Influence of anisotropy on velocity and age distribution at Scharffenbergbotnen blue ice area

Abstract

Abstract. We use a full-Stokes thermo-mechanically coupled ice-flow model to study the dynamics of the glacier inside Scharffenbergbotnen valley, Dronning Maud Land, Antarctica. The domain encompasses a high accumulation rate region and, downstream, a sublimation-dominated bare ice ablation area. The ablation ice area is notable for having old ice at its surface since the vertical velocity is upwards, and horizontal velocities are almost stagnant there. We compare the model simulation with field observations of velocities and the age distribution of the surface ice. No satisfactory match using an isotropic flow law could be found because of too high vertical velocities and much too high horizontal ones in simulations despite varying enhancement factor, geothermal heat flux and surface temperatures over large ranges. However, the existence of a pronounced ice fabric may explain the observed present-day surface velocity and mass balance distribution in the inner Scharffenbergbotnen blue ice area. Near absence of data on the temporal evolution of Scharffenbergbotnen since the Late Glacial Maximum necessitates exploration of the impact of anisotropy using prescribed ice fabrics: isotropic, single maximum, and linear variation with depth, in both two-dimensional and threedimensional flow models. The realistic velocity field simulated with a noncollinear orthotropic flow law, however, produced surface ages in significant disagreement with the few reliable age measurements and suggests that the age field is not in a steady state and that the present distribution is a result of a flow reorganization at about 15 000 yr BP. In order to fully understand the surface age distribution, a transient simulation starting from the Late Glacial Maximum including the correct initial conditions for geometry, age, fabric and temperature distribution would be needed. This is the first time that the importance of anisotropy has been demonstrated in the ice dynamics of a blue ice area and demonstrates the need to understand ice flow in order to better interpret archives of ancient ice for paleoclimate research.

Insights into anisotropy development and weakening of ice from in situ P wave velocity monitoring during laboratory creep

“…An improved knowledge of large‐scale ice dynamics, and thus our ability to predict future changes of ice sheets, is limited by our understanding of the microstructural mechanics of ice [ Alley , ; Treverrow et al , ]. Constraining the mechanisms and rates by which CPOs and corresponding mechanical behavior evolve will be crucial to understanding how ice flow properties develop as boundary conditions change (e.g., grounding zones where ice streams flow into ice shelves), in the Antarctic or Greenland ice sheets [ Bamber et al , ; Rignot et al , ; Zwinger et al , ]. It is fundamentally important to understand the response of ice sheets to climate change and their future contributions to sea level rise [ Pollard and DeConto , ].…”

Evaluating the Duration and Continuity of Potential Climate Records From the Allan Hills Blue Ice Area, East Antarctica

“…Together with previous modeling studies in blue ice areas (Grinsted et al, ; Moore et al, ), our results show that surface velocities were slower in the past, and consequently, ice older than expected from modern ice flow observations is likely present. Blue ice areas, however, are also frequently characterized by complex flow as ice interacts with steep basal topography (Zwinger et al, ), which can result in spatial variations in the extent of old ice (Figure d) or stratigraphically disturbed ice (Higgins et al, ). Hence, even with evidence of very old ice from exposed tephra layers, meteorites, and discrete ice samples (Dunbar et al, ; Higgins et al, ; Yan et al, ), careful site selection is necessary to ensure the retrieval of a continuous, undisturbed ice core record.…”

Implementing an empirical scalar constitutive relation for ice with flow-induced polycrystalline anisotropy in large-scale ice sheet models

“…To date, flow relations utilising a fabric description that relies on fabric evolution equations or that is imposed as a function of location within the ice sheet have been restricted to regional simulations (e.g. Seddik et al, 2011;Martín and Gudmundsson, 2012;Zwinger et al, 2014).…”