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 CO 2 –Demise

Moon-forming impact aftermath Earth radiates at ~2300 K –Detectable – Gas + dust: ∼ 12 MYR HD 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 – °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 – 142 Nd 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 NaAl 3 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 CO 2 –Vast sink pH ~6 Ocean No left-over cations in accessible crust Need to subduct CO 2

Global CO 2 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 pCO 2 : Y carb factor at ridge is independent of ocean concentration Fraction subducted 1- F arc depends on mantle temperature No obvious buffer at most of CO 2 in mantle Modern Y carb is buffer proportional to ocean concentration

Climate and CO 2 CO 2 -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 CO 2 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 CO 2 clement atmosphere –Need to subduct almost all the CO 2 –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 CO 2 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 O 2 in air pCO 2 < 10 PAL Crust and mantle already affected by Life Crust becomes oxidised –Biotic CH 4 : H 2 to space 2 bars of N 2 –NH 4 + subduction –Biological Gaia buffer

Arc volcanoes CO 2 –Slab and biological carbonate –Organic carbon – 13 C Mantle gets CO 2 that platforms won’t take Ocean gets water that mantle and crust won’t take SO 2 –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 pCO 2 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) –CO 2 sink (pre- habitability)