Glacier Dynamics and Sea Level Change

Week 1 Sea level changes: Two articles that focus on the past contribution of West Antarctica to changes in sea level appeared in the 19 March 2009 issue of Nature. The first, by Naish et al. discusses the evidence from an ANDRILL sediment core at the edge of the Ross Ice Shelf. This sets the stage for a major modeling effort by Pollard and DeConto that attempts to describe changes in ice sheet volume over the past 5 million years. This second paper touches on many issues that we will come across again in later readings. From the Nature website be sure to also access the supplementary information for animations and more details on the modeling procedure. A perspectives piece by Huybrechts gives useful background material for both these articles. (Note: these links only work from campus IP addresses.)

I'd suggest focussing on the Pollard and DeConto paper, but for further reading about the problem of West Antarctica you may also be interested in Oppenheimer, 2004 and the projections of Bamber et al., 2009, which are put in context by Ivins. A recent discussion of observed flow changes in Greenland is given by Rignot and Kanagaratnam, 2006 and a broader overview of projections for the future is given by Alley et al., 2005. A longer review of sea-level reconstructions during the last interglacial, referred to by glaciologists as the Eemian, and by many others as Marine Isotope Stage (MIS) 5e, is provided by Hearty et al., 2007. For further reading on the long-term glacial history of Greenland, see Lunt et al., 2008.

Week 2 Isostasy: The 2004 review article by Peltier includes a concise description of ice distribution during the last glacial period (i.e. the late Pleistocene, also known as the Wisconsonin period). A more detailed analysis of post-glacial rebound on the Cascadia margin is provided by James et al., 2009a. I'd suggest focussing mainly on this second paper for our discussion.

For further background on isostatic rebound you may find the review article by Walcott, 1973 helpful. More recent inferences for the geometry of the Laurentide ice sheet are discussed by Tamisiea et al., 2007. A recent review of the coastal geometry during the late pleistocene in southern British Columbia is contained in another article by James et al., 2009b.

Week 3 Glacier flow: Marshall, 2005 wrote a nice tutorial on the main ingredients contained within models of ice sheet dynamics. These lecture notes by Hulbe from a modeling school this past summer might also be useful. One example of an ongoing project aimed at modeling the behavior of ice sheets is discussed by Rutt et al., 2009.

We'll concentrate on Marshall's paper for our discussion this week. Section 4 makes mention of the ongoing controversy concerning the actual mechanism of ice deformation and the appropriate value of Glen's flow law exponent. An important contribution to this debate by Goldsby and Kohlstedt, 2001 touched off a comment and reply that make for interesting reading. A different Marshall et al., 2002 obtained an impressive set of field data to weigh in on this topic.

Week 4 Glacier sliding: Near its melting temperature ice is very slippery, yet temperate glaciers somehow manage to cling to the sides of mountains and only slide downwards at rates that are typically measured in meters to tens of meters per year - with maximum rates for the faster glaciers and ice streams of order a few kilometers per year. The theory of glacier sliding has developed primarily from studies of valley glaciers, where the key role of basal roughness has long been recognized e.g., see Weertman, 1957; Nye, 1969; and Kamb, 1970. In practice, seasonal variations in sliding rate are common and there is both observational and theoretical support for the hypothesis that subglacial water is an important factor in determining how quickly glaciers slide. Basal ice often entrains solid debris that are expected to alter the rate at which a glacier can slide under a given driving stress (i.e. consider glacial striations). Rapid sliding occurs over sediments that have the potential to deform and/or be eroded so that the basal roughness can be a dynamic quantity. All of these issues serve to complicate the physics of glacier sliding and no comprehensive theory for this important process is in current widespread use. Instead, large scale ice sheet models including that described by Rutt et al., 2009, and that used by Pollard and DeConto, 2009, assume that the sliding speed is directly related to the basal shear stress through a "sliding parameter" that must be specified (though this sliding parameter is often treated as a simple scalar, it can instead be a function).

This week, let's begin by focussing on Weertman's paper. I'd suggest we also read Zwally et al., 2002 to provoke further discussion.

Week 5 Subglacial hydrology and glacier surges:

Except in circumstances where sliding rates are minimal because of cold-based conditions, the glacier bed marks the interface between ice and liquid water. Despite (or perhaps because of) our limited access to this important boundary, numerous drainage theories have been developed to describe how subglacial water flows. A reasonably concise review of the hydrology of alpine glaciers is given by Hubbard and Nienow, 1997, and a broader overview of glacier drainage is found in Fountain and Walder, 1998. For an informative discussion of a range of subglacial processes see Clarke, 2005.

The review papers listed above are a useful entry into the vast literature on subglacial hydrology. To focus our discussion this week, let's read about some of the potential consequences of changes in drainage systems as put forth in the classic study by Kamb et al., 1985. Charlie Raymond's super-8 movie of the surge is worth a look.

Week 6 Ice streams and Heinrich events:

The major ice sheets are drained by outlet glaciers and ice streams that flow at a much faster rate than the surrounding ice. This motion is primarily due to sliding and the rapid speeds seem to be associated with high subglacial pore pressures. This week, we'll go over a recent review paper by Bennett that describes much of what we understand about ice stream behavior. Much remains to be learned, and ongoing research will undoubtedly lead to changes in this understanding. For example, a recent paper by Hindmarsh overcomes some of the numerical difficulties that have been recognized in generating model ice streams.

Week 7 Grounding line dynamics:

Contributions of glaciated regions to sea level rise come from both surface (e.g. run-off), and dynamic (e.g. ice flow) processes. In Greenland, a recent report found that these two sources contribute equally. In Antarctica, the flow of ice across the grounding line into the sea is of paramount importance. The so-called "marine ice sheet problem" poses considerable challenges to modelers, both because of the change in the dominant stress balance that occurs at the transition from grounded to floating ice, and because of the need to track the moving boundary that is defined by the flotation condition. Schoof has offered a solution to these difficulties for an idealized case where the floating ice shelf is not supported by "buttressing". This theoretical work forms the basis for a major model intercomparison project, known as "MISMIP".

This week, we'll focus on Schoof (2007), and read a prespectives piece by Vaughan and Arthern for further context.

Week 8 Subglacial and supraglacial lakes:

Technological innovations and increased ice-sheet monitoring have drawn attention to the transient drainage of lakes at the ice surface and at the bed interface. We've already talked about the potential for lakes at the surface to drain suddenly to the ice base, as was observed recently in Greenland by Das et al., 2008. A more surprising discovery was the apparent interconnection of a series of lakes near the Siple coast; this is discussed by Fricker and Scambos, 2009. The potential importance of such behavior is hinted at by Bell et al.'s 2007 detection of lakes between another set of Antarctic ice streams.

This week, we look at two short papers by Sergienko et al., 2007 and Winberry et al., 2009 that discuss how further information about lake behavior can be obtained.

References:

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Bamber, J. L., R. E. M. Riva, B. L.A. Vermeersen, A. M. LeBrocq, (2009) Reassessment of the potential sea level rise from a collapse of the West Antarctic Ice Sheet, Science, 324, 901-903.

Bell, R. E., M. Studinger, C.A. Shuman, M. A. Fahnestock, I. Joughin (2007)Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams, Nature, 445, 904-907.

(*6)Bennett, M. R. (2003), Ice streams as the arteries of an ice sheet: their mechanics, stability and significance, Earth-Sci. Rev., 61, 309-339.

Clarke, G.K.C. (2005) Subglacial processes, Annu. Rev. Earth Planet. Sci., 33, 247-276.

Das, S. B., I. Joughin, M. D. Behn, I.M. Howat, M.A. King, D. Lizarralde, M.P. Bhatia (2008)Fracture propagation to the base of the Greenland Ice Sheet during supraglacial lake drainage, Science, 320, 778-781.

Duval, P., M. Montagnat (2002) Comment on "Superplastic deformation of ice: Experimental observations" by D. L. Goldsby and D. L. Kohlstedt, J. Geophys. Res., 107, 2082.

Fountain, A.G., J.S. Walder (1998) Water flow through temperate glaciers Rev. Geop., 36, 299-328.

Fricker, H. A., T. Scambos (2009)Connected subglacial lake activity on lower Mercer and Whillans Ice Streams, West Antarctica, 2003-2008, J. Glaciol., 55, 190, 303-315.

Goldsby, D. L., D. L. Kohlstedt (2001) Superplastic deformation of ice: Experimental observations J. Geophys. Res., 106, 11017-11030.

Goldsby, D. L., D. L. Kohlstedt (2002) Reply to comment by P. Duval and M. Montagnat on "Superplastic deformation of ice: Experimental observations", J. Geophys. Res., 107, 2313.

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Hindmarsh, R.C.A. (2009)Consistent generation of ice-streams via thermo-viscous instabilities modulated by membrane stresses, Geophys. Res. Lett., 36, L06502.

Hubbard, B., P. Nienow (1997) Alpine subglacial hydrology Quaternary Science Reviews, 16, 939-955.

Huybrechts, P. (2009) West-side story of Antarctic ice, Nature, 418, 295-296.

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(*2)James, T. S., E. J. Gowan, I. Wada, K. Wang, (2009a) Viscosity of the asthenosphere from glacial isostatic adjustment and subduction dynamics at the northern Cascadia subduction zone, British Columbia, Canada, J. Geophys. Res., 114, B04405.

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(*3)Marshall, S. J. (2005) Recent advances in understanding ice sheet dynamics, Earth Plan. Sci. Lett., 240, 191-204.

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(*1)Pollard, D., R. M. DeConto, (2009) Modelling West Antarctic ice sheet growth and collapse through the past five million years, Nature, 458, 329-332.

Rignot, E., P. Kanagaratnam, (2006) Changes in the velocity structure of the Greenland Ice Sheet, Science, 311, 986-990.

Rutt, I. C., M. Hagdorn, R. R. J. Hulton, A. J. Payne (2009)The Glimmer community ice sheet model, J. Geophys. Res., 114, F02004.

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(*7) Sergienko, O.V., D. R. MacAyeal, R.A. Bindschadler (2007)Causes of sudden, short-term changes in ice-stream surface elevation, Geophys. Res. Lett., 34, L22503.

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(*7)Winberry, J.P., S. Anandakrishnan, R.B. Alley (2009)Seismic observations of transient subglacial water-flow beneath MacAyeal Ice Stream, West Antatctica, Geophys. Res. Lett., 36, L11502.

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