2001). Key issues concerning these ice sheets were their height, because it impacted on atmospheric circulation; their volume, because it impacted on global sea level; and their potential for unstable behaviour, because such behaviour may have been a driver for climate change (Hulbe et al., 2004). All three issues are strongly linked to the spatial organisation of basal thermal zones or STO, and in particular the proportions of frozen and thawed bed. Unstable behaviour can only be initiated in thawed- bed areas under ice sheets because a frozen basal interface inhibits the rapid-flow mechanisms of sliding and subglacial sediment deformation (Alley et al., 1986, 1987; Iverson et al., 1995; Christoffersen and Tulaczyk, 2003). Ice-sheet STO is therefore critical to palaeoglacio- logy, and in particular to understanding the operation of palaeo-ice sheets and their interaction with the climate system. Previous studies of the beds of former ice sheets have either used the landform record to infer glaciological process (e.g. Stokes and Clark, 2002a; Roberts and Long, ARTICLE IN PRESS 0277-3791/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2006.12.010 Ã Corresponding author. E-mail address: johan.kleman@natgeo.su.se (J. Kleman).