# Presentation: 404817

## Special thanks to:

- Michael Studinger, John Anderson, and Cindy Alley for figures;
- Many colleagues for ideas and discussions;
- NSF OPP for funding.

## Slide 2

## Outburst Floods from Big Ice:Normal but Important?

- R.B. Alley, T.K. Dupont, B.R. Parizek, S. Anandakrishnan, E.B. Evenson, D.E. Lawson & G.J. Larson

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## Hypothesis in Brief

- Glacier overdeepens;
- Climate change causes retreat; later, cooling, ice-shelf formation and grounding;
- Freeze-on to sill and surface-slope reversal occur, trapping water;
- Ice thickens, thaws, loses slope reversal;
- Outburst flood and surge follow.

## Evidence?

- Certainly not proven;
- But, each piece of hypothesis observed in nature and supported by models;
- And, geomorphic features support former occurrence of such floods.

## Can glaciers overdeepen?

- Certainly;
- Basins with closed contours widely observed;
- Mechanisms rather well understood (see Alley et al., Stabilizing feedbacks in glacier-bed erosion, Nature, 2003, with cover and News & Views); also from isostatic depression.

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## Can climate fluctuations cause retreat, advance?

- Surely, this one is easy;
- While we haven’t observed an ice-shelf advance into an overdeepening, we are in a warm time with almost no advances of cold ice occurring;
- Any advance of numerous ice masses could place an ice shelf over an overdeepening.

## Can an advancing ice shelf ground on a basin sill?

- Highly likely;
- Haven’t observed it, but again, we are in a retreat phase now;
- Certainly, local grounding of ice shelves common;
- Bindschadler interpreted Crary Ice Rise temperature profile as showing grounding.

## Can the ice then thicken over the water upglacier of the sill?

- Highly likely;
- If an ice shelf grounds, it may already have been thickening, with continuation likely;
- Grounding will increase resistance to flow, hence favor thickening.

## Can the water behind the sill remain trapped for a while?

- Highly likely.
- Trapping by freeze-on and by development of reversed ice-air surface slope.
- One may be sufficient to cause trapping; both together almost certainly sufficient.

## Can the ice freeze to the sill?

- Highly likely;
- Siple Coast ice streams show tendency for fast-flowing ice to freeze to bed;
- Ice shelf with basal melting more likely than ice stream to freeze to bed if grounded;
- Crary Ice Rise seems to have done this;
- Ice stream C trying to freeze on, maybe others also.

## Can a slope reversal develop?

- Highly likely;
- Strong contrast in topography or basal slipperiness favors rafting of ice over through longitudinal stresses rather than sudden thinning;
- Dupont has modeled, and works well;
- Observed over Lake Vostok, in ice rumples, in other places.

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## Why need both sealing mechanisms?

- D. Mickelson showed Green Bay lobe frozen-toe outbursts, but not huge;
- Some frozen toes drain (e.g., Trapridge);
- Reversal trapping known (e.g. Vatnajokull);
- But tunneling through possible (e.g., along fracture zones, etc.);
- With both, more likely to hold water (e.g., Lake Vostok?).

## Can one or both sealing mechanisms be lost?

- Highly likely;
- Thickening and rising shear stress favor thawing (propagation of Trapridge bulge, Svalbard surges, ice stream C bulge);
- Thicker ice less likely to have surface-slope reversal (Dupont);
- And, thawing reduces contrast hence likelihood of slope reversal.

## Can sealing loss trigger a flood?

- Highly likely;
- If ice thickened upglacier, trapped water at higher head than ocean;
- Jokulhlaups well-known, well-characterized and modeled.

## Can sealing loss trigger a surge?

- Likely;
- Thawing over a sill must speed ice flow;
- If the sill is sufficiently lubricated (say, by till) the speed-up may be large;
- Not observed yet, but seems likely.

## Where might this have occurred?

- Proximal Heinrich layers meltwater-washed silts despite cold conditions; J. Andrews reports channel across Hudson Strait sill;
- J. Anderson et al. have >100 m channels in 1400 m water in front of PIG, which is overdeepened and silled;
- G. Denton & D. Sugden have Labyrinth, others indicating subglacial outburst floods.

## Now what?

- Haven’t observed it all, but we shouldn’t have because we’re in a warm phase now;
- Haven’t modeled it all, but Dupont has made important chunks of the model work;
- Nature has all the elements needed to make this go;
- Explanatory; important geomorphically, possibly also glaciologically & climatically.

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