Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The other volatile: O2 What is the mantle/surface/biology connection? Charles H. Langmuir Harvard University [email protected] Major Questions • Why is the mantle slightly oxidized? • Why isn’t it more oxidized? • Does oxygenation of the surface oxidize the mantle? • Has oxidation state changed through Earth history? • What’s is happening today? Planetary Evolution as Energy Transformation From an initial reduced oxidation state --To a “planetary fuel cell” that permits greater access to energy and efficient transfer and processing of stellar energy Planets are Initially Reduced • Solar nebula has excess of hydrogen and Fe metal, no free oxygen Meteorites that make up planets or reveal their interior have reduced minerals Carbonaceous chondrite Pallasite – Fe as Fe, FeO and FeS – S as FeS – C in meteorites is reduced, CO2 in ancient atmosphere Solar System objects with truncated evolution remain reduced Origin of Life requires reducing conditions • precursor organic molecules can form and survive only under reducing conditions • Carbon can vary from +4 to -4 in its oxidation state! • CO2 +4, • CO +2, • C, CH2O 0 • CH4 Organic molecules all have reduced carbon and hydrogen bonds -4 Current upper mantle has about 3% Fe3+ • Not in equilibrium with metallic Fe • Did photosynthesis do it? Life produces an Electric Current that makes reduced molecules and oxidized complements • CO2 + electron donor + hydrogen → CH2O + oxidized by-product Carbon changes valence from 4+ to neutral or negative: electron flow • Over Earth history this current has created larges masses of organic matter and a complementary oxidized surface reservoir Rise of O2 permitted Eukaryotic Cells and Multicellular Life: Aerobic Respiration The full potential of aerobic respiration requires high O2 Aerobic (1-2% O2?) EUKARYOTES PROKARYOTES Anaerobic • One trillion of these working together with active oxygen transport: 21% O2 QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Hydrogen Fuel Cell Modern Earth’s Fuel Cell Aerobic Life, Weathering Reduced C, Fe, S Modern Earth as Planetary Fuel Cell +O 2 + C and CO2 FeO FeS Fe, Ni + Permits far greater energy flow than earlier in Earth history Electron mass balance means every oxidized element must be matched by a reduced element. Net O2 production is the excess of organic matter production over destruction, and this organic matter has to end up somewhere, unoxidized From this perspective the current Earth has zero net O2 production O2 is actively consumed QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. So the rise of oxygen and the creation of the planetary fuel cell involves sources and sinks and their evolution through Earth history. When and how did it all happen? Mass Independent Sulfur Isotope Fractionation Some atmospheric oxygen beginning at 2.4Ga From Farquhar Pyrite Sulfur Isotopes Seawater sulfate? QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. From Lyons (2010) Canfield (2004) Mo abundances QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. From Scott et al 2008 One possibility from Snowball Earth Proponents www.snowballearth.org Deep ocean is source of oxidzied materials for subduction (a) Did not exist before ~600Ma (b) Cannot have caused significant mantle oxidation There must be mass balance between oxidized and reduced reservoirs Reservoirs of Carbon Total organic carbon is 600 - 1250 *1018 moles Oxidized Reservoirs 2% of oxidizing power produced by organic life resides as O2 in the atmosphere. 98% is in oxidized Fe and S. Most of the story is in rocks. Mantle carbon output through time? Implies that MOST “oxygen production” occurred early; Gobbled up by Fe and S From Hayes and Waldbauer (2006) QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Mass balance problem = 500 - 1000 * 1018 moles of reduced carbon equivalent Implies a large reduced reservoir somehere: (a) Subducted organic carbon (b) Hydrogen loss to space Reservoirs of Carbon on the Mantle Earth Total organic carbon is 600 - 1250 *1018 moles Simple mass balance constraints: • One possible reduced reservoir is subduction of organic carbon. Happening today. Earlier oceans were reduced, permitting organic matter accumulation. – Many others propose hydrogen loss from upper atmosphere. Unobservable and untestable? • Simple mass balance constraints: • To increase upper mantle Fe3+/Fe2+ by 1% requires 2 billion years of present Fe3+ subduction. – Data suggest deep ocean not oxidized prior to 700Ma – Even small increase of mantle Fe3+ requires thousands of examoles of subducted oxidized material-- makes mass balance problem MUCH worse • Production of oxidized species can have had only a negligible impact on mean upper mantle oxidation state Elements with variable oxidation states record mantle conditions QuickTime™ and a decompressor are needed to see this picture. Delano 2001 QuickTime™ and a decompressor are needed to see thi s picture. Oxidation State of Upper Mantle Source Regions Has Not Changed Since Archean QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Delano 2001 Changing the Oxidation State Requires Electron Transport • 3FeO → Fe + Fe2O3 Iron changes valence from +2 to neutral and +3: electron flow occurs if the Fe metal is segregated to the core • This process could oxidize the mantle if it were significant. Might it occur progressively over Earth history? Is the solid Earth important today? Ocean crust is oxidized as it interacts with seawater. QuickTime™ and a decompressor are needed to see this picture. Photo from Alt et al. ODP hole 504b. Ferric iron increases by about 1 wt%. Subduction flux is 8 * 1012 moles per year, which is four times the estimated organic carbon burial rate. Is atmospheric O2 decreasing? Essential feedback on O2? Present Earth Is Out of Balance • Current burial rate of organic carbon (= O2 production) is 0.68 * 1012 moles/yr • Current flux of subducting Fe3+ is 2*1012 equivalent moles • Suggests plate tectonic feedback on O2 Modern Convergent Margins QuickTime™ and a decompressor are needed to see this picture. • Kelley and Cottrell (2009) Fe-Mn Differences MORB/Back-Arc 0.21 FeO/MnO= 54+- 1/sqrt(210) 0.2 MnO 0.19 EPR ELSC 0.18 0.17 FeO/MnO= 57+- 2/sqrt(30) 0.16 0.15 8 9 10 11 12 Fe2O3 Total 13 14 15 Reflections Mantle was slightly oxidized early and has maintained that state within tight bounds Life did not oxidize the mantle-- it may have slightly reduced it Life today is changing mantle oxidation state Mantle plays a critical role in the oxygen story