Seismic Monitoring of a Subarctic River: Seasonal Variations in Hydraulics, Sediment Transport, and Ice Dynamics

Is the regolith hypothesis consistent with the physics of glacial removal of mechanically weak surface material?&#160;&#160;The&#160; mid-Pleistocene transition (MPT) from small 40 kyr glacial cycles to large, abruptly terminating 100 kyr ones represents a major climate system reorganization for which a clear understanding is lacking. A leading mechanism for this transition is a stabilization of ice sheets due to a shift to higher friction substrate. The Pleistocene saw the removal of deformable regolith -- laying bare hard higher-friction bedrock that would help preserve regional ice during warm interstadials. This is the regolith hypothesis.&#160;&#160;The removal of regolith by Pleistocene ice sheets remains poorly constrained. To date, only models with a forced change in area of regolith cover or 1D flow line models with simplistic sediment transport have been used to probe the role of regolith in the MPT. It is therefore unclear if the appropriate amount of regolith removal can occur within the time-frame of the MPT.&#160;To properly test the hypothesis, at least three components are required: capable model, observational constraint, and a probe of uncertainties. A capable model must explicitly represent relevant processes in a fully coupled self-consistent manner. We have therefore configured a state of the art 3D glacial systems model (GSM). The GSM incorporates a state-of-the-art fully coupled sediment production/transport model, subglacial hydrology, visco-elastic glacial isostatic adjustment, 3D thermomechanically coupled hybrid shallow ice/shallow shelf ice dynamics, and internal climate solution from an energy balance model. The model generates sediment by quarrying and abrasion, and both subglacial and englacial sediment transport. The subglacial hydrology model employs a linked-cavity system with a flux based switch to tunnel drainage, giving dynamic effective pressure needed for realistic sediment and sliding processes. The coupled model is driven only by prescribed atmospheric CO2 and orbitally derived insolation.&#160;The required observational constraints include present-day regolith distribution and inferred Pleistocene ice volume from proxy records.&#160;The final component is&#160; a large ensemble of full Pleistocene simulations that probe both initial regolith distribution uncertainties and model parametric uncertainties. We present the results of such an ensemble, examining both rates of computed regolith removal and changes in ice volume cycling across the MPT interval.

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Increased Subglacial Sediment Discharge in a Warming Climate: Consideration of Ice Dynamics, Glacial Erosion, and Fluvial Sediment Transport

Geophysical Research Letters ◽

10.1029/2019gl085672 ◽

2020 ◽

Vol 47 (7) ◽

Cited By ~ 4

Author(s):

Ian Delaney ◽

Surendra Adhikari

Keyword(s):

Sediment Transport ◽

Sediment Discharge ◽

Glacial Erosion ◽

Ice Dynamics ◽

Fluvial Sediment ◽

Warming Climate ◽

Fluvial Sediment Transport

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Seismic monitoring in the Gugla rock glacier (Switzerland): ambient noise correlation, microseismicity and modelling

Geophysical Journal International ◽

10.1093/gji/ggaa097 ◽

2020 ◽

Vol 221 (3) ◽

pp. 1719-1735

Author(s):

Antoine Guillemot ◽

Agnès Helmstetter ◽

Éric Larose ◽

Laurent Baillet ◽

Stéphane Garambois ◽

...

Keyword(s):

Wave Velocity ◽

Active Layer ◽

Seismic Wave ◽

Seasonal Variations ◽

Ambient Noise ◽

Seismic Velocity ◽

Seismic Monitoring ◽

Structural Variations ◽

Rock Glacier ◽

Velocity Changes

SUMMARY A network of seismometers has been installed on the Gugla rock glacier since October 2015 to estimate seismic velocity changes and detect microseismicity. These two processes are related to mechanical and structural variations occurring within the rock glacier. Seismic monitoring thus allows a better understanding of the dynamics of rock glaciers throughout the year. We observed seasonal variations in seismic wave velocity and microseismic activity over the 3 yr of the study. In the first part of our analysis, we used ambient noise correlations to compute daily changes of surface wave velocity. In winter, seismic wave velocities were higher, probably due to refreezing of the permafrost active layer and cooling of the uppermost permafrost layers, leading to increased overall rigidity of the medium. This assumption was verified using a seismic model of wave propagation that estimates the depth of P- and S-wave velocity changes from 0 down to 10 m. During melting periods, both a sudden velocity decrease and a decorrelation of the seismic responses were observed. These effects can probably be explained by the increased water content of the active layer. In the second part of our study, we focused on detecting microseismic signals generated in and around the rock glacier. This seismic activity (microquakes and rockfalls) also exhibits seasonal variations, with a maximum in spring and summer, which correlates principally with an exacerbated post-winter erosional phase of the front and a faster rock glacier displacement rate. In addition, we observed short bursts of microseismicity, both during snowfall and during rapid melting periods, probably due to pore pressure increase.

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