Effects of atmospheric composition on apparent activation energy of silicate weathering: II. Implications for evolution of atmospheric CO2 in the Precambrian

Abstract

The apparent activation energy of silicate weathering is a key parameter for understanding the regulation of atmospheric CO2 and surface-temperature of the Earth. Combining the atmospheric composition effects on the apparent activation energy with the compensation law for silicate-weathering flux, the relationship between the temperature dependence of atmospheric CO2 (ΔHCO2′), temperature (T) and silicate-weathering flux (FCO2) has been recently established (Kanzaki and Murakami, 2018). The present study examined the effects of atmospheric CO2 and CH4 on silicate weathering in the Precambrian based on the above T-ΔHCO2′-FCO2 relationship and the greenhouse effects of CO2, which represent ΔHCO2′ on the global scale, with and without the presence of CH4. Calculation of the ratio of the change in FCO2 to the corresponding change in the partial pressure of atmospheric CO2 (PCO2) as an indicator of the silicate-weathering feedback on CO2 revealed hitherto unknown weathering-climate interplays. The states where PCO2 < 10−0.5 atm and T > ∼30 °C are unstable due to the positive feedback, and immediately change with slight CO2 changes to either the states of PCO2 > 10−0.5 atm or those of PCO2 < 10−0.5 atm and T < ∼30 °C, both of which are stable due to the negative feedback. The latter states are especially stable against glaciations, because the feedback becomes more negative as temperature decreases, possibly explaining the stable climates in the Mesoproterozoic. When CH4 is present in atmosphere with CH4/CO2 ratio within a limited range (∼0.03–0.15), a positive feedback operates at low temperatures (<∼5 °C) and thus global glaciations can occur, which may explain the glaciation in the late Archean.The temperature and PCO2 transitions in the Precambrian were finally calculated based on the relationship between ΔHCO2′, T and FCO2 and the greenhouse effects of CO2. The calculated CO2 levels are high enough that the temperature could have been maintained at >0 °C only by CO2 through the Precambrian. The consistent PCO2 estimates from paleosols (fossil weathering profiles) in the literature support the argument. The calculated temperatures suggest that the Earth could have been cool to hot until around the end of Archean and cool to moderate afterwards.