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Iceberg Calving Dynamics of Jakobshavn Isbrę, Greenland

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Iceberg Calving Dynamics of Jakobshavn Isbrę, Greenland ICEBERG CALVING DYNAMICS OF JAKOBSHAVN ISBRÆ, GREENLAND By Jason Michael Amundson RECOMMENDED: Advisory Committee Chair Chair, Department of Geology and Geophysics APPROVED: Dean, College of Natural Science and Mathematics Dean of the Graduate School Date ICEBERG CALVING DYNAMICS OF JAKOBSHAVN ISBRÆ, GREENLAND A THESIS Presented to the Faculty of the University of Alaska Fairbanks in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY By Jason Michael Amundson, B.S., M.S. Fairbanks, Alaska May 2010 iii Abstract Jakobshavn Isbræ, a fast-flowing outlet glacier in West Greenland, began a rapid retreat in the late 1990’s. The glacier has since retreated over 15 km, thinned by tens of meters, and doubled its discharge into the ocean. The glacier’s retreat and associated dynamic adjustment are driven by poorly-understood processes occurring at the glacier-ocean in- terface. These processes were investigated by synthesizing a suite of field data collected in 2007–2008, including timelapse imagery, seismic and audio recordings, iceberg and glacier motion surveys, and ocean wave measurements, with simple theoretical considerations. Observations indicate that the glacier’s mass loss from calving occurs primarily in sum- mer and is dominated by the semi-weekly calving of full-glacier-thickness icebergs, which can only occur when the terminus is at or near flotation. The calving icebergs produce long-lasting and far-reaching ocean waves and seismic signals, including “glacial earth- quakes”. Due to changes in the glacier stress field associated with calving, the lower glacier instantaneously accelerates by ∼3% but does not episodically slip, thus contradicting the originally proposed glacial earthquake mechanism. We furthermore showed that the pre- dominance of calving during summer can be attributed to variations in the strength of the proglacial ice mélange (dense pack of sea ice and icebergs). Sea ice growth in winter stiffens the mélange and prevents calving; each summer the mélange weakens and calving resumes. Previously proposed calving models are unable to explain the terminus dynam- ics of Jakobshavn Isbræ (and many other calving glaciers). Using our field observations as a basis, we developed a general framework for iceberg calving models that can be applied to any calving margin. The framework is based on mass continuity, the assumption that calving rate and terminus velocity are not independent, and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset. Al- though the calving framework does not constitute a complete calving model, it provides a guide for future attempts to define a universal calving law. iv Table of Contents Page Signature Page . i Title Page . ii Abstract . iii Table of Contents . iv List of Figures . vii List of Tables . ix List of Other Materials . x List of Appendices . xi Acknowledgements . xii 1 Introduction 1 1.1 Background . 1 1.2 Project and objectives . 3 2 Glacier, fjord, and seismic response to recent large calving events, Jakobshavn Isbræ, Greenland 5 Abstract . 5 2.1 Introduction . 5 2.2 Methods . 7 2.3 Description of Calving Events . 8 2.4 Calving-Induced Glacial Earthquakes . 12 2.5 Conclusions . 13 Acknowledgements . 14 References . 15 Appendix . 18 3 Ice mélange dynamics and implications for terminus stability, Jakobshavn Isbræ, Greenland 27 Abstract . 27 v Page 3.1 Introduction . 28 3.2 Methods . 30 3.3 Results . 32 3.3.1 Temporal Variations in Terminus and Ice Mélange Dynamics . 32 3.3.2 Glaciogenic Ocean Waves . 35 3.3.3 Seismic and Acoustic Signals Emanating from the Fjord . 35 3.4 Discussion of Calving Events . 38 3.5 Simple Force Balance Analysis of Calving . 41 3.6 Interpretation . 45 3.6.1 Mélange and Fjord Dynamics . 45 3.6.2 Mélange Influence on Glacier Dynamics . 47 3.6.3 Sequence of Calving Events and Glacial Earthquakes . 48 3.6.4 Floatation Condition for Calving . 50 3.7 Conclusions . 51 Acknowledgements . 53 References . 54 Appendix . 60 4 A unifying framework for iceberg calving models 61 Abstract . 61 4.1 Introduction . 61 4.2 Steady-state calving rate . 64 4.2.1 Continuous calving . 65 4.2.2 Discrete calving . 66 4.2.3 Comparison with observations . 70 4.3 Calving framework . 71 4.3.1 General framework . 71 4.3.2 Case studies . 73 vi Page 4.3.3 Parameterization of self-sustaining processes . 75 4.4 Application of calving framework . 77 4.4.1 Seasonal variations in terminus position . 78 4.4.2 Incorporating existing calving criteria into the calving framework . 80 4.5 Conclusions . 81 Acknowledgements . 82 References . 83 5 Conclusions 87 References . 90 vii List of Figures Page 2.1 Jakobshavn Isbræ and motion surveying data . 6 2.2 Imagery of calving events . 9 2.3 Seismogram from the 4 July 2007 calving event . 11 2.A-1 Iceberg motion recorded with a GPS . 20 2.A-2 Example of a wave in Ilulissat Harbor, near the fjord mouth, that was pro- duced by a calving event . 21 2.A-3 Seismogram generated by the overturning of a large iceberg . 22 2.A-4 Comparison of seismograms recorded during a calving event to those recorded during known teleseismic glacial earthquakes . 23 2.A-5 Glacier motion at one of the optical survey markers . 24 2.A-6 Glacier motion at one of the GPS sites . 25 3.1 MODIS image of the terminal region of Jakobshavn Isbræ . 29 3.2 Timelapse imagery of the ice mélange . 33 3.3 Measurements of iceberg and glacier motion . 34 3.4 Ocean waves produced by a calving event on 19 July 2008 . 36 3.5 Examples of seismic signals originating in the fjord and terminus area . 37 3.6 Seismic and acoustic waveforms from a calving event on 15 July 2008 . 38 3.7 Temporal variations in the rates of short seismic events . 39 3.8 Diagrams used for the force balance analysis of calving icebergs . 42 3.9 Force from the ice mélange (per meter lateral width) for various e and q that is required to decelerate an already overturning iceberg or to prevent an iceberg from overturning in the first place . 44 3.10 Minimum b (water depth divided by ice thickness) for which buoyant forces will cause a grounded iceberg with width eH and tilt from vertical q to overturn . 46 viii Page 4.1 Schematic diagram of a glacier terminus . 64 4.2 Contours of H0=H1 for various along-flow thickness gradients (¶h=¶x) and normalized calving retreat lengths (Dx=H1) .................. 67 4.3 Contours of H0=H1 for various strain rates (e˙zz), time periods between calv- ing events (Dt), ice thicknesses (H1), and along-flow melt rates (b˙ and m˙ ) . 69 4.4 Theoretical steady-state thickness profiles of a 20 km wide and 100 km long ice shelf for various grounding line thicknesses and velocities (Hg and ug), melt rates (b˙), and lateral shear stresses (t) . 75 −1 4.5 Terminus position versus time for a glacier with ut = 10 km a , e˙zz = −1 a−1, ¶h=¶x = −0:1 (rough values for rapidly flowing outlet glaciers in Greenland), and b˙ = m˙ =0 ............................ 79 ix List of Tables Page 2.A-1 List of all recorded calving events and associated seismograms (UTC time) from Jakobshavn Isbrae between 13 May 2007 and 14 May 2008 . 18 x List of Other Materials A CD containing timelapse imagery of Jakobshavn Isbræ . Pocket xi List of Appendices Page Appendix 2.A . 18 Appendix 2.B . 26 Appendix 3.A . 60 xii Acknowledgements I write this as I am halfway through my eighth year in Fairbanks. Even though I’ve lived most of this time in what some people might say are dry, poorly-insulated, and poorly- heated cabins, I really “can’t complain” about the time that I’ve spent here (pardon my Minnesotan). In fact, I know that I’m going to miss this place and its people a whole heck of a lot once I leave. I’ve been fortunate to have many colleagues that I also consider friends, including my advisor Martin Truffer. Martin encouraged me to develop my own ideas and challenged me to finish them; I find it difficult to imagine an advisor that could have better prepared me for the future. (Thanks also Dana for letting us have so much fun in Greenland!) The rest of my committee was equally supportive. Roman Motyka taught me to remember the data, Ed Bueler (excitedly) answered my trivial math questions and advised me on framing my work in the context of ice sheet models, Regine Hock offered her brutally honest opin- ions of my manuscript drafts (“This is boring!”), and Erin Pettit rightfully questioned some of my ideas. My thesis also strongly benefited from discussions with Mark Fahnestock, who at times was essentially a co-advisor. He taught me something about networking and creativity, and never rejected any of my ideas outright, even when he maybe should have. Field work and data analysis would not have been as successful or enjoyable.
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