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Greenbaum-Dissertation-2015 Copyright by Jamin Stevens Greenbaum 2015 The Dissertation Committee for Jamin Stevens Greenbaum Certifies that this is the approved version of the following dissertation: Geophysical Investigations of the Coastal Evolution of the Totten Glacier System, East Antarctica Committee: Donald D. Blankenship, Supervisor Stephen P. Grand Patrick Heimbach Charles S. Jackson Mrinal K. Sen Clark R. Wilson Duncan A. Young Geophysical Investigations of the Coastal Evolution of the Totten Glacier System, East Antarctica by Jamin Stevens Greenbaum, B.S.A.S.E, M.S.E DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY The University of Texas at Austin December 2015 Dedication To Gloria James and Claudia Stevens, who have given me a lifetime of inspiration to pursue an education at every level as a critical dimension of a purpose-driven life. Acknowledgements I wish to thank: Donald Blankenship for teaching me to prioritize problems that I care about, to work with people that possess not only competence but compassion and curiosity, and to value platforms, analyses, and synthesis equally in the service of disciplined hypothesis testing. Claudia Stevens and Gloria James for having a vision for my life and enabling my education from elementary school to graduate school. Murray and Nessa Grainger for instilling in me the belief that anything is possible with practice and a commitment to excellence. Richard and Kathleen Greenbaum for the ethical and organizational training that enabled habitual academic and athletic achievement beginning in High School. Kevin Tuerff for suggesting that I consider pursuing a career in science and for his ongoing mentorship and friendship. D. Young for his scientific mentorship and, specifically, for teaching me that geophysical interpretations are always guilty until proven innocent. D. Blankenship, D. Young, S. Grand, P. Heimbach, C. Jackson, M. Sen, and C. Wilson for their encouragement, insight, and engagement in my scientific and professional development. D. Schroeder for showing me the function and value of the academy, the importance of hypothesis-driven instrumentation and technique development, and for teaching me that the most meaningful rewards require discipline, focus, and a long view. M. Siegert, T.v. Ommen, J. Roberts, R. Warner, B. Legresy, E. Lemeur, A. Aitken, J. Dowdeswell, A. Leventer, A. Orsi, B. Huber, S. Gulick, A. Shevenell, E. Domack, R. Forsberg, A. Olesen, M. Ghidella, G. Ng, T. Richter, S. Kempf, A. Mironov, C. Grima, K. Soderlund, S. Zedler, B. Schmidt, F. Habbal, L. Lindzey, E. Quartini, M. Cavitte, C. Greene, B. Gooch, and B. Frederick for their partnership in scientific discovery. v Geophysical Investigations of the Coastal Evolution of the Totten Glacier System, East Antarctica Jamin Stevens Greenbaum, Ph.D. The University of Texas at Austin, 2015 Supervisor: Donald D. Blankenship The Sabrina Coast is the primary outlet of ice from the Aurora Subglacial Basin, draining more than 3.5 meters of eustatic sea level potential into the Indo-Pacific sector of the Southern Ocean. Recent work has shown that the Aurora Subglacial Basin has drained and filled many times since large scale glaciation began including evidence that it collapsed during the Pliocene. Ice predominantly flows from the interior to the coast through two outlet glaciers, Totten Glacier and the unnamed glacier feeding the Moscow University Ice Shelf. Steady thinning rates near the grounding line of Totten Glacier are the largest in East Antarctica and the nature of the thinning suggests that it is driven by enhanced basal melting due to ocean processes while thinning rates are modest for the glacier feeding the Moscow University Ice Shelf. Warm modified Circumpolar Deep Water (mCDW), which has been linked to glacier retreat in West Antarctica, has been observed in summer and winter on the Sabrina Coast continental shelf in the 300-600 m depth range. Here we show that entrances to the cavity deeper than this range of thermocline depths indicate that the TGIS is vulnerable to intrusions of mCDW. We relate areas of elevated basal specularity and reflectivity (flatness and brightness of the ice-ocean interface, respectively), supported by independent model predictions and ice flow divergence estimates of basal vi melt, to ice draft and ice thinning rates, demonstrating that the current climatological regime is enhancing melting of ice deeper than 800 mbsl in the cavity, possibly resulting in a bifurcated coastal ice thinning signal. For the first time in East Antarctica, and the first time using ice sounding radar, we identify large basal channels and flat terraces several kilometers wide and several hundred meters deep produced by complex ice-ocean melt processes in the deep, warm cavity waters. Finally, by analyzing new physical oceanographic data recently acquired along the Sabrina Coast, we show that the depths of deep, warm mCDW observed on the continental shelf relative to the coastal bathymetry control the coastal thinning of ice between the Moscow University Ice Shelf and Totten Glacier; moderate shoaling the average depth of the thermocline would allow the observed mCDW to breach this topography and enhance coastal melting. The lack of dense, cool Shelf Water at depth on the continental shelf indicates that that variability in heat content along the Sabrina Coast is driven by far-field oceanic boundary conditions, not to polynya activity as has been proposed. Predicting future change in this important area of the coastline requires improved knowledge of the sub-ice shelf boundary conditions and processes, and controls on continental shelf circulation and heat transfer. vii Table of Contents List of Figures ........................................................................................................ xi Chapter 1 Introduction .............................................................................................1 1.1 Ice Sheet Contributions to Sea Level .....................................................1 1.2 East Antarctic Submarine Basins ...........................................................2 1.3 The Sabrina Coast, East Antarctica .......................................................6 1.4 Controls on Basal Melting of Ice Shelves..............................................8 1.4.1 Geophysical Context .....................................................................9 1.4.2 Seafloor bathymetry ....................................................................11 1.4.3 Ice shelf cavity morphology and roughness................................13 1.4.4 Cross-continental shelf heat exchange ........................................15 1.5 Observing and Inferring Ice Shelf Properties and Processes ...............18 1.6 Scope of Chapters ................................................................................21 Chapter 2 Ocean Access to Totten Glacier in East Antarctica ..............................24 2.1 Introduction ..........................................................................................24 2.2 Results ..................................................................................................25 2.3 Discussion ............................................................................................31 2.4 Conclusions ..........................................................................................33 2.5 Methods................................................................................................34 2.5.1 Data Acquisition Overview.........................................................35 2.5.2 Data Analysis Overview .............................................................36 2.5.3 Bathymetry ..................................................................................37 2.5.3.1 Introduction .....................................................................37 2.5.3.2 Gravity Data ....................................................................38 2.5.3.3 Gravity modeling and inversion approach ......................39 2.5.3.4 3D Forward Model ..........................................................43 2.5.3.5 3D Inversion Uncertainty Estimation .............................45 2.5.4 Trough Gravity Data ...................................................................47 2.5.5 Grounded icebergs ......................................................................49 viii 2.5.6 Bed Reflection Coefficients: .......................................................50 2.5.7 Magnetics data and depth to magnetic basement solutions ........50 2.5.8 Sea Level Potential for Totten Glacier and the Aurora Subglacial Basin ...........................................................................................50 Chapter 3 ................................................................................................................52 Discovery and context for large sub-ice shelf channels and basal terraces beneath Totten Glacier, East Antarctica .....................................................................52 3.1.1 Mass loss and bifurcated thinning of Totten Glacier ..................52 3.1.2 Availability of warm water for ice shelf basal melting...............53 3.1.3 Observing and inferring ice shelf basal melting .........................58 3.2 Methods...................................................................................................61 3.2.1 Reflectivity of the basal interface ...............................................61
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