hadean to archean some hell-on-earth norman h. sleep
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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 - PowerPoint PPT PresentationTRANSCRIPT
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• Continue with warm
greenhouse– 500 K
– 100 bar CO2
– Demise
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
Hot greenhouse
• Surface mostly molten• Atmosphere radiates at
cloud-top temperature• Heat escapes at
greenhouse limit• Takes 10 Ma to get heat
out
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
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
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
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)
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
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
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
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
€
∂Rsurf
∂t=∂A
∂tCmanYridge(Tman ) −Ycarb(Cocean )[1 − Farc(Tman )][ ]
Climate and CO2• CO2-rich mantle
domains aftermath of moon-forming impact
• Subduction maintains situation– Nice climate– Buffered
• Kimberlites treasure trove of geological records
Fate of slab carbonate
• Carbonate in thin zone
• CO2 stays in slab
• Most of mantle carbon
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
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
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
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
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
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
Some geochemistry
• Detrital magnetite unstable at high pCO2
• Detrital quartz dissolves at high T
• Banal and not reported much
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)