Quantifying ancient atmospheric pCO_2 provides valuable insights into the interplay between greenhouse gases and global climate. Beyond the 800-ky history uncovered by ice cores, discrepancies in both the trend and magnitude of pCO_2 changes remain among different proxy-derived results. The traditional paleosol pCO_2 paleobarometer suffers from largely unconstrained soil-respired CO_2 concentration (S(z)). Using finely disseminated carbonates precipitated in paleosols from the Chinese Loess Plateau, here we identified that their S(z) can be quantitatively constrained by soil magnetic susceptibility. Based on this approach, we reconstructed pCO_2 during 2.6–0.9 Ma, which documents overall low pCO_2 levels (<300 ppm) comparable with ice core records, indicating that the Earth system has operated under late Pleistocene pCO_2 levels for an extended period. The pCO_2 levels do not show statistically significant differences across the mid-Pleistocene Transition (ca. 1.2–0.8 Ma), suggesting that CO_2 is probably not the driver of this important climate change event
