Chris D. Clark

Palaeo-ice streams

Abstract The location and behaviour of ice streams is one of the most important controls on ice sheet configuration and stability. In order to reconstruct former ice sheets we need to know ice stream location and timing. Once identified, the beds of palaeo-ice streams provide an unprecedented opportunity to glean information about their basal environment, something that remains very difficult under contemporary ice streams. This paper represents the first synthesis and discussion of palaeo-ice stream research from a variety of former ice sheets and includes new insights relating their configuration and activity to the evidence they leave behind. The ambiguous use of the term ‘ice stream’ is addressed and four possible configurations for ice streams are presented. Diagnostic landforms and landform assemblages that we would expect an ice stream to produce are discussed and the temporal context of bedform generation is described as either a ‘rubber stamped’ or ‘smudged’ bedform imprint, resulting from isochronous or time-transgressive landform generation. In particular, we focus on the configuration of terrestrially terminating ice streams, for which there are no modern analogues. Technological advances in marine geophysics are helping to generate a convergence of interest between Antarctic glaciology and palaeo glaciology. It is suggested that investigations around the fringes of contemporary ice sheets, (i.e. West Antarctica) can provide evidence that directly links the geomorphological record of palaeo-ice streams with their contemporary counterparts. In addition, computational advances and modelling adaptations have permitted the incorporation of ice streams within ice sheet models. We argue that data from palaeo-ice stream beds are invaluable to ice sheet/stream modelling experiments and will help us understand ice stream operation and the linkages between climate perturbations and both palaeo and contemporary ice sheets.

Geomorphological criteria for identifying Pleistocene ice streams

Ice streams are critical regulatory mechanisms in contemporary ice sheets. It has been inferred that they also had a significant effect on the dynamics of former ice sheets. Subsequently, many people have invoked their widespread occurrence from a variety of formerly glaciated areas. Hypothesised locations, however, have often outweighed meaningful evidence. This paper addresses the problem, using the characteristics of contemporary ice streams as a basis for their identification from former ice-sheet beds. A convergence of knowledge gained from contemporary ice-stream research, coupled with theories of glacial geomorphology, allows several geomorphological criteria to be identified as suggestive signatures of ice-stream activity. It is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams. The criteria are used to construct conceptual land-system models of the beds of former ice streams, and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based.

Remote sensing techniques for mangrove mapping

Different approaches to the classification of remotely sensed data of mangroves are reviewed, and five different methodologies identified. Landsat TM, SPOT XS and CASI data of mangroves from the Turks and Caicos Islands, were classified using each method. All classifications of SPOT XS data failed to discriminate satisfactorily between mangrove and non-mangrove vegetation. Classification accuracy of CASI data was higher than Landsat TM for all methods, and more mangrove classes could be discriminated. Merging Landsat TM and SPOT XP data improved visual interpretation of images, but did not enhance discrimination of different mangrove categories. The most accurate combination of sensor and image processing method for mapping the mangroves of the eastern Caribbean islands is identified.

A review of remote sensing for the assessment and management of tropical coastal resources

This article reviews applications of remote sensing to the assessment of tropical coastal resources. These applications are discussed in the context of specific management objectives and sensors used. Remote sensing remains the only way to obtain synoptic data for large coastal areas uniformly in time and space, repeatedly and nonintrusively. Routine applications to tropical coastal management include the mapping of littoral and shallow marine habitats, change detection, bathymetry mapping, and the study of suspended sediment plumes and coastal currents. The case studies reviewed suggest that wider use of remote sensing in tropical coastal zone management is limited by (1) factors that affect data availability, such as cloud cover and sensor specification; and (2) the problems that decision makers face in selecting a remote sensing technique suitable to their project objectives. These problems arise from the difficulty in comparing the capabilities of different sensors and the limited amount of published ...

Benefits of water column correction and contextual editing for mapping coral reefs

Classification accuracy of coral reefs can be increased significantly by compensation for light attenuation in the water column and contextual editing to account for generic patterns of reef distribution. Both processes are easily implemented and collectively constitute an accruement in accuracy of 22 per cent for airborne multispectral imagery (CASI) and up to 17 per cent for satellite sensor imagery, for each extra days effort using the technique (up to maximum accuracies of 89 and 73 per cent respectively).

Reconstructing the evolutionary dynamics of former ice sheets using multi-temporal evidence, remote sensing and GIS

Abstract Recent advances have been made in the way in which former ice sheets are reconstructed from the fragmentary geomorphological and geological evidence that they leave behind. The use of satellite images to view large scale glacial geomorphology has revealed cross-cutting patterns that record changing flow configurations through time. These flow changes may arise from migrating ice divides, the switching on and off of ice streams, margin retreat or flow patterns from more than one glaciation. Such multi-temporal evidence permits more detailed reconstruction of the evolution of former ice sheets which contrasts with previous reconstructions of just glacial maxima or deglacial retreat. Increased complexity of ice flow and the realisation that evidence may relate to different time-slices of ice sheet history have led to the use of geographical information systems (GIS) to integrate a wide variety of different lines of evidence, to permit the construction of ice dynamic scenarios. The use of remote sensing and GIS for palaeoglaciological reconstruction is reviewed, and its impact assessed. Recommendations for processing of remotely sensed data (optical and radar) for use in glacial geomorphological mapping are outlined. An overall reconstruction strategy which uses remote sensing to acquire ice flow information and a GIS to integrate it with field evidence is suggested. Reconstructions based on geological and geomorphological evidence are now capable of providing rigorous tests for ice sheet modelling experiments. It is anticipated that future links between modelling and ice sheet wide reconstructions will yield major advances in our understanding of both the physics of ice sheets and the behaviour of former ice sheets.

Digital analysis of multispectral airborne imagery of coral reefs

Abstract The digital airborne sensor, CASI (Compact Airborne Spectrographic Imager) has considerable potential for mapping marine habitats. Here we present an account of one of the first coral reef applications. The CASI was flown over reefs of the Turks and Caicos Islands (British West Indies) and set to view 1 m pixels in 8 spectral bands. In addition, reef habitats were sampled in situ by visual assessment of percent cover in 1 m quadrats. Seagrass standing crop was assessed using a calibrated visual scale. Benthic habitats were classified using hierarchical cluster and similarity percentage analyses of the field survey data. Two levels of habitat discrimination were assessed: a coarse level (corals, algae, sand, seagrass) and a fine level which included nine reef habitats. Overall accuracies of CASI-derived habitat maps were 89% and 81% for coarse and fine levels of habitat discrimination, respectively. Accuracies were greatest once CASI data had been processed to compensate for variations in depth and edited to take account of generic patterns of reef distribution. These overall accuracies were significantly (P<0.001) better than those obtained from satellite imagery of the same site (Landsat MSS, Landsat TM, SPOT XS, SPOT Pan, merged Landsat TM/SPOT Pan). Results from CASI were also significantly better than those from interpretation of 1:10 000 colour aerial photographs of reefs in Anguilla (Sheppard et al. 1995). However, the studies may not have been entirely comparable due to a disparity in the areas mapped.

A groove-ploughing theory for the production of mega-scale glacial lineations, and implications for ice-stream mechanics

Mega-scale glacial lineations (MSGLs) are longitudinally aligned corrugations (ridge-groove structures 6-100 km long) in sediment produced subglacially. They are indicators of fast flow and a common signature of ice-stream beds. We develop a qualitative theory that accounts for their formation, and use numerical modelling, and observations of ice-stream beds to provide supporting evidence. Ice in contact with a rough (scale of 10-10 3 m) bedrock surface will mimic the form of the bed. Because of flow acceleration and convergence in ice-stream onset zones, the ice-base roughness elements experience transverse strain, transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards. Where such keels slide across a soft sedimentary bed, they plough through the sediments, carving elongate grooves, and deforming material up into intervening ridges. This explains MSGLs and has important implications for ice-stream mechanics. Groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream. Keels may provide basal drag in the force budget of ice streams, thereby playing a role in flow regulation and stability. We speculate that groove ploughing permits significant ice-stream widening, thus facilitating high-magnitude ice discharge.

Large-scale ice-moulding: a discussion of genesis and glaciological significance

Abstract Landsat images reveal a previously unsuspected large-scale pattern of streamlining within drift which comprises a number of components. Drumlins and megaflutes form part of the pattern, but in addition there are two previously undocumented ice-moulded landform elements: streamlined lineations of much greater proportions, referred to as mega-scale glacial lineations, and a distinctive cross-cutting topology within the grain. Consideration of the genesis and glaciological significance of such landforms leads to a number of conclusions. It is suggested that mega-scale glacial lineations were formed under conditions of fast ice flow and their presence may thus record former locations of ice streams or surge events. Many lineations of varying scales display a pattern that reveals that they were not formed in sub-marginal positions. Extensive sets of lineations must have been formed approximately synchronously, thus indicating that lineation generation occurs over a wide range of glaciodynamic conditions, from sub-marginal positions to interior portions of ice sheets. Subglacial deformation of tills has been widely invoked to account for rapid glacier motion. Criteria for identifying such tills are restricted to structures viewed in cross-section, and it is thus hard to assess how widespread the processes of deformation may have been. Lineations provide a surface “marker” of structure that records subsequent deformation from other ice-flow events. It is argued that the palaeo-flow record including cross-cutting patterns provides the surface or plan expression of subglacial deformation. The degree of modification to pre-existing lineations should permit the identification of zones favoured by deformation, and thus assist in an assessment of how pervasive deforming bed processes were in influencing ice sheet dynamics.

The Laurentide ice sheet through the last glacial cycle: the topology of drift lineations as a key to the dynamic behaviour of former ice sheets

Study of satellite images from most of the area of the Canadian mainland once covered by the Laurentide ice sheet reveals a complex pattern of superimposed drift lineations. They are believed to have formed subglacially and parallel to ice flow. Aerial photographs reveal patterns of superimposition which permit the sequence of lineation patterns to be identified. The sequential lineation patterns are interpreted as evidence of shifting patterns of flow in an evolving ice sheet. Flow stages are recognised which reflect roughly synchronous integrated patterns of ice sheet flow. Comparison with stratigraphic sections in the Hudson Bay Lowlands suggests that all the principal stages may have formed during the last, Wisconsinan, glacial cycle. Analogy between Flow stage lineation patterns and the form and flow patterns of modern ice sheets permits reconstruction of patterns of ice divides and centres of mass which moved by 1000–2000 km during the glacial period. There is evidence that during the early Wisconsinan, ice sheet formation in Keewatin may have been independent of that in Labrador–Quebec, and that these two ice masses joined to form a major early Wisconsinan ice sheet. Subsequently the western dome decayed whilst the eastern dome remained relatively stable. A western dome then re-formed, and fused with the eastern dome to form the late Wisconsinan ice sheet before final decay. Because of strong coupling between three-dimensional ice sheet geometry and atmospheric circulation, it is suggested that the major changes of geometry must have been associated with large scale atmospheric circulation changes. Lineation patterns suggest very little erosional/depositional activity in ice divide regions, and can be used to reconstruct large scale patterns of erosion/deposition. The sequence of flow stages through time provides an integrative framework allowing sparse stratigraphic data to be used most efficiently in reconstructing ice sheet history in time and space.