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 CO 2 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 CO 2 ( Δ H C O 2 ′ ), temperature ( T ) and silicate-weathering flux ( F C O 2 ) has been recently established ( Kanzaki and Murakami, 2018 ). The present study examined the effects of atmospheric CO 2 and CH 4 on silicate weathering in the Precambrian based on the above T - Δ H C O 2 ′ - F C O 2 relationship and the greenhouse effects of CO 2 , which represent Δ H C O 2 ′ on the global scale, with and without the presence of CH 4 . Calculation of the ratio of the change in F C O 2 to the corresponding change in the partial pressure of atmospheric CO 2 ( P C O 2 ) as an indicator of the silicate-weathering feedback on CO 2 revealed hitherto unknown weathering-climate interplays. The states where P C O 2 −0.5 atm and T  > ∼30 °C are unstable due to the positive feedback, and immediately change with slight CO 2 changes to either the states of P C O 2  > 10 −0.5 atm or those of P C O 2 −0.5 atm and T 4 is present in atmosphere with CH 4 /CO 2 ratio within a limited range (∼0.03–0.15), a positive feedback operates at low temperatures ( The temperature and P C O 2 transitions in the Precambrian were finally calculated based on the relationship between Δ H C O 2 ′ , T and F C O 2 and the greenhouse effects of CO 2 . The calculated CO 2 levels are high enough that the temperature could have been maintained at >0 °C only by CO 2 through the Precambrian. The consistent P C O 2 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.