Archean Atmospheric Ammonia: Biogeochemical Constraints on Early Earth Habitability

Ammonia (NH ₃ ) is the best greenhouse gas, with a broad absorption feature co-incident with the 8-14 micron water vapour window. It was the original suggestion for resolving the Faint Young Sun Paradox [1], with few other greenhouse gases capable of doing so [2]. However, recent notions of a weakly-reducing Archean atmosphere, coupled with ammonia's high solubility and susceptibility to photolytic destruction, have lead to ammonia falling out of fashion as a solution to the FYSP.

Here, we produce a new box-model for an Archean ocean, biosphere, and atmosphere, including solute and photolytic reactions, illustrating that ammonia was likely present as a trace atmospheric gas. In a preliminary model of only nitrogen biogeochemical cycles, nominal runs have a steady state atmospheric reservoir of 0.1 ppm NH ₃ , which gives a radiative forcing of 3 W/m ² . This is a non-trivial contribution to the total of 50 W/m ² which are required [3]; when accounting for cloud feedbacks, ammonia contributes even more to the 30 W/m ² required. Including carbon system interactions with this nominal result yields a steady state ammonia reservoir of around 10 ppm, enhancing the likelihood of elevated surface temperatures in the Archean.

More interesting still are feedbacks between these two greenhouse gases. For example, CO ₂ incursions reduce ocean pH, in turn shifting the ocean equilibrium to favour ammonium over ammonia, incurring ammonia draw down from the atmosphere. Thus, in response to perturbation, we find a feedback relationship between the CO ₂ and NH ₃ greenhouses. Our coupled nitrogen and carbon biogeochemical model for the Archean earth has indicated a potential ammonia greenhouse solution to the FYSP, controlled strongly by the partial pressure of carbon dioxide.

[1] Sagan, C. & Mullen, G. (1972). Earth and Mars: Evolution of atmospheres and surface temperatures. Science, 177(4043), 52-56.

[2] Byrne, B & Goldblatt, C. Clim. Past, 10, 1779-1801, 2014, doi:10.5194/cp-10-1779-2014.

[3] Goldblatt, C. & Zahnle, K., Clim. Past, 7, 203-220, 2011, doi:10.5194/cp-7-203-2011.