The importance of Antarctic blue ice for understanding the tropical ocean of Snowball Earth

Stephen WarrenUniversity 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 1-billion-year absence Repeated glaciations for ~200 Myr Carbon isotopes 13C=-5‰ in cap carbonates indicating no biological fractionation

2. TheoryPositive feedback of snow albedo results in an instability in many climate models. (Budyko, 1969; Manabe 1975; . . . )  

Albedo (percent)(reflectance for solar radiation)

Dry snow 80Bare glacier ice 60Bare cold thick sea ice 50Ocean water 7

Latitudinal distribution of solar radiation (annual)

Latitude

Dis

trib

utio

n of

Ins

olat

ion

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. PuzzleSome surface life continued through these episodes.  Photosynthetic eukaryotic algae require both liquid water and sunlight. 

5. CoincidenceShortly 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 lifeExample 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 iceS = solar heating below level zFL = 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.

What’s needed to calculate ice thickness: Sublimation rate at topFreezing rate at baseHeat conduction through iceComposition of sea ice, ice shelvesAlbedos of snow and icePenetration 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 SmithGet snowball if CO2 drops to 1700 ppm

(sea-ice albedo 0.65) Crowley and BaumGet 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 snow-free “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 80Snow containing 10 ppm dust 77*Sea ice covered with 1 cm of cold snow 78Bubbly blue-white glacier ice 57Low-latitude ocean water (before freezing) 7Bare non-melting sea ice, Ts>-23C 47

Bare sub-eutectic sea ice, Ts<-23C 71

Opaque layer of NaCl2H2O 75*

Salt with 0.1% dust 58*Opaque layer of soil-dust 40Shallow brine pool, Ts>-23C 23

Melting sea ice, granular surface layer 60Marine ice 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 Mawson Station (coastal East Antarctica). Weller, 19680.55 Dronning Maud Land (DML) ~1250 m.

Bintanja & van den Broeke0.58-0.61 DML coastal. Reijmer, Bantanja, Greuell0.53 (average 0.55 but partly snow-covered) DML.

Bintanja, Jonsson, Knap0.65 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.