Transcript Sun Strength - Miami University

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.
What Evidence Supports the theory
of Snowball Earth
• Sun’s radiation/Earth’s Albedo
• Glacial Deposits
• Paleomagnetism
• Carbon Dioxide
• Isotopes
• Evolutionary burst
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).
Heat Balance
• The earth’s surface temperature is
governed by the heat balance between
incoming solar radiation and outgoing
radiation emitted by surface or near
surface.
• In layman’s terms: heat absorbed should
equal heat emitted
Heat Balance
R2Es(1-)=4R2(fTs4)
• R is the planetary radius
• Es is the solar irradiance
•  is the planetary albedo
• f is the effective infrared transmission
factor (greenhouse effect)
•  is the Steffan Boltzman constant
(5.67 x 10-8 Wm-2 x K-2)
• Ts is the surface temperature
(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.
• So according to the formula, if the
planetary albedo where to increase,
what would happen to surface
temperatures?
R2Es(1-)=4R2(fTs4)
Albedo
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).
Runaway Ice Albedo
• In a simulation done by Budyko, once ice
formed beyond a critical latitude (30
North or South- half of the Earth’s surface
area), the positive feedback became so
strong that temperatures plummeted and
the entire earth froze over (up to 1 km
thick in oceans).
Runaway Ice Albedo
Runaway Ice Albedo
First Clues
• Thick layers of ancient rock hold clues to
climate of Neoproterozoic
– Occurrence of glacial debris near sea level in
tropics?
– Unusual deposits of iron-rich rock should only
form when there is little to no oxygen in the
oceans and atmosphere.
– Rocks known to form in warm water seem to
have accumulated right after glaciers receded.
• 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.
• 1964, Brian Harland pointed out that
glacial deposits dot Neoproterozoic rock
outcrop virtually every continent.
• Joseph Kirschvink promoted
Neoproterozoic deep freeze because of
iron deposits found mixed with glacial
debris.
– Millions of years of ice could readily create
this situation. Therefore dissolved iron
expelled from seafloor hot springs could
accumulate in water.
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.
Banded iron-formation with icerafted carbonate dropstone in
Mackenzie Mtns, Canada
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.
• Harland’s Evidence
– 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.
• 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.
• Kenneth Calderia and James Kasting
estimated to overcome a runaway freeze it
would require roughly 350 times present
day concentration of CO2.
• Once melting begins, low albedo seawater
replaces high albedo ice.
• Greenhouse atmosphere helps to drive
surface temperatures to almost 50
degrees Celcius
• Resumed evaporation helps warm
atmosphere.
• Torrential rain would scrub some of CO2
out of air in form of carbonic acid
• Chemical erosion products would quickly
build up in ocean water which would lead
to precipitation
– Oxygen would again mix with ocean
– Force iron to precipitate out with debris once
carried by sea ice and glaciers.
Isotopes
What is an isotope?
• An atom always has the same number
of protons, or positively charged
particles, but the number of electrons
and neutrons may differ. An isotope of
an atom contains more or fewer
neutrons than the average. To see if an
atom is an isotope, look at the atomic
mass.
Isotopes
Carbon
• Carbon supplied
to the ocean and
atmosphere
comes from
outgassing of
carbon dioxide by
volcanoes, and
contains about
1% carbon-13 and
99% carbon-12.
But that’s not the whole story…
Carbon
• In the oceans, carbon is removed by the
burial of calcium carbonate. If this were
the only process in effect, calcium
carbonate would have the same ratio of
carbon-13 and carbon-12 as the volcanic
output.
Carbon
• BUT, carbon is also removed from the
ocean in the form of organic matter, and
organic carbon is depleted in carbon-13
(2.5% less than in calcium carbonate).
Modern calcium carbonate is enriched in
carbon-13 by approximately 0.5% relative
to the volcano source (Hoffman).
Carbon
• So, if there was less biological productivity,
would carbonate records show higher or
lower carbon-13 values?
Carbon
• According to
Hoffman and
Schrag, “Even the
meteorite impact
that killed off the
dinosaurs 65 million
years ago did not
bring about such a
prolonged collapse
in activity.”
Organisms
The Neoproterozoic “Freeze-Fry”
• What implication does this have for the
evolution of life?
• Could organisms have survived?
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)
Extremophiles
Evolution of Life
Could the freeze-fry events have actually
encouraged evolution?
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).
Post-Snowball
Environment
• Snowball seawater was laden with
nutrients due to hydrothermal activity
and limited organic productivity.
• Once the snowball oceans began to
melt, productivity and burial of organic
matter increased, and oxygen was
released to the atmosphere.
• This rise in free oxygen could be the
cause of the explosion of life after the
snowball events.
Emergence of Animals
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).
Varanger simulation which combines reduced solar
luminosity, 40 ppm of CO2, and reduced ocean heat
transport. The simulation runs 60 model years from
initial, non-snowball conditions until an equilibrium
result is obtained.
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).
Future Snowball Earth?
• Coldest state since the Neoproterozioc
for the last millions years
• Approximately 80,000 years from the
next glacial maximum
• Evidence suggests that the last several
cycles have been getting stronger and
stronger (Hoffman)
Future Snowball Earth?
• During the last glacial maximum
(20,000 years ago), the deep ocean
cooled to near its freezing points, and
sea ice reached latitudes as low as 40
to 45 North
• Could the next ice age reach the critical
latitude?