Overview of “Snowball Earth” (observation, theory, hypothesis, puzzle, coincidence)    1. Observation Glacial deposits at sea level within 10° of 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 d13C=-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

Latitudinal distribution of solar radiation (annual) Distribution of Insolation 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).

Environmental conditions just before transition to Snowball Earth Sun 6% dimmer than today. Continents small and near equator Lots of shallow-sea continental shelf near Equator.   Cause of Snowball onset Probably atmospheric CO2 dropped from 1000 to 100 ppm, perhaps because methane had taken over CO2’s role in maintaining the greenhouse; then if methane production slowed CO2 would not be able to rise fast enough to compensate.

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 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 Dz 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 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 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 –23°C, 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 80 Snow containing 10 ppm dust 77* Sea ice covered with 1 cm of cold snow 78 Bubbly blue-white glacier ice 57 Low-latitude ocean water (before freezing) 7 Bare non-melting sea ice, Ts>-23°C 47 Bare sub-eutectic sea ice, Ts<-23°C 71 Opaque layer of NaCl.2H2O 75* Salt with 0.1% dust 58* Opaque layer of soil-dust 40 Shallow brine pool, Ts>-23°C 23 Melting sea ice, granular surface layer 60 Marine ice 25

Proposal: (laboratory, modeling, fieldwork)   1. Laboratory work in cold room (Bonnie Light) formation of salt crust inclusion/migration of brine and bubbles dust on glacier ice? 2. Modeling of radiation and heat transfer (Richard Brandt) two-layer model more-transparent ice parameterizations for climate models of albedo as function of bubble content, ice thickness transmittance as function of bubble content, ice thickness 3. Modeling of sea-glacier flow (Ed Waddington?) through Gibraltar, Bosporus

4. Fieldwork in Antarctica a. bare subeutectic sea ice: McMurdo Sound in September b. salt crust on a dry-valley lake c. blue ice at meteorite-sites  

Collaborators at UW Richard Brandt Bonnie Light Ed Waddington student (will propose)   Potential collaborations/interactions at other ABI nodes Kasting (Penn State University)

Funding This project was not included in 2000 ABI proposal. ABI funding in Years 4 & 5 for Bonnie Light's laboratory work, which has just begun Accomplishments: drafted paper "Ocean surfaces on Snowball Earth" parameterization of albedo and transmittance for climate models laboratory work on salt   Future funding from ABI and/or NSF Polar Programs