Seismic and radar observations of subglacial bed forms beneath the onset zone of Rutford Ice Stream, Antarctica

Abstract

ABSTRACT. We present seismic and radar data from the onset region of Rutford Ice Stream, West Antarctica, which show the form and internal structure of a variety of bed forms beneath an active ice stream. The ice flow in the area of our survey accelerates from 72 to >200 m a -1 , the ice is 2200-3200 m thick, and the bed of the ice stream lies up to 2000 m below present sea level. We have imaged the internal structure of the bed forms with seismic reflection techniques and also observed radar reflections from below the bed in some circumstances. We observed a transverse moraine 2 km wide and 1.5 km long beneath the slower-flowing part of the ice stream, which we interpret to be composed of unconsolidated sediment undergoing active deformation near the ice-sediment interface. We observed drumlins of classical form with elongation ratios of between 1 : 1.5 and 1 : 4.0 where the surface flow speed exceeded 95 m a -1 . The conformity of the internal structure of the bed forms with the ice base suggests that the bed forms are active depositional features in congruence with the observation of a contemporary drumlin-forming episode in the distal part of the same ice stream. These observations provide the first direct evidence of the association between ice-stream flow speed and bed-form shape.

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…Specifically, drumlins appear to be recorded in the onset zone of Rutford Ice Stream, West Antarctica, where ice velocities accelerate from 72 to >200 m a -1 in the transition from slower ice sheet flow to more rapid stream flow (King et al, 2007). Further down the ice stream, where velocities increase to around 375 m a -1 , King et al (2009) reported the presence of mega-scale glacial lineations, which are indistinguishable from those reported on palaeo-ice stream beds.…”

“…However, recent advances in geophysical observations have allowed bedforms to be imaged beneath existing ice sheets at depths of up to 2 km below the ice surface (King et al, 2007;Smith et al, 2007).…”

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

“…They are probably either purely erosional bedrock features or comprise tails of stiff-to-soft till seeded from large bedrock cores. Traditionally, drumlins have been related to deforming bed conditions and ice flow acceleration at substrate boundaries (Wellner et al, 2001), and were mapped in zones of increasing velocity beneath modern Antarctic ice streams (King et al, 2007). The increasing mean elongation ratio of the drumlins in the western ASE documents ice flow acceleration or rapid flow over a large w70 km long zone of the inner shelf ( Fig.…”

“…Many studies of northern hemisphere palaeo-ice sheets adopt a rigorous mapping approach for dividing the temporal and spatial relationships of glacial landforms (Stokes and Clark, 2003a;Stokes et al, 2009). While many of these have recognised complex patterns of flow related to palaeo-ice streams (Clark, 1997), Antarctic case-studies have not followed suite, and the local complexity of subglacial conditions hinted at by recent studies on modern ice stream beds (King et al, 2007(King et al, , 2009Smith and Murray, 2009) has not been replicated for Antarctic palaeo-ice stream beds thus far. We map subglacial bedforms with aims to: (i) characterise the geomorphology of the western ASE continental shelf; (ii) reconstruct the LGM flow pattern of this sector of the Fig.…”

“…basal drag/flow mechanics), ice dynamics, and bedforms is therefore required. Exposed palaeo-ice stream beds provide an opportunity to improve our understanding of complex bed controls on ice flow King et al, 2007;Ottesen et al, 2008;) by investigating sediment and bedform signatures over wide areas and over varying substrates.…”