Quantifying ancient atmospheric pCO2 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 pCO2 changes remain among different proxy-derived results. The traditional paleosol pCO2 paleobarometer suffers from largely unconstrained soil-respired CO2 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 pCO2 during 2.6–0.9 Ma, which documents overall low pCO2 levels (<300 ppm) comparable with ice core records, indicating that the Earth system has operated under late Pleistocene pCO2 levels for an extended period. The pCO2 levels do not show statistically significant differences across the mid-Pleistocene Transition (ca. 1.2–0.8 Ma), suggesting that CO2 is probably not the driver of this important climate change event.

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Pedogenic carbonate is an invaluable archive for reconstructing continental paleoclimate and paleoecology. The δ13C of pedogenic carbonate (δ13Cc) has been widely used to document the rise and expansion of C4 plants over the Cenozoic. This application requires a fundamental presumption that in soil pores, soil-respired CO2 dominates over atmospheric CO2 during the formation of pedogenic carbonates. However, the decoupling between δ13Cc and δ13C of soil organic matter (δ13CSOM) have been observed, particularly in arid regions, suggesting that this presumption is not always valid. To evaluate the influence of atmospheric CO2 on soil δ13Cc, here we performed systematic δ13C analyses of paleosols across the Chinese Loess Plateau, with the sample ages spanning three intervals: the Holocene, the Late Pleistocene, and the mid-Pliocene warm period. Our paired δ13Cc and δ13CSOM data reveal broadly divergent trending patterns. Using a two-component CO2-mixing model, we show substantial incorporations of atmospheric CO2 (up to 60%) into soil pore space during carbonate precipitation. This result readily explains the enrichment of δ13Cc and its divergence from δ13CSOM. As a consequence, δ13C of pedogenic carbonates formed under semiarid and/or arid conditions are largely driven by regional aridity through its control on soil CO2 composition, and thus cannot be used to evaluate the relative abundance of C3 versus C4 plants. Nonetheless, these carbonates can be applied for atmospheric CO2 reconstructions, even for periods with low CO2 levels.

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Soils represent the largest terrestrial carbon pool, a majority of which come from atmospheric CO 2 that is fixed through photosynthesis by the terrestrial biosphere (Stockmann et al., 2013). The exchange of carbon between the atmosphere and the terrestrial biosphere makes soils an archive of past environmental changes (Pachauri et al., 2014). For example, the stable carbon isotopic composition (δ 13 C) of soil organic matter (SOM) has been used to approximate the δ 13 C of CO 2 produced by soil respiration-a key parameter in using paleosols to reconstruct paleoatmospheric CO 2 levels (Breecker, 2010;Cerling, 1992). Moreover, the δ 13 C SOM of ancient soils, or paleosols, have been widely used to narrate the evolution history of aboveground vegetation habitat as well as hominin environments (e.g., Cerling, 1991;Ehleringer et al., 1997).The decomposition of SOM represents a significant obstacle when using δ 13 C SOM records to reconstruct paleoenvironments (Bowen & Beerling, 2004). SOM is a mixture of organic compounds (e.g., lipids, lignins) of both plant and microbial origins, featuring distinct δ 13 C signals, physicochemical properties, and reaction kinetics (Ehleringer et al., 2000). Changes in the relative abundances of those compounds during SOM degradation could cause 13 C fractionation, altering the δ 13 C signal of fresh, undegraded soils. In well-drained, modern soil profiles, the decrease of total organic carbon content (TOC) along depth usually occurs concomitantly with a progressive increase of δ 13