The importance of Antarctic blue ice for understanding
The importance of Antarctic blue ice for understanding the tropical ocean of Snowball Earth Stephen Warren University of Washington Seattle, USA Overview of Snowball Earth
(observation, theory, hypothesis, puzzle, coincidence) 1. Observation Glacial deposits at sea level within 10of the paleoequator, in the Neoproterozoic, 750-700 and 620-580 Ma (Harland, 1964; Evans, 2000).
Observations explained by Snowball Earth hypothesis Glacial deposits at low paleolatitudes Thick carbonate layers capping glacial deposits Iron deposits (banded iron formations) after 1billion-year absence Repeated glaciations for ~200 Myr Carbon isotopes 13C=-5 in cap carbonates
indicating no biological fractionation 2. Theory Positive feedback of snow albedo results in an instability in many climate models. (Budyko, 1969; Manabe 1975; . . . ) Albedo (percent)
(reflectance for solar radiation) Dry snow 80 Bare glacier ice 60 Bare cold thick sea ice 50 Ocean water
7 Distribution of Insolation Latitudinal distribution of solar radiation (annual)
Latitude 3. Hypothesis This runaway albedo feedback catastrophe actually occurred during the Neoproterozoic. Each event lasted ~10 Ma, and was ended by the greenhouse effect due to buildup of atmospheric CO2
from volcanoes (Kirschvink 1992; Hoffman & Schrag 1998). 4. Puzzle Some surface life continued through these episodes. Photosynthetic eukaryotic algae require both liquid water and sunlight.
5. Coincidence Shortly after the final Snowball event: The Cambrian Explosion 575-525 Ma. Numerous animal phyla first appear as fossils. The NASA Astrobiology Roadmap
Des Marais et al., 2003 Goal 4: Understand how past life on Earth interacted with its changing planetary and Solar System environment. Investigate the historical relationship between Earth and its biota by integrating evidence from both the geologic and biomolecular records of ancient life and its environments. Background. . . . How did life respond to major planetary disturbances,
such as bolide impacts, sudden atmospheric changes, and global glaciations . . . Objective 4.2. Foundations of complex life Example investigations. Study . . . proxies of environmental change in Neoproterozoic rocks to better understand the history of global climatic perturbations that may have influenced the early evolution of complex life.
If the oceans did indeed freeze to the Equator, where did surface life survive? 1. At local geothermal hotspots 2. Under thin tropical snowfree sea ice 3. In unfrozen parts of the tropical ocean 4. In water-filled crevasses at shear margins
of sea-glaciers (ice shelves) 5. Under thin ice on deep tropical lakes 6. . . . k dT/dz = S(z) + FL + Fg k = thermal conductivity of ice S = solar heating below level z
FL = latent heat released by freezing at base Fg = geothermal heat flux Ice thickness z is inversely proportional to these heat fluxes. Ice can be kept thin if sunlight penetrates through ice; absorbed heat must be conducted upward. Difficulty of maintaining thin ice in tropics: To keep temperature below freezing, albedo must be high, so ice must
contain lots of scatterers (e.g. bubbles). But these same scatterers impede transmission of light through ice. Whats needed to calculate ice thickness: Sublimation rate at top Freezing rate at base Heat conduction through ice
Composition of sea ice, ice shelves Albedos of snow and ice Penetration of solar radiation into snow and ice Albedo of ice and snow on the ocean surface determines: Drawdown of atmospheric CO2 necessary to initiate
snowball Critical latitude for ice-albedo instability Surface temperatures of Snowball Earth Duration of a snowball event (how much volcanic emission of CO2 is required to warm the climate to melt the ice)
Climate modeling of Snowball Earth Jenkins and Smith Get snowball if CO2 drops to 1700 ppm (sea-ice albedo 0.65) Crowley and Baum Get snowball only if CO2 drops to 40 ppm (sea-ice albedo 0.5)
Surface Types on the Snowball Ocean Snow-covered oceans at high and middle latitudes. Where precipitation exceeds evaporation, the surface will be dry snow with albedo about 0.8. Snow-free glacier ice exposed in the subtropics. This ice will resemble the snowfree blue ice surfaces found near Antarctic mountains. This ice has a high albedo (about 0.6) because it contains numerous bubbles, since its origin was compression of snow.
Frozen seawater exposed at the equator. If the sublimation rate exceeds the net inflow of sea-glaciers, frozen seawater will reach the surface. The albedo of bare non-melting first-year sea ice is about 0.5, but it rises to 0.7 if the temperature drops below 23C, because salts precipitate in the brine inclusions. Development of salt crust. The initial catastrophic freezing of the low-latitude ocean surface will result in sea ice with salinity 4-6. After 200-2000 years the top 3 meters of ice would sublimate away, leaving a salt crust with albedo 0.75.
Surface Type Albedo (percent) Ice shelf covered with thick cold snow
Snow containing 10 ppm dust Sea ice covered with 1 cm of cold snow Bubbly blue-white glacier ice Low-latitude ocean water (before freezing) Bare non-melting sea ice, Ts>-23C Bare sub-eutectic sea ice, Ts<-23C Opaque layer of NaCl2H2O
Salt with 0.1% dust Opaque layer of soil-dust Shallow brine pool, Ts>-23C Melting sea ice, granular surface layer Marine ice 80
77* 78 57 7 47 71 75*
58* 40 23 60 25 Pollard and Kasting (JGR, 2005): Thin ice is possible at the equator if
the albedo of snow-free sea-glaciers is reduced to 0.47. Modern examples of bare cold glacier ice exposed by sublimation: Blue-ice surfaces in Antarctica Albedos: 0.66 0.55
Mawson Station (coastal East Antarctica). Weller, 1968 Dronning Maud Land (DML) ~1250 m. Bintanja & van den Broeke 0.58-0.61 DML coastal. Reijmer, Bantanja, Greuell 0.53 (average 0.55 but partly snow-covered) DML. Bintanja, Jonsson, Knap 0.65 DML (low elevation). Liston, Winther, et al.
0.63 Trans-Antarctic mountains. Warren et al. 1993 Conclusion Measurements of albedo on Antarctic blue ice are important not only for the local surface energy budget; they also may explain why there are animals on Earth.
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