Estimates of atmospheric CO2 in the Neoarchean–Paleoproterozoic from paleosols

Abstract

Atmospheric CO2 levels reflect Earth’s surface temperatures directly, and have been discussed especially in association with the faint young Sun and Snowball Earth in the Precambrian. In addition, atmospheric O2 levels in the Precambrian have been estimated from paleosols, fossil weathering profiles, based on the estimates of atmospheric CO2 levels. Nevertheless, atmospheric CO2 levels in the Neoarchean and the Paleoproterozoic have remained as a debatable topic. In order to precisely estimate atmospheric CO2 levels in the Neoarchean–Paleoproterozoic, we developed a new method that calculates CO2 levels from the chemical compositions of paleosols. The new method (i) calculates the cation concentrations in porewaters at the time of weathering from those of paleosols, (ii) describes the relationships between partial pressure of atmospheric CO2 (PCO2), pH and cation concentrations based on the charge balance between the cations and anions including carbonate species in porewaters, and (iii) finally calculates PCO2 levels at a given temperature constraining pH by thermodynamics of weathering secondary-minerals. By applying the new method to modern weathering profiles, we obtained a good agreement between the calculated and observed PCO2 levels. The weathering rate deduced from the new method was proportional to PCO2 with fractional dependence of 0.18 and the apparent activation energy of weathering was 40–55kJmol−1, which is consistent with the laboratory and field results. The application to modern weathering and the formulated characteristics of weathering strongly indicate that the new method is valid and robust.The new method was then applied to eight paleosols formed in the Neoarchean–Paleoproterozoic. We made constraints on the local temperatures, at which the paleosols were formed, mainly by the temperature and solute-concentration relationships in the literature, because they should have been different between the paleosols and from the average global surface temperatures. Under the constrained local temperatures, the PCO2 levels were calculated to be 85–510times the present day atmospheric level (PAL) at ∼2.77Ga, 78–2500PAL at ∼2.75Ga, 160–490PAL at ∼2.46Ga, 30–190PAL at ∼2.15Ga, 20–620PAL at ∼2.08Ga and 23–210PAL at ∼1.85Ga. The estimated PCO2 levels are higher than those to maintain the average global surface temperature of the Earth above the freezing point of water only by CO2 itself. The newly estimated PCO2 levels probably imply that atmospheric CO2 decreased gradually in long term in the Neoarchean–Paleoproterozoic and that the glaciations at ∼2.9 and ∼2.4–2.2Ga were differently triggered.