The snowball Earth hypothesis: testing the limits of global change

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The snowball Earth hypothesis testing the limits
of global change

• Paul Hoffman and Daniel Schrag
• Earth and Planetary Sciences, Harvard
• Terra Nova, 14, 129-155, 2002

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Fig 1. (a) Global distribution of Proterozoic
glacier deposits, based on Natural Remanent
Magnetization, or paleomagnetic data (direction
of Earths magnetic field in distant past)
indicative of latitude.(b) histogram based on
paleolatitude
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Fig 2. Neoproterozoic glacial features
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(a) striated pavement, overlain by diamictite - a
mixture of fine material, stones, and boulders -
deposits (similar to tillite), northern
Norway(b) multi-striated stone diamictite,
Mauritania
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(c) diamictite, distinctly surrounded by
carbonate strata (indicative of warm climates),
in Mackenzie mountains, NW Canada(d) dolomite
dropstone (rock fragments dropped through water
to ground) in banded iron formations, MacKenzie
NW Canada
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(e) ice-rafted dropstone in a diamictite,
Namibia, impact of dropstone on sea-bed caused
the folded layering(f) contact between upper
dropstone (diamictite) and cap dolomite - a
sediment formed from carbonate elements - Namibia
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Fig 4. Geology in the Mackenzie Mountains, NW
Canada(a) Keel Formation is probably
glacial(d) zone of storm wave currents in
glacial deposits
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Fig 5. Cap carbonate sequences in Namibia,
showing glacial layers overlain by warmer
carbonates
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Fig. 6. Ice-line latitudes (at sea level) as a
function of solar flux (up to 30 change), or
equivalent pCO2 there are 3 stable points
(yellow dots) Lower section shows instability lt
30 degrees drives system to ice-covered branch
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- Earth is now at point 1- lowering pCO2
causes ice lines to migrate stably towards
equator (2)- runaway albedo-feedback causes
instability to ice-covered Earth (3-4) - normal
outgassing over My causes increase of pCO2 back
to 4- reverse albedo effect deglaciates Earth
to point 5where a high pCO2 causes transient
ultra- greenhouse 5-enhanced silicate
weathering reduces pCO2 to 6-polar caps re-form
and Earth returns to 7 (1)
Note small ice-cap instability (between 1 and 6)
predicts ice-free Arctic due to anthropogenic
warming
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Fig. 6. Ice-line latitudes (sea level) as a
function of solar flux, or equivalent pCO2 there
are 3 stable points (yellow dots )
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Changes in mean surface global temperature
through 1 snowball cycle
1 cycle 5 My
numbers 1-7 refer to points on previous figure
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Frequency of occurrence of iron formations, and
constraints on atmospheric oxygen (note age axis
is reversed)
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end of main argument
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additional figures from paper

• Paul Hoffman and Daniel Schrag
• Earth and Planetary Sciences, Harvard
• Terra Nova, 14, 129-155, 2002

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