[MIS07-07] Potential Positive Feedback Mechanisms in an Anoxic Environmental System of a Planet with CO2-CH4 Atmosphere
キーワード：炭素循環、気候安定性
A system of aqua planet like the Earth has several important feedback mechanisms, including a negative silicate weathering feedback (Walker, 1981) and a positive ice-albedo feedback (e.g., North, 1975), which affect the climate stability, hence habitability of life. In the case of the Earth, anoxic marine microbial ecosystem is considered to have dominated for more than ~1.0 Gyr (e.g., Olson, 2006). On such a planet, carbon dioxide (CO2) and biogenic methane (CH4) might be two of the most likely greenhouse gases (Haqq-Misra et al., 2008). High concentrations of both CO2 and CH4 are considered to be detectable biosignatures in exoplanets thanks to chemical disequilibrium between them (e.g., Krissansen-Totton et al., 2018); thus, the climate stability of anoxic exoplanetary environmental systems is important for future explorations. One of the characteristic features of the anoxic environmental system of the Archean Earth is that biological activity may have been limited by the electron donor (i.e. H2, Fe2+, and CO) (e.g. Kharecha et al., 2005). This means that, the production of methane by decomposition of organic carbon through methanogenesis is controlled by the amount of the electron donor and the burial efficiency of organic carbon. The burial efficiency of organic carbon depends on temperature, which might have worked as a positive feedback in the climate system through an effect on global carbon cycle (Stanley, 2010; Finnegan et al., 2012). The other important characteristic feature is that aerosol particles composed of hydrocarbons ("haze") are formed in the atmosphere once methane exceeds some critical level (CH4/CO2 > ~0.1) (e.g. Trainer et al., 2006; Pavlov et al., 2001). Previously, formation of the hydrocarbon haze on the Earth-like planet is considered to work as a negative feedback, which stabilizes the planetary climate (Pavlov et al., 2001; Zerkle et al., 2012). However, how these feedback mechanisms affect the global carbon cycle in the anoxic environmental system of the planet has not been investigated. Here we investigated possible feedback mechanisms in the anoxic environmental systme of the Earth-like planet, using a coupled model of atmosphere photocheistry-marine microbial ecosystem-global carbon cycle, in which the photochemical model "Atmos" (Arney et al., 2016) is coupled with an anoxic marine microbial ecosystem model (Ozaki et al., 2018). We show that, because a decrease of temperature promotes burial of organic carbon, consumption of carbon from the atmosphere-ocean system is promoted and methane production is suppressed. Because both processes lead to cooling of the planet, this mechanism could work as a positive feedback in the anoxic environmental system of the planet. Further, if haze is formed in the atmosphere under low pCO2 condition, removal of haze particles from the atmosphere decreases the pCO2, which also works as a positive feedback in the climate system. We discuss how much these potential positive feedback mechanisms could have affected the climate stability together with the silicate weathering feedback and the ice-albedo feedback in the anoxic environmental system of the planet, and how they could have helped the onset of glacial events in the Early Earth.