commit 8d086944cd053fd51067e225c55ac920f0891a3c
parent 09b71369ea5df67fdf7b767018ea2636727864e0
Author: Anders Damsgaard <anders@adamsgaard.dk>
Date: Thu, 14 Nov 2019 15:16:45 +0100
Improve abstract and intro further
Diffstat:
2 files changed, 36 insertions(+), 24 deletions(-)
diff --git a/BIBnew.bib b/BIBnew.bib
@@ -9220,4 +9220,17 @@ Winton and A. T. Wittenberg and F. Zeng and R. Zhang and J. P. Dunne},
author = {S. Palmer and M. McMillan and M. Morlighem},
title = {Subglacial lake drainage detected beneath the Greenland ice sheet},
journal = {Nature Commun.}
+}
+@article{Humphrey1993,
+ doi = {10.1029/92jb01869},
+ url = {https://doi.org/10.1029%2F92jb01869},
+ year = 1993,
+ month = {jan},
+ publisher = {American Geophysical Union ({AGU})},
+ volume = {98},
+ number = {B1},
+ pages = {837--846},
+ author = {N. Humphrey and B. Kamb and M. Fahnestock and H. Engelhardt},
+ title = {Characteristics of the bed of the Lower Columbia Glacier, Alaska},
+ journal = {J. Geophys. Res.: Solid Earth}
}
\ No newline at end of file
diff --git a/continuum-granular-manuscript1.tex b/continuum-granular-manuscript1.tex
@@ -48,14 +48,13 @@ maxcitenames=2, backend=bibtex8]{biblatex}
\maketitle
\begin{abstract}
-The dynamic interplay between fast ice flow, meltwater drainage, and till deformation is crucial for understanding glacier and ice-sheet behavior.
-Subglacial sediment transport constructs landforms that influence glacier stress balance and post-glaciation geomorphology.
-Till yield strength is highly dependent on water pressure and follows the Mohr-Coulomb rheology.
-However, the physical transport of till during subglacial shear is not well understood.
-Prognostic ice-sheet models assume that fast flow is produced by shear on the ice-bed interface, while geological observations indicate that internal deformation within till layers is common.
-We present a water-saturated continuum model that is both inspired by the granular nature of till layers and is consistent with Mohr-Coulomb mechanics, allowing
-modeling of the critical aspects of glacier-sediment-hydrology system.
-Model results indicate that pulses in water pressure can shift deformation away from the ice-bed interface and far into the bed, resulting in significant till advection.
+The dynamic interplay between ice flow, meltwater drainage, and basal sediment deformation is crucial for understanding glacier and ice-sheet behavior.
+Field observations indicate that subglacial sediment (till) layers often are mobile, and that a significant amount of the ice translation is contributed by till deformation.
+In turn, subglacial sediment transport constructs landforms that influence glacier stress balance, hydrology, and geomorphology.
+However, many numerical and theoretical models ignore till deformation and assume ice movement is contributed by interficial slip between ice and bed.
+Geotechnical analyses concluded that till deforms according to Mohr-Coulomb plasticity, which proved difficult to describe in models.
+Here we present a new continuum model that is both inspired by the granular nature of till layers and is consistent with Mohr-Coulomb mechanics, allowing modeling of the critical aspects of glacier-sediment-hydrology system.
+Our model results indicate that pulses in water pressure can shift deformation away from the ice-bed interface and far into the bed, causing episodes of significant till advection.
Deep deformation is most likely in tills with relatively high hydraulic permeability, forced by long-lasting and large water-pressure variations.
\end{abstract}
@@ -65,27 +64,23 @@ Deep deformation is most likely in tills with relatively high hydraulic permeabi
Fast glacier and ice-sheet flow often ocurrs over weak sedimentary deposits, where basal slip accounts for nearly all movement \citep[e.g.,][]{Cuffey2010}.
Basal sediments, called subglacial till, are diamictons commonly consisting of reworked sediments and erosional products \citep[e.g.,][]{Evans2006}.
-%\citet{Hooke1995} demonstrated that the grain-size distribution is fractal.
-Water is typically generated at the ice-bed interface from frictional heating, and fully saturates the pore space.
-Variations in subglacial water pressure are common and can be caused by internal and external factors.
-For example, \citet{Kavanaugh2009} showed from field observations that hydraulic flow paths and sediment deformation can constantly rearrange.
-Large pressure variations can occur through episodic drainage of ice-surface lakes \citep[e.g.,][]{Christoffersen2018} or subglacial lakes \citep[e.g.,][]{Palmer2015}.
-The pore pressure relieves some of the overburden ice weight and reduces the compressive solid stress on the granular skeleton lowering Terzaghi's effective stress \citep{Terzaghi1943}.
-Pore-pressure induced stress modifications control the kinematics of the ice and sub-ice sediments, which in turn may feed back into the dynamics of the system.
-The interplay of ice, water, and sediment is therefore important for glacier and ice-sheet dynamics, but remains poorly understood \citep[e.g.,][]{Clarke2005}.
-
-\citet{Boulton1979} presented the pioneering idea that sedimentary beds and hydrological processes can significantly influence glacier and ice-sheet flow and stability.
-This idea sparked interest in understanding the associated sediment physics.
-\citet{Boulton1987} argued that a viscous rheological model with mild stress non-linearity appropriately described their in-situ observations of a deforming bed.
+Meltwater fully saturates the pore space, and variations in subglacial water pressure are common and can be caused by internal variability \citep[e.g.,][]{Kavanaugh2009} or external water input \citep[e.g.,][]{Andrews2014, Christoffersen2018}.
+%For example, large basal pressure variations can occur from daily meltwater input \citep[e.g.,][]{Andrews2014}, or through episodic drainage of ice-surface lakes \citep[e.g.,][]{Christoffersen2018} or subglacial lakes \citep[e.g.,][]{Palmer2015}.
+%The pore pressure relieves some of the overburden ice weight on the bed and reduces the compressive solid stress on the granular skeleton lowering Terzaghi's effective stress \citep{Terzaghi1943}.
+%Pore-pressure induced stress variations control the kinematics of the ice and sub-ice sediments, which in turn may feed back into the dynamics of the system.
+%The interplay of ice, water, and sediment is therefore important for glacier and ice-sheet dynamics, but remains poorly understood \citep[e.g.,][]{Clarke2005}.
+%
+%\citet{Boulton1979} presented the pioneering idea that sedimentary beds and hydrological processes can significantly influence glacier and ice-sheet flow and stability.
+%This idea sparked interest in understanding the associated sediment physics.
+In-situ field observations demonstrate that deformation of this layer can contribute significantly to the glacier movement \citep[e.g.,][]{Boulton1979, Humphrey1993, Truffer2000}.
+\citet{Boulton1987} argued that a viscous rheological model with mild stress non-linearity appropriately describes subglacial till deformation.
A viscous rheology implies that the stress required to deform the till is strongly dependent on how fast it is deformed.
-Secondly, viscous materials have no upper bound to their strength with increasing strain rates.
-However, this result was hard to reproduce.
-\citet{Kamb1991}, \citet{Iverson1998}, and \citet{Tulaczyk2000} demonstrated from laboratory shear tests that rate-independent Mohr-Coulomb plasticity, as common for sedimentary materials, is a far better rheological description for subglacial till.
+However, \citet{Kamb1991}, \citet{Iverson1998}, and \citet{Tulaczyk2000} demonstrated from laboratory shear tests that rate-independent Mohr-Coulomb plasticity, as common for sedimentary materials, is a far better rheological description for subglacial till.
Mohr-Coulomb plastic materials have a yield strength that linearly scales with effective stress, regardless of strain rate.
\citet{Iverson2010} reviewed possible viscous contributions during till-water deformation, but deemed them to be of minor importance.
In spite of a limited observational basis, viscous rheologies continued to be applied as they allow for mathematical modeling of till advection.
Tills with viscous rheology were used to explain coupled ice-bed processes including subglacial sediment transport \citep[e.g.,][]{Jenson1995}, landform formation \citep[e.g.,][]{Hindmarsh1999, Fowler2000}, localization of water drainage \citep[e.g.,][]{Walder1994, Ng2000b}, and ice-sheet behavior in a changing climate \citep[e.g.,][]{Pollard2009}.
-Meanwhile, the Mohr-Coulomb plastic model continued to gain mounting empirical support from further laboratory testing \citep[e.g.,][]{Rathbun2008, Iverson2015}, as well as field observations on mountain glaciers \citep[e.g.,][]{Hooke1997, Truffer2006, Iverson2007}, mountain glaciers \citep[e.g.,][]{Kavanaugh2006}, and ice sheets \citep[e.g.,][]{Tulaczyk2006, Gillet-Chaulet2016, Minchew2016}.
+Meanwhile, the Mohr-Coulomb plastic model continued to gain further empirical support from laboratory testing \citep[e.g.,][]{Rathbun2008, Iverson2015}, as well as field observations on mountain glaciers \citep[e.g.,][]{Hooke1997, Truffer2006, Iverson2007}, mountain glaciers \citep[e.g.,][]{Kavanaugh2006}, and ice sheets \citep[e.g.,][]{Tulaczyk2006, Gillet-Chaulet2016, Minchew2016}.
Inconveniently, the plastic rheology caused a deadlock for the typical continuum modeling of ice and till, as the Mohr-Coulomb constitutive model offers no direct relation between stress and strain rate.
The deadlock was partially resolved when \citet{Schoof2006} and \citet{Bueler2009} showed that Mohr-Coulomb friction can be included in ice-sheet models through mathematical reguralization.
While the methods describe the mechanical effect of the bed on the flowing ice, they offer no treatment of sediment erosion, transport, and deposition as strain in the sedimentary bed is not included.
@@ -547,6 +542,10 @@ Deep deformation may be common in coarse-grained subglacial tills with strong an
Similarly, sudden water-pressure pulses are powerful drivers for single events of deep deformation.
+\section*{Acknowledgements}%
+A.D. benefited from conversations with Dongzhuo Li, Indraneel Kasmalkar, Jason Amundson, Martin Truffer, and Lucas Zoet during model development.
+Analysis and visualization of model output was performed with Gnuplot.
+
\section*{Appendix}%
\label{sec:appendix}
The grain-water model is written in C and is available under free-software licensing at \url{https://src.adamsgaard.dk/1d_fd_simple_shear}.
