Ice on Earth: An overview and examples on physical properties - Ice on Earth during the Pleistocene - Present-day polar and temperate ice masses - Transformation of snow to ice - Mass balance, ice deformation, flow of glaciers and ice sheets. Thorsteinn Thorsteinsson Session # 23, Monday 1/17/05 Arctic: Up to 4 km deep ice-covered Antarctic: Up to 4 km thick ice ocean surrounded by land masses. sheet surrounded by oceans. Permanent snow and ice in the Northern Hemisphere: Area Vol. Sea level 106 km2 106 km3 eq. (m) Greenland 1.73 3.0 7.5 Other locations 0.5 0.12 0.3 Sea ice 8.87 Total NH 11.0 Greenland: Research activity focussed on ice core studies and estimation of current mass balance. Antarctica: Research focus on: - Climate history from ice cores SP Vostok - Subglacial lakes - Possible instability of West Antarctic ice sheet Dome C Permanent snow and ice cover in the Southern Hemisphere: Area Volume Sea level 106 km2 106 km3 eq. (m) Antarctica 13.0 29.4 73.5 Other locations 0.032 <0.01 <0.02 Sea ice 4.2 Total SH 17.23 Total entire globe 28.3 The Pleistocene: Last 2-3 million years (except Holocene) Characterized by large variations in ice volume on the continents. At least 20 glacial-interglacial cycles, each lasting 100-150 ka. Eemian Last interglacial (110-135 ka BP) Wisconsin/Weichselian: Last glacial period (110-11.5 ka BP) Holocene: Present interglacial (11.5 ka BP – present) The Quaternary: Pleistocene + Holocene Past ice sheets: Laurentide ice sheet (covering northern part of Northern America) Scandinavian ice sheet Barents and Kara sea ice sheets Siberian ice sheet? Svendsen et al., 2004 A reconstruction of Eurasian ice sheets at the Last Glacial Maximum (LGM, about 20,000 years before present - 20 ka/kyr BP). Long-term changes in ice-sheet extent generally believed to be due to insolation variations resulting from changes in Earth´s orbital parameters: eccentricity obliquity precession Schematic relationship between orbital forcing and preserved paleoclimate signal in a sedimentary record (marine sediment, ice core, pollen record, etc.) MIS (Marine Isotope Stage) Summer sea-surface temperature reconstruction for the North Atlantic Ocean over the last 140,000 years. Eemian interglacial: Holocene Wisconsin Sea level up to 6 m higher than during the Holocene Sea level varies with changes in ice volume on the continents. Data for last 135,000 yr, radiometrically dated coral reef terraces in New Guinea. In studies of past and present terrestrial ice masses we distinguish between: Polar ice: Temperature below freezing point in entire ice mass Temperate ice: Entire glacier/ice cap at freezing point, except top 15 m during winter. Polythermal ice: Part polar / part temperate Surface layers Polar ice: Seasonal variation in snow Temperate ice: Winter cold wave temperature down to 15 m depth eliminated by latent heat of refreezing (model result). Temperature below 15 m meltwater. All ice below 10 m at equal to mean annual temperature. melting point. Temperature profiles through polar ice sheets: GRIP, Central Greenland (accumulation rate: 0.23 m ice/yr) This profile can be modelled using the heat transfer equation, which takes into 500 account heat conduction in the ice, and advection of heat as ice moves downward. 1000 Near-constant temperature in upper 1500 10 kyr 1500 m. 2000 Ice warms towards bedrock due to the geothermal heat flux. 2500 100 kyr 3000 Temperature profiles through polar ice sheets: Vostok, East Antarctica (accumulation rate: 0.025 m/yr) 10 kyr Much lower annual accumulation and thus much slower transport of cold surface ice downwards. Temperature increases steadily with depth towards bottom, where melting point is reached. 100 kyr Transformation of snow to ice: Rounding and settling of crystals, gradual increase in crystal size and formation of bonds between crystals. Air space thus gradually decreases and density increases. Typical densities: Snow: 50-400 kg/m3 Firn: 400-830 kg/m3 Glacier ice: 830-920 kg/m3 Glacier ice (ρ > 830 kg/m3) has formed when remaining air is sealed off in bubbles. The rate of transformation is highly temperature dependent: Temperate ice (0 °C) Completed at 15-35 m depth, takes 10-20 years. Greenland ice sheet (-30 °C): 70 m, 200 years East Antarctica (-55°C): 110 m, 2000 years Density variation in a deep Greenland core: 0-3000 m Air bubbles have entirely disappeared due to pressure of overlying ice. Volume expansion due to warming of ice near bedrock. Density (g/cm3) Schematic of an ice sheet: Velocity vectors in a glacier: A glacier is a mass of ice that deforms under its own weight! . n The “flow law for ice”: ε = Αοexp(-Q/RT)σ . σ = stress, ε = strain rate, Q = activation energy, R = gas constant, T = temperature is commonly used to describe the relationship between applied stress and resulting strain rate. Laboratory experiments and studies of glacier flow indicate that n~3. The deformation rate is highly dependent on temperature (5 times higher at -10 °C than at -25 °C). The factor Αο varies with impurity content, crystal size and c-axis orientation. Elastic deformation: Linear relationship between stress and strain (σ=Eε) Newtonian viscous deformation: . Linear relationship between stress and strain rate (σ= ηε) Perfectly plastic deformation: Material does not deform until high stress (yield stress) is applied, then deforms very rapidly. Ice deformation intermediate between Newtonian viscous and perfectly plastic behaviour (n=3 curve). Two mechanisms of glacier/ice sheet movement: A volume element of ice can be Ice at melting point can slide deformed into a different shape, over the bed. without changing the total volume (incompressible material). No sliding occurs if ice is frozen to the bed. The regelation mechanism of basal sliding: Ice flows from left to right and encounters a bedrock bump (L < 1 m). Melting point depressed on upstream side, increased on downstream side. Heat flow through bump melts ice on upstream side and refreezing occurs on downstream side. Confirmed by observations..