Millennial-scale migration of the frozen/melted basal boundary, western Greenland ice sheet

Aidan Stansberry,Toby Meierbachtol,Joel Harper,Jesse V Johnson

doi:10.1017/jog.2021.134

Abstract

AbstractThe geometry and thermal structure of western Greenland ice sheet are known to have undergone relatively substantial change over the Holocene. Evolution of the frozen and melted fractions of the bed associated with the ice-sheet retreat over this time frame remains unclear. We address this question using a thermo-mechanically coupled flowline model to simulate a 11 ka period of ice-sheet retreat in west central Greenland. Results indicate an episode of ~100 km of terminus retreat corresponded to ~16 km of upstream frozen/melted basal boundary migration. The majority of migration of the frozen area is associated with the enhancement of the frictional and strain heating fields, which are accentuated toward the retreating ice margin. The thermally active bedrock layer acts as a heat sink, tending to slow contraction of frozen-bed conditions. Since the bedrock heat flux in our region is relatively low compared to other regions of the ice sheet, the frozen region is relatively greater and therefore more susceptible to marginward changes in the frictional and strain heating fields. Migration of melted regions thus depends on both geometric changes and the antecedent thermal state of the bedrock and ice, both of which vary considerably around the ice sheet.

Highlights

As ice flows from the cold interior of the Greenland ice sheet (GrIS) to the margins, it exchanges energy with surrounding ice and the underlying bedrock, and it undergoes heating as it deforms and generates friction at the basal boundary
As the ice margin initially retreats between 11.4 and 8.7 ka BP, the surface slope increases across the profile
Our transient and thermo-mechanically coupled model with thermally active bedrock simulates the last 11.4 ka along a flowline in western GrIS, with constraints provided by prior work on ice margin chronology and climate forcing

Summary

Introduction

As ice flows from the cold interior of the Greenland ice sheet (GrIS) to the margins, it exchanges energy with surrounding ice and the underlying bedrock, and it undergoes heating as it deforms and generates friction at the basal boundary. At the bed of the ice sheet, these processes result in a thermal pattern where the central core of GrIS is mainly frozen, and the outer flanks are at the pressure melting point (MacGregor and others, 2016). The frozen/melted boundary signifies the initiation of melting basal ice, and a transition point on the flowline to basal processes involving liquid water and decoupling of the ice and bedrock. Flowers, 2015) such that the areal distribution of melted ice-sheet bed governs regional hydrologic processes in the terrestrial and marine areas adjacent to the ice sheet (DeFoor and others, 2011). Regions of the ice sheet with melted bed generally have higher velocities (MacGregor and others, 2016) and greater advection of ice to the margins of the ice sheet, where mass is lost via marine calving or melting in warmer temperatures

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