Seasonal and Diurnal Dynamics of Subglacial Channels: Observations Beneath an Alpine Glacier

Abstract

Abstract. Water flowing below glaciers exerts a major control on glacier basal sliding speeds. However, our knowledge on the physics of subglacial hydrology and its link with sliding is limited by lacking observations. Here we use a two-year long dataset made of on-ice measured seismic and in-situ measured glacier basal sliding speed records on the Glacier d’Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels hydraulic radius and hydraulic pressure gradient. At seasonal timescales we observe, for the first time, that hydraulic radius and hydraulic pressure gradient present a four-fold increase from spring to summer, followed by a comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels behaving at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behavior that is consistent with channels being out-of-equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which demonstrate that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speeds, supporting that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel-system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speeds) through an efficient hydraulic connection between the two systems. Using the two regimes herein observed in channels seasonal-dynamics as constraints for subglacial hydrology/ice dynamics models may allow to strengthen our knowledge on the physics of subglacial processes.

Seasonality of Solute Flux and Water Source Chemistry in a Coastal Glacierized Watershed Undergoing Rapid Change: Wolverine Glacier Watershed, Alaska

“…Hydrologically, glaciers are important for short‐term storage and release of precipitation, and as long‐term supply of water during dry intervals during the late summer (Figure 2, Fountain & Walder, 1998). A large fall rain event that delivers a substantial water volume to the subglacial drainage network can cause a reorganization of channels and result in slower draining locations with longer residence times to connect to the faster draining system (Nanni et al., 2019; Rada & Schoof, 2018; Rutter, 2005; Wadham et al., 1997). When these areas are drained, they can increase total solute concentrations at the glacier outlet and contribute to observed outlet signatures (Figures 4d and 6d).…”

“…Snowmelt should be a strong signal in the Wolverine watershed and we expect increasing Q and decreasing EC in P3. During this time, strong daily increases in temperature and radiation result in daily snow and ice melt pulses that are commonly reflected in Q and EC records (Nanni et al., 2019; Singley et al., 2017; Swift et al., 2005). In coastal Alaska (and other coastal glacierized regions), a late‐season rainy period characterized by an event‐response based hydrograph occurs regularly as storms increase in frequency (Curran & Biles, 2021; Déry et al., 2009).…”

On the Green's function emergence from interferometry of seismic wave fields generated in high-melt glaciers: implications for passive imaging and monitoring

“…Higher acausal amplitudes observed at larger distances are evidence for a higher density of sources located downstream of the array, according to our cross-correlation definition. More sources downstream are likely generated by faster water flow running into subglacial conduits toward the glacier icefall (Gimbert et al, 2016;Nanni et al, 2019b). Looking closer at NCC for individual receiver pairs, we sometimes observe spurious arrivals around time 0 (marked as green dots in Fig.…”

“…1b). Seismic noise power shows diurnal variations that are correlated with higher discharge during daytime and reduced water pressure at night Nanni et al, 2019b).…”

Repeating Low Frequency Icequakes in the Mont‐Blanc Massif Triggered by Snowfalls

“…A few seismic stations operated between October 2017 and May 2020 (stations B01‐B04), with several gaps in the acquisition. The first station B01 was installed on Argentière glacier on 4 October 2017 (Nanni et al., 2020). Then station B02 was installed at 70 m depth in a borehole close to B01 on 20 April 2018, followed by B03 on 23 October 2018 and B04 on 26 June 2019.…”