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Could subglacial Lake Vostok survive the buildup of the Antarctic ice sheet?

Could subglacial Lake Vostok survive the buildup of the Antarctic ice sheet?. Frank PATTYN Department of Geography, Vrije Universiteit Brussel, Brussels, Belgium. Overview. Background The subglacial lake effect and Lake Vostok Advance of an ice cap over a slippery spot

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Could subglacial Lake Vostok survive the buildup of the Antarctic ice sheet?

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  1. Could subglacial Lake Vostok survive the buildup of the Antarctic ice sheet? Frank PATTYN Department of Geography, Vrije Universiteit Brussel, Brussels, Belgium

  2. Overview • Background • The subglacial lake effect and Lake Vostok • Advance of an ice cap over a slippery spot • Grounding line advance over a preglacial lake • Similarities and dissimilarities with other Antarctic subglacial lakes • Conclusions

  3. N. Duxbury et al.JGR 106 (E1), 2001 • If Lake Vostok existed as a preglacial lake (15 Ma ago) it survived ice sheet growth and remained stable under the Antarctic ice cover • As long as lake > 53 m deep • Lake Vostok considered as closed system, which is dynamically inconsistent, as closure must have happened between initiation and development of ice sheet

  4. M. SiegertJGR 109 (E02007), 2004 • Ice would ground in Lake Vostok during buildup, and lake could not survive • Water flow beneath ice sheet is controlled by hydraulic potential gradient • If surface slope > 1/10 basal slope, water can flow ‘uphill’ • Steep margin existed during icesheet buildup close to Lake Vostok (DeConto & Pollard, 2003) • So, water was expelled from lake due to high subglacial hydraulic potential gradient

  5. F. PattynJGR 109 (E11004), 2004 • Mechanism for survival of Lake Vostok during buildup • Above models do not include interaction between ice sheet and lake water surface (pre- and sub-): = Slippery spot • Examined with a 3D higher-order ice-sheet model

  6. 3D higher order ice sheet model Ice-sheet modeling based on 3 conservative equations. Assuming constant ice density: Mass Momentum Energy Constitutive eq.

  7. Momentum: force balance Higher-order model including vertical shearing, longitudinal and transverse stress gradients Pattyn, 2003, JGR Solid Earth

  8. Subglacial lake effect Modeled domain Pattyn et al., JGLAC, in press.

  9. Basal boundary condition β2≤ ∞ Siegert (1999) β2= 0 Vostok Station Lake Vostok = slippery spot RADARSAT

  10. Surface velocity (color, white contours); surface topography (black dashed) DIAGNOSTIC EXPERIMENTS DNL = No Lake: ice flow in direction of steepest surface slope Local ice divide DLE = With Lake: ice flow is also flux dependent

  11. Surface elevation Ice thickness change Surface velocity Ice flow direction Basal friction field Effect of varying basal friction on the velocity and surface elevation of an ice mass

  12. Advance of an ice cap over a slippery spot • Rectangular domain of 1500 x 1500 km • Advance of ice sheet determined by surface mass balance equation

  13. Cross sectional view in time: NO LAKE

  14. Cross sectional view in time: LAKE

  15. Surface slope evolution across the lake Subglacial hydraulic potential gradient remains low in case of lake

  16. Grounding line advance over a preglacial lake • Criterion of hydraulic potential gradient is valid as long as lake is completely sealed • During ice sheet advance glacier terminus might be calving in the lake • Ice tongues or shelves are much thinner than sheets, so complete grounding is not likely

  17. Alternative explanation • Doran et al. (2003): Lake Vida, Dry Valleys, Antarctica • Ice sealed lake covered by 19m of lake ice • If Lake Vostok existed as a preglacial lake it would shift to an ice-sealed lake before being overridden by glaciers

  18. Similarities and dissimilarities with other Antarctic subglacial lakes • Astrolabe subglacial basin: at present devoid of water, although similar mechanism could apply • Advance of ice sheet occurred much later in time than at Lake Vostok • Antarctic ice sheet was bigger and climate cooler, so maybe there existed no preglacial lake • Furthermore, much more dynamism of Antarctic ice sheet edge in recent Cenozoic (no stable configuration compared to Lake Vostok)

  19. Grounding line problem • Grounding line dynamics are not yet understood (Vieli & Payne 2005) • Plays a decisive role in dynamics of advance of ice sheets in preglacial lakes or even oceans • Problem of calving… • ASPI – IPY • ISMIP (ice sheet model intercomparison project)

  20. Conclusions • If Lake Vostok existed as a preglacial lake (>15 Ma BP), it could have survived mid-Miocene glaciation • Interaction of ice sheet with water surface leads to a flattening of ice/air interface of ice sheet • This keeps subglacial hydraulic potential gradients low, so water can remain in subglacial cavity • But … problem of grounding line migration not fully understood

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