Sediment redistribution beneath the terminus of an advancing glacier, Taku Glacier (T'aakúKwáan Sít'i), Alaska

Abstract

The recently-advancing Taku Glacier is excavating subglacial sediments at high rates over multi-decadal timescales. However, sediment redistribution over shorter timescales remains unquantified. We use a variety of methods to study subglacial and proglacial sediment redistribution on decadal, seasonal, and daily timescales to gain insight into sub- and proglacial landscape formation. Both excavation and deposition were observed from 2003 to 2015 (2.8 ± 0.9 m a−1 to +2.9 ± 0.9 m a−1). The observed patterns imply that a subglacial conduit has occupied the same site over the past decade. Outwash fans on the subaerial end moraine experience fluvial sediment reworking almost year-round, with net sediment gain in winter and net sediment loss in summer, and an overall mass gain between 2005 and 2015. We estimate that tens of meters of sediment still underlie the glacier terminus, sediments which can be remobilized during future activity. However, imminent retreat from the proglacial moraine will limit its sediment supply, leaving the moraine vulnerable to erosion by bordering rivers. Retreat into an over-deepened basin will leave the glacier vulnerable to increased frontal ablation and accelerating retreat.

Comprehensive Radar Mapping of Malaspina Glacier (Sít' Tlein), Alaska—The World's Largest Piedmont Glacier—Reveals Potential for Instability

“…As with Zechmann et al. (2021), we compare subglacial water routing for hydraulic potential surfaces corresponding to ice flotation fractions of 0.50, 0.75, and 1.0. For each fractional ice flotation the D$$\infty $$ flow accumulation algorithm (Tarboton, 1997) is used to determine subglacial drainage such that water is routed to the steepest down‐gradient slope of the eight triangular facets centered on each grid cell of the hydraulic potential surface.…”

“…In reality, an ever-evolving subglacial hydraulic system implies that the ice flotation fraction varies spatiotemporally, but here we choose a spatially constant value of 𝐴𝐴 𝐴𝐴 to provide insight into the sensitivity of water routing with changes in 𝐴𝐴 𝐴𝐴 . As with Zechmann et al (2021), we compare subglacial water routing for hydraulic potential surfaces corresponding to ice flotation fractions of 0.50, 0.75, and 1.0. For each fractional ice flotation the D 𝐴𝐴 ∞ flow accumulation algorithm (Tarboton, 1997) is used to determine subglacial drainage such that water is routed to the steepest down-gradient slope of the eight triangular facets centered on each grid cell of the hydraulic potential surface.…”

Modeling the spatially distributed nature of subglacial sediment transport and erosion

“…Additional processes may also be omitted due to model inputs. For instance, the evolving surface topography, not considered here, may cause alternative flow paths below the glacier (Fischer et al, 2005) and exposes new patches of the glacier bed to sediment transport or the relocation of channels (e.g., Zechmann et al, 2021). Furthermore, glacier sliding remains constant over the model run, so the results do not explicitly account for seasonal or interannual variability in bedrock erosion (e.g., Herman et al, 2015;Ugelvig et al, 2018); however, temporal variations in bedrock erosion are calculated through changing till thickness (Eq.…”

“…Therefore, it does not explicitly simulate the evolution of efficient and inefficient subglacial drainage systems over the course of the season or the inheritance of existing subglacial canals or channels (Fig. 3;e.g., Werder et al, 2013;Zechmann et al, 2021). In addition, a response time of the subglacial channel is chosen prior to simulations; this could be compared to a more sophisticated, but computationally more expensive, representation of processes in an R-channel model (e.g., Röthlisberger, 1972).…”