Accretionary Pedogenesis and Holocene Climate Evolution in Black Soil Region of Northeast China: Evidence from a Sedimentary Profile

The black soil region of the Songnen Plain, one of the world's three major black soil belts, is critical for China's grain security, yet the formation mechanism of its thick, organic-rich soils remains insufficiently quantified. In this study, we investigate the ZYHPM01 profile in the eastern Songnen Plain using grain-size end-member analysis (EMA), multi-proxy geochemical tracers, and coarse-grained quartz optically stimulated luminescence (OSL) dating. The EMA identifies three sedimentary components: EM1 represents distal background dust transported via high-altitude suspension; EM2 and EM3 indicate proximal dust inputs driven by the winter monsoon. Based on the OSL-based chronostratigraphy constructed using piecewise linear regression, the Holocene evolution is divided into five stages. Following rapid accumulation in the Early Holocene (11.63–10.31 ka BP), the region experienced a climatic setback during the transition to the Middle Holocene, broadly consistent with high-latitude cooling episodes such as the 8.2 ka event. The Mid-Holocene Climatic Optimum (6.72–5.11 ka BP) established a warm-humid environment, with peak CIA values (~68.8) and minimum Sr/Cu ratios (~7.0) confirming intense chemical weathering and leaching, which served as the primary environmental window for black soil initiation. This period was terminated by an abrupt shift to cold-dry conditions in the Late Holocene, characterized by enhanced coarse-grained dust inputs driven by a strengthened winter monsoon. We propose a Formation-Preservation mechanism: the Mid-Holocene provided optimal hydrothermal conditions for intense chemical weathering, high biological productivity, and active pedogenesis (Formation Phase), while the subsequent Late Holocene climatic deterioration inhibited microbial decomposition and restricted chemical leaching, effectively preserving the accumulated organic carbon (Preservation Phase). These findings reveal that modern black soil in the Songnen Plain is a legacy of peak Mid-Holocene productivity preserved by climatic cooling, providing new insights for understanding critical zone evolution at monsoon margins.