Snowball Earth Presented by Mindi Purdy and Jen Ulrich
Theory of Snowball Earth • Many lines of evidence support a theory that the 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-)=4R2(fTs4) • 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-)=4R2(fTs4)
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).
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 ice-rafted carbonate dropstone in Mackenzie Mtns, Canada
Critical Element: Location of the Continents • Harland’s idea based on assumption that 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 ice-free 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 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.
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)
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.
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.
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: • Simulationsfound 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