Transcript Observations explained by “Snowball Earth”

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 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 1billion-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
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 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 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 –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
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>-23°C
Bare sub-eutectic sea ice, Ts<-23°C
Opaque layer of NaCl.2H2O
Salt with 0.1% dust
Opaque layer of soil-dust
Shallow brine pool, Ts>-23°C
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.