Stokes, Christopher Richard (2001) The geomorphology of palaeo-ice streams : identification, characterisation and implications for ice stream functioning. PhD thesis, University of Sheffield.
Abstract
Ice streams are the dominant drainage pathways of contemporary ice sheets and their
location and behaviour are viewed as key controls on ice sheet stability. Identifying
palaeo-ice streams is of paramount importance if we are to produce accurate
reconstructions of former ice sheets and examine their critical role in the oceanclimate
system.
Many workers have invoked palaeo-ice streams from a variety of former ice sheets,
despite a limited understanding of their glacial geomorphology. This thesis addresses
the problem by predicting several diagnostic geomorphological criteria indicative of
ice stream activity. These are developed objectively from the known characteristics of
contemporary ice streams and can be summarised as: large flow-set dimensions (>20
km wide and >150 km long), highly convergent flow patterns, highly attenuated
subglacial bedforms (length:width >10: 1), Boothia-type dispersal plumes, abrupt
lateral margins «2 km), ice stream marginal moraines, evidence of pervasively
deformed till, and submarine sediment accumulations (marine-terminating ice streams
only). Collectively, the criteria are used to construct conceptual landsystems of
palaeo-ice stream tracks. Using satellite imagery and aerial photography to map
glacial geomorphology, identification of the criteria is used to validate the location of
a previously hypothesised ice stream and identify a hitherto undetected palaeo-ice
stream from the former Laurentide Ice Sheet. Implications for ice stream basal
processes are explored and their ice sheet-wide significance is assessed.
On Victoria Island (Arctic Canada) five of the geomorphological criteria are identified
and the extent of the marine-based M'Clintock Channel Ice Stream is reconstructed at
720 km in length and 140 km in width. The ice stream (operating between 10,400 and
10,000 yr BP) was located within a broad topographic trough, but internal
glaciological processes, rather than properties of the bed controlled the margin
locations. It eroded into pre-existing unconsolidated sediments and left a spectacular
pattern of subglacially-produced landforms, recording a snapshot view of the bed prior
to ice stream shut-down. Sediment availability appears critical to its functioning
(deformable bed?) and the debris flux of the ice stream is inferred to have been high.
Frictional shut-down occurred once down-cutting through sediments reached hard
bedrock close to the terminus.
The presence of four of the geomorphological criteria are used to identify a terrestrial
ice stream which drained the Keewatin Sector of the Laurentide Ice Sheet between ca.
10,000 and 8,500 yr BP. Its size is reconstructed at over 450 km in length and 140 km
in width, and it left behind a subglacial bedform pattern consisting of highly
attenuated drumlins (length:width ratios up to 48: 1) displaying exceptional parallel
conformity. This represents an isochronous bedform pattern and variations in
lineament elongation ratio are thought to be a useful proxy for ice velocity. Highest
elongation ratios occur immediately downstream of a topographic step where the ice
stream entered a sedimentary basin. It is inferred that the ice stream was triggered by
climatic warming which altered the ice sheet configuration and the thermal state of the
bed. A switch from cold to warm-based conditions probably triggered rapid basal
sliding. The ice stream (and a tributary) shut down when it ran out of ice, causing
widespread thinning of the ice sheet and subsequent deglaciation.
These ice streams denote considerable ice sheet instability over both hard and soft
(deformable) beds and emphasise the enormous effects that ice streams had in
controlling the deglaciation of the Laurentide Ice Sheet.
Metadata
Keywords: | Sheets; Stability; Glacial; Flow; Deglaciation |
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Awarding institution: | University of Sheffield |
Academic Units: | The University of Sheffield > Faculty of Social Sciences (Sheffield) > Geography (Sheffield) |
Identification Number/EthosID: | uk.bl.ethos.340136 |
Depositing User: | EThOS Import Sheffield |
Date Deposited: | 18 Jan 2017 16:56 |
Last Modified: | 18 Jan 2017 16:56 |
Open Archives Initiative ID (OAI ID): | oai:etheses.whiterose.ac.uk:14815 |
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