Temporal Evolution of the Termini and Subaqueous Morphologies of Lake-Calving Glaciers in Aoraki/Mount Cook National Park, New Zealand
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Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author. Temporal evolution of the termini and subaqueous morphologies of lake-calving glaciers in Aoraki/Mount Cook National Park, New Zealand A thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Geography at Massey University, Palmerston North, New Zealand. Clare Margaret Robertson 2012 This is for you Dad. I wish you were here to see it. i ii Abstract The potential impact that subaqueous mass loss may have on glacier mass balance and volume reduction is unclear, primarily due to a lack of quantitative data. Therefore, in order to fully understand the potential contribution subaqueous calving and melting may make to glacier mass loss, it is important to understand how submerged extensions of glacier fronts (“ice ramps”) are created and maintained. This study improves the understanding of the controls on subaqueous ice ramp development and evolution at debris-covered, lake-calving margins by investigating the temporal evolution and subaqueous morphology of the termini of lake-calving glaciers in Aoraki/Mount Cook National Park, New Zealand. These glaciers are in close proximity to each other, yet each represents different stages on the glacier retreat-proglacial lake development continuum. Through a combination of field-based data collection and remote sensing, the calving margins of Mueller, Hooker, Tasman, Murchison, Classen, Grey, Maud and Godley glaciers were examined over a variety of time scales (days to years). This research indicates that the evolution of subaqueous ice ramps is intrinsically linked to subaerial retreat and that temporal changes in subaqueous morphologies are driven by subaerial calving, subsequent subaqueous calving, and sedimentation. The study highlighted that it is vital to understand the subaqueous morphologies of glaciers, how these evolve over time and what controls this evolution. In addition, when predicting glacier retreat it is important to consider subaqueous morphologies, through the incorporation of quantitative data and waterline melt rates, in order to more accurately predict retreat and hence mass loss from a glacier. Glacier retreat and concurrent proglacial lake expansion were also found to vary significantly within a single mountain belt. The identification of subaqueous ice ramps extending from lake-calving debris- covered glaciers, along with the examination of controls on ice ramp development and evolution, contributes significantly to the understanding of subaqueous morphologies and potential mass loss from these sections of the glacier. These results also lead to a better understanding of how subaqueous sections influence overall glacier retreat. iii iv Thesis structure and note on authorship This thesis consists of traditional thesis-style chapters (chapters 1, 2 and 7) and manuscripts written for publication in scientific journals (chapters 3-6). As a result, there is some repetition in chapters 1 to 6, particularly in the introduction, methods and study area sections. Some journal specific restrictions, such as abstract length and key words, are retained in the thesis. I completed all the field work at Aoraki/Mount Cook National Park, analysed data and wrote all the text in this thesis. Manuscripts are however co-authored to acknowledge the input of others into the PhD, as appropriate. My supervisors, Martin Brook, Ian Fuller, Douglas Benn, and Kat Holt, contributed to developing the project concept, editing manuscripts and providing general advice. Statements (Doctoral Research Committee form 16) regarding the contribution made by myself to chapters 3-6 and consent of the co-authors to include their work in the thesis are contained in the Appendices. Clare Robertson v vi Acknowledgements My PhD research would not have been possible without the support, guidance and advice of a number of people. Acknowledgements relating to individual chapters are included at the end of those chapters. Firstly, I wish to thank my family. You lot have been my rock for the past 3 years. You may not have completely understood my ‘nerd-talk’ but you still listened, appeared interested, and encouraged me. Dad, I wish you were here to see the final product. You always knew I could do it and that helped me push through the obstacles. I finished this for you. John, you have listened to me complain for the past 3 years! I couldn’t have kept going if it wasn’t for your encouragement and support. I would like to thank the Ryoichi Sasakawa Young Leaders Fellowship Fund for awarding me a fellowship. I could not have started or finished this work without this funding. I am also grateful for receiving the Harrette Jenkins Award from the Manawatu Branch of the New Zealand Federation of Graduate Women. I could not have completed my fieldwork without the enthusiastic bunch of people that helped me. David Feek’s expertise and help were often the difference between success and failure in the field, while Rob Dykes often acted as a sounding board for field frustrations. John Appleby, Jess Costal, Jingjing Wang, Alastair Clement, Sam Benny and Rob Eastham all provide invaluable help on the lakes and shared in the suffering of carrying equipment to and from Mueller lake. I would like to thank Charlie Hobbs for being so enthusiastic about Mueller lake and this research. I knew I always had eyes on the lake when I wasn’t there. Thanks for all those hot chocolates too! Thanks to The Helicopter Line Mount Cook and Tekapo Helicopters who flew on the boundaries of vii weather and weight limits to get my equipment where I needed it. Field accommodation help from The New Zealand Alpine Club was also greatly appreciated. I wish to thank my supervisory team whose encouragement, guidance and patience through difficult times pushed me through to the end. In particular, I would like to thank Dr. Ian Fuller and Dr. Kat Holt who agreed to be a part of the team even though they were from different research areas, and Prof. Doug Benn, who provided invaluable advice and critical insight from the other side of the world and boosted my confidence in the process. viii Table of Contents Abstract ............................................................................................................................ iii Thesis structure and note on authorship ............................................................................ v Acknowledgements ......................................................................................................... vii List of Figures ................................................................................................................ xiv List of Tables................................................................................................................ xxiii Chapter 1: Introduction ............................................................................................... 1 1.1 Background ........................................................................................................ 1 1.2 Aim, scope and objectives .................................................................................. 3 1.3 Thesis structure ................................................................................................... 5 Chapter 2: Lake-calving glaciers ................................................................................ 7 2.1 Introduction ........................................................................................................ 7 2.2 The significance of calving glaciers ................................................................... 7 2.3 Influence of the proglacial water body ............................................................. 11 2.3.1 Development of proglacial lakes ............................................................... 12 2.3.2 Waterline melt ........................................................................................... 14 2.3.3 Subaqueous melt ....................................................................................... 15 2.3.4 Thermal regime ......................................................................................... 18 2.3.5 Circulation and convection ....................................................................... 20 2.3.6 Lake level .................................................................................................. 22 2.4 Calving processes ............................................................................................. 23 2.4.1 The role of crevasses ................................................................................. 23 2.4.2 Hierarchy of mechanisms that trigger calving .......................................... 28 ix 2.4.2.1 Stretching in response to large-scale velocity gradients .................... 28 2.4.2.2 Force imbalances at unsupported ice cliffs ........................................ 29 2.4.2.3 Undercutting by subaqueous melting ................................................ 30 2.4.2.4 Torque arising from buoyant forces .................................................. 30 2.4.2.5 Coseismic triggers ............................................................................. 31 2.4.3 Calving types............................................................................................. 31 2.4.3.1 Buoyant calving