Radar investigation of water distribution beneath Rutford Ice

Stream, West Antarctica

Ed King

British Antarctic Survey

Water beneath AntarcticaThings we have known about for a long time:

•Sub-glacial lakes

•Water-saturated till as a lubricant for ice streams

New insights in the past 5 years:

•A 1.5 m deep water channel imaged by borehole video on Ice Stream C•Gray’s bullseyes – inflation and deflation of surface depressions on Ice Stream D due to large water fluxes•Wingham’s flood – draining one subglacial lake into another

•King’s canal – seismic evidence of free water beneath Rutford Ice Stream

There is a lot more water moving around beneath the Antarctic ice sheet than anyone thought even a few years

ago.

Can we detect the water and map it’s distribution?

Rutford Ice Stream

Radarsat mosaic

Ellsworth

Mountains

Rutford I.S.

Kealey I.R. Carls

on In

let

100 km

N

MODIS / MOA , 125m pixel

Ellsworth

Mountains

Rutford I.S.

Kealey I.R. Carls

on In

let

100 km

N

Bedrock elevation from BAS aerogeophysicsGold to brown = above sealevel Violet to green = below sealevel

W

Ice flow

2900 m

E

DELORES radar data 2005/06 season

Assumption:

high amplitude bed reflection = water

Question:

Why does the water concentrate where it does?

Hydraulic potential gradient

igzszb

surface elevation

1020

1040

1060

1080

1100

1120

1140

1160

10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000

surface slope

-0.02

-0.01

0

0.01

0.02

0.03

0.04

10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000

Ice flow

Bed reflection amplitude

1 2 34

Hydraulic potential gradient4

2

0

-2

-4

X10

kg m

s4

-3

-2

Distance

Meltwater production:

1. Geothermal heat

2. Frictional melting due to flow

3. Thick ice

Is there evidence of basal melting beneath Rutford ice Stream?

Whence the water?

Ice flow

EW

450m

250m

Melting? Relict or contemporary?

S N

Ice flow out of page

Good story so far,

But……

Bed reflection amplitude-2

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

14000 15000 16000 17000 18000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

seismic

radar

5 per. Mov. Avg. (radar)

5 per. Mov. Avg. (seismic)

Comparison of reflection amplitudes: seismic vs radar

Water

Summary

•Radar shows strong variation in bed reflection amplitude

•Interpreted here (and elsewhere) as an indication of water

•Water distribution controlled by hydraulic potential

•Evidence for melting of basal ice

•Seismic reflections also show high amplitudes, interpreted as a canal system

•However, seismic and radar high amplitudes not coincident

Discussion

•Acoustic and electromagnetic signals reacting to water in different ways?

•Canals vs thin films?

•Flowing vs still water?

•Distribution of suspended fines in the water?

•Variations in dissolved salts?

Isochrons

Not isochrons?

Ice flow

2002-03 high-energy seismic data

Radar high amplitude

Is this evidence of re-freezing?Is this a mechanism for debris entrainment in basal ice?

But what about freezing rates?

Water

Water, water everywhereNor any drop to drink…