Quantitative Reconstruction Of Paleoatmospheric CO₂ Levels Using Pedogenic Carbonates From The Chinese Loess Plateau

Atmospheric CO₂ acts as one of the most important forcings on global climate changes.Due to anthropogenic activities since the Industrial Revolution,CO₂ level continues rising far beyond the range?ca.180-280 ppm?over the past 800 thousand years.Compared to decadal instrumental records,paleoclimate reconstruction provides quantitative information on the Earth system's response to climate forcings on time scales longer than a few centuries,which helps the refinement of climate models and facilitates a better constraint on future climate changes.With similar-to-modern boundary conditions as well as CO₂ levels,the late Cenozoic is considered an important analog for future climate.However,the causal effects between these climate transitions and CO₂ forcing remain unclear,as individual CO₂ estimates,mainly from the marine realm,differ in both magnitudes and detailed variation trends.Based on secondary carbonates formed during pedogenesis?pedogenic carbonate?,the paleosol CO₂ paleobarometer has been widely used for paleo-CO₂ reconstruction throughout the geological history.However,due to the difficulty in constraining one of the key variables — soil-respired CO₂ concentration during the formation time of pedogenic carbonate?S?z??,uncertainty related to this method remains large.The eolian deposits on the Chinese Loess Plateau?CLP?contains abundant pedogenic carbonates of various morphologies.Upon previous CLP studies,the ?13Cc is commonly regarded as a paleovegetation proxy,and has been widely used to track the evolution of C4 biomass and the East Asian summer monsoon.However,published records show that the spatiotemporal distributions of ?13Cc do not always track those of ?13C of soil organic matter??13CSOM?,indicating the potential influence of atmospheric CO₂ on the ?13Cc.Through systematic investigations of modern soil observation data and paleosol ?13C records,in combination with multiple mineralogical and geochemical methodologies,this study quantitatively evaluated the controlling factors of the ?13Cc.Using an empirical model to constrain S?z?,we were able to increase the precision of the paleosol-CO₂ estimates,and further reconstructed paleo-CO₂ levels.The main conclusions from this study are:1)Due to the deep formation depth and discontinuous distribution of calcite nodules — the traditional materials used for the paleosol-CO₂ proxy,their formation ages are difficult to constrain,and are thus inappropriate for the reconstruction of highresolution,continuous p CO₂ curve.Instead,this study focuses on paleosol layers in which dolomite was completely leached away but calcite remains.The finely disseminated calcites in these bulk paleosol samples are mainly composed of needle fiber calcites,and are identified as typical pedogenic carbonates through both mineralogical and geochemical analyses.Their shallow formation and continuous distribution throughout the paleosol profile effectively circumvent the problems related to calcite nodules.2)The spatiotemporal decoupling between the ?13C of pedogenic carbonates and soil organic matter widely exists,challenging the ?13Cc as a pure paleovegetation indicator.Results from both modern soil observations and paleosol ?13C records suggest that,under semiarid or arid environment such as the CLP,a weaker soil productivity?i.e.lower S?z??leads to a higher relative abundance of atmospheric CO₂ in the soil system,thereby affecting the ?13Cc.Estimates based on the two-component mixing model show that,atmospheric constitutes for a major portion of the total soil-CO₂?¹0-50%?.3)Modern soil CO₂ flux data from the CLP region show significant correlations with rainfall amounts,indicating that S?z?are predominantly controlled by the summer monsoonal rainfall.Consequently,the bulk soil magnetic susceptibility?MS?of the past 800-ky paleosols show positive correlations with S?z?.The MS-S?z?model can be used to quantitatively constrain paleo-S?z?levels,thereby increasing the precision of the paleosol-CO₂ method.4)Applying the MS-S?z?proxy on early Pleistocene paleosol samples,we were able to calculate high-precision paleosol-based CO₂ estimates.The results yield a p CO₂ range of 180-300 ppm during 2.6-0.8 Ma,and is in concert with snapshots of CO₂ levels retrieved from “blue ice” found in Antarctic Allan Hills,indicating that the Earth system has been operating under low-p CO₂ condition throughout the entire Pleistocene epoch.