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Hadean to Archean Some Hell-on-Earth Norman H. Sleep

Hadean to Archean Some Hell-on-Earth Norman H. Sleep. Sequence of events. Moon-forming impact Prior status? Bad aftermath Earth starts hot Clement by start of Archean. Moon-forming impact. ~4.5 Ga Earth vapourized ~1 ka Internal hot and tidal greenhouse ~10 Ma

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Hadean to Archean Some Hell-on-Earth Norman H. Sleep

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  1. Hadean to ArcheanSome Hell-on-EarthNorman H. Sleep

  2. Sequence of events • Moon-forming impact • Prior status? • Bad aftermath • Earth starts hot • Clement by start of Archean

  3. Moon-forming impact • ~4.5 Ga • Earth vapourized ~1 ka • Internal hot and tidal greenhouse ~10 Ma • Continue with warm greenhouse • 500 K • 100 bar CO2 • Demise

  4. Moon-forming impact aftermath • Earth radiates at ~2300 K • Detectable • Gas + dust: ∼12 MYR HD172555 SYSTEM Lisse et al. (2009) • Material must pass repeatedly through thin photosphere to radiate heat • Moon disk and Earth exchange oxygen but may be not W

  5. Hot greenhouse • Surface mostly molten • Atmosphere radiates at cloud-top temperature • Heat escapes at greenhouse limit • Takes 10 Ma to get heat out

  6. Hot greenhouse-Moon • Tidal dissipation major heat source • Liquid does not dissipate • Solid does dissipate • Buffer • Takes 10 Ma to get heat out through clouds • Moon orbit climate controlled Observable in present orbit

  7. Hot greenhouse-Moon • Tidal dissipation major heat source • Mantle cools slowly • Mantle freezes from middle up & down • Liquid adiabat parallel to melting curve • Moon orbit climate controlled

  8. Moon-forming impact aftermath • Cooling follows P-T paths in modern peridotite and basalt systems at ridge axis • 1000°C NaCl-rich Pt-bearing ore fluid • 500-600°C Dense NaCl brine • 350°C seawater

  9. Demise of warm greenhouse • Carbonates are stable in basalt • Hard to subduct into hot mantle initially • Subducted material persists in mantle • 4.26 Ga India • 142Nd subduction age • In lithosphere at 3.6 Ga • Igneous age 1.48 Ga • Upadhyay, D., E. E. Scherer, K. Mezger (2009)

  10. Surface ocean chemistry • Bicarbonate ocean impossible on Earth • NaAl and NaAl3 silicates insoluble • Na:Al ~ 1:3 • Cannot get soluble Na silicate • Possible with solar Na:Al ~1:1 • Europa, Enceladus, Titan • Evapouration = rain

  11. Surface ocean chemistry • Mineral acid ocean impossible on Earth • Basalt and peridotite are buffers • Na >> Cl • NaCl fluids by 1000°C • No titration of HCl like in old textbooks

  12. End of hot greenhouse • Oceanic crust takes ~10 bars CO2 • Vast sink • pH ~6 Ocean • No left-over cations in accessible crust • Need to subduct CO2

  13. Global CO2 balance Rate of change in surface reservoir = Global spreading rate * (ridge flux factor – subduction flux factor) Ridge flux depends on mantle concentration and mantle temperature At high pCO2: Ycarb factor at ridge is independent of ocean concentration Fraction subducted 1- Farc depends on mantle temperature No obvious buffer at most of CO2 in mantle Modern Ycarb is buffer proportional to ocean concentration

  14. Climate and CO2 • CO2-rich mantle domains aftermath of moon-forming impact • Subduction maintains situation • Nice climate • Buffered • Kimberlites treasure trove of geological records

  15. Fate of slab carbonate • Carbonate in thin zone • CO2 stays in slab • Most of mantle carbon

  16. Fate of slab carbonate • Modern carbonate in thin zone • Excess cations in crust • Ocean pH ~8 • Earth passes through pH ~6 ocean; 1 bar CO2 clement atmosphere • Need to subduct almost all the CO2 • Earth does not linger from balance of internal processes

  17. Info from slab carbonate • Carbonate starts in thin zone in subduction • Stays as concentrated zone through geological time • Treasure trove for mantle paleontology and environmental geology

  18. Earth at 3.8 Ga • Photosynthesis needed for black shale • Land weathering • Marine deposition • Banded Iron formation (BIF) • FeO-based photosynthesis • Sulphur cycle • Sulphide-based photosynthesis • Ocean pH ~8 (REE & Y in BIF) • Near modern CO2 in air 3.8 Ga Isua, Greenland: Black shale turbidites

  19. Earth at 3.8 Ga • Photosynthesis • Ocean • Land • Full C, S, Fe cycles • Ocean pH 8 • Clement • No O2 in air • pCO2 < 10 PAL • Crust and mantle already affected by Life • Crust becomes oxidised • Biotic CH4: H2 to space • 2 bars of N2 • NH4+ subduction • Biological Gaia buffer

  20. Arc volcanoes • CO2 • Slab and biological carbonate • Organic carbon • 13C • Mantle gets CO2 that platforms won’t take • Ocean gets water that mantle and crust won’t take • SO2 • Subducted sulphate important

  21. Sulphur (brimstone), fire, and oxygen Strong biological control of mantle cycle. Sulphide in slab comes from sulphate from photosynthesis. Less brimstone (sulphur) in Hadean arc volcanoes before photosynthesis Durable tracer of life Cotton Mather

  22. Some geochemistry • Detrital magnetite unstable at high pCO2 • Detrital quartz dissolves at high T • Banal and not reported much

  23. Other geochemistry • Ocean buffered by basaltic crust • Passes through pH ~6 and clement • pH ~8 by 3.85 Ga • Redox state (pre-biotic) • CO2 sink (pre-habitability)

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