Revisiting the Early Earth's Methanogen Biosphere

Earth's geological history suggests that methane (CH4) concentrations were considerably higher billions of years ago, which would have significant effects on climate and atmospheric photochemistry. The largest source of Archean (2.5-4 billion years ago) CH4 was probably biological production, as it is on Earth today (Kharecha et al 2005). Methanogenesis, a primitive methane-producing metabolism that evolved early in Earth's history (Ueno et al 2006), requires hydrogen (H2) and carbon dioxide (CO2) to generate energy, releasing methane as a byproduct. We have used an ecology model that tracks volcanically produced H2 and dissolved oceanic CO2 to estimate plausible biogenic CH4 production rates during the Archean . Methane is then released into the water column and will eventually make its way into the atmosphere. We use biogenic methane fluxes out of the ocean as input to simulations using a 1-D photochemical-climate model of Archean Earth's atmosphere to test the implications for photochemistry and climate. High concentrations of methane can generate organic hazes, which can strongly impact habitability, climate, and planetary spectral appearance (Arney et al 2016). Preliminary analysis of our data shows haze formation when higher levels of methane are present. Hazy atmospheres produce temperature profiles that display strong temperature inversions in the upper stratosphere atmosphere and cooling at the surface level. Also, with thicker hazes there is less methane photolysis in the lower atmosphere which in turn decreases ethane formation, a byproduct of methane and important greenhouse gas. Lastly, we generate spectra from the results of these simulations using the Spectral Mapping and Atmospheric Radiative Transfer model (SMART; Meadows and Crisp 1996) to see what planets with similar spectra will look like to future telescopes. This work represents a self-consistent synthesis of planetary processes from the biosphere, to the atmosphere, to the telescope.