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?
Evidence for glaciation in South Africa, 2.3 Ga
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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
What is the evidence for sea
level glaciation?
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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
Courtesy of J. Kirschvink, Caltech
Polteau et al., 2006
What is the evidence that South
Africa was at the equator at 2.2 GA?
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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
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
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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
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.
Intermediate
O2 ?
Manganese deposits: origin and significance
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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
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Mn2+ oxidation is possible after 2.45 Ba,
because there is O2in air
No more
MIF’s; O2 in air
So what happened? - a hypothesis
• Before about 2.45 Ga, Earth was warmed partly by a CH4 greenhouse
• Between about 2.4 - 2.2 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?
Neoproterozoic Snowball Earth
1.
2.
3.
4.
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
Slushball hypothesis
Continents were glaciated to equator, but there was open ocean
Events were cyclic (glaciers grew and retreated)
Climate models of snowball events
Earth was repeatedly glaciated during the Neoproterozoic
Slushball view: Multiple glaciations with open water
Age of individual glacial event (Ma)
780
750
720
690
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)*
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
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
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
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!
Snowball Earth: Kirschvink/Schrag and Hoffman hypothesis-2
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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
CO2
CO2
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.
Soils
MgSiO3 + 2 CO2 + H2O
--> Mg2+ + SiO2 + 2 HCO3-
The solid Earth
(Contains most of the CO2 on the planet)
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
Snowball cycles according to
Hoffman and Schrag
1.
Starting point
2.
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.
1. Starting
condition
2
5
3
4
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.
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
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
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
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
Snowball hypothesis: is there evidence for the predicted
CO2 changes?
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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‰
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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‰