The influence of basal disturbances on the steady state surface topography and on surface velocities of glaciers and ice sheets is investigated numerically. This is done for finite amplitude basal perturbations in both bed topography and basal slipperiness using a nonlinear ice rheology and a nonlinear sliding law. The effects of varying the exponent n in Glen's flow law on transfer characteristics are mainly quantitative and do not affect qualitative aspects of the transfer amplitudes and phase shifts, such as the number of maxima and inflection points when plotted as functions of wavelength. In particular, the well-known maximum in bed-to-surface transfer amplitude for a Newtonian medium at sufficiently high slip ratios (ratio between mean sliding velocity and mean ice deformational velocity) also forms for n > 1. Transfer amplitudes generally become smaller with increasing n for wavelengths less than about 3 times the mean ice thickness (h). For larger wavelengths the situation is reversed and transfer amplitudes increase with n. For active ice streams, characterized by high basal slipperiness, low surface slopes ( 0.5) and fairly constant for all wavelengths longer than about 3-5h. With increasing n and decreasing surface slope the lower limit of wavelengths over which horizontal stress gradients can be ignored increases markedly. Perturbation solutions for Newtonian medium are found to give an accurate description of the transfer characteristics for bedrock amplitudes up to 50% ice thickness and for fractional amplitudes of slipperiness perturbations up to 0.5.

On the relationship between surface and basal properties on glaciers, ice sheets, and ice streams

Ice stream discharge responds to a balance between gravity, basal friction and lateral drag. Appreciable viscous heating occurs in shear margins between ice streams and adjacent slow-moving ice ridges, altering the temperature-dependent viscosity distribution that connects lateral drag to marginal strain rates and ice stream velocity. Warmer ice deforms more easily and accommodates faster flow, whereas cold ice supplied from ice ridges drives advective cooling that counteracts viscous heating. Here, we present a two-dimensional (three velocity component), steady-state model designed to explore the thermal controls on ice stream shear margins. We validate our treatment through comparison with observed velocities for Bindschadler Ice Stream and verify that calculated temperatures are consistent with results from previous studies. Sweeping through a parameter range that encompasses conditions representative of ice streams in Antarctica, we show that modeled steady-state velocity has a modest response to different choices in forcing up until temperate zones develop in the shear margins. When temperate zones are present, velocity is much more sensitive to changes in forcing. We identify key scalings for the emergence of temperate conditions in our idealized treatment that can be used to identify where thermo-mechanical feedbacks influence the evolution of the ice sheet.

/pdf/thermal-controls-on-ice-stream-shear-margins-4ometxobom.pdf

Thermal controls on ice stream shear margins

Abstract Accurate representation of the viscous flow of ice is fundamental to understanding glacier dynamics and projecting sea-level rise. Ice viscosity is often described by a simple but largely untested and uncalibrated constitutive relation, Glen’s Flow Law, wherein the rate of deformation is proportional to stress raised to the power n . The value n = 3 is commonly prescribed in ice-flow models, though observations and experiments support a range of values across stresses and temperatures found on Earth. Here, we leverage recent remotely-sensed observations of Antarctic ice shelves to show that Glen’s Flow Law approximates the viscous flow of ice with n = 4.1 ± 0.4 in fast-flowing areas. The viscosity and flow rate of ice are therefore more sensitive to changes in stress than most ice-flow models allow. By calibrating the governing equation of ice deformation, our result is a pathway towards improving projections of future glacier change. 

/pdf/ice-viscosity-is-more-sensitive-to-stress-than-commonly-3ekt6h8i.pdf

Ice viscosity is more sensitive to stress than commonly assumed

Supplemental materials for preprint: A new approach to inferring basal drag and ice rheology in ice streams, with applications to West Antarctic ice streams

We investigate the errors caused by neglecting the crystal-orientation fabric when inferring the basal friction coefficient field, and whether such errors can be alleviated by inferring an isotropic enhancement factor field to compensate for missing fabric information. We calculate the steady states that arise from ice flowing over a sticky spot and a bedrock bump using a vertical-slab numerical ice-flow model, consisting of a Weertman sliding law and the anisotropic Johnson flow law, coupled to a spectral fabric model of lattice rotation and dynamic recrystallisation. Given the steady or transient states as input for a canonical adjoint-based inversion, we find that Glen's isotropic flow law cannot necessarily be used to infer the true basal drag or friction coefficient field, which are obscured by the orientation fabric, thus potentially affecting vertically integrated mass fluxes. By inverting for an equivalent isotropic enhancement factor, a more accurate mass flux can be recovered, suggesting that joint inversions for basal friction and the isotropic flow-rate factor may be able to compensate for mechanical anisotropies caused by the fabric. Thus, in addition to other sources of rheological uncertainty, fabric might complicate attempts to relate subglacial conditions to basal properties inferred from an inversion relying on Glen's law.

/pdf/inferred-basal-friction-and-mass-flux-affected-by-crystal-i1li1lm1lt.pdf

Inferred basal friction and mass flux affected by crystal-orientation fabrics

Inequalities persist in the geosciences. Women and people of color remain underrepresented at all levels of the academic faculty, including positions of power. The proportion of women among geoscie...

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2021AV000436

Trends in the Representation of Women Among US Geoscience Faculty From 1999 to 2020: The Long Road Toward Gender Parity

Drag at the bed and along the lateral margins are the primary forces resisting flow in outlet glaciers. Simultaneously inferring these parame- ters is challenging since basal drag and ice viscosity are coupled in the momen- tum balance, which governs ice flow. Here, we test the ability of adjoint-based inverse methods to infer the slipperiness coefficient in a power-law sliding law and the flow-rate parameter in the constitutive relation. We modify existing inversions by including surface strain rates into the regularization of the in- version. Using synthetic data generated with physically-motivated variations in basal drag and ice rheology, we show that this allows for more accurate in- ferences. We apply this method to Bindschadler and MacAyeal Ice Streams in West Antarctica. Our results show relatively soft ice in the shear margins and spatially varying basal drag, with an increase in drag with distance upstream of the grounding line punctuated by localized areas of relatively high drag. We interpret the former to reflect a combination of heating through viscous dissipation and changes in the crystalline structure. These results suggest that adjoint-based inverse methods can provide inferences of basal drag and ice rheology when the regularization is informed by strain rates.

/pdf/a-new-approach-to-inferring-basal-drag-and-ice-rheology-in-44fov4skez.pdf

A new approach to inferring basal drag and ice rheology in ice streams, with applications to West Antarctic ice streams

The initiation of fractures and fast flow in floating regions of Antarctica have the potential to destabilize large regions of the grounded ice sheet, leading to significant sea-level rise. While observations have shown rapid, localized deformation and damage in the margins of fast-flowing glaciers, there remain gaps in our understanding of how rapid deformation affects the viscosity and toughness of ice. Here we derive a model for dynamic recrystallization of ice that includes a novel representation of migration recrystallization. This mechanism is absent from existing models and is likely dominant in warm areas undergoing rapid deformation, such as shear margins in ice sheets. While solid earth studies find fine-grained rock in shear zones, here we find elevated ice grain sizes (>10 mm) due to warmer temperatures and high strain rates activating migration recrystallization. Large grain sizes implies that ice in shear margins deforms primarily by dislocation creep, suggesting a flow-law stress exponent of n=4 rather than the canonical n=3. Further, we find that this increase in grain size results in a decrease in tensile strength of ice by ~75% in the margins of glaciers. Thus, this increase in grain size softens the margins of fast-flowing glaciers and makes ice shelf margins more vulnerable to fracture than previously supposed. These results also suggest the need to consider the effects of dynamic recrystallization in large-scale ice-sheet modeling.

https://eartharxiv.org/repository/object/2151/download/5406/

Meghana Ranganathan

Papers

Trends in the Representation of Women Among US Geoscience Faculty From 1999 to 2020: The Long Road Toward Gender Parity

A new approach to inferring basal drag and ice rheology in ice streams, with applications to West Antarctic ice streams

Recrystallization of ice enhances the creep and vulnerability to fracture of ice shelves

Recrystallization of ice enhances the creep and vulnerability to fracture of ice shelves