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

Abstract

Late 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.

Insights into glacial processes from micromorphology of silt-sized sediment

“…6). The high abundance of microtextures imparted through sustained stress and grinding (e.g., fracture types, edge abrasion) coupled with the low frequency of fluvial textures (e.g., v-shaped percussions) on both grain populations strongly suggests the grain size production of the ∼ 10 µm meltwater-silt mode results from abrasion and grinding at the base of glacial ice rather than the comminution of grains during subglacial hydrologic transport (e.g., Schroeder et al, 2019). Witus et al (2014) reached a similar conclusion after examining sand grain microtextures from MPDs and tills collected offshore from Pine Island Glacier (samples which we also include in this study -Table A1).…”

“…Model results (e.g., Carter et al, 2011) and satellite observations (Wingham et al, 2006;Fricker et al, 2007;Bowling et al, 2019;Hoffman et al, 2020) indicate that subglacial water can be stored in and actively transmitted between subglacial basins, demonstrating connected subglacial plumbing that mirrors basin-channel systems preserved on deglaciated continental shelves (e.g., Lowe and Anderson, 2003;Anderson and Fretwell, 2008;Kuhn et al, 2017;Simkins et al, 2017;Kirkham et al, 2019;Hogan et al, 2020b). The importance of subglacial lakes as reservoirs of glacial melt and sediments has been evoked to explain discrepancies between the annual production of basal melt and the volume of water required to mobilize quantities of MPDs observed offshore (e.g., Witus et al, 2014;Schroeder et al, 2019;Lepp et al, 2022). For example, the distribution of ∼ 120 km 3 of silts deposited offshore from Pine Island Glacier is interpreted to have been sourced in part by high-magnitude purging events of subglacial reservoirs of water and sediments (Witus et al, 2014).…”

“…This similarity suggests the glacial and/or glaciofluvial processes that produce these sedimentary deposits operate on an ice-sheet-wide scale, but such processes are poorly understood. Furthermore, hypothesized mechanisms for subglacial mobilization and sorting of MPDs observed offshore (e.g., Schroeder et al, 2019) have not been empirically evaluated.…”