The Hadean-Archaean environment

Abstract

A sparse geological record combined with physics and molecular phylogeny constrains the environmental conditions on the early Earth. The Earth began hot after the moon-forming impact and cooled to the point where liquid water was present in approximately 10 million years. Subsequently, a few asteroid impacts may have briefly heated surface environments, leaving only thermophile survivors in kilometer-deep rocks. A warm 500 K, 100 bar CO(2) greenhouse persisted until subducted oceanic crust sequestered CO(2) into the mantle. It is not known whether the Earth's surface lingered in a approximately 70 degrees C thermophile environment well into the Archaean or cooled to clement or freezing conditions in the Hadean. Recently discovered approximately 4.3 Ga rocks near Hudson Bay may have formed during the warm greenhouse. Alkalic rocks in India indicate carbonate subduction by 4.26 Ga. The presence of 3.8 Ga black shales in Greenland indicates that S-based photosynthesis had evolved in the oceans and likely Fe-based photosynthesis and efficient chemical weathering on land. Overall, mantle derived rocks, especially kimberlites and similar CO(2)-rich magmas, preserve evidence of subducted upper oceanic crust, ancient surface environments, and biosignatures of photosynthesis.

Figure 1.

Schematic diagram shows cross section of ridge axis (left). Hot black smoker vents cool the magma lens and underlying mush chamber. Warm 20–60°C vents occur off axis. Carbonate minerals remove CO₂ from seawater. CO₂ is highly concentrated in a thin layer. The concentration before subduction (solid line) is somewhat greater than the concentration after subduction (dashed line). The concentration of water decreases slowly with depth (solid line). The subduction process removes most of the water into arc volcanics and returns it to the surface (dashed line). Mantle CO₂ hence resides at high concentrations in rocks formed from the upper oceanic crust. These domains are the source for kimberlites and other CO₂-rich magmas. Not to scale.