"Snowball Earth (Part 2)" PDF

Breathe Deep and then… freeze ... repeat Present “Snowball Earth Period” Neo Proterozoic Proterozoic Paleo Proterozoic Huronian Glaciation Archean likely warm, stable Textbook Figure 12-11 ~600 Ma ¡ Massive cap carbonates ¡ Sediments deposited under ice ¡ Banded Iron Formations ~ 800 Ma...

Breathe Deep and then… freeze ... repeat Present “Snowball Earth Period” Neo Proterozoic Proterozoic Paleo Proterozoic Huronian Glaciation Archean likely warm, stable Textbook Figure 12-11 ~600 Ma ¡ Massive cap carbonates ¡ Sediments deposited under ice ¡ Banded Iron Formations ~ 800 Ma Banded Iron Formations (BIFs) ¡ Require alternation between oxic and anoxic ocean waters ¡ Common in the early geologic record ¡ Disappeared from the rock record at ~1.9 Ga ¡ BIFs briefly re-appear in the Neo Proterozoic ¡ With dropstones! www-eps.harvard.edu/.../faculty/ hoffman/snowball_paper.html Dropstone, Namibia Paul Hoffmann Daniel Schrag ¡ They are not odd in polar regions. ¡ They are unknown today in low-latitude regions (except at high altitudes). inclination angle declination angle ¡ Natural remnant magnetism (NRM) is the magnetism of a rock or sediment and preserves paleo latitude at the time of formation. ¡ But magnetism can be “reset” by heating of rocks and sediments. ¡ If magnetism is reset, then it cannot be used to infer position of rocks at time of formation! ¡ How do we know whether magnetism has been reset? ¡ Examine magnetism in folded rock layers. ¡ If magnetism follows folds, it is original. ¡ If magnetism cuts across folds, it has been reset. Original Magnetism Reset Magnetism Original Fold TestMagnetism confirmed that all continents were located well within 60 degrees of the equator. Reset Magnetism And some continents were right on the equator! ¡ Massive cap carbonates ¡ Sediments deposited under ice ¡ Banded Iron Formations (re-) occur briefly ¡ Cap-Carbonates resulted from carbonate precipitation that was not biologically mediated. ¡ Non-biologically mediated carbonate formation occurs today only in warm water environments (e.g., the Bahamas). ¡ How can warm water deposits overlie glacial deposits? Cap carbonates are isotopically light! www-eps.harvard.edu/.../faculty/ hoffman/snowball_paper.html ¡ These d13C data are from carbonates. ¡ Not to be confused with d13C data from organic carbon. d13C (per mil) Photosynthesis organic carbon Methanogenesis CH4 -25 to -30 -60 d13C (per mil) sea water DIC ~ 0 (-2 to 2) Cap carbonates are isotopically light! www-eps.harvard.edu/.../faculty/ hoffman/snowball_paper.html ¡ How could ice form at equator? ¡ Why were the oceans swinging between oxic and anoxic? ¡ How could warm water carbonates form directly on top of glacial deposits? ¡ Why was ocean productivity reduced? ¡ The Big Tilt ¡ The Snowball Earth ¡ During the Neo Proterozoic, Earth’s axial tilt was substantially greater than it is today. ¡ As a result, mean annual temperatures in equatorial regions were cooler than at the poles. Axial Tilt = 23.5 degrees Northern Summer Northern Winter Axial Tilt = 60+ degrees Northern Summer Extremely Hot Northern Winter Extremely cold Despite strong seasonal contrasts at the poles, equatorial regions have lower annual average temperatures. ¡ The early Earth was subject to heavy bombardment by asteroids and comets. ¡ Maybe a major collision tipped the Earth more than it is tipped today. ¡ The planet Uranus has a large axial tilt (~98o) ¡ Evidence for this tilt is from observations of the planet’s many moons. ¡ Tilt likely caused by major collision. http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=422 http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=422 ¡ Uranus is rotating on it side ¡ Large tilt was likely caused by major collision. ¡ An axial tilt more extreme than today’s could produce ice at sea level in the tropics ¡ What caused the axial tilt? The heavy bombardment and the moon formation took place long before the Neo Proterozoic. ¡ What caused the axial “un-tilt”? ¡ The Big Tilt Hypothesis explains ice at sea level on the equator. ¡ But what about banded iron formations, cap carbonates and light carbon isotopes ???