Elevation Change Anomalies in West Antarctica - New Windows Into Dynamics of

Subglacial Water Flow

S. Tulaczyk, UCSC

L. Gray, CCRS

I. Joughin, APL, UW

K. Jezek, BPRC, OSU

V.B. Spikes, Earth Science Agency

R. Bindschadler, NASA

Acknowledgements:

Financial support from NASA

What causes fast ice motion?

Lubrication of the bed by water.

Borehole to the ice stream bed (A. Behar)Borehole drill and drillers (A. Behar)

Bindshadler Ice Stream Margin (N. Nereson)

Until recently the theory of subglacial water flow assumed that meltwater gradually trickles from areas of basal melting to areas of basal freezing

Joughin, MacAyeal, Tulaczyk (2004)

Recent satellite and airborne measurements of ice surface elevation changes suggest that water collects in, and drains from,

small subglacial lakes

Area A1 includes an area of the Kamb Ice Stream (“C”).

Area A2 includes a tributary of the Bindschadler Ice Stream (“D”).

L1 and L2 are lidar flight lines flown by SOAR in 1998 and 2000.

Gray and others, 2005

Airborne laser altimetry shows localized surface uplifts and depressions.

Lidar data courtesy of the SOAR facility, U Texas, and NSF.

Data from Spikes et al. Thickness changes on Whillans Ice Stream and Ice Stream C, West Antarctica, derived from laser altimeter measurements. J. Glaciol. 49, 165, 2003.

Gray and others, 2005

A: Horizontal velocity field interpolated from repeated GPS measurements*.

B: This was used to estimate the 24-day horizontal displacements which was then combined with the radar line-of-sight displacement from an interferometric pair (from Sept. 26 and Oct. 20, 1997) to derive the local vertical displacement. The red and blue areas represent uplift and subsidence, respectively.

*  Chen, X., Bindschadler, R. A. & Vornberger, P. L. Determination of velocity field and strain-rate field in West Antarctica using high precision GPS measurements. Surveying and Land Information Systems. 58, 247-255 (1998).

A

B

Gray and others, 2005

All elevation-change anomalies occur at regions of ‘reversed’ slopes, the features are on or just upstream from ice moving slightly ‘uphill’. This suggests that subglacial water could ‘pond’ in the hydrologic potential well defined primarily by the surface slopes. Assuming that initially inflow exceeds outflow then the surface could be lifted as water collects.

Drainage of these small subglacial lakes requires discharge of up to ~0.3 cubic kilometers of water, which is similar to the total estimated annual water production in an individual drainage basin.

Increased pressure on bed due to ice rigidity

Link to Subglacial water system…

Circular bed depression

€

Z = Zd 1− 8r

l

⎛

⎝ ⎜

⎞

⎠ ⎟2

+16r

l

⎛

⎝ ⎜

⎞

⎠ ⎟4 ⎡

⎣ ⎢

⎤

⎦ ⎥

ECA = Water volume gauge Incredibly Amazing Fact #1!As water volume changes, ice lid flexes => we can measure subglacial water fluxes by looking at ECAs

€

Pi − Pw = −1024δ

l4Zd

… it gets even better …ECA = Pressure gauge Incredibly Amazing Fact #2!As water volume changes, ice lid flexes => ice surface level is a proxy for water pressure (elastic fiber stresses support part of ice weight)

System of two coupled Ordinary Differential Equations describing temporal evolution of depression depth and outflow conduit diameter:

€

∂Zd∂t

= −12

πl2q−

5φw

12lηD 3 hρ ig+

1024δ

l4Zd − po

⎛

⎝ ⎜

⎞

⎠ ⎟

⎡

⎣ ⎢

⎤

⎦ ⎥

€

∂D∂t

=2

3ρ iλ l2ηD3 hρ ig+

1024δ

l4Zd − po

⎛

⎝ ⎜

⎞

⎠ ⎟2

−φw

4

hsρ ig− Zd1024δ l4 − ponb

⎛

⎝ ⎜

⎞

⎠ ⎟

n

Theory of subglacial lake drainage indicates that such drainage should take form of relatively rapid ‘subglacial floods’

Output looks very much ‘jokulhlaup-ish’ - long periods of filling are followed by short periods of drainage

Large Antarctic subglacial lakes ~10,000 cubic km (Dowdeswell and Siegert, 2003)Q: Do they drain catastrophically?A: Great question! Largest historic floods were subglacial floods in Iceland (~3 cubic km of water discharged in a single event)

Q: Is rapidly varying subglacial water flow forcing ice flow to vary?A: Another great question! If yes, we have to re-evaluate our predictions.

Kamb Ice Stream may restart if re-lubricated by water. Whillans Ice Stream may reverse its slow down. Our assessment of future drainage from other ice streams may be change as well. Possibly greater contribution of Antarctica to near-future sea-level

rise.

Image courtesy of Ian Joughin, JPL (red colors ~1000 of m/yr, blue ~100 m/yr, yellowish-green ~10 m/yr

Take home messages

ECAs can be used as gauges of subglacial water flux and pressure

They provide cheap windows into a problem that is difficult to constrain otherwise but crucial to understanding WAIS behavior