Increased channelization of subglacial drainage during deglaciation of the Laurentide Ice Sheet

R D Storrar,D J A Evans,C R Stokes

doi:10.1130/g35092.1

Abstract

The configuration of subglacial meltwater is a critical control on ice sheet dynamics, and the presence of pressurized water distributed across the bed can induce dynamic instabilities. However, this process can be offset by efficient evacuation of water within large subglacial channels, and drainage systems beneath alpine glaciers have been shown to become increasingly channelized throughout the melt season in response to the increased production of meltwater. This seasonal evolution has recently been inferred beneath outlet glaciers of the Greenland Ice Sheet, but the extent to which this process occurs across much larger spatial and temporal scales is largely unknown, introducing considerable uncertainty about the evolution of subglacial drainage networks at the ice sheet scale and associated ice sheet dynamics. This paper uses an unprecedented data set of over 20,000 eskers to reconstruct the evolution of channelized meltwater systems during the final deglaciation of the Laurentide Ice Sheet (13–7 kyr B.P.). We demonstrate that eskers become more frequent during deglaciation and that this coincides with periods of increased rates of ice margin recession and climatic warming. Such behavior is reminiscent of the seasonal evolution of drainage systems observed in smaller glaciers and implies that channelized drainage became increasingly important during deglaciation. An important corollary is that the area of the bed subjected to a less efficient pressurized drainage system decreased, which may have precluded dynamic instabilities, such as surging or ice streaming.

Highlights

Recent estimates have shown that mass loss from the Greenland Ice Sheet and West Antarctic Ice Sheet is accelerating (Rignot et al, 2011)
The Northern Hemisphere temperature record shows pronounced warming from 12.5 kyr B.P., following the Younger Dryas, to ca. 9 kyr B.P. (Fig. 2). This warming would have led to a negative mass balance, with an associated increase in the production of surface meltwater (Carlson et al, 2009). It may be manifest as an increase in the rate of ice margin retreat, which does occur, e.g., from 100–200 m yr−1 to almost 400 m yr−1 from 11 to 9 kyr B.P. in Keewatin, and from 0–100 to over 400 m yr−1 from 8.5 to 7.5 kyr B.P. in Labrador
We suggest that the increase in esker density during deglaciation is a reflection of an increased volume of surface meltwater entering the subglacial system via moulins and englacial channels, similar to that observed in the ablation zone of the Greenland Ice Sheet (Catania and Neumann, 2010) and earlier hypothesized to explain some esker patterns by Brennand (2000)

Summary

Introduction

Recent estimates have shown that mass loss from the Greenland Ice Sheet and West Antarctic Ice Sheet is accelerating (Rignot et al, 2011). Others have argued that an increased melt flux may be counteracted by the evolution of the drainage system, which responds by generating large channels that efficiently evacuate excess surface meltwater (Bartholomew et al, 2010; Schoof, 2010; Sundal et al, 2011; Shannon et al, 2013), similar to the seasonal evolution inferred beneath alpine glaciers (Hubbard and Nienow, 1997; Iken and Truffer, 1997) These potential switches in subglacial drainage introduce uncertainty when attempting to predict the future behavior of ice sheets: Could increased surface melt lead to more widespread dynamic instabilities, or will subglacial drainage systems become more efficient? Our understanding of how drainage systems might evolve over time scales of centuries to millennia, the impact this may have on ice dynamics, and our ability to model future ice sheet behavior remains limited

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