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)