Clerc Fiona

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Clerc
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Fiona
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  • Article
    Marine ice cliff instability mitigated by slow removal of ice shelves
    (American Geophysical Union, 2019-10-21) Clerc, Fiona ; Minchew, Brent M. ; Behn, Mark D.
    The accelerated calving of ice shelves buttressing the Antarctic Ice Sheet may form unstable ice cliffs. The marine ice cliff instability hypothesis posits that cliffs taller than a critical height (~90 m) will undergo structural collapse, initiating runaway retreat in ice‐sheet models. This critical height is based on inferences from preexisting, static ice cliffs. Here we show how the critical height increases with the timescale of ice‐shelf collapse. We model failure mechanisms within an ice cliff deforming after removal of ice‐shelf buttressing stresses. If removal occurs rapidly, the cliff deforms primarily elastically and fails through tensile‐brittle fracture, even at relatively small cliff heights. As the ice‐shelf removal timescale increases, viscous relaxation dominates, and the critical height increases to ~540 m for timescales greater than days. A 90‐m critical height implies ice‐shelf removal in under an hour. Incorporation of ice‐shelf collapse timescales in prognostic ice‐sheet models will mitigate the marine ice cliff instability, implying less ice mass loss.
  • Thesis
    Insights from geodynamic models into ice flow, mantle magmatism, and their interactions
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-02) Clerc, Fiona ; Behn, Mark D. ; Minchew, Brent M.
    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.
  • Article
    Causes of oceanic crustal thickness oscillations along a 74-M Mid-Atlantic ridge flow line
    (American Geophysical Union, 2019-11-19) Shinevar, William J. ; Mark, Hannah F. ; Clerc, Fiona ; Codillo, Emmanuel A. ; Gong, Jianhua ; Olive, Jean-Arthur ; Brown, Stephanie M. ; Smalls, Paris T. ; Liao, Yang ; Le Roux, Véronique ; Behn, Mark D.
    Gravity, magnetic, and bathymetry data collected along a continuous 1,400‐km‐long spreading‐parallel flow line across the Mid‐Atlantic Ridge indicate significant tectonic and magmatic fluctuations in the formation of oceanic crust over a range of time scales. The transect spans from 28 Ma on the African Plate to 74 Ma on the North American plate, crossing the Mid‐Atlantic Ridge at 35.8°N. Gravity‐derived crustal thicknesses vary from 3–9 km with a standard deviation of 1.0 km. Spectral analysis of bathymetry and residual mantle Bouguer anomaly show a diffuse power at >1 Myr and concurrent peaks at 390, 550, and 950 kyr. Large‐scale (>10 km) mantle thermal and compositional heterogeneities, variations in upper mantle flow, and detachment faulting likely generate the >1 Myr diffuse power. The 550‐ and 950‐kyr peaks may reflect the presence of magma solitons and/or regularly spaced ~7.7 and 13.3 km short‐wavelength mantle compositional heterogeneities. The 390‐kyr spectral peak corresponds to the characteristic spacing of faults along the flow line. Fault spacing also varies over longer periods (>10 Myr), which we interpret as reflecting long‐lived changes in the fraction of tectonically versus magmatically accommodated extensional strain. A newly discovered off‐axis oceanic core complex (Kafka Dome) found at 8 Ma on the African plate further suggests extended time periods of tectonically‐dominated plate separation. Fault spacing negatively correlates with gravity‐derived crustal thickness, supporting a strong link between magma input and fault style at mid‐ocean ridges.
  • Dataset
    Causes of oceanic crustal thickness oscillations along a 74-Myr Mid-Atlantic Ridge flow line
    ( 2019-11-12) Shinevar, William J. ; Mark, Hannah F. ; Clerc, Fiona ; Codillo, Emmanuel A. ; Gong, Jianhua ; Olive, Jean-Arthur ; Brown, Stephanie M. ; Smalls, Paris T. ; Liao, Yang ; Le Roux, Véronique ; Behn, Mark D.
    Gravity, magnetic, and bathymetry data collected along a continuous 1400-km-long spreading-parallel flow line across the Mid-Atlantic Ridge indicate significant tectonic and magmatic fluctuations in the formation of oceanic crust over a range of timescales. The transect spans from 28 Ma on the African Plate to 74 Ma on the North American plate, crossing the Mid-Atlantic Ridge at 35.8 ºN. Gravity-derived crustal thicknesses vary from 3–9 km with a standard deviation of 1 km. Spectral analysis of bathymetry and residual mantle Bouguer anomaly (RMBA) show diffuse power at >1 Myr and concurrent peaks at 390, 550, and 950 kyr. Large-scale (>10-km) mantle thermal and compositional heterogeneities, variations in upper mantle flow, and detachment faulting likely generate the >1 Myr diffuse power. The 550- and 950-kyr peaks may reflect the presence of magma solitons and/or regularly spaced ~7.7 and 13.3 km short-wavelength mantle compositional heterogeneities. The 390-kyr spectral peak corresponds to the characteristic spacing of faults along the flow line. Fault spacing also varies over longer periods (>10 Myr), which we interpret as reflecting long-lived changes in the fraction of tectonically- vs. magmatically- accommodated extensional strain. A newly discovered off-axis oceanic core complex (Kafka Dome) found at 8 Ma on the African plate further suggests extended time periods of tectonically dominated plate separation. Fault spacing negatively correlates with gravity-derived crustal thickness, supporting a strong link between magma input and fault style at mid-ocean ridges.
  • Article
    Deglaciation-enhanced mantle CO2 fluxes at Yellowstone imply positive climate feedback
    (Nature Research, 2024-02-20) Clerc, Fiona ; Behn, Mark D. ; Minchew, Brent M.
    Mantle melt generation in response to glacial unloading has been linked to enhanced magmatic volatile release in Iceland and global eruptive records. It is unclear whether this process is important in systems lacking evidence of enhanced eruptions. The deglaciation of the Yellowstone ice cap did not observably enhance volcanism, yet Yellowstone emits large volumes of CO2 due to melt crystallization at depth. Here we model mantle melting and CO2 release during the deglaciation of Yellowstone (using Iceland as a benchmark). We find mantle melting is enhanced 19-fold during deglaciation, generating an additional 250–620 km3. These melts segregate an additional 18–79 Gt of CO2 from the mantle, representing a ~3–15% increase in the global volcanic CO2 flux (if degassed immediately). We suggest deglaciation-enhanced mantle melting is important in continental settings with partially molten mantle – including Greenland and West Antarctica – potentially implying positive feedbacks between deglaciation and climate warming.