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…