Constraining Archean Earth’s Atmosphere with the Geological Record

Abstract:

A warm, water-bearing Archean Earth, when the Sun was young and faint, remains a paradox to the scientific world. Abundant geological data suggests that Archean Earth had standing water at the surface, despite the fainter Sun. An explanation of this paradox is vital to the understanding of Earth’s history and coevolution with life. If the surface of the planet was not being kept warm by the Sun, which was 25% less luminous than now, it must have been kept warm a different way—by an atmospheric composition high in greenhouse gases. Constraints on these gases come from the geological record, which have provided proxies for the redox state of the atmosphere (limiting H₂ and O₂), the total atmospheric pressure, and the partial pressure of certain gases such as carbon dioxide (CO₂) and methane (CH₄). Previous attempts at solutions to the paradox are consistent with some, but not all, of the geological proxies.

 The constraints are used as inputs for a 1-D photochemical code, which calculates atmospheric composition and predicts the abundances of atmospheric gases that affect climate, particularly methane (CH₄) and gaseous hydrogen (H₂). A coupled 1-D radiative-convective climate code is then used to calculate the corresponding surface temperature. Critically, the improved photochemical code maintains strict redox boundary conditions, and is being further updated to ensure that the redox fluxes from volcanoes and mid-ocean ridge vents are consistent with both each other and the redox state of the mantle. These code improvements will lead to changes in both the inputs to the atmosphere from volcanoes and the sink for oxidants at mid-ocean ridges, in turn affecting the abundance of redox-sensitive greenhouse gases such as CH₄ and H₂. The main purpose of this project is to extend simulations of the Archean surface environment down into the mantle, and to search for a solution to the faint young sun paradox that is consistent with the geological proxies. Beyond having interesting implications on Archean Earth’s climatic history, this study effectively uses Archean Earth as an analog environment in which to test a new tool for simulating the effects of mantle geochemistry on (exo)planetary atmospheres and environments.