Abstract
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.
Highlights
Quantifying ancient atmospheric pCO2 provides valuable insights into the interplay between greenhouse gases and global climate
The soil δ13Cs is calculated from δ13Cc using a temperaturedependent fractionation factor[8]; the soil-respired δ13Cr is approximated by δ13C value of coeval soil organic matter (SOM) preserved in the same paleosol strata[9]; and the carbon isotopic composition of atmospheric CO2 (δ13Ca) can be estimated from marine carbonate records[10]
Unlike the more humid southeastern Chinese Loess Plateau (CLP), where extensive leaching completely decalcified the paleosols, or the more arid northwestern CLP (e.g. Huanxian, Jingyuan), where large amount of detrital carbonates remained due to insufficient rainfall[24], our sampling location, the Luochuan section (35.76°N, 109.42°E) located in the central CLP (Supplementary Fig. 1), is characterized by the appropriate level of pedogenesis
Summary
Quantifying ancient atmospheric pCO2 provides valuable insights into the interplay between greenhouse gases and global climate. The soil δ13Cs is calculated from δ13Cc (the carbon isotopic composition of pedogenic carbonate) using a temperaturedependent fractionation factor[8]; the soil-respired δ13Cr is approximated by δ13C value of coeval soil organic matter (SOM) preserved in the same paleosol strata[9]; and the carbon isotopic composition of atmospheric CO2 (δ13Ca) can be estimated from marine carbonate records[10]. This approach has been extensively applied to pedogenic carbonates worldwide[11] to reconstruct pCO2 over the geological past. Significant uncertainties still remain in this paleosol CO2 barometer, which is mainly sourced from the S(z) term that describes soil respiration[12]
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