Decoupled paleosol-based proxies in Chinese loess deposits: Role of leaching and illuviation processes

Abstract

The paleosol-based proxies derived from the ancient aeolian deposits on the Chinese Loess Plateau (CLP) are commonly used to infer past changes in climate and environment, but those proxies often display distinct spatiotemporal trends. Several mechanisms have been proposed to explain the differences between the proxy records from the CLP. However, no existing mechanism satisfyingly reconciles all discrepancies, and a process-based understanding is still lacking. Leaching-illuviation processes can modify the physicochemical properties of modern soil profiles, but their role in paleosol–loess proxy systems has not been assessed systematically. This work evaluates the influences of post-pedogenic leaching-illuviation processes on paleosol-based proxy systems in multiple loess-paleosol sections since the middle Pleistocene spanning a rainfall gradient across the CLP. We focus on a classical pedogenic proxy (magnetic susceptibility) and a suite of chemical weathering proxies (redness, carbonate content, and elemental ratios). The correlations between the studied pedogenic and chemical weathering proxies vary significantly across broad spatiotemporal scales. The studied proxies have strong correlations under low-to-moderate rainfall in the dry northwestern (NW) CLP but show weak correlations or decorrelate under high rainfall in the wet southeastern (SE) CLP. We compile multiple lines of evidence to demonstrate that leaching-illuviation processes operate to different extents in those sections and can reconcile the behaviors of those proxy systems across space and time. In addition, we propose that the correlation coefficient between magnetic susceptibility and chemical weathering proxies helps to assess the pristineness of paleosol-based proxies and the intensity of leaching-illuviation processes, which in turn reflects rainfall magnitude. We summarize our findings in a conceptual model describing how rainfall-facilitated leaching-illuviation processes affect these paleosol-based proxies. Overall, our work provides a comprehensive investigation of the influences on paleosol-based proxy systems from leaching-illuviation processes in aeolian deposits, advancing our understanding of how those proxy systems behave under changing climates.