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Studies of Antarctic Ice Shelf Stability: Surface Melting, Basal Melting, and Ice Flow Dynamics

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Studies of Antarctic Ice Shelf Stability: Surface Melting, Basal Melting, and Ice Flow Dynamics Studies of Antarctic ice shelf stability: surface melting, basal melting, and ice flow dynamics by Karen E. Alley B.A., Colgate University, 2012 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Geological Sciences 2017 This thesis entitled: Studies of Antarctic ice shelf stability: Surface melting, basal melting, and ice flow dynamics written by Karen E. Alley has been approved for the Department of Geological Sciences James White Ted Scambos Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above-mentioned discipline. ii Alley, Karen Elizabeth (Ph.D., Department of Geological Sciences) Studies of Antarctic ice shelf stability: Surface melting, basal melting, and ice flow dynamics Thesis directed by Senior Research Scientist T.A. Scambos and Professor J.W.C. White Abstract: Floating extensions of ice sheets, known as ice shelves, play a vital role in regulating the rate of ice flow into the Southern Ocean from the Antarctic Ice Sheet. Shear stresses imparted by contact with islands, embayment walls, and other obstructions transmit “backstress” to grounded ice. Ice shelf collapse reduces or eliminates this backstress, increasing mass flux to the ocean and therefore rates of sea level rise. This dissertation presents studies that address three main factors that regulate ice shelf stability: surface melt, basal melt, and ice flow dynamics. The first factor, surface melt, is assessed using active microwave backscatter. Combined with measurements of annual melt, backscatter values provide insights into the state of the upper layers of the ice shelf, indicating whether melt ponds, which can destabilize ice shelves, are likely to form on the ice shelf surface. We present a map of the relative vulnerability of ice shelves to hydrofracture collapse caused by surface melt ponding. As many authors have recently performed large-scale assessments of basal melt, the second factor is addressed at a smaller scale, through the study of channels that form on the undersides of ice shelves. These basal channels are mapped using visible-band imagery, and shown statistically to be related to the presence of warm ocean water. Landsat imagery and ICESat laser altimetry provide evidence that basal channels can in some cases change very rapidly and cause weakening of ice shelf structures. The final study addresses the calculation of surface strain iii rates from velocity fields. This common calculation, which is integral to understanding of flow patterns and stresses on both grounded and floating ice, can be achieved using a variety of approaches. We examine two commonly used algorithms and the differences in results produced by the different methods. We also present a Matlab code for calculating strain rates and a data product of strain rates across the Antarctic continent. All three studies contribute to the knowledge needed to comprehensively assess ice shelf stability; proposed future studies that continue toward this goal are discussed in the final chapter. iv Acknowledgments The years of work represented by this dissertation would not have been possible without the unwavering support of many family, friends, and colleagues. First, I would like to thank both of my advisors, Ted Scambos and Jim White, who have supported me generously through their time, advice, and funding over the last five years. My sincere gratitude also goes to all my committee members. To Bob Anderson, for being an outstanding professor and keeping me excited about everything to do with science. To Waleed Abdalati, for helping me to become a better teacher and communicator. And to Helen Fricker, for inspiring me and helping to guide me through meetings and publications. I have also received considerable support from Dr. Hari Rajaram at CU Boulder and Dr. Doug MacAyeal at the University of Chicago, who gave me confidence to learn how to apply math and coding to my science. I am indebted to several fellow CU Boulder grad students/post-docs who kept me going throughout the process: To Mike MacFerrin, for leading an outstanding Greenland field season and giving freely of friendship and help with research afterwards; to Mahsa Moussavi for her kindness and advice and great listening ear; and to Allen Pope, who was the main factor in keeping me sane during many stressful moments. I would also like to acknowledge my family, who got me to where I am today. To my sister and life coach Janet Jones, whose Skype conversations keep me in touch with reality; to my mother Cindy Alley who inspires my creativity and encourages me every step of the way; and to my father, Richard Alley: Your enthusiasm and ability to communicate science brought me inexorably into the field of glaciology, and inspire me every single day to pass on that excitement to others. Finally, I would like to thank my husband Alex, who has shown me love and support beyond anything I could have imagined, and who has probably heard more about this dissertation than he could have imagined. v CONTENTS CHAPTER 1: .......................................................................................................................................... 1 1.1: ICE SHELVES AND THEIR ROLE IN THE CRYOSPHERE ........................................................................................ 1 1.2: DISSERTATION GOALS AND OUTLINE .......................................................................................................... 3 CHAPTER 2: .......................................................................................................................................... 6 2.1: SCATTEROMETRY AND ITS APPLICATIONS TO SNOW AND ICE .......................................................................... 6 2.1.1: Available scatterometer data .................................................................................................... 8 2.1.2 Scatterometer measurements of sea ice .................................................................................... 9 2.1.3 Scatterometer measurements of snow ..................................................................................... 12 2.1.4 Scatterometer melt detection ................................................................................................... 13 2.1.5 Other scatterometer measurements of ice sheets and glaciers ................................................ 15 2.1.6 Strengths and limitations of scatterometry in cryospheric science ........................................... 16 2.2 SUBMITTED PAPER ON ICE SHELF VULNERABILITY TO HYDROFRACTURE ............................................................ 17 2.3. SUPPORTING INFORMATION .................................................................................................................. 37 CHAPTER 3: ........................................................................................................................................ 47 3.1: INTRODUCTION ................................................................................................................................... 47 3.2: BASAL CHANNELS IN THE LITERATURE ...................................................................................................... 48 3.2.1: Basal channel distribution ....................................................................................................... 49 3.2.2: Basal channels beneath the Petermann Glacier ice shelf ........................................................ 50 3.2.3: Basal channels beneath the Pine Island Glacier ice shelf ......................................................... 52 3.2.4: Basal channels in other locations ............................................................................................ 54 3.2.5: Basal channel modeling ........................................................................................................... 57 3.3: ARTICLE PUBLISHED IN NATURE GEOSCIENCE, MARCH 2016 ....................................................................... 61 3.4: SUPPLEMENTARY INFORMATION ............................................................................................................. 74 3.4.1. Identifying basal channels ....................................................................................................... 74 3.4.2. Basal channel origins ............................................................................................................... 79 3.4.3. Basal channel density and grounding line depth statistics ...................................................... 84 3.4.4. Persistent polynyas .................................................................................................................. 89 3.4.5. Basal channel growth and corrections ........................................................................................................................................ 90 3.4.6. Basal channels and ice shelf stability ....................................................................................... 96 CHAPTER 4: ...................................................................................................................................... 100 4.1: INTRODUCTION ................................................................................................................................. 100 4.2:
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