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

摘要

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 (similar to 5 degrees C) and thus global glaciations can occur, which may explain the glaciation in the late Archean.