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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.
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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.
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 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 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.
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.
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).
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 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 post-glacial 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 • 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 margins- increased 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 • 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
