The history of soil gleyization in the Songnen Plain over the last 195 ka, as revealed by the Harbin loess-paleosol sequence, NE China

Abstract

Understanding the spatiotemporal evolution patterns of East Asian summer monsoon (EASM) precipitation and its forcing mechanisms is essential for comprehending global climate change and regional environmental shifts. Despite this significance, a lack of well-exposed outcrop sections has constrained our understanding of the evolution and forcing mechanisms of the EASM in the Songnen Plain, NE China. To address this critical issue, this study conducted a high-resolution analysis of magnetic susceptibility, goethite (Gt), and hematite (Hm) in the Harbin loess-paleosol sequence located in the southeastern Songnen Plain. In addition, a close-spaced OSL and ESR dating was employed, with the primary aim of unraveling the paleoclimatic significance of these proxies in the uniquely developed black paleosol regions of the cold climate. The results indicate that the black paleosol layers exhibit significantly lower values of low-frequency magnetic susceptibility (χ_lf), goethite (Gt) and hematite (Hm) compared to the loess layers, indicating a decoupling between these indicators and the pedogenic processes. These findings imply that gleyization has influenced the Harbin black paleosol layers during their development. Furthermore, a negative correlation was observed (R² = 0.65 for χ_lf and R² = 0.42 for Hm/Gt ratio) between the χ_lf and the Hm/Gt ratio of the topsoil in potential dust sources, and mean annual precipitation (MAP). Consequently, the χlf-MAP and Hm/Gt-MAP climofunctions were proposed to reconstruct the paleoprecipitation recorded by the Harbin loess-paleosol sequence. The differences in precipitation, as constructed by χ_lf and Hm/Gt, can be used to characterize the degree in gleyization of the black paleosol. Based on the quantitative reconstruction of paleoprecipitation and paleogleyization, the paleoclimatic evolution of the Songnen Plain since 195 ka can be categorized into three stages: from 195 to 160 ka, the East Asian summer monsoon (EASM) was strong, characterized by a cold and wet climate and a low degree of soil gleyization; from 160 ka to 123 ka, the intensity of the EASM weakened, with the warm and dry climate, as well as an increased degree of soil gleyization; after 123 ka, monsoon precipitation continued to decline, the climate became cool and dry, and the rate of permafrost melting slowed; however, from 105 to 100 ka, a modest increase in temperature and monsoon rainfall occurred, accompanied by intensified soil gleyization; in contrast, from 65 to 55 ka, the climate resembled that of the earlier cold interval. Overall, the evolution of summer monsoon precipitation in the Songnen Plain is jointly regulated by both high- and low-latitude processes. The summer solar radiation and changes in Arctic ice volume are the main factors affecting the evolution of the EASM. Additionally, the Western Pacific Subtropical High (WPSH) or El Niño-Southern Oscillation (ENSO) events also contribute to the variations in monsoon precipitation to some extent.