Ground-penetrating radar profiles of the McMurdo Shear Zone, Antarctica, acquired with an unmanned rover: Interpretation of crevasses, fractures, and folds within firn and marine ice

Abstract

The crevassed firn of the McMurdo shear zone (SZ) within the Ross Ice Shelf may also contain crevasses deep within its meteoric and marine ice, but the surface crevassing prevents ordinary vehicle access to investigate its structure geophysically. We used a lightweight robotic vehicle to tow 200- and 400-MHz ground-penetrating radar antennas simultaneously along 100 parallel transects over a [Formula: see text] grid spanning the SZ width. Transects were generally orthogonal to the ice flow. Total firn and meteoric ice thickness was approximately 160 m. Firn crevasses profiled at 400 MHz were up to 16 m wide, under snow bridges up to 10 m thick, and with strikes near 35°–40° to the transect direction. From the top down, 200-MHz profiles revealed firn diffractions originating to a depth of approximately 40 m, no discernible structure within the meteoric ice, a discontinuous transitional horizon, and at least 20 m of stratified marine ice; 28–31 m of freeboard found more marine ice exists. Based on 10 consecutive transects covering approximately [Formula: see text], we preliminarily interpreted the transitional horizon to be a thin saline layer, and marine ice hyperbolic diffractions and reflections to be responses to localized fractures, and crevasses filled with unstratified marine ice, all at strikes from 27° to 50°. We preliminarily interpreted off-nadir, marine ice horizons to be responses to linear and folded faults, similar to some in firn. The coinciding and synchronously folded areas of fractured firn and marine ice suggested that the visibly unstructured meteoric ice beneath our grid was also fractured, but either never crevassed, crevassed and sutured without marine ice inclusions, or that any ice containing crevasses might have eroded before marine ice accretion. We will test these interpretations with analysis of all transects and by extending our grid and increasing our depth ranges.

Five decades of radioglaciology

“…Rodriguez-Morales and others, 2013; Kjær and others, 2018). Ultimately, new platforms, such as rovers, drones and satellites stand to transform the way radar-sounding observations are made (Jezek and others, 2006; Koh and others, 2010; Arcone and others, 2016; Freeman and others, 2017; Dall and others, 2018; Carrer and others, 2018; Gogineni and others, 2018; Culberg and Schroeder, 2019; Arnold and others, 2020).…”

Brine, englacial structure and basal properties near the terminus of McMurdo Ice Shelf, Antarctica

“…Geometric spreading results in hyperbola footprints which mask the ice shelf base near each fracture. However, up-glacier results from Arcone and others (2016) suggest that these basal fractures are likely filled with marine ice and that tens of meters of freeze-on marine ice exists below the meteoric ice. The fractures appear to be consistently ~120–140 m in width (Fig.…”

“…Vaughan and others, 2012; Le Brocq and others, 2013; Sergienko, 2013); however, the ice flow dynamics of this region are well known (e.g. Glasser and others, 2006; Glasser and others, 2014; Arcone and others, 2016) and do not support a hypothesis of subglacial channels. Therefore, we interpret these features to represent rifts initially caused by the ice flow velocity gradient near White Island, which have been re-sutured within McMurdo Sound.…”

“…The ice shelf currently terminates in front of Scott Base in a north–south orientation that is perpendicular to Hut Point shoreline. Ice velocities transition from 260 to 380 m a −1 within the shear zone upstream of White Island (Arcone and others, 2016) to 100–230 m a −1 between Hut Point and White Island. The slower velocities within McMurdo Sound are attributed to buttressing of Hut Point which we suggest causes compression and therefore re-suturing of rifts created by the velocity gradient up-glacier of White Island and Mina Bluff.…”

“…The shear zone consists of ~150 m of meteoric ice and 20–30 m of stratified marine ice at the ice shelf base. Buried surface crevasses to 30 m depth and bottom crevasses or rifts, both caused by a combination of tensile and shear stress, are prevalent in profiles collected by Arcone and others (2016).…”