Download Snowball Earth

Snowball Earth
Presented by Mindi Purdy and Jen Ulrich
Theory of Snowball Earth
• Many lines of evidence support a theory that the
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entire Earth was ice-covered for long periods
600-700 million years ago.
Each glacial period lasted for millions of years
and ended violently under extreme greenhouse
conditions.
Proposes that these climate shocks triggered the
evolution of multicellular animal life and
challenge long-held assumptions regarding the
limits of global change.
Sun Strength
• Main sequence
•
stars: radiate more
energy as their
helium cores grow
more massive.
The sun’s luminosity
in the
Neoproterozioc
period was only
93% - 94% of its
present value
(Hoffman).
Albedo
• Planetary albedo is defined as the
fraction of incoming radiation that is
reflected back to space. It could also
be considered in terms of the degree of
whiteness.
Ice-Albedo Feedback
• For any imposed
cooling, the
resulting higher
albedo would
cause further
cooling. This
positive
feedback also
applies to
warming.
Runaway Ice Albedo
• If Earth’s climate cooled, and ice formed
at lower and lower latitudes, the planetary
albedo would rise at a faster and faster
rate because there is more surface area
per degree of latitude as one approaches
the equator (Hoffman).
• Carbon Dioxide absorbs infrared radiation
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emitted from the Earth’s surface.
Key to reversing Runaway freeze
It is emitted from volcanoes
– Offset by erosion or silicate rocks
• Chemical breakdown of the rocks converts CO2
to bicarbonate and is washed into oceans.
• Bicarbonate combines with Calcium and
Magnesium ions to produce carbonate
sediments.
• Joseph Kirschvink pointed out that during
Snowball Earth shifting tectonic plates
would continue to build volcanoes and to
supply the atmosphere with CO2.
• At same time liquid water needed to erode
rocks and bury Carbon is trapped in ice.
• Eventually CO2 level would get high
enough that it would heat up planet and
end Snowball Earth.
Paleomagnetism
• uses the alignment of magnetic minerals in rock
deposits (termed natural remnant magnetization)
to determine where the deposits were formed.
• Before rocks harden, grains aligned themselves
with magnetic field.
– If formed near poles, magnetic orientation
would be nearly vertical
– Instead found the grains dipped only slightly
relative to horizontal because of their position
near the equator.
• Nambia’s Skeleton Coast
– Provides evidence of glaciers in rocks formed
from deposits of dirt and debris left behind
when ice melted.
– Also found rocks dominated by calcium and
magnesium just above debris.
– Chemical evidence that a hothouse could
have followed.
Critical Element: Location of the
Continents
• Harland’s idea based on assumption that
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continents were all located near the equator
during the Neoproterozoic period.
Reasoning
– When continents near poles, CO2 in atmosphere
remains high enough to keep planet warm.
– If continents cluster in tropics, they would remain icefree as the earth grew colder and approached critical
threshold for Runaway freeze.
– In other words, the CO2 “safety switch” would fail
because carbon burial continues unchecked.
Carbonate Clues
• Neoproterozoic blanketed by carbonate
rocks which form in warm shallow seas.
• Transition from glacial deposits to cap
carbonates abrupt and lacks evidence
significant time passed
• Thick sequence of extreme greenhouse
conditions unique to transient aftermath of
Snowball Earth.
Extremophiles
Extremophiles are organisms that live in
extreme conditions. Evidence for
survival of these organisms during
snowball earth events are found in
these areas:
• Hydrothermal vent communities
• Hot springs
• Very cold areas - cold-loving organisms
(psychrophilic)
Bottleneck Effect
• Population bottleneck and flushes
(environmental filters) are observed to
accelerate evolution in some species
(Hoffman).
• It is known that various organisms
undergo chromosomal reorganization in
the face of environmental crisis
(Carson).
Arguments Against
Obliquity/Seasonality:
• A high obliquity (greater than 54)
would allow the poles to receive more
energy than the equator, and ice could
form at the equator
• But high obliquity enhances seasonality.
Stronger seasonality increases summer
ablation and also decreases
accumulation of winter snow because
colder air tends to be drier.
Obliquity/Seasonality
Obliquity/Seasonality
Arguments Against
Inertial-Interchange True Polar
Wander:
• Entire crust and mantle rotates relative
to Earth’s spin axis
• Rapid transitions from low-latitude to
high latitude
• Explains how equatorial glaciation could
have occurred without a deep freeze
Arguments Against
Evidence for open ocean at equator:
• Simulations found that an area of open
water in the equatorial oceans is
consistent with the evidence for
equatorial glaciation at sea level
• In a more complex model, Earth was
able to freeze over in a slab ocean, but
in the real ocean model, it transports
enough heat in currents to the ice
margin to hold the ice off (Kerr).
Arguments Against
Survival of life without
sunlight/oxygen:
• organic photosynthesis would be
severely reduced for millions of years
because ice cover would block out
sunlight
• Meltwater pools
• Bare ground
Arguments Against
• Strontium:
• 87Sr/86Sr should decline during snowball
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events due to hydrothermal dominance and
decreased riverine input and organic
productivity
87Sr/86Sr is sensitive to buffering by carbonate
dissolution and has a long residence time
Evidence has found that glacial and postglacial 87Sr/86Sr ratios were not significantly
different from preglacial values (Hoffman).
Isotope Analysis
Research Question: Is there a correlation
between isotope data and the Varanger
Glaciation in the Vendian Period?
Isotope Analysis
• Isotope data from “The Vendian Record of Sr and C
Isotopic Variations in Seawater: Implications for
Tectonics and Paleoclimate” (Kaufman, et. al.)
• Three different locations:
– Polarisbreen Group and equivalents, Svalbard and East
Greenland (A)
– Nama and Witvlei Groups, Namibia and South Africa (B)
– Windermere Supergroup, Arctic Canada (C)
• Vendian Period- a time period from 540 million years
ago (Ma) to 610 Ma; Varanger Glaciation- ~585 Ma
to 610 Ma
Isotope Analysis
• Carbon-13
– Outgassing of carbon from
volcanoes- 99% C-12, 1%
C-13
– Removed from oceans by
burial calcium carbonate
and in the form of organic
matter (depleted in C-13)
– When biological productivity
is high, oceans are enriched
in C-13; when productivity
is low, oceans are depleted
in C-13
Isotope Analysis
• Oxygen-18
– Evaporation preferentially removes water with
Oxygen-16, which makes this isotope abundant in the
atmosphere
– Precipitation and runoff returns water high in O16 to
the Earth’s surface, but during glacial periods, O16 is
preferentially stored in ice
– O-18 levels are higher in the sediments during colder
(particularly glacial) periods
– Difference in O18 and O16 ratios between carbonates
and the water from which they precipitate is a
function of temperature (Willson)
Isotope Analysis
• Sr87/Sr86
– The ratio of the heavier Sr87 to the lighter Sr86
reflects climate change
– A higher value indicates increased terrestrial
erosion and decreased hydrothermal activity,
whereas a lower ratio denotes hydrothermal
dominance and a decreased riverine input
Isotope Analysis
Isotope Analysis
Isotope Analysis
Isotope Analysis
Isotope Analysis
• From the correlation graph, as C13 and O18
levels plummeted, the Sr87/Sr86 ratio
increased significantly. Therefore, it seems
that the Sr ratio has an inverse
relationship with C13 and O18 levels.
Isotope Analysis
• Carbon-13
– Breakup of supercontinent- more continental marginsincreased drawdown of organic carbon- high C13 values
observed before the glaciations at approximately 610 Ma
– Major drop in C13 levels correlates with enhanced ocean
circulation, increased erosion of sedimentary carbon
(Kaufman, Jacobsen, and Knoll), and the (almost)
cessation of biological productivity associated with such
an extreme glaciation.
– Large positive excursion around 580 Ma is consistent with
the hothouse theory in that a large amount of carbon
dioxide (350 times current value) would be required to
melt a snowball earth, and biological productivity would
increase significantly.
Isotope Analysis
• Oxygen-18
– Oxygen-18 levels should be higher during
colder periods, because O16 is preferentially
stored in ice, and seawater is enriched with
O18.
– The rise in O18 levels occurs around 580 to
585 Ma, which could correlate with the height
of the Varanger Glaciation if measurement
uncertainty is taken into account.
Isotope Analysis
Isotope Analysis
• Sr Ratio
– Large overall increase in the Sr ratio would
have been created by a combination of :
• erosion of the aging upper crust and
• tectonic events- continent-continent collisions
(Pan-African and Himalayan-Tibetan)
• decrease in the hydrothermal contribution to the
Sr87/Sr86 budget