A subglacial hydrologic drainage hypothesis for silt sorting and deposition during retreat in Pine Island Bay

Dustin M Schroeder,Emma J Mackie,John B Anderson,Timothy T Creyts

doi:10.1017/aog.2019.44

Dustin M Schroeder, Emma J Mackie + Show 2 more

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https://doi.org/10.1017/aog.2019.44

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Abstract

AbstractLate Holocene sediment deposits in Pine Island Bay, West Antarctica, are hypothesized to be linked to intensive meltwater drainage during the retreat of the paleo-Pine Island Ice Stream after the Last Glacial Maximum. The uppermost sediment units show an abrupt transition from ice-proximal debris to a draped silt during the late Holocene, which is interpreted to coincide with rapid deglaciation. The small scale and fine sorting of the upper unit could be attributed to origins in subglacial meltwater; however the thickness and deposition rate for this unit imply punctuated- rather than continuous-deposition. This, combined with the deposit's location seaward of large, bedrock basins, has led to the interpretation of this unit as the result of subglacial lake outbursts in these basins. However, the fine-scale sorting of the silt unit is problematic for this energetic interpretation, which should mobilize and deposit a wider range of sediment sizes. To resolve this discrepancy, we present an alternative mechanism in which the silt was sorted by a distributed subglacial water system, stored in bedrock basins far inland of the grounding line, and subsequently eroded at higher flow speeds during retreat. We demonstrate that this mechanism is physically plausible given the subglacial conditions during the late Holocene. We hypothesize that similar silt units observed elsewhere in Antarctica downstream of bedrock basins could be the result of the same mechanism.

Highlights

Thwaites and Pine Island glaciers dominate mass loss from the West Antarctic Ice Sheet (Rignot and others, 2019) and are vulnerable to climate forcings that threaten ice-sheet stability (Hughes, 1981; Joughin and others, 2014)
Ice-sheet reconstructions of Pine Island Bay (PIB) have shown that Thwaites and Pine Island glaciers were once merged into a single ice stream that was grounded to the outer shelf (Lowe and Anderson, 2002; Evans and others, 2006; Graham and others, 2010; Jakobsson and others, 2011, 2012; Hillenbrand and others, 2013; Nitsche and others, 2013)
Retreat across inner PIB is thought to have been driven by intense meltwater drainage events, driven by a change in basal conditions (Kirshner and others, 2012; Witus and others, 2014; Kirkham and others, 2019), though the mechanisms remain poorly understood

Summary

Introduction

Thwaites and Pine Island glaciers dominate mass loss from the West Antarctic Ice Sheet (Rignot and others, 2019) and are vulnerable to climate forcings that threaten ice-sheet stability (Hughes, 1981; Joughin and others, 2014). The low velocities of such a distributed system (Le Brocq and others, 2009; Creyts and Schoof, 2009) could transport only silt and clay, based on formulas from Fredsøe and Deigaard (1992), effectively harvesting and sorting sediments with diameters below 10 μm from the unsorted till across the catchment This stage of the proposed mechanism achieves the fine sorting of the observed silt units in PIB (Witus and others, 2014), Marguerite Bay (Kennedy and Anderson, 1989) and Ross Sea (Prothro and others, 2018) through the steady-state, slow-flowing water over a large area for a long period of time. Our mechanism would need to produce upstream velocities (Fig. 2a) between ∼10−2 m s−1 and 10−1 m s−1, those velocities need to drop an order of magnitude or less below ∼10−2 m s−1 in the bedrock basins (Fig. 2c) or lakes (Fig. 2b) and increase an order of magnitude or more to above ∼10−1 m s−1 during retreat (Fig. 2e)

Discussion

Conclusion