Lecture 23Recap on oxygen history of Earth’s atmosphereSnowball Earth(p. 237-240, textbook &Scientific American article by Paul Hoffman & Dan Schragabout the Neoproterozoic “Snowball Earth”)History of Earth’s O2Time before present (Ga)Oxygen level, log10(pO2) (bar)charcoaldetrital sideriteBeggiatoaanimalsred bedsiron formationsprebioticocean Fe2+environmentalchange{ocean SO42-high ocean SO42-Time before present (Ga)Oxygen level, log10(pO2) (bar)charcoaldetrital sideriteBeggiatoaanimalsred bedsiron formationsprebioticTime before present (Ga)Oxygen level, log10(pO2) (bar)charcoaldetrital sideriteBeggiatoaanimalsred bedsiron formationsprebioticocean Fe2+environmentalchange{ocean SO42-high ocean SO42-(Downward arrows indicate upper limits on O2; upward arrows, lower limits on O2, from geochemicals)Snowball Earth episodesCH4 greenhouseLater O2 historyA SECOND RISE OF O2 ~800-600 Ma also appears to have occurred, marked by increased sulfate (SO42-) in theoceans. Sulfate derives from O2 reacting with sulfur mineralson land; dissolved sulfate is transported by rivers to the ocean.The earliest animal fossils (575-555 Ma) appear after the 2ndrise of O2. Such fossils were first identified in the Ediacaran hills,southern Australia.“Ediacaran” fossils appear on 6 continents.70 cmIn the Carboniferous (~300 Ma), O2 mayhave been a higher concentration than today. Insects rely on diffusion of O2 for respiration. O2 was high enough then tosupport giant (70 cm wingspan) dragonfliesthat would be unable to function today.Snowball EarthLow-latitude glaciations occur in Earth history during two times:2.4-2.2 Ga: there are 3 tropical glaciations during this time(part of the Paleoproterozoic era (2.5-1.6 Ga))0.8-0.6 Ga: there are 2 to 4 tropical glaciations during this time(part of the Neoproterozoic era (0.9-0.544 Ga))Theory: Once ice extends to within 30° of the equator, there isrunaway ice-albedo feedback. (Colder => higher albedo => colder)Some argue for a totally ice-covered Earth on this basis. Otherscientists disagree and argue for a “Slushball” Earth with areasof open water near the equator.Evidence for glaciation:1) tillites2) glacial striations3) dropstonesGlacial geology is one of manyindicators of paleoclimate, i.e.,past climate1) TillitesMixtures of pebbles, sand and mud packed together to formrock. Formed from debris when glaciers grind up rocks. Thedebris is dragged along and deposited as rubble, ormoraines along the flanks of the ice sheetFig.12-72) Glacial striationsParallel scratches where the glacier has dragged rocksFig.12-73) DropstonesChunks of rock dropped from melting icebergs that end up inotherwise finely laminated marine sedimentsFig.12-7History of the Snowball idea…1) Geology.Sir Douglas Mawson (U. of Adelaide) in the 1940s noted low latitudeglaciation in Neoproterozoic rocks.Found dropstones of limestone (CaCO3) and dolomite (CaMg(CO3)2), thatform primarily in tropical waters. Suggested glaciers had advanced overtropical carbonate platforms like today’s Bahamas.Brian Harland (U. of Cambridge) used paleomagnetism to infer a low latitudelocation. However, at the time (1960s), most considered his paleomagneticmeasurements suspect since the technique was new.2) Climate science. Michail Budyko (1969) showed that the Earthfreezes over if edges of the polar ice caps wander equatorwardfarther than 30º latitude. Mean global temperatures of –35 to -50ºCwere predicted with an equatorial ice thickness of 1.5 km, up to 3 kmat the poles. There was no conceivable escape. Thus, it could nothave ever happened. Right?The Snowball idea largely melted away. Then…(here and henceforth, we deal with the Neoproterozoic episodes)Evidence for low-latitude glaciation becomes unequivocalField lines of Earth’s magnetic field areperpendicular to the surface at the poles andparallel to the surface near the equator. Volcanicrocks containing iron minerals, such asmagnetite (Fe3O4), become magnetized in thedirection of the magnetic field before they hardeninto rock.Thus, rocks in which the remnant magnetic fieldis parallel to the sediment beds would haveformed in the tropics.1992 – Kirschvink postulates that the Earth had frozen over and escapedthrough build-up of volcanic CO2. He calls the concept “Snowball Earth”.1987 – Joe Kirschvink (Caltech) shows S. Australia magnetization in glacialdeposits indicate the deposits formed within a few degrees of the equator.Possible continental positions during Late Proterozoic Glaciations: Fig 12-10Critical latitude for "runaway" ice-albedo (~30 degrees):Why would ice-albedo feedback get stronger as the ice-line gotto lower and lower latitudes???Answer: - more sunlight gets reflected if lower latitudes are ice-covered- the amount of area per degree latitude gets much larger (major factor)Iron Solubility andDeposition in Relation to aSnowball Eventhttp://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.htmlBanded Iron Formations(BIFs) re-occur during SnowballSnowball initiationLoss of CO2 withcontinental weathering? Or lossof CH4 with a rise of O2?Runaway ice-albedoGeneral Circulation Model (GCM) simulationOne problem: the authors of this study used a value for albedo which atits highest was only 0.44! Yet snow and ice is much higher: 0.6-0.9; sea-ice=0.62Chandler, M.A., and L.E. Sohl 2000. Climate forcings and the initiation of low-latitude ice sheets during the Neoproterozoic Varanger glacial interval.J. Geophysical Research 105, 20737-20756.initiationdeep freeze warm-upfry upHoffman and Schrag modelCap carbonates above dropstonescap carbonatesdropstoneCarbon isotopesSee p.222, Kump. The delta notation is defined as:d13C = (13C/12Csample) - (13C/12Cstandard) x 1000 (‰)13C/12CstandardValue of volcanic gas d13C = -6‰ Value of carbonate d13C just before Snowball glaciation -6‰ - attributed to loss of organic productivity by Hoffman and SchragApproximately, the same d13C value is seen in the cap carbonatesTypical value of ancient marine carbonate d13C = 0 ‰Cap carbonatesVariable thickness of carbonate, (limestone (CaCO3) ordolomite (MgCa(CO3)2)), that sit on top ofglacial dropstone and tillite deposits.Cap carbonate formation is attributed to the loss of CO2 builup in the atmosphere during the glaciation, through weatheringand deposition.Eventually, biology recovers and values of d13C go back to normal.Biology and the SnowballHoffman & Schrag speculate that refuges in a “Snowball Earth”led to speciation of