Characteristics of the deforming bed: till properties on the deglaciated Antarctic continental shelf

Anna Ruth W Halberstadt,Lindsay O Prothro,John B Anderson,Philip J Bart,Lauren M Simkins

doi:10.1017/jog.2018.92

Abstract

ABSTRACTContemporary ice stream flow is directly linked to conditions at the ice/bed interface, yet this environment is logistically difficult to access. Instead, we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated Antarctic continental shelf. We test currently-accepted hypotheses surrounding subglacial processes and till properties with a Ross Sea dataset. Till shear strengths indicate a continuum of simultaneous processes acting at the bed, rather than discrete ‘deformation’ and ‘lodgement’ end-members. We identify a threshold water content representing saturated pore spaces, leading to basal sliding and meltwater channelization. Based on observations of till properties relative to glacial landforms, we challenge the assumption that low shear strength is linked to intense deformation. Spatial variability in landform morphology reflects variability in deforming processes at the sub-ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line. Regional till properties generally correlate with seafloor geology and deglacial history; the western Ross Sea is characterized by higher and more variable shear strengths and water contents, while lower-shear strength till was preserved in the Eastern Basin. These observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves.

Highlights

The discovery of deforming sediments underlying ice streams prompted a fundamental change in the understanding of ice-sheet dynamics
The eastern Ross Sea was characterized by many fastflowing ice streams likely seamed together (e.g., Anderson and others, 2014), preventing the formation of significant shear margins, so we suggest that the difference between outer-trough and inner-trough flow velocities was not large enough to noticeably affect lineation morphology
By integrating analyses of glacial landform morphology and till characteristics, we challenge commonly-accepted paradigms surrounding subglacial processes and till properties. Specific observations from this investigation include the following: (1) The lack of discrete clusters of ‘deformation’ and ‘lodgement’ till end-members implies that paleo-ice stream trough beds hosted a transient mosaic of sediment deformation and stable spots, producing tills that represent an amalgamation of subglacial processes

Summary

Introduction

The discovery of deforming sediments underlying ice streams prompted a fundamental change in the understanding of ice-sheet dynamics. Tills collected from sediment cores on the Antarctic continental shelf are matrix-supported diamictons that are typically massive, with textural homogeneity (Anderson, 1999) they can display micromorphological variability (e.g., Menzies, 2000; Menzies and others, 2006; Reinardy and others, 2011). Ross Sea tills are thickest in troughs, locally approaching 80 m, and thin abruptly against banks and trough flanks (Shipp and others, 1999), consistent with other observations around Antarctica (Bart and Anderson, 1995; Klages and others, 2014). These till units were constructed throughout multiple phases of subglacial deformation during the last glacial cycle. A modern ‘snapshot’ shows deforming sediments below the contemporary Whillans Ice Stream with an average thickness of 6 m (Alley and others, 1987)

Methods

Results

Discussion

Conclusion