Causes and consequences of low atmospheric pCO2 in the Late Mesoproterozoic

- New paleosol-based estimates of Mesoproterozoic atmospheric CO2 have been made.
- The CO2 level (< 5 × Preindustrial) represents a newly recognized Proterozoic minimum.
- Atmospheric CO2 drawdown from 1.8-1.1 Ga ago is accompanied by increased C burial.
- Long-term changes in C burial likely resulted in slowly increasing atmospheric O2.
- Enhanced C burial was probably triggered by enhanced productivity by the biosphere.

Based upon various proxy, theoretical, and model constraints, Paleoproterozoic atmospheric pCO2 was much higher than Phanerozoic levels. However, relatively little is known about the transition between the two climate states. Here, geochemical mass-balance from ~ 1.1 Ga old Midcontinent Rift System (USA) paleosols is used to reconstruct atmospheric pCO2 during the Mesoproterozoic. The calculations robustly indicate low atmospheric pCO2 (< 10 times Preindustrial levels). Results are consistent between seven paleosols at one site, between paleosols at different Midcontinental Rift sites, and between the new results and previously published penecontemporaneous paleosol and microfossil reconstructions. The newly recognized Mesoproterozoic pCO2 minimum is best explained as the culmination of a long-term C burial event by the biosphere that is also indicated by marine carbonate δ13C changes during the Mesoproterozoic, and which is consistent with changes in the biosphere including increased stromatolite abundance and diversity, evolution of sulfur-utilizing bacteria, and the spread of microbial mats into continental environments.