A quantitative reconstruction of a loess–paleosol sequence focused on paleosol genesis: An example from a section at Chaoyang, China

Abstract

A typical loess–paleosol sequence at Chaoyang in China, deposited continuously since 423ka BP, was examined to quantitatively reconstruct a loess–paleosol sequence. First, particle size distribution was evaluated based on clay-free fractions (where the sum of sand+silt=100%) and fine-earth fractions and used to characterize the parent material uniformity as well as field morphology, geochemical features, and micro-morphology characteristics. Second, mass balance (MB) and soil reconstruction (SR) approaches were employed to examine the linkage between pedogenesis and loess deposition. The results indicated a uniform loess deposit below 228cm depth, and the presence of four reddish stratigraphic layers interbedded with four yellowish stratigraphic layers, all derived mostly from silt eolian deposits. The main soil pedogenic processes following silt deposition were clay eluviation–illuviation, silification, and ferrallitization. The strongest pedogenesis occurred for the S3 deposit layer during 225–243ka BP and was associated with the greatest mass loss of non-clay (7.6%), Na2O (42%), CaO (21%), MgO (28%), SiO2 (10%), and at the same time with the greatest gains in clay (42%), Fe2O3 (8%), and Al2O3 (4%) together with mass loss changes (MC) of 620g/100cm2. The greatest mass gain changes (234g/100cm2) occurred in the L4 deposit layer during 243–311ka BP and were associated with gains in Na2O (18%) and P2O5 (4.5%) indicating the fastest deposition rate. During the long period of sedimentation (423ka BP) loess deposition was predominant during dry-cold periods, whereas paleosol formation from pedogenic processes was predominant during wet-warm periods. Thus, the formation of paleosols and loess deposition were dependent on the relative rate of pedogenesis and deposition as controlled by climate change. Under an East Asian summer monsoon (EAS) paleosol formation dominated, whereas loess deposition dominated under an East Asian winter monsoon (EAW). Quantification of the degree of loess development (paleosols) or loess deposits aids reconstructing soil genesis and understanding the loess–paleosol formation sequence and its linkage to climate change.