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

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 (Delta H'(CO2)), temperature (T) and silicate-weathering flux (F-CO2) 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-Delta H'(CO2)-F-CO2 relationship and the greenhouse effects of CO2, which represent Delta H'(CO2) on the global scale, with and without the presence of CH4. Calculation of the ratio of the change in F-CO2 to the corresponding change in the partial pressure of atmospheric CO2 (P-CO2) as an indicator of the silicate-weathering feedback on CO2 revealed hitherto unknown weathering-climate interplays. The states where P-CO2 < 10(-0.5) atm and T> similar to 30 degrees C are unstable due to the positive feedback, and immediately change with slight CO2 changes to either the states of P-CO2 > 10(-0.5) atm or those of P-CO2 < 10(-0.5) atm and T < similar to 30 degrees 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 (similar to 0.03-0.15), a positive feedback operates at low temperatures (0 degrees C only by CO2 through the Precambrian. The consistent P-CO2 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. (C) 2018 Elsevier Ltd. All rights reserved.