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208_Lect09_color - Formation of Solar System Layered Earth...

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Unformatted text preview: Formation of Solar System Layered Earth, Oceans Atmosphere Origin of Moon Archean Tectonics The Archean Spiral Galaxy Crab Nebula Nebular Nutshell 1 Supernova plasma Solar wind 2 Impacts add volatiles Impacts add water Canadian Shield 3 Origin of Water: Where does it come from? More complex than addition by comet and meteorite impacts… Earth didn’t have oceans 4.5 b.y. ago So, where did ‘surface ’ So, water come from? It came from Earth ’s interior and was released during mantle melting (seafloor spreading). Accretion of Earth & Origin of Water Planetesimals collide to build Earth Then mantle degassing Initially, depleted bodies form terrestrial planets. Late in process, Chondritic Planetesimals (18% H 2O) impact Earth. Early on H 2O lost during accretion (why? Earth too small and too hot) H2O from chondrites and comets retained in mantle plate tectonics of early Earth Liberates mantle water to surface during seafloor spreading = oceans & atmosphere! Let’s check this hypothesis. How How much water is in the earth? in Estimated from eruptions of mid-ocean ridges = Upper mantle Hot Spots = Lower mantle Water Content of Earth Upper Mantle is degassed (lost water from melting beneath ridges) Could degassing of UM account for oceans? Assume that originally UM = LM •Multiply ratio of mass UM/LM (0.4) by the Ocean masses contained in the lower mantle (3) •Result: UM originally contained 1.2 ocean volumes (it now contains 0.2, thus lost 1 ocean volume to make the oceans!) 4 Archean Tectonics Archean Eon: 4.55 to 2.5 Ga Archean = high heat flow Greater concentration of radioactive isotopes Lots of initial heat from condensation and core segregation Therefore, Vigorous mantle convection Many mantle plumes Likely more plates and moving ‘rapidly ’ Dikes in the Canadian Shield Big problem in geology: how to make a continent Continents are Felsic It is unreasonable to fractionate mantle melts (ultramafic) to produce such vast quantities of granite ~95% fractionation Where ’s all the mafic cumulate? Archean Shields: Granite-Greenstone Belts greenstone granite Classic Interpretation: Granites = very old microcontinents. Greenstones = metamorphosed forearc clastic and volcaniclastic sediments Continental growth by arc accretion 5 Here’s an idea (Gunnarson et al ‘98) Gunnarson Distillation of mafic crust Start with a hotspot (like iceland ) The magma partially melts the basaltic crust (these melts will be less mafic than the basalt) Some magmas fractionate to granite (felsic bodies in image) These are re-melted by latter mafic intrusions and mix (magmas become more felsic) Some of this erupts Some remains in crust Overall result: crust becomes more felsic by this distillation process Archean Shields Shield = exposed Precambrian basement Platform = Phanerozoic strata on PC basement Craton = Shield + Platform (not in orogeny for > 1 Ga Shields and Cratons Oldest rocks = Acosta Gneiss of Canadian Sheild: 3.8-4.0 Ga Oldest thing = zircon from Acosta Gneiss: 4.2 Ga (continent & ocean) 6 Archean Life = Bacteria First fossils are 3.2 Ga bacteria. Hypothesis: life evolved as chemotrophic bacteria in deep oceans (near vents) protected from UV-B. Bacteria reign on earth as dominant life form for > 2Ga. Origin of Life on Earth: Traditional Concept Oldest fossils are 3.2 Ga Early atmosphere First evidence for life is 3.9 Ga organic molecules in 12 sediment with abundant 12C sediment - a property of organic C Life began 1 b.y. after planet condensation Reducing environment (No O2) No ozone, UV-B radiation! Rich in CH 4, NH 3, CO2, N 2 ‘Primordial Soup ’... video Origin of Life: Traditional Concept Miller & Urey, 1953 Simulated a reducing atmosphere and ‘primordial ’ atmosphere ocean added energy Lightening, volcanic, solar radiation, etc. Results: Formed 12 of 20 amino acids in life Other organic molecules too (HCN, Formaldehyde) Conclusion: Note: 74 amino acids found in Chondritic meteorites. Not hard to make simple organic molecules 7 Origin of Life: Traditional Concept Later experiments (Fox) Created polymers of amino acids Proteins RNA! Fatty acids & lipids - required for cells Note: also in chondritic meteorites! No replication of these polymers in the lab, NOT Life Origin of Life: Traditional Concept Conclusion of Experiments Early atmosphere + energy results in amino acids - and maybe life Or: (not traditional concept) meteorites add amino acids and lipids to early Earth (extraterrestrial origin of life) Problem: that nasty UV-B! Turn to new discoveries for origin of life... New discovery Vent Communities Discovered 1976 on the Galapagos Spreading Center Now known on all midocean ridge systems What are all these heterotrophs eating? Chemotrophic Bacteria Get energy from reducing methane and sulfur Do not rely on sunlight! Base of vent food web 8 Origin of Life: New hypothesis Amino acids Extraterrestrial, or Generated at vent (heat Energy) as in Miller-Urey experiments Why at vent and not surface? Deep sea is shielded from UV-B radiation! Pyrite (FeS2) is abundant in this environment and can serve as catalyst for replicating RNA Gunter Gunter Wächtersh äuser ’s new new idea Origin of Life: New Hypothesis Carl Woese : new discovery Carl Woese Why are they called Archeabacteria Why Archeabacteria (old bacteria)? DNA of chemotrophic bacteria indicate they are different than eubacteria They diverged from the bush of life long ago (genetically, they are varied varied and complex - i.e. ‘been around a while) Possibly back when life evolved on earth in the deep oceans. Question: when did Earth first have oceans? 4.3 Ga Maybe life started long before first fossil (3.2 b.y.) At some point in Archean, life becomes photosynthetic (e.g. cyanobacteria) 3.45 Ga Stromatolite 3.8 Ga BIF 9 ...
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