Hydrological networks beneath Antarctica: New signals from altimetry

1. Hydrological networks beneath Antarctica: New signals from altimetry Duncan Wingham1, Andrew Shepherd2, Martin Siegert3 , Alan Muir1 Centre for Polar Observation and Modelling 1University College London 2Scott Polar Research Institute 3University of Bristol

5. Radio echo soundings

6. Subglacial lakes

7. Elevation trend

8. Elevation trend

9. Lowering hydrograph

10. 10 km2 elevation trend

11. Raising hydrograph

12. Hydrograph

13. Cartoon

14. Subglacial hydrology

15. Subglacial hydrology

16. Lake topography

17. Lake area Area ~ 600 km2 Deflation ~ 3 m Volume ~ 1.8 km3 Q ~ 50 m3 s-1

18. Lake area Area ~ 600 km2 Deflation ~ 3 m Volume ~ 1.8 km3 Q ~ 50 m3 s-1

19. Hydraulic model • Uphill flow forced by overburden • Ice modelled as two heavy plugs • Walls generate shear forces TL and TU opposing flow

20. Hydraulic model • Uphill flow forced by overburden • Ice modelled as two heavy plugs • Walls generate shear forces TL and TU opposing flow Force balance Df=hydraulic potential across tunnel =1.5 x 106 Pa

21. Hydraulic model • Uphill flow forced by overburden • Ice modelled as two heavy plugs • Walls generate shear forces TL and TU opposing flow Force balance Df=hydraulic potential across tunnel =1.5 x 106 Pa Rothlisberger channel geometry Q=50 m3 s-1 S = 26 m2

22. Hydraulic model • Uphill flow forced by overburden • Ice modelled as two heavy plugs • Walls generate shear forces TL and TU opposing flow Force balance Df=hydraulic potential across tunnel =1.5 x 106 Pa Rothlisberger channel geometry Q=50 m3 s-1 S = 26 m2 Nye’s jökulhlaup closure rate dS/dt < 0 if peff > 590 kPa >> DpL = 26 kPa i.e. closure unlikely

23. What role is the roof playing?

24. Potential Energy released ~ 2 X 1015 J What role is the roof playing?

25. Potential Energy released Energy used in heating water ~ 2 X 1015 J ~ 2 X 1011 J What role is the roof playing?

26. Potential Energy released Energy used in heating water Energy used in deforming ice ~ 2 X 1015 J ~ 2 X 1011 J ~ 2 X 1013 J What role is the roof playing?

27. Potential Energy released Energy used in heating water Energy used in deforming ice ~ 2 X 1015 J ~ 2 X 1011 J ~ 2 X 1013 J What role is the roof playing? Tunnel area according to energy ~ 28 m2 Cf. Channel Theory ~ 26 m2

28. Potential Energy released Energy used in heating water Energy used in deforming ice ~ 2 X 1015 J ~ 2 X 1011 J ~ 2 X 1013 J What role is the roof playing? Tunnel area according to energy ~ 28 m2 Cf. Channel Theory ~ 26 m2 Roof is playing a small role

29. Other signals within hydrograph

30. Other signals within hydrograph How many channels? Single channel velocity of 2.1 ms-1, or 1.6 days transit time.

31. Other signals within hydrograph How many channels? Single channel velocity of 2.1 ms-1, or 1.6 days transit time. What shape are channels? Semi-circular channel has a -t-3 rise

32. Other signals within hydrograph How many channels? Single channel velocity of 2.1 ms-1, or 1.6 days transit time. What shape are channels? Semi-circular channel has a -t-3 rise How frequent are outbursts Upstream catchment = 50,000 km2 Basal melt rate = 1 mm yr-1 Periodicity ~ 36 years

33. Implications • Lakes not closed, but regularly flushed • Periodicity far shorter than ice sheet lifetime • Large lakes (e.g. Vostok, 5400 km3) pose considerable threat • Mechanism may be related to Dansgaard-Oeschger events • Outbursts affect lake habitats • Periodic exchange reduces gas solute • Impacts on microbial diversity • Complicates isotopic analyses • Exploration risks contaminating downstream lakes