SNOWBALL EARTH (or Popsicle Planet)

• R. Monastersky, 1998. Popsicle Planet. Science News.
• P. F. Hoffman and D. P. Schrag, 2000, Snowball Earth, Scientific American, January 2000.

• Did Earth freeze over almost completely, almost killing off all life, between about 850-550 Ma?
• Did Earth alternate between almost frozen-almost boiling during that period?
• How can we use carbon isotopes (see last lecture) to find out what happened at that time?

In order to find out, we need to revisit one of the topics of last lecture: does the Cenozoic carbon and oxygen isotope records:linkage of carbon cycle and climate? Specifically, what do these records show at about 55 Ma, which might help us understand what happened during 'Snowball Earth' episodes?

How to explain these rapid carbon isotope shifts?

Whole ocean d¹³C lighter by 2 ^o/_oo, over a few thousand years. Whole deep ocean warmer by ~ 5^oC. (looks like fossil fuel buring). The usual three possible causes do not work:

1. Weathering of organic-rich rocks (oil, coal) takes hundreds of thousands of years!
2. Killing off whole biosphere: not enough to have that much effect; no evidence.
3. Volcanoes? Not rapid enough, d¹³C not light enough.

What we need to explain these isotope changes is:

Source of ¹²C (such as organic matter). From that source in the lithosphere, the carbon must be able to escape into ocean-atmosphere rapidly.

Answer: Clathrates (methane hydrate)

What are Methane Hydrates?

• Methane Hydrates are one example of 'clathrates'
• Clathrates are compounds which consist of a 'cage structure', in which one type of molecule is trapped inside a cage of other molecules; in this case: Methane (CH₄) is trapped in Water Ice (H₂O)

Grey=carbon

Green=hydrogen in CH₄

Red = oxygen

White= hydrogen in H₂O

Most common: bacterial degradation of organic matter in low-oxygen environments (within sediments). Carbon isotope composition: ~-60 ^o/_oo

Where do clathrates occur? Wherever there is:

• Methane (organic matter; in oceans mainly close to continents (on continental margins) where biological productivity is highest.
• Water
• Clathrate must be stable (ice): cold and/or high pressure

How much methane is there in gas hydrates?

Estimates vary; many indicate VERY large amounts, of about 10,000 Gigaton carbon (1 Gigaton = 10¹⁵ ton), that is at least twice as much as the carbon in all known and estimated fossil fuel (coal, oil, gas) reservoirs.

Hydrate Stability

If we could suddenly destabilize a large amounts of methane hydrate ice, we would:

Rapidly add huge amounts of methane to the atmosphere

• Methane is a strong green house gas, is oxidized to CO₂
• Positive feedback: blow up gas hydrates -> liberate methane -> warms up earth -> blows up more gas hydrates

So we would cause rapid and possibly extreme global warming, which can be recognized in the rock record by a large, negative 'spike' in d¹³C values in carbonates, because bacterially produced methane in gas hydrates has very negative d¹³C values.

• Over 3.85 billion years, life on Earth never became extinct
• All that time there must have been liquid water on Earth
• Surface temperature of Earth between 0 and 100^oC.

Thermostat of the Earth: BLAG model of long carbon cycle.

But: did the thermostat work efficiently when life was not very abundant?? Note that in the above figure living organisms are a necessary link!

• In carbonate records, there are VERY large variations in d¹³C, >500 Ma
• Did that indicate unstable climate?

There is considerable evidence for 'strange' climate: SNOWBALL EARTH (or rather: freeze-fry Earth)

• 605-595 Ma: Marinoan Glacial (SNOWBALL II); 700-690 Ma: Sturtian glacial (SNOWBALL I)
• Glacial deposits at low latitudes (paleomagnetic evidence) and at low altitude (sea level)
• VERY rapid changes from very cold to very warm climate

Development of Snowball Earth hypothesis:

Stage 1: Snowball Earth Prologue

• Breakup of a single land mass ~770 Ma: small continents near equator, high rainfall.
• Rapid weathering of rocks (wet); CO2 out of atmosphere.
• No burrowing animals; organic carbon produced, does not get from sediments back into oceans
• Ice packs at poles; ice reflects solar energy-> even colder ( + feedback)
• Planet iced over within a millennium.

Stage 2: Snowball Earth at its Coldest

• Global temperatures to -50^oC ; oceans ice over (to what depth?), limited by heat from Earth's interior.
• Much life dies; simple life forms survive at hot springs.
• Cold and dry: stops growth of land glaciers; dry: not much weathering.
• No more photosynthesis: oceans iced over; d¹³C values get more negative (to ~-6^o/_oo, volcanic values)
• Oceans go anoxic
• CO₂ emitted from volcanoes is not removed from the atmosphere; planet warms, sea ice thins.

 Stage 3: Snowball Earth as it Thaws

•  CO₂ increases 1000 fold in ~10 million years (volcanoes): warming.
• Moisture feeds land glaciers; they grow
• Open water in tropics: less heat reflected back into space -> fast rise in global temperatures.
• In centuries: a brutally hot, wet world

Stage 4: Hothouse Aftermath

• Oceans thaw, sea water evaporates, water vapor with CO₂ produces intense greenhouse
• Temperatures to ~50^oC, intense evaporation, rainfall.
• Rivers: ions into the oceans, form carbonate sediments.

BUT: does the record support this? Do carbon isotope values show the expected pattern: very low values during the coldest stage, when the oceans were iced over and there could not be any photosynthesis?

• Negative d¹³C values NOT in glacials (very few data available), but directly above, in carbonates
• 'Cap carbonates' have structures as limestones in contact with gas hydrates, and very low carbon isotope values.
• Carbon isotope swings: gas hydrate dissociation: as earth warms out of the snowball stage, gas hydrates dissociate as the oceans begin to warm slightly, and cause + feedback, rapid and extreme warming. The oceans were probably never fully frozen over (no need to fully stop photosynthesis: low carbon isotope values explained by methane hydrate dissociation, not by total end of photosynthesis). 

• Good evidence for extreme climate swings (hot to cold), cold extreme (ice in tropics)
• Not clear to which depths, over how much area, oceans were frozen: climate models say tropics remained open sea
• Life forms: 750-580 million years ago there were large (up to ~ 3 ft), multicellular organisms    ( which are called Vendobionta: 670-545 Ma). If oceans froze until depths of 1000 m: Vendobionta would not have survived, which they did.
• 'Slushball' Earth: oceans at least partially ice-free