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From 2005/04/01
Abstract of the Doctoral Dissertation
Ph.D, Hokkaido University, 1997

Subglacial Hydraulic Interpretation of
Streamlined Glacial Erosional Bedforms
along the Soya Coast, East Antarctica.

by
Takanobu SAWAGAKI
in englishin japanese


Aims and Structure of This Thesis

I pay a particular attention to the question of whether erosion was mainly by glacier ice or melt water. The purpose of this study is thus to draw an attention to the significance and implications of subglacial melt water erosion on the marginal area of the Antarctic Ice Sheet in the past. This is the first attempt to consider the glacial erosional bedforms as water products in the Antarctica.

After an introduction (Chapter 1), the physical setting of the Antarctica and the present interest area of the Queen Moud Land, East Antarctica are briefly described (Chapter 2).

Chapter 3 is devoted to describe the classification and distribution of glacial landforms along the Soya Coast. Results of the geomorphological field surveys on the ice-free areas in JARE-34th and 35th are described, and the distribution and alignment of large-scale forms are mapped from aerial photographs. According to these investigations, the genesis of the erosional bedforms is discussed.

In Chapter 4, an examination will be performed to clarify the possibility of the formation of subglacial lakes and subglacial water transfer from the inner ice sheet to the terminus, which support the subglacial melt water interpretation of the bedforms.

In Chapter 5, the attention will be concentrated in a regional scale. Concerning the formation of streamlined bedforms, I examine a theoretical calculation to estimate the conformity of water flow beneath the ice sheet to the underlying bed topography.

On the basis of the results of Chapters 3, 4 and 5, the historical development of the landforms associated with deglaciation, and physical conditions affecting subglacial melt water produce are discussed in Chapter 6.

The interpretation of the sculptured bedforms and theoretical calculations of water flow beneath an ice sheet indicate that the subglacial water flow beneath the marginal ice sheet took part in the development of the bedforms, and the subglacial water flow could be controlled by a single ice mass in short period without significant change of the ice surface. This is the main conclusion of this study and is summarized in Chapter 7.


Abstract

This study aims at reconstructing the historical development of the glacial erosional bedforms in the Soya Coast of Lutzow-Holm Bay, East Antarctica, and at drawing attention to the significance and implications of subglacial melt water erosion on the marginal area of the Antarctic Ice Sheet in the past. For these purposes, descriptions of sculptured and streamlined bedforms in the present ice-free areas, hydrostatic investigations on the subglacial ponding along the Shirase drainage basin, and a theoretical calculation to estimate the conformity of water flow beneath ice sheet to underlying topography were carried out. Streamlined bedforms in the studied area are classified into crescentic transverse ridges and tadpole rocks, and these bedforms are accompanied by small erosional marks which support the interpretation of subglacial melt water erosion. Some tadpole rocks are superimposed on large roches moutonnees, and these two kinds of landforms were interpreted to have different modes of formation.

The source of the subglacial melt water was attributed to the subglacial lakes formerly existed beneath the upper reaches of the Mizuho Plateau. Hydrostatic investigation along the Shirase drainage basin revealed that several subglacial lakes are expected to have been formed on the bed depressions with the water surfaces determined by englacial equipotential surfaces.

For the theoretical calculation of water flow beneath an ice sheet, it was assumed that water flow at a glacier bed was controlled by ambient pressure gradients beneath the ice mass, and that the former ice sheet responsible for the sculptured bedforms extended to the western edge of the presently ice-free areas along the Soya Coast. Once a suitable ice surface model is determined by trial-and-error, good conformity of hydraulic gradients to the bedform distribution is recognized. A strong control of the gneissic structure on erosion is revealed by the regional change of the long axis' trend of streamlined bedforms. However, it is emphasized that the conformable ice surface represents the former ice sheet that took part in the development of the hills, and the subglacial water flow could be controlled by a single ice mass in short period without significant change of the ice surface profile.

These results finally lead to the following reconstruction of the bedform evolution along the Soya Coast: An initial episode of glacial plucking and abrasion produced roches moutonnees and basic large-scale landforms. Melt water flowing periodically into the Lutzow-Holm Bay sculptured s-forms and streamlined bedforms in bedrock much of the areas. During this period, except for water flowing phases, ice again come contact with the bedrock to form striations superimposed on the s-forms and the hillocks.

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