Ice Streams und Isbr 1 Ice Streams: Causes for Rapid Ice Flow Fast - PowerPoint PPT Presentation

Ice Streams und Isbræ 1

Ice Streams: Causes for Rapid Ice Flow Fast ice flow is controlled by many different factors which change on different time scales. Panel Controlling Factor Time Scale 7 meltwater fluxes short < 1 - 100 a 4 position of calving front short < 1 - 100 a 3 bed roughness middle 100-1000 a 6 geothermal heat flux middle 100-1000 a 5 softness of subglacial sediments middle 100-1000 a 2 topographic steps middle 100-1000 a 1 topographic focussing long 1000-100’000 a 2 Winsborrow (2019)

Ice Stream Archetypes Isbræ: e.g. Jakobshavn • topographically fixed • deep fjord in bedrock • fast ice deformation • rapid basal motion Ice streams: e.g. Siple Coast • flat sediment bed • sediment deformation controls flow • position variable • Shutdown possible 3 Raymond & Echelmeyer (2001)

Marginal Zone of Ice Stream B / Whillans Ice Stream • almost no inclination • surface very flat • bed mostly flat • localized, fast flow → wide crevasse zones • Influence on dynamics and ice temperatures (cooling) 4

Jakobshavn Isbræ: Kalbungsfront 2004 5 Photo: A. Bauder

Greenland: Fjord Systems Under the Ice Eisschild • marginal mountain ranges → "bath tub" • large ice streams are mostly topographically fixed Jakobshavn Isbræ • deepest outlet glacier fjord • up to 1500 m below sea level 6

Greenland: Ice Stream Flow Patterns • (a) Jakobshavn Isbræ • (b) Peterman Glacier • (c) Nioghalvfjerdsbræ (79N) • (d) Zachariæ Isbræ 7 Lemos (2018)

Greenland: Ice Stream Flow Patterns 8 Lemos (2018)

Greenland: Ice Stream Flow Patterns (2) 9 Lemos (2018)

Siple Coast: Ice Streams, Ice Rises 10 USGS

Antarctica: Cross Sections Through Ice Streams • Siple Coast: A-E: not topographically fixed (B = Whillans; D = Raymond; E= Mac Ayeal) • Lambert, Byrd: topographically fixed 11 Bennet (2003)

Ice Streams / Isbræ: Temperatures Whillans Ice Stream (IS B) Jakobshavn Isbræ • width 30 km / thickness 800 m • width 5 km / thickness 2500 m • vertically ∼ homogeneous • ∼ half circle • abrupt horizontal transition • dynamic influence on margins 12 Truffer & Echelmeyer (2003)

Ice Streams / Isbræ: Velocities and Stresses Whillans Whillans Jakobshavn Jakobshavn 13 Truffer & Echelmeyer (2003)

Theoretical Velocities Whillans Ice Stream Jakobshavn Isbræ • only lateral stress transfer • stress transfer to sides and bed • almost friction-free at bed • half width W = H = 2 . 5 km • shape factor f = 1 . 0 • shape factor f = 0 . 5 • half width W = 15 km • slope α = 1 . 2 ◦ • slope α = 0 . 1 ◦ • temperate bed → A = 75 MPa − 3 a − 1 • temperature − 20 ◦ C → A = 3 . 79 MPa − 3 a − 1 2 A n + 1 ( ρ g sin α ) 3 W 4 v = 2 A n + 1 ( ρ g sin α ) 3 W 4 v = ∼ 1150 m a − 1 ∼ 350 m a − 1 14