How to draw down CO2 from severe Hadean to habitable Archean?

Abstract:

It has been hypothesized that as the magma ocean crystallized in the Hadean, volatiles such as CO₂ and H₂O were released to the surface culminating with the formation of a liquid ocean by about 4.4 Ga [1] and hot CO₂-rich atmosphere [2]. The resulting late Hadean atmospheric pCO₂ may have been as high as 100 bars [3] with corresponding surface temperatures ~500 K [4]. Geological evidence suggests that by the early-to-mid Archean, atmospheric pCO₂ became less than 1 bar [5]. However, the mechanisms responsible for the great amount of CO₂ drawdown in a relatively short period of time remain enigmatic. To identify these possible mechanisms, we have developed a box model during the CIDER 2015 Summer Program that takes into account geological constraints on basalt alteration [6, 7] and possible rate of new oceanic crust formation [8] for the Archean. Our model integrates geodynamic and geochemical approaches of interaction between the Hadean atmosphere, hydrosphere, oceanic crust, and mantle to drawdown CO₂. Our primary assumption for the Hadean is the absence of the continental crust and thus continental weathering. Therefore in the model we present, the level of CO₂ in the atmosphere is regulated by the formation of oceanic crust (OC), rate of the interaction between the ocean and OC, and carbonate subduction/CO₂ degassing. Preliminary results suggest that it would take about 1 billion years for the atmospheric CO₂ to decrease to 1 bar if the production of oceanic crust was 10 times more than today and the pH of the ocean was less than 7, making the basalt alteration more efficient. However, there is evidence that some continental crust began to form as early as 4.4 Ga [9] and therefore the role of continental weathering and its rate of CO₂ drawdown will need to be further explored.

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