Earth’s mantle was mostly molten after the Moon-forming impact. We assume that Earth had total CO2 and water inventories comparable to the modern ones. These volatiles entered the atmosphere, which soon became opaque and radiated at the runaway greenhouse threshold. The Moon caused strong Earth tides until it retreated well away from Earth. Partially crystalline mush dissipated tidal energy and buffered internal temperature so that mush was present. Available internal and tidal energy limited the greenhouse epoch to ~10 m.y. The uppermost mantle was slurry above mostly solid mush at the end of the epoch and the deep mantle solid. Solid crust froze soon after this. The crust was out of chemical equilibrium with the massive atmosphere and later with the ocean. Such water-rock reaction continued over geological time. Subduction sequestered CO2 into the mantle. Thick oceanic crust retarded subduction so overall plate rates were less than modern rate. Brief local periods of rapid subduction occurred when old lithosphere descended with its crust becoming dense eclogite. Old ocean crust remains cold during fast subduction and retains its CO2. Emplacement of CO2 into the mantle required much of the Hadean. Earth was inhabited with moderate CO2 atmospheric levels when the first evidence of life is found at ~3.9 Ga. Arc volcanics formed at the expense of highly carbonated seafloor were inevitable at the time the Earth became habitable. These rocks are an attractive prebiotic environment that was reducing and K-rich.
Reichman Family Lecture Hall (ES1050)
Norman Sleep (Stanford)
January 24, 2014
14:00 - 15:00