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Insights from Geodynamic Models Into Ice Flow, Mantle Magmatism, and Their Interactions

Fiona Clerc, Ph.D., 2023
Mark Behn, Co-Advisor
Brent Minchew, Co-Advisor
In this thesis, I use geodynamic models to study processes within the Earth’s mantle and cryosphere.
I begin by quantifying previously unconsidered sources of magmatic CO2. In Chapter 2, I predict how small concentrations of CO2 found in passively upwelling mantle throughout ocean basins may generate low-degree carbonate melting. I find the flux of CO2 segregated by these melts rivals the flux from mid-ocean ridges. In Chapter 3, I model how the deglaciation of the Yellowstone ice cap caused a reduction in mantle pressures and enhanced melting 19-fold. I predict the additional melting segregates a globally-significant mass of CO2, potentially playing a role in positive feedbacks between deglaciation and climate. I suggest enhanced melting may be important in other magmatically-active, continental settings undergoing rapid deglaciation — for instance, under the collapse of the West Antarctic Ice Sheet (WAIS).
This thesis next explores glaciological factors controlling WAIS stability, associated with the fracturing of ice sheet margins supported by floating ice shelves. The Marine Ice Cliff Instability posits ice cliffs above a critical height collapse under their own weight, initiating runaway ice sheet retreat. In Chapter 4, I model the formation of marine ice cliffs, as an Antarctic ice shelf is removed. I show that over ice-shelf collapse timescales longer than a few days (consistent with observations), ice cliffs comprised of intact ice are more stable, undergoing viscous flow rather than brittle fracture. I next investigate interactions between viscous and brittle processes, guided by observations on a modern Antarctic ice shelf. In Chapter 5, I model deformation at the McDonald Ice Rumples (MIR), formed as the Brunt Ice Shelf is grounded into a bathymetric high. The MIR are characterized by concentric folds intersected by radial fractures, implying viscous and brittle behavior, respectively. I interpret these features to constrain ice rheology and strength. More broadly, this final chapter highlights how leveraging glaciological observations as natural experiments places constraints on the phenomenological laws which govern ice and (analogously) mantle flow.
In summary, jointly developing models of both ice and mantle flow better constrains the dynamics of each system (solid Earth and cryosphere) and their interactions.