Formation and evolution of an extensive blue ice moraine in central Transantarctic Mountains, Antarctica

Christine M Kassab,Joseph A Graly,Kathy J Licht,Katrin Lindbäck,Rickard Petersson,Michael R Kaplan

doi:10.1017/jog.2019.83

Abstract

AbstractMount Achernar moraine is a terrestrial sediment archive that preserves a record of ice-sheet dynamics and climate over multiple glacial cycles. Similar records exist in other blue ice moraines elsewhere on the continent, but an understanding of how these moraines form is limited. We propose a model to explain the formation of extensive, coherent blue ice moraine sequences based on the integration of ground-penetrating radar (GPR) data with ice velocity and surface exposure ages. GPR transects (100 and 25 MHz) both perpendicular and parallel to moraine ridges at Mount Achernar reveal an internal structure defined by alternating relatively clean ice and steeply dipping debris bands extending to depth, and where visible, to the underlying bedrock surface. Sediment is carried to the surface from depth along these debris bands, and sublimates out of the ice, accumulating over time (>300 ka). The internal pattern of dipping reflectors, combined with increasing surface exposure ages, suggest sequential exposure of the sediment where ice and debris accretes laterally to form the moraine. Subsurface structure varies across the moraine and can be linked to changes in basal entrainment conditions. We speculate that higher concentrations of debris may have been entrained in the ice during colder glacial periods or entrained more proximal to the moraine sequence.

Highlights

With the increasing focus on how Earth’s ice sheets will respond to future changes in our climate, we must gain a more thorough understanding of how they have responded during previous climate fluctuations
We describe new findings based on ground-penetrating radar (GPR) data from the Mount Achernar moraine along upper Law Glacier that reveals the internal stratigraphy of this blue ice moraine, and we develop a model explaining the temporal evolution of moraine formation
GPR transects were collected in two areas at Mount Achernar: area 1 where debris bands emerge on the surface of the margin of Law Glacier; and area 2, which encompasses the upstream half of the moraine (Fig. 1)

Summary

Introduction

With the increasing focus on how Earth’s ice sheets will respond to future changes in our climate, we must gain a more thorough understanding of how they have responded during previous climate fluctuations. Distinct lithofacies in a sediment core collected in the Ross Sea record successive glacial advances and retreats on the continental shelf (Krissek and others, 2007) This provides important information about what happens near the ice-sheet margin, but it cannot provide information about the interior of the ice sheet. Sinisalo and Moore (2010) classified blue ice areas into two types, open and closed, based on the absence or presence of bedrock obstacles impeding ice flow, respectively. In both systems, subsurface ice is brought to the surface as a result of sublimation, but the difference lies in whether the deepest layers reach the surface. As Corti and others (2008) show in a laboratory model, it is the combination of stresses related to ice flow around a barrier and high sublimation rates that pulls the oldest ice to the surface

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