Morainal Bank Evolution and Impact on Terminus Dynamics During a Tidewater Glacier Stillstand

Abstract

Sedimentary processes are known to help facilitate tidewater glacier advance, but their role in modulating retreat is uncertain and poorly quantified. In this study we use repeated seafloor bathymetric surveys and satellite-derived terminus positions from LeConte Glacier, Alaska, to evaluate the evolution of a morainal bank and related changes in terminus dynamics over a 17-year period. The glacier experienced a rapid retreat between 1994 and 1999, before stabilizing at a constriction in the fjord. Since then, the glacier terminus has remained stabilized while constructing a morainal bank up to 140 m high in water depths of 240-260 m, with rates of sediment delivery of 3.3 × 10 5 to 3.8 × 10 5 m 3 a −1. Based on repeated interannual surveys between 2016 and 2018, the moraine is a dynamic feature characterized by push ridges, evidence of active gravity flows, and bulldozing by the glacier at rates of up to meters per day. Beginning in 2016, the summertime terminus has become increasingly retracted, revealing a newly emerging basin potentially signaling the onset of renewed retreat. Between 2000 and 2016, the growing moraine reduced the exposed submarine area of the terminus by up to 22%, altered the geometry of the terminus during seasonal advances, and altered the terminus stress balance. These feedbacks for calving, melting, and ice flow likely represent mechanisms whereby moraine growth may delay glacier retreat, in a system where readvance is unlikely.

Subglacial Discharge Reflux and Buoyancy Forcing Drive Seasonality in a Silled Glacial Fjord

“…Model bathymetry was constructed using data from a 5 m resolution multibeam echosounder (Reson SeaBat 7111) survey in August 2016, with data gaps filled in using two 20 m resolution fathometer (Furuno 528L) surveys in August 1999 and September 2000 (Eidam et al., 2020). Observed fjord bathymetry was then interpolated onto the 50 × 50 m model grid (Figure 1d).…”

The Relationship Between Submarine Melt and Subglacial Discharge From Observations at a Tidewater Glacier

“…Thus, the volume flux of Q M is a more robust estimate of the total meltwater flux, while the average terminus melt rate should be considered an upper limit due to the unresolved contribution of icebergs. Additionally, by using a constant terminus surface area, the melt rate neglects the seasonal changes in submarine surface area (Eidam et al., 2020). The runoff flux, Q R , could include any inputs of freshwater that do not require latent heat from the ocean to undergo phase change, but we assume that runoff is dominated by subglacial discharge that drains at the bed of the glacier.…”

“…Adapted from Eidam et al. (2020). (b) Zoom of near‐glacier region with locations of the ocean transect for budget calculations and the glacier transect for ice flux measurements—for all surveys except September 2018—and the mooring location.…”

“…LeConte is the southernmost tidewater glacier in the northern hemisphere and has an annual ice flux of ∼1 Gt a −1 (McNabb et al., 2015). LeConte Bay is a narrow, sinuous fjord—between 1 and 2 km wide and 18 km long—with several deep subbasins separated by three morainal banks: MB1, MB2, and LIA (Little Ice Age) as shown in Figure 1 (Eidam et al., 2020; Motyka et al., 2003). The glacier currently terminates in water depths ranging from 170 to 200 m and undergoes seasonal advance/retreat oscillations of ∼300 m (Eidam et al., 2020).…”

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

“…Without this important sediment source, the present-day Taku Glacier foreland may then suffer net mass loss due to erosion from the Taku and Norris Rivers. In an over-deepened basin and without a pinning point, it is unlikely that Taku Glacier will form a new proglacial shoal to slow the retreat (Eidam and others, 2020). The nearest most likely pinning point is in the vicinity of the 1890 terminus location (~7 km from the 2015 terminus) where the glacier valley narrows and undergoes a sharp turn.…”