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. Here, 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 associated with the winter monsoon. A Bayesian age-depth model constructed from five OSL ages establishes the chronostratigraphic framework, dividing the Holocene evolution into five stages. Following rapid accumulation in the Early Holocene (12.07-10.31 ka BP), the region experienced a cooling episode during the Early-to-Middle Holocene transition, broadly consistent with high-latitude cooling events such as the 8.2 ka event. The Mid-Holocene Climatic Optimum (6.72-5.11 ka BP) established a warm-humid environment, with elevated Chemical Index of Alteration (CIA) values (avg. ~ 64.4) and minimum Sr/Cu ratios (~6.88) indicating moderate chemical weathering and leaching; this interval served as the primary environmental window for black soil initiation. An abrupt shift to cold-dry conditions in the Late Holocene, marked by enhanced coarse-grained dust inputs driven by a strengthened winter monsoon, terminated this optimum. We propose a formation-preservation hypothesis: Mid-Holocene productivity formed the organic-rich substrate, while subsequent climatic cooling likely inhibited microbial decomposition and restricted chemical leaching, thereby favoring preservation of the accumulated organic carbon. These findings suggest that the black soil at ZYHPM01 represents Mid-Holocene productivity preserved by Late Holocene climatic cooling, offering new insights into critical zone evolution at monsoon margins.