ABSTRACT. The surface of the Ross Ice Shelf (RIS) is textured by flow stripes, crevasses and other features related to ice flow and deformation. Here, moderate resolution optical satellite images are used to map and classify regions of the RIS characterized by different surface textures. Because the textures arise from ice deformation, the map is used to identify structural provinces with common deformation history. We classify four province types: regions associated with large outlet glaciers, shear zones, extension downstream of obstacles and suture zones between provinces with different upstream sources. Adjacent provinces with contrasting histories are in some locations deforming at different rates, suggesting that our province map is also an ice fabric map. Structural provinces have more complicated shapes in the part of the ice shelf fed by West Antarctic ice streams than in the part fed by outlet glaciers from the Transantarctic Mountains. The map may be used to infer past variations in stress conditions and flow events that cannot be inferred from flow traces alone.

Observations of ice shelf anisotropy on borehole seismic data are presented. Hot-water-drilledboreholes were made by the Aotearoa New Zealand Ross Ice Shelf Programme through a grounding-line proximal site at Windless Bight and the central Ross Ice Shelf site HWD-2. The boreholes were used to freeze seismometers into the ice at different depths. Seismic observations of shear wave splitting were made on the borehole seismometers using active sources deployed at the surface. These shear wave splitting data were used to constrain anisotropic ice crystallographic preferred orientations (CPO) within the ice column. Forward models of seismic properties from different CPO geometries are compared to the observations and a best fitting CPO model is found to explain the seismic anisotropy at HWD-2. This model consists of a vertical girdle of ice c axes that constitute 80% of the CPO in combination with tight horizontal clusters, which contribute 20% of ice c axes. The origin of the modeled CPO is discussed with regard to calculated strain rates at the site and found to be indicative of the current shear kinematics with vertical shear plane and horizontal shear direction. At HWD-2 the 370 m thick ice shelf is calculated to consist of at least 197 m of anisotropic ice.

Accelerating ice loss from Antarctica's ice sheets is projected to contribute 13-42 cm of sea level rise by the end of the century (Edwards et al., 2021). This contribution is mainly driven by an increase in ice flowing off the continent and into the ocean. Most Antarctic ice discharge becomes part of a floating ice shelf (Rignot et al., 2013), half of which will melt before it reaches the open ocean, while the other half will eventually calve as icebergs (Liu et al., 2015;Rignot et al., 2013). Ice shelves slow the discharge of glacial ice into the ocean. When ice shelves drag past coastlines, islands, and pinning points, they generate back stresses and buttress against ice flow (e.g., Dupont & Alley, 2005;Fürst et al., 2016). This buttressing is an important control on the rate of ice loss from Antarctica. The removal of buttressing when an ice shelf retreats or disintegrates leads to the acceleration of ice loss (e.g., Berthier et al., 2012;Rignot et al., 2004;Scambos et al., 2004).Melt at the base of ice shelves is largely controlled by ocean circulation in the sub-ice-shelf cavity. Theoretical frameworks of ocean circulation under ice shelves were first developed from the interpretation of direct oceanographic observations at ice shelf fronts (e.g., S. S. Jacobs et al., 1979). At a large scale, currents under an ice shelf follow a circulation-cell, described in detail by S. Jacobs et al. (1992) and well approximated by the ice pump mechanism (Lewis & Perkin, 1986). Sea ice formation releases high salinity water which sinks and flows down-

We use high resolution, ground-based observations of ice displacement to investigate ice deformation across the floating left-lateral shear margin of Priestley Glacier, Terra Nova Bay, Antarctica. Bare ice conditions allow us to fix survey marks directly to the glacier surface. A combination of continuous positioning of a local reference mark, and repeat positioning of a network of 33 stakes installed across a 2 km width of the shear margin are used to quantify shear strain rates and the ice response to tidal forcing over an 18-day period. Along-flow velocity observed at a continuous Global Navigation Satellite Systems (GNSS) station within the network varies by up to ∼30% of the mean speed (±28 m a−1) over diurnal tidal cycles, with faster flow during the falling tide and slower flow during the rising tide. Long-term deformation in the margin approximates simple shear with a small component of flow-parallel shortening. At shorter timescales, precise optical techniques allow high-resolution observations of across-flow bending in response to the ocean tide, including across-flow strains on the order of 10–5. An elastodynamic model informed by the field observations is used to simulate the across-flow motion and deformation. Flexure is concentrated in the shear margin, such that a non-homogeneous elastic modulus is implied to best account for the combined observations. The combined pattern of ice displacement and ice strain also depends on the extent of coupling between the ice and valley sidewall. These conclusions suggest that investigations of elastic properties made using vertical ice motion, but neglecting horizontal displacement and surface strain, will lead to incorrect conclusions about the elastic properties of ice and potentially over-simplified assumptions about the sidewall boundary condition.

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Wilkes Land and Totten Glacier (TG) in East Antarctica (EA) have been losing ice mass significantly since 1989. There is a lack of knowledge of long-term mass balance in the region which hinders the estimation of its contribution to global sea level rise. Here we show that this acceleration trend in TG has occurred since the 1960s. We reconstruct ice flow velocity fields of 1963–1989 in TG from the first-generation satellite images of ARGON and Landsat-1&4, and build a five decade-long record of ice dynamics. We find a persistent long-term ice discharge rate of 68 ± 1 Gt/y and an acceleration of 0.17 ± 0.02 Gt/y2 from 1963 to 2018, making TG the greatest contributor to global sea level rise in EA. We attribute the long-term acceleration near grounding line from 1963 to 2018 to basal melting likely induced by warm modified Circumpolar Deep Water. The speed up in shelf front during 1973–1989 was caused by a large calving front retreat. As the current trend continues, intensified monitoring in the TG region is recommended in the next decades.

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