1. Paleoproterozoic Snowball Earth: Earth is glaciated to the equator 2
Paleoproterozoic Snowball Earth: Earth is glaciated to the equator 2.2 Byr ago
At about 2.2 Ga:
Earth was glaciated at the equator, at sea level
Evidence for the equator; evidence for sea level
Sulfur isotopes indicate a change in atmospheric chemistry
Manganese ores are deposited in large amounts
Are these phenomena connected?
What caused the glaciation?

3. Evidence for glaciation in South Africa, 2.3 Ga
Makganyene diamictite is a deposit of glacial debris left when glaciers melted
Ongeluk volcanics allow dating and determining latitude
Hotazel dropstones are rocks dropped by icebergs into fine marine muds
Kalahari MnO2 deposit appears after ice age ends
Cobble with glacial grooves, Makganyene diamictite, J. Kirschvink, Caltech
Kirschvink et al., 2000

4. What is the evidence for sea level glaciation?
Upper right: glacial rocks within marine sediments, Makganyene Formation
Lower right: Pebble-scored rock, Makganyene Formation
Below: glacial dropstones are present in the bottom of the overlying Hotazel Formation
Dropstones are iceberg-transported and dropped upon melting into marine sediments
Fig. 4. (A and B) Clast bearing stromatilitic bioherms within the Makganyene diamictites. (C) Clast-free stromatolitic bioherm within the Makganyene
diamictites. (D) Large striated pebble from the Makganyene Formation at the Griquatown hinge zone.
Polteau et al., 2006
Courtesy of J. Kirschvink, Caltech

5. What is the evidence that South Africa was at the equator at 2.2 GA?
Continents “drift” over Earth’s surface
Global rearrangements take about 200 Myr
Examples:
At 200 Ma, India was stuck to Antarctica
At 50 Ma, Australia was stuck to Antarctica
We need to find out where South Africa was at 2.2 Ga
Use the imprint of Earth’s ancient magnetic field
Volcanic rocks have magnetite (magnetic)
Grains align along Earth’s magnetic field lines
Field orientation and grain orientation depend on latitude
Grains now exert their own magnetic field
Direction reflects latitude when rocks cooled (magma solidified)
Right: Ongeluk pillow lavas formed under water, used for magnetic orientation studies
Courtesy Joe Kirschvink, Caltech

6. Switch gears: O2 photosynthesis evolved or became a lot more prevalent at about 2.45 Ma: there is the first evidence for some O2 in air
Photosynthesis: CO2 + H2O  CH2O + O2
Plants develop the ability to photosynthesize between 3.7 – 2.5 Byr ago
The ability improves until photosynthesis is rapid by 2.45 Ba
O2 becomes more abundant in air at this time, with effects on climate

7. Mass independent sulfur isotope fractionation through time suggests that UV radiation was intercepted above the troposphere by about 2.45 Ga (i. e., ~ 10 ppm O2 in air by 2.45 Ga)
Small ∆33S,
No UV reaching troposphere,
O2>10-5* present conc.
Large ∆33S,
UV reaches troposphere,
O2<10-5 * present conc.
Inter-mediate O2?

8. Manganese deposits: origin and significance
Black beds in photo below, from Hotazel Formation, are MnO2 deposits
These are the oldest known
Formation: Mn+2 is soluble, reaches high concentrations in O2-free seawater
In presence of O2:
2 Mn2++O2 + 2H2O --> 2 MnO2 + 4 H+
MnO2 insoluble, precipitates as black mineral
Mn2+ oxidation is possible after 2.45 Ba, because there is O2in air
No more MIF’s; O2 in air

9. So what happened? - a hypothesis
Before about 2.45 Ga, Earth was warmed partly by a CH4 greenhouse
Between about Ga, O2 levels in the atmosphere rose
By 2.2 Ga, O2 was high enough that CH4 was rapidly oxidized
CH4 + 2 O2 --> CO2 + 2 H2O
And/or, O2 in air oxidizes OCS and we lose its greenhouse
3 O2 + 2 OCS --> 2 CO2 + 2 SO2
Without CH4 and/or OCS greenhouse, CO2 alone is too low to keep Earth from freezing over
Global “Snowball” glaciation results
How does a global snowball glaciation end?

10. Neoproterozoic Snowball Earth
Earth was glaciated to sea level at the equator (Snowball events”) at ~ 715 Ma (Sturtian) and again ~ 635 Ma (Marinoan)
Snowball hypothesis
Earth was completely covered with ice
Events lasted millions of years
Snowball terminated by atmospheric CO2 buildup
3. Slushball hypothesis
Continents were glaciated to equator, but there was open ocean
Events were cyclic (glaciers grew and retreated)
4. Climate models of snowball events

11. Slushball view: Multiple glaciations with open water
Earth was repeatedly glaciated during the Neoproterozoic
Slushball view: Multiple glaciations with open water
Age of individual glacial event (Ma)
690
780
750
720
660
630
Dates of glacial events
Colored bars indicate periods of glaciation
Allen and Etienne, 2008
1 or 2 glaciations may have been snowball events
Sturtian (715 Ma)
*Marinoan (635 Ma)*

12. Evidence for sea level glaciation at the equator
Evidence for glaciation comes from looking at the rocks
Dropstones are released by icebergs, indicate local sea level at equator
Diamictites are deposits of glacially ground dirt and rocks dropped when the glacier melts
Evidence for tropical location comes from paleomagnetics

13. Some evidence for tropical glaciation during the Marinoan
Striated stone from Mauritania, pebbles in ice cut into rock Hoffman and Schrag, 2002
Dropstone in Namibian sediments; transported by iceberg, fell when ice melted Hoffman and Schrag, 2002
Dropstones in Namibian sediments, ovelain by white carbonate deposit Hoffman and Schrag, 2002

14. The Snowball Earth hypothesis
1969: Mikhail Budyko suggests that Earth can enter a Snowball state but never exit: Snowball reflects sunlight and maintains itself
1992: Joe Kirschvink writes 2-page paper outlining argument for Snowball Earth and explaining how Earth reverts to a normal climate
1998: Dan Schrag and Paul Hoffman expand on Kirschvink’s ideas and present experimental evidence for a Snowball cycle

15. Snowball Earth: Kirschvink/Schrag and Hoffman hypothesis-1
Some feature associated with the concentration of continents in the tropics leads to the planet becoming very cold (note continental positions and dominance of tropics)
Ice line extends equatorward as Earth cools
Ice line reaches 30˚ latitude; remainder of Earth freezes as absorbed sunlight is insufficient to prevent freezing
Snowball Earth ensues, persists due to high albedo
Hydrologic cycles slows, glaciers almost cease to flow
Animals face some challenges!

16. Snowball Earth: Kirschvink/Schrag and Hoffman hypothesis-2
The natural cycle of CO2 on the planet leads to the end of the Snowball, warm overshoot, and the eventual restoration of equable climates
The atmosphere
What sets the level of CO2 in air?
If addition is faster than uptake, concentration rises.
Temperature rises.
Uptake is faster.
Uptake comes into balance with input.
CO2
CO2
Soils
MgSiO3 + 2 CO2 + H2O
--> Mg2+ + SiO2 + 2 HCO3-
The solid Earth
(Contains most of the CO2 on the planet)

17. Snowball Earth: Kirschvink/Schrag and Hoffman hypothesis-3
Volcanic CO2 accumulates in the atmosphere, Earth gradually warms
Warming melts ice at equator
Albedo decreases, earth warms, more ice melts
Snowball state catastrophically ends
Hothouse state: no ice, high CO2, high temperatures
Weathering is very rapid, high delivery of chemicals to oceans
Massive amounts of CaCO3, MgCa(CO3)2 precipitate among other things
CO2 is drawn down, Earth returns to “normal” state

18. Snowball cycles according to Hoffman and Schrag
Starting point
Continents around equator take up CO2; T falls, ice line extends.
3. When ice reaches ~ 30˚, planet is cold, ice line immediately extends to the equator (Earth is snowball). Glacial deposits form.
4. CO2 uptake stops but release continues to atmosphere. Ice reflection keeps planet frozen but CO2 rises in air.
5. CO2 rises high enough to melt ice; albedo decreases and planet warms abruptly. Weathering is rapid and cap carbonates form.
1. Starting condition 2 5 3 4

19. Snowball hypothesis: is there evidence for the predicted CO2 changes?
No direct record of Neoproterozoic concentrations of atmospheric CO2
13C gives indirect information
2 carbon isotopes, 12C (98.9 %) and 13C (1.1 %)
13C = [ {(13C/12C)sample / (13C/12C)standard} - 1] *1000, per mil (‰)
Relevant information about carbon isotopes
CO2 entering the atmosphere from volcanoes has 13C~ - 5‰
Over geologic timescales, CO2 exchanges between ocean and atmosphere
In the ocean, there is equilibrium between CO2, HCO3-, and CO32-; the three species together form “dissolved inorganic carbon”
13C of CaCO3 ~ 13C of dissolved inorganic carbon
13C of organic carbon ~ 13C of dissolved inorganic carbon - 20 ‰
13C of CaCO3 depends on the ratio in which organic/CaCO3 are buried

20. Snowball hypothesis: is there evidence for the predicted CO2 changes
Snowball hypothesis: is there evidence for the predicted CO2 changes? - the data-3
Later after Snowball: Ocean recovers,13C increases as organic matter forms
Immediately after Snowball: CaCO3 precipitates with 13C at seawater/volcanic value (-5‰)
Snowball isolation: add volcanic CO2, 13C shifts to volcanic value
Pre/early Snowball: 13C~+5‰
~50% of CO2 removed as organics

21. Snowball hypothesis: is there evidence for the predicted CO2 changes
Snowball hypothesis: is there evidence for the predicted CO2 changes? - the data-1
Outcrop of rock in Namibia recording a complete Snowball Earth cycle
Elevation ~ 500 m

22. Snowball hypothesis: is there evidence for the predicted CO2 changes
Snowball hypothesis: is there evidence for the predicted CO2 changes? - the data-3
Extensive CaCO3 deposits
Postglacial cap CaCO3/MgCO3
Glacial deposits from Snowball time

23. Snowball hypothesis: is there evidence for the predicted CO2 changes?
No direct record of Neoproterozoic concentrations of atmospheric CO2
13C gives indirect information
Relevant information about carbon isotopes
CO2 entering the atmosphere from volcanoes has 13C~-5‰
Over geologic timescales, CO2 exchanges between ocean and atmosphere
In the ocean, there is equilibrium between CO2, HCO3-, and CO32-; the three species together form “dissolved inorganic carbon”
13C of CaCO3 ~ 13C of dissolved inorganic carbon
13C of organic carbon ~ 13C of dissolved inorganic carbon - 20 ‰
There is more information:
If no organic matter forms, 13C of CaCO3 is -5‰
If nearly all DIC is removed as organic carbon
13C of organic C = -5‰
13C of dissolved inorganic carbon = +15‰
13C of CaCO3 = +15‰
If half of C is removed as CaCO3 and half as organic C
13C of organic C = -15‰
13C of CaCO3 = +5‰